JP2002017082A - Power supply apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain circuit efficiency from lowering by load fluctuations or power voltage fluctuations with a simple structure, in a power supply apparatus capable of performing highly efficient power conversion by reducing switching loss. SOLUTION: This power supply unit formed with a power converter circuit having at least an inductor L1, a switching device Q1, and a rectifying device D1 between a DC power E and a load circuit LOAD, comprises a means of generating magnetic fluxes of the same polarity as the polarity of the magnet fluxes generated by an inductor current having a polarity applying negative- polarity current to the load circuit LOAD on a magnetic path forming the inductor L1 during the period except while the current running through the inductor L1 has such a polarity as to apply positive-polarity current to the load circuit LOAD. Preferably, a means for generating magnetic fluxes in the inductor L1 serves as a means for applying current to a secondary winding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device using a switching element, and more particularly to a technique for reducing switching loss and enabling high-efficiency power conversion.

[0002]

2. Description of the Related Art (Conventional Example 1) FIG. 10 shows an example of a conventional example. This example is shown in U.S. Pat. No. 4,727,308, in which a flywheel diode of a buck converter is replaced with an auxiliary switching element Q2. Main switch element Q1 and auxiliary switch element Q2 are alternately turned ON / O.
Repeat FF. The auxiliary switch element Q2 is turned off after a predetermined time when the polarity of the current of the inductor L1 is inverted.
When the main switch element Q1 is turned on by the energy stored in the inductor L1 due to the reverse polarity current, the voltage across the main switch element Q1 is reduced to zero volt. Thereby, the switching loss of main switching element Q1 can be suppressed. However, as compared with a normal buck converter, an auxiliary switch element Q2 having a breakdown voltage equivalent to that of the main switch element Q1 requiring high-side drive is required.

FIG. 11 shows an operation waveform of this conventional example. In the figure, IL1 is the current of the inductor L1, Vds is the voltage across the main switch element Q1, Vn2 is the voltage of the secondary winding of the inductor L1, Iq1 is the current of the main switch element Q1, Iq
2 is a current of the auxiliary switch element Q2, Vgs (q1) is a drive signal of the main switch element Q1, and Vgs (q2) is a drive signal of the auxiliary switch element Q2.

(Conventional Example 2) FIG. 12 shows a second conventional example. A rectifying element D
1 to form a normal buck converter. However, the voltage across the main switch element Q1 is indirectly detected via the secondary winding n2 of the inductor L1, and when the voltage across the main switch element Q1 is near the minimum, the main switch element Q1 is turned on. Thus, the switching loss at the time of turn-on is reduced. FIG. 13 shows the waveform of each part. In the figure, IL1 is the current of the inductor L1, Vds is the voltage across the main switch element Q1, and Vn2 is the inductor L1.
Is the drive signal for the main switch element Q1.

In the prior art 2, the ratio between the input voltage Vi and the output voltage Vo must be 2: 1 in order for the voltage across the main switch element Q1 to reach the vicinity of zero most efficiently. By the fluctuation of the voltage Vi and the output voltage Vo,
Efficiency deteriorates. FIG. 14 shows the waveform of each part when the output voltage Vo is low.

(Conventional Example 3) FIG. 15 shows a third conventional example. This example is disclosed in Japanese Patent Application No. 11-212719, in which a boost converter as a PFC function is provided in a stage preceding the buck converter, and the output voltage of the boost converter (capacitor EC
Changes). However, since the output voltage of the boost converter has a lower limit (input voltage Vi of the boost converter), if the output voltage Vo of the buck converter is extremely low, the efficiency deteriorates.

(Conventional Example 4) FIG. 16 shows a second conventional example. This example is disclosed in Japanese Patent Application No. 11-117066, and in this example, when the output voltage Vo becomes higher than the auxiliary power supply voltage Es set near 1/2 of the input voltage Vi, It works so as to suppress an increase in the current for making the voltage between both ends of the switch element Q1 zero volt. FIG.
17A and 17B are current loops of a normal buck converter. FIG. 17C shows a resonance loop that regenerates a necessary current in a loop of the auxiliary power supply Es, the diode Ds, the inductor L1, the switch element Q1, and the main power supply E in order to generate a necessary condition for zero voltage switching. Is shown. As a result, the resonance condition does not change with respect to changes in the power supply voltage Vi and the load voltage Vo, and the condition of zero voltage switching is secured.

FIG. 18 shows an operation waveform of this conventional example. In the figure, IL1 is the current of the inductor L1, Vds is the voltage across the main switch element Q1, Vn2 is the voltage of the secondary winding of the inductor L1, Iq1 is the current of the main switch element Q1, and Id.
1 is a current of the diode D1, Id2 is a current of the diode D2, and Vgs is a drive signal of the main switch element Q1.

In this embodiment, the resonance condition for zero voltage can be kept constant in a region where the output voltage Vo is equal to or higher than the auxiliary voltage Es. However, the output voltage Vo is lower than the auxiliary power supply voltage E.
If it changes in the region of s or less, the amplitude of resonance for zero voltage changes, and the condition of zero voltage switching may not be ensured in some cases.

[0010]

In each of the above-mentioned prior arts, the circuit efficiency is reduced due to load fluctuations and power supply voltage fluctuations, or the application range of a countermeasure circuit for suppressing the efficiency reduction is partial. There has been a problem that a countermeasure circuit for suppressing a decrease in efficiency is complicated.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide a power supply device capable of performing high-efficiency power conversion by reducing switching loss with a simple configuration. An object of the present invention is to suppress a decrease in circuit efficiency due to a load fluctuation and a power supply voltage fluctuation.

[0012]

According to the power supply device of the present invention, in order to solve the above problems, as shown in FIG.
In a power supply device including at least a power conversion circuit having at least an inductor L1, a switching element Q1, and a rectifying element D1 between a DC power supply E and a load circuit LOAD, at least a current flowing through the inductor L1 causes a positive current to flow to the load circuit LOAD. During a period excluding the period in which the polarity occurs, the load circuit LOAD is applied to the magnetic path forming the inductor L1.
And a means for generating a magnetic flux having the same polarity as a magnetic flux generated by an inductor current having a polarity for applying a negative current to the magnetic field.

[0013]

(Embodiment 1) FIG. 1 shows a first embodiment of the present invention. Hereinafter, the circuit configuration will be described. One end of a load LOAD is connected to a positive terminal of the input DC power supply E. The other end of the load LOAD is connected to one end of the inductor L1. The other end of the inductor L1 is connected to the anode of the diode D1 and one end of the main switch element Q1. The cathode of the diode D1 is connected to the positive terminal of the input DC power supply E. The other end of the main switch element Q1 is connected to the negative terminal of the input DC power supply E. A capacitor C1 is connected in parallel to both ends of the main switch element Q1. One end of the secondary winding n2 of the inductor L1 is grounded, and the other end is connected to the control circuit CNT1 via the resistor R1. The drive signal output from the control circuit CNT1 is supplied to the main switch element Q1. The connection point between the secondary winding n2 of the inductor L1 and the resistor R1 is connected to the cathode of the diode D2. The anode of the diode D2 is connected to one end of the auxiliary switch element Q2, and the other end of the auxiliary switch element Q2 is connected to the positive terminal of the auxiliary voltage source E2. The negative terminal of the auxiliary voltage source E2 is connected to the ground line (the negative terminal of the input DC power supply E). A second control circuit CN includes a diode D2, an auxiliary switch element Q2, and an auxiliary voltage source E2.
T2 is configured.

FIG. 2 shows operation waveforms of the present embodiment. In the figure, IL1 is the current of the inductor L1, Vds is the voltage across the main switch element Q1, Vn2 is the voltage of the secondary winding n2 of the inductor L1, Iq1 is the current of the main switch element Q1,
Iq2 is the current of the auxiliary switch element Q2, Vgs (q1)
Is a drive signal of the main switch element Q1, and Vgs (q2) is a drive signal of the auxiliary switch element Q2.

The control circuit CNT1 is connected to the inductor L1.
By detecting the voltage Vn2 of the next winding n2, the voltage across the main switch element Q1 is indirectly detected. When the voltage across the main switch element Q1 is minimized, the main switch element Q1 is turned on. One end of the secondary winding n2 of the inductor L1 is connected to the ground of the control circuit CNT1. The other end of the secondary winding n2 of the inductor L1 is input to the control circuit CNT1 via the resistor R1 as a source signal for detecting the switching timing. The auxiliary voltage source E2 is connected to the other end of the secondary winding n2 of the inductor L1 via the diode D2 and the auxiliary switch element Q2. At this time, the voltage of the auxiliary voltage source E2 is
0 volts and the output voltage V of the buck converter
A relationship is maintained such that the voltage is equal to or less than a voltage (Von2 / n1) obtained by multiplying the number of turns of the secondary winding n2 with respect to the primary winding n1 of the inductor L1. Further, by selecting the turns ratio of the secondary winding n2 so that the auxiliary voltage source E2 also serves as the power supply circuit of the control circuit CNT1, a simpler configuration can be achieved. The auxiliary switch element Q2 turns on after the main switch element Q1 turns off. However, since the voltage of the secondary winding n2 is higher than the voltage of the voltage source E2 during the period when the current flows through the first diode D1, the second diode D2 turns off and the secondary winding n2
No current flows through When the current in the inductor L1 returns to zero, the voltage across the secondary winding n2 starts to decrease. When the voltage of the auxiliary voltage source E2 is reached, from the auxiliary voltage source E2,
A current flows through the secondary winding n2 via the auxiliary switch element Q2 and the second diode D2.

When the calculation is performed by focusing on the energy of each part, I = √ (Vi · (Vi−2 · Vo) · C / L) (1) where, Vi: voltage of input power supply E Vo: output voltage C : Combined capacitance of capacitor C1 and main switch element Q1 L: Inductance value of inductor L1

By passing the above current through the secondary winding n2,
The voltage across main switch element Q1 can be set to zero volts. The auxiliary switch element Q2 turns off when the current detecting means of the secondary winding n2 detects that the secondary winding current is equal to or greater than the current given by the above equation. Thereafter, when the voltage between both ends of the main switch element Q1 falls below a predetermined value, the control circuit C
NT1 turns on main switching element Q1.

According to the present embodiment, a function equivalent to that of the prior art 1 requiring a high breakdown voltage element and a driving circuit can be realized by a low breakdown voltage element. The time ratio of the period during which the current flows from the control power supply E2 to the secondary winding n2 to the switching period is extremely short. Therefore, the average value of the currents involved in the secondary winding supply is very small, and the control power supply E2 does not become extremely large as compared with the conventional example. At this time, the energy supplied from the control power supply E2 to the secondary winding n2 of the inductor L1 is fed back to the main power supply E together with the energy of the electric charge accumulated in the output capacitance of the main switch element Q1 at the time of off. That is, the energy is fed back to the main power supply in the loop of the switch output capacitance (Q1, C1) → the primary winding n1 of the inductor L1 → the load circuit LOAD → the main power supply E, and the returned energy is indirectly applied to the load. In principle, there is no loss at all.

Further, the value of the equation (1) is negative, that is, 2
Even if no current is applied to the next winding n2, the main switching element Q1
By stopping the drive signal of the auxiliary switch element Q2 during the period in which the voltage across the terminal can be reduced to zero volt, the voltage across the main switch element Q1 can be reduced to zero volt, and the operation becomes exactly the same as the conventional operation. During this operation, the auxiliary switching element Q2 and the diode D2 are open to the secondary winding n2, and thus have no effect on the operation of the main circuit.

(Embodiment 2) FIG. 3 shows a second embodiment of the present invention. Hereinafter, the circuit configuration will be described. One end of an inductor L1 is connected to a positive terminal of the input DC power supply E. The other end of the inductor L1 is connected to one end of the main switch element Q1 and to the anode of the diode D1. The cathode of the diode D1 is connected to one end of the load LOAD. The other end of the load LOAD and the other end of the main switch element Q1 are connected to the negative terminal of the input DC power supply E. A capacitor C1 is connected in parallel to both ends of the main switch element Q1. Inductor L1
Of the secondary winding n2 is grounded, and the other end is connected to a resistor R2.
1 is connected to the control circuit CNT1. The drive signal output from the control circuit CNT1 is the main switch element Q
1 is supplied. The connection point between the secondary winding n2 of the inductor L1 and the resistor R1 is connected to the cathode of the diode D2. The anode of the diode D2 is connected to one end of the auxiliary switch element Q2.
Is connected to the positive terminal of the auxiliary voltage source E2.
The negative terminal of the auxiliary voltage source E2 is connected to the ground line (the negative terminal of the input DC power supply E). Diode D2
And the auxiliary switch element Q2 and the auxiliary voltage source E2
Of the control circuit CNT2.

FIG. 4 shows operation waveforms of the present embodiment. In the figure, IL1 is the current of the inductor L1, Vds is the voltage across the main switch element Q1, Vn2 is the voltage of the secondary winding n2 of the inductor L1, Iq1 is the current of the main switch element Q1,
Iq2 is the current of the auxiliary switch element Q2, Vgs (q1)
Is a drive signal of the main switch element Q1, and Vgs (q2) is a drive signal of the auxiliary switch element Q2.

In this embodiment, the present invention is applied to a boost converter configuration. However, the voltage of the auxiliary voltage source E2 exceeds 0 volt, and the difference between the output voltage Vo and the input voltage Vi of the buck converter is multiplied by the turns ratio of the secondary winding n2 to the primary winding n1 of the inductor L1. (Vo−Vi) · (n2 / n1) or less. According to the present embodiment, the same effect as in the first embodiment can be obtained in the boost converter configuration.

(Embodiment 3) FIG. 5 shows a third embodiment of the present invention. Hereinafter, the circuit configuration will be described. One end of an inductor L1 and one end of a load LOAD are connected to a positive terminal of the input DC power supply E. The other end of the inductor L1 is connected to one end of the main switch element Q1 and the anode of the diode D1. The cathode of the diode D1 is connected to the other end of the load LOAD. The other end of the main switch element Q1 is connected to the negative terminal of the input DC power supply E. A capacitor C1 is connected in parallel to both ends of the main switch element Q1. One end of the secondary winding n2 of the inductor L1 is grounded, and the other end is connected to the control circuit CNT1 via the resistor R1. The drive signal output from the control circuit CNT1 is supplied to the main switch element Q1. The connection point between the secondary winding n2 of the inductor L1 and the resistor R1 is connected to the cathode of the diode D2. The anode of the diode D2 is connected to one end of the auxiliary switch element Q2, and the other end of the auxiliary switch element Q2 is connected to the positive terminal of the auxiliary voltage source E2. The negative terminal of the auxiliary voltage source E2 is connected to the ground line (the negative terminal of the input DC power supply E). A second control circuit CN includes a diode D2, an auxiliary switch element Q2, and an auxiliary voltage source E2.
T2 is configured.

FIG. 6 shows operation waveforms of the present embodiment. In the figure, IL1 is the current of the inductor L1, Vds is the voltage across the main switch element Q1, Vn2 is the voltage of the secondary winding n2 of the inductor L1, Iq1 is the current of the main switch element Q1,
Iq2 is the current of the auxiliary switch element Q2, Vgs (q1)
Is a drive signal of the main switch element Q1, and Vgs (q2) is a drive signal of the auxiliary switch element Q2.

In the present embodiment, the present invention is applied to a buck-boost converter configuration, and the same effect as in the first embodiment can be obtained in the buck-boost converter configuration.

(Embodiment 4) FIG. 7 shows a fourth embodiment of the present invention. This embodiment is different from the first embodiment shown in FIG. 1 in that the switch element Q3 is connected in series with the capacitor C1 connected in parallel with the main switch element Q1, and the operation mode detecting means CNT3 of the converter is connected to the main switch element Q1. Have. The operation mode detecting means CNT3 detects a current immediately after the main switch element Q1 is turned on, and when the detected value is equal to or more than a predetermined value, it is regarded as a continuous current mode.
Otherwise, it is assumed to be in discontinuous current mode. In the case of the continuous current mode, the switching element Q3 is turned off. In the case of the discontinuous current mode, the switching element Q3 is turned on.

According to the configuration of the present embodiment, in the discontinuous current mode, the voltage between both ends of the main switch element Q1 at the time of turn-on is reduced by the resonance between the capacitor C1 and the inductor L1. In the case of the continuous current mode, the current of the inductor L1 always flows in the positive direction. Therefore, as in the case of the discontinuous current mode, it is not possible to use a means for reducing the voltage across the main switch element Q1 by the current in the reverse direction. The charge stored in the capacitor C1 connected to both ends during the off period of the main switch element Q1 is short-circuited by the main switch element Q1 when the main switch element Q1 is turned on, so that switching loss increases. Therefore,
In the case of the continuous current mode, by turning off the main switching element Q3, it is possible to prevent the charge of the capacitor C1 from being short-circuited and to suppress the switching loss.

(Embodiment 5) FIG. 8 shows a fifth embodiment of the present invention. This embodiment is different from the first embodiment shown in FIG. 1 in that a series circuit of a capacitor C4 and a switch element Q4 is connected in parallel with the main switch element Q1 and the capacitor C1, and the output current is directly / indirectly changed. It has means for detecting CNT4. When the output current means CNT4 detects that the output current is equal to or more than a predetermined value, the switch element Q4 is turned on and the on-period of the auxiliary switch element Q2 is lengthened.

When the output power is large, the ratio of the turn-off switching loss to the total switching loss becomes large. For this reason, in this method, as the output power increases, the effect of improving the efficiency of the conventional buck converter diminishes. In order to suppress the turn-off switching loss, a capacitor C connected in parallel with the main switch element Q1 is used.
It is better to increase the capacity of 1. This is because the rise of the drain voltage at the time of turn-off becomes gentle, and the fall of the drain current becomes sharp. However, if the capacitance of the capacitor C1 is simply increased, the resonance frequency of the capacitor C1 and the inductor L1 decreases, and the time required to reduce the voltage across the main switch element Q1 to zero volt by the resonance current becomes longer. This means that the time ratio of the time during which the current flows in the inductor L1 in the positive direction becomes shorter. Therefore, to get the same output,
It is necessary to increase the peak current of the inductor L1.
This can lead to new losses, such as inductors, that offset the suppressed turn-off switching loss. Therefore, by increasing the on-period of the auxiliary switching element Q2, it is possible to increase the energy for generating a negative current in the inductor L1, and to shorten the time until the voltage across the main switching element Q1 reaches zero volt. it can. By the above operation, it is possible to realize a power converter in which the efficiency improvement effect is less changed with respect to the output current fluctuation.

(Embodiment 6) FIG. 9 shows a sixth embodiment of the present invention. Hereinafter, the circuit configuration will be described. A power factor correction circuit PFC is connected to the AC power supply AC.
The output is connected to a smoothing capacitor EC. The power factor improvement circuit PFC includes a boost chopper circuit and the like, and has a function of rectifying and smoothing the AC power supply AC to obtain a boosted DC voltage, and reducing input current harmonic distortion. One end of a load LOAD is connected to a positive terminal of the smoothing capacitor EC. The load LOAD includes a series circuit of a discharge lamp LAMP and an inductor Lf and a parallel circuit of a capacitor Cf.
f constitutes a low-pass filter circuit. Load LOA
The other end of D is connected to one end of inductor L1. The other end of the inductor L1 is connected to the anode of the diode D1 and one end of the main switch element Q1. Diode D
One cathode is connected to the positive terminal of the smoothing capacitor EC. The other end of the main switch element Q1 is connected to the negative terminal of the smoothing capacitor EC. Main switch element Q1
, A series circuit of a capacitor C1 and a switch element Q3 is connected in parallel. Secondary winding n of inductor L1
One end of 2 is grounded, and the other end is connected to a control circuit CNT1 via a resistor R1. The drive signal output from the control circuit CNT1 is supplied to the main switch element Q1. The connection point between the secondary winding n2 of the inductor L1 and the resistor R1 is connected to the cathode of the diode D2. The anode of the diode D2 is connected to one end of the auxiliary switching element Q2, and the other end of the auxiliary switching element Q2 is connected to the positive terminal of the auxiliary voltage source E2. Auxiliary voltage source E
The negative electrode terminal 2 is connected to a ground line (a negative electrode terminal of the smoothing capacitor EC). A second control circuit CNT2 is constituted by the diode D2, the auxiliary switch element Q2, and the auxiliary voltage source E2. The switching element Q3 connected in series with the capacitor C1 connected in parallel with the main switching element Q1 is controlled to be turned on or off by the output of the operation mode detecting means CNT3 of the converter. The operation mode detecting means CNT3 detects a current immediately after the main switch element Q1 is turned on, and when the detected value is equal to or more than a predetermined value, it is regarded as a continuous current mode. Otherwise, it is assumed to be in discontinuous current mode. In the case of the continuous current mode, the switching element Q3 is turned off. In the case of the discontinuous current mode, the switching element Q3 is turned on.

This embodiment is a combination of a power factor correction circuit PFC, a buck converter, and a discharge lamp as a load, and is particularly preferable as a lighting device for a high-intensity discharge lamp in which the load has a low impedance in the starting process. It is a form.

[0032]

According to the present invention, in a power supply apparatus including at least a power conversion circuit having at least an inductor, a switch element, and a rectifying element between a DC power supply and a load circuit, at least a current flowing through the inductor is positive in the load circuit. A magnetic flux having the same polarity as a magnetic flux generated by an inductor current having a polarity for applying a negative current to a load circuit is generated in a magnetic path constituting the inductor during a period excluding a period having a polarity that generates a current in a direction. The provision of the squeezing means has an effect of suppressing a change in efficiency due to a load change and a power supply voltage change in a high-efficiency switching power supply with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

FIG. 2 is an operation waveform diagram according to the first embodiment of the present invention.

FIG. 3 is a circuit diagram according to a second embodiment of the present invention.

FIG. 4 is an operation waveform diagram according to the second embodiment of the present invention.

FIG. 5 is a circuit diagram of a third embodiment of the present invention.

FIG. 6 is an operation waveform diagram according to the third embodiment of the present invention.

FIG. 7 is a circuit diagram of a fourth embodiment of the present invention.

FIG. 8 is a circuit diagram according to a fifth embodiment of the present invention.

FIG. 9 is a circuit diagram of a sixth embodiment of the present invention.

FIG. 10 is a circuit diagram of Conventional Example 1.

FIG. 11 is an operation waveform diagram of Conventional Example 1.

FIG. 12 is a circuit diagram of a second conventional example.

FIG. 13 is an operation waveform diagram of Conventional Example 2.

FIG. 14 is an operation waveform diagram of the conventional example 2 when the output voltage is low.

FIG. 15 is a circuit diagram of a third conventional example.

FIG. 16 is a circuit diagram of a fourth conventional example.

FIG. 17 is a circuit diagram showing a current path of Conventional Example 4.

FIG. 18 is an operation waveform diagram of Conventional Example 4.

[Explanation of symbols]

 E input power supply Vi input voltage Vo output voltage Q1 main switch element D1 diode L1 inductor C1 capacitor Q2 auxiliary switch element E2 auxiliary power supply D2 diode

Claims (13)

[Claims] 1. A power supply apparatus comprising a power conversion circuit having at least an inductor, a switch element, and a rectifier element between a DC power supply and a load circuit, wherein at least a current flowing through the inductor generates a positive current in the load circuit. In a period excluding the period of the polarity, the magnetic path constituting the inductor is provided with means for generating a magnetic flux having the same polarity as the magnetic flux generated by the inductor current of the polarity for applying the negative current to the load circuit. A power supply device, characterized in that: 2. The power supply device according to claim 1, wherein the configuration of the power conversion circuit is a buck converter. 3. The power supply device according to claim 1, wherein the magnetic flux generating means for the inductor is a current applying means for a secondary winding provided on the inductor. 4. The current application means is connected to a secondary winding of the inductor via an auxiliary switching element, and is connected to a primary winding of the inductor exceeding 0 volts and an output voltage of a buck converter. 4. The power supply device according to claim 3, wherein the auxiliary voltage source is set to a voltage equal to or less than a voltage obtained by multiplying a turn ratio of a secondary winding with respect to the auxiliary voltage source. 5. The power conversion circuit according to claim 1, wherein the power conversion circuit is a boost converter, and has an auxiliary voltage source connected to a secondary winding of the inductor via an auxiliary switch element. Power supply. 6. The auxiliary voltage source of claim 5, wherein the difference between the output voltage and the input voltage of the boost converter exceeds 0 volts and the ratio of the number of turns of the secondary winding to the primary winding of the inductor is multiplied. A power supply characterized by being set to a voltage or lower. 7. The power conversion circuit according to claim 1, wherein the configuration of the power conversion circuit is a buck-boost converter, and further includes an auxiliary voltage source connected to a secondary winding of the inductor via an auxiliary switch element. The power supply as described. 8. The auxiliary voltage source according to claim 7, wherein the auxiliary voltage source is set to a voltage exceeding 0 volt and a voltage obtained by multiplying an output voltage of the buck-boost converter by a turns ratio of a secondary winding to a primary winding of the inductor. A power supply device characterized in that: 9. The switch element of the power conversion circuit,
9. The power supply device according to claim 1, wherein the power supply device is driven by a drive signal generated in a control circuit based on a voltage between both ends of a secondary winding of the inductor. 10. A power conversion circuit comprising: means for detecting an operation mode of the power conversion circuit; and when the operation mode is a continuous current mode, means for reducing capacitance at both ends of a switch element of the power conversion circuit. The power supply device according to any one of claims 1 to 9, wherein: 11. A power conversion circuit comprising means for detecting an output current of the power conversion circuit, and when it is detected that the output current is equal to or more than a predetermined value, the capacitance at both ends of the switch element of the power conversion circuit is increased. 8. The power supply device according to claim 4, wherein an on-period of said auxiliary switch element is extended. 12. The power supply device according to claim 1, wherein the load circuit has at least a discharge lamp. 13. A buck converter power conversion circuit having at least an inductor, a main switch element and a rectifier element is provided between a DC power supply circuit for rectifying and smoothing an AC power supply and a load circuit having at least a discharge lamp. In the power supply device, an auxiliary voltage source is connected to a secondary winding provided in an inductor forming the converter via an auxiliary switching element, and the auxiliary switching element is turned on at least during an off period of the main switching element forming the converter. A power supply device, wherein the main switch element is turned on at least in a state where the voltage between both ends of the main switch element constituting the converter is lower than when the auxiliary switch element is turned on.