CN109194111B

CN109194111B - LLC full-bridge converter soft start control method based on variable duty ratio

Info

Publication number: CN109194111B
Application number: CN201811283815.XA
Authority: CN
Inventors: 杜贵平; 赖娜; 雷雁雄; 黎嘉健
Original assignee: South China University of Technology SCUT
Current assignee: South China University of Technology SCUT
Priority date: 2018-10-31
Filing date: 2018-10-31
Publication date: 2020-02-18
Anticipated expiration: 2038-10-31
Also published as: CN109194111A

Abstract

The invention discloses a soft start control method of an LLC full-bridge converter based on a variable duty ratio, which is characterized in that 4 MOSFETs on the primary side of the LLC full-bridge converter are controlled to ensure that signals of an upper switching tube and a lower switching tube of the same bridge arm are complementary, the upper tube of a left bridge arm leads the upper tube of a right bridge arm by 50% of a period, and meanwhile, the soft start control method is carried out according to outputVoltage derived start-up time t_rThe duty ratio of the switching device is adjusted to change along with time, so that the duty ratio of the driving signal of the switching device is gradually increased to a specified value according to a specified curve, and the output voltage is increased from 0 to a given voltage to achieve soft start. The method weakens the current impact in the starting process of the traditional LLC full-bridge converter, reduces the overshoot of the output voltage, enables the circuit to be more stable and reliable in starting, has higher engineering use value, and is reliable, simple and easy to realize.

Description

LLC full-bridge converter soft start control method based on variable duty ratio

Technical Field

The invention relates to the technical field of LLC full-bridge converter soft start, in particular to an LLC full-bridge converter soft start control method based on a variable duty ratio.

Background

LLC resonant converters have advantages such as high operating efficiency and are widely used in various fields, but during the start-up of the circuit, excessive current surges are a problem that cannot be ignored. On one hand, when the bidirectional LLC resonant converter is analyzed, it is assumed that the output filter capacitor is large enough to clamp the secondary voltage of the transformer when the diode is turned on, but the large output filter capacitor needs to be charged when the actual converter is started (assuming that the capacitor is completely discharged when the converter is started), so that a large inrush current is generated; on the other hand, the resonant cavity impedance in an LLC resonant converter is very small due to operation at the resonant frequency, resulting in a large current being produced by the resonant cavity. The overlarge impact circuit in the circuit easily causes the heating damage of the power device, and influences the normal work of the system.

Disclosure of Invention

The invention aims to overcome the defects of the prior art, provides a soft start control method of an LLC full-bridge converter based on variable duty ratio, weakens the current impact in the starting process of the traditional LLC full-bridge converter, reduces the overshoot of output voltage, enables the circuit to be more stable and reliable in starting, has higher engineering use value, and is reliable, simple in circuit and easy to realize.

In order to achieve the purpose, the technical scheme provided by the invention is as follows: aThe LLC full-bridge converter is a main power circuit and is matched with a peripheral control circuit consisting of a pulse width modulator and a driving module which are connected; the method comprises the steps of controlling 4 MOSFETs on the primary side of the LLC full-bridge converter to enable signals of an upper switching tube and a lower switching tube of the same bridge arm to be complementary, enabling an upper tube of a left bridge arm to lead a tube of a right bridge arm by 50% of a period, and obtaining starting time t according to output voltage_rThe duty ratio of the switching device is adjusted to change along with time, so that the duty ratio of the driving signal of the switching device is gradually increased to a specified value according to a specified curve, and the output voltage is increased from 0 to a given voltage to achieve soft start.

Further, the duty ratio D of the switching device is adjusted_yThe method of variation with time t is: in the starting process of the circuit, the duty ratio D is in the early stage of soft start_yVaries exponentially with time t; at the later stage of soft start, duty ratio D_yAnd the duty ratio of the primary side device changes along with time until the voltage of the output end of the rectifying side of the LLC full-bridge converter rises to a specified voltage, so that the whole soft start process is completed.

Further, an obtained curve of the duty ratio changing along with time is a splicing curve obtained by directly splicing a duty ratio linear changing curve and a duty ratio exponential function changing curve and is obtained by proportional shrinkage; the splicing curve is at T_changBefore the moment, duty ratio D_yThe expression of the curve, which varies exponentially with time t, is:

at T_changAfter the moment, duty cycle D_yThe expression of the curve which changes linearly with time t is as follows:

the splicing duty ratio D is obtained by the expression of the two curves_yThe change curve of (a) is expressed as follows:

wherein D is_{y_max}Is the rated duty cycle of the converter; t is t₁The moment when the stitching curve rises to the nominal duty cycle.

Further, said T_changThe moment is the moment when the speed when the duty ratio changes exponentially with time is equal to the speed when the duty ratio changes linearly with time, so that the duty ratio change curve is in smooth transition, namely:

ΔD_{y_exp}＝ΔD_{y_line}

by calculating T_chang＝0.8425t_r。

Compared with the prior art, the invention has the following advantages and beneficial effects:

1. the method reduces the current impact in the circuit by adjusting the duty ratio of the primary side switching tube to change along with the time, achieves the soft start of the circuit, and has the advantages of high response speed, simple control, simple circuit structure and the like through off-line control.

2. During soft start, the output voltage basically has no overshoot, the charging current impact of the output filter capacitor is small, and the soft start process of the circuit is more stable and reliable.

Drawings

FIG. 1 is a schematic diagram of a control strategy according to the present invention.

Fig. 2 is a graph of duty cycle versus time.

Detailed Description

The following describes the present invention in further detail with reference to the accompanying drawings. But practice of the invention is not limited thereto.

As shown in fig. 1, which is a schematic diagram of a soft start control strategy of an LLC full-bridge converter based on variable duty ratio, the main power circuit shown in the diagram is the LLC full-bridge converter, and the peripheral control circuit includes a pulse width modulator and a driving module connected to each other. The control method provided by the embodiment mainly controls the LLC full-bridge converter4 MOSFETs on the primary side of the converter enable signals of an upper switching tube and a lower switching tube of the same bridge arm to be complementary, the upper tube of a left bridge arm is advanced by 50% of a period compared with the upper tube of a right bridge arm, and meanwhile, starting time t is obtained according to output voltage_rThe duty ratio of the switching device is adjusted to change along with time, so that the duty ratio of the driving signal of the switching device is gradually increased to a specified value according to a specified curve, and the output voltage is slowly increased from 0 to a given voltage to achieve soft start.

Duty ratio D of the above regulating switch device_yThe method of variation with time t is: in the starting process of the circuit, the duty ratio D is in the early stage of soft start_yVaries exponentially with time t; at the later stage of soft start, duty ratio D_yAnd changing linearly along with the time t to obtain a curve of the duty ratio changing along with the time. The duty ratio of the primary side device changes along with time until the voltage of the output end of the rectifying side of the full-bridge LLC resonant converter rises to a specified voltage, and the whole soft start process is completed.

The obtained curve of the duty ratio changing with time is obtained by directly splicing a duty ratio linear changing curve and a duty ratio exponential function changing curve and performing proportional contraction, and a curve graph of the duty ratio changing with time in the starting process is shown in fig. 2. The splicing curve is at T_changBefore the moment, duty ratio D_yThe curve that varies exponentially with time t is shown in fig. 2 b and has the expression:

at T_changAfter the moment, duty cycle D_yThe curve which varies linearly with time t is shown in fig. 2 as a and has the expression:

the splicing duty ratio D is obtained by the expression of the two curves_yIs shown as c in fig. 2, and its expression is as follows:

the variable duty ratio strategy is obtained by proportionally contracting the change curve of the splicing duty ratio Dy, and the duty ratio D of the variable duty ratio strategy_yIs shown as d in fig. 2.

Above D_{y_max}Is the rated duty cycle of the converter; t is t₁The moment when the stitching curve rises to the nominal duty cycle. The duty cycle is set to 43.75% in this implementation. In the starting process, the duty ratio of a primary side switching tube of the full-bridge LLC resonant converter is gradually changed along with the increase of time until the rectified output voltage of the bidirectional LLC converter rises to a specified voltage, and the whole starting process is completed.

The T is_changThe time is the time when the speed of the duty ratio changing in an exponential form along with the time is equal to the speed of the duty ratio changing in a straight line along with the time, so that the duty ratio changing curve is in smooth transition, namely

ΔD_{y_exp}＝ΔD_{y_line}

T can be obtained by calculation from the above formula_chang＝0.8425t_r。

Various modifications, additions and substitutions for the specific embodiments described herein may be made by those skilled in the art without departing from the spirit and scope of the invention, which is within the ambit of the following claims. The technical scope of the present invention is not limited to the above-described embodiments.

Claims

1. A LLC full-bridge converter soft start control method based on variable duty ratio, said LLC full-bridge converter is the main power circuit, and is matched with the peripheral control circuit composed of pulse width modulator and drive module connected; the method is characterized in that: the method comprises the steps of controlling 4 MOSFETs on the primary side of the LLC full-bridge converter to enable signals of an upper switching tube and a lower switching tube of the same bridge arm to be complementary, enabling an upper tube of a left bridge arm to lead a tube of a right bridge arm by 50% of a period, and obtaining starting time t according to output voltage_rThe duty ratio of the switching device is adjusted to vary with time so that the duty ratio of the driving signal is adjustedThe designated curve is gradually increased to a designated value, so that the output voltage is increased from 0 to a given voltage to achieve soft start;

wherein the duty ratio D of the switching device is adjusted_yThe method of variation with time t is: in the starting process of the circuit, the duty ratio D is in the early stage of soft start_yVaries exponentially with time t; at the later stage of soft start, duty ratio D_yChanging linearly along with time t to obtain a curve of duty ratio changing along with time, wherein the duty ratio of the primary side device changes along with time until the voltage of the output end of the rectifying side of the LLC full-bridge converter rises to a specified voltage, and the whole soft start process is completed;

the obtained curve of the duty ratio changing along with time is a splicing curve obtained by directly splicing a duty ratio linear changing curve and a duty ratio exponential function changing curve and is obtained by proportional shrinkage; the splicing curve is at T_changBefore the moment, duty ratio D_yThe expression of the curve, which varies exponentially with time t, is:

wherein D is_{y_max}Is the rated duty cycle of the converter; t is t₁The moment when the splicing curve rises to the rated duty ratio;

the T is_changThe time is the speed when the duty ratio changes exponentially with time and the duty ratio changes at any timeWhen the speed is equal when the duty ratio changes linearly, the duty ratio change curve is in smooth transition, namely:

ΔD_{y_exp}＝ΔD_{y_line}

by calculating T_chang＝0.8425t_r。