JP2007244075A - Synchronous rectification forward converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To construct a synchronous rectification forward converter wherein self-oscillation in reverse current operation is prevented and output stabilizing response in normal operation is enhanced. <P>SOLUTION: In the synchronous rectification forward converter, the tertiary winding N3 of a transformer T1 is provided with a tertiary rectifying and smoothing circuit 22 for detecting output voltage between output terminals 32a and 32b. The forward converter is provided with: a steady-state voltage generation circuit 27 that applies certain direct-current voltage from its output voltage or steady-state voltage regulated to an upper-limit voltage to a switching control circuit 23; and a varied voltage detection circuit 28 that detects a varied voltage of the output voltage of the tertiary rectifying and smoothing circuit 22 and supplies it to the switching control circuit 23. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a forward type converter that performs synchronous rectification, and more particularly to a synchronous rectification type forward converter that eliminates a problem during reverse current operation.

  Generally, a synchronous rectification type forward converter has a circuit configuration in which, when a voltage higher than the voltage that the converter intends to generate between output terminals is applied between the output terminals from a load (external), the output of the converter A so-called “reverse current” phenomenon occurs in which a current flows backward from the input side to the input side.

  Such an operation when a reverse current is generated (hereinafter referred to as “reverse current operation”) causes various problems. Conventionally, the circuit operation is controlled by detecting the reverse current.

  Patent Document 1 discloses a synchronous rectification type forward converter including a reverse current detection circuit. Here, the operation of the reverse current detection circuit of Patent Document 1 will be described with reference to FIG. FIG. 1 shows the main part of the circuit shown in FIG.

  In FIG. 1, a main switch element Q1 is connected in series to a primary winding N1 of a transformer T1. A choke coil L1 and a rectifying switch element Q2 are connected in series to the secondary winding N2 of the transformer T1, and a smoothing capacitor C2 is connected in parallel between the output terminals 32a and 32b. The choke coil L1 and the smoothing capacitor C2 form a loop, and the commutation switch element Q3 is provided at a position that becomes a commutation path when the excitation energy of the choke coil L1 is released. Further, a commutation switch turn-off control switch element Q4 is connected between the gate and source of the commutation switch element Q3.

  The tertiary winding N3 of the transformer T1 is provided with a tertiary rectifying / smoothing circuit including a rectifying diode D2, a commutation diode D1, a choke coil L2, and a smoothing capacitor C3. The output voltage of the tertiary rectifying / smoothing circuit is divided by resistors R1 and R2, and is given as a control voltage to the switching control circuit 23. The output point of this resistance voltage dividing circuit is called X point.

  The reverse current detection circuit 25 is a circuit that charges the capacitor C4 through the resistors R3 and R4 and the diodes D3, D4, and D5, and one end of the resistor R3 is connected to the anodes of the diodes D1 and D2 of the tertiary rectification smoothing circuit 22. One end of the resistor R4 is connected to the gate of the main switch element Q1, and the anode of the diode D5 is connected to the X point.

FIG. 2 is a diagram showing voltage waveforms at various parts shown in FIG. During normal operation, as shown in FIG. 2A, the gate-source voltage of the main switch element Q1 and the anode voltages of the diodes D1 and D2 have complementary waveforms, and the voltage at the X point (hereinafter, “ X point voltage ") becomes a constant voltage as shown in the figure (it can be made constant by setting circuit constants).
On the other hand, in the switching control circuit 23, since the charging voltage of the capacitor C4 of the reverse current detection circuit 25 and the X point voltage are substantially equal during normal operation, the diode D5 connecting them is in the OFF state, and the X point voltage is the switching control. It is applied as it is as a control voltage for the circuit 23.

  Here, when an overvoltage is applied from the outside (load) between the output terminals 32a and 32b of the synchronous rectification forward converter 100 shown in FIG. 1, a reverse current is generated. Even if such a reverse current occurs, the switching control circuit 23 continues to operate. FIG. 2B is a voltage waveform diagram of each part during reverse current operation. When a reverse current is generated in this way, the voltage application time Ton for turning on Q1 with respect to the gate of the main switch element Q1 is shortened, and the Q1 gate-source voltage and the anode voltages of the diodes D1 and D2 are complementary. It ’s not right. As a result, a non-charging period P of the capacitor C4 occurs, and a periodic depression (voltage drop) occurs in the X point voltage. This occurs every switching period.

Since this point X voltage is a control voltage applied to the switching control circuit 23, when the point X voltage decreases, the switching control circuit 23 assumes that the output voltage to the output terminals 32a-32b has decreased, and outputs the output voltage. The main switch element Q1 is controlled in the upward direction. That is, the on-duty ratio is increased. As a result, the output voltage rises, the output voltage rise stops at a predetermined time, and the reverse current operation stops.
Japanese Patent No. 3391320

  However, during reverse current operation, the voltage between the output terminals is higher than the set value, so that a voltage approximately proportional to the voltage between the output terminals is output from the tertiary rectification smoothing circuit, and the average voltage at the point X increases. To go. FIG. 2C shows this state. The switching control circuit 23 performs switching control of the main switch element Q1 in the direction of decreasing the output voltage with the average voltage increase at the point X. This operation is an inconsistent operation opposite to the operation at the time of detecting the reverse current. Even if the control operation is opposite, that is, in the direction of lowering the output voltage, there is no problem when the reverse current is small. However, when the reverse current becomes large, the reverse current may not be suppressed due to the contradiction. This problem occurs because the X-point voltage tends to rise without limit due to the reverse current.

  If the voltage at the point X continues to rise, the on-period of the main switch element Q1 is finally set to 0 (hereinafter referred to as “disappearance of the gate pulse”) by control in a direction to lower the output voltage. This means that the control by the switching control circuit 23 is stopped. In this state, self-excited oscillation is generated by the energy of the secondary choke coil L1. Since the switching frequency during the self-excited oscillation is considerably lower (for example, about half) than the switching frequency during normal operation, various problems arise. For example, the operation of the switching control circuit 23 becomes unstable and the excitation time of the transformer is extended, so that an excessive drain voltage is applied to the main switch element Q1, the rectifying switch element Q2, and the commutation switch element Q3, and the drain current flows to cause stress. May cause destruction.

  Further, if there is a sudden rise in the input voltage of the synchronous rectification type forward converter 100 or a sudden drop in the load current, the output voltage rises temporarily, and the X point voltage rises accordingly, and the switching control circuit 23 Works negative feedback control to lower the output voltage. However, since the point X voltage is the output voltage of the tertiary rectification smoothing circuit, that is, the voltage itself that rectifies and smooths the induced voltage of the tertiary winding, the input voltage suddenly rises or the load current suddenly declines. Even if the above occurs, the transient increase in the voltage at the point X is small and a delay occurs. Therefore, the response of stabilizing the output voltage is poor. This phenomenon is not limited to the presence or absence of the self-excited oscillation operation.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a synchronous rectification forward converter that prevents self-excited oscillation during reverse current operation and has improved output stabilization response during normal operation.

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

[1] A transformer (T1) having a primary winding, a secondary winding, and a tertiary winding, respectively, and a main switch element (Q1) connected in series to the primary winding (N1) of the transformer, The choke coil (L1) connected in series to the secondary winding of the transformer, the smoothing capacitor (C2) connected in parallel between the output terminals (32a-32b), and the secondary winding of the transformer The rectifying switch element (Q2) connected in series and turned on / off in synchronization with the on / off of the main switch element, and turned off / on in synchronization with the on / off of the main switch element. The commutation switch element (Q3) constituting the excitation energy discharge path of the choke coil and the switching control (on-duty ratio control) of the main switch element so as to perform a negative feedback operation with a control voltage applied to the control voltage input section. ) A switching control circuit (23) performing, in the synchronous rectification type forward converter with,
A steady voltage generation circuit (27) for applying a constant DC voltage to the control voltage input section of the switching control circuit, and a tertiary rectification smoothing circuit (22) for rectifying and smoothing the induced voltage of the tertiary winding (N3) of the transformer And a change voltage detection circuit (28) for detecting a transient change voltage of the output voltage of the tertiary rectification smoothing circuit and supplying the change voltage to the control voltage input section of the switching control circuit. It is a feature.

  [2] A tertiary rectification smoothing circuit for rectifying and smoothing the induced voltage of the tertiary winding of the transformer, and a transient voltage change component of the output voltage of the tertiary rectification smoothing circuit are detected, and the change voltage is calculated. The change voltage detection circuit (28) applied to the control voltage input unit of the switching control circuit and the upper limit of the output voltage of the tertiary rectification smoothing circuit are limited to a predetermined upper limit value and to the control voltage input unit of the switching control circuit And a steady voltage generation circuit (27) for providing.

  [3] The steady voltage generation circuit is constituted by a regulator circuit that stabilizes the output voltage of the tertiary rectification smoothing circuit by comparing the output voltage of the tertiary rectification smoothing circuit with a reference voltage (Vr), for example. .

  [4] Further, for example, a reverse current detection circuit (30) that detects a reverse current state in which a current flows backward from a load circuit connected between the output terminals and operates the regulator circuit only in the reverse current state. Provide.

  [1] The steady voltage generation circuit (27) applies a constant DC voltage to the control voltage input section of the switching control circuit (23), and the change voltage detection circuit (28) outputs the output voltage of the tertiary rectification smoothing circuit (22). Since the transient change voltage is detected and the change voltage is applied to the control voltage input unit of the switching control circuit, the control voltage applied to the control voltage input unit of the switching control circuit (22) is, on average, the constant voltage. Since it is a direct current voltage or does not exceed this constant direct current voltage, an operation in which the control voltage of the switching control circuit increases without limit during reverse current operation can be avoided, and self-excited oscillation does not occur.

  Further, since the transient change voltage of the output voltage of the tertiary rectification smoothing circuit is given to the control voltage input section of the switching control circuit, feedback is applied according to the input voltage of the synchronous rectification forward converter and the transient fluctuation of the load. Therefore, it is possible to maintain high responsiveness to stabilization of the output voltage.

  [2] Since the output voltage of the tertiary rectification smoothing circuit (22) is limited to a predetermined upper limit value by the steady voltage generation circuit, and the voltage is applied to the control voltage input unit of the switching control circuit (23), Negative feedback is applied according to fluctuations in the transformer tertiary winding voltage (output voltage to the load). For this reason, the output voltage can be stabilized even when the input voltage of the synchronous rectification type forward converter and the load are varied except during the reverse current operation.

  [3] The steady voltage generation circuit (27) is configured by a regulator circuit that stabilizes the output voltage of the tertiary rectification smoothing circuit by comparing the output voltage of the tertiary rectification smoothing circuit (22) with the reference voltage. The output voltage can be further stabilized.

  [4] When the reverse current detection circuit (30) detects the reverse current state and operates the regulator circuit, the upper limit of the output voltage of the tertiary rectification smoothing circuit is limited only during reverse current operation, and during reverse current operation. It is not limited except. Therefore, even if the output voltage of the tertiary rectification smoothing circuit rises due to factors other than reverse current such as input voltage and load fluctuation, the voltage is not limited to the upper limit value, and the output voltage can be stabilized by normal operation. .

<< First Embodiment >>
The synchronous rectification type forward converter according to the first embodiment will be described with reference to FIGS.
FIG. 3 is a circuit diagram of a synchronous rectification forward converter partially blocked. As shown in FIG. 3, the transformer T1 includes a primary winding N1, a secondary winding N2, and a tertiary winding N3. A main switch element Q1 is connected in series to the primary winding N1. A synchronous rectifier circuit 26 is connected to the secondary winding N2 of the transformer T1, and an output voltage is supplied from the output terminals 32a and 32b to the load. A tertiary rectification smoothing circuit 22 is connected to the tertiary winding N3 of the transformer T1. The steady voltage generating circuit 27 receives the output voltage of the tertiary rectifying / smoothing circuit 22 to generate a steady voltage (DC voltage), and supplies it to the control voltage input unit (point X) of the switching control circuit 23 as a control voltage. The change voltage detection circuit 28 detects a transient change voltage of the output voltage of the tertiary rectification smoothing circuit 22 and supplies it to the control voltage input section (point X) of the switching control circuit 23.

  The switching control circuit 23 outputs switching control signals to the main switch element Q1 and the synchronous rectifier circuit 26 to perform switching thereof. Further, the switching control circuit 23 controls the on-duty ratio of the main switch element Q1 so as to perform negative feedback operation with the control voltage applied to the point X so that the output voltage between the output terminals 32a and 32b is stabilized. To control.

  The synchronous rectification forward converter shown in FIG. 3 is different from the conventional synchronous rectification forward converter in that a steady voltage generation circuit 27 that generates a steady voltage from the output voltage of the tertiary rectification smoothing circuit 22 and a change voltage are detected. The change voltage detection circuit 28 for outputting is provided.

Next, a specific circuit configuration of the synchronous rectification forward converter will be described with reference to FIG.
A primary switch element Q1 is connected in series to the primary winding N1 of the transformer T1, and a smoothing and filter capacitor C1 is connected between the input terminals 21a-21b. A choke coil L1 and a rectifying switch element Q2 are connected in series to the secondary winding N2 of the transformer T1, and a smoothing capacitor C2 is connected in parallel between the output terminals 32a and 32b. The choke coil L1 and the smoothing capacitor C2 form a loop, and the commutation switch element Q3 is provided at a position that becomes a commutation path when the excitation energy of the choke coil L1 is released. A commutation switch turn-off control switch element Q4 is connected between the gate and source of the commutation switch element Q3. A diode D6 is provided in the direction from the drain of the rectifying switch element Q2 to the gate of the commutation switch element Q3. In this way, the synchronous rectification circuit 26 is configured.

  A choke coil L2 and a rectifier diode D2 are connected in series to the tertiary winding N3 of the transformer T1, and a smoothing capacitor C3 is connected to the output section. Further, the choke coil L2 and the smoothing capacitor C3 constitute a loop, and the commutation diode D1 is provided at a position serving as a commutation path when the excitation energy of the choke coil L2 is released. In this way, the tertiary rectification smoothing circuit 22 is configured.

  The steady voltage generation circuit 27 receives a voltage output circuit 29 that receives the output voltage of the tertiary rectifying and smoothing circuit 22 and generates a steady voltage (a predetermined DC voltage in this example), and resistors R1 and R2 that divide the output voltage. I have. The change voltage detection circuit 28 includes a capacitor C5 and a resistor R5. The change voltage detection circuit 28 detects a transient change voltage from the output voltage of the tertiary rectification smoothing circuit 22 and supplies it to the control voltage input section of the switching control circuit 23. The combined voltage of the output voltage of the steady voltage generation circuit 27 and the output voltage of the change voltage detection circuit 28 is given to the switching control circuit 23 as a control voltage.

The switching control circuit 23 outputs a Q1 on / off control pulse to the gate of the main switch element Q1. The signal transmission circuit 24 applies the switching pulse output from the switching control circuit 23 to the gate of the switching element Q4 for commutation switch turn-off control.
In this way, the synchronous rectification type forward converter 101 is configured.

  The operation of the synchronous rectification type forward converter shown in FIGS. 3 and 4 will be described with reference to FIG. First, the main switch element Q1 is turned on by a positive 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 transformer T1. Along with this, the rectifying switch element Q2 is turned on by the induced voltage of the secondary winding N2, and the current flows through the path of the secondary winding N2, the capacitor C2, the choke coil L1, the rectifying switch element Q2, and the secondary winding N2. As a result, C2 is charged and excitation energy is accumulated in L1. At this time, the output of the signal transmission circuit 24 is at a high level, the commutation switch turn-off control switch element Q4 is on, and the commutation switch element Q3 is off.

Next, when the main switch element Q1 is turned off under the control of the switching control circuit 23, the induced voltage of the secondary winding N2 of the transformer T1 is inverted and the gate voltage of the rectifying switch element Q2 is inverted, so that Q2 is turned off. . Further, the commutation switch turn-off control switch element Q4 is turned off by the output signal from the signal transmission circuit 24. Therefore, a high level voltage is applied to the gate of the commutation switch element Q3 via the diode D6, and the commutation switch element Q3 is turned on. As a result, commutation occurs in the path of choke coil L1 → commutation switch element Q3 → capacitor C2 → choke coil L1.
The rectification and commutation are repeated by turning on and off the main switch element Q1.

  The tertiary rectifying / smoothing circuit 22 connected to the tertiary winding N3 of the transformer T1 also operates as a forward converter. That is, during the ON period of the main switch element Q1, a current flows through the path of the tertiary winding N3 → capacitor C3 → choke coil L2 → diode D2 → tertiary winding N3, C3 is charged and excitation energy is accumulated in L2. Is done. When the main switch element Q1 is turned off, the voltage of the tertiary winding N3 is inverted, the diode D2 is turned off, and commutation occurs in the path L2-> D1-> C3. As a result, the output voltage of the tertiary rectifying / smoothing circuit 22 becomes a voltage substantially proportional to the voltage between the output terminals 32a and 32b.

  FIG. 5 is a waveform diagram of each part shown in FIG. Q1 is turned on in a period Ton when the gate-source voltage of the main switch element Q1 is at a high level, and Q1 is turned off in a period Toff at a low level. Since the switching control circuit 23 controls the on-duty ratio of Q1 at a constant switching frequency, the period Ton · Toff changes according to the output voltage.

  When the voltage output circuit 29 is a circuit that outputs the output voltage of the tertiary rectification smoothing circuit 22 almost as it is within a range where the output voltage of the tertiary rectification smoothing circuit 22 does not exceed a predetermined upper limit, the tertiary rectification smoothing The on-period Ton changes according to the output voltage of the circuit 22, that is, according to the voltage between the output terminals 32a and 32b. This stabilizes the output voltage.

  When the output voltage of the tertiary rectifying / smoothing circuit 22 exceeds a predetermined upper limit value, the voltage output circuit 29 outputs the upper limit voltage. Therefore, the ON period Ton of Q1 does not become shorter than that.

  Therefore, when an overvoltage is applied from the outside (load) between the output terminals 32a and 32b, even if the output voltage of the tertiary rectifying and smoothing circuit 22 rises excessively, it is given from the steady voltage generating circuit 27 to the switching control circuit 23. The controlled voltage does not exceed the upper limit value. Therefore, the disappearance phenomenon of the gate pulse can be prevented, and the self-oscillation operation can be prevented.

  Further, by taking the control voltage applied to the switching control circuit 23 from the output voltage of the tertiary rectifying and smoothing circuit 22, it is possible to respond in response to a steady voltage change between the output terminals 32a and 32b.

  Further, since the change voltage detection circuit 28 gives the change voltage corresponding to the transient voltage change of the output voltage to the switching control circuit 23, it can respond to the transient voltage fluctuation. Incidentally, when the output voltage of the tertiary rectifying and smoothing circuit is fed back to the switching control circuit 23 as it is without dividing the steady voltage component (DC component) and the change voltage component (AC component) and without setting an upper limit, It responds as it is to a transient voltage rise due to a sudden change, a sudden change in load, etc., and the recovery time from the output voltage rise to the normal output voltage becomes longer. That is, the responsiveness is deteriorated.

<< Second Embodiment >>
Next, a synchronous rectification forward converter according to a second embodiment will be described with reference to FIG.
In the second embodiment, the steady voltage generation circuit 27 always generates a constant DC voltage. That is, in FIG. 6, the steady voltage generating circuit 27 is composed of a voltage source Es that generates a constant voltage and resistors R1 and R2 that divide the voltage. Others are the same as those shown in FIG.

  Thus, by making the steady voltage (DC) component constant among the control voltages for the switching control circuit 23, the control voltage for the switching control circuit 23 due to the increase in the output voltage of the tertiary rectification smoothing circuit 22 during the reverse current operation. Excessive voltage rise is eliminated, and disappearance of gate pulses and self-excited oscillation can be prevented. Further, regarding the transient fluctuation of the output voltage of the tertiary rectification smoothing circuit 22 with respect to the sudden change of the input voltage and the load, the switching control circuit 23 responds according to the output voltage of the change voltage detection circuit 28. The voltage between the output terminals 32a and 32b is stabilized against the fluctuation.

<< Third Embodiment >>
Next, a synchronous rectifying forward converter according to a third embodiment will be described with reference to FIG.
In the third embodiment, as the steady voltage generating circuit 27, a transistor Q5 connected in series to the output of the tertiary rectifying and smoothing circuit 22, resistors R1 and R2, and a resistor R6 connected between the base and collector of the transistor Q5. The diode D7 and the Zener diode ZD1 are connected between the base of the transistor Q5 and the ground (the reference voltage terminal of the tertiary rectification smoothing circuit 22). Here, the diode D7 is provided for correcting the temperature drift of the Zener diode ZD1. Other configurations are the same as those in the first and second embodiments.

  By configuring the steady voltage generation circuit 27 in this way, in the normal voltage range where the Zener diode ZD1 is not conductive, the base current of Q5 flows through the resistor R6, so that the divided voltage (X The point voltage is substantially equal to the output voltage of the tertiary rectifying / smoothing circuit 22. Therefore, the voltage at the point X varies according to the variation in the output voltage of the tertiary rectification smoothing circuit 22 (voltage variation between the output terminals 32a and 32b). In this state, the steady voltage between the output terminals 32a and 32b is stabilized by the control of the switching control circuit 23.

  During reverse current operation, in a state where the output voltage of the tertiary rectifying and smoothing circuit 22 rises and the Zener diode ZD1 becomes conductive, the voltage applied across the series circuit of the resistors R1 and R2 is equal to the Zener voltage of the Zener diode ZD1. Become. Therefore, the steady voltage generating circuit 27 acts as a voltage regulator circuit, and even if the output voltage of the tertiary rectifying and smoothing circuit 22 rises further, the X point voltage becomes constant at its upper limit value, and the on-duty of the main switch element Q1 The ratio does not fall below the lower limit, and so-called gate pulse disappearance and self-excited oscillation problems can be avoided.

<< 4th Embodiment >>
Next, a synchronous rectifying forward converter according to a fourth embodiment will be described with reference to FIG.
In this example, as shown in FIG. 8, a regulator circuit is configured using an operational amplifier 31 as the steady voltage generation circuit 27. In the example shown in FIG. 7, the regulator circuit is configured by a circuit including the Zener diode ZD1 and the transistor Q5. However, in the example shown in FIG. 8, an operational amplifier 31, a transistor Q6 and their peripheral circuits are further added to form a regulator circuit. It is composed. The reference voltage Vr is applied to the non-inverting input terminal of the operational amplifier 31 and the divided voltage of the tertiary rectification smoothing circuit 22 by the resistors R8 and R9 is applied to the inverting input terminal, and is connected between the Zener diode ZD1 and the ground. The circuit is configured such that the transistor Q6 is driven by the operational amplifier 31. Other configurations are the same as those in the first to third embodiments.

  With this configuration, when the output voltage of the tertiary rectifying / smoothing circuit 22 rises excessively, the divided voltage by the resistors R8 and R9 rises above the reference voltage Vr and the output of the operational amplifier 31 becomes low level. Q6 is turned on and the Zener diode ZD1 becomes conductive. As a result, the on-resistance of the transistor Q5 increases, and the rise of the divided output (point X voltage) of the resistors R1 and R2 is limited. Thereby, the disappearance of the so-called gate pulse and the self-excited oscillation can be prevented.

  Thus, by using the operational amplifier that differentially amplifies compared with the reference voltage, it is possible to increase the voltage regulation during the reverse current operation as compared with the third embodiment.

<< Fifth Embodiment >>
Next, a synchronous rectifying forward converter according to a fifth embodiment will be described with reference to FIGS.
As shown in FIG. 9, this synchronous rectification type forward converter constitutes a regulator circuit including transistors Q5 and Q6 and a Zener diode ZD1 as a steady voltage generation circuit 27, and is provided with a reverse current detection circuit 30. The configuration of the tertiary rectifying / smoothing circuit 22 is the same as that shown in FIG. Other configurations are the same as those in the first to fourth embodiments.

  FIG. 10 is a specific example of the reverse current detection circuit 30 shown in FIG. Here, the point A is connected to the base of the transistor Q6 shown in FIG. L2 is connected to one end of the choke coil L2 in the tertiary rectifying / smoothing circuit 22 (respective anodes of the rectifying diode D2 and the commutation diode D1). The resistor R4 is connected to the gate of the main switch element Q1 or the output portion of the same timing signal as the gate signal. The configuration of the reverse current detection circuit 30 is the same as that of the conventional reverse current detection circuit 25 shown in FIG. 1, except that the point A is connected to the base of the transistor Q6 of the regulator circuit shown in FIG. .

  In a state where no reverse current is generated, a predetermined voltage is charged in the capacitor C4. Therefore, the potential at the point A is maintained at a high level, and the transistor Q6 shown in FIG. 9 is maintained in an off state. Therefore, the transistor Q5 is turned on, and the divided voltage (X-point voltage) by the resistors R1 and R2 varies according to the output voltage of the tertiary rectification smoothing circuit 22. When a reverse current is generated, the potential at the point A is lowered and the transistor Q6 is turned on. Therefore, the on-resistance of the transistor Q5 is increased, and further increase in the voltage at the point X is restricted. That is, it operates as a regulator circuit. Since the regulator circuit operates only during reverse current operation in this way, it is possible to prevent malfunction when the output voltage of the tertiary rectification smoothing circuit 22 rises due to other factors such as a sudden change in input voltage or a sudden change in load. That is, when the output voltage of the tertiary rectifying / smoothing circuit 22 increases due to a factor other than the reverse current, the voltage is not limited to the upper limit value, and the switching is performed in response to the output voltage increase of the tertiary rectifying / smoothing circuit 22. Control is performed properly. Therefore, it is possible to stabilize the output voltage over a wide range.

  FIG. 11 shows waveforms of the output voltage of the tertiary rectifying / smoothing circuit 22 shown in FIG. 10, the voltage (Y-point voltage) Vy of the capacitor C4, and the X-point voltage Vx in FIG. Here, (A) is during reverse current operation, and (B) is during transient response. However, the span of the time axis in (A) is on the order of switching cycles, and (B) is on the order of several times to several tens of times longer than that, for example. During the reverse current operation, the voltage of the capacitor C4 of the reverse current detection circuit 30 shown in FIG. 10 decreases with the period of the switching frequency, but the X point voltage Vx maintains a constant voltage by the regulator operation of the steady voltage generation circuit 27. On the other hand, when the output voltage of the tertiary rectifying / smoothing circuit fluctuates transiently, the X point voltage Vx changes according to the voltage signal from the change voltage detection circuit 28. That is, it responds to the transient fluctuation.

  FIG. 12 shows a configuration example of a reverse current detection circuit different from that shown in FIG. Here, the point A is a point A connected to the base of the transistor Q6 shown in FIG. In a normal state, a current flows from the source of the main switch element Q1 toward the input terminal 21b (direction indicated by the solid line arrow). At this time, since the base potential of the transistor Q7 is lower than the emitter potential, Q7 is kept off. Therefore, the transistor Q6 shown in FIG. 9 is kept off.

  During reverse current operation, current flows from the negative input terminal 21b toward the source of the main switch element Q1, as indicated by the dashed arrow in FIG. Further, since the output voltage of the tertiary rectifying / smoothing circuit is also higher than normal, a current flows in the direction of the resistor R11 → the transistor Q7 → the main switch element Q1. As a result, the transistor Q7 becomes conductive, and the point A becomes a low level. Therefore, the steady voltage generation circuit 27 shown in FIG. 9 operates as a regulator. During reverse current operation, a current also flows in the direction of the resistor R11 → the diode D8 → the resistor R10 → the main switch element Q1. The diode D8 limits the upper limit of the base potential of the transistor Q7 and prevents reverse current from Q1 to the tertiary rectifying and smoothing circuit direction via the resistors R10 and R11 during reverse current operation.

10 is a circuit diagram illustrating a configuration of a converter according to Patent Document 1. FIG. It is a wave form diagram of each part of the converter. It is a block diagram of the synchronous rectification type forward converter concerning a 1st embodiment. It is a specific circuit diagram of the converter. 4 is a waveform diagram of each part. It is a circuit diagram of the synchronous rectification type | mold forward converter which concerns on 2nd Embodiment. 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 figure which shows the structure of a reverse current detection circuit. 9 is a waveform diagram of each part. It is a figure which shows the other structure of a reverse current detection circuit.

Explanation of symbols

21-input terminal 22-3 tertiary rectification smoothing circuit 23-switching control circuit 24-signal transmission circuit 25, 30-reverse current detection circuit 26-synchronous rectification circuit 27-steady voltage generation circuit 28-change voltage detection circuit 29-voltage output Circuit 31-Operational amplifier 32-Output terminal 100, 101-Synchronous rectification type forward converter T1-Main transformer N1-1 Primary winding N2-2 Secondary winding N3-3 Tertiary winding Q1-Main switch element Q2-Rectification switch element Q3 -Commutation switch element Q4- Commutation switch turn-off control switch element L1, L2- Choke coil C2- Smoothing capacitor

Claims (4)

A transformer having a primary winding, a secondary winding, and a tertiary winding, a main switch element connected in series to the primary winding of the transformer, and a series connection to the secondary winding of the transformer The choke coil, the smoothing capacitor connected in parallel between the output terminals, and the rectifier switch connected in series to the secondary winding of the transformer and turned on / off in synchronization with the on / off of the main switch element And a commutation switch element that forms an excitation energy discharge path of the choke coil by being turned on and off in synchronization with on and off of the main switch element, and a control voltage applied to a control voltage input unit In a synchronous rectification forward converter comprising a switching control circuit that performs switching control of the main switch element so as to perform negative feedback operation,
A steady voltage generation circuit for applying a constant DC voltage to the control voltage input unit of the switching control circuit;
A tertiary rectifying and smoothing circuit for rectifying and smoothing the induced voltage of the tertiary winding of the transformer;
A change voltage detection circuit that detects a transient change voltage of the output voltage of the tertiary rectification smoothing circuit and applies the change voltage to a control voltage input unit of the switching control circuit;
A synchronous rectification type forward converter characterized by comprising: A transformer having a primary winding, a secondary winding, and a tertiary winding, a main switch element connected in series to the primary winding of the transformer, and a series connection to the secondary winding of the transformer The choke coil, the smoothing capacitor connected in parallel between the output terminals, and the rectifier switch connected in series to the secondary winding of the transformer and turned on / off in synchronization with the on / off of the main switch element And a commutation switch element that forms an excitation energy discharge path of the choke coil by being turned on and off in synchronization with on and off of the main switch element, and a control voltage applied to a control voltage input unit In a synchronous rectification forward converter comprising a switching control circuit that performs switching control of the main switch element so as to perform negative feedback operation,
A tertiary rectifying and smoothing circuit for rectifying and smoothing the induced voltage of the tertiary winding of the transformer;
A change voltage detection circuit that detects a transient voltage change component of an output voltage of the tertiary rectification smoothing circuit and applies the voltage change component to a control voltage input unit of the switching control circuit;
A steady voltage generating circuit that limits the upper limit of the output voltage of the tertiary rectifying and smoothing circuit to a predetermined upper limit value and that supplies the control voltage input unit of the switching control circuit;
A synchronous rectification type forward converter characterized by comprising:   3. The synchronous rectification type circuit according to claim 2, wherein the steady voltage generation circuit is a regulator circuit that compares the output voltage of the tertiary rectification smoothing circuit with a reference voltage and stabilizes the output voltage of the tertiary rectification smoothing circuit. Forward converter.   4. The synchronous rectification according to claim 3, further comprising a reverse current detection circuit that detects a reverse current state in which a current flows backward from a load circuit connected between the output terminals and operates the regulator circuit only in the reverse current state. Type forward converter.

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