JP4485404B2 - Self-excited switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device that converts a DC input voltage into a desired output voltage and supplies it to an electronic device, and relates to a self-excited switching power supply device that turns on a control terminal of a switching element by a resonance voltage of a sub winding.

  As shown in FIG. 5 and Patent Document 1, the conventional self-excited switching power supply device has a circuit configuration between a pair of input terminals Vin to which a DC voltage is input, a primary winding Np of a transformer T, a MOS-FET, and the like. Switching elements Tr1 are connected in series. Furthermore, between the pair of input terminals Vin, one end of the resistor R1 is connected to the + side, the other end of the resistor R1 is connected to the cathode of the Zener diode ZD1, and the anode is connected to the − side of the input terminal Vin. . Furthermore, the cathode of the Zener diode ZD1 is connected to the gate which is the control terminal of the switching element Tr1 via the resistor R2.

  The secondary winding Ns of the transformer T is adjacent to the primary winding Np, the terminal on the non-dotted side of the secondary winding Ns is connected to the anode of the diode D2, and the cathode is the + of the pair of output terminals Vo. Connected to the side. A capacitor C4 is connected between the cathode of the diode D2 and the dot-attached terminal of the secondary winding Np, and a series circuit composed of the resistor R9, the LED of the photocoupler PC, and the shunt regulator SR is in parallel with the capacitor C4. It is connected to the. In this series circuit, the anode terminal of the LED in the photocoupler PC is connected to the resistor R9, and the cathode is connected to the cathode of the shunt regulator SR. The anode of the shunt regulator SR is connected to the dot side of the secondary winding Ns. Further, output resistors R10 and R11 are connected in series between the output terminals Vo, and the midpoint thereof is connected to the control terminal of the shunt regulator SR.

  The transformer T is provided with a sub-winding Nsub. Both ends of the sub-winding Nsub are connected to a series circuit of a capacitor C2, a resistor R7, and a diode D1, and the anode of the diode D1 has no dot of the sub-winding Nsub. And a negative terminal of the input terminal Vin. One terminal of a series circuit of a capacitor C3 and a resistor R3 is connected to the terminal on the dot side of the sub winding Nsub, and the other end of this series circuit is connected to the control terminal of the switching element Tr1.

  The control terminal of the switching element Tr1 is connected to the negative side of the input terminal Vin via a resistor R8, the collector of the npn transistor Tr2 is connected, and the emitter thereof is connected to the negative input terminal -Vin. ing. The base of the transistor Tr2 is connected to the dot-side terminal of the sub-winding Nsub via the resistor R4 and the Zener diode ZD2, and is connected to the dot-side of the sub-winding Nsub via the resistor R5. ing. Further, the base of the transistor Tr2 is connected to the input terminal −Vin via the capacitor C1 and to the emitter of the phototransistor of the photocoupler PC. The collector of the phototransistor of the photocoupler PC is connected to the cathode of the diode D1, and is connected to the dot-side of the sub winding Nsub via the resistor R6.

  As shown in FIG. 6, the operation of this conventional self-excited switching power supply device is as follows. When an input voltage is applied between a pair of input terminals Vin during startup, the input voltage is applied to the switching element Tr1 via resistors R1 and R2. In addition to the control terminal, the switching element Tr1 is turned on and a current Id flows. As a result, the input voltage Vi is applied to the primary winding Np, a voltage is induced between the sub-windings Nsub, a voltage due to positive feedback is applied to the control terminal of the switching element Tr1 via the capacitor C3 and the resistor R3, and the ON Maintain state. At this time, a reverse voltage is applied to the secondary diode D2, and no current flows through the secondary winding Ns. The exciting current flowing through the primary winding Np increases in proportion to time, the magnetic flux density in the transformer T increases, and is stored as energy.

  Further, when the switching element Tr1 is in the ON state, the voltage generated in the sub winding Nsub charges the capacitor C1 through the resistor R5, the Zener diode ZD2, and the resistor R4. As a result, the base voltage of the transistor Tr2 rises, and when it exceeds a predetermined threshold, the transistor Tr2 is turned on. When the transistor Tr2 is turned on, the control terminal potential of the switching element Tr1 is lowered to near 0 V, so that the switching element Tr1 is turned off and the current Id does not flow. When the current Id stops flowing, the voltage between the primary winding Np and the sub-winding Nsub is inverted. Thereby, the switching element Tr1 is turned off.

  When the switching element Tr1 is turned off, a current flows through the secondary winding Ns due to the energy accumulated in the transformer T. When the current flows through the secondary winding Ns and the energy accumulated in the transformer T is released, the voltage between the windings of the transformer T is the inductance between the windings, the line capacitance of the transformer T, and the snubber circuit. It resonates with the capacitance.

  Further, the sub-winding Nsub resonates with a voltage amplitude proportional to the output around 0V, and when the resonance voltage becomes higher than the ON voltage of the control terminal of the switching element Tr1, the switching element Tr1 is rapidly turned on. . The self-excited switching power supply circuit oscillates by performing the above operation.

Further, in the control of the output voltage of the power supply device, when the voltage applied to the control terminal of the shunt regulator SR exceeds a predetermined threshold, the LED of the photocoupler PC is turned on to turn on the phototransistor and charge the capacitor C1. As a result, the transistor Tr2 is turned on to turn off the switching element Tr1, and the on-time of the switching element Tr1 is controlled to keep the output voltage constant as described above.
Japanese Patent Laid-Open No. 10-108462

  In the case of the above conventional self-excited switching power supply device, self-excited oscillation is caused by applying positive feedback to the control terminal of the switching element Tr1 by the resonance voltage of the sub-winding Nsub. In this case, as indicated by the dotted line in FIG. 6, since the minus potential of the sub winding Nsub approaches 0V, the amplitude of resonance vibration may be reduced and may not reach the threshold voltage of the control terminal of the switching element Tr1. As a result, the self-excited oscillation is not successful, and there is a problem that the output current is greatly reduced as shown in FIG. Further, when the bias is applied from the resistor R2 so as to compensate for the resonance amplitude of the sub-winding Nsub, there is a problem that an excessive current flows in the short-circuit current as shown in FIG. 7B.

  Therefore, conventionally, in order to solve these problems, there is a case in which an element that selects the threshold value of the control terminal of the switching element and turns it on reliably is used. However, in this case, there is a problem that the cost is increased by selecting the elements.

  Also, a secondary side rectifier diode having a high forward voltage may be used to increase the voltage remaining in the secondary winding even if the output decreases, thereby increasing the resonance amplitude of the sub-winding Nsub. In this case, there is a problem that the efficiency is lowered and the shape of the element is increased.

  In addition, it is conceivable to increase the impedance on the secondary side. In this case, however, there is a problem that the efficiency of the power supply device is reduced, and it is also effective to increase the coupling between the secondary winding and the sub winding of the transformer. However, in this case, the transformer becomes complicated, leading to an increase in cost. In addition, a large transformer or a semiconductor element that does not break against a short-circuit current may be used, but there is a problem that the apparatus becomes large and the cost is increased.

  The present invention has been made in view of the above-mentioned problems of the prior art. The operation at the time of overcurrent is stabilized with a simple circuit configuration, short-circuit current and the like are prevented, and the component cost and the power supply are reduced. An object of the present invention is to provide a possible self-excited switching power supply.

The present invention includes a transformer having a primary winding, a secondary winding and a sub-winding, and a switching element such as a MOS-FET connected in series to the primary winding. A self-excited switching power supply device that feeds back a voltage generated in the primary winding and sub-winding to a control terminal to turn on and off the switching element, wherein the transformer within a period in which the switching element is off And a forcible drive circuit that turns on the switching element with a predetermined voltage that does not reach the threshold voltage of the control terminal of the switching element after the energy accumulated in the sub-winding is released. This is an excitation type switching power supply device. For example, the forced drive circuit detects that the voltage generated in the sub-winding has become 0V and turns on the switching element.

  In addition, there is an operation stop circuit that stops the operation of the forced drive circuit when the switching element is turned on. Further, it may have an operation start circuit for operating the forced drive circuit when the output voltage by the secondary winding becomes a predetermined value or less.

  In the self-excited switching power supply device of the present invention, even when the output voltage drops due to overcurrent or the like and the feedback by the sub-winding becomes weak, the switching element is reliably turned on by the forcible drive circuit, and the self-excited oscillation is successful. It can be prevented from disappearing. Furthermore, the on-timing of the switching element can be set as appropriate, and the on-timing of the switching element can be delayed by the resonance time. Thereby, the cross loss of a switching element can be reduced.

  According to the third and fourth aspects of the present invention, the forced drive circuit is operated only at the time of abnormality due to overcurrent or the like by the operation stop circuit or the operation start circuit of the forced drive circuit, thereby improving the efficiency of the power supply device. Is also possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a circuit of a self-excited switching power supply apparatus according to a first embodiment of the present invention. The same components as those of the conventional circuit of FIG. In this circuit, a primary winding Np of a transformer T and a switching element Tr1 such as a MOS-FET are connected in series between a pair of input terminals Vin to which a DC voltage is input, and a resistor R1 and a Zener diode ZD1 are connected to each other. The cathode of the Zener diode ZD1 is connected to the control terminal of the switching element Tr1 via the resistor R2.

  The secondary winding Ns of the transformer T is adjacent to the primary winding Np, the terminal on the non-dotted side of the secondary winding Ns is connected to the anode of the diode D2, and the cathode is the + of the pair of output terminals Vo. Connected to the side. A capacitor C4 is connected between the pair of output terminals Vo, and a series circuit of a resistor R9, an LED in the photocoupler PC, and a shunt regulator SR is connected in parallel. The anode of the shunt regulator SR is connected to the dot side of the secondary winding Ns, and the midpoint of the output resistors R10 and R11 between the output terminals Vo is connected to the control terminal of the shunt regulator SR.

  The transformer T is provided with a sub-winding Nsub, and both ends of the sub-winding Nsub are connected to a series circuit of a capacitor C2, a resistor R7, and a diode D1, and the anode of the diode D1 is connected to the dot of the sub-winding Nsub. It is connected to the terminal on the non-side and connected to the negative side of the input terminal Vin. One terminal of a series circuit of a capacitor C3 and a resistor R3 is connected to the terminal on the dot side of the sub winding Nsub, and the other end of this series circuit is connected to the control terminal of the switching element Tr1.

  The control terminal of the switching element Tr1 is connected to the negative side of the input terminal Vin via the resistor R8, and is connected to the collector of the npn transistor Tr2, and its emitter is connected to the negative input terminal -Vin. ing. The base of the transistor Tr2 is connected to the dot-side terminal of the sub-winding Nsub via the resistor R4 and the Zener diode ZD2, and is connected to the dot-side of the sub-winding Nsub via the resistor R5. ing. Further, the base of the transistor Tr2 is connected to the input terminal −Vin via the integrating capacitor C1 and to the emitter of the phototransistor of the photocoupler PC. The collector of the phototransistor of the photocoupler PC is connected to the cathode of the diode D1, and is connected to the dot-side of the sub winding Nsub via the resistor R6.

  Further, the cathode of the diode D3 that constitutes the forced drive circuit 10 that forcibly drives the switching element Tr1 is connected to the control terminal of the switching element Tr1. The anode of the diode D3 is connected to the collector of the pnp transistor Tr3, and the emitter of the transistor Tr3 is connected to the cathode of the Zener diode ZD1. A resistor R15 is connected between the emitter and base of the transistor Tr3, and the base of the transistor Tr3 is connected to the negative side of the input terminal Vin via the resistor R16 and an npn transistor Tr4. The transistor Tr4 has a collector connected to the resistor R16 and an emitter connected to the negative input terminal -Vin.

  The base of the transistor Tr4 is connected to the cathode side of the Zener diode ZD1 via the resistor R14 and to the emitter of the pnp type transistor Tr5. The transistor Tr5 has a collector connected to the negative input terminal -Vin and a base connected between the resistors R12 and R13. The other end of the resistor R12 is connected to the dot-side of the sub-winding Nsub, and the other end of the resistor R13 is connected to the negative input terminal -Vin.

  As shown in FIG. 6, the operation of the self-excited switching power supply device of this embodiment is substantially the same as that of the conventional power supply device described above, and when an input voltage is applied between a pair of input terminals Vin at the time of startup. The input voltage is applied to the control terminal of the switching element Tr1 via the resistors R1 and R2, the switching element Tr1 is turned on, and the current Id flows. As a result, an input voltage is applied to the primary winding Np, a voltage is induced between the sub-windings Nsub, and a voltage due to positive feedback is applied to the control terminal of the switching element Tr1 via the capacitor C3 and the resistor R3. To maintain. At this time, a reverse voltage is applied to the secondary diode D2, and no current flows through the secondary winding Ns.

  Further, when the switching element Tr1 is in the ON state, the voltage generated in the sub winding Nsub charges the capacitor C1 through the resistor R5, the Zener diode ZD2, and the resistor R4. As a result, the base voltage of the transistor Tr2 rises, and when it exceeds a predetermined threshold, the transistor Tr2 is turned on. When the transistor Tr2 is turned on, the switching element Tr1 is turned off and the current Id does not flow. When the current Id stops flowing, the voltage between the primary windings Np and the voltage between the sub windings Nsub are inverted. Thereby, the switching element Tr1 is turned off. When the switching element Tr1 is turned off, a current flows through the secondary winding Ns due to the energy accumulated in the transformer T. When the current flows through the secondary winding Ns and the energy has been released, the inter-winding voltage of the transformer T changes to 0V.

  Further, the sub-winding Nsub resonates with a voltage amplitude proportional to the output centered on 0V. When the resonance voltage exceeds 0 V, the forced drive circuit 10 injects charges into the control terminal of the switching element Tr1 in an operation described later, and turns off the switching element Tr1 rapidly. The operation of the self-excited switching power supply device of this embodiment performs self-excited oscillation by performing the above operation.

  Further, the output voltage is controlled when the voltage applied to the control terminal of the shunt regulator SR exceeds a predetermined threshold value, the LED of the photocoupler PC is turned on to turn on the phototransistor, and the capacitor C1 is charged. The switching element Tr1 is turned off by turning on Tr2, the on-time of the switching element Tr1 is controlled, and the output voltage is adjusted.

  The operation of the forced drive circuit 10 is as follows. When the switching element Tr1 shifts from OFF to ON, as shown in FIG. 6, the voltage across the sub-winding Nsub shifts from a minus voltage to a plus voltage with an amplitude proportional to the output voltage. To do. At this time, the potential divided by the resistors R12 and R13 also shifts from minus to plus, and when the base potential of the transistor Tr5 becomes 0V, the base current Ib5 of the transistor Tr5 does not flow and the transistor Tr5 is turned off. When the transistor Tr5 is turned off, the base potential of the transistor Tr4 rises at the same time, and when the current Ib4 flows through the base of the transistor Tr4, the transistor Tr4 is turned on. When the transistor Tr4 is turned on, the base potential of the transistor Tr3 is lowered to the potential divided by the resistors R15 and R16, and the transistor Tr3 is turned on. As a result, a current Ic3 flows through the transistor Tr3, charges the control terminal of the switching element Tr1, and turns on the switching element Tr1. The transistor Tr3 flows charge until the switching element Tr1 is reliably turned on.

  In the self-excited switching power supply device of this embodiment, the forced drive circuit 10 can reliably turn on the switching element Tr1 even when the positive feedback by the sub-winding Nsub is weakened by an overcurrent or the like. As a result, it is possible to prevent the switching element Tr1 from being turned on and the output current from rapidly decreasing or an excessive short-circuit current from flowing. In addition, the forced drive circuit 10 has a simple configuration, allows parts to be miniaturized, the apparatus itself can be miniaturized, and contributes to cost reduction.

  Next, a second embodiment of the present invention will be described with reference to FIG. Here, the same components as those in the above embodiment are denoted by the same reference numerals, and description thereof is omitted. In the forced drive circuit 12 of this embodiment, the middle point of the resistors R12 and R13 is connected to the base of the transistor Tr4.

  In the self-excited switching power supply device of this embodiment, when the switching element Tr1 is turned on from off, the voltage of the sub-winding Nsub is divided by resistors R12 and R13 and applied to the base of the transistor Tr4, and the divided potential is obtained. Reaches the ON potential of the transistor Tr4, the base current Ib4 flows, the transistor Tr4 is turned ON, and the switching element Tr1 is turned ON as described above.

  According to the self-excited switching power supply device of this embodiment, the timing at which the transistor Tr4 is turned on can be arbitrarily set by R12 and R13, and the on-timing of the switching element Tr1 can be appropriately set. Accordingly, it is possible to delay the on-timing of the switching element Tr1 by the resonance time. By delaying this, the drain-source voltage Vds of the switching element Tr1 can be turned on and the switching element Tr1 can be turned on. Cross loss can be reduced.

  Next, a third embodiment of the present invention will be described with reference to FIG. Here, the same components as those in the above embodiment are denoted by the same reference numerals, and description thereof is omitted. In this embodiment, an operation stop circuit 14 for stopping the operation of the forced drive circuit 10 is connected to the base of the transistor Tr4 of the forced drive circuit 10 in FIG.

  The operation stop circuit 14 includes an npn-type transistor Tr6 having a collector connected to the base of the transistor Tr4, and the base of the transistor Tr6 is connected to the middle points of the resistors R17 and R18. The other end of the resistor R17 is connected to the dot-side of the sub-winding Nsub, and the other end of the resistor R18 is connected to the negative input terminal -Vin.

  In the self-excited switching power supply device of this embodiment, when the switching element Tr1 is turned on, the voltage of the sub-winding Nsub suddenly increases and becomes a voltage proportional to the input voltage. Then, the potential of the sub winding Nsub is divided by the resistors R17 and R18 and applied to the base of the transistor Tr6 to turn on the transistor Tr6. When the transistor Tr6 is turned on, the base current Ib6 flows and the transistor Tr4 is turned off. As a result, the forced drive circuit 10 is stopped.

  According to the self-excited switching power supply device of this embodiment, when the switching element Tr1 is turned on, the forced drive circuit 10 is stopped. Therefore, when the switching element Tr1 is turned off, a current is supplied to the transistor Tr2 via the transistor Tr3. The drive loss of the forced drive circuit 10 can be suppressed without flowing.

  Next, a fourth embodiment of the present invention will be described with reference to FIG. Here, the same components as those in the above embodiment are denoted by the same reference numerals, and description thereof is omitted. In this embodiment, an operation start circuit 16 for starting the operation of the forced drive circuit 10 is connected to the base of the transistor Tr4 of the forced drive circuit 10 of FIG.

  The operation start circuit 16 includes an npn transistor Tr7 whose collector is connected to the base of the transistor Tr4, and the base of the transistor Tr7 is connected to the middle point of the resistors R19 and R20. The other end of the resistor R19 is connected to the-side input terminal -Vin, the other end of the resistor R20 is connected to the emitter of the transistor Tr7, and is connected to the-side input terminal -Vin via the capacitor C5. It is connected. Further, the emitter of the transistor Tr7 is connected to the anode of the diode D4, and the cathode of the diode D4 is connected to the dot-side of the sub winding Nsub.

  In the self-excited switching power supply device of this embodiment, the forced drive circuit 10 is operated only when the output voltage drops below a set value due to overcurrent or the like. In the operation start circuit 16, current flows from the sub-winding Nsub through the path of the capacitor C5 and the diode D4 while the switching element Tr1 is off, and a voltage proportional to the output voltage is peak-held in the capacitor C5. The peak-held voltage is divided by resistors R19 and R20 and applied to the base of transistor Tr7. Thus, while the transistor Tr7 is on, the transistor Tr4 is off and the forced drive circuit 10 is in a stopped state.

  When the output voltage is decreased due to overcurrent or the like, the voltage of the capacitor C5 is decreased, the base potential of the transistor Tr7 is decreased, and the transistor Tr7 is turned off below a predetermined potential. As a result, the transistors Tr4 and Tr5 can be turned on, and the forced drive circuit 10 operates as described above. The potential at which the transistor Tr7 is turned off can be set as appropriate.

  According to the self-excited switching power supply device of this embodiment, since the forced drive circuit 10 does not operate during normal operation, it is possible to prevent the efficiency of the power supply device from decreasing.

  The self-excited switching power supply device of the present invention is not limited to the above-described embodiment, and the configuration of the forced drive circuit can be set as appropriate, even if both the operation stop circuit and the operation start circuit are connected. good. The transistor Tr6 may be replaced with a MOS-FET, and the diode D3 may be a Schottky barrier diode.

1 is a schematic circuit diagram of a self-excited switching power supply device according to a first embodiment of the present invention. It is a schematic circuit diagram of the self-excited switching power supply device of 2nd embodiment of this invention. It is a schematic circuit diagram of the self-excited switching power supply device of 2nd embodiment of this invention. It is a schematic circuit diagram of the self-excited switching power supply device of 2nd embodiment of this invention. It is the schematic of the conventional self-excited switching power supply device. It is a timing chart which shows operation | movement of the conventional self-excited switching power supply device. An abnormal operation of a conventional self-excited switching power supply device is shown. It is a waveform diagram.

Explanation of symbols

10, 12 Forced drive circuit 14 Operation stop circuit 16 Operation start circuit Np Primary winding Ns Secondary winding Nsub Subwinding T Transformer Tr1 Switching element

Claims (4)

A transformer having a primary winding, a secondary winding, and a sub-winding; and a switching element connected in series to the primary winding; and the primary winding and the control terminal of the switching element In a self-excited switching power supply device that feeds back a voltage generated in a sub-winding to turn on / off the switching element , after energy accumulated in the transformer is released within a period in which the switching element is off , the self-excited switching power supply device and a voltage that is generated in the sub-windings provided with a forced drive circuit for turning on said switching element at a predetermined voltage that does not reach the threshold voltage of the control terminal of the switching element.   2. The self-excited switching power supply device according to claim 1, wherein the forced drive circuit detects that a voltage generated in the sub-winding has become about 0 V and turns on the switching element. 3.   3. The self-excited switching power supply device according to claim 1, further comprising an operation stop circuit that stops the operation of the forced drive circuit when the switching element is turned on. 4. The self-excited switching power supply device according to claim 1, further comprising an operation start circuit that operates the forced drive circuit when an output voltage of the secondary winding becomes a predetermined value or less.

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