JP5012404B2 - Synchronous rectification type DC-DC converter - Google Patents

Description

  The present invention relates to a synchronous rectification type DC-DC converter in which a main switch element is connected to a primary winding of a transformer, and a rectification switch element and a commutation switch element are provided in a secondary winding of the transformer.

  For example, in a power supply device for information communication equipment such as an optical fiber transmission / reception device, a mobile phone base station, or a PC server, normally, a plurality of loads are operating, so that the load fluctuation when viewed as a whole device is large. High responsiveness is required even when the load changes suddenly. For this reason, feedback control using a photocoupler has problems in output voltage stability and responsiveness.

  In particular, in the power supply of information communication equipment, in recent years, there has been a special request that the power supply voltage temporarily decreases to reduce power consumption, while the current temporarily requires about 10 to 30 A.

The forward type synchronous rectification type DC-DC converter is one of the converters suitable for the above requirements, and for example, Patent Document 1 has been disclosed.
Here, the configuration of the synchronous rectification type DC-DC converter disclosed in Patent Document 1 will be described with reference to FIG.

  The circuit shown in FIG. 1 constitutes a synchronous rectifier circuit generally called “self-driven type”. The “self-driven type” synchronous rectifier circuit is a circuit in which a synchronous rectifier element is directly turned on / off by a voltage excited by a transformer.

In FIG. 1, when the primary side switch element 4 is on, an upward voltage is excited on the secondary side of the transformer 5, and the rectifying side synchronous rectifier element 6 is turned on by the excitation voltage. When the primary side switching element 4 is turned off, a downward voltage is excited on the secondary side of the transformer 5, and this voltage is applied to the commutation side synchronous rectification element 7 through the diode 8 to be commutated side synchronous rectification element 7. Turns on. (At the same time, the rectification side synchronous rectification element 6 is turned off.)
In this way, the synchronous rectifier is directly turned on / off with the voltage excited by the transformer 5.
Japanese Patent Laying-Open No. 2005-80342

  As described above, the self-driving type that directly uses the voltage excited by the transformer 5 in FIG. 1 to turn on / off the synchronous rectifier element has the following major problems.

  In order to increase the total output current capacity of the DC-DC converter having such a configuration, when a plurality of DC-DC converters are connected in parallel to a common load, the converter itself shown in FIG. In other words, when the primary side switching element 4 is constantly in the OFF state, the voltage output from the other converter is input from the output terminal 25 in reverse. In this state, a voltage is applied by the route of the output terminal 25 → the inductor 20 → the gate of the rectifying side synchronous rectifying element 6 and the rectifying side synchronous rectifying element 6 is turned on, although the primary side is not operating. A phenomenon occurs in which self-excited oscillation (free resonance) starts without permission on the secondary side.

  When the self-excited oscillation occurs, even if the primary side switch element 4 is turned on as required to start the power supply device, the secondary side has already resonated and thus does not start normally.

  In addition, due to self-excited oscillation, an overvoltage may be applied to the gate and drain of the rectifying side synchronous rectifying element and the drain of the main switch element, and the converter may be destroyed.

  Accordingly, an object of the present invention is to suppress the self-excited oscillation operation due to an external voltage or the like input from the output terminal when the primary side operation is stopped, thereby preventing the breakdown of the circuit and starting the primary side. It is an object of the present invention to provide a synchronous rectification type DC-DC converter that operates normally at the time and after startup.

  In order to solve the above-mentioned problem, the present invention outputs the output by forcibly turning off the rectifying side synchronous rectifying element simultaneously with the turning off of the commutating side synchronous rectifying element when the primary side operation is stopped. Prevents self-excited oscillation due to external voltage input from the terminal.

[1] Specifically, a transformer (T1) including at least a primary winding (n1) and a secondary winding (n2);
A main switch element (Q1) connected in series to the primary winding (n1) of the transformer (T1);
A choke coil (L1) connected in series to the secondary winding (n2) of the transformer (T1);
A smoothing capacitor (C2) connected in parallel to the output unit;
A rectifying side synchronous rectification element (Q2) connected in series to the secondary winding (n2) of the transformer (T1) and turned on / off in synchronization with the on / off of the main switch element (Q1);
An excitation energy discharge path of the choke coil (L1) is formed by being connected in parallel to the output unit and turning on / off at a timing inverted in synchronization with the on / off of the main switch element (Q1). A commutation side synchronous rectifier element (Q3),
A pulse signal generation circuit (31) for performing switching control of the main switch element (Q1);
Timing signal transmission means for transmitting an on-timing signal output from the pulse signal generation circuit (31) to the main switch element (Q1) to the secondary side in an insulated state;
In the synchronous rectification type DC-DC converter provided with
A time constant circuit for detecting that the gate voltage of the commutation side synchronous rectifier element (Q3) continuously exceeds a predetermined threshold for a predetermined time or more, and the commutation after the elapse of time determined by the time constant circuit. A turn-off circuit for turning off the commutation side synchronous rectification element (Q3) by discharging electric charge accumulated between the gate and source of the side synchronous rectification element (Q3);
A gate bias voltage limiting circuit is provided for limiting the gate bias voltage of the rectifying side synchronous rectifying element (Q2) to be less than a predetermined threshold in synchronization with the operation of the turn-off circuit.

  With this configuration, although it is a forward DC-DC converter including a self-driven synchronous rectifier circuit, it is possible to prevent the secondary side circuit from self-oscillating due to the voltage applied from the output terminal after the converter is stopped.

  [2] In the gate bias voltage limiting circuit, a source terminal is connected to the gate terminal of the rectifying side synchronous rectifier element (Q2), and a drain terminal is connected to one end of the secondary winding (n2). The second switch element (Q6) is provided at least, and the gate-source charge of the second switch element (Q6) is discharged in synchronization with the turn-off circuit.

  [3] The gate terminal side of the second switch element (Q6) is a cathode between the gate terminal of the second switch element (Q6) and the source terminal of the rectifier side synchronous rectifier element (Q2). A constant voltage diode (ZD1) is connected so as to become, and a discharge path for turning off the commutation side synchronous rectifier element (Q3) during steady operation is connected to the cathode of the constant voltage diode (ZD1). The configuration is as follows.

  With this configuration, the gate voltage of the second switch element (Q6) is clamped by the constant voltage diode (ZD1), so that the voltage applied between the drain and source of the second switch element (Q6) is made constant. Therefore, the gate of the rectifying side synchronous rectifier element (Q2) can be protected (prevented from destruction) from an overvoltage caused by an increase in input voltage or a spike voltage caused by a leakage inductance of the transformer.

  [4] The gate bias voltage limiting circuit is connected between one end of the secondary winding (n2) of the transformer (T1) and the gate terminal of the rectifying side synchronous rectifying element (Q2). A third capacitor (D5) connected between the first capacitor (C5), a connection point between the gate terminal of the rectifying side synchronous rectifier (Q2) and the first capacitor (C5), and the turn-off circuit. And a second capacitor (C4) connected between the cathode of the third diode (D5) and the source terminal of the rectifying side synchronous rectifier (Q2).

  With this configuration, the rectifying side synchronous rectifying element (Q2) can be reliably kept off when the primary side operation is stopped. In addition, since the capacitor (C5) is connected between one end of the secondary winding (n2) of the transformer (T1) and the gate terminal of the rectifying side synchronous rectifying element (Q2), the secondary coil of the transformer (T1) When the voltage generated at (n2) is higher than the optimum gate drive voltage of the rectifying side synchronous rectifying element (Q2), the capacitance ratio between the capacitor (C5) and the input capacity of the rectifying side synchronous rectifying element (Q2) When the drive voltage is adjusted to an optimum value, the capacitance is equivalently reduced by the capacitor series connection, and the drive loss of the rectifying side synchronous rectifier element (Q2) can be reduced.

  [5] The time constant circuit is connected between the gate terminal and the source terminal of the commutation side synchronous rectification element (Q3), and the gate terminal and the source of the commutation side synchronous rectification element (Q3). The time constant circuit is connected to the control terminal of the third switch element (Q5) for turning on / off the conduction with the terminal.

  [6] The time constant circuit is a CR time constant circuit including at least one resistor and at least one capacitor, and a connection point of the at least one resistor and at least one capacitor is a third point. It is assumed that it is connected to the control terminal of the switch element (Q5).

  With the configurations [5] and [6], since the time constant operation is not performed during the steady operation, there is no wasteful power consumption during the steady operation and the circuit efficiency is improved.

  [7] Further, a backflow prevention diode (D3) is connected between the connection point of the resistor and the capacitor of the CR time constant circuit and the gate terminal of the commutation side synchronous rectifier (Q3). The side synchronous rectifier element (Q3) is connected using the gate terminal side as a cathode.

  With this configuration, when the gate-source charge of the commutation side synchronous rectifier element (Q3) is charged, the charge of the capacitor of the time constant circuit is discharged, and a stable time constant operation is always performed.

  [8] The transformer (T1) includes an auxiliary winding (n3), one end of the auxiliary winding (n3) is connected to the gate terminal of the commutation side synchronous rectifier (Q3), and the other end is It is connected to the source terminal of the commutation side synchronous rectifier element (Q3) via the first switch element (Q4), and the control terminal of the first switch element (Q4) is supplied from the timing signal transmission means. A signal is applied.

  With this configuration, the commutation-side synchronous rectifier is turned on and off at an almost optimal timing, so that it has a unique effect that a highly efficient power conversion operation is possible. That is, when the off-timing of the commutation-side synchronous rectifier is too early, the parasitic diode becomes conductive and conduction loss increases, and the operation when the short-circuit current flows when the off-timing of the commutation-side synchronous rectifier is too late can be avoided. .

  [9] The timing signal transmission means is a pulse transformer that transmits the pulse signal output from the pulse signal generation circuit (31) to the secondary side.

  With this configuration, a small and low-cost synchronous rectification type DC-DC converter can be configured.

  In a forward converter using a self-driven synchronous rectification element, when the primary side operation is stopped, the rectification side synchronous rectification element is forcibly turned off at the same time as the commutation side synchronous rectification element is turned off. Thus, free resonance due to an external voltage or the like input from the output terminal can be prevented.

<< First Embodiment >>
FIG. 2 is a circuit diagram of the synchronous rectification type DC-DC converter according to the first embodiment. FIG. 3 is a voltage waveform diagram of each part.

  In FIG. 2, a synchronous rectification type DC-DC converter 101 has a main switch element Q1 connected in series to a primary winding n1 of a transformer T1, and this series circuit is input from an input terminal (+ Vin · −Vin). A voltage is applied, and a capacitor C1 is connected between the input terminals (+ Vin · −Vin). The secondary winding n2 of the first transformer T1 is turned on / off in synchronization with the on / off of the rectifying side synchronous rectification element Q2 that is turned on / off in synchronization with the on / off of the main switching element Q1. A synchronous rectifier circuit including a commutation side synchronous rectifier element Q3, a choke coil L1, and a smoothing capacitor C2 is connected.

  One end of the auxiliary winding n3 of the first transformer T1 is connected to the low-voltage side terminal of the commutation side synchronous rectifier element Q3 via the first switch element Q4 that is instantaneously turned on at the ON timing of the main switch element Q1. The other end is connected to the control terminal of commutation side synchronous rectification element Q3.

  The control terminal of the commutation side synchronous rectification element Q3 is connected to the anode of the first diode D4, and is electrically connected between the cathode of the first diode D4 and the low voltage terminal after a certain time from voltage application. A turn-off circuit SW having a circuit TC is provided.

  One end of the secondary winding of the second transformer T2, which is a pulse transformer, is connected to the low voltage side terminal of the commutation side synchronous rectifier element Q3, and the other end is connected to the control terminal of the first switch element Q4.

  A clamp circuit CC including a second switch element Q6 that clamps the potential of the control terminal of the rectifying side synchronous rectifying element Q2 to a constant potential is connected to the control terminal of the rectifying side synchronous rectifying element Q2.

  Between the control terminal of the second switch element Q6 and the turn-off circuit SW, a diode D5 is provided as a discharge path for discharging the charge accumulated in the control terminal of the second switch element Q6.

  The pulse signal generation circuit 31 includes a main switch drive signal output terminal OUT, a feedback terminal FB, and a ground terminal GND, and sends a drive pulse to the gate of the main switch element Q1 through the primary winding of the second transformer T2. give. In this example, a reset diode D1 is connected to the primary winding of the second transformer T2, and a pulse is generated only at the rising timing of Q1 on the secondary winding of the second transformer T2. It is configured.

  The pulse signal generation circuit 31 detects the output voltage of the voltage dividing circuit composed of the resistors R3 and R4 connected between the output terminals (+ Vout · −Vout), and maintains the predetermined value of the main switch element Q1. Controls the on-duty ratio of the pulse signal applied to the gate.

  The time constant circuit TC includes resistors R1, R2 and a capacitor C3, and the turn-off circuit SW includes the time constant circuit TC, the third switch element Q5, and a diode D3.

  The clamp circuit CC includes a resistor R5, a Zener diode ZD1, a capacitor C4, and a second switch element Q6, and clamps the gate voltage of the rectifying side synchronous rectifier element Q2.

The operation of the synchronous rectification type DC-DC converter shown in FIG. 2 is as follows.
First, in steady operation, DC power is input between + Vin and -Vin, and is smoothed by the capacitor C1 of the input filter. The main switch element Q1 is switched by a square wave signal output from the OUT terminal of the pulse signal generation circuit 31, and converts the DC power into AC power. The AC power is transmitted from the primary winding n1 of the first transformer T1 to the secondary winding n2, rectified by the rectifying side synchronous rectifying element Q2 and the commutating side synchronous rectifying element Q3, and the choke coil L1 and the capacitor C2 It is smoothed by the output filter, and is output as a DC power between + Vout and -Vout.

  The output voltage is divided by resistors R3 and R4, input to the FB terminal of the pulse signal generation circuit 31 and fed back, and the pulse signal generation circuit 31 performs PWM control on the square wave signal based on the feedback to thereby convert the output voltage of the converter. Stabilize. When a square wave signal output from the OUT terminal of the pulse signal generation circuit 31 is applied to the gate of the main switch element Q1 via the primary winding of the second transformer T2, the primary winding of the second transformer T2 Is formed with a pulse signal substantially synchronized with the ON timing of the main switch element, and is transmitted from the primary side to the secondary side by the second transformer T2. The rectifying side synchronous rectifying element Q2 constitutes a self-driven circuit that charges and discharges the gate using a change in voltage induced in the first transformer T1, and Q6 is connected in series to the gate of Q2. Connected.

  The gate voltage of Q6 in the steady operation is that the reset voltage induced in the auxiliary winding n3 of the first transformer is charged through the diode D4 and the resistor R5, and becomes the Zener voltage of the Zener diode ZD1 by the smoothing action of the capacitor C4. stable. When the Zener voltage of the Zener diode ZD1 is Vz and the threshold voltage of Q6 is Vgs, the gate drive voltage of the rectifying side synchronous rectifier element Q2 is clamped at (Vz−Vgs).

  The gate of the commutation side synchronous rectifier element Q3 is charged by a reset voltage generated in the auxiliary winding n3 of the first transformer T1 immediately after the main switch element Q1 is turned off, and the second transformer T2 is turned on when the main switch element Q1 is turned on. Is applied to the first switch element Q4 to discharge. As a result, the commutation side synchronous rectification element Q3 is driven in a substantially inverted manner with respect to the main switch element Q1.

  When the input voltage decreases or a stop signal is input from the outside, the switching operation of the main switch element Q1 stops as shown in FIG. When the main switch element Q1 stops, there is no pulse signal for the first switch element Q4 as shown in FIG.

  As shown in FIG. 3C, the gate of the commutation side synchronous rectifier element Q3 is charged by the reset voltage generated in the auxiliary winding n3 of the transformer T1 immediately after the switching operation of the main switch element Q1 is stopped, and is turned on. Become. Since the time constant (t4 to t6) of the time constant circuit TC is set longer than the off period (t2 to t3, t4 to t5) in the steady operation, the third switch element Q5 maintains the off state in the steady operation. However, when the switching operation is stopped, the commutation side synchronous rectifier element Q3 is kept on for a longer period than the off period, and therefore, as shown in FIG. 3 (f), the B− The voltage between E reaches the threshold voltage at t6 and turns on.

  As shown in FIG. 3G, when the third switch element Q5 is turned on, the gate accumulated charge of the commutation side synchronous rectifier element Q3 is discharged via the diode D4, and the capacitor C4 is accumulated via the diode D5. The electric charge is also discharged, and the gate of the commutation side synchronous rectification element Q3 and the gate of the second switch element Q6 are lowered at substantially the same voltage.

  When the gate voltage of the commutation side synchronous rectification element Q3 decreases and reaches the threshold voltage Vgs at t7, the commutation side synchronous rectification element Q3 is turned off. At the moment when the gate voltage of the commutation side synchronous rectifier Q3 drops to Vgs, the gate voltage of the second switch element Q6 also becomes substantially the same Vgs.

  When the commutation-side synchronous rectifier element Q3 is turned off by the action of the turn-off circuit SW, as shown in FIG. 3D, the on-period of the main switch element Q1 is set in the first transformer T1 by the energy stored in the choke coil L1. A voltage having the same polarity is induced, but the threshold voltage of the rectifying side synchronous rectifying element Q2 is not reached by the clamping action of the second switching element Q6, and Q2 is not turned on.

  At the moment when the gate voltage of Q3 drops to the threshold voltage Vgs of the second switch element Q6, the gate voltage of the rectifying side synchronous rectifier element Q2 is clamped to (Vgs of Q3−Vgs of Q6).

  As described above, when the turn-off circuit SW is operated, charging of the gate of the rectifying side synchronous rectifying element Q2 is hindered by the clamping action of the second switch element Q6 whose gate voltage is reduced together with the gate of the commutating side synchronous rectifying element Q3. Self-excited oscillation can be prevented beforehand.

  At the moment when the commutation side synchronous rectifier Q3 is turned off by the turn-off circuit SW, a voltage change of the secondary winding n2 of the first transformer T1 is also induced in the gate of Q6 through the feedback capacitance of the second switch element Q6. Due to the presence of the capacitor C4, it is possible to suppress a change in the voltage of the gate to a level where there is no problem.

  Further, when the switching operation is stopped, no voltage is induced in the auxiliary winding n3 of the first transformer T1, and the gate of the commutation side synchronous rectification element Q3 is not charged, so that the off state of the commutation side synchronous rectification element Q3 is maintained. The At this time, since the voltage across the capacitor C4 is zero volts, the second switch element Q6 prevents the rectifying side synchronous rectifying element Q2 from being charged with the gate, and the rectifying side synchronous rectifying element Q2 also maintains the off state.

<< Second Embodiment >>
FIG. 4 is a circuit diagram of a synchronous rectification type DC-DC converter according to the second embodiment.
In FIG. 4, this synchronous rectification type DC-DC converter 102 has a main switch element Q1 connected in series to a primary winding n1 of a transformer T1, and is input to this series circuit from an input terminal (+ Vin · −Vin). A voltage is applied, and a capacitor C1 is connected between the input terminals (+ Vin · −Vin). The secondary winding n2 of the first transformer T1 is turned on / off in synchronization with the on / off of the rectifying side synchronous rectification element Q2 that is turned on / off in synchronization with the on / off of the main switching element Q1. A synchronous rectifier circuit including a commutation side synchronous rectifier element Q3, a choke coil L1, and a smoothing capacitor C2 is connected.

As shown in FIG. 4, a commutation side synchronous rectifier element is connected to one end of the auxiliary winding n3 of the first transformer T1 via a first switch element Q4 that is instantaneously turned on at the on timing of the main switch element Q1. The other end is connected to the control terminal of the commutation side synchronous rectification element Q3.
The configuration of the turn-off circuit SW is the same as that of the first embodiment shown in FIG.

  The power conversion operation of the synchronous rectification type DC-DC converter 102 according to the second embodiment is the same as the power conversion operation of the synchronous rectification type DC-DC converter 101 shown in the first embodiment. In the synchronous rectification type DC-DC converter 102, the induced voltage of the secondary winding n2 of the first transformer T1 is applied to the gate of the rectification side synchronous rectification element Q2 via the capacitor C5.

  The gate of the commutation side synchronous rectifier element Q3 is charged by a reset voltage generated in the auxiliary winding n3 of the first transformer T1 immediately after the main switch element Q1 is turned off, and is a pulse transformer at the on timing of the main switch element Q1. The pulse signal of the second transformer T2 is discharged by applying it to the switch element Q4, and the commutation side synchronous rectifier element Q3 is driven in a substantially inverted manner with respect to the main switch element Q1.

  If the input voltage decreases or a stop signal is input from the outside, the switching operation of the main switch element Q1 stops. When the main switch element Q1 stops, the pulse signal disappears. Immediately after the switching operation of the main switch element Q1 is stopped, the gate of the commutation side synchronous rectifier element Q3 is charged by the reset voltage generated in the auxiliary winding n3 of the transformer T1, and is turned on. Since the time constant of the time constant circuit TC is set longer than the off period in the steady operation, the switch element Q5 maintains the off state in the steady operation. However, when the switching operation is stopped, the time constant circuit TC is switched for a period longer than the off period. Since the on-state of the flow side synchronous rectification element Q3 continues, the voltage between B and E of the switch element Q5 reaches the threshold voltage and turns on.

  When the switch element Q5 is turned on, the gate accumulated charge of the commutation side synchronous rectification element Q3 is discharged through the diode D4, and when the gate voltage is lowered to the threshold voltage, the commutation side synchronous rectification element Q3 is turned off.

  When the commutation side synchronous rectification element Q3 is turned off in this way, the voltage having the same polarity as the on period of the main switch element Q1 is induced in the first transformer T1 by the energy stored in the choke coil L1, but the commutation side synchronization is performed. Since the gate of the rectifying element Q2 is short-circuited to the source via the diode D5 and the switching element Q5, the threshold voltage is not reached, and the rectifying side synchronous rectifying element Q2 is not turned on.

  As described above, the gate charge of the commutation side synchronous rectification element Q3 is discharged and the gate charge of the rectification side synchronous rectification element Q2 is hindered, so that self-excited oscillation can be prevented.

<< Third Embodiment >>
FIG. 5 is a circuit diagram of a synchronous rectification type DC-DC converter according to the third embodiment.
In FIG. 5, this synchronous rectification type DC-DC converter 103 has a main switch element Q1 connected in series to a primary winding n1 of a transformer T1, and this series circuit is inputted from an input terminal (+ Vin · −Vin). A voltage is applied, and a capacitor C1 is connected between the input terminals (+ Vin · −Vin). The secondary winding n2 of the first transformer T1 is turned on / off in synchronization with the on / off of the rectifying side synchronous rectification element Q2 that is turned on / off in synchronization with the on / off of the main switching element Q1. A synchronous rectifier circuit including a commutation side synchronous rectifier element Q3, a choke coil L1, and a smoothing capacitor C2 is connected.

  As shown in FIG. 5, one end of the auxiliary winding n3 of the first transformer T1 is connected to the commutation side synchronous rectifier Q3 via the first switch element Q4 that instantaneously turns on at the ON timing of the main switch element Q1. Connect to the low voltage side terminal, connect the other end to the control terminal of the commutation side synchronous rectification element Q3, and connect one end of the secondary winding of the second transformer T2 to the low voltage side terminal of the commutation side synchronous rectification element Q3 The other end is connected to the control terminal of the first switch element Q4.

  A clamp circuit CC including a second switch element Q6 that clamps the potential of the control terminal of the rectifying side synchronous rectifying element Q2 to a constant potential is connected to the control terminal of the rectifying side synchronous rectifying element Q2.

  A first diode D4 is connected between a connection point between one end of the auxiliary winding n3 of the first transformer T1 and the first switch element Q4 and a control terminal of the second switch element Q6.

  A first capacitor C5 is connected between the high-voltage side terminal of the second switch element Q6 and one end of the secondary winding n2 of the first transformer T1.

  A second diode D6 is connected between the connection point of the second switch element Q6 and the first capacitor C5 and the low voltage side terminal of the rectifying side synchronous rectifying element Q2.

  The power conversion operation of the synchronous rectification DC-DC converter 103 according to the third embodiment is the same as the power conversion operation of the synchronous rectification DC-DC converter 101 shown in the first embodiment. In the synchronous rectification type DC-DC converter 103, the induced voltage of the secondary winding n2 of the first transformer T1 is applied to the gate of the rectification side synchronous rectification element Q2 via the capacitor C5 and the switch element Q6. As a result, the capacitance of the capacitor C5 and the input capacitance of the rectifying side synchronous rectifying element Q2 are connected in series, so that the amplitude of the induced voltage of the secondary winding n2 is greater than the voltage required for driving the rectifying side synchronous rectifying element Q2. If it is large, the equivalent loss can be reduced to reduce drive loss.

  The diode D6 clamps the lower limit of the driving signal of the rectifying side synchronous rectifying element Q2 at approximately zero volts. After the reset of the first transformer T1 during the OFF period of the main switch element Q1, the output capacitance of the first switch element Q4 is charged with the gate accumulated charge of the commutation side synchronous rectifier element Q3, and the first switch element A voltage appears between D and S of Q4. When the circuit constant is set so that the peak value of the reset voltage appearing in the auxiliary winding n3 of the first transformer T1 is higher than the Zener voltage Vz of the Zener diode ZD1, the voltage between D and S of the first switch element Q4 is almost equal. Clamped at Vz.

  Once clamped at Vz, the accumulated charge is discharged through the resistor R6, so that the gate voltage of the second switch element Q6 slightly decreases during the OFF period of the main switch element Q1, but the rectifying side synchronous rectifier element The gate drive voltage of Q2 is clamped approximately at (Zener voltage Vz of ZD1−threshold voltage Vgs of Q6).

  The gate of the commutation side synchronous rectification element Q3 is charged by a reset voltage generated in the auxiliary winding n3 of the transformer T1 immediately after the main switch element Q1 is turned off, and the pulse signal of the second transformer T2 is turned on when the main switch element Q1 is turned on. Is applied to the first switch element Q4, and the commutation side synchronous rectifier element Q3 is driven in a substantially inverted manner with respect to the main switch element Q1.

  If the input voltage decreases or a stop signal is input from the outside, the switching operation of the main switch element Q1 is stopped, and there is no pulse signal for the first switch element Q4.

  The commutation side synchronous rectification element Q3 is turned on when the gate of the commutation side synchronous rectification element Q3 is charged by the reset voltage generated in the auxiliary winding n3 of the first transformer T1 immediately after the switching operation of the main switch element Q1 is stopped. It becomes a state.

  The capacitor C4 and the resistor R6 constitute a time constant circuit TC, and the time constant circuit TC and the diode D4 constitute a turn-off circuit SW.

  According to a time constant determined by the product of the input capacitance of commutation side synchronous rectification element Q3 and the capacitance of capacitor C4 and the resistance R6, the gate accumulated charge of commutation side synchronous rectification element Q3 is gradually discharged. However, since the time constant is set longer than the off period of the steady operation, in the steady operation, the commutation side synchronous rectification element Q3 maintains the on state until it is turned off by the first switch element Q4. If the switching operation on the primary side stops, the ON state of the commutation side synchronous rectifier element Q3 continues for a period longer than the OFF period.

  Therefore, when the gate accumulated charge of the commutation side synchronous rectification element Q3 is discharged according to the time constant and becomes the threshold voltage Vgs of Q3, the commutation side synchronous rectification element Q3 is turned off. Assuming that the forward voltage drop of the diode D4 is Vfd4, the gate voltage of the second switch element Q6 becomes (Vgs−Vfd4 of Q3) at the moment when the gate voltage of the commutation side synchronous rectifier element Q3 drops to the threshold voltage Vgs. .

  When the commutation-side synchronous rectifier element Q3 is turned off in this way, a voltage having the same polarity as the on-period of the main switch element Q1 is induced in the first transformer T1 by the energy stored in the choke coil L1, The threshold voltage of the rectifying side synchronous rectifying element Q2 is not reached by the clamping action of the switch element Q6, and the rectifying side synchronous rectifying element Q2 is not turned on. Assuming that the threshold voltage of the second switch element Q6 is Vgs, the gate voltage of the rectifier side synchronous rectifier element Q2 is (Vgs of Q3 −Vfd4 − Clamped to Vgs of Q6).

  As described above, since the gate charging of the rectifying side synchronous rectification element Q2 is hindered by the clamping action of the second switching element Q6 whose gate voltage is reduced together with the gate of the commutation side synchronous rectification element Q3, self-excited oscillation is prevented in advance. it can.

  According to each embodiment described above, self-oscillation can be prevented with a self-driven simple circuit configuration, and the gate and drain of the rectifying side synchronous rectification element Q2 and the drain of the main switch element Q1 generated by the self-excitation oscillation This prevents over-voltage against the converter and prevents the converter from being destroyed, and limits the backflow current when the converter is stopped to prevent undershoot of the output voltage.

  Furthermore, according to the first and third embodiments, the gate drive voltage of the rectifying side synchronous rectifier element Q2 is clamped to an appropriate value by the clamp circuit CC, thereby causing an overvoltage due to an increase in input voltage or a leakage inductance of the transformer. Therefore, it is possible to protect the gate of the rectifying side synchronous rectifying element Q2 from the spike voltage, and to prevent the occurrence of a backflow current in an operation mode in which a DC voltage is applied from the converter output when the switching operation is stopped.

  With these effects, a highly reliable synchronous rectification type DC-DC converter can be configured with a simple circuit configuration.

  Incidentally, when compared with the circuit shown in FIG. 1 of Patent Document 1, the switch elements 9 and 14 are directly driven by the voltage excited by the drive transformer 24 in FIG. Generates up to voltage. If such elements are to be driven, a large value is required for the inductance value of the drive transformer 24, and the transformer cannot be reduced in size. On the other hand, according to the present invention, since the first switch element Q4 is only driven by the second transformer T2, the inductance value of the second transformer T2 can be small. Therefore, not the drive transformer but a pulse transformer that only needs to transmit the rising / falling of the pulse can be used as the second transformer.

  Further, the resistor 10 in FIG. 1 of Patent Document 1 is a resistor for discharging the charge accumulated between the gate and the source of the switch element 7 when the converter is stopped, and a current flows through the resistor 10 even during steady operation. As a result, the circuit efficiency is deteriorated. On the other hand, the time constant circuit TC including the resistors R1 and R2, the capacitor C3, and the third switch element Q5 in the present invention does not operate at the time of steady operation, so that the circuit efficiency is not deteriorated.

In addition, this invention is not limited to each embodiment shown above, A various structure can be taken.
For example, the time constant may be configured using a timer circuit instead of the CR integrating circuit.
Further, the switch element (Q6) of the clamp circuit CC may be formed not by an N-channel MOSFET but by an NPN bipolar transistor. In this case, a diode in the reverse direction is connected in parallel with respect to CE of the NPN transistor. do it. The driving power source for the rectifying side synchronous rectifying element Q2 and the commutation side synchronous rectifying element Q3 may be obtained from the choke coil L1 instead of the first transformer T1.

  Further, the first transformer T1 and the second transformer T2 are composed of a composite magnetic component sharing the same core (for example, a coil device disclosed in Japanese Patent Laid-Open No. 2000-260639), and the size and cost of the component are reduced. It is also possible to make it easier.

  Further, an SR driving method that does not use a pulse transformer may be applied. For example, a circuit is configured to turn off the commutation side synchronous rectification element Q3 by an induced voltage of an inductor connected in series with the rectification side synchronous rectification element Q2. Also good.

1 is a circuit diagram of a synchronous rectification type DC-DC converter disclosed in Patent Document 1. FIG. 1 is a circuit diagram of a synchronous rectification type DC-DC converter according to a first embodiment. FIG. It is a voltage waveform figure of each part of the circuit. It is a circuit diagram of the synchronous rectification type DC-DC converter which concerns on 2nd Embodiment. It is a circuit diagram of the synchronous rectification type DC-DC converter which concerns on 3rd Embodiment.

Explanation of symbols

31-pulse signal generation circuit 101-103-synchronous rectification type DC-DC converter C2-smoothing capacitor CC-clamp circuit D1-reset diode D3-backflow prevention diode D4-first diode D5-third diode D6-second Diode L1-choke coil n1-1 primary winding n2-2 secondary winding n3-auxiliary winding Q1-main switch element Q2-rectifier side synchronous rectifier element Q3-commutation side synchronous rectifier element Q4-first switch element Q5-third switch element Q6-second switch element SW-turn-off circuit T1-first transformer T2-second transformer (pulse transformer)
TC-Time constant circuit ZD1 ... Zener diode

Claims (9)

A transformer having at least a primary winding and a secondary winding;
A main switch element connected in series to the primary winding of the transformer;
A choke coil connected in series with the secondary winding of the transformer;
A smoothing capacitor connected in parallel to the output section;
A rectifying side synchronous rectifying element connected in series to the secondary winding of the transformer and turned on / off in synchronization with on / off of the main switch element;
The commutation-side synchronous rectification that is connected in parallel to the output unit and that is turned on / off at a timing that is inverted in synchronization with the on / off of the main switch element, thereby forming an excitation energy discharge path of the choke coil Elements,
A pulse signal generation circuit for performing switching control of the main switch element;
Timing signal transmission means for transmitting an ON timing signal output from the pulse signal generation circuit to the main switch element to the secondary side in an insulated state;
In the synchronous rectification type DC-DC converter provided with
A time constant circuit for detecting that the gate voltage of the commutation side synchronous rectifier element is continuously greater than a predetermined threshold for a predetermined time or more, and the commutation side synchronous rectification after elapse of a time determined by the time constant circuit; A turn-off circuit for turning off the commutation-side synchronous rectifier element by discharging electric charge accumulated between the gate and source of the element;
A DC-DC converter comprising a gate bias voltage limiting circuit for limiting a gate bias voltage of the rectifying side synchronous rectifying element to be less than a predetermined threshold in synchronization with the operation of the turn-off circuit.   The gate bias voltage limiting circuit includes at least a second switch element having a source terminal connected to a gate terminal of the rectifying side synchronous rectifier element and a drain terminal connected to one end of the secondary winding, and the turn-off circuit. 2. The DC-DC converter according to claim 1, wherein the charge between the gate and the source of the second switch element is discharged in synchronization with the second switch element.   A constant voltage diode is connected between the gate terminal of the second switch element and the source terminal of the rectifying side synchronous rectifying element so that the gate terminal side of the second switch element becomes a cathode, and the constant voltage 3. The DC-DC converter according to claim 2, wherein a discharge path for turning off the commutation side synchronous rectifying element during steady operation is connected to a cathode of the diode.   The gate bias voltage limiting circuit includes a first capacitor connected between one end of a secondary winding of the transformer and a gate terminal of the rectifying side synchronous rectifying element, and a gate terminal of the rectifying side synchronous rectifying element. And a first diode connected between the connection point of the first capacitor and the turn-off circuit, a cathode of the third diode, and a source terminal of the rectifying side synchronous rectifying element. The DC-DC converter according to claim 1, further comprising at least a second capacitor.   The time constant circuit is connected between a gate terminal and a source terminal of the commutation side synchronous rectification element, and turns on / off conduction between the gate terminal and the source terminal of the commutation side synchronous rectification element. The DC-DC converter according to any one of claims 1 to 4, wherein the time constant circuit is connected to a control terminal of a third switch element.   The time constant circuit is a CR time constant circuit including at least one resistor and at least one capacitor, and a connection point of the at least one resistor and at least one capacitor is connected to the third switch element. The DC-DC converter according to claim 5, wherein the DC-DC converter is connected to a control terminal.   Between the connection point of the resistor and the capacitor of the CR time constant circuit and the gate terminal of the commutation-side synchronous rectification element, a backflow prevention diode has the gate terminal side of the commutation-side synchronous rectification element as a cathode. The DC-DC converter according to claim 6 connected.   The transformer includes an auxiliary winding, and one end of the auxiliary winding is connected to the gate terminal of the commutation side synchronous rectification element, and the other end is connected to the source of the commutation side synchronous rectification element via a first switch element. The DC-DC converter according to claim 1, wherein the DC-DC converter is connected to a terminal and configured to apply a signal from the timing signal transmission means to a control terminal of the first switch element. .   The synchronous rectification type DC-DC converter according to any one of claims 1 to 8, wherein the timing signal transmission means is a pulse transformer for transmitting a pulse signal output from the pulse signal generation circuit to a secondary side.

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