Switching Power Supply

Self-oscillation type switching power supply having time constant circuit electronic switch an external voltage and having charging time variable in response to output voltage

Switching Power Supply Abstract
A self-oscillation switching power supply including a transformer having a primary winding, a secondary winding, and a feedback winding, and further including a control winding if necessary; a switching transistor for turning on and off the current of the primary winding; a control transistor for controlling a positive feedback signal from the feedback winding or control winding to the switching transistor; and a time constant circuit which is charged by a voltage generated across the feedback winding or control winding and which supplies a control voltage to the control transistor. The self-oscillation switching power supply further comprises an electronic switch which is opened and closed in response to the positive feedback signal from the control winding or feedback winding wherein an external voltage source is connected to the time constant circuit via said electronic switch so that the charging time of the time constant circuit is varied according to an output voltage detection signal.

Switching Power Supply Claims
What is claimed is:

1. A self-oscillation switching power supply comprising: a transformer including a primary winding, a secondary winding and at least one control winding, a switching transistor for turning on and off the current flowing in said primary winding; a control transistor for controlling a feedback signal from said at least one control winding to said switching transistor; and a time constant circuit having a charging time which is charged by a voltage generated across said at least one control winding and which supplies a control voltage to said control transistor, and further comprising an electronic switch which is opened and closed in response to the feedback signal from said at least one control winding wherein an external voltage source is connected to said time constant circuit by said electronic switch so that the charging time of said time constant circuit is varied according to an output voltage detection signal dependent on an output voltage of the power supply.

2. A self-oscillation switching power supply according to claim 1, wherein said time constant circuit includes a series circuit of a resistor and a capacitor and the voltage across the charged capacitor is output as the control voltage supplied to said control transistor and wherein said electronic switch is disposed between said external voltage source and said capacitor.

3. A self-oscillation switching power supply according to claim 2, wherein said electronic switch comprises a transistor which is turned on by the feedback voltage of said at least one control winding and wherein there is provided a diode for discharging the charge of said capacitor into said at least one control winding when said transistor turns off.

4. A self-oscillation switching power supply according to claim 1, wherein there is provided an impedance circuit between said time constant circuit and a control voltage input part of said control transistor so that said impedance circuit prevents said control transistor from being reverse-biased by said at least one control winding.

5. A self-oscillation switching power supply according to claim 2, wherein there is provided an impedance circuit between said time constant circuit and a control voltage input part of said control transistor so that said impedance circuit prevents said control transistor from being reverse-biased by said at least one control winding.

6. A self-oscillation switching power supply comprising: a transformer including a primary winding, a secondary winding and at least one control winding, a switching transistor for turning on and off the current flowing in said primary winding; a control transistor for controlling a feedback signal from said at least one control winding to said switching transistor; and a time constant circuit having a charging time which is charged by a voltage generated across said at least one control winding and which supplies a control voltage to said control transistor, and further comprising an electronic switch which is opened and closed in response to the feedback signal from said at least one control winding wherein an external voltage source is connected to said time constant circuit by said electronic switch so that the charging time of said time constant circuit is varied according to an output voltage detection signal dependent on an output voltage of the power supply;

wherein said time constant circuit includes a series circuit of a resistor and a capacitor and the voltage across the charged capacitor is outout as the control voltage supplied to said control transistor and wherein said electronic switch is disposed between said external voltage source and said capacitor;

wherein said electronic switch comprises a transistor which is turned on by the feedback voltage of said at least one control winding and wherein there is provided a diode for discharging the charge of said capacitor into said at least one control winding when said transistor turns off; and further

wherein there is provided an impedance circuit between said time constant circuit and a control voltage input part of said control transistor so that said impedance circuit prevents said control transistor from being reverse-biased by said at least one control winding.

7. A self-oscillation switching power supply according to any of claim 1, wherein a control signal input part of the switching transistor is connected to a delay transistor for causing said switching transistor to have a delay in its turning-on timing and wherein there is provided an impedance circuit between a control voltage input part of said delay transistor and said at least one control winding so that said impedance circuit prevents said delay transistor from being reverse-biased by said at least one control winding.

8. A self-oscillation switching power supply according to claim 1, wherein there is provided an impedance circuit between a control voltage input part of said control transistor and said at least one control winding so that said impedance circuit prevents said control transistor from being reverse-biased by said at least one control winding.

9. A self-oscillation switching power supply according to claim 1, wherein a control signal input part of the switching transistor is connected to a delay transistor for causing said switching transistor to have a delay in its turning-on timing and wherein there is provided a delay circuit for delaying the feedback signal from said at least one control winding by an amount corresponding to a fixed time constant and supplying the resultant delayed signal as a control signal to said delay transistor.

10. A self-oscillation switching power supply according to claim 1, wherein there is provided a delay circuit for delaying the feedback signal from said at least one control winding by an amount corresponding to a fixed time constant and supplying the resultant delayed signal as a control signal to said control transistor.

11. A self-oscillation switching power supply comprising: a transformer including a primary winding, a secondary winding and at least one control winding, a switching transistor for turning on and off the current flowing in said primary winding; a control transistor for controlling a feedback signal from said at least one control winding to said switching transistor; and a time constant circuit having a charging time which is charged by a voltage generated across said at least one control winding and which supplies a control voltage to said control transistor, and further comprising an electronic switch which is opened and closed in response to the feedback signal from said at least one control winding wherein an external voltage source is connected to said time constant circuit by said electronic switch so that the charging time of said time constant circuit is varied according to an output voltage detection signal dependent on an output voltage of the power supply;

wherein a control signal input part of the switching transistor is connected to a delay transistor for causing said switching transistor to have a delay in its turning-on timing and wherein there is provided a bias voltage generating circuit between a control voltage input part of said delay transistor and said at least one control winding so that said bias voltage generating circuit is charged by a voltage generated across said at least one control winding thereby providing a DC bias voltage to a control voltage applied to said delay transistor.

12. A self-oscillation switching power supply according to claim 1, wherein there is provided a bias voltage generating circuit between a control voltage input part of said control transistor and said at least one control winding so that said bias voltage generating circuit is charged by a voltage generated across said at least one control winding thereby providing a DC bias voltage to the control voltage applied to said control transistor.

13. A self-oscillation switching power supply according to claim 1, further comprising a switch for disabling operation of said switching transistor.

14. A self-oscillation switching power supply according to claim 13, wherein the switch comprises a switch disposed remotely from the power supply.

15. A self-oscillation switching power supply according to claim 1, wherein the at least one control winding comprises a first feedback winding and a second control winding.

16. A self-oscillation switching power supply according to claim 1, wherein the at least one control winding comprises a single control winding.

17. A self-oscillation switching power supply according to claim 1, wherein the external voltage source is generated by an error amplifier comparing said output voltage detection signal comprising a signal related to an output voltage of said power supply to a reference voltage.

Switching Power Supply Description
capacitor.

The electronic switch may be formed with a transistor which is turned on by the positive feedback voltage of the control winding or feedback winding and there is provided a diode for discharging the charge of the capacitor into the control winding or feedback winding when the transistor turns off.

In this circuit configuration, the charging time of the time constant circuit is controlled by applying a voltage from the external voltage source to the time constant circuit, and the charging can be performed without causing the feedback winding to be short-circuited. Therefore, the control transistor does not have a premature operation due to a delay in the supply of the positive feedback signal from the feedback winding to the switching transistor. This ensures that the charging time can be controlled in a stable fashion without encountering an intermittent operation.

The self-oscillation switching power supply may further comprises an impedance circuit between the time constant circuit and the control voltage input part of the control transistor so that the impedance circuit prevents the control transistor from being reverse-biased by the feedback winding or control winding. In this circuit configuration, because the control transistor is prevented from being reverse-biased, its response ability at high frequencies becomes low and thus the switching transistor is prevented from oscillating at a high frequency. This makes it possible to vary the output voltage (current) over a wide range in a stable fashion. Furthermore, the switching loss due to the high-frequency oscillation of the switching transistor can also be prevented.

The control signal input part of the switching transistor may be connected to a delay transistor for causing the switching transistor to have a delay in the turning-on timing and there is provided an impedance circuit between the control voltage input part of the delay transistor and the control winding or feedback winding so that the impedance circuit prevents the delay transistor from being reverse-biased by the control winding or feedback winding. Alternatively, there may be provided an impedance circuit between the control voltage input part of the control transistor and the control winding or feedback winding so that the impedance circuit prevents the control transistor from being reverse-biased by the control winding or feedback winding. In either circuit configuration, the control transistor or the delay transistor is prevented from being reverse-biased. That is, when the switching transistor turns on, a voltage is generated across the control winding or feedback winding, which would otherwise cause the control transistor or the delay transistor to be reverse-biased. However, the impedance circuit prevents the control transistor or the delay transistor from being reverse-biased, and thus the control transistor or the delay transistor turns off after a short delay due to the effect of carrier accumulation in the transistor. During the above process, the switching transistor is maintained in the off-state. When the voltage applied to the switching transistor becomes low enough, the switching transistor turns on. This suppresses an excess current which charges the capacitance associated with the primary winding of the high-voltage transformer thereby ensuring that the output voltage (current) can be controlled over a wide range in a stable fashion by controlling the on-period of the switching transistor using the control transistor. Furthermore, the switching loss which occurs when the switching transistor turns on is also reduced.

In the self-oscillation switching power supply, the control signal input part of the switching transistor may be connected to a delay transistor for causing the switching transistor to have a delay in the turning-on timing and there is provided a delay circuit for delaying the positive feedback signal from the control winding or feedback winding by an amount corresponding to a fixed time constant and supplying the resultant delayed signal as the control signal to the delay transistor. Alternatively, there may be provided a delay circuit for delaying the positive feedback signal from the control winding or feedback winding by an amount corresponding to a fixed time constant and supplying the resultant delayed signal as the control signal to the control transistor. In either circuit configuration, the control transistor is driven by a voltage having a phase delay relative to the voltage induced across the control winding or the feedback winding and thus the switching transistor turns on after a delay corresponding to the delay of the delay circuit. As a result, the switching transistor turns on when the voltage applied to the switching transistor becomes low enough. This suppresses an excess current which charges the capacitance of the high-voltage transformer and also suppresses the amplitude of a ringing component flowing through the switching transistor during the on-period thereby ensuring that the output voltage (current) can be controlled over a wide range in a stable fashion by controlling the on-period of the switching transistor using the control transistor. Furthermore, the switching loss which occurs when the switching transistor turns on is also reduced.

Furthermore, the control signal input part of the switching transistor may be connected to a delay transistor for causing the switching transistor to have a delay in the turning-on timing and there is provided a bias voltage generating circuit between the control voltage input part of the delay transistor and the control winding or feedback winding so that the bias voltage generating circuit is charged by the voltage generated across the control winding or feedback winding thereby providing a DC bias voltage to the control voltage applied to the delay transistor. Alternatively, there may be provided a bias voltage generating circuit between the control voltage input part of the control transistor and the control winding or feedback winding so that the bias voltage generating circuit is charged by the voltage generated across the control winding or feedback winding thereby providing a DC bias voltage to the control voltage applied to the control transistor. In either circuit configuration, the control transistor or the delay transistor is controlled by the sum of the voltage induced across the control winding and the negative DC component superimposed on the induced voltage. Therefore, the switching transistor turns on when the voltage applied to the switching transistor becomes low enough. This suppresses an excess current which charges the capacitance of the high-voltage transformer and also suppresses the amplitude of a ringing component flowing through the switching transistor during the on-period thereby ensuring that the output voltage (current) can be controlled over a wide range in a stable fashion by controlling the on-period of the switching transistor using the control transistor. Furthermore, the switching loss which occurs when the switching transistor turns on is also reduced.

For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a general example of a circuit configuration of a self-oscillation switching power supply according to a first embodiment.

FIG. 2 is a circuit diagram illustrating another general example of a circuit configuration of a self-oscillation switching power supply according to the first embodiment.

FIG. 3 is a circuit diagram illustrating a specific example of a self-oscillation switching power supply according to the first embodiment.

FIG. 4 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a second embodiment.

FIG. 5 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a third embodiment.

FIG. 6 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a fourth embodiment.

FIG. 7 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a fifth embodiment.

FIG. 8 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a sixth embodiment.

FIG. 9 is a diagram illustrating the voltage waveforms for various points in the circuit of FIG. 8.

FIG. 10 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a seventh embodiment.

FIG. 11 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to an eighth embodiment.

FIG. 12 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a ninth embodiment.

FIG. 13 is a diagram illustrating the voltage waveforms for various points in the circuit of FIG. 12.

FIG. 14 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to a tenth embodiment.

FIG. 15 is a circuit diagram illustrating an example of a self-oscillation switching power supply according to an eleventh embodiment.

FIG. 16 is a circuit diagram illustrating a conventional self-oscillation switching power supply.

FIG. 17 is a circuit diagram of a transformer.

FIGS. 18A to 18C are diagrams illustrating the changes in the waveform of the collector-emitter voltage of a switching transistor, which occurs when the load changes.

FIG. 19 illustrates an equivalent circuit of a transformer and a switching transistor.

FIGS. 20A to 20D are diagrams illustrating the changes in the waveform of the collector-emitter voltage of the switching transistor, which occurs with the change in the on-period of the switching transistor.

FIGS. 21A to 20D are diagrams illustrating the voltage and current waveforms for various points in FIG. 19.

FIGS. 22A and 22B are diagrams illustrating the voltage and current waveforms for various points in FIG. 19.

FIG. 23 is a diagram illustrating a conventional high-voltage switching power supply.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, the preferred embodiments of the present invention are explained in detail with reference to the drawings.

FIG. 1 is a circuit diagram illustrating a general example of a circuit configuration of the self-oscillation switching power supply according to a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an input power supply, T denotes a high-voltage transformer including a primary winding Lp, a secondary winding Ls, and a feedback winding Lf. Q1 denotes the switching transistor connected to the input power supply via the primary winding Lp of the high-voltage transformer T. A starting resistor R1 is connected to the base of the switching transistor Q1. A current limiting resistor R2, a speed-up capacitor C2, and a diode D2 are disposed between the feedback winding Lf and the base of the switching transistor Q1. The control transistor Q2 is connected between the base and the emitter of the switching transistor Q1. The feedback winding Lf is connected to a time constant circuit 4 so that a voltage generated by the time constant circuit 4 is applied to the base of the control transistor Q2. Reference numeral 3 denotes an external voltage source for varying the charging time of the time constant circuit 4 via an electronic switch 5. The electronic switch 5 is turned on by a positive feedback signal generated by the feedback winding Lf. The second winding Ls of the high-voltage transformer T is connected to a rectifying and smoothing circuit 2 comprising a rectifying diode D1 and a smoothing capacitor C1. A resistance voltage divider comprising resistors R3 and R4 is connected to the output side of the rectifying and smoothing circuit. Depending on the detected value of the output voltage of the resistance voltage divider, the external voltage source 3 controls the charging time of the time constant circuit 4.

FIG. 2 is a circuit diagram illustrating another general example of a circuit configuration of the self-oscillation switching power supply according to the first embodiment. In FIG. 2, the time constant circuit 4 includes a resistor R5 and a capacitor C3 wherein the voltage across the capacitor C3 is applied to the base of the control transistor Q2.

FIG. 3 is a circuit diagram of a specific example of a self-oscillation switching power supply according to the first embodiment. In FIG. 3, there is provided a transistor Q3 which corresponds to the electronic switch 5 shown in FIG. 1 or 2 and which turns on in response to the positive feedback voltage of a feedback winding Lf. A diode D3 serves to form a path through which a capacitor C3 is charged (discharged). Furthermore, in FIG. 3, there is provided an error amplifier 7 which corresponds to the external voltage source 3 shown in FIG. 1 or 2. This error amplifier 7 employs a reference voltage supplied by a reference voltage source Vr, and a voltage obtained by dividing the output voltage via resistors R3 and R4 is applied to an input of the error amplifier. The error amplifier amplifies the difference between the input voltage and the reference voltage by a predetermined amplification factor. The resultant voltage signal is applied to the collector of the transistor Q3 via a resistor R6. The non-inverting input terminal of the error amplifier 7 is pulled up via a resistor R22 and a diode D9. A remote switch is disposed between ground and the node between R11 and D9. A series circuit comprising a diode D10, a Zener diode DZ, and a resistor R12 is connected between the output of the error amplifier 7 and the base of a control transistor Q2.

The circuit shown in FIG. 3 operates as follows. If a DC voltage is applied from an input power supply 1, a small starting current flows into the base of a switching transistor Q1 via a starting resistor R1. As a result, a current flows through the collector of Q1. This causes a reduction in the collector-emitter voltage and thus a voltage is applied between the terminals of the primary winding Lp of a high-voltage transformer T. In proportion to this voltage, a voltage is induced across the feedback winding Lf. The induced voltage causes a positive feedback current to be supplied to the base of the switching transistor Q1 via a current limiting resistor R2, a speed-up capacitor C2, and a diode D2. As a result, the transistor Q1 is turned on (into a saturated state). In response to the transition of Q1 into the on-state, a DC voltage is applied between the terminals of the primary winding Lp of the high-voltage transformer T and a current flows through the primary winding Lp. As a result, the high-voltage transformer is excited. At the same time, a voltage is induced across the feedback winding Lf whereby the capacitance C3 is charged via the resistor R5 and the base-emitter path of the transistor Q3. Furthermore, depending on the output voltage of the error amplifier 7, a charging current flows into the capacitor C3 via a resistor R6 and the collector-emitter path of the transistor Q3. When the charging voltage across the capacitor C3 reaches a threshold value (about 0.6 V) of the base-emitter voltage of the control transistor Q2, the base and the emitter of the switching transistor Q1 are short-circuited by Q2 and thus the base current of the switching transistor Q1 is cut off. As a result, Q1 quickly turns off. When the switching transistor Q1 turns off, the base of the switching transistor Q1 is reverse-biased to a negative voltage by the induced voltage of the feedback winding Lf. At the same time, the capacitor C3 is forced to be discharged (reversely charged) by the feedback winding Lf via the diode D3 and the resistor R5. As a result, the base of the control transistor Q2 is reverse-biased to a negative voltage. Thus, the transistor Q2 is maintained in the off-state. During the time period in which the switching transistor Q1 is in the off-state, the high-voltage transformer T freely oscillates at a resonance frequency. As a result, a voltage is induced across the feedback winding whereby the base of the switching transistor Q1 is forward-biased. Thus, the switching transistor Q1 again turns on. The above-described turning on and off occurs periodically and the oscillatory operation grows into a continuous oscillation.

If an increase occurs in the output voltage, the voltage at the non-inverting terminal of the error amplifier 7 increases and a corresponding increase occurs in the collector voltage of the transistor Q3. This results in an increase in the charging current flowing into the capacitor C3 via the resistor R6 and the collector-emitter path of the transistor Q3. Thus, the increase in the output voltage results in an increase in the increasing rate at which the voltage across the capacitor C3 increases, and thus the capacitor C3 is charged in a shorter charging time. As a result, the on-period of the switching transistor Q1 becomes shorter. That is, if the switching transistor Q1 turns off and a positive feedback signal is generated across the feedback winding Lf, then the control transistor Q2 turns on and the switching transistor Q1 turns off in a short time. The turning-off of the switching transistor Q1 causes a negative voltage to be induced across the feedback winding Lf and thus the capacitor C3 is discharged (reversely charged) via the diode D5, the resistor R5, and the feedback winding Lf. Conversely, if a reduction occurs in the output voltage, the process occurs in an opposite fashion. That is, the charging time becomes longer and thus the on-period of the switching transistor Q1 becomes longer. In this circuit, as described above, the charging time associated with the time constant circuit including the resistor R5, the transistor Q3, and the capacitor C3 is controlled depending on the output voltage of the error amplifier 7 serving as the external voltage source so that even when the on-period of the switching transistor Q1 becomes short due to a reduction in the load current, a sufficient amount of positive feedback current is supplied into the base of the switching transistor Q1 from the feedback winding Lf during the period in which the capacitance is charged, without causing the feedback winding Lf to be short-circuited by the transistor Q3. This prevents the control transistor Q2 from turning on earlier than the transistor Q1, and thus it becomes possible that the switching transistor Q1 can operate in a stable fashion over a wide operating region from the saturation region to the unsaturated region without encountering an intermittent operation.

Furthermore, in FIG. 3, if the remote switch is turned off, the non-inverting terminal of the error amplifier 7 is pulled up to a high level and thus the output voltage of the error amplifier 7 is maintained at a maximum value. As a result, the Zener diode DZ turns on and thus the control transistor Q2 turns on. As a result, the switching transistor Q1 turns off and it is maintained in the off-state. If the remote switch is turned on, then the diode D9 is turned off and thus the non-inverting terminal of the error amplifier 7 is released from the pulled-up state. As a result, the voltage applied to the non-inverting terminal of the error amplifier 7 becomes equal to the voltage provided by the voltage dividing resistors R3 and R4. The output voltage of the error amplifier 7 decreases from the maximum value toward a particular control voltage. As a result, the switching transistor Q1 starts to operate from a state in which the on-period has a minimum value, and the on-period quickly increases to the normal on-period value. Thus, the output voltage quickly rises up without generating overshoot. This technique makes it possible to quickly start the switching power supply circuit using an inexpensive remote control circuit without producing overshoot.

FIG. 4 is a circuit diagram illustrating a second embodiment of a self-oscillation switching power supply. The difference from the circuit shown in FIG. 3 is that there is provided an additional diode D5 between the capacitor C3 of the time constant circuit and the base of the control transistor Q2. Although not shown in FIG. 4, the switching power supply also includes a remote control circuit. The diode D5 additionally disposed in the circuit serves to cut off a reverse bias current which would otherwise flow into the control transistor Q2 from the feedback winding Lf when the switching transistor Q1 turns off. As a result, high-frequency response is prevented by the carrier accumulation effect in the control transistor Q2. This limits the maximum oscillation frequency of the switching transistor Q1. As a result, the high-frequency oscillation at the series resonance frequency associated with the circuit formed by the leakage inductance and the distributed capacitance of the high-voltage transformer is suppressed. Therefore, an unstable operation such as an intermittent operation is prevented and it becomes possible to vary the output voltage (current) over a wide range without encountering instability.

FIG. 5 is a circuit diagram illustrating a third embodiment of a self-oscillation switching power supply. The difference from the circuit shown in FIG. 3 is that there is provided a delay transistor Q4 between the base and the emitter of the switching transistor Q1 and there are also provided a control winding Lc and an impedance circuit 8 so that a control signal is generated by the control winding Lc and applied to the delay transistor Q4 via the impedance circuit 8. The polarity of the voltage induced across the control winding Lc is opposite to that of the feedback winding Lf. More specifically, a positive voltage is induced during the off-period of the switching transistor Q1 whereby the delay transistor Q4 is forward-biased between its base and emitter, via the impedance circuit including the current limiting resistor R7 and the diode D4. After that, a voltage is induced across the feedback winding by the resonant oscillation of the high-voltage transformer T whereby the base of the switching transistor Q1 is forward-biased. In response to the forward bias voltage, the switching transistor Q1 attempts to turn on. However, the diode D4 prevents the carriers accumulated in the delay transistor Q4 from being swept out, and thus the delay transistor Q4 is maintained in the on-state for a short time. As a result, the switching transistor Q1 turns on after a short delay. Therefore, an unstable operation such as an intermittent operation is prevented and it becomes possible to vary the output voltage (current) over a wide range without encountering instability.

FIG. 6 illustrates a fourth embodiment of a self-oscillation switching power supply. In this fourth embodiment, unlike the embodiment described above with reference to FIG. 5, a PNP transistor is employed as the delay transistor Q4 and the diode D4 of the impedance circuit 8 is disposed in an opposite direction thereby achieving similar effects without using the control winding. In this circuit, as in the circuit shown in FIG. 5, when the switching transistor Q1 is going to turn on, the diode D4 prevents the carriers accumulated in the delay transistor Q4 from being swept out, and thus the delay transistor Q4 is maintained in the on-state for a short time. As a result, the switching transistor Q1 turns on after a short delay. Therefore, an unstable operation such as an intermittent operation is prevented and it becomes possible to vary the output voltage (current) over a wide range without encountering instability.

FIG. 7 is a circuit diagram illustrating a * fifth embodiment of a self-oscillation switching power supply. In this fifth embodiment, unlike the embodiment described above with reference to FIG. 5, similar effects are achieved without using the delay transistor Q4 shown in FIG. 5. In FIG. 7, an impedance circuit 8 is connected between a control winding Lc and the base of a control transistor Q2. Furthermore, a reverse current cutting-off diode D7 is disposed between a time constant circuit 4 and the base of the transistor Q2. In this circuit configuration, the control transistor Q2 also plays a role which is played by the delay transistor Q4 in the circuit shown in FIG. 5.

Referring now to FIGS. 8 and 9, a sixth embodiment of a self-oscillation switching power supply is described below.

This sixth embodiment is different from that shown in FIG. 5 in the circuit configuration of the part between the control winding Lc and the delay transistor Q4. In this sixth embodiment, a delay circuit 9 is realized using an integrating circuit including a resistor R8 and a capacitor C4. In FIG. 8, the control winding Lc is formed to have a polarity so that the delay transistor Q4 is reverse-biased during the on-period of the switching transistor Q1 and so that the delay transistor Q4 is forward-biased during the off-period of the switching transistor Q1. The delay circuit 9 integrates the induced voltage of the control winding Lc using the resistor R8 and the capacitor C4. As a result, the voltage across the capacitor C4 has a phase delay of about 90.degree. relative to the induced voltage of the control winding Lc, and the waveform of the voltage across the capacitor C4 decreases in amplitude with a gain determined by the resistance of the resistor R8 and the capacitance of the capacitor C4. FIG. 9 illustrates the waveforms of the collector-emitter voltage Vce of the switching transistor Q1, the induced voltage V.sub.Lf of the feedback winding Lf, the induced voltage V.sub.Lc of the control winding Lc, and the voltage Vc across the capacitor C4. As can be seen from FIG. 9, the voltage Vc having a phase delay relative to the phase of the voltage V.sub.Lf induced across the feedback winding Lf is applied to the base of the delay transistor Q4. Therefore, when a positive feedback voltage, which would enhance the turning-on of the switching transistor Q1, is induced across the feedback winding Lf, the base and the emitter of the switching transistor Q1 are further short-circuited by the delay transistor Q4 until the voltage across the capacitor C4 decreases to a value lower than the threshold voltage (about 0.6 V) of the delay transistor Q4. Thus, when the switching transistor Q1 turns on, a positive feedback current is supplied after Vc has become substantially equal to 0 V as a result of a resonant oscillation thereby producing a delay in the turning-on timing of the switching transistor Q1 thus suppressing an initial excess current into the collector of the switching transistor Q1. As a result, a great reduction in the switching loss is achieved. Furthermore, the amplitude of the ringing component superimposed on the collector current of Q1 during the on-period of transistor Q1 is suppressed and thus the on-period of the switching transistor is properly controlled depending on the output voltage detected.

FIG. 10 is a circuit diagram illustrating a seventh embodiment of a self-oscillation switching power supply. This circuit is different from that shown in FIG. 8 in that a PNP transistor is employed as the delay transistor Q4 and similar effects are achieved without using the control winding. In FIG. 10, a capacitor C4 is charged so that the delay transistor Q4 is reverse-biased during the on-period of the switching transistor Q1 and so that the delay transistor Q4 is forward-biased during the off-period of the switching transistor Q1 thereby producing a delay in the turning-on timing of the switching transistor Q1 thus suppressing an initial excess current into the collector of the switching transistor Q1. As a result, a great reduction in the switching loss is achieved. Furthermore, the amplitude of the ringing component superimposed on the collector current of Q1 during the on-period of Q1 is suppressed and thus the on-period of the switching transistor is properly controlled depending on the output voltage detected.

FIG. 11 is a circuit diagram illustrating an eighth embodiment of a self-oscillation switching power supply. This circuit is different from that shown in FIG. 8 in that similar effects are achieved without using the delay transistor Q4 shown in FIG. 8. In FIG. 11, the output of a delay circuit 9 realized by an integrating circuit is connected to the base of a control transistor Q2 via a reverse current cutting-off diode D6. Furthermore, a reverse current cutting-off diode D7 is disposed between a time constant circuit and the base of the transistor Q2. In this circuit configuration, the control transistor Q2 also plays a role which is played by the delay transistor Q4 in the circuit shown in FIG. 8.

Referring now to FIGS. 12 and 13, a ninth embodiment of a self-oscillation switching power supply is described below.

FIG. 12 is a circuit diagram of this power supply. This circuit is different from that shown in FIG. 8 in the circuit configuration of the part between the control winding Lc and the delay transistor Q4. In FIG. 12, a bias voltage generating circuit 10 is formed using a capacitor C5, a diode D8, and a resistor R9. FIG. 13 illustrates the waveforms of the collector-emitter voltage Vce of the switching transistor Q1, the induced voltage V.sub.Lf of the feedback winding Lf, the induced voltage V.sub.Lc of the control winding Lc, and the cathode voltage Vd of the diode D8. The sum of the induced voltage V.sub.Lc of the control winding and the voltage (Vsf) across the capacitor C5 is applied to the base of the delay transistor Q4, as represented by Vd in FIG. 13. Therefore, when a positive feedback voltage, which would enhance the turning-on of the switching transistor Q1, is induced across the feedback winding Lf, the base and the emitter of the switching transistor Q1 are further short-circuited by the delay transistor Q4 until the voltage across the capacitor C4 decreases to a value lower than the threshold voltage (about 0.6 V) of the delay transistor Q4. Therefore, when the switching transistor Q1 turns on, a positive feedback current is supplied after Vc has become substantially equal to 0 V as a result of a resonant oscillation thereby producing a short delay in the turning-on timing of the switching transistor Q1 thus suppressing an initial excess current into the collector of the switching transistor Q1. As a result, a great reduction in the switching loss is achieved. Furthermore, the amplitude of the ringing component superimposed on the collector current of transistor Q1 during the on-period of transistor Q1 is suppressed and thus the on-period of the switching transistor is properly controlled depending on the output voltage detected.

FIG. 14 is a circuit diagram illustrating a tenth embodiment of a self-oscillation switching power supply. In this tenth embodiment, unlike the embodiment described above with reference to FIG. 12, a PNP transistor is employed as the delay transistor Q4 and the diode D8 of the bias voltage generating circuit 10 is disposed in an opposite direction thereby achieving similar effects without using the control winding. As can be seen from FIG. 14, the polarity of the bias voltage generated by the bias voltage generating circuit is opposite to that shown in FIG. 12. In this circuit configuration, as in the circuit shown on FIG. 12, a short delay is produced in the turning-on timing of the switching transistor Q1 thereby suppressing an initial excess current into the collector of the switching transistor Q1. As a result, a great reduction in the switching loss is achieved. Furthermore, the amplitude of the ringing component superimposed on the collector current of transistor Q1 during the on-period of transistor Q1 is suppressed and thus the on-period of the switching transistor is properly controlled depending on the output voltage detected.

FIG. 15 is a circuit diagram illustrating an eleventh embodiment of a self-oscillation switching power supply. In this eleventh embodiment, unlike the embodiment described above with reference to FIG. 12, similar effects are achieved without using the delay transistor Q4. In FIG. 15, the output of a bias voltage generating circuit 10 is connected to the base of a control transistor Q2 via a reverse current cutting-off diode D6. Furthermore, a reverse current cutting-off diode D7 is disposed between the time constant circuit and the base of the transistor Q2. In this circuit configuration, the control transistor Q2 also plays a role which is played by the delay transistor Q4 in the circuit shown in FIG. 12.

In the embodiments described above, the discrete transistor Q3 serving as the electronic switch may be replaced with the phototransistor of a photocoupler. For example, in FIG. 3, Q3 may be replaced with the phototransistor of a photocoupler, the anode of the light emitting diode of the photocoupler may be connected to the node between the resistor R5 and the diode D3, and the cathode of the light emitting diode may be connected to either terminal of the capacitor C3.

In the embodiments described above, a capacitor may be connected in parallel to the resistor R5 of the time constant circuit so that the transistor Q3 can be driven even when the positive feedback voltage signal from the feedback winding Lf has high-frequency components. This allows a further expansion of the range over which the output can be controlled.

Furthermore, in the embodiments described above, if the resistor R6 disposed in the charging path extending from the error amplifier 7 is replaced with a series circuit of a resistor and a Zener diode, then the series circuit behaves as a variable impedance element in the range close to the Zener voltage whereby, when the charging time is set to be long, the charging current caused by the output of the error amplifier can be suppressed to an infinitely low level. This allows a further expansion of the range over which the output can be controlled.

Furthermore, the delay transistor Q4 in the discrete form shown in various figures may be replaced with the phototransistor of a photocoupler. For example, in FIG. 5, Q4 may be replaced with the phototransistor of a photocoupler, the anode of the light emitting diode of the photocoupler may be connected to the output of the impedance circuit, and the cathode of the light emitting diode may be connected to the emitter of the phototransistor.

Furthermore, the transistors of the bipolar type employed in the embodiments described above may be replaced with transistors of the unipolar type.

Although, in the embodiments described above, the time constant circuit for outputting a control signal to the control transistor Q2 is connected to the feedback winding, similar operations and advantages can also be achieved by connecting the time constant circuit to a control winding which is additionally provided so that the control winding has the same polarity as that of the feedback winding.

Furthermore, similar operations and advantages can also be achieved by connecting the time constant circuit to a control winding which is additionally provided so that the control winding has an opposite polarity to that of the feedback winding and replacing the control transistor Q2 with a PNP transistor.

Although, in the embodiments described above, the power supply circuit is designed to output a constant voltage, the present invention may also be applied to a power supply circuit in which the output current is detected and the feedback control is performed so as to output a constant current.

While preferred embodiments of the invention have been disclosed, various modes of carrying out the principles disclosed herein are contemplated as being within the scope of the following claims. Therefore, it is understood that the scope of the invention is not to be limited except as otherwise set forth in the claims.