CN117439419A

CN117439419A - Flyback conversion circuit, control method thereof and control chip

Info

Publication number: CN117439419A
Application number: CN202311418259.3A
Authority: CN
Inventors: 林思聪
Original assignee: Fanta Semiconductor Technology Hangzhou Co ltd
Current assignee: Fanta Semiconductor Technology Hangzhou Co ltd
Priority date: 2023-10-28
Filing date: 2023-10-28
Publication date: 2024-01-23

Abstract

The embodiment of the invention provides a flyback conversion circuit, a control method and a control chip thereof. The circuit comprises an energy storage tube, a continuous tube and an excitation inductance, and the control method comprises the following steps: acquiring a first voltage; the first voltage is the voltage on the excitation inductor when the energy storage tube is conducted; acquiring a first conduction time; the first conduction time is the conduction time of the energy storage tube; calculating a first volt-second product of the excitation inductance according to the first voltage and a first conduction time; compensating the first volt-second product to obtain a compensation volt-second product, wherein the compensation volt-second product is larger than the first volt-second product; acquiring a second voltage; the second voltage is the voltage on the excitation inductor when the energy storage tube is turned off; and calculating a second conduction time according to the compensation volt-second product and a second voltage, wherein the second conduction time is the conduction time of the follow-up pipe. The control method can realize zero-voltage turn-on control of the energy storage tube.

Description

Flyback conversion circuit, control method thereof and control chip

[ field of technology ]

The invention relates to the field of switch circuit control, in particular to a flyback conversion circuit, a control method thereof and a control chip.

[ background Art ]

With the development of consumer electronics applications, requirements for device charging are continuously increasing, and USB charging standards are continuously updated. The latest USB PD3.1 increases the extended power range (Extended Power Range, EPR for short), and the maximum power is extended from 100W to 240W. The PD3.1 provides a more flexible charging scheme for devices supporting higher power, such as game books, displays, printers, even electric vehicles, etc.

The circuit topology adopted by the current mainstream portable charging equipment mainly comprises a flyback converter. The device has the advantages of simple topological structure and low cost, can realize the electrical isolation of primary and secondary sides, and provides good electrical and safety performance. However, the transformer of the flyback converter has a large volume and low power efficiency, and is difficult to meet the requirements of people on high efficiency and miniaturized charging equipment. LLC resonant converters enable ZVS (Zero Voltage Switch, zero voltage switching) with higher power density and efficiency, but in a scenario with a wide output voltage range like PD, the efficiency is not always kept optimal. The output voltage changes and the efficiency is reduced. And compared with a flyback converter, the cost is greatly improved.

Common flyback converters include an AHB flyback converter (asymmetric half-bridge flyback converter), an ACF flyback converter (active clamp flyback converter), and a ZVS flyback converter (flyback converter with zero voltage switching function), and how to control the flyback converter to achieve higher efficiency is a technical problem to be solved.

[ invention ]

In view of this, the embodiment of the invention provides a flyback converter circuit, a control method thereof and a control chip thereof, so as to realize the ZVS function of the energy storage tube and improve the circuit efficiency.

In order to solve the technical problems, the application adopts the following technical scheme:

the control method of the flyback conversion circuit comprises the following steps of:

acquiring a first voltage; the first voltage is the voltage on the excitation inductor when the energy storage tube is conducted;

acquiring a first conduction time; the first conduction time is the conduction time of the energy storage tube;

calculating a first volt-second product of the excitation inductance according to the first voltage and a first conduction time;

compensating the first volt-second product to obtain a compensation volt-second product, wherein the compensation volt-second product is larger than the first volt-second product;

Acquiring a second voltage; the second voltage is the voltage on the excitation inductor when the energy storage tube is turned off;

and calculating a second conduction time according to the compensation volt-second product and a second voltage, wherein the second conduction time is the conduction time of the follow-up pipe.

According to the control method, when the energy storage tube is conducted, the first volt-second product of the excitation inductor is compensated to obtain the compensation volt-second product, and then the conduction time of the continuous flow tube is calculated according to the compensation volt-second product, so that ZVS control of the energy storage tube can be achieved, and the circuit efficiency is improved.

The control chip of the flyback conversion circuit comprises an energy storage tube, a continuous flow tube, an excitation inductance and an auxiliary winding; the control chip at least comprises an auxiliary winding pin, a voltage acquisition module, a compensation module, a timing module and a PWM logic module;

the auxiliary winding pin can be electrically connected with one end of the auxiliary winding and is at least used for acquiring the voltage of the auxiliary winding;

the voltage acquisition module is electrically connected with the auxiliary winding pin and is at least used for acquiring a first voltage and a second voltage according to the auxiliary winding voltage; the first voltage is the voltage on the excitation inductor when the energy storage tube is conducted; the second voltage is the voltage on the excitation inductor when the energy storage tube is turned off;

The compensation module is used for compensating the first volt-second product to obtain a compensation volt-second product; the first volt-second product is the product of a first conduction duration and the first voltage; the compensation volt-second product is greater than the first volt-second product;

the timing module is used for acquiring the first conduction time length and acquiring the second conduction time length of the continuous flow pipe according to the compensation volt-second product and the second voltage; the first conduction time is the conduction time of the energy storage tube; the second conduction time is the conduction time of the continuous flow pipe;

the PWM logic module is electrically connected with the timing module and is used for generating PWM signals for controlling the operation of the freewheel according to at least the second conduction duration.

The control chip can realize the control method, so that the energy storage tube can realize ZVS control, and the circuit efficiency is improved.

A flyback conversion circuit comprises a main topology unit, a secondary side unit and the control chip of the flyback conversion circuit; the main topology unit of the flyback conversion circuit comprises an energy storage tube, a continuous flow tube, a transformer, leakage inductance, an excitation inductance, a resonance capacitor and an auxiliary winding; the control chip is electrically connected with at least the auxiliary winding, the energy storage tube and the freewheel tube.

Under the control of the control chip, the flyback conversion circuit can control the on-time of the freewheel tube, thereby realizing ZVS control of the energy storage tube and being beneficial to improving the circuit efficiency.

[ description of the drawings ]

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings can be obtained according to these drawings without inventive effort for a person skilled in the art, but shall fall within the scope of protection of the present application.

FIG. 1 is a schematic diagram of an AHB flyback converter circuit in the prior art;

FIG. 2 is a schematic diagram of an ACF flyback converter circuit according to the prior art;

fig. 3 is a flowchart of a control method of a flyback converter circuit according to an embodiment of the present application;

FIG. 4 is a schematic diagram of an asymmetric half-bridge flyback circuit according to the present disclosure;

FIG. 5 is a schematic diagram of waveforms in CRM mode for an asymmetric half-bridge flyback circuit;

FIG. 6 is a schematic diagram of waveforms in DCM mode;

FIG. 7 is a schematic diagram of the principle of charging and discharging the junction capacitance of the energy storage tube and the freewheel tube with exciting inductance current;

FIG. 8 is a schematic waveform diagram for ZVS implementation in CRM mode;

FIG. 9 is a schematic diagram of a control method for adaptive adjustment of compensation coefficients;

FIG. 10 is a waveform schematic diagram of a ZVS detection method based on auxiliary winding voltage drop time;

FIG. 11 is a waveform diagram of ZVS pulse segment control in DCM mode;

FIG. 12 is a schematic block diagram of a control chip according to an embodiment of the present invention;

FIG. 13 is a schematic waveform diagram of the auxiliary winding voltage in CRM mode;

FIG. 14 is a schematic block diagram of a control chip according to another embodiment of the present application;

FIG. 15 is a schematic waveform diagram of the control chip of FIG. 14;

FIG. 16 is a schematic block diagram of a compensation module;

fig. 17 is a schematic block diagram of a first ZVS decision unit;

fig. 18 is a schematic block diagram of a second ZVS decision unit;

fig. 19 is a schematic block diagram of a third ZVS decision unit;

fig. 20 is a schematic block diagram of a fourth ZVS decision unit;

FIG. 21 is a schematic block diagram of a control chip according to another embodiment of the present invention;

fig. 22 is a schematic diagram of an asymmetric half-bridge flyback circuit according to an embodiment of the present invention.

[ detailed description ] of the invention

For a better understanding of the technical solution of the present invention, the following detailed description of the embodiments of the present invention refers to the accompanying drawings.

It should be understood that the described embodiments are merely some, but not all, embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The terminology used in the embodiments of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in this application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Electrical connections primarily represent the transfer of signals, including direct electrical connections and indirect electrical connections.

It should be understood that the term "and/or" as used herein is merely one way of describing an association of associated objects, meaning that there may be three relationships, e.g., a and/or b, which may represent: the first and second cases exist separately, and the first and second cases exist separately. In addition, the character "/" herein generally indicates that the front and rear associated objects are an "or" relationship.

As shown in fig. 1, the asymmetric half-bridge flyback converter circuit has advantages such as simple structure, low cost, ZVS realization, wide output voltage range adaptation, and the like, and is favored in recent years. As shown in fig. 2, the ACF flyback converter circuit has wider input voltage regulation capability, higher efficiency and switching frequency. How to simply and reliably control a flyback converter to realize ZVS and improve the self-adaptation degree or efficiency of the flyback converter is a technical problem to be solved.

The asymmetric half-bridge flyback circuit shown in fig. 1 comprises an energy storage tube Q1, a freewheel tube Q2, leakage inductance Lr, excitation inductance Lm, a resonance capacitor Cr and a transformer T; the first end of the energy storage tube Q1 is electrically connected with the high-potential end of the input voltage Vin, the second end of the energy storage tube Q1 is electrically connected with the first end of the follow current tube Q2, and the second end of the follow current tube Q2 is electrically connected with the low-potential end of the input voltage Vin; the resonance unit of the asymmetric half-bridge flyback circuit comprises a leakage inductance Lr, an excitation inductance Lm and a resonance capacitor Cr which are connected in series; one end of the resonance unit is electrically connected with the first end of the follow current tube Q2, and the other end of the resonance unit is electrically connected with the second end of the follow current tube; the primary winding of the transformer is connected in parallel with the excitation inductance Lm, and the secondary winding of the transformer and the secondary circuit form a secondary unit. The control method of the asymmetric half-bridge flyback circuit mainly comprises a complementary control strategy of the energy storage tube Q1 and the freewheel tube Q2 and a non-complementary control strategy of the energy storage tube Q1 and the freewheel tube Q2. The energy storage tube Q1 generally adopts a peak current control mode or an on-time control mode. Since the complementary control strategies of Q1 and Q2 result in excessive negative system current, the system loop is energy intensive and overall efficiency is reduced, and therefore, is rarely employed.

The ACF flyback conversion circuit shown in fig. 2 also comprises an energy storage tube Q1, a freewheel tube Q2, leakage inductance Lr, excitation inductance Lm, a resonance capacitor Cr and a transformer T; the resonant unit of the ACF flyback circuit also comprises a leakage inductance Lr, an excitation inductance Lm and a resonant capacitor Cr which are connected in series. The ZVS flyback converter circuit may implement ZVS by means of a ZVS auxiliary winding, which comprises at least a capacitor and a shunt tube.

The leakage inductance Lr and the excitation inductance Lm are only schematic in circuit schematic, and do not necessarily include the device, and may be implemented by a transformer design, or may be implemented by an inductance, which is not limited in this application.

In order to realize the control of the flyback converter circuit with the resonant unit, the application provides a control method of the flyback circuit shown in fig. 1, wherein the control method is shown in fig. 3 and comprises the following steps:

step S1: acquiring a first voltage V1; the first voltage V1 is the voltage on the excitation inductance Lm when the energy storage tube Q1 is turned on;

step S2: acquiring a first conduction time T1; the first conduction time T1 is the conduction time of the energy storage tube Q1; the conducting time length is determined by the peak current in the peak current control mode;

step S3: calculate the first volt-second product: calculating a first volt-second product S1 of the excitation inductance according to the first voltage V1 and the first conduction time T1; where s1=v1×t1.

Step S4: and (3) compensation: compensating the first volt-second product S1 to obtain a compensation volt-second product Sc, wherein the compensation volt-second product Sc is larger than the first volt-second product S1; specifically, the first voltage may be compensated, or the first conduction period may be compensated, or the product of the first voltage and the first conduction period may be compensated, but the embodiment of the compensation is that the volt-second product on the excitation inductance Lm when Q1 is turned on is increased no matter how the compensation is performed.

Step S5: acquiring a second voltage V2; the second voltage V2 is the voltage on the excitation inductor when the energy storage tube Q1 is turned off;

step S6: calculating a second conduction duration T2: and calculating a second conduction time length T2 according to the compensation volt-second product Sc and the second voltage V2, wherein the second conduction time length T2 is the conduction time length of the freewheel Q2.

In this embodiment, the off-time of Q1 is determined by the peak current, the off-time of Q2 is determined by steps S1-S6, and the on-times of Q1 and Q2 are determined by the current zero-crossing point, the on-valley bottom, the switching frequency, or the dead time of two switching transistors. The first voltage and the second voltage may be obtained by adding an auxiliary winding as shown in fig. 4. In addition, if the secondary winding is not provided, the first voltage and the second voltage may also be obtained by:

when Q1 is turned on, neglecting the influence of leakage inductance, the first voltage V1 is the difference between the input voltage Vin and the resonant capacitor voltage Vcr, and can be expressed by the formula: v (V) ₁ ＝V _in -V _cr Acquiring a first voltage; when Q1 is off, a second voltageWhere Vo is the output voltage, np is the primary side turn ratio of the transformer, ns is the secondary side turn ratio. The first voltage and the second voltage may be obtained by sampling, and the application is not limited thereto.

The existing control method is that for the control of the Q2 tube, a curve of output voltage and on time is configured, the complexity of the system is increased, the self-adaption degree is low, and the influence of main circuit parameters is easy. In addition, in order to ensure that the system always realizes ZVS, a self-adaptive regulation strategy of a Q2 pipe is introduced. The regulation control strategy is adaptively regulated according to the occurrence time of the ZCD signal. However, the time at which the ZCD signal appears is not completely related to whether the Q1 pipe achieves ZVS. Thus, repeated trial and error adjustments are required, which results in further increased difficulty in use. The control method provided by the application determines the conduction time of the Q2 according to the parameters related to the field inductance Lm volt-second product when the Q1 is conducted, is simple and reliable in control and high in self-adaptation degree, can well exert the topological advantages, and achieves higher efficiency.

The compensation method in step S4 may be superposition or product, taking the product as an example:

the compensation volt-second product and the second conduction time length are obtained by compensation and calculation through the following formula:

Sc＝k*S1＝k*V1*T1；

t2=sc/V2; wherein k is a compensation coefficient and k is greater than 1, sc is a compensation volt-second product, S1 is a first volt-second product, V1 is a first voltage, T1 is a first conduction period, V2 is a second voltage, and T2 is a second conduction period.

In one embodiment, the compensation coefficient k is unchanged if the input voltage Vin and/or the output voltage range of the flyback converter circuit is within a preset range. That is, the on time of the flywheel Q2 is relatively fixed within a certain input/output voltage range.

In one embodiment, the compensation coefficient k is set to be positively correlated with the input voltage Vin of the flyback converter circuit and/or the capacitance value C1 of the junction capacitance q1_coss of the energy storage tube Q1, i.e. k=f (Vin), or k=f (C1), or k=f (Vin, C1), the specific form of the function f is not limited, but the dependent variable of the function f is positively correlated with the magnitude of at least one independent variable.

Taking an asymmetric half-bridge flyback circuit as an example, the main structure and main key signals in the topology are shown in fig. 5, wherein Q1 is an energy storage tube, Q2 is a continuous tube, lr is leakage inductance of a transformer, lm is excitation inductance of the transformer, and Cr is resonance capacitance. Lr, lm and Cr together form a resonant cavity of the asymmetric half-bridge flyback converter. Wherein the resonant capacitance Cr also can transfer energy to the secondary side, thus contributing to a reduction in the volume of the system transformer. When Q1 is turned on, the topology absorbs energy from the input voltage Vin and stores the energy in the excitation inductance Lm and the resonance capacitance Cr. When Q1 is off, the topology begins to transfer energy to the secondary side. Q2 is a freewheeling tube, when Q1 is closed and the driving level of Q2 is low, the body diode of Q2 conducts freewheeling, and when the driving level of Q2 is high, the channel in the Q2 tube is conducted, so that the on-resistance can be effectively reduced, the efficiency is improved, and at the moment, the energy stored on Lr, lm and Cr is transferred to the secondary side through Q3. The control method of the asymmetric half-bridge flyback circuit provided by the application is a non-complementary control method, which is beneficial to improving the system efficiency, and the basic working modes comprise: CRM (critical) mode and DCM (discontinuous) mode; drive waveform G of energy storage tube Q1 _Q1 Drive waveform G of freewheel Q2 _Q2 Current waveform I of leakage inductance Lr _Lr And a current waveform I of the excitation inductance Lm _Lm Midpoint voltage V of the sum midpoint HB _HB Wave of (2)As shown in fig. 5 and 6, the waveforms in the CRM mode are shown in fig. 5, and the waveforms in the DCM mode are shown in fig. 6. For DCM mode, if Q1 turns on near the peak of resonance for HB, then it again belongs to quasi-resonant mode of operation. During the control of the freewheel Q2, the current I is caused by controlling its on-time _Lm To a negative value, ZVS of the tank Q1 tube is facilitated.

The leakage inductance Lr is usually much smaller than the excitation inductance Lm, so when the follow-up tube Q2 is turned off, I _Lm And I _Lr Will coincide soon and be equal in value. Neglecting the influence of leakage inductance, this time can be regarded as the current I of the excitation inductance _Lm The junction capacitance q1_coss of Q1 is discharged and the junction capacitance q2_coss of Q2 is charged as shown in fig. 7.

E when the resonance unit resonates _Lm Energy stored for exciting inductance E _coss The energy corresponding to the charge and discharge of the resonant capacitor is only the negative current I on Lm _neg Corresponding E _Lm Greater than E _coss Then during the dead zone of Q1 and Q2, the junction capacitance of Q1 and Q2 can end up charging and discharging, enabling ZVS conduction of Q1, thereby improving system efficiency. In order to reliably realize ZVS of the energy storage tube, I is arranged _neg A certain size requirement needs to be met. When negative current I is applied to Lm _neg When the absolute value of (a) meets a certain requirement, the stored energy can completely charge and discharge the junction capacitance of Q1 and Q2, so that ZVS is realized, as shown in fig. 8. According to the method, the conduction time of the Q2 is increased, so that negative current in Lm resonance is large enough, enough energy is stored in the excitation inductor, the junction capacitance of the Q1 and the Q2 can be successfully charged, ZVS can be better realized by the Q1, and the efficiency of the system is improved. In the embodiment, a compensation coefficient k is introduced, and control of Q2 is indirectly realized by controlling k, so that negative current of the excitation inductor is increased.

Based on the above principle, the embodiment of the present application further provides an adaptive control method of the flyback converter circuit, which adjusts the compensation coefficient k according to the ZVS state of the system (i.e., the ZVS state of Q1), and then controls the conduction time of the freewheel (i.e., the second conduction period T2) based on k, as shown in fig. 9. Wherein the first voltage and the second voltage can be obtained through the auxiliary winding AUX.

In the control method with the self-adaptive adjustment compensation coefficient, the compensation coefficient is obtained at least through the following steps:

judging whether the energy storage tube Q1 realizes ZVS, if so, controlling the compensation coefficient of the next switching period to be smaller than that of the current switching period, reducing Lm negative current and reducing system circulation; if not, the compensation coefficient of the next switching period is controlled to be larger than that of the current switching period, and Lm negative current is increased to realize ZVS.

Specifically, in one embodiment, controlling the compensation coefficient of the next switching cycle to be smaller than the compensation coefficient of the present cycle includes the steps of: reducing the step length on the basis of the compensation coefficient of the current switching period to obtain the compensation coefficient of the next switching period; the step of controlling the compensation coefficient of the next switching period to be larger than the compensation coefficient of the current switching period comprises the following steps: step length is increased on the basis of the compensation coefficient of the current switching period to obtain the compensation coefficient of the next switching period; the step length is a preset value. The step size may be one thousandth, or other value. The larger the step size setting, the faster the regulation speed of system k, and when an input voltage change or an output voltage change occurs, the system can quickly enter the ZVS state again. However, this value is too large, which may cause instability of the system, and needs to be set according to actual conditions.

Determining whether the energy storage tube Q1 achieves ZVS may be obtained by:

mode one: obtaining midpoint voltage V when the energy storage tube is conducted _HB The method comprises the steps of carrying out a first treatment on the surface of the Midpoint voltage V _HB Is the common point voltage of the energy storage tube Q1 and the freewheel tube Q2;

will midpoint voltage V _HB Subtracting the first threshold value to obtain a reference voltage; the first threshold may be preset to a small value, such as less than 1V.

When the energy storage tube is conducted again, the midpoint voltage V of the energy storage tube Q1 is obtained again _HB The midpoint voltage V to be acquired again _HB ' comparing with the reference voltage;

if the midpoint voltage V is acquired again _HB And if' is greater than or equal to the reference voltage, the midpoint voltage can reach or approach Vin, the energy storage tube Q1 is judged to realize ZVS, otherwise, the energy storage tube Q1 is judged to not realize ZVS.

Mode two: preset midpoint voltage V _HB Is a reference change slope of (2); midpoint voltage V _HB Is the common point voltage of the energy storage tube Q1 and the freewheel tube Q2;

acquiring the change slope of the midpoint voltage in the dead time of the energy storage tube and the follow-up tube;

comparing the change slope of the midpoint voltage with a reference change slope of the midpoint voltage;

if the change slope of the midpoint voltage is larger than or equal to the reference change slope of the midpoint voltage, the midpoint voltage can reach or approach Vin, and then the energy storage tube is judged to realize ZVS; otherwise, judging that the energy storage tube does not realize ZVS. In this embodiment, the transformation slope may be obtained by sampling the midpoint voltage, and performing signal conditioning and differentiation.

Mode three: if the flyback converter circuit includes an auxiliary winding, referring to fig. 10, determining whether the energy storage tube achieves ZVS includes the steps of:

Acquiring a first time length; the first time period is the time (e.g. t) when the auxiliary winding voltage Vaux is less than the voltage threshold Vth after the freewheel Q2 is turned off _cnt1 /t _cnt2 Shown). The voltage threshold Vth is preset to obtain a threshold, specifically, vth can be obtained by subtracting a customizable value Δv from Vaux1, and one practical parameter is set to Δv to be (2-12) V, where Vaux1 is the voltage on winding AUX when Q1 is turned on.

Judging the first time length and the expected time length T _{fall_time} Is of a size of (2); desired duration T _{fall_time} The expected duration is obtained through presetting or the expected duration is in positive correlation with the input voltage according to the self-adaptive adjustment of the input voltage of the flyback conversion circuit;

if the first time is t _cnt1 Less than or equal to the expected time length T _{fall_time} The auxiliary winding voltage can be quickly reduced to Vaux1, and the energy storage tube is judged to realize ZVS; otherwise, judging that the energy storage tube does not realize ZVS.

FIG. 10 is a schematic waveform diagram based on auxiliary winding fall time detection, wherein the fall time t _cnt2 >t _cnt1 The auxiliary winding is longer in falling time, the negative current on the exciting inductance is smaller, the stored energy is less, the junction capacitance reversing capability is weaker finally, and the system cannot realize ZVS. In this embodiment, to further optimize system performance, T _{fall_time} The self-adaptive adjustment is carried out according to the input voltage Vin, and the required falling time is relatively longer as the input voltage Vin is higher, the expected duration T can be set _{fall_time} Is positively correlated, e.g., proportional, to the input voltage Vin to simultaneously compromise system efficiency and meet ZVS conditions. If the desired duration is set to a fixed value, when the input voltage increases, the negative current on the exciting inductance may be excessively large, resulting in an increase of the system loop current, and a decrease of the system efficiency.

Mode four: if the flyback converter circuit includes an auxiliary winding, determining whether the energy storage tube achieves ZVS includes the steps of:

presetting a reference change slope of auxiliary winding voltage;

acquiring the change slope of the auxiliary winding voltage in the dead time of the energy storage tube and the continuous tube;

comparing the change slope of the auxiliary winding voltage with a reference change slope of the auxiliary winding voltage;

if the change slope of the auxiliary winding voltage is larger than or equal to the reference change slope of the auxiliary winding voltage, judging that the energy storage tube realizes ZVS; otherwise, it is determined that the energy storage tube does not achieve ZVS.

The method is similar to the second mode in terms of the slope of the voltage change at the middle point of the bridge arm, and the compensation coefficient k is adjusted by detecting the slope of the voltage drop moment of the auxiliary winding to obtain whether the system realizes the ZVS state. Wherein the reference change slope of the auxiliary winding voltage can be configured by itself according to the system state. The first mode and the second mode are suitable for the asymmetric half-bridge flyback conversion circuit.

In the CRM mode, the on time of the flyback conversion circuit Q1 is controlled by the peak current comparator; the on-time of Q2 is controlled by the compensation coefficient k, which in turn can be adaptively adjusted by the four above-described means. In DCM mode, ZVS pulse can be added to achieve ZVS of Q1, as the system will enter discontinuous operation mode, as shown in fig. 11. The second conduction period T2 comprises an energy transmission period and a ZVS period; the energy transmission time length is V1T 1/V2; the ZVS duration is (k-1) V1T 1/V2; the ZVS duration stage generates a ZVS Pulse square wave to achieve ZVS of Q1. The starting point of the ZVS duration is determined by the DCM valley number or the switching frequency of the flyback converter circuit.

In order to implement the above control method, the embodiment of the present application further provides a control chip, which is adapted to implement the above control method, and is suitable for a flyback converter circuit including an auxiliary winding, for example, an asymmetric half-bridge flyback converter circuit as shown in fig. 4, and as shown in fig. 12, the control chip IC at least includes an auxiliary winding pin 11, a voltage acquisition module 12, a timing module 13, a compensation module 14, and a PWM logic module 15;

the auxiliary winding pin 11 can be electrically connected to one end of the auxiliary winding AUX for at least acquiring the auxiliary winding voltage V _aux ；

The voltage acquisition module 12 is electrically connected with the auxiliary winding pin 11 and is at least used for controlling the auxiliary winding voltage V _aux Acquiring a first voltage V1 and a second voltage V2; the first voltage V1 is the voltage on the excitation inductance Lm when the energy storage tube Q1 is turned on; the second voltage V2 is the voltage on the excitation inductance Lm when the Q1 is turned off;

the compensation module 14 is configured to compensate the first volt-second product S1 to obtain a compensated volt-second product Sc; the first volt-second product S1 is the product of the first conduction time period T1 and the first voltage V1; the compensation result is Sc > S1.

The timing module 13 is configured to obtain a first conduction duration T1, and obtain a second conduction duration T2 of the continuous tube Q2 according to the compensation volt-second product Sc and the second voltage V2; the first conduction time T1 is the conduction time of the energy storage tube Q1; the second conduction period T2 is the conduction period of the freewheel Q2.

The PWM logic module 15 is electrically connected to the timing module 13, and is configured to generate a PWM signal for controlling the continuous flow tube to operate according to at least the second on-time period T2, which is also referred to as a control signal or a driving signal, and specifically, the second on-time period is used for generating a signal for controlling the Q2 to be turned off.

In one embodiment, compensation is performed with a compensation coefficient k, compensating volt-second product sc=kv1×t1; t2=sc/V2; where k is the compensation coefficient and k >1. The compensation module is used for determining a compensation coefficient k according to at least one parameter of the input voltage Vin, the output voltage Vo, the capacitance value C1 of the energy storage tube junction capacitor Q1_coss and the ZVS signal of the flyback conversion circuit. Wherein the ZVS signal characterizes whether the energy storage tube has realized ZVS function in the last switching cycle; the specific determination manner may refer to the description of the control method embodiment, and this is not repeated in this application. The following description is made with reference to the waveform diagram of fig. 13 and the circuit diagram of fig. 4:

Vaux1 is the voltage of the auxiliary winding when Q1 is on; vaux2 is the voltage of the auxiliary winding when Q2 is on; np is the number of turns of the primary winding, N _aux For auxiliary winding turns, ns is the secondary winding turns, then:

Sc＝k·V _aux1 ·T ₁ ＝V _aux2 ·T ₂ k > 1 … … equation 1.

In one embodiment, as shown in fig. 14, the voltage acquisition module 12 includes a signal conditioning unit 121, a sampling control unit 122, a sampling processing unit 123, and a gate 124; the timing module 13 comprises a counter;

one end of the signal conditioning unit 121 is electrically connected with the auxiliary winding pin 11, the other end of the signal conditioning unit is electrically connected with the input end of the sampling processing unit 123, the control end of the sampling processing unit 123 is electrically connected with the sampling control unit 122, the first output end of the sampling processing unit 123 is electrically connected with the first input end of the gating device 124 through the compensation module 14, the second output end of the sampling processing unit 123 is electrically connected with the second input end of the gating device 124, and the sampling control unit 12 is electrically connected with the control end of the gating device 124; for sampling control unit 122Receiving control signals G of the energy storage tube Q1 and the freewheel tube Q2 _Q1 And G _Q2 Delay the first delay time t after receiving the on signal of the energy storage tube Q1 _sam1 The sampling processing unit 123 is controlled to perform sampling processing to obtain a first digital quantity Dvaux1 representing the first voltage V1, and delay the second delay time t after receiving the on signal of the follow-up tube Q2 _sam2 Controlling the sampling processing unit 123 to perform sampling processing to obtain a second digital quantity Dvaux2 representing the second voltage V2;

the sampling control unit 122 is further configured to input a k-times first digital quantity (k×dvux 1) to a timer as a timing step when the energy storage tube Q1 is turned on, and input a second digital quantity (dvux 2) to the timer as a timing step when the freewheel tube Q2 is turned on;

the counter 13 is electrically connected with the sampling control unit 122 and receives a zero crossing signal CSZERO, and is configured to count up after the energy storage tube Q1 is turned on and the zero crossing signal is received, so as to obtain a first count n1 representing a first conduction duration; and start counting downwards when the freewheel Q2 is turned on until the timer is counted down to 0, and send a signal representing the counter of 0 to the PWM logic module 15, and the PWM logic module 15 generates a control signal Q for turning off the freewheel Q2 according to the signal representing the counter of 0 _{2_RST1} 。

In this embodiment, the sampling processing unit 123 is mainly used for analog-to-digital conversion and filtering, that is, the sampling processing unit 123 includes an ADC conversion module and a low-pass filter; specifically, according to the driving signals of Q1 and Q2, respectively triggering ADC conversion after Q1 is conducted and tsam1 is delayed, so as to obtain an ADC value of Vaux 1; and triggering ADC conversion after Q2 is turned on and tsam2 is delayed, so as to obtain an ADC value of Vaux 2. The two sets of ADC data are filtered by low pass filters to obtain dvax 1 and dvax 2, respectively, and input to the gate 124 of the subsequent stage. The gate 124 is controlled by the sampling control unit 122, and when Q1 is on, k·dvax 1 is selected as the step size of the counter, and when Q2 is on, dvax 2 is selected as the step size of the counter. The counter is triggered by a clock clk, which has a clock period Tclk. The following transformations may be performed according to equation 1:

k·D _aux1 ·(n ₁ ·T _clk )＝D _aux2 ·(n ₂ ·T _clk )，k＞1；

Wherein n1 is the number of times the counter counts up, n2 is the number of times the counter counts down, and the following relationship can be further obtained:

for the counter, dir (i.e. G _Q1 And G _Q2 ) It is determined whether the counter counts up or down, step is the Step size corresponding to each count, upEn is the up enable bit of the counter, CSZERO is the zero crossing signal of the resonant cavity current, and to avoid the system runaway, the counter can count up only when the current crosses zero, i.e. the CSZERO flag bit is 1. The waveform of this method is shown in fig. 15, and the number of saw teeth of Cnt can be understood as the count number, and Q2 is turned off after Cnt of the counter returns to 0.

Further, in one embodiment, as shown in fig. 16, the compensation module 14 includes a ZVS judgment unit 141 and a compensation coefficient adjustment unit 142; the ZVS judging unit 141 is electrically connected to the compensation coefficient adjusting unit 142; the ZVS determining unit 141 is configured to determine whether the energy storage tube Q1 implements ZVS in a previous switching period, and if yes, send a first flag signal to the compensation coefficient adjusting unit 142; if not, the second flag signal is sent to the compensation coefficient adjustment unit 142. The compensation coefficient adjusting unit is preset with an adjusting step length and is used for reducing the adjusting step length on the basis of the compensation coefficient of the last switching period according to the first mark signal to serve as the compensation coefficient of the current switching period; and the step length is further used for increasing the adjusting step length to be used as the compensation coefficient of the switching period according to the second mark signal on the basis of the compensation coefficient of the previous switching period.

In this embodiment, whether the ZVS is implemented by the system is detected first, then the compensation coefficient k is adjusted cycle by cycle in a stepping manner, and then the on time of Q2 is controlled according to the compensation coefficient k. Taking the first flag signal as zvs_flag=1 as an example, when Q1 is turned on, the state of zvs_flag is read, if the flag bit is 1, it indicates that the system achieves ZVS, otherwise, ZVS is not achieved. When the system realizes ZVS, k is reduced by one adjustment step; when the system does not achieve ZVS, k is increased by one adjustment step. The adjustment step may be one thousandth or other value. The larger the adjustment step size setting, the faster the adjustment speed of the compensation system k, and when the input voltage change or the output voltage change occurs, the system can quickly enter the ZVS state again. However, this value is too large, which may cause instability of the system, and needs to be set according to actual conditions.

Further, the ZVS judgment unit may be implemented in the following 4 ways:

mode one: as shown in fig. 17, ZVS judgment unit 141 includes midpoint voltage sampling subunit 1411, analog-to-digital conversion subunit 1412, operation subunit 1213, digital-to-analog conversion subunit 1414, and comparator 1415;

the midpoint voltage sampling subunit 1411 is configured to obtain a midpoint voltage V when the energy storage tube Q1 is turned on _HB ；

Analog-to-digital conversion subunit 1412 is configured to convert midpoint voltage V _HB Converted into a characteristic midpoint voltage V _HB A mid-point voltage digital quantity;

an operator subunit 1413 is configured to subtract a first threshold digital quantity Δd from the midpoint voltage digital quantity _HB Obtaining a reference voltage digital quantity;

digital-to-analog conversion subunit 1414 is used for converting reference voltage digital quantity into reference voltage V _{HB_REF} ；

The reference end of the comparator 1415 is electrically connected with the output end of the digital-to-analog conversion subunit 1414, the sampling end is electrically connected with the midpoint voltage sampling subunit 1411, and is used for downsampling the current switching period to obtain midpoint voltage and reference voltage V _{HB_REF} Comparing to generate the first and second flag signals; reference voltage V _{HB_REF} Subtracting the first threshold digital quantity according to the midpoint voltage digital quantity in the last switching period, and then performing digital-to-analog conversion to obtain the voltage-to-analog converter; when the midpoint voltage is greater than or equal to the reference voltage, a first flag signal is generated, and when the midpoint voltage is less than the reference voltage, a second flag signal is generated.

In this embodiment, the method is consistent with the Vaux1 acquisition methodAs shown in fig. 13, the circuit is turned on at Q1 and delayed by t _sam1 After triggering ADC conversion, obtaining the corresponding midpoint voltage V at the moment _HB Subtracting a certain threshold value delta D from the value _HB After V is obtained by DAC _{HB_REF} . And then, when the Q1 is turned on, reading the state of the ZVS_flag, and adjusting the compensation coefficient k.

Mode two: as shown in fig. 18, ZVS judgment unit 141 includes a midpoint voltage sampling subunit 1411, a differentiating circuit 1416, and a comparator 1415;

the midpoint voltage sampling subunit 1411 and the differentiating circuit 1416 are configured to obtain a change slope of the midpoint voltage within dead time of the energy storage tube Q1 and the freewheel tube Q2, and send the change slope of the midpoint voltage to a sampling end of the comparator 1415;

the comparator 1415 receives the reference change slope of the midpoint voltage and compares the reference change slope of the midpoint voltage with the change slope of the midpoint voltage to produce a first flag signal and a second flag signal; if the change slope of the midpoint voltage is greater than or equal to the reference change slope of the midpoint voltage, generating a first flag signal; and if the change slope of the midpoint voltage is smaller than the reference change slope of the midpoint voltage, generating a second mark signal.

In the embodiment, the compensation coefficient k is adjusted based on the voltage slope of the middle point of the bridge arm, and when the change slope of HB voltage is larger, the larger the current for charging and discharging the junction capacitor is, the easier the system realizes ZVS. Wherein the reference change slope V of the midpoint voltage _{HBdv/dt_REF} Can be configured by itself according to the actual system. In this method, for the distinguishing position of the zvs_flag, the dead time for turning off Q1 and Q2 is changed, as in the interval t 1-t 2 in fig. 9/10, so long as the zvs_flag is at an excessively high level during this period, the system realizes ZVS.

Mode three: as shown in fig. 19, the ZVS judgment unit 141 includes a first time length acquisition subunit 1431 and a judgment subunit 1432:

a first time length acquisition subunit 1431 configured to acquire a first time length; the first duration is the time when the voltage of the auxiliary winding is less than a voltage threshold value after the freewheel is closed; the voltage threshold value is preset;

a judging subunit 1432, configured to judge the first duration and the expected duration, and generate a first flag signal and a second flag signal according to a judgment result; if the first time length is smaller than or equal to the expected time length, generating a first mark signal; if the first time length is longer than the expected time length, generating a second mark signal; the expected duration is obtained through presetting or the expected duration is in positive correlation with the input voltage according to the self-adaptive adjustment trend of the input voltage of the flyback conversion circuit.

In this embodiment, when Q2 is turned off in conjunction with fig. 4 and 19, the timer starts to count, and when the auxiliary winding voltage Vaux is smaller than the set voltage threshold Vth, the timer stops. The output time length of the timer is recorded as a first time length t _cnt (t in FIG. 10) _cnt1 、t _cnt2 ) A first time length is compared with a predefined expected time T _{fall_time} In contrast, the fall time t _cnt Less than the expected value T _{fall_time} When the system is illustrated as implementing ZVS, otherwise ZVS is not implemented. The setting of the voltage threshold Vth can be seen in fig. 10 and the corresponding description thereof, and will not be described herein.

Mode four: as shown in fig. 20, the ZVS judging unit 141 includes an auxiliary winding voltage sampling subunit 1441, a differentiating circuit 1442, and a comparator 1443;

the auxiliary winding voltage sampling subunit 1441 and the differentiating circuit 1442 are configured to obtain a change slope of the auxiliary winding voltage in a dead time of the energy storage tube and the continuous tube, and send the change slope of the auxiliary winding voltage to a sampling end of the comparator 1443;

a comparator 1443 reference terminal receives the reference change slope of the auxiliary winding voltage and is configured to compare the reference change slope of the auxiliary winding voltage with the change slope of the auxiliary winding voltage to produce a first flag signal and a second flag signal; generating a first flag signal if the change slope of the auxiliary winding voltage is greater than or equal to the reference change slope of the auxiliary winding voltage; and generating a second sign signal if the change slope of the auxiliary winding voltage is smaller than the reference voltage of the auxiliary winding voltage.

The embodiment is similar to a method based on bridge arm midpoint voltage differentiation, and the compensation coefficient k is adjusted by detecting the slope of the auxiliary winding voltage falling moment to obtain whether the system realizes the ZVS state. Wherein the reference change slope V of the auxiliary winding voltage _{AUXdv/dt_REF} Can be configured by itself according to the system state.

Further, referring to fig. 11 and 21, when the flyback converter circuit operates in DCM, the compensation coefficient is a sum of a first compensation coefficient and a second compensation coefficient, where the first compensation coefficient is 1, and the second compensation coefficient is (k-1); the first compensation coefficient is used for energy transfer, and the second compensation coefficient is used for ZVS; the timer as in fig. 21 is also used to receive ZVS pulse signals (ZVS pulses); the timer is also used for timing when the ZVS pulse signal is received, and when the timing time length reaches the on time length corresponding to the second compensation coefficient, the flywheel tube is controlled to be turned off so as to realize ZVS; the ZVS pulse signal is determined by the DCM valley count in the DCM mode of the flyback converter circuit or the switching frequency of the flyback converter circuit.

In this embodiment, the on time of Q2 (second on time period) is divided into two segments, the first segment is the energy transfer time, and the second segment is the ZVS achieving time. At this time, the compensation coefficient k can be regarded as 1 for control, and the control process is still as shown in FIG. 14, and Q is generated _{2_rst1} To turn off Q2. The second period of time can be controlled by considering the compensation factor as (k-1), as shown in FIG. 21, the gate includes 3 inputs, the counter includes a port Down EN receiving ZVS pulses, so that the PWM logic module 15 is not only capable of generating the first segment Q _{2_rst1} Q also enables generation of ZVS stage _{2_rst2} . The difference between the signal before the gate and fig. 14 is that the input signal here also includes (k-1) ·dvax 1; yet another difference is that the counter performs a subtraction only when the ZVS pulse flag bit is high, thereby generating the q2_rst2 signal. The gate 124 of fig. 21 is merely a functional example, and may be implemented with a multi-port gate, or with a plurality of two-port gates.

Based on the control method and the control chip, the present application further provides a flyback conversion circuit, as shown in fig. 22, including a main topology unit 100, a secondary side unit 200 of the flyback conversion circuit, and the control chip 300 provided in any of the foregoing embodiments; the main topology unit of the flyback conversion circuit 100 comprises an energy storage tube Q1, a freewheel tube Q2, a transformer T, leakage inductance Lr, an excitation inductance Lm, a resonance capacitor Cr and an auxiliary winding AUX; the control chip 300 is electrically connected to at least the auxiliary winding AUX, the energy storage tube Q1 and the freewheel tube Q2, at least for the above-mentioned control purposes. The master topology may be in the form of an AHB, ACF or ZVS flyback converter.

In one embodiment, the control chip 300 not only integrates a control chip for implementing the control strategy, but also has a control function for the energy storage tube Q1, and adjusts the on time of Q1 and Q2 according to the sampling signal Vcomp of Vout, so as to implement control for the output voltage Vout, where the sampling signal Vcomp of Vout can be input to the control chip 300 through the FB pin. By utilizing the scheme provided by the patent, the reliable and stable operation of the system can be ensured, the system efficiency can be improved, and the topological advantage can be exerted. Functions of the respective pins of the control chip 300:

ZCD: the auxiliary winding zero crossing signal is obtained, and the voltage signals during the conduction period of the Q1 and the Q2 are obtained, wherein the signals can represent terminal voltages (namely a first voltage V1 and a second voltage V2) in two time periods on the exciting inductance.

And (B): and acquiring an externally compensated voltage signal so as to control the working mode of the system and the command value of peak current control.

CS: and sampling the current, and controlling and protecting the peak current.

HV: and the high-voltage starting of the system is realized.

GH. GL: respectively drive signals of an energy storage tube and a freewheel tube in the flyback conversion circuit.

The switching tube of the flyback conversion circuit is controlled to be soft switching control, the self-adaptation degree is high, the topology advantage can be fully utilized, and higher efficiency is realized. It should be noted that the structure of the secondary side unit 200 in the drawings is only an example, and may be designed according to the use situation, which is not limited in this application.

The foregoing disclosure is directed to the preferred embodiment of the present application and is not intended to limit the scope of the claims, but rather to cover any and all modifications, equivalents, alternatives, and improvements within the spirit and principles of the present application.

Claims

1. The control method of the flyback conversion circuit is characterized by comprising an energy storage tube, a continuous flow tube and an excitation inductance, and comprises the following steps of:

2. The control method according to claim 1, wherein the compensation volt-second product and the second on-time are compensated and calculated by the following formula:

Sc＝k*S1＝k*V1*T1；

t2=sc/V2; wherein k is a compensation coefficient and k is greater than 1, sc is the compensation volt-second product, S1 is the first volt-second product, V1 is the first voltage, T1 is the first conduction duration, V2 is the second voltage, and T2 is the second conduction duration.

3. The control method according to claim 2, wherein the compensation coefficient is unchanged if the input voltage and/or the output voltage range of the flyback converter circuit is within a preset range;

alternatively, the compensation coefficient is positively correlated with the input voltage of the flyback converter circuit;

alternatively, the compensation coefficient is positively correlated with the capacitance value of the storage tube junction capacitance.

4. The control method according to claim 2, characterized in that the compensation coefficient is obtained at least by:

judging whether the energy storage tube realizes ZVS or not, and if so, controlling the compensation coefficient of the next switching period to be smaller than that of the current switching period; if not, the compensation coefficient of the next switching period is controlled to be larger than that of the current switching period.

5. The control method according to claim 4, wherein the step of controlling the compensation coefficient of the next switching cycle to be smaller than the compensation coefficient of the present cycle comprises the steps of: reducing the step length on the basis of the compensation coefficient of the current switching period to obtain the compensation coefficient of the next switching period;

The step of controlling the compensation coefficient of the next switching period to be larger than the compensation coefficient of the current switching period comprises the following steps: adding the step length on the basis of the compensation coefficient of the current switching period to obtain the compensation coefficient of the next switching period;

the step length is a preset value.

6. The control method according to claim 4 or 5, wherein when the control method is applied to an asymmetric half-bridge flyback converter circuit, the determining whether the energy storage tube realizes ZVS comprises the steps of:

acquiring the midpoint voltage of the energy storage tube when the energy storage tube is conducted; the midpoint voltage is the common point voltage of the energy storage tube and the freewheel tube;

subtracting a first threshold value from the midpoint voltage to obtain a reference voltage;

when the energy storage tube is conducted again, acquiring the midpoint voltage of the energy storage tube when the energy storage tube is conducted again, and comparing the acquired midpoint voltage with the reference voltage;

and if the obtained midpoint voltage is larger than or equal to the reference voltage, judging that the energy storage tube realizes ZVS, otherwise, judging that the energy storage tube does not realize ZVS.

7. The control method according to claim 4 or 5, wherein when the control method is applied to an asymmetric half-bridge flyback converter circuit, the determining whether the energy storage tube realizes ZVS comprises the steps of:

Presetting a reference change slope of the midpoint voltage; the midpoint voltage is the common point voltage of the energy storage tube and the freewheel tube;

if the change slope of the midpoint voltage is larger than or equal to the reference change slope of the midpoint voltage, judging that the energy storage tube realizes ZVS; otherwise, judging that the energy storage tube does not realize ZVS.

8. The control method according to claim 4 or 5, wherein the flyback conversion circuit includes an auxiliary winding, and wherein the determining whether the energy storage tube achieves ZVS includes the steps of:

acquiring a first time length; the first duration is the time when the auxiliary winding voltage is smaller than a voltage threshold value after the freewheel is closed; the voltage threshold value is preset;

judging the sizes of the first time length and the expected time length; the expected duration is obtained through presetting or is subjected to self-adaptive adjustment according to the input voltage of the flyback conversion circuit, wherein the adjustment trend is that the expected duration is positively related to the input voltage;

If the first time length is smaller than or equal to the expected time length, judging that the energy storage tube realizes ZVS; otherwise, judging that the energy storage tube does not realize ZVS.

9. The control method according to claim 4 or 5, wherein the flyback conversion circuit includes an auxiliary winding, and wherein the determining whether the energy storage tube achieves ZVS includes the steps of:

presetting a reference change slope of auxiliary winding voltage;

if the change slope of the auxiliary winding voltage is larger than or equal to the reference change slope of the auxiliary winding voltage, judging that the energy storage tube realizes ZVS; otherwise, judging that the energy storage tube does not realize ZVS.

10. The control method of claim 2, wherein the second on-time period includes an energy transfer time period and a ZVS time period when the flyback converter circuit is operating in DCM; the energy transmission duration is V1 x T1/V2; the ZVS duration is (k-1) V1T 1/V2, and when the timing starting point of the ZVS duration is determined by the number of DCM valleys or the switching frequency of the flyback converter circuit.

11. The control chip of the flyback conversion circuit is characterized by comprising an energy storage tube, a continuous flow tube, an excitation inductance and an auxiliary winding; the control chip at least comprises an auxiliary winding pin, a voltage acquisition module, a compensation module, a timing module and a PWM logic module;

12. The control chip of claim 11, wherein the compensation module compensates the first volt-second product with a compensation coefficient; the compensation volt-second product sc=k×v1×t1;

t2=sc/V2; wherein k is a compensation coefficient and k is greater than 1, sc is the compensation volt-second product, S1 is the first volt-second product, V1 is the first voltage, T1 is the first conduction time period, V2 is the second voltage, and T2 is the second conduction time period;

the compensation module is used for determining the compensation coefficient according to at least one parameter of input voltage, output voltage, capacitance value of the energy storage tube junction capacitor and ZVS signal of the flyback conversion circuit; the ZVS signal characterizes whether the energy storage tube has achieved ZVS functionality in the last switching cycle.

13. The control chip of claim 12, wherein the voltage acquisition module comprises a signal conditioning unit, a sampling control unit, a sampling processing unit, and a gate; the timing module comprises a counter;

one end of the signal conditioning unit is electrically connected with the auxiliary winding pin, the other end of the signal conditioning unit is electrically connected with the input end of the sampling processing unit, the control end of the sampling processing unit is electrically connected with the sampling control unit, the first output end of the sampling processing unit is electrically connected with the first input end of the gating device through the compensation module, the second output end of the sampling processing unit is electrically connected with the second input end of the gating device, and the sampling control unit is electrically connected with the control end of the gating device;

The sampling control unit is used for receiving control signals of the energy storage tube and the follow-up tube, delaying a first delay time after receiving a conduction signal of the energy storage tube, controlling the sampling processing unit to conduct sampling processing to obtain a first digital quantity representing the first voltage, and delaying a second delay time after receiving a conduction signal of the follow-up tube, controlling the sampling processing unit to conduct sampling processing to obtain a second digital quantity representing the second voltage;

the sampling control unit is used for inputting a first digital quantity which is k times of the first digital quantity into the timer to be used as a timing step when the energy storage tube is conducted, and inputting a second digital quantity into the timer to be used as a timing step when the freewheel tube is conducted;

the counter is electrically connected with the sampling control unit and receives a zero crossing signal, and is used for counting upwards after the energy storage tube is conducted and the zero crossing signal is received, so as to obtain a first count n1 representing the first conduction duration; and starting counting downwards when the freewheel is conducted until the timer is classified as 0 and stopping counting, and sending a signal representing that the counter is 0 to the PWM logic module, wherein the PWM logic module generates a control signal for turning off the freewheel according to the signal representing that the counter is 0.

14. The control chip according to claim 13, wherein the compensation module includes a ZVS judgment unit and a compensation coefficient adjustment unit; the ZVS judging unit is electrically connected with the compensation coefficient adjusting unit;

the ZVS judging unit is used for judging whether the energy storage tube realizes ZVS in the last switching period, if so, a first mark signal is sent to the compensation coefficient adjusting unit; if not, a second mark signal is sent to the compensation coefficient adjusting unit;

the compensation coefficient adjusting unit is preset with an adjusting step length and is used for reducing the adjusting step length on the basis of the compensation coefficient of the last switching period according to the first mark signal to serve as the compensation coefficient of the current switching period; and the step length is further used for increasing the adjusting step length on the basis of the compensation coefficient of the last switching period according to the second mark signal to serve as the compensation coefficient of the current switching period.

15. The control chip of claim 14, wherein the ZVS decision unit comprises a midpoint voltage sampling subunit, an analog-to-digital conversion subunit, an operation subunit, a digital-to-analog conversion subunit, and a comparator;

the midpoint voltage sampling subunit is used for acquiring midpoint voltage when the energy storage tube is conducted; the midpoint voltage is the common point voltage of the energy storage tube and the freewheel tube;

The analog-to-digital conversion subunit is used for converting the midpoint voltage into a midpoint voltage digital quantity representing the midpoint voltage;

the operation subunit is used for subtracting the first threshold digital quantity from the midpoint voltage digital quantity to obtain a reference voltage digital quantity;

the digital-to-analog conversion subunit is used for converting the reference voltage digital quantity into a reference voltage;

the reference end of the comparator is electrically connected with the output end of the digital-to-analog conversion subunit, the sampling end is electrically connected with the midpoint voltage sampling subunit, and the comparator is used for performing downsampling in the current switching period to obtain the midpoint voltage and the reference voltage, so that the first sign signal and the second sign signal are generated;

or,

the ZVS judging unit comprises a midpoint voltage sampling subunit, a differentiating circuit and a comparator;

the midpoint voltage sampling subunit and the differential circuit are used for acquiring the change slope of the midpoint voltage in the dead time of the energy storage tube and the continuous tube and sending the change slope of the midpoint voltage to the sampling end of the comparator;

the comparator reference terminal receives a reference change slope of the midpoint voltage and is used for comparing the reference change slope of the midpoint voltage with the change slope of the midpoint voltage to produce a first flag signal and a second flag signal;

Or,

the ZVS judging unit includes a first time length acquiring subunit and a judging subunit:

the first time length obtaining subunit is used for obtaining a first time length; the first duration is the time when the auxiliary winding voltage is smaller than a voltage threshold value after the freewheel is closed; the voltage threshold value is preset;

the judging subunit is used for judging the first time length and the expected time length and generating a first mark signal and a second mark signal according to a judging result; the expected duration is obtained through presetting or is subjected to self-adaptive adjustment according to the input voltage of the flyback conversion circuit, wherein the adjustment trend is that the expected duration is positively related to the input voltage;

or,

the ZVS judging unit comprises an auxiliary winding voltage sampling subunit, a differentiating circuit and a comparator;

the auxiliary winding voltage sampling subunit and the differential circuit are used for acquiring the change slope of the auxiliary winding voltage in the dead time of the energy storage tube and the continuous tube and sending the change slope of the auxiliary winding voltage to the sampling end of the comparator;

the comparator reference terminal receives a reference change slope of the auxiliary winding voltage and is configured to compare the reference change slope of the auxiliary winding voltage with the change slope of the auxiliary winding voltage to produce a first flag signal and a second flag signal.

16. The control chip of claim 13, wherein when the flyback converter circuit is operating in DCM, the compensation factor is a sum of a first compensation factor and a second compensation factor, the first compensation factor being 1, the second compensation factor being (k-1); the first compensation coefficient is used for energy transfer, and the second compensation coefficient is used for ZVS; the timer is also used for receiving a ZVS pulse signal; the timer is also used for timing when the ZVS pulse signal is received, and when the timing time length reaches the conduction time length corresponding to the second compensation coefficient, the freewheel is controlled to be turned off; the ZVS pulse signal is determined by the DCM valley count or the switching frequency of the flyback converter circuit.

17. A flyback converter circuit comprising a main topology unit, a secondary unit and a control chip according to any one of claims 11-16; the main topology unit of the flyback conversion circuit comprises an energy storage tube, a continuous flow tube, a transformer, leakage inductance, an excitation inductance, a resonance capacitor and an auxiliary winding; the control chip is electrically connected with at least the auxiliary winding, the energy storage tube and the freewheel tube.