JP2007082354A - Synchronous rectification version forward converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To constitute a synchronous rectifying-type forward converter to be stably operated by reducing a stress on a switching element for synchronous rectification even if a reverse operation by an output overvoltage or a self-oscillation occurs. <P>SOLUTION: A switching element Q5 for communication switch turn-off control is connected to an auxiliary winding N4 of a main transformer T1 in series. A charged electric charge of a gate/source capacitance of a communication switching element Q3 is controlled by the switching element Q5. A control switching element driving circuit 24A turns on the switching element Q5 when a main switching element Q1 is turned on. A primary-side control stop detection circuit 25A detects a control stopping state of a switching control circuit 23 on the basis of an output signal from the control switching element driving circuit 24A. If the primary-side control is stopped, an on-signal intermittent switching element Q7 is conducted to turn on the switching element Q5, so that the electric charge of the gate/source capacitance of the communication switching element Q3 is immediately discharged. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a forward converter that synchronously rectifies an output current.

  Patent Document 1 is disclosed as a conventional synchronous rectification type forward converter. FIG. 1 shows a circuit of the converter of Patent Document 1.

  In the circuit shown in FIG. 1, when the primary side main switch element 2 of the transformer 4 is turned on, the secondary side rectification side synchronous rectification element 5 is turned on and the commutation side switch element 6 is turned off. Here, if the turn-off of the commutation side switch element 6 is delayed, a short-circuit path passing through the two switch elements 5 and 6 is formed. Therefore, the drive switch element 7 is provided in series with the tertiary winding 4c of the transformer 4, The drive switch element 7 is turned on by a signal for turning on the primary side main switch element 2. With this configuration, the charge of the gate capacitance of the commutation side switch element 6 is drawn out via the drive switch element 7 immediately before the primary side main switch element 2 is turned on, and the commutation side switch element 6 is quickly turned off. To prevent short circuit.

  Thus, a synchronous rectification type forward having a choke coil on the secondary side and turning on / off the secondary side synchronous rectification element and the commutation side switch element by the voltage generated in the secondary winding and the auxiliary winding. In the converter, when the switching operation of the main switch is stopped or a relatively high voltage is applied to the output terminal, the secondary side self-oscillates and power flows back from the secondary side to the primary side (regeneration is performed). Problem arises.

  Therefore, Patent Document 2 includes a self-excited oscillation detection circuit that detects self-excited oscillation of the commutation-side synchronous rectifier caused by the switching operation of the main switch being stopped. Shown is a circuit that stops the self-excited oscillation by turning on the switch provided between the gate and source of the rectifier and lowering the gate voltage below the threshold voltage value to turn off the commutation side synchronous rectifier. Yes.

Patent Document 3 shows a configuration in which an auxiliary switch element is connected between the gate and source of the commutation side switch element, and the gate of the commutation side switch element is turned off by the gate signal of the rectification side switch element. . As a result, the frequency of the self-excited oscillation is set high, the current flowing back from the outside is suppressed small, and the current stress of the synchronous rectifying element due to the backflow / self-excited oscillation is reduced.
JP 2000-262051 A JP 2003-304684 A JP 2003-244952 A

  However, the converter shown in Patent Document 2 requires two auxiliary switches, that is, an off-timing switch for the commutation side synchronous rectification element and a stop switch for self-excited oscillation, which complicates the circuit.

  In the converter disclosed in Patent Document 3, when an auxiliary switch element is provided between the gate and source of the commutation side switch element, the off timing of the commutation side switch element is set to the half-on state of the commutation side switch element. Therefore, excessive stress is applied to the switch element (main switch element, synchronous rectification switch element) in the operation as the backflow prevention circuit. Furthermore, there is a problem that the auxiliary switch element may short-circuit both ends of the winding of the transformer due to the delay in turning off the auxiliary switch when the rectifying side synchronous rectifier element is turned off.

  Accordingly, an object of the present invention is to eliminate the above-mentioned problems and to reduce the stress on the synchronous rectification switch element even when a backflow operation and self-excited oscillation due to output overvoltage occur, or to stop self-excited oscillation. Another object is to provide a synchronous rectification forward converter.

  In order to solve the above problems, a synchronous rectification type forward converter of the present invention is configured as follows.

(1) A transformer (T) having a primary winding (N1) and a secondary winding (N2), and a main switch element (Q1) connected in series to the primary winding (N1) of the transformer (T) ), A choke coil (L2) connected in series with the secondary winding (N2) of the transformer (T), a smoothing capacitor (C1) connected in parallel between output terminals, and the transformer (T) A rectifying switch element (rectifying-side synchronous rectifying element Q2) connected in series to the secondary winding (N2) of the main switching element (Q1) and turned on / off in synchronization with the on / off of the main switching element (Q1); A commutation switch element (commutation side synchronous rectification element Q3) that is turned on / off in synchronization with the on / off of the switch element (Q1), and forms a discharge path of the excitation energy of the choke coil by turning on, and the main switch Performs switching control of element (Q1) A switching control circuit (23), in the synchronous rectification type forward converter with,
Application of an electromotive voltage of the auxiliary winding (N4) of the transformer (T) connected in series to the auxiliary winding (N4) of the transformer (T) to the control terminal of the commutation switch element (Q3) A switching element (Q5) for commutation switch turn-off control for controlling,
A control switch element drive circuit (24) for turning on the commutation switch turn-off control switch element (Q5) when the main switch element (Q1) is turned on;
A control voltage signal generation circuit (26) for generating a control voltage signal for the commutation switch turn-off control switch element (Q5);
A primary control stop detection circuit (25) for detecting a control stop state of the switching control circuit (23);
Based on the output of the primary side control stop detection circuit (25), when the primary side control is stopped, an on control voltage signal is sent from the control voltage signal generation circuit (26) to the commutation switch turn-off control switch element (Q5). A primary side control stop control circuit (27) to be applied;
Is provided.

(2) The primary side control stop time control circuit (27) includes an on signal intermittent switch element (Q7) for supplying an on control voltage signal to the commutation switch turn-off control switch element (Q5),
The control switch element drive circuit (24) is driven on the primary side by a drive signal to the main switch element (Q1) by the switching control circuit (23), and the commutation switch turn-off control switch element from the secondary side. A pulse transformer (T2) for generating a control voltage signal to (Q5);
The primary-side control stop detection circuit (25) rectifies the secondary-side output of the pulse transformer (T2) and charges it at a rate exceeding its discharge rate, and supplies the voltage generated by the charging to the on-signal intermittent switch element. (Q7) provided with a charging circuit,
On the secondary side of the pulse transformer (T2), between the control terminal of the commutation switch turn-off control switch element (Q5) and the ground of the charging circuit, the commutation switch turn-off control switch element (Q5). It is assumed that an impedance circuit (D9) is provided to prevent the control terminal from being grounded to the ground.

(3) The charging circuit is obtained by performing both-wave rectification on the secondary output of the pulse transformer.

(4) The control voltage signal generation circuit (26) is, for example, a charging circuit that charges a voltage signal that drives the rectifying switch element (Q2).

(5) The control voltage signal generation circuit (26) is, for example, a charging circuit that charges the flyback voltage of the auxiliary winding (N4).

(6) The control voltage signal generation circuit (26) is, for example, a charging circuit that charges a voltage generated in the choke coil (L2).

(7) Driven by the ON control voltage signal supplied to the commutation switch turn-off control switch element (Q5) via the primary side control stop time control circuit (27), the rectification switch element (Q2) It is assumed that a rectifying switch turn-off control switch element (Q6) for controlling the control voltage is further provided.

(1) When the overvoltage is applied to the output terminal, the synchronous rectifier circuit causes self-excited oscillation to stop the control on the primary side, or the input to the primary side is turned off. When the control is stopped, an on-control voltage is applied to the commutation switch turn-off control switch element (Q5), the gate parasitic capacitance charge of the commutation switch element Q3 is quickly extracted, and the commutation switch Q3 is accelerated. As a result, the self-excited oscillation frequency is prevented from decreasing. As a result, the stress of the rectifying switch element and the commutation switch element is reduced.

(2) On the secondary side of the pulse transformer (T2), by providing an impedance circuit (D9) between the control terminal of the commutation switch turn-off control switch element (Q5) and the ground of the charging circuit, When the control on the primary side is stopped, the signal current of the control voltage signal generation circuit (26) is prevented from flowing to the ground, so that the commutation switch turn-off control switch element (Q5) can operate correctly.

(3) The charging circuit double-waves the voltage generated on the secondary side of the pulse transformer (T2) to double the charging speed, and conversely, the discharging speed of the charging circuit can be set faster. When the control is stopped, the commutation switch turn-off control switch element (Q5) can be turned on quickly so that the self-excited oscillation frequency reduction suppressing function can be exhibited quickly.

(4) By configuring the control voltage signal generation circuit (26) with a charging circuit that charges a voltage signal that drives the rectifying switch element (Q2), it is suitable for controlling the switch element (Q5) for commutation switch turn-off control. A voltage signal can be easily obtained.

(5) The control voltage signal generation circuit (26) may be configured by a charging circuit that charges the flyback voltage of the auxiliary winding (N4).

(6) Alternatively, the control voltage signal generation circuit (26) may be configured by a charging circuit that charges a voltage generated in the choke coil (L2).

(7) The rectifier switch turn-off control switch element (Q6) controls the control voltage of the rectifier switch element (Q2) by the on-control signal from the commutation switch turn-off control switch element (Q5) on-signal generation circuit. Self-excited oscillation stops and backflow can be stopped completely.

First, with reference to FIG. 2, the common structure of the synchronous rectification type | mold forward converter which concerns on each embodiment shown below is demonstrated.
FIG. 2 is a circuit diagram of a synchronous rectification forward converter partially blocked and symbolized.

  As shown in FIG. 2, the main transformer T1 includes a primary winding N1, a secondary winding N2, a tertiary winding N3, and an auxiliary winding N4. A main switch element Q1 is connected in series to the primary winding N1, and a capacitor is connected between the input terminals 21 (21a, 21b). A choke coil L2 and a rectifying switch element Q2 are connected in series to the secondary winding N2 of the main transformer T1, and a smoothing capacitor C1 is connected between the output terminals 32 (32a, 32b). Further, a commutation switch element Q3 is provided at a position that forms a commutation path when the excitation energy of the choke coil L2 is released (forms a loop together with the choke coil L2 and the smoothing capacitor C1).

  A tertiary rectifying / smoothing circuit 22 including diodes D1 and D2, a choke coil L1, and a capacitor C2 is connected to the tertiary winding N3 of the main transformer T1. The switching control circuit 23 receives the output from the tertiary rectifying / smoothing circuit 22 as a power source and also as an output voltage detection signal, and outputs a switching control signal to the main switch element Q1. A circuit is configured to apply an electromotive voltage of the secondary winding N2 of the main transformer T1 to the control terminal of the rectifying switch element Q2. A rectifier switch turn-off control switch element Q6 for controlling the control voltage is connected to the control terminal of the rectifier switch element Q2.

  One end of the auxiliary winding N4 of the main transformer T1 is connected to the control terminal of the commutation switch element Q3, and the switching element Q5 for commutation switch turn-off control is connected to the other end of the auxiliary winding N4. .

  A control switch element driving circuit 24 is connected to control terminals of the rectifier switch turn-off control switch element Q6 and the commutation switch turn-off control switch element Q5. The control switch element driving circuit 24 receives a switching control signal for the main switch element Q1 output from the switching control circuit 23, and is used for the commutation switch turn-off control switch element Q5 and the rectifier switch turn-off control at a timing synchronized with the switching control signal. The switch element Q6 is driven.

  Specifically, the commutation switch turn-off control switch element Q5 is turned on when the main switch element Q1 is turned on. Further, the rectifier switch turn-off control switch element Q6 is turned on when the main switch Q1 is turned off.

  Further, the voltage generated by the control voltage signal generation circuit 26 is applied to the control terminals of the rectifier switch turn-off control switch element Q6 and the commutation switch turn-off control switch element Q5 via the ON signal intermittent switch element Q7. The circuit is configured.

  The primary-side control stop detection circuit 25 detects the operation stop state of the switching control circuit 23 via the control switch element drive circuit 24, and turns on the on signal intermittent switch element Q7 in the stop state.

The operation of the synchronous rectification type forward converter shown in FIG. 2 is as follows.
<Normal operation>
First, the main switch element Q1 is turned on by a voltage applied from the switching control circuit 23 to the gate of the main switch element Q1. When Q1 is turned on, a current flows through the primary winding N1 of the main transformer T1. Along with this, the rectifying switch element Q2 is turned on by the electromotive voltage of the secondary winding N2, a current flows through the path of N2, C1, L2, Q2, and N2, C1 is charged, and excitation energy is accumulated in L2. Is done. At this time, since the electromotive voltage of the auxiliary winding N4 is opposite, the commutation switch element Q3 remains off.

When the main switch element Q1 is turned off under the control of the switching control circuit 23, the electromotive voltage of the secondary winding N2 is inverted and the control terminal voltage of Q2 is inverted, so that Q2 is turned off. Further, the control terminal voltage of the commutation switch element Q3 becomes positive due to the electromotive voltage generated in the auxiliary winding N4, and Q3 is turned on. That is, the control voltage is applied to the control terminal of Q3 through the path of the control terminal of diode Ds (parasitic diode of Q5) → N4 → Q3, and Q3 is turned on. As a result, commutation occurs along the route L2->Q3->C1-> L2.
The rectification and commutation are repeated by turning on and off the main switch element Q1.

  Further, since Q5 is turned on when Q1 is turned on by a signal output from the control switch element drive circuit 24, the charge charged in the control terminal of Q3 (the gate-source capacitance if Q3 is a MOS-FET) is charged. The battery is immediately discharged and Q3 is immediately turned off. Since Q6 is turned on when Q1 is turned off, the charge charged in the control terminal of Q2 (the gate-source capacitance if Q2 is a MOS-FET) is quickly discharged, and Q2 is immediately turned off. In this way, a state where both the rectifying switch element Q2 and the commutation switch element Q3 are on is avoided.

<Backflow operation>
When a mode in which the choke coil L2 is excited by an overvoltage generated at the secondary output when the rectification method is set to synchronous rectification, the synchronous rectification switch elements Q2 and Q3 are MOS-FETs and bidirectional currents are generated. In this mode, power is regenerated from the secondary output to the input side. In this mode, current flows from the output to the converter (backflow), so this mode of operation is referred to as backflow operation.

  The backflow operation depends on the capability of the input power supply (the input power supply of this synchronous rectification type forward converter). Basically, the operation differs depending on whether the power regenerated to the input is not absorbed or not. If the input power at the time of backflow is not absorbed, the input voltage rises, so a protection circuit at the time of input overvoltage is required. Here, it is assumed that the input power supply can also sink current and the voltage can be kept constant.

  During the reverse flow operation, a state occurs in which the on period of Q1 is shorter than the on period of the main transformer T1. In normal operation, L2 is excited from the primary side, so when power supply from the primary is lost, the L2 voltage is inverted, the transformer voltage is inverted, and Q2 is turned off. For example, it is excited during this OFF period by another power source connected in parallel to the output terminal, and the excitation state is reset at the ON timing of Q1. In this circuit, since the commutation switch element Q3 is controlled by the secondary winding N2 of the transformer, Q2 is controlled to be off by the excitation state of L2.

  In this way, during the backflow operation, the operation is performed at the switching frequency by the switching control circuit 23, and thus the operation frequency is fixed.

<Self-excited oscillation operation>
When the control by the switching control circuit 23 is completely eliminated from the state of the backflow operation (the switching of the main switch element Q1 is stopped), the self-oscillation operation state in which the operation frequency of the converter is controlled by the voltage applied from the output It becomes.

  When switching control is stopped in the circuit of FIG. 2, the commutation switch element Q3 is turned on by the voltage generated in the auxiliary winding N4. Thereby, a current (back flow) flows through Q3 and the choke coil L2. This current increases with time.

  Here, if the control voltage signal generation circuit 26 and the ON signal intermittent switch element Q7 are not present, the primary side control is stopped during the self-excited oscillation, and therefore the commutation switch turn-off control switch element Q5 is in the OFF state. It is. Therefore, the electric charge charged in the control terminal (gate-source capacitance) of Q3 is only gradually discharged, and the voltage at the control terminal of Q3 is only gradually reduced.

  When the discharge proceeds and Q3 is turned off, the voltage of the choke coil L2 is inverted, and Q2 is turned on. As a result, a current flows through the path of the secondary winding N 2 → Q 2 → L 2, and power is regenerated to the primary side. When the excitation of the choke coil L2 is reset, the voltage of the choke coil L2 is inverted and Q2 is turned off, thereby generating a flyback voltage in the secondary winding N2 and the auxiliary winding N4 and turning on Q3. . By repeating this operation, self-oscillation occurs.

  As described above, if the commutation switch turn-off control switch element Q5 does not exist, the charge charged in the control terminal of Q3 is discharged only gradually, and the oscillation frequency of self-oscillation becomes low. For example, with 35 to 75 V input and 1.2 to 5 V output, the switching frequency at the time of control is 640 kHz, and the self-excited oscillation frequency when the frequency reduction is not suppressed is as low as about 300 kHz.

  Therefore, when the primary side control is stopped, the ON signal intermittent switch element Q7 is turned ON, and the voltage of the control voltage signal generation circuit 26 is applied to the control terminals of Q5 and Q6. When Q5 is turned on, the on period of Q3 is limited, and the self-excited oscillation frequency is maintained at the same level as the normal switching frequency. Since Q2 is turned off when Q6 is turned on, the main transformer T1 is not excited, no forward voltage for resetting the transformer excitation is generated in the auxiliary winding N4, Q3 is not turned on again, and self-oscillation occurs. Stop.

Next, the configuration of the synchronous rectification forward converter according to the second embodiment will be described with reference to FIGS.
FIG. 3 is a circuit diagram of a synchronous rectification type forward converter according to the second embodiment, and FIG. 4 is a waveform diagram of main parts thereof.

  As shown in FIG. 3, the switching control circuit 23 operates using the output from the tertiary rectifying / smoothing circuit 22 as a power source, and resistances R2 and R3 for dividing the voltage, and for switching control for inputting the divided voltage. It consists of IC230. The switching control circuit 23 outputs a switching control signal to the gate of the main switch element Q1 through the primary winding of the pulse transformer T2. A diode D10 for resetting the pulse transformer is connected to the primary winding of the pulse transformer T2.

  A capacitor C4 is connected to the gate of the rectifying switch element Q2 so that the electromotive voltage of the secondary winding N2 of the main transformer T1 is applied via the capacitor C4. A diode D3 is connected between the drain and source of the rectifying switch element Q2.

  One end of the auxiliary winding N4 of the main transformer T1 is connected to the gate of the commutation switching element Q3, and the switching element Q5 for commutation switch turn-off control is connected to the other end of the auxiliary winding N4.

  One end of the secondary winding of the pulse transformer T2 is connected to the gate of the commutation switch turn-off control switch element Q5.

  A charging circuit including a diode D13, a capacitor C6, and a resistor R13 is provided as a control voltage signal generation circuit 26A between the connection point between the gate of Q2 and the capacitor C4 and the ground on the secondary side. A primary-side control stop time control circuit 27 composed of an ON signal intermittent switch element Q7 and a resistor R10 is provided so that the voltage (charge voltage of C6) from the control voltage signal generation circuit 26A is applied to the gate of Q5. Yes.

  One end of the secondary winding of the pulse transformer T2 (the side where a positive voltage is generated when the main switch element Q1 is turned on) is connected to the gate of the switch element Q5 for commutation switch turn-off control. The other end of the secondary winding is connected to the secondary ground via a diode D9. The meaning of providing the diode D9 not provided in FIG. 1 will be described later. The pulse transformer T2, the diode D10, and the resistor R5 constitute a control switch element driving circuit 24A.

  On the secondary side of the pulse transformer T2, a primary-side control stop detection circuit 25A including a diode D14, a capacitor C5, and resistors R11 and R12 is provided. The primary side control stop detection circuit 25A is a charging circuit for the secondary winding voltage of the pulse transformer T2, and when the primary side control is performed, a predetermined voltage (charging voltage of C5) appears and discharges. When the primary side control is stopped, the charging voltage is almost zero. As shown in FIG. 3, charging is performed by connecting the ground of the charging circuit to the point where the source of Q5 is connected, that is, the connection point between the rectifying switch element Q2 and the choke coil L2, that is, the secondary side ground. The charging voltage of the circuit is applied to the gate of the on signal intermittent switch element Q7. Since the ON signal intermittent switch element Q7 is a depletion type p-channel MOS-FET, it is turned on when the gate voltage is low, and is turned off when the positive voltage is equal to or higher than a predetermined value. Therefore, it becomes conductive when the primary side control is stopped.

As described above, since the other end of the secondary winding of the pulse transformer T2 is connected to the secondary side ground, the output of the control voltage signal generation circuit 26A is supplied to the Q5 via the primary side control stop time control circuit 27. Since the voltage to be applied to the gate of Q5 is grounded when the control on the primary side is stopped simply by connecting to the gate of the primary side, the current of the pulse transformer T2 is prevented in the direction to prevent the current from flowing to the ground. A diode D9 is inserted between the secondary winding and the ground.
In the example shown in FIG. 2, the control switch element Q6 that turns off the rectifying switch element Q2 when the main switch Q1 is turned off is provided, but the circuit is not provided in the example of FIG. The other configuration is the same as that shown in FIG. 2, and the operation thereof is also the same as that described based on FIG.

The operation of the synchronous rectification type forward converter shown in FIG. 3 is as follows.
First, the control voltage signal generation circuit 26A holds the peak value of the gate voltage of Q2 in the capacitor C6. However, it does not have a special meaning in maintaining the peak value, but it means that the capacitor C6 is charged to a voltage higher than a predetermined value. The voltage charged in the capacitor C6 is referred to as “Q5 ON signal”. Since the signal for driving Q2 is generated not only during normal operation (a state in which the control on the primary side is performed and the signal for controlling Q5 is transmitted via the pulse transformer T2) but also during self-oscillation, 1 The control voltage signal generation circuit 26A functions even when the secondary side control is stopped.

  On the other hand, the primary-side control stop detection circuit 25A holds a peak value (of a signal transmitted from the primary side via the pulse transformer T2 to drive Q5) in the capacitor C5. This also does not mean that the peak value has a special meaning, but it makes sense that C5 is charged to a voltage higher than a predetermined value. Since the signal for driving Q5 is also stopped when the primary side control is stopped, C5 is no longer charged, the already charged electric charge is discharged through the resistor R12, and the charging voltage is lowered.

  Now, during normal operation, the charging voltage of the capacitor C5 becomes equal to or higher than a predetermined value, and the ON signal intermittent switch element Q7 is cut off. Therefore, the Q5 on signal is not output.

  If the primary side control is stopped for some reason at the timing indicated by t0 in FIG. 4, the signal transmitted to Q5 via T2 stops, and Q5 is not turned on by the signal from the primary side. Then, since there is no signal transmitted via T2, C5 of the control stop detection circuit is not charged. Since the charge charged in C5 during normal operation is discharged through R12, the charging voltage of C5 gradually decreases. When the charging voltage of C5 becomes equal to or lower than the threshold value of Q7 at t1 shown in FIG. 4, Q7 becomes conductive, and the Q5 ON signal is applied to the gate of Q5 via Q7 and R10. Turn on. As a result, the charge of the gate-source capacitance of the commutation switch element Q3 is quickly discharged, the turn-off of Q3 is accelerated, and a decrease in the oscillation frequency of the self-excited oscillation is suppressed.

  By the way, the gate of Q5 is connected to one end of the secondary winding in order to be controlled by a signal originally generated in the secondary winding of the pulse transformer. The other end of the secondary winding is connected to the source of Q5, that is, the secondary side ground. That is, the gate of Q5 is connected to the secondary side ground via a secondary winding in terms of DC. Therefore, Q5 cannot be controlled by simply applying the Q5 ON signal to the gate of Q5.

  Therefore, as described above, the diode D9 is provided between the secondary winding and the secondary ground to prevent the gate of Q5 from being grounded during self-excited oscillation. During normal operation, the pulse for controlling Q5 becomes a forward voltage for D9, so D9 does not get in the way.

  However, by providing D9 in this way, it is possible to prevent the gate of Q5 from falling to the ground, but since there is no loss of wiring connecting the pulse transformer T2 and the gate of Q5, there is no current from the gate of Q5 to the pulse transformer T2 side. Can not be prevented from flowing. As described above, the primary-side control stop detection circuit 25A is connected to the secondary winding of the pulse transformer. Therefore, in this state, during the self-excited oscillation, the current due to the Q5 ON signal flows into the primary-side control stop detection circuit 25A and charges C5. In this case, there is a possibility that the charged voltage of C5 rises and Q7 turned on with great effort is turned off. Therefore, R11 is provided in front of the primary side control stop detection circuit 25A to increase the charging time constant of C5 to suppress the rising speed of the charging voltage (so as not to exceed the discharge due to R12), and at least Q7 is complete. It is made not to turn off. Even if Q7 has a certain resistance value, it can achieve its intended purpose as long as it is conductive.

  Further, in the example shown in FIG. 3, the Q5 ON signal is applied to the gate of Q5 (one end side of the secondary winding of the pulse transformer T2). However, since the secondary winding of the pulse transformer T2 can be regarded as a simple wiring during self-excited oscillation, the Q5 ON signal is actually applied to any part from the gate of Q5 to the cathode of D9 (the other end of the secondary winding). You may comprise so that it may apply. The place where the Q5 ON signal is applied is a path (control signal transmission path) through which a signal for controlling Q5 from the primary side passes during normal operation. Therefore, the Q5 ON signal only needs to be applied to this control signal transmission path.

Next, the configuration of the synchronous rectification type forward converter according to the third embodiment will be described with reference to FIG.
In the third embodiment, as shown in FIG. 5, a rectifying switch turn-off control switch element Q6 for turning off Q2 during self-excited oscillation is provided, and the gate of Q6 is connected to the secondary winding of the pulse transformer T2. Connected to the other end (cathode side of D9). The Q5 ON signal is applied to the gate of Q6 (that is, the other end of the secondary winding of the pulse transformer T2 (included in the control signal transmission path of Q5)). The switch element Q6 for rectifying switch turn-off control is a switch element corresponding to Q6 shown in FIG. Other configurations are the same as those in the second embodiment.

  Since the gate of Q5 and the gate of Q6 are connected (short-circuited) via the secondary winding of the pulse transformer T2, which can be regarded as a simple wiring when the primary side control is stopped, the Q5 ON signal is applied to the gate of Q6. Even in this configuration, the control of Q5 is the same as in the second embodiment.

  With this configuration, Q2 is turned off by turning on Q5 and turning on Q6 with the Q5 on signal during self-excited oscillation.

  When Q6 is turned on at the same time as Q5 with the Q5 on signal, it is likely to simply connect the gates of Q5 and Q6, but then Q6 will be turned on simultaneously with Q5 during normal operation. It is not preferable. Therefore, it is connected to the other end of the secondary winding of the pulse transformer T2 so that Q6 is not turned on by a signal that turns on Q5 during normal operation. As a result, the signal that turns on Q5 has the opposite polarity to Q6, and Q6 does not turn on. In addition, since the diode D10 is connected to the primary winding of the pulse transformer T2, a signal that turns on Q6 is not generated during normal operation in terms of circuit. That is, during normal operation, Q6 is always off and does not exist equivalently.

  In the third embodiment, since Q2 is turned off when Q6 is turned on, self-oscillation itself does not continue and stops. When the self-excited oscillation stops, no voltage is generated to turn on Q3 in the auxiliary winding N4 of the main transformer T1, so Q3 is also turned off (without being turned on again), and the backflow is completely stopped.

  In the third embodiment, the self-excited oscillation finally stops, so the function of suppressing the self-excited oscillation frequency (the function of turning on Q5) seems unnecessary at first glance. However, even though self-excited oscillation does not continue, it oscillates at least until Q6 is turned on, and as a result, a current load is applied to Q2 and Q3. .

Next, the configuration of a synchronous rectification forward converter according to a fourth embodiment will be described with reference to FIG.
The fourth embodiment is another variation of the control voltage signal generation circuit. As shown in FIG. 6, the control voltage signal generation circuit 26B includes a diode D13, a capacitor C6, and a resistor R13, and charges the flyback voltage of the auxiliary winding N4 of the main transformer T1. Even with such a configuration, control voltages for the commutation switch turn-off control switch element Q5 and the rectifier switch turn-off control switch element Q6 can be generated. Other configurations and operations are the same as those in the third embodiment.

  The configuration of the control voltage signal generation circuit 26B can also be applied to the circuit shown in the second embodiment shown in FIG.

Next, the configuration of the synchronous rectification forward converter according to the fifth embodiment will be described with reference to FIG.
The fifth embodiment is another variation of the control voltage signal generation circuit. As shown in FIG. 7, the control voltage signal generation circuit 26C includes a diode D13, a capacitor C6, and resistors R13 and R14, and charges the voltage generated in the choke coil L2. Even with such a configuration, it is possible to generate control voltages for the commutation switch turn-off control switch element Q5 and the rectifier switch turn-off control switch element Q6. Other configurations and operations are the same as those in the third embodiment.

  The configuration of the control voltage signal generation circuit 26C can also be applied to the circuit shown in the second embodiment shown in FIG.

Next, the configuration of a synchronous rectification forward converter according to a sixth embodiment will be described with reference to FIG.
In the sixth embodiment, the diode D10 provided in parallel with the primary winding of the pulse transformer T2 shown in the second to fourth embodiments is removed. Instead, a diode D12 that clamps the negative pulse to ground and a diode D11 that clamps the positive pulse to VCC are provided to clamp the voltage and remove noise.

  Since the parallel diode of the primary winding of the pulse transformer T2 is removed, not only a signal for turning on Q1 during normal operation but also a signal for turning off (a signal in the reverse direction) is transmitted to the secondary side via the pulse transformer T2. . Therefore, when a signal for turning off Q1 is output on the primary side, a signal for turning on Q6 is output to the secondary side, and Q2 is turned off.

  Originally, when Q1 is off, Q2 is also controlled to turn off at the voltage generated in the secondary winding N2 of the main transformer T1 by turning off Q1, but before that, Q2 is turned off by turning on Q6. Yes. That is, the on-timing of Q3 may be shifted with respect to the off-timing of Q2 due to leakage inductance of the transformer T1, and there is a possibility that the timing of turning on Q2 and Q3 at the same time may occur. Control can be prevented, so that the simultaneous ON can be more reliably prevented. However, since there is no current path from the ground for the signal in the reverse direction as it is, the diode D8 is provided in order to secure the current path in the reverse direction.

FIG. 9 is a circuit diagram of a portion where the secondary side signal of the pulse transformer T2 is applied to the gates of Q5 and Q6. FIG. 10 is a waveform diagram thereof.
Thus, since signals in both directions are output from the secondary winding of the pulse transformer T2, the primary side control stop detection circuit may be provided on either side thereof. In the example shown in FIG. 8, the primary-side control stop detection circuit 25A is connected to the other end different from those in the second to fourth embodiments. Other configurations and operations are the same as those of the fourth embodiment.

Next, the configuration of the synchronous rectification forward converter according to the seventh embodiment will be described with reference to FIG.
In the synchronous rectification forward converter according to the seventh embodiment, in the circuit shown in FIG. 8, by further providing a resistor R15 and a diode D15, the voltage in which direction generated in the secondary winding of the pulse transformer T2 is increased. In other words, the capacitor C7 can be charged by the two-wave rectification of the pulse transformer T2. In this case, the so-called double-wave rectification doubles the charging speed of C7. When the charging speed is high, even if the resistance value of R14 is slightly reduced to increase the discharging speed, it is possible to avoid that the charging voltage of C7 decreases and Q7 is turned on during normal operation. If the resistance value of R14 is small, when the primary side control is stopped, the charging voltage drop speed of C7 is increased and Q7 is turned on earlier. In other words, Q5 can be turned on early so that the self-excited oscillation frequency reduction suppressing function can be exhibited quickly.
Others are the same as in the case of the sixth embodiment.

  In each of the embodiments described above, the diode D9 is provided as an impedance circuit between the secondary winding of the pulse transformer T2 and the secondary-side ground of the main transformer T1, but the commutation switch turn-off control switch Since the gates of the element Q5 and the switch element Q6 for turning off the rectifying switch do not have to fall to the ground, for example, a resistor having an appropriate value may be used. Further, the impedance circuit may be configured by another switch element that can be synchronously controlled.

10 is a circuit diagram illustrating a configuration of a converter according to Patent Document 1. FIG. It is a circuit diagram of a synchronous rectification type forward converter concerning a 1st embodiment. It is a circuit diagram of the synchronous rectification type | mold forward converter which concerns on 2nd Embodiment. It is a wave form diagram of the principal part of the converter. It is a circuit diagram of the synchronous rectification type | mold forward converter which concerns on 3rd Embodiment. It is a circuit diagram of a synchronous rectification type forward converter concerning a 4th embodiment. It is a circuit diagram of the synchronous rectification type | mold forward converter which concerns on 5th Embodiment. It is a circuit diagram of the synchronous rectification type | mold forward converter which concerns on 6th Embodiment. It is a partial circuit diagram of the converter. 9 is a waveform diagram of each part. It is a circuit diagram of the synchronous rectification type | mold forward converter which concerns on 7th Embodiment.

Explanation of symbols

T1-main transformer 21-input terminal 22-3 tertiary rectifying / smoothing circuit 23-switching control circuit 24-control switch element driving circuit 25-1 primary side control stop detection circuit 26-control voltage signal generation circuit 27-1 primary side control Stop-time control circuit 32-output terminal Q1-main switch element Q2-commutation switch element Q3-commutation switch element Q5-commutation switch turn-off control switch element Q6-commutation switch turn-off control switch element Q7-on signal intermittent switch element N1-1 primary winding N2-2 secondary winding N3-3 tertiary winding N4-auxiliary winding L1, L2-choke coil C1-smoothing capacitor D9-diode (impedance circuit)

Claims (7)

A transformer having a primary winding and a secondary winding; a main switch element connected in series to the primary winding of the transformer; and a choke coil connected in series to the secondary winding of the transformer; A smoothing capacitor connected in parallel between the output terminals, a rectifying switch element connected in series with the secondary winding of the transformer, and turned on / off in synchronization with on / off of the main switch element; A commutation switch element that is turned off / on in synchronization with the on / off of the switch element, and that configures a discharge path of the excitation energy of the choke coil by the on state, and a switching control circuit that performs switching control of the main switch element, In the synchronous rectification type forward converter provided,
A switch element for commutation switch turn-off control that is connected in series to the auxiliary winding of the transformer and that controls the application of the electromotive voltage of the auxiliary winding of the transformer to the control terminal of the commutation switching element,
A control switch element driving circuit for turning on the commutation switch turn-off control switch element when the main switch element is on;
A control voltage signal generation circuit for generating a control voltage signal for the commutation switch turn-off control switch element;
A primary control stop detection circuit for detecting a control stop state of the switching control circuit;
Based on the output of the primary side control stop detection circuit, a primary side control stop time control circuit that gives an on control voltage signal from the control voltage signal generation circuit to the commutation switch turn-off control switch element when the primary side control stops. When,
Synchronous rectification type forward converter. The primary side control stop time control circuit includes an on signal intermittent switch element that gives an on control voltage signal to the commutation switch turn-off control switch element,
The control switch element drive circuit is driven on the primary side by a drive signal to the main switch element by the switching control circuit, and generates a control voltage signal from the secondary side to the commutation switch turn-off control switch element. With a transformer
The primary-side control stop detection circuit includes a charging circuit that rectifies and charges the secondary-side output of the pulse transformer and applies a voltage generated by the charging to the ON signal intermittent switch element,
On the secondary side of the pulse transformer, the control terminal of the commutation switch turn-off control switch element is grounded between the control terminal of the commutation switch turn-off control switch element and the ground of the charging circuit. The synchronous rectification forward converter according to claim 1, further comprising an impedance circuit that prevents the occurrence of the impedance rectification.   The synchronous rectification type forward converter according to claim 2, wherein the charging circuit is obtained by performing both-wave rectification on the secondary output of the pulse transformer.   4. The synchronous rectification forward converter according to claim 1, wherein the control voltage signal generation circuit is a charging circuit that charges a voltage signal for driving the rectifying switch element. 5.   4. The synchronous rectification forward converter according to claim 1, wherein the control voltage signal generation circuit is a charging circuit that charges a flyback voltage of the auxiliary winding. 5.   4. The synchronous rectification forward converter according to claim 1, wherein the control voltage signal generation circuit is a charging circuit that charges a voltage generated in the choke coil.   A rectifier switch turn-off control switch that is driven by the on-control voltage signal supplied to the commutation switch turn-off control switch element via the primary side control stop time control circuit and controls the control voltage of the rectifier switch element. The synchronous rectification type forward converter of any one of Claims 1-6 which provided the element.

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