JP3045215B2 - Resonant switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonance type switching power supply using resonance of an inductance and a capacitor connected in series to the inductance.

[0002]

FIG. 1 shows a conventional resonance type switching power supply. This switching power supply includes a series circuit of transistors Q1 and Q2 as first and second switching elements connected between one end and the other end of a DC power supply 1, an output transformer T1, and an output transformer T1. A first capacitor C1 for resonance connected in series to the primary winding N1, second and third capacitors C2 and C3 connected in parallel to the first and second transistors Q1 and Q2, And the first and second clamping diodes D1, D2 connected in parallel in the reverse direction to the second transistors Q1, Q2.
2, a control circuit 2 connected to the bases (control terminals) of the first and second transistors Q1 and Q2, the secondary windings N2a and N2b of the output transformer T1, and the secondary windings N2a and N2b.
Rectifying and smoothing circuit composed of diodes D3 and D4 and a smoothing capacitor C4, an error amplifier 3, a reference voltage source 4, and a current detector 5.

[0003] The series circuit of the primary winding N1 and the capacitor C1 is connected in parallel to the second transistor element Q2. The transformer T1 is a leakage transformer, and the primary winding N1 and the secondary windings N2a and N2b are loosely coupled via a core 9. The secondary windings N2a and N2b are connected in a center tap manner. A load (not shown) is connected to the output terminal 6 connected to the smoothing capacitor C4. One input terminal of the error amplifier 3 is connected to the output terminal 6, the other input terminal is connected to the reference voltage source 4, and the error output line 7 is connected to the control circuit 2. This error amplifier 3
Outputs a voltage corresponding to the difference between the output detection voltage and the reference voltage. A current detector 5 comprising a current transformer is connected to the control circuit 2 by a line 8.

As shown in FIG. 2, the control circuit 2 includes a VCO (voltage controlled oscillator) 11, a waveform shaping circuit 12, a NOT circuit 13, a rectifying / smoothing circuit 14 for current detection, and a comparator 15
And an overcurrent detection reference voltage source 16 and an off control transistor 17.

[0005] The VCO 11 is connected to the output line 7 of the error amplifier 3 of FIG. 1, and its oscillation frequency becomes higher when the detection voltage at the output terminal 6 is higher than the reference value, and when the output detection voltage is lower than the reference value. Is configured to be low. The output of the VCO 11 is shaped into a square wave by the waveform shaping circuit 12 and sent to the base (control terminal) of the first transistor Q1 via the line 18 and via the NOT circuit 13 to the base of the second transistor Q2 via the line 19. (Control terminal).

The transistor 17 controls the second transistor Q2 to be turned off in the event of an overcurrent, and is connected between the line 19 and the ground, that is, the other end of the power supply 1 in FIG. The rectifying / smoothing circuit 14 includes a diode 14a and a capacitor 14b, and the output line 8 of the current detector 5
Is rectified and sent to one input terminal of the comparator 15.
The comparator 15 is a reference voltage source 1 connected to the other input terminal.
The reference voltage indicating the overcurrent of No. 6 is compared with the output voltage of the rectifying / smoothing circuit 14, and the transistor 17 is turned on when the output voltage of the rectifying / smoothing circuit 14 becomes higher than the reference voltage. As a result, the second transistor Q2 is turned off.

In the switching power supply shown in FIG. 1, the first and second transistors Q1 and Q2 are connected to the switching power supply shown in FIG.
They are turned on alternately as shown in FIG. Now, when the transistor Q1 is on, the power supply 1 and the first transistor Q
A current flows through a circuit consisting of a primary winding N1 having an inductance and a capacitor C1. This current is a current based on the series resonance of the primary winding N1 and the capacitor C1, has a waveform approximate to a sine wave, enables zero current switching at turn-on, and reduces switching loss. When the first transistor Q1 is turned off,
Instead, the second transistor Q2 is turned on, and the capacitor C1, the primary winding N1, and the second transistor Q2 are turned on.
A resonant current flows through the circuit consisting of By repeating the above operation, currents in the first and second directions alternately flow through the primary winding N1 of the output transformer T1, and corresponding output voltages are obtained through the secondary windings N2a and N2b. D3
, D4 and the capacitor C4. FIG. 6 shows the first
And the collector-emitter voltages VCE1 and VCE2 of the second transistors Q1 and Q2, the collector currents Ic1 and Ic2, and the current In1 of the primary winding N1 of the output transformer T1. The current in the region E1 of the current In1 in FIG. 6E is a parallel circuit composed of the DC power supply 1, the capacitor C2, the primary winding N1, and the resonance circuit including the capacitor C1, and the capacitor C1, the capacitor C3, and the primary winding N1. It flows based on the resonance circuit. The current in the region E2 of the current In1 depends on the capacitor C of the resonance circuit.
It flows through diode D2 instead of 3. The current in the regions E3, E4 in the negative half-wave flows through the capacitor C1, the primary winding N1, the capacitors C2 and C3, and then through the diode D1 instead of the capacitor C2.

In the device shown in FIG. 1, when the voltage at the output terminal 6 becomes higher than a predetermined value, the output frequency f2 of the VCO 11 shown in FIG.
1, the on / off repetition frequency of Q2 is increased. Conversely, when the voltage at the output terminal 6 is lower than the predetermined value, the operation is the opposite of the above.

The voltage V of the primary winding N1 of the output transformer T1
The amplitude of n1 varies depending on the on / off frequency f2 of the first and second transistors Q1, Q2. FIG. 3 shows the response of the resonance circuit including the inductance Ln1 of the primary winding N1 and the capacitor C1. The transistor Q has a higher frequency than the inherent resonance frequency f1 determined by Ln1 and C1.
1. When Q2 turns on and off, the response decreases.
FIG. 4 explains this. When f2 shown in the first half of FIG. 4 is low, the amplitude of the voltage Vn1 of the primary winding N1 is large, but when f2 shown in the second half is high. Voltage V
The amplitude of n1 decreases. As a result, the frequency f of the VCO 11
Voltage control is achieved by controlling in the range of 2 to f2 '.

[0010]

By the way, in the device shown in FIG. 1, the primary winding N is used to perform overcurrent protection or current control.
A current detector is connected in series with 1. Current detectors are not only expensive but also occupy a large area. A current detection resistor may be connected in series to the primary winding N1 instead of the current detector 5, but has the same drawbacks as the current detector 5, but also has the drawback of causing power loss.

An object of the present invention is to provide a resonance type switching power supply device capable of reducing the cost and size of the current detecting means.

[0012]

In order to achieve the above object, the present invention provides a series circuit of a DC power supply and first and second switching elements connected between one end and the other end of the DC power supply. A primary winding connected in parallel to the second switching element, and a secondary winding electromagnetically coupled to the primary winding.
An output transformer having a secondary winding; an output rectifying / smoothing circuit connected to the secondary winding; and an output rectifying / smoothing circuit connected in series to the first and second switching elements.
On-off control of the first and second switching elements alternately so as to keep the output voltage of the output rectifying / smoothing circuit constant, and the series resonance capacitor and inductance connected in series also to the next winding. A switching circuit for controlling the resonance, comprising: a rectifying / smoothing circuit for current detection connected to both ends of the resonance capacitor or the inductance for detecting currents of the series resonance capacitor and the inductance; Source
A comparator connected to the rectifying / smoothing circuit for current detection and the reference voltage source, and comparing a detection voltage obtained from the rectifying / smoothing circuit for current detection with a reference voltage of the reference voltage source,
A circuit for controlling at least one of the first and second switching elements to be in an off state or to reduce an on-time width thereof by an output of the comparator indicating that the detection voltage is higher than the reference voltage; The present invention relates to a resonance type switching power supply device characterized by comprising: In addition , the current detection may be performed by providing a tertiary winding as described in claims 3 and 4 . Further, c as shown in claims 5 and 6 - can be performed current detection-bridge resonant-type switching power supply device. Claim 2
As shown in FIGS. 4 and 6 , the resonance inductance can be obtained by the primary winding.

[0013]

The functions and effects of the present invention are realized by focusing on the fact that the voltage of the resonance circuit is proportional to the output power. The rectifying and smoothing circuit of each claim obtains a detection voltage corresponding to the output power. This detection voltage corresponds to the output current when the output voltage is constant. Therefore, the current and output current of the resonance circuit can be known from the detected voltage, and the rectifying / smoothing circuit functions as current detection means. Claim 1
In the case of 6 , a special current detector is not required, and the cost and size of the current detecting means can be reduced. In the third and fourth aspects, the tertiary winding is provided, but the cost and size can be reduced as compared with the case where the current detector is independently provided.

[0014]

First Embodiment Next, a resonance type switching power supply according to a first embodiment of the present invention will be described with reference to FIG. However,
In FIG. 6, portions common to FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 6, the current detector 5 provided in FIG. 1 is omitted. Instead, the voltage of the resonance capacitor C1 is detected by a line 8 as a voltage detection circuit, and the detected voltage is sent to the control circuit 2. The other configuration of FIG.
Is the same as In the resonance type switching power supply having the configuration shown in FIG. 6, when the output circuit of the output stage of the secondary windings N2a and N2b, that is, the power of the load connected to the output terminal 6 increases,
In proportion to this, the voltage of the primary winding N1 and the capacitor C1
The rectified smoothed value or the peak hold value of the voltage of the second voltage increases.
If the voltage at the output terminal 6 is controlled to a constant value, the rectified output voltage of the capacitor C1 and the primary winding increases in proportion to the load current. That is, the peak value of the resonance current flowing in a sine wave form in the series resonance circuit of the capacitor C1 and the primary winding N1 having the inductance L changes depending on the magnitude of the load connected to the output terminal 6, and the output current is reduced. When it gets bigger,
The peak value of the resonance current increases, and the peak value of the voltage of the primary winding N1 having the capacitor C1 or the inductance also increases. Therefore, the voltage of the capacitor C1 is
Thus, current detection is achieved with a simple configuration of detecting the current, and cost reduction and miniaturization are achieved. The voltage of the capacitor C1 detected on the line 8 is equal to the rectifying and smoothing circuit 1 of FIG.
4, a comparator 15 and a transistor 17, and the second transistor Q
Used for off control of 2.

[0015]

Second Embodiment Next, a resonance type switching power supply according to a second embodiment will be described with reference to FIG. However, in FIG. 7, portions common to FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In this embodiment, the primary winding N1 is moved to the ground side, and the voltage of the primary winding N1 is detected on the line 8 and sent to the control circuit 2. 7 is the same as FIG.
It is configured identically. Since the rectified smoothed voltage of the voltage of the primary winding N1 is proportional to the load current, the same operation and effect as in the second embodiment can be obtained.

[0016]

Third Embodiment Next, a resonance-type switching power supply according to a third embodiment of the present invention will be described with reference to FIG.
However, in FIG. 8, portions common to FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In this embodiment, a tertiary winding N3 is provided in the transformer T1 for current detection. This tertiary winding N3 is tightly electromagnetically coupled to the primary winding N1. Therefore, a voltage proportional to the primary winding N1 is obtained in the tertiary winding N3. Therefore, the voltage of the tertiary winding N3 can be used similarly to the current detection by the primary winding N1 of the second embodiment.

In FIG. 8, the first and second transistors Q1
, Q2, and an off-control circuit 2b for the second transistor Q2.
The on / off control circuit 2a shown in FIG. 8 shows a portion including the VCO 11, the waveform shaping circuit 12, and the NOT circuit 13 shown in FIG. The off control circuit 2b is provided instead of the rectifying / smoothing circuit 14, the comparator 15, and the transistor 17 shown in FIG.

The off control circuit 2b includes a rectifying / smoothing circuit 14 connected to the tertiary winding N3. The output terminal of the rectifying / smoothing circuit 14 is connected to the base of the transistor 17 via a Zener diode 16a functioning as a reference voltage source and a comparator. The transistor 17 is connected between the base and the emitter of the second transistor Q2 via a diode 31. In order to keep the transistor 17 turned on at the time of overcurrent, a Zener diode 32 is connected in parallel to the power supply 1 via a resistor 33, and the cathode of the Zener diode 32 is connected to the emitter of the latch transistor 34. The collector of the transistor 34 is connected to the base of the transistor 17, and the base of the latching transistor 34 is connected to the collector of the transistor 17. Note that a resistor 35 is connected between output lines of the rectifying / smoothing circuit 14.

When the voltage of the tertiary winding N3 increases in response to an increase in the load current, the output voltage of the rectifying / smoothing circuit 14 also increases. When the output voltage exceeds the Zener voltage of the Zener diode 16a, the Zener diode 16a turns on. , The base current of the transistor 17 is supplied, which is also turned on. As a result, the second transistor Q1 is turned off. When the transistor 17 is turned on, the latch transistor 34 is also turned on, and the base current of the transistor 17 is supplied from the power supply 1 and is kept on.
The diode 31 prevents the base current of the second transistor Q2 from flowing through the emitter / base of the latch transistor 34.

In the third embodiment, the tertiary winding N3 is provided, but the tertiary winding N3 is integrated with the transformer T1, so that the cost and size can be reduced compared to the case where the current detector 5 is provided alone as shown in FIG. You.

[0021]

Fourth Embodiment Next, a resonance type switching power supply according to a fourth embodiment will be described with reference to FIG. However, in FIG. 9, portions common to FIGS. 1 and 6 are denoted by the same reference numerals, and description thereof is omitted. In the embodiment of FIG. 9, the first and second switching power supply circuits 4
1, 42 are connected in parallel. The first and second switching power supply circuits 41 and 42 have the same circuit configuration. Instead of one resonance capacitor C1 in FIG.
Of the capacitors C1a and C1b. First and second
The series circuit of the resonance capacitors C1a and C1b is connected between one end and the other end of the power supply 1, and the primary winding N1 having the inductance of the transformer T1 is connected to the first and second switching elements Q1 and Q2. It is connected between the connection midpoint and the interconnection midpoint of the first and second resonance capacitors C1a and C1b. Therefore, the circuit of FIG. 9 is formed in a half-bridge type.

The first and second switching elements Q1, Q2 of FIG. 9 instead of the transistors Q1, Q2 of FIG.
Insulated gate type (M
OS type) field-effect transistor, equivalently including first and second control switches S1, S2 between a drain and a source.
2 and the first and second diodes connected in anti-parallel thereto.
De Da and Db. The first and second switching elements Q1 and Q2 can be composed of a bipolar transistor and a diode connected in anti-parallel to the bipolar transistor as shown in FIG. Also, the capacitors C2 and C2 are connected in parallel with the first and second switching elements Q1 and Q2 as in FIG.
3 can be connected.

First and second switching power supply circuits 4
Capacitors 46 and 47 are connected in parallel to the first resonance capacitor C1b via diodes 44 and 45 in order to detect a current supplied from the first and the second 42 to the load 43. A resistor 48 is connected in parallel with the capacitors 46 and 47.
A series circuit of 49 and a series circuit of resistors 50 and 51 are connected. A current balance detection resistor 52 is connected between the voltage dividing points of the resistors 48 and 49 set to the same voltage dividing ratio and the voltage dividing points of the resistors 50 and 51. This current detecting resistor 5
Both ends of 2 are connected to a differential amplifier 53 for current balance detection, and this output terminal is connected to control circuits 6a and 6b. The control circuits 6a and 6b respond to the output of an error amplifier (not shown) provided in the same manner as the error amplifier 3 of FIG. 1 to control the voltage of the load 43 to be constant, and the first and second switching elements. The on-time width of Q1 and Q2 is controlled. The control circuits 6a and 6b respond to the output of the differential amplifier 53 so that the output currents of the first and second switching power supply circuits 41 and 42 are balanced so that the first and second switching elements Q1 and Q2 Control the on-time width. The output of the differential amplifier 53 changes according to the current flowing through the resistor 52. When the output currents of the first and second switching power supply circuits 41 and 42 are the same, the voltages of the capacitors 46 and 47 become the same, and the divided output of the resistors 48 and 49 and the divided voltage of the resistors 50 and 51 are obtained. The output becomes the same, and no current flows through the resistor 52. However, when the output current becomes unbalanced, a current flows through the resistor 52.

In the first and second switching power supply circuits 41 and 42 of FIG. 9, series resonance is performed by the capacitance of the parallel circuit of the first and second resonance capacitors C1a and C1b and the inductance of the primary winding N1. Occurs.

In the circuit of FIG. 9, as in FIG. 6, it is not necessary to connect a current detector to the main circuit through which the resonance current flows, and the same operation and effect as in the circuit of FIG. 6 can be obtained. Also,
Output current adjustment of the first and second switching power supply circuits 41 and 42 can be easily achieved.

[0026]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) Two different outputs can be obtained by changing the number of turns of the secondary windings N2a and N2b. Also, the output transformer T1
, One of the secondary windings N2a and N2b can be omitted. (2) Transistors Q1, Q2 of FIGS. 6, 7 and 8
Can be replaced with another switching element such as a field effect transistor. (3) Transistor Q2 in the circuits of FIGS. 6 and 7
Can be changed to the off control circuit 2b of FIG. Conversely, the off control circuit 2b in FIG. 8 can be changed to the off control circuit in FIG. (4) The present invention can be applied to the case where the currents of the first and second transistors Q1 and Q2 are fed back using a mag amplifier to control the first and second transistors Q1 and Q2 in a self-excited manner. it can. (5) The switching power supply circuits of FIGS. 6, 7 and 8 can be connected in parallel as shown in FIG. 9 to provide a current balance detection circuit similar to that of FIG. (6) In the circuits of FIGS. 6, 7, 8 and 9, 1
A resonance inductance can be connected in series with the next winding N1. The output current can be known by detecting the voltage of the resonance inductance. In addition, the resonance inductance and the inductance of the primary winding may be summed up to form a series resonance inductance. (7) In the circuits of FIGS. 6, 7, 8 and 9, 1
In parallel with the secondary winding N1, an inductance greater than the inductance of the primary winding N1 or the inductance in series therewith can be connected. (8) In FIG. 9, the capacitance of the capacitors C1a and C1b can be increased, and a resonance capacitor having a smaller capacitance can be connected in series to the primary winding N1. (9) In FIG. 6, FIG. 7 and FIG. 8, the output current is cut off by controlling the second transistor Q2 to be off by the voltage detection by the line 8 or the tertiary winding N3. It can be turned off, or the first and second transistors Q1, Q2 can be turned off. Further, by detecting the current through the line 8 or the tertiary winding N3, the on-time width of the first and second transistors Q1 and Q2 can be reduced to reduce the output current.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a conventional switching power supply device.

FIG. 2 is a diagram illustrating a control circuit of FIG. 1;

FIG. 3 is a diagram for explaining the relationship between the on / off frequencies of Q1 and Q2 and the response of the resonance circuit of N1 and C1 in FIGS.

FIG. 4 shows the ON / OFF frequency of Q1 and Q2 and the primary winding N1.
FIG. 4 is a diagram showing a relationship with the voltage of FIG.

FIG. 5 is a waveform chart showing a state of each unit in FIG. 1;

FIG. 6 is a circuit diagram showing a switching power supply device according to the first embodiment.

FIG. 7 is a circuit diagram illustrating a switching power supply device according to a second embodiment.

FIG. 8 is a circuit diagram illustrating a switching power supply device according to a third embodiment.

FIG. 9 is a circuit diagram showing a switching power supply device according to a fourth embodiment.

[Explanation of symbols]

 T1 output transformer N11a, N11b Primary winding C1 Resonant capacitor 2 Control circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28

Claims (6)

(57) [Claims] 1. A DC power supply; a series circuit of first and second switching elements connected between one end and the other end of the DC power supply; and a parallel connection to the second switching element. An output transformer having a primary winding and a secondary winding electromagnetically coupled to the primary winding; an output rectifying / smoothing circuit connected to the secondary winding; A series resonance capacitor and an inductance respectively connected in series to the switching element and also in series to the primary winding; and the first so as to make the output voltage of the output rectifying and smoothing circuit constant. And a control circuit for alternately turning on / off the second switching element. The resonance type switching power supply device further comprising: A rectifying / smoothing circuit for current detection connected to both ends of the capacitor or the inductance; a reference voltage source; and a rectifying / smoothing circuit for current detection connected to the rectifying / smoothing circuit for current detection and the reference voltage source. A comparator for comparing the detected voltage with a reference voltage of the reference voltage source, and an output of the comparator indicating that the detected voltage is higher than the reference voltage, among the first and second switching elements. And a circuit for controlling at least one of them to be in an off state or to reduce an on-time width thereof. 2. A DC power supply; a series circuit of first and second switching elements connected between one end and the other end of the DC power supply; An output transformer having a primary winding connected in parallel to the secondary winding and a secondary winding electromagnetically coupled to the primary winding; an output rectifying / smoothing circuit connected to the secondary winding; A series resonance capacitor connected in series to the first and second switching elements and also connected in series to the primary winding; and making the output voltage of the output rectifying / smoothing circuit constant. And a control circuit for alternately turning on and off the first and second switching elements in order to detect the current of the series resonance capacitor and the primary winding. A rectifying / smoothing circuit for current detection connected to both ends of the resonance capacitor or the primary winding; a reference voltage source; and a rectifying / smoothing circuit for current detection connected to the rectifying / smoothing circuit for current detection and the reference voltage source. A comparator for comparing the detection voltage obtained from the rectifying / smoothing circuit with a reference voltage of the reference voltage source; and an output of the comparator indicating that the detection voltage is higher than the reference voltage. And a circuit for controlling at least one of the switching elements to be in an off state or to reduce an on-time width of the switching element. 3. A DC power supply; a series circuit of first and second switching elements connected between one end and the other end of the DC power supply; and a parallel connection to the second switching element. An output transformer having a primary winding and a secondary winding electromagnetically coupled to the primary winding; an output rectifying / smoothing circuit connected to the secondary winding; A series resonance capacitor and an inductance respectively connected in series to the switching element and also in series to the primary winding; and the first so as to make the output voltage of the output rectifying and smoothing circuit constant. And a control circuit for alternately turning on and off the second switching element. A tertiary winding densely electromagnetically coupled to the primary winding; For detecting current A rectifying / smoothing circuit for current detection connected to both ends of the tertiary winding; a reference voltage source; and a rectifying / smoothing circuit for current detection connected to the rectifying / smoothing circuit for current detection and the reference voltage source. A comparator for comparing the obtained detection voltage with the reference voltage of the reference voltage source; and an output of the comparator indicating that the detection voltage is higher than the reference voltage. And a circuit for controlling at least one of them to be in an off state or to reduce an on-time width thereof. 4. A DC power supply; a series circuit of first and second switching elements connected between one end and the other end of the DC power supply; An output transformer having a primary winding connected in parallel to the secondary winding and a secondary winding electromagnetically coupled to the primary winding; an output rectifying / smoothing circuit connected to the secondary winding; A series resonance capacitor connected in series to the first and second switching elements and also connected in series to the primary winding; and making the output voltage of the output rectifying / smoothing circuit constant. A control circuit for alternately turning on and off the first and second switching elements as described above, a tertiary winding tightly electromagnetically coupled to the primary winding, Primary winding A rectifying / smoothing circuit for current detection connected to both ends of the tertiary winding, a reference voltage source, a rectifying / smoothing circuit for current detection and the reference voltage source, A comparator for comparing the detection voltage obtained from the rectifying / smoothing circuit with a reference voltage of the reference voltage source; and an output of the comparator indicating that the detection voltage is higher than the reference voltage. And a circuit for controlling at least one of the switching elements to be in an off state or to reduce an on-time width of the switching element. 5. A DC power supply; a series circuit of first and second switching elements connected between one end and the other end of the DC power supply; and a series connection between one end and the other end of the DC power supply. An output transformer having a primary winding and a secondary winding electromagnetically coupled to the primary winding; and an output connected to the secondary winding. A rectifying / smoothing circuit; and an inductance connected via the primary winding between an interconnection midpoint of the first and second switching elements and an interconnection midpoint of the first and second capacitors. A control circuit that alternately turns on and off the first and second switching elements so as to keep the output voltage of the output rectifying / smoothing circuit constant. The first to detect current A rectifying / smoothing circuit for current detection connected to both ends of the capacitor or the second capacitor or the inductance; a reference voltage source; and a rectifying / smoothing circuit for current detection connected to the rectifying / smoothing circuit for current detection and the reference voltage source. A comparator for comparing a detection voltage obtained from a smoothing circuit with a reference voltage of the reference voltage source; and a first and a second output signal from the comparator indicating that the detection voltage is higher than the reference voltage. A circuit for controlling at least one of the switching elements to be in an off state or to reduce an on-time width thereof. 6. A DC power supply; a series circuit of first and second switching elements connected between one end and the other end of the DC power supply; and a series connection between one end and the other end of the DC power supply. A series circuit of the first and second capacitors, and a leakage connected between an interconnection midpoint of the first and second switching elements and an interconnection midpoint of the first and second capacitors. A primary winding of a transformer having an inductance; a secondary winding electromagnetically coupled to the primary winding; an output rectifying / smoothing circuit connected to the secondary winding; and an output voltage of the output rectifying / smoothing circuit. And a control circuit for alternately turning on and off the first and second switching elements so as to make the first constant. The capacitor or the second A rectifying / smoothing circuit for current detection connected to both ends of a capacitor or the primary winding; a reference voltage source; and a rectifying / smoothing circuit for current detection connected to the rectifying / smoothing circuit for current detection and the reference voltage source. And a comparator for comparing the detected voltage obtained from the above with the reference voltage of the reference voltage source; and the first and second switching elements based on an output of the comparator indicating that the detected voltage is higher than the reference voltage. And a circuit for controlling at least one of them to be in an off state or to reduce an on-time width thereof.