JP2010124613A - Switching power supply unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small switching power supply unit which can quickly converge the self-oscillating phenomenon of a synchronous rectifying circuit to be stopped, regardless of the operation state, immediately prior to stoppage of the on/off operation of a main switching element, even if it is stopped, the setting of an output smoothing circuit and other conditions. <P>SOLUTION: The switching power supply unit includes a first diode DB6 in which a cathode terminal is connected to the gate terminal of a commutation-side switching element TR2 and an anode terminal is connected to a source terminal. The unit further includes a third auxiliary switching element TR5, which is inserted into the connection point between an auxiliary winding T1c and the source terminal of the commutation side switching element TR2, wherein a drain terminal is connected to one end of the auxiliary winding side T1c and the source terminal is connected to the source terminal side of the commutation side switching element TR2. The output from an auxiliary pulse generation circuit DRV is connected between the gate and source of the third auxiliary switching element TR5. A discharge resistor Rg2 is provided for forming the charge/discharging channel of the capacitor C2, connected between the auxiliary winding T1c and the gate terminal of the commutation-side switching element TR2. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a switching power supply apparatus that converts an input voltage into a desired DC voltage and supplies electric power to an electronic device, and more particularly to a single forward switching power supply apparatus that includes a synchronous rectifier circuit.

  Conventionally, in a switching power supply device, in order to reduce the loss of the output rectifier circuit, an N-channel MOS field effect transistor (hereinafter referred to as FET) having a small conduction resistance is used as the rectifier element, and the rectifier element is used as the main switching element. Synchronous rectifier circuits that are turned on and off in synchronization with each other are used.

  In recent years, a rectifying switch that is a rectifying element using a synchronous rectifier circuit also in a single forward type isolated switching power supply device in which a main switching element is arranged on the primary side of a main transformer and a rectifying element is arranged on a secondary side. A switching power supply device in which a drive circuit for the element and the commutation side switch element is simply configured and a drive circuit that prevents an abnormal operation when the power supply is stopped have been actively proposed.

  As this type of single forward switching power supply, for example, as disclosed in Patent Document 1, the drain and gate of a rectifying switch are connected to a secondary winding of a transformer, and the drain and transformer of a commutation switch are connected. Connect one end of the secondary winding, connect the source of the commutation switch to the source of the rectification switch, connect the tertiary winding of the transformer between the gate and source of the commutation switch, and between the gate and source of the commutation switch Connect the drain and source of the first auxiliary switch to the gate, connect the drive circuit between the gate and source of this auxiliary switch, connect the capacitor for the charge pump through the tertiary winding from the gate of the commutation switch, A second auxiliary switch is provided between the gate and source of this switch, and the gate of this auxiliary switch is connected to the gate of the commutation switch. During the ON period, the gate and source of the rectifying switch are short-circuited, the source and gate of the third auxiliary switch are connected between the source and drain of the second auxiliary switch, and the drain of this third auxiliary switch is connected to the tertiary winding. There is a switching power supply device provided with a synchronous rectification drive circuit connected to.

According to this synchronous rectification drive circuit, during the normal operation in which the primary side main switch is turned on / off, the charge pump capacitor is repeatedly charged by the tertiary winding voltage when the primary side main switch is turned on. When the switch continues to be turned off (for example, when the input power is shut off and the power is stopped), the operation of charging the charge pump capacitor is stopped by continuously turning off the third auxiliary switch. As a result, the voltage between the gate and the source of the commutation switch is quickly lowered, and the self-oscillation phenomenon of the synchronous rectifier circuit is difficult to continue.
JP 2007-49800 A

  However, the synchronous rectification driving circuit of the switching power supply device disclosed in Patent Document 1 is dependent on the operation state immediately before the primary-side main switch on / off operation is stopped and the conditions such as the setting of constants of the output smoothing circuit and the like. In some cases, the self-oscillation phenomenon of the circuit cannot be quickly suppressed.

  During normal operation when the main switch turns on and off, if the commutation side switch is on and the main switch is kept off, the commutation switch will go from on to off because its gate voltage is discharged by the discharge resistor. To do. When the current of the output inductor flows from the drain terminal to the source terminal of the commutation switch immediately before turning off, the drain voltage of the commutation switch increases due to the magnetic action of the output inductor. In particular, when the commutation switch is turned off sharply, the drain voltage of the commutation switch increases with a large surge. On the other hand, the gate of the third auxiliary switch is connected to the drain of the second auxiliary switch, and is further connected to the drain of the commutation switch via a capacitor. Therefore, the drain voltage of the commutation switch accompanied by a large surge greatly increases the gate voltage of the third auxiliary switch, and eventually exceeds the gate threshold, and the third auxiliary switch is turned on. Then, the charge pump capacitor is charged, and the next turn-on is induced for the commutation switch that is turned off. As a result, the self-oscillation phenomenon of the synchronous rectifier circuit may continue. It was.

  If this self-oscillation phenomenon continues, a large current that is difficult to suppress is generated, or the main transformer is excited at a frequency lower than the normal switching frequency, causing magnetic saturation, thereby damaging the internal elements of the switching power supply device There was a fear.

  Furthermore, this self-oscillation phenomenon is more likely to occur as the output inductor is made smaller, and the self-oscillation phenomenon is more likely to occur as the capacitor capacity for absorbing general ripple / ripple noise connected to the output end or load end of the switching power supply increases. The oscillation phenomenon tends to continue. Therefore, in order to avoid the self-oscillation phenomenon, there is a problem that the output inductor is enlarged and certain restrictions are imposed on noise countermeasures for the load.

  The present invention has been made in view of the above-described background art, and when the on / off operation of the primary-side main switching element is stopped, regardless of the operation state immediately before that, the setting of the output smoothing circuit, and other conditions. An object of the present invention is to provide a small switching power supply device capable of quickly converging and stopping the self-oscillation phenomenon of a synchronous rectifier circuit.

  The present invention is connected in series to an input power source, a main switching element for intermittent input voltage, a control circuit for generating a control pulse for turning on and off the main switching element, and connected in series to the main switching element, A primary transformer provided with a primary winding to which an intermittent voltage generated by the interruption of the main switching element is applied, and a secondary winding for generating an AC voltage obtained by transforming the intermittent voltage, and the secondary winding Output terminal of the secondary winding, the drain terminal being connected to one end which is on the low potential side during the ON period of the main switching element and the gate terminal connected to the other end, and the output of the secondary winding A commutation side switch element having a drain terminal connected to the other end of the end and a source terminal connected to a source terminal of the rectifying side switch element; a smoothing inductor; A voltage dividing circuit comprising a slipping capacitor, a switching circuit including a smoothing circuit having an input end connected between a drain and a source of the commutation side switch element and having both ends of the smoothing capacitor as output ends It is a power supply device. Further, the control pulse generated by the control circuit shows a low level during the low level, and an auxiliary pulse for generating an auxiliary pulse that is inverted to a high level with a time difference as short as possible immediately before the control pulse is inverted to a high level. A pulse generator and a third winding provided in the main transformer, one end of which is on the low potential side when the main switching element is on, the gate of the commutation side switching element via a capacitor An auxiliary winding connected to the terminal, the other end connected to the source terminal of the commutation side switch element, a drain terminal connected to the gate terminal of the rectification side switch element, and a source terminal connected to the rectification side switch A first auxiliary switch element connected to the source terminal of the element; a cathode terminal connected to the gate terminal of the first auxiliary switch element; and an anode terminal A backflow prevention diode connected to the gate terminal of the commutation side switch element, a drain terminal connected to the gate terminal of the first auxiliary switch element, and a source terminal connected to the source terminal of the first auxiliary switch element A second auxiliary switch element in which the output of the auxiliary pulse generation circuit is connected between the gate terminal and the source terminal, a cathode terminal is connected to the gate terminal of the commutation side switch element, and an anode terminal is the commutation A first diode connected to the source terminal of the side switch element, and the main switching element, the rectifier side switch element, and the commutation side switch element are turned on and off in a complementary manner It is a power supply device. And it is inserted at the connection point between the auxiliary winding and the source terminal of the commutation side switch element, the drain terminal is connected to one end of the auxiliary winding side, and the source terminal is the source terminal of the commutation side switch element A third auxiliary switch element connected between the gate and the source, the cathode terminal being connected to the drain terminal of the third auxiliary switch element, and the anode terminal being A second diode connected to the source terminal of the third auxiliary switch element, and a discharge path for the capacitor charge connected between the auxiliary winding and the gate terminal of the commutation side switch element are formed. A switching power supply device including a discharging resistor.

  Further, a time constant setting resistor connected in series with the backflow prevention diode, a drain terminal is connected to a gate terminal of the commutation switch element, a source terminal is connected to a source terminal of the commutation switch element, and a gate A fourth auxiliary switch element having an output of the auxiliary pulse generation circuit connected between a terminal and a source terminal, and the control circuit outputs a control pulse capable of turning on and off the main switching element The time constant setting resistor limits the rate of rise of the gate-source voltage of the first auxiliary switch element and continues turning off the first auxiliary switch element.

  The first diode is a parasitic diode formed between a drain and a source in the fourth auxiliary switch element. The second diode is a parasitic diode formed between a drain and a source in the third auxiliary switch element.

  According to the switching power supply device of the present invention, the third auxiliary switch element has a configuration in which the gate terminal is connected to the output of the auxiliary pulse generation circuit and is driven by the auxiliary pulse. The three auxiliary switch elements are reliably kept off, and the charging operation to the capacitor connected to the gate terminal of the commutation side switch element is stopped. Therefore, the decrease in the gate-source voltage of the commutation side switch element is accelerated, and the self-oscillation phenomenon of the synchronous rectifier circuit can be quickly converged and stopped.

  Also, after providing a fourth auxiliary switch element that short-circuits between the gate and source of the commutation side switch element at high speed, a time constant setting resistor is inserted in series with the reverse current prevention diode, and during normal operation of the power supply If the first auxiliary switch element is always turned off and can be turned on only when the power is stopped, the malfunction of the rectifying switch element due to the delay in turn-off of the first auxiliary switch element during normal operation While the power supply is stopped, the first auxiliary switch element is turned on to suppress the turn-on of the rectifying switch element, and the self-oscillation phenomenon can be quickly converged and stopped.

  Hereinafter, a switching power supply device 10 according to a first embodiment of the present invention will be described with reference to FIGS. 1 and 2. As shown in the circuit diagram of FIG. 1, in the switching power supply device 10, a series circuit of a primary winding T1a of a transformer T1 and a main switching element TR0 that is an FET is connected to both ends of a DC input power supply Ein. Further, the transformer T1 is provided with a secondary winding T1b that generates an AC voltage obtained by transforming a rectangular intermittent voltage applied to the primary winding T1a by the on / off operation of the main switching element TR0, and both ends thereof are provided. The rectification side switch element TR1 and the commutation side switch element TR2 for rectifying the AC voltage are connected. The rectifying switch element TR1 has a drain terminal connected to one end that is on the low potential side during the ON period of the main switching element TR0 among the output ends of the secondary winding T1b. The commutation side switch element TR2 has a drain terminal connected to the other end of the output end of the secondary winding T1b, and a source terminal connected to the source terminal of the rectification side switch element TR1. Further, a series circuit of a smoothing inductor Lo and a smoothing capacitor Co is connected between the drain and source of the commutation side switch element. A DC output voltage Vout is generated at both ends of the smoothing capacitor Co, and a load LD is connected to supply an output current. That is, as described above, the switching power supply device 10 has a configuration in which the power conversion method is the single forward method and the FET is used as the rectifying element on the secondary side.

  A control circuit CON is connected to the gate of the main switching element TR0, and the control circuit CON outputs a control pulse VCON to control the main switching element TR0 on and off. The control pulse VCON is a rectangular pulse voltage that is pulse width modulated to stabilize the output voltage Vout to a predetermined voltage value, and turns on the main switching element TR0 when it is at a high level and turns it off when it is at a low level. be able to.

  In order to perform the single forward type rectification operation, the rectification side switch element TR1 is turned on / off in substantially the same phase as the main switching element TR0, and the commutation side switch element TR2 is turned on / off complementarily with the main switching element TR0. Drive like so. Hereinafter, a configuration of a circuit portion related to driving of the switch elements TR1 and TR2 will be described.

  The switching power supply device 10 includes an auxiliary pulse generation circuit DRV, and as shown in FIG. 1, its input terminal is connected to the control circuit CON, and its output terminal is between the gates and sources of auxiliary switch elements TR4 to TR6 described later. It is connected. The auxiliary pulse generation circuit DRV receives the control pulse VCON generated by the control circuit CON, outputs an auxiliary pulse VDRV insulated from the primary side circuit via an insulating means such as a transformer, and outputs auxiliary switch elements TR4 to TR6 described later. It works to turn on and off. The auxiliary pulse VDRV is a rectangular pulse voltage that exhibits a low level during the period when the control pulse VCON is at a low level, and that is inverted to a high level with a time difference as short as possible immediately before the control pulse VCON is inverted to a high level. is there.

  The gate terminal of the rectifying switch element TR1 is connected to one end of the output terminal of the secondary winding T1b which is on the high potential side while the main switching element is on, and the speed-up capacitor C1 and the peak current limiter. The resistors R1 are connected via a series circuit. Note that the capacitor C1 and the resistor R1 do not affect the basic operation of the present invention, and therefore may be short-circuited if necessary.

  The main transformer T1 is provided with an auxiliary winding T1c, which is a third winding, and one end on the low potential side during the period when the main switching element TR0 is on is connected in series with a capacitor C2 and a resistor R2 for suppressing surge current. The other terminal is connected to the source terminal of the commutation side switch element TR2 via a third auxiliary switch TR4 described later via a circuit. The resistor R2 has a small resistance value that does not affect the basic operation of the present invention, and may be removed as necessary.

  A drain terminal and a source terminal of the first auxiliary switch TR3 are connected between the gate and source of the rectifying switch element TR1, and a resistor Rg1 for absorbing noise such as static electricity is further connected. The resistor Rg1 has a large resistance value that does not affect the basic operation of the present invention, and may be removed as needed.

  A series circuit of a backflow prevention diode D1 and a time constant setting resistor R3 is connected in the direction from the gate terminal of the commutation switch element TR2 to the gate terminal of the first auxiliary switch element TR3. The time constant setting resistor R3 is in a current flow path for charging a parasitic capacitor Cg3 formed between the gate and the source in the first auxiliary switch element TR3. By adjusting the resistance, The rate at which the gate-source voltage Vg3 of the auxiliary switch element TR3 rises can be changed.

  The drain terminal and the source terminal of the second auxiliary switch element TR4 are connected between the gate and source of the first auxiliary switch element TR3, and further, the auxiliary terminal is connected between the gate and source of the second auxiliary switch element TR4. The output terminal of the pulse generation circuit DRV is connected.

  A third auxiliary switch element TR5 is inserted between the auxiliary winding T1c and the source terminal of the commutation side switch element TR2. The third auxiliary switch element TR5 has a drain terminal connected to the auxiliary winding T1c, a source terminal connected to the source terminal side of the commutation side switch element TR2, and an auxiliary pulse generating circuit between the gate and the source. The output terminal of DRV is connected.

  Between the gate and source of the commutation side switch element TR2, a fourth auxiliary switch element TR6 and a discharge resistor Rg2 that forms a path for discharging the charge of the capacitor C2 are connected. The fourth auxiliary switch element TR6 has a drain terminal connected to the gate terminal of the commutation switch element TR2 via a resistor R4 for suppressing surge current, and a source terminal connected to the source terminal of the commutation switch element TR2. The output terminal of the auxiliary pulse generating circuit DRV is connected between the gate and the source. The discharge resistor Rg2 may be any resistor that forms the discharge path of the capacitor C2, and may be a resistor connected to both ends of the capacitor C2, for example. The resistor R4 has a small resistance value that does not affect the basic operation of the present invention, and may be short-circuited if necessary.

  The main switching element TR0, the rectifying side switching element TR1, the commutation side switching element TR2, the first auxiliary switching element TR3, the second auxiliary switching element TR4, the third auxiliary switching element TR5, and the fourth auxiliary switching element. All TR6 are FETs, and in each element, parasitic diodes DB0, DB2, DB3, DB4, DB5, DB6 are formed in the direction from the source to the drain. Therefore, if this parasitic diode is used, for example, the trouble of adding a dedicated diode from the outside to the locations of the parasitic diodes DB5 and DB6 can be saved. The parasitic diode DB6 indicates the first diode of the present invention, and the parasitic diode DB5 indicates the second diode.

  Next, the operation of the switching power supply device 10 having the above configuration will be described based on the time chart of FIG. In period 1, the control pulse VCON generated by the control circuit CON repeats high / low levels, whereby the main switching element TR1 is turned on / off, and the switching power supply device 10 performs a normal power conversion operation. The auxiliary pulse generation circuit DRV indicates a low level during a period in which the control pulse VCON is at a low level, and is inverted to a high level a little earlier than the control pulse VCON is inverted to a high level. Providing this slight time difference is a well-known technique for preventing the main switching element TR0 and the commutation side switching element TR2 from being turned on simultaneously. Further, the time for which the high level is continued does not need to be strictly defined as long as it is a certain time or longer. The time for which the high level is continued will be described later.

  The gate-source voltage Vg1 of the rectifying switch element TR1 is generated using a rectangular AC voltage generated in the secondary winding T1b. Therefore, the rectifying switch element TR1 is turned on / off at substantially the same timing and in the same phase as the main switching element TR0.

  On the other hand, the timing at which the gate-source voltage Vg2 of the commutation side switch element TR2 is inverted from the high level to the low level is determined according to the timing at which the auxiliary pulse VDRV generates the high level. On the other hand, the timing of inversion from the low level to the high level is determined according to the timing at which the substantially rectangular voltage generated in the auxiliary winding T1c generates a high potential on the capacitor C2, that is, the timing at which the main switching element TR0 is turned on. The

  When the auxiliary pulse VDRV is inverted from the low level to the high level, the fourth auxiliary switch element TR6 is turned on, and the gate and the source of the commutation side switch element TR4 are short-circuited via the resistor R4. Since the resistor R4 has a sufficiently small resistance value, the gate-source voltage Vg2 drops to a low level in a very short time.

  In addition, when the auxiliary pulse VDRV is inverted from the low level to the high level, the third auxiliary switch element TR5 is also turned on, and a current in the direction of the source terminal of the commutation side switch element flows from one end of the auxiliary winding T1c. It becomes possible.

  When the auxiliary pulse VDRV is inverted from the low level to the high level, the second auxiliary switch element TR4 is also turned on, and the gate-source voltage Vg3 of the first auxiliary switch element TR3 is instantaneously lowered to the low level.

  After the auxiliary pulse VDRV is inverted from the low level to the high level, after a short time, the control pulse VCON is inverted to the high level, and the main switching element TR0 is turned on. Then, the gate-source voltage Vg1 of the rectifying switch element TR1 becomes high level, and the rectifying switch element TR1 is turned on.

  At the same time, a voltage is generated at both ends of the auxiliary winding T1c so that the drain terminal side of the third auxiliary switch element TR5 has a high potential. Then, current flows through the path of the auxiliary winding T1c, the third auxiliary switch element TR5, the fourth auxiliary switch element TR6 or the parasitic diode DB6, the resistor R4, the resistor R2, and the capacitor C2, and charges the capacitor C2. The charging operation is completed in a very short time, and the voltage at both ends of the capacitor C2 is approximately equal to the voltage generated by the auxiliary winding T1c and has a high potential on the gate terminal side of the commutation side switch element TR2. Will occur.

The above-described high level duration of the auxiliary pulse VDRV is set so that the on state is maintained at least until the charging operation of the capacitor C2 is completed after the third auxiliary switch element TR5 is turned on. Has been. Then, when the high level time of the control pulse VDVR elapses and the level is inverted to the low level, each auxiliary switch element TR4, TR5, TR6 turns off, but changes to the operation of other switch elements at the timing when they turn off. Does not occur. For example, when the third auxiliary switch element TR5 is turned off, as shown in FIG. 2, a predetermined voltage is induced in the drain-source voltage Vd5, but the on / off state of other switch elements is reversed. Next, when the control pulse VCON is inverted from the low level to the high level, the main switching element TR0 is turned off. Then, the gate-source voltage Vg1 of the rectifying switch element TR1 becomes low level, and the rectifying switch element TR1 is turned off.

  At the same time, a voltage having a high potential on the capacitor C2 side is generated at both ends of the auxiliary winding T1c. Then, current flows between the auxiliary winding T1c, the capacitor C2, the resistor R2, the gate-source of the commutation side switch element TR2, and the path of the parasitic diode DB5, and the commutation side switch element TR2 has a gate-source voltage Vg2. High level turns on. The gate-source voltage Vg2 instantaneously rises to a voltage obtained by adding the voltage generated in the auxiliary winding T1c and the voltage across the capacitor C2 charged while the main switching element TR0 is on. Thereafter, the accumulated charge in the capacitor C2 is gradually discharged by the resistor Rg2 and the like, and the gate-source voltage Vg2 gradually decreases, but the voltage value sufficient to continue the ON state of the commutation side switch element TR2 is maintained. is doing.

  As described above, the gate-source voltage Vg1 of the rectifying switch element TR1 is generated using a rectangular AC voltage generated in the secondary winding T1b, and the first auxiliary switch element TR3 contributes. Not done. In the first auxiliary switch element TR3, as the gate-source voltage Vg2 of the commutation side switch element TR2 increases, the parasitic capacitor Cg3 is charged by the current flowing through the path of the backflow prevention diode D1 and the time constant setting resistor R3. The gate-source voltage Vg3 increases. However, the time constant determined by the time constant setting resistor R3 and the parasitic capacitor Cg3 is set to a relatively large value, thereby limiting the rising speed of the gate-source voltage Vg3. Therefore, the gate-source voltage Vg3 does not reach the gate threshold value Vth3 at which the first auxiliary switch element TR3 can be turned on until the second auxiliary switch element TR4 is turned on. That is, during the normal operation in the period 1, the first auxiliary switch element TR3 continues to be in the off state and is not involved in the operation of turning off the rectifying side switch element TR1.

  Next, an operation in the period 2 is described. In the period 2, for example, the supply of the input power source Ein of the switching power supply device 10 is interrupted, the control circuit CON stops generating the control pulse VCON (continues the low level), and the auxiliary pulse VCRV is also at the low level. It is a continuing period. Period 2 starts from a state in which the control pulse VCON is inverted from the high level to the low level. In the commutation side switch element TR2, the high-level gate-source voltage Vg2 is gradually discharged and continues to decrease. During this time, a large drain current Id2 in the direction from the drain terminal to the source terminal flows through the commutation side switch element TR2 due to the magnetic action of the smoothing inductor. On the other hand, the gate-source voltage Vg3 of the first auxiliary switch element TR3 continues to rise gently, and stops rising when it reaches the gate-source voltage Vg2 of the commutation side switch element TR2 that continues to fall. Even when the gate-source voltage Vg2 further decreases, the voltage value of the gate-source voltage Vg2 is maintained by the function of the backflow prevention diode D1.

  When the gate-source voltage Vg2 drops to a predetermined voltage value, the commutation side switch element TR2 shifts from on to off. Here, a large drain current Id2 flows in the direction from the drain terminal to the source terminal of the commutation side switch element TR2 immediately before turning off, and the current is cut off sharply by the commutation side switch element TR2. The drain-source voltage Vd2 of the commutation side switch element TR2 rises sharply with a large surge due to the magnetic action of the smoothing inductor Lo. The voltage accompanying this surge is transmitted to the gate terminal of the rectifying switch element TR1 through the capacitor C1 and the resistor R1. At this time, the first auxiliary switch TR3 is turned off because the gate-source voltage Vg3 has not yet reached the threshold value Vth3. Therefore, the gate-source voltage Vg1 of the rectifying switch element TR1 easily rises, and the rectifying switch element TR1 shifts from off to on.

  When the rectifying switch element TR1 is turned on, a large current flows through the path of the secondary winding T1b, the rectifying switch element TR1, and the smoothing capacitor Co due to the magnetic action of the smoothing inductor Lo, and the primary side via the transformer T1. To discharge current.

  Further, when the rectifying switch element TR1 is turned on, a voltage is generated at both ends of the auxiliary winding T1c with the drain terminal side of the third auxiliary switch element TR5 at a high potential. However, since the auxiliary pulse VDRV is stopped, the third auxiliary switch element TR5 continues to be turned off, and the capacitor C2 is not charged by the auxiliary winding T1c.

  When the smoothing inductor Lo releases the current as described above, the drain-source voltage of the commutation side switch element TR2 eventually decreases, and accordingly, the rectification side switch element TR1 has its gate-source voltage Vg1 Decrease and turn off. At the same time, a voltage having a high potential on the capacitor C2 side is generated at both ends of the auxiliary winding T1c. Then, current flows between the auxiliary winding T1c, the capacitor C2, the resistor R2, the gate-source of the commutation side switch element TR2, and the path of the parasitic diode DB5, and the commutation side switch element TR2 has a gate-source voltage Vg2. High level turns on. The gate-source voltage Vg2 rises to a voltage obtained by adding the voltage generated in the auxiliary winding T1c and the voltage across the capacitor C2. However, since the capacitor C2 is not recharged while the rectifying switch element TR1 is off, only a low voltage remains at both ends of the capacitor C2. As a result, the gate-source voltage Vg2 is lower than the previous period. It only rises to a voltage value. Therefore, the drain current id2 flowing from the drain terminal to the source terminal flowing in the commutation side switch element TR2 is suppressed from increasing from the previous period. Thereafter, the accumulated charge in the capacitor C2 is gradually discharged by the resistor Rg2 and the like, and the gate-source voltage Vg2 gradually decreases. On the other hand, the gate-source voltage Vg3 of the first auxiliary switch element TR3 rises gently, exceeds the gate threshold value Vth3 of the first auxiliary switch element TR3, continues to increase, and continues to decrease. The rise stops when the gate-source voltage Vg2 is reached. When the first auxiliary switch TR3 is turned on, the gate and the source of the rectifying switch element TR1 are almost short-circuited.

  When the gate-source voltage Vg2 drops to a predetermined voltage value, the commutation side switch element TR2 shifts from on to off. Just before turning off, the drain current Id2 flowing in the direction from the drain terminal to the source terminal of the commutation side switch element TR2 is still large as in the previous period. Therefore, the drain-source voltage Vd2 of the commutation side switch element TR2 rises sharply with a large surge. This voltage with a large surge is transmitted to the gate terminal of the rectifying switch element TR1 through the capacitor C1 and the resistor R1. However, the first auxiliary switch TR3 is turned on when the gate-source voltage Vg3 reaches the threshold value Vth3. Accordingly, since the gate-source voltage Vg1 is prevented from rising, the rectifying switch element TR1 is turned on incompletely, leaving a slightly large conduction resistance.

  When the above self-oscillation phenomenon is repeated, the rectification side switch element TR1 and the commutation side switch element TR2 cannot continue the on / off operation, and the self oscillation phenomenon converges and stops quickly.

  Next, the operation of the switching power supply device 10 according to the present embodiment is compared with the operation of the switching power supply device 20 having a configuration that does not include the configuration that characterizes the present invention. As shown in FIG. 3, the switching power supply device 20 of the comparative example does not include the third auxiliary switch TR5 and the parasitic diode DB5, which are characteristic configurations of this embodiment, and the drain terminal and the source of the third auxiliary switch TR5. The locations where the terminals were connected are directly connected. Since the other configurations are the same, the same reference numerals as those of the switching power supply device 10 of this embodiment are given and the description thereof is omitted.

  FIG. 4 shows operation waveforms of the switching power supply device 20 of the comparative example. In the switching power supply 20, the gate-source voltage Vg <b> 2 decreases to a predetermined voltage value in the period 2 (the period in which the control circuit CON stops generating the control pulse and keeps the low level), and the commutation side The switch element TR2 shifts from on to off, and the rectifier side switch element TR1 is turned on by the rise of the gate-source voltage Vg1 of the rectifier side switch element TR1. At the same time, a voltage with a high potential on the drain terminal side of the third auxiliary switch element TR5 is generated at both ends of the auxiliary winding T1c. At this time, since the switching power supply device 20 is not provided with the third auxiliary switch element TR5, the path of the auxiliary winding T1c, the fourth auxiliary switch element TR6 or the parasitic diode DB6, the resistor R4, the resistor R2, and the capacitor C2 Current flows, and the operation of charging the capacitor C2 is performed. Therefore, when the self-oscillation phenomenon occurs, a voltage obtained by adding the voltage generated in the auxiliary winding T1c and the voltage across the capacitor C2 charged in the above operation is added between the gate and source of the commutation side switch element TR2. That is, a voltage that allows the commutation side switch element TR2 to be sufficiently turned on is repeatedly supplied, and the increase in the drain current Id2 is accelerated. In the switching power supply device 20, any of the internal components exceeds the absolute maximum rating and eventually breaks.

  On the other hand, in the switching power supply device 10 of the first embodiment of the present invention, in the period 2, the auxiliary winding T1c, the off operation of the third auxiliary switch element TR5 and the parasitic diode DB5 as the second diode are provided. The current path passing through the fourth auxiliary switch element TR6 or the parasitic diode DB6 as the first diode, the resistor R4, the resistor R2, and the capacitor C2 is cut off, and the charging operation of the capacitor C2 is stopped. Further, the ON operation of the first auxiliary switch TR3 functions effectively, and even if the self-oscillation phenomenon occurs, the ON / OFF operation of the rectifying side switch element TR1 and the commutation side switch element TR2 is suppressed, and promptly The self-oscillation phenomenon can be converged and stopped.

  Next, a switching power supply device 30 according to a second embodiment of the present invention will be described with reference to FIG. In the switching power supply 30, the fourth auxiliary switch TR6 (including the parasitic diode DB6) and the resistor R4 are opened and removed, and the diode D2 extends from the source terminal to the gate terminal between the gate and source of the commutation side switch element TR2. Is connected, and the configuration of the switching power supply device 10 is different in that the time constant setting resistor R3 is short-circuit removed. That is, in the switching power supply device 10, the function of the second auxiliary switch element TR4 that short-circuits between the gate and source of the first auxiliary switch TR3 and the fourth that short-circuits between the gate and source of the commutation side switch element TR2. In the switching power supply device 30, the functions of the auxiliary switch TR6 are concentrated in the second auxiliary switch element TR4. Further, in the switching power supply device 30, the time constant setting resistor R3 is removed by short-circuiting (or made to have a sufficiently small resistance value), and the commutation side switch element TR2 is maintained at a high speed of turn-off and deleted. The same function as that performed by the parasitic diode DB6 is secured by adding the diode D2. In addition, since the other structure is the same, the same code | symbol as the switching power supply device 10 is attached | subjected and description is abbreviate | omitted.

  In general, many small and inexpensive FETs have a relatively large conduction resistance Ron. However, the switching power supply 10 is a case where such a general FET is used for the first auxiliary switch TR3. In addition, the circuit is configured so that the rectifying switch element TR1 can operate as intended. Therefore, the degree of freedom in selecting the FET is high.

  On the other hand, the switching power supply 30 has a circuit configuration that can reduce the number of parts by using an FET having a relatively low conduction resistance Ron as the first auxiliary switch TR3.

  In the case of the switching power supply 30, during normal operation (corresponding to period 1 in FIG. 2), if the auxiliary switch TR3 has a delay in turn-off, the turn-on of the rectifying switch element TR1 is delayed, which hinders the power conversion operation. In many cases, the high-level peak value of the gate-source voltage Vg3 is limited to a low level.

  On the other hand, the FET has a general property that the conduction resistance Ron relatively increases as the applied voltage between the gate and the source decreases. Therefore, for example, if the conduction resistance Ron of the auxiliary switch TR3 is too large, when the self-oscillation phenomenon occurs, the short circuit between the gate and the source of the rectifying switch element TR1 becomes insufficient, and the drain of the commutation switch element TR2 The gate-source voltage Vg1 rises due to the surge voltage generated between the sources, and the rectifying switch element TR1 is likely to be turned on, and the effect of suppressing the self-oscillation phenomenon may be reduced.

  Therefore, in the switching power supply device 30, if a FET having a sufficiently small conduction resistance Ron is selected even if the gate-source voltage is low, the circuit configuration is simplified in relation to the setting of the gate-source voltage Vg 3. During normal operation, the same operation as that of the switching power supply device 10 is performed when the power supply is stopped while avoiding the malfunction of the rectifying switch element TR1 due to the turn-off delay of the first auxiliary switch element TR3. And self-oscillation can be quickly converged and stopped.

1 is a circuit diagram showing a first embodiment of a switching power supply device of the present invention. It is a time chart which shows operation | movement of 1st embodiment. It is a circuit diagram which shows the switching power supply device of the comparative example which is not provided with the characteristic structure of the switching power supply device of 1st embodiment. It is a time chart which shows operation | movement of the switching power supply device of the comparative example shown in FIG. It is a circuit diagram which shows 2nd embodiment of the switching power supply device of this invention.

Explanation of symbols

10, 20, 30 Switching power supply device CON Control circuit C1, C2 Capacitor Cg3 Parasitic capacitor Co Smoothing capacitor D1 Backflow prevention diode D2 Diode DB1, DB2, DB3, DB4, DB5, DB6 Parasitic diode DRV Correction pulse generation circuit Ein Input power supply Lo Smoothing inductor LD Load R1, R2, R4 Resistor R3 Time constant setting resistor Rg2 Discharging resistor T1 Main transformer T1a Primary winding T1b Secondary winding T1c Auxiliary winding TR0 Main switching element TR1 Rectification side switch element TR2 Commutation side Switch element TR3 First auxiliary switch TR4 Second auxiliary switch TR5 Third auxiliary switch TR6 Fourth auxiliary switch

Claims (4)

A main switching element connected in series to an input power source, and a control circuit for generating a control pulse for turning on and off the main switching element, and a main switching element connected in series. A primary winding to which an intermittent voltage generated by the intermittent is applied, a main transformer provided with a secondary winding for generating an AC voltage obtained by transforming the intermittent voltage, and an output terminal of the secondary winding. Among them, the drain terminal is connected to one end which is on the low potential side while the main switching element is on, and the rectifying side switch element connected to the gate terminal to the other end, and the other end of the output of the secondary winding A commutation-side switch element having a drain terminal connected to one end and a source terminal connected to the source terminal of the rectifying-side switch element, a smoothing inductor, and a smoothing capacitor A voltage dividing circuit including a smoothing circuit having an input end connected between a drain and a source of the commutation-side switch element and having both ends of the smoothing capacitor as output ends, and the control circuit generates An auxiliary pulse generating circuit that generates an auxiliary pulse that is inverted to a high level with a time difference as short as possible immediately before the control pulse is inverted to a high level. A third winding provided in the transformer, one end of which is on the low potential side while the main switching element is on is connected to the gate terminal of the commutation-side switch element via a capacitor; One end of the auxiliary winding connected to the source terminal of the commutation side switch element, the drain terminal connected to the gate terminal of the rectification side switch element, and the source terminal connected to the rectification side The first auxiliary switch element connected to the source terminal of the switch element, the cathode terminal is connected to the gate terminal of the first auxiliary switch element, and the anode terminal is connected to the gate terminal of the commutation side switch element A backflow prevention diode, a drain terminal is connected to a gate terminal of the first auxiliary switch element, a source terminal is connected to a source terminal of the first auxiliary switch element, and the auxiliary pulse is generated between the gate terminal and the source terminal A second auxiliary switch element connected to the output of the circuit; a cathode terminal connected to the gate terminal of the commutation side switch element; and an anode terminal connected to the source terminal of the commutation side switch element. A diode, and the main switching element, the rectifying side switching element, and the commutation side switching element are complementarily turned on / off In the synchronous rectification type switching power supply
Inserted at the connection point between the auxiliary winding and the source terminal of the commutation side switch element, the drain terminal is connected to one end of the auxiliary winding side, and the source terminal is connected to the source terminal side of the commutation side switch element A third auxiliary switch element connected and connected between the gate and source of the output of the auxiliary pulse generation circuit;
A second diode having a cathode terminal connected to a drain terminal of the third auxiliary switch element and an anode terminal connected to a source terminal of the third auxiliary switch element;
A switching power supply device comprising: a discharge resistor that forms a discharge path for the capacitor charge connected between the auxiliary winding and the gate terminal of the commutation-side switch element. A time constant setting resistor connected in series with the backflow prevention diode;
A drain terminal is connected to the gate terminal of the commutation switch element, a source terminal is connected to a source terminal of the commutation switch element, and an output of the auxiliary pulse generation circuit is connected between the gate terminal and the source terminal. Auxiliary switch elements are provided,
When the control circuit outputs a control pulse capable of turning on and off the main switching element, the time constant setting resistor limits the rate of rise of the gate-source voltage of the first auxiliary switch element. 2. The switching power supply device according to claim 1, wherein the first auxiliary switch element is continuously turned off.   3. The switching power supply device according to claim 2, wherein the first diode is a parasitic diode formed between a drain and a source in the fourth auxiliary switch element. 4. The switching power supply device according to claim 1, wherein the second diode is a parasitic diode formed between a drain and a source in the third auxiliary switch element. 5.

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