JP6281748B2 - DC-DC converter - Google Patents

Description

  The present invention relates to a DC-DC converter, and more particularly to a snubber circuit that suppresses a surge voltage of a diode.

  DC-DC converters are widely used to drive electronic devices. These DC-DC converters convert the input DC voltage into a rectangular wave by on / off control with a switching element, and convert the rectangular wave into a stable predetermined DC voltage by rectifying and smoothing. However, since the DC-DC converter controls the switching elements on and off, noise is generated. This noise is applied as a surge voltage to a semiconductor element that is switched between conductive and non-conductive, such as a diode used in a switch element or a rectifying and smoothing circuit, and may damage the semiconductor element. In order to suppress the surge voltage, generally, a CR snubber circuit configured by connecting a snubber resistor and a snubber capacitor in series is inserted at both ends of the semiconductor element.

  Taking a diode of a rectifying and smoothing circuit as an example, a CR snubber circuit can suppress a surge voltage because a snubber capacitor is charged via a snubber resistor when a semiconductor element is turned off and a voltage is applied. Further, when the semiconductor element becomes conductive, the energy charged in the snubber capacitor is released through the snubber resistor. For this reason, the CR snubber circuit is effective in suppressing the surge voltage, but the suppressed surge is consumed by the resistor, and loss occurs.

  Therefore, an active snubber circuit composed of a capacitor and an auxiliary switch has been proposed instead of the CR snubber circuit. FIG. 14 shows an example of a conventional DC / DC converter that suppresses a surge voltage of a semiconductor element using an active snubber circuit (Patent Document 1). In FIG. 14, the main switch FET 111 is connected in series to the DC power supply 110, and the primary winding 112 a of the transformer 112 is connected in series to the main switch FET 111. A rectifying diode 113 and an output capacitor 114 are connected in series to the secondary winding 112 b of the transformer 112, and a load 115 is connected in parallel to the output capacitor 114. A series circuit including a capacitor 116 and an auxiliary switch FET 117 is connected in parallel with the main switch FET 111. Similarly, a series circuit including a capacitor 118 and an auxiliary switch FET 119 is connected in parallel to the diode 113 of the secondary circuit.

The auxiliary switch FET 117 is controlled to be in a non-conductive state when the main switch FET 111 is in a conductive state and to be in a conductive state when the main switch FET 111 is in a non-conductive state. Further, the auxiliary switch FET 119 performs control so as to be in a non-conductive state when the diode 113 is in a conductive state and to be in a conductive state when the diode 113 is in a non-conductive state. For example, when the main switch FET111 is switched to a non-conductive state, the auxiliary switch FET17 is switched to a conductive state. Here, since the capacitor 116 is set to several tens of times the stray capacitance of the main switch FET 111, both ends of the main switch FET 111 are short-circuited to a low frequency and low impedance state, and the voltage between both ends of the main switch FET 111 is reduced. Is kept at an almost constant value to suppress the surge voltage. On the other hand, when the main switch FET 111 is switched from the non-conductive state to the conductive state, the auxiliary switch FET 17 is switched to the non-conductive state. Therefore, the voltage of the capacitor 116 is maintained as it is and no power loss occurs.
Similarly, when the diode 113 of the secondary side circuit is in the conductive state and then switched to the non-conductive state, the FET 119 is switched to the conductive state. Since the capacitance of the capacitor 118 is set to several tens of times the stray capacitance of the diode 113, the voltage across the diode 113 is maintained at a substantially constant value, and a voltage surge can be suppressed.

  When the main switch FET 111 becomes non-conductive, the voltage between both ends rises by charging the stray capacitance of the main switch FET 111. However, in the conventional example shown in FIG. 14, the auxiliary switch FET 117 conducts before the voltage across the main switch FET 111 reaches the voltage charged in the capacitor 116, and therefore, the path of the capacitor 116 → FET 111 → FET 117. The capacitor 116 discharges and charges the stray capacitance of the main switch FET111. This discharge current further increases when the capacity of the capacitor 116 is large, and an element having a large current capacity is required for the auxiliary switch FET 117. Further, in the conventional example shown in FIG. 14, since the source of the auxiliary switch FET 117 and the source of the main switch FET 111 have different potentials, a new drive circuit for driving the auxiliary switch FET 117 is required, and the circuit becomes complicated.

  Similarly, when the secondary-side diode 113 is switched from the conductive state to the non-conductive state, when the voltage of the diode 113 is lower than the voltage charged in the capacitor 118, the discharge current of the capacitor 118 flows and the auxiliary switch FET 119 Charge the stray capacitance. For this reason, like the auxiliary switch FET 117, the auxiliary switch FET 119 requires an element having a large current capacity, and further requires a new drive circuit.

The present invention provides a DC converter that performs voltage conversion by repeating conduction and non-conduction of a semiconductor switch element.
In a DC / DC converter, a snubber circuit having a series circuit composed of a snubber capacitor and a snubber diode connected in parallel to both ends of a semiconductor switch element, and an auxiliary switch connected in parallel to both ends of the snubber diode , applied to the snubber circuit The voltage to be given is
A voltage detection circuit that detects that the value exceeds the value and a snubber circuit according to the voltage detection circuit.
When the voltage is below the specified value, a power supply for driving the auxiliary switch is generated, and the snubber circuit
Auxiliary switch to discharge the snubber capacitor when the applied voltage is above a predetermined value
A DC / DC converter comprising: a snubber circuit having a drive circuit for driving the drive circuit .

  In a DC-DC converter having a snubber circuit that suppresses a surge voltage of a semiconductor element, a surge voltage is suppressed without causing a snubber capacitor to be unnecessarily discharged, and a DC-DC converter that is smaller and more efficient than conventional ones is realized. With the goal.

  The present invention relates to a DC-DC converter that performs voltage conversion by repeating conduction and non-conduction of a semiconductor switch element, a series circuit including a snubber capacitor and a snubber diode connected in parallel to both ends of the semiconductor switch element, and a snubber diode A snubber circuit having an auxiliary switch connected in parallel to both ends is provided, and the auxiliary switch is turned on when the voltage across the semiconductor switch element becomes equal to or higher than a predetermined value.

  The DC-DC converter according to the present invention suppresses a surge voltage by a series circuit composed of a snubber capacitor and a snubber diode when the semiconductor switch element is turned off, and assists when the voltage across the semiconductor switch element exceeds a predetermined value. Since the switch is turned on, the snubber capacitor is not discharged more than necessary, and the energy charged by the surge voltage is discharged. For this reason, since the surge voltage is suppressed with a small energy loss, noise can be reduced efficiently.

FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 2 is a timing chart showing the operation of the auxiliary switch when the on-timing of the auxiliary switch is not optimal in the first embodiment of the present invention. FIG. 3 is a timing chart showing the operation of the auxiliary switch when the on-timing of the auxiliary switch is optimized in the first embodiment of the present invention. FIG. 4 is a circuit configuration diagram when the first embodiment of the present invention is applied to a rectifier. FIG. 5 is a timing chart when the first embodiment of the present invention is applied to a rectifier. FIG. 6 is a circuit diagram showing a second embodiment in which the present invention is applied to a synchronous rectifier circuit. FIG. 7 is a timing chart showing the operation of the auxiliary switch when the second embodiment of the present invention is not applied. FIG. 8 is a timing chart showing the operation of the auxiliary switch in the second embodiment of the present invention. FIG. 9 is a circuit diagram showing a third embodiment of the present invention. FIG. 10 is a timing chart showing the operation of the auxiliary switch in the third embodiment of the present invention. FIG. 11 is a circuit diagram showing a fourth embodiment of the present invention. FIG. 12 is a block diagram when the present invention is applied to a center tap type rectifier circuit. FIG. 13 is a block diagram when the present invention is applied to a rectifier circuit of a forward converter. FIG. 14 is a circuit diagram showing a conventional example.

  FIG. 1 is a block diagram showing a first embodiment of the present invention. DC power supply 1, primary winding of transformer T, and main switch Q0 are connected in series. The main switch Q0 is ON / OFF controlled by the control circuit 6, and intermittently applies a voltage to the primary winding of the transformer T. For this reason, a rectangular wave voltage is induced in the secondary winding Ns of the transformer T. This voltage is rectified by the diode D0, smoothed by the smoothing capacitor Co, and the DC voltage Vo is output to Vo + and Vo−. Further, in the first embodiment, the snubber circuit 5 is connected to both ends of the diode D0 and the snubber circuit 51 is connected to both ends of the main switch Q0.

  The snubber circuit 5 includes a first series circuit including a snubber capacitor C1 and a snubber diode D1, a second series circuit including a resistor R1 and an auxiliary switch Q1 connected in parallel to the snubber diode D1, and a snubber control circuit 7. Is done. The first series circuit constitutes a snubber unit that suppresses the surge voltage of the semiconductor element, and the second series circuit constitutes a discharge circuit that discharges the charge charged in the snubber capacitor C1 by the surge voltage, and a snubber control circuit Detects the voltage applied to the snubber circuit 5, and drives the auxiliary switch in accordance with this voltage to control on / off. Resistor R1 limits the discharge current of snubber capacitor C1. The snubber circuit 51 includes a first series circuit including a snubber capacitor C51 and a snubber diode D51, a second series circuit including a resistor 51 and an auxiliary switch Q51 connected in parallel to the snubber diode D51, and a snubber control circuit 71. Is done.

  The snubber circuit 5 connected to the diode D0 and the snubber circuit 51 connected to the main switch Q0 perform the same operation to suppress the surge voltage. Hereinafter, the operation of the diode D0 and the snubber circuit 5 will be described as an example. When the main switch Q0 is turned on, a voltage in the opposite direction to the diode D0 is generated in the secondary winding Ns of the transformer T, and the voltage across the diode D0 rises. When the voltage across the diode D0 becomes higher than the voltage of the snubber capacitor C1, a current flows through the snubber capacitor C1 and the snubber diode D1 to charge the snubber capacitor C1. Thereby, the surge voltage applied to both ends of the diode D0 is suppressed. The auxiliary switch Q1 is turned on when the voltage applied to the snubber circuit 5 exceeds a predetermined value. After that, when the surge voltage disappears, the snubber capacitor C1 is discharged through a path of C1, Ns, Co, Q1, and R1, and the voltage at both ends of the snubber capacitor C1 drops to the voltage applied to the snubber circuit 5 at this time. In other words, the snubber capacitor C1 discharges the voltage that has risen due to the surge voltage. Next, the auxiliary switch Q1 is turned off when the diode D0 becomes conductive and the voltage at both ends becomes a predetermined value or less. The discharge path of the snubber capacitor C1 disappears, and the snubber capacitor C1 maintains a voltage while the diode D0 is in a conductive state.

Here, when the auxiliary switch Q1 is turned on at the same time that the diode D0 is turned off, the auxiliary switch Q1 is turned on before the voltage applied to the snubber circuit 5 rises to the voltage of the snubber capacitor C1. Therefore, the snubber capacitor C1 discharges along a path of C1 → D0 stray capacitance → Q1 → resistor R1. In particular, when the auxiliary switch Q1 is turned on in a state where the voltage of the diode D0 is close to zero, an almost short-circuited state occurs and a large current flows.
FIG. 2 shows the voltage VD across the diode D0 and the current waveform IQ1 of the auxiliary switch Q1 at this time. In FIG. 2, the auxiliary switch Q1 of the snubber circuit 5 is turned on based on the ON signal of the main switch Q0. Therefore, an excessive current flows through the auxiliary switch Q1 immediately after the diode D0 is turned off. On the other hand, FIG. 3 shows a case where the auxiliary switch Q1 is turned on when the voltage applied to the snubber circuit 5 becomes 90% of the voltage applied to the snubber circuit 5 when the surge voltage ends. The voltage VD across the diode D0 and the current waveform IQ1 of the auxiliary switch Q1. As shown in FIG. 3, when the auxiliary switch Q1 is turned on after the voltage applied to the snubber circuit 5 has sufficiently risen, an excessive current flows to the auxiliary switch Q1 immediately after the diode D0 is turned off. Absent.

  If the auxiliary switch Q1 is turned on when the voltage applied to the snubber circuit 5 exceeds the voltage of the snubber capacitor C1, the current that flows immediately after the diode D0 becomes non-conductive disappears, but the voltage of the input power supply or the output voltage, etc. It depends on the fluctuation of Further, it is necessary to discharge the voltage charged and increased thereafter with the surge voltage until the voltage applied to the snubber circuit when the surge voltage is finished. In the present invention, the voltage of the snubber capacitor C1 is surely discharged by turning on the auxiliary switch Q1 until the diode D0 becomes conductive. For this reason, the voltage for turning on the auxiliary switch Q1 is set lower than the voltage applied to the snubber circuit when the surge voltage ends. As a result, the auxiliary switch Q1 does not flow an excessive current immediately after the diode D0 becomes non-conductive, and can turn on the auxiliary switch Q1 while the diode D0 is non-conductive, so that the voltage of the snubber capacitor C1 is reduced. The portion raised by the surge voltage can be discharged. Even if the voltage for turning on the auxiliary switch Q1 is about 50% of the voltage applied to the snubber circuit when the surge voltage is finished, an excessive current does not flow immediately after the diode D0 is turned off. In consideration of variations and the like, the voltage for turning on the auxiliary switch Q1 may be set to about 50% to 90% of the voltage applied to the snubber circuit when the surge voltage ends.

  FIG. 4 shows details of the snubber circuit according to the first embodiment of the present invention shown in FIG. FIG. 4 shows an example of the secondary side rectifier circuit, but the main switch Q0 of the switch circuit 6 operates in the same manner. The secondary side rectifier circuit is constituted by a diode D0. A series circuit composed of resistors R3 and R2 is connected to both ends of the diode D0, a gate of the FET switch Q2 is connected to a connection point between the resistors R2 and R3, and a source of the resistor R2 and the FET switch Q2 is connected to the connection point. Is connected to the anode of the diode D0 to constitute the voltage detection circuit 8. The drain of the FET switch Q2 is connected to the positive side of the voltage source V1 via the resistor R4, and the negative side of the voltage source V1 is connected to the anode of the diode D0. Further, the drain of the FET switch Q2 is connected to the anode of the diode D3 via the capacitor C2, and the cathode of the diode D3 is connected to the negative side of the voltage source V1. The anode of the diode D3 is further connected to the negative side of the voltage source V1 through a series circuit of a resistor R5 and a resistor R6, and constitutes a drive circuit for the auxiliary switch Q1. The voltage detection circuit 8 and the drive circuit 9 constitute a snubber control circuit. A discharge circuit configured by a snubber capacitor C1 and a snubber diode D1 for suppressing a surge voltage of a semiconductor element, a resistor R1 connected in parallel with the snubber diode D1 and an auxiliary switch Q1, and discharging the snubber capacitor C1 is shown in FIG. Is the same.

  The operation of the snubber circuit according to the first embodiment will be described with reference to FIGS. First, when the diode D0 is in a conducting state, a low voltage is generated in the diode D0 due to a forward voltage drop. Therefore, the voltage VR2 generated in the resistor R2 by dividing the voltage across the diode D0 by the resistor R3 and the resistor R2. Does not become a voltage for turning on the FET switch Q2, and the FET switch Q2 is turned off. At this time, the voltage source V1 that outputs the voltage V1 flows through the path of R4 → C2 → D3, charges the capacitor C2, and the both ends of the capacitor C2 become the voltage V1. At this time, since the forward voltage drop of the diode D3 is applied to the resistors R5 and R6, the voltage VR6 generated at the resistor R6 does not turn on the auxiliary switch Q1, and the auxiliary switch Q1 is turned off.

  Thereafter, at time t1, the diode D0 changes from the conductive state to the nonconductive state, and the surge voltage generated by the voltage VNs generated in the secondary winding Ns of the transformer T, the voltage Vo of the smoothing capacitor Co, the leakage inductance, and the like is diode D0. And the stray capacitance of the diode D0 is charged. The resistor R2 and the resistor R3 are set so that the FET switch Q2 is turned on when the diode voltage VD applied across the diode D0 becomes equal to or higher than the voltage Vth. For this reason, when the voltage VD becomes equal to or higher than the voltage Vth at time t2, the FET switch Q2 is turned on, and the capacitor C2 is discharged along the path C2-> Q2-> R6-> R5-> C2. Here, since the resistor R5 and the resistor R6 are set so that the voltage VR6 generated in the resistor R6 turns on the auxiliary switch Q1, the auxiliary switch Q1 is turned on. Capacitor C2 is charged when diode D0 is conductive, and generates a power source for driving auxiliary switch Q1 when diode D0 is non-conductive. At this time, since the difference between the voltage VD across the diode D0 and the capacitor voltage VC charged in the snubber capacitor C1 is small, no large current flows through the auxiliary switch Q1.

  The diode voltage VD further rises due to the surge voltage. When the diode voltage VD becomes equal to or higher than the capacitor voltage VC, a current flows through the path of the snubber capacitor C1 and the snubber diode D1, the snubber capacitor C1 is charged, and the voltage applied to the diode D0 is increased. Suppress. When the surge voltage disappears at time t3, the diode voltage VD becomes the voltage Vr. The voltage Vr is determined by the voltage VNs generated in the secondary winding Ns of the transformer T and the voltage Vo of the smoothing capacitor Co. Since the capacitor voltage VC of the snubber capacitor C1 is charged with a surge voltage and VC> Vr, the snubber capacitor C1 is discharged through a path of C1, Ns, Co, Q1, and C1. Since VD> Vth and the diode D0 are in the non-conductive state, this discharge continues, so that the capacitor voltage VC becomes substantially the same as the voltage Vr.

  Thereafter, at time t4, when the voltage VNs of the secondary winding of the transformer T is generated in the reverse direction, the voltage is applied to the diode D0 in the forward direction, so that the voltage charged in the stray capacitance of the diode D0 is discharged. Then, the voltage VD drops. When the voltage VD becomes equal to or lower than the voltage Vth, the FET switch Q2 is turned off, and the capacitor C2 is charged by the current flowing from the voltage source V1 through the resistor R4, capacitor C2, and diode D3, and the auxiliary switch Q1 is turned off. When the discharge of the stray capacitance is finished, the diode D0 is applied with a voltage in the forward direction and becomes conductive. The above operation is repeated.

A second embodiment of the present invention is shown in FIG. 6 is obtained by replacing the diode D0 with the synchronous rectifier circuit in the rectifier circuit 3 of FIG. The synchronous rectification circuit includes a synchronous rectification switch Q31 and a synchronous rectification control circuit that controls on / off of the synchronous rectification switch. This method is particularly effective in the case of a synchronous rectifier circuit. Normally, in the case of the synchronous rectification method, the synchronous rectification signal is generated based on the timing at which the primary main switch Q0 is switched between the conductive state and the non-conductive state, and the synchronous rectification switch Q31 is driven. However, if the synchronous rectification switch Q31 is performed simultaneously with the switching of the main switch Q0, the synchronous rectification switch Q31 is turned on and off when the voltage is changed, and a short-circuit current may flow. Thus, the synchronous rectification switch Q31 is provided with a dead time so as to be turned off earlier than the synchronous rectification signal and turned on later than the synchronous rectification signal. Although it is conceivable to use the driving signal of the synchronous rectification switch Q31 for driving the auxiliary switch Q1, if the driving signal of the synchronous rectification switch Q31 is used, the auxiliary switch Q1 is turned on earlier by the dead time, and the dead time is reduced. The auxiliary switch Q1 is turned off later by the time. FIG. 7 shows the waveforms of the respective parts when the auxiliary switch Q1 is driven using the gate signal of the synchronous rectification switch. Q31Vds is the drain-source voltage of the synchronous rectification switch Q31, Q31Vgs is the gate voltage of the synchronous rectification switch Q31, Q1Vgs is the gate voltage of the auxiliary switch Q1, and IQ1 is the drain current of the auxiliary switch Q1. As shown in FIG. 7, when the synchronous rectification switch Q31 is turned on and the auxiliary switch Q1 is turned off at the same timing, or when the synchronous rectification switch Q31 is turned off and the auxiliary switch Q1 is turned on at the same timing, the discharge of the snubber capacitor C1 As a result, a large current flows through the auxiliary switch Q1.
FIG. 8 shows operation waveforms of the second embodiment of the present invention. Each waveform is the same as FIG. As shown in FIG. 8, in the second embodiment, when the voltage across the snubber circuit 5 becomes equal to or higher than a predetermined voltage (voltage Vth shown in FIG. 5), the auxiliary switch Q1 is switched to the conductive state. Even if there is, a large current does not flow through the auxiliary switch Q1 during the dead time period. That is, the snubber capacitor C1 is not unnecessarily discharged.
As described above, the same effect as that of the first embodiment is obtained in the second embodiment.

  A third embodiment of the present invention is shown in FIG. 9 is different from the first embodiment of the present invention shown in FIG. 6 in that the FET switch Q2, the resistor R4, and the voltage source V1 are deleted, and the buffer Q3 is inserted between the connection point of the resistor R2 and the resistor R3 and both ends of the capacitor C2. Is. The buffer Q3 has a predetermined threshold value. Since the voltage generated in the resistor R2 when the diode D0 is in a conductive state is less than the threshold value of the buffer Q3, the buffer Q3 outputs a high level, and a current flows through the capacitor C2 and the diode D3 to charge the capacitor C2. When the diode D0 becomes non-conductive and the voltage generated in the resistor R2 exceeds the threshold value, the buffer outputs a low level. Therefore, the capacitor C2 is discharged along the path C2-> Q3-> R6-> R5-> C2, and the resistor R6 The auxiliary switch Q1 is turned on with the generated voltage. FIG. 10 shows operation waveforms of the third embodiment shown in FIG. VD is a voltage applied to the diode D0, VR2 is a voltage generated in the resistor R2, Q1Vgs is a gate voltage of the auxiliary switch Q1, and IQ1 is a current of the auxiliary switch Q1. As described above, the third embodiment also has the same effect as the first embodiment shown in FIG.

  A fourth embodiment of the present invention is shown in FIG. FIG. 11 shows an auxiliary switch that is a P-channel MOFEST, with the FET switch Q2, the resistor R4, the voltage source V1, the capacitor C2, the diode D3, the resistor R5, and the resistor R6 removed from the first embodiment of the present invention shown in FIG. A series circuit of an auxiliary switch Q4, which is an N-channel MOSFET instead of Q1, and a resistor R1 is connected in parallel with the snubber diode D1. Further, the resistor R9 is connected to the cathode of the diode D0, the emitter of the PNP transistor Q6 is connected to the resistor R9, and the collector of the transistor Q6 is connected to the anode of the diode D0 via the resistor R10. The resistor R8 is connected from the cathode of the diode D0, the emitter of the PNP transistor Q5 is connected to the resistor R8, and the resistor R7 and the gate of the auxiliary switch Q4 are connected to the collector of the transistor Q5. The other end of the resistor R7 is connected to the source of the auxiliary switch Q4, and the resistor R7 is connected between the gate and the source of the auxiliary switch Q4. The collector and base of the transistor Q6 are connected, and the bases of the transistors Q5 and Q6 are connected to each other to form a current mirror circuit. The source of the auxiliary switch Q4 is connected to the connection point of the snubber capacitor C1 and the snubber diode D1, and the drain of the auxiliary switch Q4 is connected to the anode of the diode D0 via the resistor R1. Snubber capacitor C1 is connected to the cathode of diode D0.

The operation of Embodiment 4 of the present invention thus connected will be described. When the diode D0 is in the conductive state, the voltage due to the forward voltage drop of the diode D0 is applied to the snubber circuit 5, so that the transistor Q6 cannot be turned on. Therefore, no current flows through the transistor Q5, and no driving power is supplied to the auxiliary switch Q4, so that the auxiliary switch Q4 is turned off. When the diode D0 is in a non-conducting state, the voltage VNs generated in the secondary winding Ns of the transformer T, the voltage Vo of the smoothing capacitor Co, the leakage voltage generated by the leakage inductance, etc. are applied to the snubber circuit 5, and the current mirror In the circuit, a current corresponding to the voltage applied to the path of the resistor R9, the transistor Q6, and the resistor R10 flows. For this reason, a current flows through the transistor Q5 through a path of R8.fwdarw.Q5.fwdarw.R7.fwdarw.D2. At this time, when the voltage generated in the resistor R7 exceeds the threshold voltage of the auxiliary switch Q4, the auxiliary switch Q4 is turned on. Since the voltage applied to the snubber circuit 5 is equal to or higher than a predetermined value (same as the voltage Vth in FIG. 5), even when the auxiliary switch Q4 is turned on, the discharge current of the snubber capacitor C1 is small. When the voltage applied to the snubber circuit 5 is further increased by the surge voltage and exceeds the voltage charged in the snubber capacitor C1, a current flows through the path of the snubber capacitor C1 and the snubber diode D1, thereby suppressing the surge voltage. When the surge voltage disappears, the snubber capacitor C1 discharges the voltage charged with the surge voltage along the path of C1, Ns, Co, R1, and Q1. After that, when the voltage of the secondary winding Ns of the transformer T is inverted, the voltage applied to the diode D0 is decreased, and when the voltage falls below a predetermined value, the auxiliary switch Q4 is turned off, so that the snubber capacitor maintains the charging voltage. .
Since it operates as described above, the same effects as in the first embodiment can be obtained in the fourth embodiment.

  As described above, the snubber circuit of the present invention suppresses the surge voltage of the semiconductor switch only by connecting to both ends of the semiconductor switch element. In addition, the snubber capacitor is not discharged when the applied voltage is changed by turning on and off the semiconductor switch, so there is no unnecessary discharge, and the charge charged by the surge voltage is discharged, so there is little power loss during discharge. . Further, since the driving power source of the auxiliary switch for discharging the snubber capacitor is generated by the snubber control circuit, a new driving circuit and driving power source are not required. Therefore, the snubber circuit of the present invention is not limited to the flyback type converter shown in FIG. 1, but the rectifier of the center tap type rectifying and smoothing circuit shown in FIG. 12, and the rectifier of the forward converter type rectifying and smoothing circuit shown in FIG. In addition, it can be easily mounted without adding a circuit.

  Since the surge voltage of the switch element can be easily and efficiently suppressed, it can be applied to a small and highly efficient switching DC / DC converter.

DESCRIPTION OF SYMBOLS 1 Input power supply 2 Switch circuit 3 Rectifier circuit 4 Smoothing circuit 5, 51 Snubber circuit 6 Control circuit 7 of switch circuit 2, Snubber control circuit 8 Voltage detection circuit 9 Drive circuit T Transformer Ns Secondary winding D0 Rectification Diode D1, D51 Snubber diode Co Smoothing capacitor R1, R51 Snubber resistance Q1, Q51 Auxiliary switch Q2, Q4 FET
R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R51 Resistor C1, C51 Snubber capacitor C2 Capacitor V1 Voltage source Q3 Buffer Q5, Q6 Transistor

Claims (6)

In a DC / DC converter that performs voltage conversion by repeating conduction / non-conduction of a semiconductor switch element,
A series circuit composed of a snubber capacitor and a snubber diode connected in parallel to both ends of the semiconductor switch element;
In snubber circuit having an auxiliary switch connected in parallel across the snubber diode
There,
A voltage detection circuit for detecting that the voltage applied to the snubber circuit has exceeded a predetermined value.
Road,
Depending on the voltage detection circuit, when the voltage of the snubber circuit is below a predetermined value, the auxiliary snubber
A power source for driving the switch is generated, and the voltage applied to the snubber circuit is less than a predetermined value.
Drive circuit for driving the auxiliary switch to discharge the snubber capacitor when on
And a snubber circuit having a DC / DC converter. The DC / DC converter according to claim 1, wherein the semiconductor switch element is a rectifier element that rectifies an intermittent voltage. 3. The DC / DC converter according to claim 1, wherein the semiconductor switch element is a synchronous rectifier circuit that rectifies an intermittent voltage. 4. The DC / DC converter according to claim 1, wherein a resistance for limiting a discharge current of the snubber capacitor is connected in series to the auxiliary switch. 5. The DC / DC converter according to claim 1, wherein the auxiliary switch is a P-channel MOSFET having a source connected to a cathode of the snubber diode and a drain connected to an anode of the snubber diode. 6. The DC / DC according to claim 1, wherein the voltage detection circuit detects that a predetermined current or more flows in a current mirror circuit connected to both ends of the snubber circuit.
converter.

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