JP2007166795A - Power supply circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power supply circuit for reducing a switching loss in a free wheeling FET, and stably and surely operating the free wheeling FET regardless of a load voltage. <P>SOLUTION: The power supply circuit is provided with a switching semiconductor element, its control circuit, first and second rectifying elements connected to a secondary winding in a transformer, a drive circuit, and a smoothing inductor. The second rectifying element comprises a field effect transistor for operating as the free wheeling FET. The drive circuit drives the free wheeling FET, and comprises a secondary winding in the smoothing inductor, a rectifying circuit connected to the secondary winding, and an inductance element and an impedance element connected to an output terminal of the rectifying circuit. A charge in a capacitance between a gate and a source of the free wheeling FET can be discharged by discharging energy accumulated in the inductance element through the rectifying circuit. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a power supply circuit including a synchronous rectification type rectifier circuit, and more particularly to a configuration of a drive circuit that drives a field effect transistor that operates as a rectifier element.

  In recent years, with the miniaturization of electronic devices, there is a demand for miniaturization of power supply circuits that supply energy to the electronic devices. In order to meet this demand, a DC / DC converter system is used as a power source. After the direct current voltage is once converted into a high frequency alternating voltage by an inverter connected to the primary winding of the transformer, synchronous rectification with low power loss is achieved. Conversion to a DC voltage is performed again by a rectifying unit of the system. That is, in order to supply sufficient energy commensurate with the load with a small power source, it is necessary to increase the efficiency of the power supply device. In order to achieve high efficiency of a power supply device, a synchronous rectifier circuit using a field effect transistor (hereinafter referred to as FET) as a rectifier is widely known as a rectifier circuit.

Although not shown, as a conventional synchronous rectifier circuit, an FET operating as a rectifier is connected in series to one end of a secondary winding of a transformer, and the gate of the rectifying FET is connected to the other end of the secondary winding. Thus, a circuit that drives the rectifying FET in synchronization with the voltage appearing in the secondary winding is widely known as a basic one (for example, see Patent Document 1). As another driving method, there is a method in which a third winding is provided in the transformer, a voltage generated in the third winding is converted to a DC voltage by a diode or the like, and applied to the gate of the rectifying FET for driving. Yes (see, for example, Patent Document 2). In addition, a third winding is provided in the transformer, a voltage generated in the third winding is converted into a DC voltage by a diode or the like, and the DC voltage is applied to the gate of the rectifying FET through a transistor and driven. There is also a method (for example, see Patent Document 3).
JP 2003-189622 A JP 2003-125579 A JP 2003-189608 A

  However, in the power supply circuit described in the above-mentioned Patent Document 1, since it is driven by the voltage generated in the secondary winding of the transformer, it is directly affected by the magnitude of the output voltage, In particular, in a power supply circuit having a low output voltage, for example, 3.3 V or less, the drive voltage for driving the rectifying FET may be insufficient. When the FET is driven with the insufficient drive voltage, the power loss of the FET is reduced. As a result, the power loss of the entire power supply circuit increases. On the other hand, when the load voltage is quite large, there is a problem that the loss of the drive circuit due to the limiting resistor increases. Further, in the region where the driving voltage is lower, there is a problem that the rectifying FET is not turned on, the synchronous rectifying operation is impossible, and the driving loss is increased. In particular, since the return FET that circulates the energy stored in the smoothing inductor is driven by the reset voltage of the transformer, when the output voltage is low, the return FET is turned off with a delay caused by the reset time of the transformer. It may be delayed and power loss will increase.

  Next, in the power supply circuit described in Patent Documents 2 and 3, the transformer is provided with the third winding, and the rectifying FET is driven by the voltage generated in the third winding. Therefore, the above-mentioned problem can be solved without being affected by the output voltage by appropriately selecting the number of turns of the third winding. However, since the voltage of the third winding is merely applied to the gate of the rectifying FET via a diode or a diode and a transistor, the voltage for the smoothing is exactly the same as that of the power supply described in the above-mentioned Patent Document 1. Since the return FET that circulates the energy stored in the inductor is driven by the reset voltage of the transformer, when the output voltage is low, the turn-off of the return FET may be delayed due to the delay caused by the reset time of the transformer. , Power loss increases.

  Accordingly, the present invention solves the above-mentioned problems, reduces the power loss by accelerating the turning-off operation of the freewheeling FET with a simple drive circuit, and also makes it a smoothing inductor even when the rated output voltage is considerably high. The main purpose is to operate the reflux FET stably without damaging it without providing a special protective member by appropriately selecting the number of turns of the secondary winding to be provided. In addition, the discharge of the charge charged in the gate-source capacitance of the rectifying FET can be accelerated, and the turn-off speed of the rectifying FET can be increased.

  According to a first aspect of the present invention, there is provided a transformer having a primary winding and a secondary winding connected to a DC input terminal, a switching semiconductor element connected in series to the primary winding, and the switching semiconductor element. A control circuit for controlling, a first rectifying element connected in series to the secondary winding, and a second rectifying element connected across the secondary winding and the first rectifying element; A power circuit comprising a primary winding connected in series to the secondary winding and a load and a smoothing inductor having a secondary winding magnetically coupled to the primary winding; The second rectifier element includes a reflux field effect transistor that circulates energy stored in the smoothing inductor, and is a circuit that drives the reflux field effect transistor. Winding and the secondary A rectifier circuit connected to a line, and a drive circuit including an inductance element and an impedance element connected to an output side of the rectifier circuit, wherein energy stored in the inductance element is switched in the drive circuit There is provided a power supply circuit characterized in that the charge of the gate-source capacitance of the reflux field effect transistor is discharged at high speed by being discharged through the rectifier circuit when the semiconductor element is turned on.

  According to a second invention, in the first invention, the first rectifier element is connected to one main terminal connected to one end side of the secondary winding of the transformer and to a DC output terminal side. There is provided a power supply circuit characterized by being a rectifying field effect transistor having the other main terminal and a gate connected to the other end of the secondary winding of the transformer.

  According to a third invention, in the first invention, the first rectifying element is a rectifying diode, a second transformer connected in parallel across the DC input terminal, and the second transformer A second switching semiconductor element connected in series to the primary winding; and a rectifying diode connected in series to the secondary winding of the second transformer; The secondary winding is connected in parallel across the secondary winding of the transformer and the first rectifier diode via the rectifier diode.

  In a fourth aspect based on the first aspect, the first rectifier element is connected to one main terminal connected to one end side of the secondary winding of the transformer and the other connected to the DC output terminal side. A rectifying field effect transistor having a main terminal, wherein the transformer has a third winding, and is a second driving circuit for driving the rectifying field effect transistor, the second driving circuit thereof. Is a third winding, a second rectifier circuit connected to the third winding, a second inductance element connected to the output side of the second rectifier circuit, and a second The rectifying device includes an impedance element, and in the second driving circuit, energy stored in the second inductance element is released through the second rectifier circuit when the switching semiconductor element is turned off. The gate of the field effect transistor - the charge of source capacitance to provide a power supply circuit, characterized in that it is discharged at high speed.

  According to a fifth aspect of the present invention, there is provided a transformer having a primary winding and a secondary winding, a switching semiconductor element connected in series to the primary winding, a control circuit for controlling the switching semiconductor element, A first rectifying element connected in series to a secondary winding; a second rectifying element connected across the secondary winding and the first rectifying element; and the secondary winding; In a power supply circuit comprising a primary winding connected in series to a load and a smoothing inductor having a secondary winding magnetically coupled to the primary winding, the first rectifying element comprises: A circuit comprising a rectifying field effect transistor and driving the rectifying field effect transistor, the secondary winding of the smoothing inductor, a rectifying circuit connected to the secondary winding, and the rectification thereof Inductor connected to the output side of the circuit A rectifying field effect transistor, comprising: a driving circuit comprising a capacitance element and an impedance element, wherein energy stored in the inductance element is released through the rectifying circuit when the switching semiconductor element is turned off; The power supply circuit is characterized in that the gate-source capacitance is discharged at high speed.

  A sixth invention includes a transformer having a primary winding and a secondary winding, a switching semiconductor element connected in series to the primary winding, a control circuit for controlling the switching semiconductor element, A rectifying field effect transistor connected in series to a secondary winding; a return field effect transistor connected across the secondary winding and the rectifying field effect transistor; and the secondary winding; A power supply circuit comprising a primary winding connected in series to a load and a smoothing inductor having a secondary winding magnetically coupled to the primary winding, wherein the 2 of the smoothing inductor The secondary winding includes first and second secondary windings connected in series with each other, and is connected to the first secondary winding of the smoothing inductor and the first secondary winding. First rectifier circuit and its first A first drive circuit comprising a first inductance element and a first impedance connected to the output side of the rectifier circuit, wherein the energy stored in the first inductor in the first drive circuit Is discharged through the first rectifier circuit when the switching semiconductor element is turned on, so that the charge of the gate-source capacitance of the rectifying field effect transistor is discharged at a high speed, and the second inductor of the smoothing inductor is discharged. A secondary winding, a second rectifier circuit connected to the second secondary winding, the second inductance element connected to the second rectifier circuit, and a second impedance A second drive circuit, wherein the energy stored in the second inductance element is transferred to the switching semiconductor. By being discharged through the second rectifier circuit at turn of the element, the gate of the return field effect transistor - to provide a power supply circuit of the source capacitance charges, characterized in that it is discharged at high speed.

  In a seventh aspect based on the sixth aspect, the inductance element and the impedance element exist between a gate and a source of the reflux field effect transistor and / or the rectification field effect transistor. A power supply circuit is provided.

  In an eighth aspect based on the sixth aspect or the seventh aspect, the field effect transistor for reflux and / or the field effect transistor for rectification comprises an N-channel field effect transistor, the rectifier circuit, The first rectifier circuit or the second rectifier circuit includes a first diode having an anode connected to one end of the secondary winding of the smoothing inductor, and an anode connected to the other end of the secondary winding. And a second diode connected across the secondary winding and the first diode, the cathodes of the first and second diodes are connected to each other, and the reflux field effect The gate of the transistor and / or the rectifying field effect transistor is connected to the side where the cathodes of the first and second diodes are connected to each other. To provide a source circuit.

  In a ninth aspect based on any one of the sixth aspect to the eighth aspect, the reflux field effect transistor and / or the rectifying field effect transistor comprises a P-channel field effect transistor, and the rectification The circuit, the first rectifier circuit, or the second rectifier circuit includes a first diode having a cathode connected to one end of the secondary winding of the smoothing inductor, and a second end of the secondary winding. A cathode is connected and the second diode is connected across the secondary winding and the first diode. The anodes of the first and second diodes are connected to each other. And / or the gate of the rectifying field effect transistor is connected to the side where the anodes of the first and second diodes are connected to each other. To provide a power supply circuit according to claim.

  According to a tenth aspect, in any one of the sixth to ninth aspects, the impedance element includes a capacitor, and the reflux FET and / or the rectifying field effect transistor are controlled at both ends of the capacitor. A power supply circuit is provided in which a secondary control circuit is connected.

  The present invention is a power supply circuit that reduces the power loss by speeding up the turn-off operation of the reflux FET that circulates the energy stored in the smoothing inductor with a drive circuit having a simple configuration, and has a low rated output voltage. In addition, by selecting the number of turns of the secondary winding provided in the smoothing inductor, it is possible to reduce the power loss by operating the reflux FET and further the rectifying FET in saturation. Even when the rated output voltage is quite high, by selecting the number of turns of the secondary winding provided on the smoothing inductor, it is possible to reduce the loss without damaging the rectifying FET without providing a special protective member. A power supply circuit that operates stably and reliably can be provided.

  According to the fifth aspect of the present invention, the turn-off operation of the rectifying FET is speeded up by a drive circuit having a simple configuration to reduce power loss, and even a power supply circuit having a low rated output voltage is provided in the smoothing inductor. By selecting the number of turns of the secondary winding and saturating the rectifying FET to reduce the power loss, the secondary winding provided in the smoothing inductor even when the rated output voltage is quite high By appropriately selecting the number of turns, it is possible to provide a power supply circuit that operates stably with low loss without damaging the rectifying FET without providing a special protective member.

  In the sixth aspect of the invention, the drive circuit with a simple configuration speeds up the turn-off operation of both the return FET and the rectifier FET to reduce power loss, and smoothes even a power supply circuit with a low rated output voltage. By selecting the number of turns of the secondary winding to be provided in the inductor, it is possible to reduce the power loss by saturating the return FET and rectifier FET, and smoothing even when the rated output voltage is quite high By appropriately selecting the number of turns of the secondary winding provided in the inductor, a power supply circuit that operates stably with low loss without damaging the reflux FET and the rectifying FET without providing a special protective member. Can be provided.

[Embodiment 1]
Embodiment 1 of the power supply device according to the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing a circuit configuration of the first power supply circuit 100, and FIG. 2 is a diagram showing voltage waveforms or current waveforms of respective parts of the power supply circuit 100. In FIG. 1, a primary winding 3 a of a transformer 3 and a switching semiconductor element 4 are connected in series with DC input terminals 1 and 2. The switching semiconductor element 4 is a general power MOSFET or the like, and a body diode (parasitic diode) is not shown. A control circuit 5 that controls the pulse width of the switching semiconductor element 4 is also connected between the DC input terminals 1 and 2. N-channel MOSFETs 6 and 7 are provided on the secondary winding 3 b side of the transformer 3. The MOSFET 6 is a rectifying FET that functions as a first rectifying element, and is connected in series to one end of the secondary winding 3b. Hereinafter, it is referred to as a rectifying FET 6. The MOSFET 7 is a reflux FET that is connected across the secondary winding 3b of the transformer 3 and the rectifying FET 6 and functions as a second rectifying element. Hereinafter, it is referred to as a reflux FET 7. The gate of the rectifying FET 6 is connected to the other end side of the secondary winding 3 b of the transformer 3 through the first limiting resistor 8. The body diodes (parasitic diodes) of the rectifying FET 6 and the reflux FET 7 are not shown. The same applies to the following embodiments. Note that the black dots of the primary winding 3a and the secondary winding 3b of the transformer 3 indicate polarity.

  Similar to a general synchronous rectifier circuit, a smoothing inductor 9 and a smoothing capacitor 10 constituting an output smoothing circuit are connected to the output side of the rectifying FET 6 and the return FET 7, and the DC output terminals 11 and 12 are connected to each other. An output voltage detection circuit 13 and a load 14 are connected between them. The output voltage detection signal detected by the output voltage detection circuit 13 is input as a feedback signal to the control circuit 5 through an insulation circuit 15 that insulates between primary and secondary, such as a photocoupler or a pulse transformer (not shown). Smoothing inductor 9 has a primary winding 9a and a secondary winding 9b magnetically coupled thereto. The black dots of the primary winding 9a and the secondary winding 9b indicate polarity, and the black dot side terminal of the secondary winding 9b is a, and the other terminal is b. The secondary winding 9b of the smoothing inductor 9 has a number of turns that can reliably turn off the smoothing FET 7 at high speed regardless of the load voltage and load current.

  A rectifier circuit 16 composed of diodes D1 and D2 is connected to the secondary winding 9b of the smoothing inductor 9. The diode D1 has its anode connected to the terminal a of the secondary winding 9b, and the diode D2 has its anode connected to the terminal b of the secondary winding 9b and straddles the secondary winding 9b and the diode D1. The two cathodes are connected to each other. An inductance element 17 and an impedance element 18 connected in series with each other are connected across the diode D2. The cathode sides of the diodes D1 and D2 are connected to the gate of the reflux FET 7 via the second limiting resistor 19. Here, the secondary winding 9 b of the smoothing inductor 9, the rectifier circuit 16, the inductance element 17, and the impedance element 18 constitute a drive circuit for the reflux FET 7.

  Next, the operation of the first power supply circuit 100 will be described with reference to FIG. When a DC voltage is applied to a DC input terminal 1 or 2 through a power switch (not shown) from a DC voltage obtained by converting a commercial AC power source (not shown) or an AC voltage of a generator into a DC by a rectifier circuit (not shown) or a storage battery, a control circuit 5 gives a control signal as shown in FIG. This control circuit 5 is a general one, and receives the output voltage detection signal from the output voltage detection circuit 13 and controls the pulse width of the switching semiconductor element 4 so that the output voltage becomes constant. For example, at time t1, the control circuit 5 applies a gate-source voltage Vg-s as shown in FIG. 2A as a control signal between the gate and the source of the switching semiconductor element 4, and turns on the switching semiconductor element 4. Let's say. In this case, the drain-source voltage Vd-s of the switching semiconductor element 4 drops to a sufficiently low voltage value at time t1 as shown in FIG. 2B, and the current flows in the primary winding 3a of the transformer 3. In the secondary winding 3b, a positive voltage is generated on the black dot side indicating the polarity. This state continues until time t2 when the control signal becomes zero again. The positive voltage is applied to the gate of the rectifying FET 6 through the first limiting resistor 8, and the rectifying FET 6 is turned on. The drain-source voltage Vd-s of the rectifying FET 6 becomes a sufficiently low voltage value at time t1-t2, as shown in FIG. Here, it is assumed that the reflux FET 7 is turned on before time t 1 and the energy accumulated in the smoothing inductor 9 is circulated through the smoothing capacitor 10 and the load 14.

  As the rectifying FET 6 is turned on at time t1, the current flows through the primary winding 9a of the smoothing inductor 9, the smoothing capacitor 10, the load 14, and the rectifying FET 6 from the black dot side terminal of the secondary winding 3b. It flows to the other end side of the next winding 3b. When a current flows through the primary winding 9a of the smoothing inductor 9, a voltage is induced in which the terminal b of the secondary winding 9b is positive with respect to the terminal a. This positive voltage reversely biases the diode D1. , Turn off. The voltage waveform of the primary winding 9a of the smoothing inductor 9 is shown in FIG. Thus, until now (until time t1), the current supplied from the secondary winding 9b to the inductance element 17 and the impedance element 18 through the diode D1 becomes zero, and the energy stored in the inductance element 17 until then is the diode. D2 and the impedance element 18 are emitted. Due to the current flowing in the forward direction through the diode D2, a forward voltage drop occurs in the diode D2. The voltage between the anode and the cathode of the diode D2 is as shown in FIG. This forward voltage drop of the diode D2 makes the gate voltage of the reflux FET 7 lower than the source voltage by the forward voltage drop, so that the charge of the gate-source capacitance of the reflux FET 7 is suddenly discharged and returned. The FET 7 is turned off at a higher speed. Therefore, the drain-source voltage Vd-s of the reflux FET 7 becomes a high voltage value at time t1-t2, as shown in FIG. This state continues until just before time t2. In the first embodiment, the power loss at the turn-off time of the return FET 7 can be reduced, and at the same time, since there is no delay in the turn-off operation of the return FET 7, the current flowing in the reverse direction can be completely eliminated and the power loss can be further reduced. .

  In the period t1-t2, as described above, the current is passed through the primary winding 9a of the smoothing inductor 9, the smoothing capacitor 10, the load 14, and the rectifying FET 6 from one end on the black spot side of the secondary winding 3b. To the other end side of the power and supplies power to the load 14. At this time, energy is stored in the smoothing inductor 9. The waveform of the current flowing through the smoothing inductor 9 is as shown in FIG. Next, when the control signal from the control circuit 5 changes and the gate-source voltage Vg-s of the switching semiconductor element 4 decreases to zero at time t2, the switching semiconductor element 4 is turned off. Accordingly, the voltage of the secondary winding 3b of the transformer 3 disappears, and the rectifying FET 6 is turned off. When no current flows from the secondary winding 3b of the transformer 3 to the primary winding 9a of the smoothing inductor 9, a voltage at which the terminal a is positive with respect to the terminal b in the secondary winding 9b of the smoothing inductor 9 This positive voltage is applied to the gate of the reflux FET 7 through the diode D1 of the rectifier circuit 16 and the second limiting resistor 19 to turn on the reflux FET 7.

  When the return FET 7 is turned on, the energy stored in the smoothing inductor 9 is released to the smoothing capacitor 10 and the load 14, and the current flowing through the smoothing inductor 9 continues as shown in FIG. On the other hand, in the drive circuit for the reflux FET 7, a current flows from the terminal a to the terminal b through the diode D 1, the inductance element 17 and the impedance element 18 of the rectifier circuit 16, and energy is stored in the inductance element 17. As described above, the energy stored in the inductance element 17 is useful for speeding up the turn-off operation of the reflux FET 7. At time t3, as shown in FIG. 2A, the control signal changes and the gate-source voltage of the switching semiconductor element rises again. Thereafter, the same operation as described above is repeated. In the power supply circuit 100, not only can the power loss due to the delay of the turn-off operation of the return FET 7 be reduced, but also the power supply having a low rated output voltage can be selected by appropriately selecting the number of turns of the secondary winding 9b. In addition, the reflux FET 7 can be switched stably and at high speed.

[Embodiment 2]
Next, the power supply circuit 200 according to the second embodiment will be described with reference to FIG. 3, the same symbols as those used in FIG. 1 indicate members having the same names as the members in FIG. Since the basic operation of the second power supply circuit 200 is almost the same as that of the first power supply circuit 100, only different points will be described. In the second embodiment, the impedance element 18 includes a capacitor 18 a and a resistor 18 b connected in parallel to each other, and a secondary side control circuit 21 is connected in parallel with the impedance element 18. A gate circuit 22 is connected to the gate of the reflux FET 7, and the secondary side control circuit 21 controls the gate circuit 22.

  The secondary side control circuit 21 operates using the charging voltage of the capacitor 18a in the impedance element 18 as a power source, receives the voltage detection signal from the output voltage detection circuit 13, and compares the set overvoltage set value or undervoltage set value. For example, when an abnormal state occurs such as when the voltage detection signal from the output voltage detection circuit 13 is larger than the overvoltage set value or lower than the undervoltage set value, A signal is supplied to the gate circuit 22. In that case, an alarm signal is generated as necessary, and an alarm such as turning on a lamp (not shown) or sounding a buzzer (not shown) is issued. When the gate circuit 22 receives the cutoff signal, the gate of the reflux FET 7 is shut off to turn off the reflux FET 7, and the reflux FET 7 is held off until the abnormal state is recovered and becomes normal. The control circuit 5 on the primary side receives the voltage detection signal from the output voltage detection circuit 13 and controls the pulse width of the switching semiconductor element 4 so that the DC output voltage becomes constant. Also in the power supply circuit 200, when the return FET 7 is turned off, the gate of the return FET 7 becomes lower than the source by the forward voltage drop of the diode D2 of the rectifier circuit 16 due to the return of energy of the inductance element 17. The charge in the gate-source capacitance of the reflux FET 7 is rapidly discharged. Therefore, also in this power supply circuit 200, not only can the power loss due to the delay in the turn-off operation of the return FET 7 be reduced, but also the power supply having a low rated output voltage can be selected by appropriately selecting the number of turns of the secondary winding 9b. Also in this case, the reflux FET 7 can be switched stably and at high speed. Further, the secondary side control circuit can be controlled and a necessary alarm can be generated without preparing a special power source for the secondary side control circuit.

[Embodiment 3]
Next, the power supply circuit 300 according to the third embodiment will be described with reference to FIG. In FIG. 4, the same symbols as those used in FIG. 1 or FIG. 3 indicate members having the same names as the members shown in those drawings. Since the main part of the third power supply circuit 300 is the same as that of the first or second power supply circuit, different points will be described. In the power supply circuit 100 or 200, a MOSFET is used as the first rectifying element of the synchronous rectifier circuit. However, in the power supply circuit 300, rectifying diodes 6A and 6B having a small forward voltage drop such as a Schottky barrier diode are used. Yes. In addition to the transformer 3, a second transformer 3 ′ having substantially the same characteristics as the transformer 3 is provided. A second semiconductor switching element 4 ′ such as a MOSFET is connected in series to the primary winding 3 ′ a of the transformer 3 ′ and is connected across the DC input terminals 1 and 2. A rectifying diode 6A is connected in series to the secondary winding 3b of the transformer 3, and a rectifying diode 6B is connected in series to the secondary winding 3'b of the transformer 3 '. . And they are connected so as to be parallel to each other.

  The control circuit 5 alternately turns on / off the switching semiconductor elements 4 and 4 ′ by pulse width control, or turns them on and off almost simultaneously. When both or only one of the switching semiconductor elements 4 and 4 ′ is ON, the secondary winding 3b of the transformer 3 and / or the secondary winding 3′b of the transformer 3 ′ A current flows through the smoothing inductor 9, the smoothing capacitor 10, and / or the rectifying diodes 6A and 6B, and energy is stored in the smoothing inductor 9 during this period. Next, when both of the switching semiconductor elements 4 and 4 ′ are turned off, energy is not supplied to the smoothing inductor 9 from the secondary winding 3b of the transformer 3 and the secondary winding 3′b of the transformer 3 ′. A voltage having a positive black dot side is generated in the primary winding 9a and the secondary winding 9b of the smoothing inductor 9. A positive voltage on the black dot side of the secondary winding 9b causes the diode D1 of the rectifier circuit 16 to conduct and is applied to the gate of the return FET 7 via the limiting resistor 19, turning on the return FET 7, and the inductance element 17 A current is passed through the impedance element 18. This current stores energy in the inductance element 17.

  Next, when the switching semiconductor element 4 or 4 ′ is turned on, a current flows from the secondary winding 3 b of the transformer 3 or the secondary winding 3 ′ b of the transformer 3 ′ to the smoothing inductor 9. Due to this current, a voltage having a negative black dot side is generated in the secondary winding 9b of the smoothing inductor 9, and this negative voltage reversely biases the diode D1 to make it non-conductive. Therefore, the energy stored in the inductance element 17 is circulated through the impedance element 18 and the diode D2 of the rectifier circuit 16, and the gate voltage of the return FET 7 is made lower than the source by the forward voltage drop of the diode D2. As a result, the charge in the gate-source capacitance of the reflux FET 7 is rapidly discharged, and the reflux FET 7 is turned off at high speed. Also in this power supply circuit 300, the power loss due to the delay in the off operation of the return FET 7 can be reduced, and the power supply with a low rated output voltage can be selected by appropriately selecting the number of turns of the secondary winding 9b. In addition, the reflux FET 7 can be switched stably and at high speed. Note that illustration of the output voltage detection circuit, the feedback circuit, and the like is omitted. Of course, if necessary, an output current detection circuit and its feedback circuit may be provided.

[Embodiment 4]
Next, the power supply circuit 400 according to the fourth embodiment will be described with reference to FIG. In FIG. 5, the same symbols as those used in FIGS. 1, 3, and 4 indicate members having the same names as the members shown in those drawings. Since the main part of the fourth power supply circuit 400 is the same as that of the first power supply circuit 100, different points will be described. The smoothing inductor 9 has a first secondary winding 9b and a second secondary winding 9c having the same polarity indicated by black dots. A second rectifier circuit 16 ', a second inductance element 17', and a second impedance element 18 'are also connected to the second secondary winding 9c. The second rectifier circuit 16 ′ includes first and second diodes d1 and d2 whose anodes are connected to the respective terminals of the second secondary winding 9c. The cathodes of the first and second diodes d1 and d2 are connected to each other, and the second diode d2 is connected across the second secondary winding 9c and the first diode d1. The first secondary winding 9b, the first rectifier circuit 16, the first inductance element 17, and the first impedance element 18 of the smoothing inductor 9 are used to drive the return FET 7 as in the above embodiment. The first drive circuit is configured. The second secondary winding 9c, the second rectifier circuit 16 ′, the second inductance element 17 ′, and the second impedance element 18 ′ of the smoothing inductor 9 are the second for driving the rectification FET 6. A drive circuit is configured.

  When the semiconductor switching element 4 is turned on, a current flows from the secondary winding 3b of the transformer 3 to the smoothing inductor 9 as described above. At this time, a voltage is generated in the first secondary winding 9b and the second secondary winding 9c of the smoothing inductor 9 so that the terminal opposite to the black dot side terminal is positive. In the first drive circuit, as described above, the voltage generated in the first secondary winding 9b reversely biases the diode D1, so that the energy stored in the first inductance element 17 is the first It is refluxed through the impedance element 18 and the diode D2. This forward voltage drop of the diode D2 due to the recirculation causes the gate-source capacitance of the recirculation FET 7 to be rapidly discharged at a high speed, and the recirculation FET 7 is turned off at a high speed. At the same time, in the second drive circuit, a current flows from the second secondary winding 9c through the diode d1, the second inductance element 17 ′, and the second impedance element 18 ′ of the second rectifier circuit 16 ′. The voltage generated between the second inductance element 17 ′ and the second impedance element 18 ′ is applied between the gate and the source of the rectifying FET 6 via the second limiting resistor 19 ′, and the rectifying FET 6 is Turn on at high speed. During this period, the energy of the first inductance element 17 is discharged, and the energy is stored in the second inductance element 17 '.

  Next, when the switching semiconductor element 4 is turned off, no current is supplied from the secondary winding 3b of the transformer 3 to the smoothing inductor 9, so that the first secondary winding 9b and the second secondary winding of the smoothing inductor 9 are not supplied. A voltage is induced in the winding 9c so that the black dot side terminal is positive with respect to the terminal on the opposite side. Therefore, in the first drive circuit, a current flows from the black dot side terminal of the secondary winding 9c through the diode D1, the first inductance element 17 and the first impedance element 18 of the first rectifier circuit 16, A voltage generated by the first inductance element 17 and the first impedance element 18 is applied between the gate and source of the reflux FET 6. Along with this, the return FET 7 is turned on rapidly, and the energy of the smoothing inductor 9 is circulated.

  On the other hand, in the second drive circuit, the diode d1 of the second rectifier circuit 16 ′ is reverse-biased by the voltage of the second secondary winding 9c, so that the second secondary winding 9c to the second No current flows through the inductance element 17 ′. Accordingly, the energy stored in the second inductance element 17 'flows back through the diode d2 and the second impedance element 18'. Due to this recirculating current, the diode d1 has a forward voltage drop, and a voltage lower than the source by the forward voltage drop of the diode d2 is applied to the gate of the rectifying FET 6 via the limiting resistor 19 '. Accordingly, the charge of the gate-source capacitance of the rectifying FET 6 is rapidly discharged, and the rectifying FET 6 is turned off at high speed. In the power supply circuit 400, both the rectifying FET 6 and the reflux FET 7 can be turned off at high speed, so that the power loss can be further reduced. Further, by appropriately selecting the number of turns of the first and second secondary windings 9b and 9c, both the rectifying FET 6 and the refluxing FET 7 can be stably and rapidly operated even in the case of a power supply having a low rated output voltage. It can be switched. Note that illustration of the output voltage detection circuit, the feedback circuit, and the like is omitted. Of course, if necessary, an output current detection circuit and its feedback circuit may be provided.

[Embodiment 5]
Next, a power supply circuit 500 according to the fifth embodiment will be described with reference to FIG. In FIG. 6, the same symbols as those used in FIGS. 1 and 3 to 5 indicate members having the same names as the members shown in the drawings. Since the main part of the fifth power supply circuit 500 is the same as that of the first power supply circuit 100, only different points will be described. The transformer 3 has a third winding 3c having a number of turns sufficient to stably drive the rectifying FET 6 with a simple circuit configuration even when the rated voltage between the DC output terminals 11 and 12 is 3.3 V or less, or 12 V or more. It has. A rectifier circuit 61 is connected to the third winding 3c. The rectifier circuit 61 includes diodes 61a and 61b having anodes connected to the terminals a and b of the third winding 3c, respectively. The cathodes of the diodes 61a and 61b are connected to each other, and the diode 61b is connected across the third winding 3c and the diode 61a. The gate of the rectifying FET 6 is connected to the connection point between the cathodes of the diodes 61a and 61b, and the inductance element 62 is connected in series with the diode 61a. An impedance element 63 is connected between the other end of the inductance element 62 and the terminal b of the third winding 3c. The impedance element 63 includes a capacitor 63a and a resistor 63b connected in parallel to each other. The source of the rectifying FET 6 is connected to the terminal b of the third winding 3c by the wiring 64.

  The switching semiconductor element 4 is turned on by the pulse width control signal from the control circuit 5, and current starts to flow through the primary winding 3 a of the transformer 3. Along with this, a voltage is induced in which the black dot side terminals of the secondary winding 3b and the third winding 3c are positive with respect to the opposite terminals. At this time, as described above, the reflux FET 7 is turned off at high speed. The current flows from the terminal a of the third winding 3c to the terminal b through the diode 61a of the rectifier circuit 61, the inductance element 62, and the impedance element 63. Therefore, since the inductance element 62 and the impedance element 63 exist between the gate and the source of the rectifying FET 6, those voltages are applied between the gate and the source of the rectifying FET 6, and the rectifying FET 6 is turned on rapidly. Along with this, load current is supplied from the transformer 3 through the smoothing inductor 9, and energy is stored in the smoothing inductor 9.

  Next, when the pulse width control signal from the control circuit 5 becomes zero and the switching semiconductor element 4 is turned off, the voltage of the third winding 3c disappears, and the current flows from the third winding 3c side to the inductance element 62. Is no longer supplied, the energy stored in the inductance element 62 is released through the diode 61b and the impedance 63 of the rectifier circuit 61 and circulates. At this time, since a forward voltage drop occurs in the diode 61b, the gate voltage of the rectifying FET 6 is lower than the source voltage by the forward voltage drop of the diode 61b. Therefore, the charge of the gate-source capacitance of the rectifying FET 6 is instantaneously discharged, and the rectifying FET 6 is turned off at a higher speed. At this time, the return FET 7 is turned on as described above to return the energy stored in the smoothing inductor 9. Even in the power supply circuit 500, both the rectifying FET 6 and the reflux FET 7 can be turned off at high speed, so that the power loss can be further reduced. In addition, by appropriately selecting the number of turns of the secondary winding 9b and the third winding 3c, both the rectifying FET 6 and the reflux FET 7 can be switched stably and at high speed even in the case of a power supply with a low rated output voltage. Can be made. Note that illustration of the output voltage detection circuit, the feedback circuit, and the like is omitted. Of course, if necessary, an output current detection circuit and its feedback circuit may be provided.

  In the embodiments described above, an N-channel type FET is used, but a P-channel type FET may of course be used. In the case of this modification, the diodes of the rectifier circuit in the drive circuits of the rectifier FET 6 and the reflux FET 7 may be reversed. For example, in the fourth power supply circuit 400 of the fourth embodiment, the cathodes of the diodes D1 and D2 of the rectifier circuit 16 are connected toward the secondary winding 9b of the smoothing inductor 9 and their anodes are connected to each other. Good. Then, the gate of the reflux FET 7 may be connected to the side where the anodes of the diodes D1 and D2 are connected to each other. Similarly, the cathodes of the diodes d1 and d2 of the rectifier circuit 16 'may be connected toward the second secondary winding 9c of the smoothing inductor 9, and their anodes may be connected to each other. By doing so, the gate voltage can be increased by the forward voltage drop of the diode D2 and the diode d2 when the rectifying FET 6 and the reflux FET 7 are turned off. Capacitance charges can be discharged at high speed. Of course, an N-channel FET and a P-channel FET may be used in combination.

  In the embodiment described above, the primary side circuit of the transformer 3 has been described as a single-ended inverter. However, the present invention is not limited to this. As another modification, although not shown, for example, a general full-bridge type inverter circuit in which four sets of switching semiconductor elements are connected in a full bridge configuration, or a pair of switching semiconductor elements and a pair of capacitors are bridged. It may be a half-bridge type inverter circuit having a general configuration connected to the inverter. Further, the secondary side circuit of the transformer 3 is not limited to the above embodiment. As another modification, although not shown in the figure, a push-pull type, for example, the transformer 3 has two secondary windings in series with each other, and is connected to one DC output terminal through a smoothing inductor at the midpoint thereof. One main terminal of each rectifying FET may be connected to the other ends of the two secondary windings, and the other main terminals may be connected together and connected to the other DC output terminal. . In this case, since the reflux FET is shared by the two rectifying FETs, a single unit may be used. In addition to the MOSFET, the switching semiconductor element 4 may be an IGBT or another transistor. The control method of the control circuit 5 does not have to be limited to pulse width control, and the same effect can be obtained even with frequency control.

  As a modification of the fourth power supply circuit 400 shown in FIG. 5, the first drive circuit for driving the reflux FET 7 is eliminated, and the forward FET 7 has a small forward voltage drop as a Schottky barrier diode. A diode may be used. Alternatively, the gate of the return FET 7 may be connected to the terminal on the opposite side of the black dot side terminal of the secondary winding 3b of the transformer 3 and driven by the voltage of the secondary winding 3b. In this case, only the rectifying FET 6 performs a high-speed turn-off operation in the drive circuit including the second secondary winding 9c of the smoothing inductor 9, the rectifying circuit 16 ', the inductance element 17', and the impedance element 18 '. Is called.

  As described above, according to the power supply circuit of the present invention, the smoothing inductor includes one or more secondary windings, and a set of diodes having anodes or cathodes connected to the secondary windings. And connecting the inductance element and the impedance element to the output of the rectifier circuit by connecting the gate of the reflux FET to the output of the rectifier circuit. The diode, the inductor, and the impedance element are present. Therefore, the return FET can be operated stably and reliably regardless of the load voltage, and the switching operation of the rectifier FET, in particular, the turn-off speed can be increased. Loss can be reduced and a low-loss power circuit can be provided. Also, the same effect can be achieved with the rectifying FET.

1 is a diagram showing a power supply circuit 100 according to Embodiment 1 of the present invention. It is a figure which shows the voltage waveform or current waveform of each part in the power supply circuit 100 of this invention. It is a figure which shows the power supply circuit 200 which concerns on Embodiment 2 of this invention. It is a figure which shows the power supply circuit 300 which concerns on Embodiment 3 of this invention. It is a figure which shows the power supply circuit 400 which concerns on Embodiment 4 of this invention. It is a figure which shows the power supply circuit 500 which concerns on Embodiment 5 of this invention.

Explanation of symbols

1, 2 ... DC input terminal 3, 3 '... Transformer 3a, 3a' ... Primary winding of transformer 3 3b, 3b '... Secondary winding of transformer 3 3c ... Transformer 3 3rd winding 4 ... Switching semiconductor element 5 ... Control circuit 6 ... FET for rectification
6A, 6B ... Rectifier diode 7 ... Reflux FET
8 ... Limiting resistor 9 ... Smoothing inductor 10 ... Smoothing capacitor 11, 12 ... DC output terminal 13 ... Output voltage detection circuit 14 ... Load 15 ... Insulation circuit 16 , 16 '... Rectifier circuit 17, 17' ... Inductance element 18, 18 '... Impedance element 21 ... Secondary side control circuit 22 ... Gate circuit 61 ... Rectifier circuit 62 ...・ Inductance element 63 ... Impedance element 64 ... Wiring

Claims (10)

A transformer having a primary winding and a secondary winding connected to a DC input terminal, a switching semiconductor element connected in series to the primary winding, and a control circuit for controlling the switching semiconductor element; A first rectifying element connected in series to the secondary winding; a second rectifying element connected across the secondary winding and the first rectifying element; and the secondary winding. And a smoothing inductor having a primary winding connected in series to the load and a secondary winding magnetically coupled to the primary winding,
The second rectifying element comprises a reflux field effect transistor that circulates energy stored in the smoothing inductor,
A circuit for driving the field effect transistor for reflux, the secondary winding of the smoothing inductor, a rectifier circuit connected to the secondary winding, and connected to an output side of the rectifier circuit A drive circuit comprising an inductance element and an impedance element,
In the drive circuit, the energy stored in the inductance element is released through the rectifier circuit when the switching semiconductor element is turned on, so that the charge of the gate-source capacitance of the reflux field effect transistor is discharged at high speed. A power supply circuit characterized by that. In claim 1,
The first rectifying element includes one main terminal connected to one end side of the secondary winding of the transformer, the other main terminal connected to a DC output terminal side, and the secondary winding of the transformer. A power supply circuit comprising a rectifying field effect transistor having a gate connected to the other end of the line. In claim 1,
The first rectifying element is a rectifying diode;
A second transformer connected in parallel across the DC input terminal; a second switching semiconductor element connected in series to a primary winding of the second transformer; and the second transformer A rectifying diode connected in series with the secondary winding;
The power supply circuit, wherein the secondary winding of the second transformer is connected in parallel across the secondary winding of the transformer and the first rectifying diode via the rectifying diode. . In claim 1,
The first rectifying element is a rectifying field effect transistor having one main terminal connected to one end side of the secondary winding of the transformer and the other main terminal connected to a DC output terminal side. ,
The transformer has a third winding;
A second driving circuit for driving the rectifying field effect transistor, wherein the second driving circuit includes the third winding and a second rectifying circuit connected to the third winding; A second inductance element and a second impedance element connected to the output side of the second rectifier circuit,
In the second drive circuit, the energy stored in the second inductance element is released through the second rectifier circuit when the switching semiconductor element is turned off, whereby the gate-source of the rectifying field effect transistor is obtained. A power supply circuit characterized in that the inter-capacitance charge is discharged at high speed. A transformer having a primary winding and a secondary winding, a switching semiconductor element connected in series with the primary winding, a control circuit for controlling the switching semiconductor element, and a series with the secondary winding The first rectifying element connected, the second rectifying element connected across the secondary winding and the first rectifying element, and the secondary winding and the load connected in series. And a smoothing inductor having a primary winding that is coupled and a secondary winding that is magnetically coupled to the primary winding,
The first rectifying element comprises a rectifying field effect transistor,
A circuit for driving the rectifying field effect transistor, the secondary winding of the smoothing inductor, a rectifying circuit connected to the secondary winding, and connected to an output side of the rectifying circuit A drive circuit comprising an inductance element and an impedance element,
In the drive circuit, the energy stored in the inductance element is discharged through the rectifier circuit when the switching semiconductor element is turned off, so that the charge of the gate-source capacitance of the rectifying field effect transistor is discharged at high speed. A power supply circuit characterized by that. A transformer having a primary winding and a secondary winding, a switching semiconductor element connected in series with the primary winding, a control circuit for controlling the switching semiconductor element, and a series with the secondary winding Connected in series to the connected rectifying field effect transistor, the return field effect transistor connected across the secondary winding and the rectifying field effect transistor, and the secondary winding and the load And a smoothing inductor having a primary winding that is coupled and a secondary winding that is magnetically coupled to the primary winding,
The secondary winding of the smoothing inductor is composed of first and second secondary windings connected in series with each other,
The first secondary winding of the smoothing inductor, a first rectifier circuit connected to the first secondary winding, and a first rectifier circuit connected to the output side of the first rectifier circuit A first drive circuit comprising one inductance element and a first impedance;
In the first drive circuit, energy stored in the first inductor is released through the first rectifier circuit when the switching semiconductor element is turned on, so that the gate-source region of the rectifying field effect transistor is discharged. The charge of the capacity is discharged at high speed,
The second secondary winding of the smoothing inductor, a second rectifier circuit connected to the second secondary winding, and the second rectifier circuit connected to the second rectifier circuit A second drive circuit comprising an inductance element and a second impedance;
In the second drive circuit, the energy stored in the second inductance element is released through the second rectifier circuit when the switching semiconductor element is turned on, whereby the gate-source of the reflux field effect transistor A power supply circuit characterized in that the inter-capacitance charge is discharged at high speed. In claim 6,
The power supply circuit, wherein the inductance element and the impedance element exist between a gate and a source of the reflux field effect transistor and / or the rectification field effect transistor. In claim 6 or claim 7,
The reflux field effect transistor and / or the rectifying field effect transistor is an N-channel field effect transistor,
The rectifier circuit, the first rectifier circuit, or the second rectifier circuit includes a first diode having an anode connected to one end of the secondary winding of the smoothing inductor, and the secondary winding. An anode is connected to the end and a second diode is connected across the secondary winding and the first diode, and the cathodes of the first and second diodes are connected to each other,
The power supply circuit, wherein the gates of the reflux field effect transistor and / or the rectifying field effect transistor are connected to the side where the cathodes of the first and second diodes are connected to each other. In any of claims 6 to 8,
The reflux field effect transistor and / or the rectifying field effect transistor comprises a P-channel field effect transistor,
The rectifier circuit, the first rectifier circuit, or the second rectifier circuit includes a first diode having a cathode connected to one end of the secondary winding of the smoothing inductor, and the secondary winding. A cathode connected to the end and a second diode connected across the secondary winding and the first diode, the anodes of the first and second diodes being connected to each other;
The power supply circuit, wherein a gate of the reflux field effect transistor and / or the rectifying field effect transistor is connected to a side where anodes of the first and second diodes are connected to each other. In any one of Claims 6 thru | or 9,
The impedance element includes a capacitor;
A power supply circuit, wherein a secondary side control circuit for controlling the reflux FET and / or the rectifying field effect transistor is connected to both ends of the capacitor.

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