JPH09121552A - Power supply apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply circuit whose output voltage is gradually increased since the power supply is closed, avoid a capacitive mode in a transient unstable state from the time of power supply closing to a steady state and reduce a stress applied to a switching device. SOLUTION: An input current harmonic is suppressed by an inverter circuit INV, a rush current at the time of power supply closing is suppressed by a step-down chopper circuit POW which is a power supply circuit and a switching device Q2 is shared by the inverter circuit INV and the step-down chopper circuit POW. A capacitive mode is avoided by the self-oscillation from the time of power supply closing until a voltage applied to the inverter circuit INV reaches a predetermined value and, when the voltage applied to the inverter circuit INV reaches the predetermined value, the self-oscillation is switched to the separate oscillation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply.

[0002]

2. Description of the Related Art A circuit diagram of a conventional example according to the present invention is shown in FIG.

In this circuit, the inverter circuit INV can suppress the input current harmonics to a low level, and the step-down chopper circuit POW as a power supply circuit can suppress the inrush current at the time of power-on, and First switching element (hereinafter referred to as switching element) Q
The inverter circuit INV including 2 and the step-down chopper circuit POW including the second switching element (hereinafter referred to as a switching element) Q2 share the switching element Q2, thereby reducing the size of the device. It is also possible to realize cost reduction.

This circuit includes a rectifier DB for full-wave rectifying the AC power supply Vs, a series connection of switching elements Q1 and Q2 connected to the output terminal of the rectifier DB, and a switching element Q.
A small-capacity (for example, about 0.47 μF) capacitor C15 connected to both ends of the series connection of 1 and Q2, a series connection of diodes D5 and D6 connected to both ends of the switching element Q2, and a diode D6. A series connection of an inductance element L2 and a smoothing capacitor C1 connected to both ends of a switching element Q1 via a capacitor, and a series connection of a capacitor C3 connected to both ends of a switching element Q2, a primary winding n1 of a transformer To, and a capacitor C4. And Trance To
Between the load Z1 connected to both ends of the secondary winding n2, the diode D3 connected to both ends of the capacitor C4, the diode D3, the contact of the primary winding n1 of the transformer To, and the negative terminal of the rectifier DB. It is composed of a diode D4 connected thereto and a control unit 10a in which the switching elements Q1 and Q2 repeat ON / OFF alternately.

Here, the load Z1 is a series connection of the discharge lamps La1 and La2 connected to both ends of the secondary winding n2 of the transformer To via a capacitor C5 and the discharge lamps La1 and La2.
Capacitor C connected between the non-power supply side terminals of the series connection of
And 2. A resonant circuit is configured by the capacitor C3, the primary winding n1 of the transformer To, and the capacitor C4.

The operation of suppressing the input current harmonics to a low level will be described below. (1) First, when the switching element Q1 is turned on, the smoothing capacitor C1 → the inductance element L2 → the switching element Q1 → the capacitor C3 → the primary winding n of the transformer To.
A current flows through the resonance circuit through the route of 1 → capacitor C4 → diode D5 → smoothing capacitor C1, and this current charges capacitor C4, resulting in voltage V across capacitor C4.
c4 (hereinafter referred to as voltage Vc4) rises. Then, from the moment when the sum of the output voltage VDB of the rectifier DB and the voltage Vc4 exceeds the voltage V1 supplied from the smoothing capacitor C1 to the inverter circuit INV, the AC power supply Vs → rectifier DB → switching element Q1 → capacitor C3 → transformer. Primary winding n1 of To → diode D4 → rectifier D
A current also flows in the resonance circuit in the path of B → AC power supply Vs, and this becomes an input current from the rectifier DB. Since the period during which the input current flows changes substantially in proportion to the magnitude of the power supply voltage Vs, the input current has a sinusoidal shape proportional to the power supply voltage Vs, and the input current harmonics can be suppressed low.

(2) Next, the switching element Q1 is turned off,
When the switching element Q2 turns on, the primary winding n1 of the transformer To → the capacitor C4 → the built-in diode D2 (not shown) of the switching element Q2 → the capacitor C3 → the primary of the transformer To is first caused by the regeneration mode of the inverter circuit INV. A current flows through the path of the winding n1 and then reverses, and with the capacitor C3 as a power supply, a current flows through the path of the capacitor C3 → the switching element Q2 → the capacitor C4 → the primary winding n1 of the transformer To → the capacitor C3. As a result, the voltage Vc4 decreases.

(3) Next, when the switching element Q2 is turned off, the regenerative mode of the inverter circuit INV is set for a while, and the primary winding n1 of the transformer To → the capacitor C3 → the built-in diode D1 (not shown) of the switching element Q1.
→ Capacitor C15 → Capacitor C4 → Current flows through the path of the primary winding n1 of the transformer To, and the voltage Vc4 continues to decrease. When the charge of the capacitor C4 is completely discharged, a current flows through the diode D3, not through the capacitor C4. Eventually, the current is reversed and the operation shown in (1) above is restored.

By repeating such an operation, the inverter circuit INV carries out an input current harmonic suppression operation and a high frequency oscillation operation.

Further, the step-down chopper circuit POW lowers the voltage V1 in the steady state, and has a low output frequency (for example, commercial frequency).
It is used for the purpose of reducing the ripple.

The control unit 10a also operates to avoid the phase advance mode, and the resistors R1, R2, NOT gates N1, N.
2, an AND gate AN1, a level shift circuit 2, and an oscillator 3. The avoidance method is to prevent the switching element from turning on unless a regenerative current flows through the diode built in the switching element or the diode connected in anti-parallel. Specifically, the switching element Q
The voltage VQ2 across the switching element Q2 is detected by the resistors R1 and R2 connected in parallel at both ends of the switch 2. When the detected voltage is at a low level, it is determined that the regenerative mode is in effect, and the switching terminal Q2 is turned on. Enables signal input. The ON signal to the control terminal of the switching element Q1 uses the inversion of the control signal to the control terminal of the switching element Q2 via the NOT gate N1 and the level shift circuit 2.

With the above configuration, the input current harmonics can be suppressed to a low level, the rush current at power-on can also be suppressed to a low level, and the phase advance mode can be avoided.

[0013]

However, the following problems occur in the above-mentioned conventional example.

Immediately after the power is turned on, the smoothing capacitor C1 is not charged and the voltage V1 is substantially zero.
The voltage VQ2 across the switching element Q2 detected by R2 becomes a low level regardless of the state of the switching element Q2, and immediately after the power is turned on the switching element Q2.
It becomes difficult to discriminate between the high level and the low level of the voltage VQ2 at both ends of No. 2, and the regeneration mode cannot be discriminated. Therefore, the control for avoiding the phase advance mode does not work effectively, and it always malfunctions that the ON signal can be input to the switching element Q2, and the phase advance mode occurs in the transient unstable state from the time when the power is turned on to the stable state. Excessive stress is applied to the switching elements Q1 and Q2.

Not only the structure shown in the above-mentioned conventional example but also the structure using the step-down chopper circuit and the step-up / down chopper circuit as the input power source of the inverter circuit INV, the smoothing capacitor C1 is not charged immediately after the power is turned on, Therefore, the voltage V
1 becomes almost zero. Therefore, with the method of avoiding the phase advance mode as described above, it is difficult to distinguish between the high level and the low level of the switching element, the control of the phase advance mode does not work effectively, and the stable state is maintained after the power is turned on. The phase leading mode is generated in the transient unstable state up to, and excessive stress is applied to the switching element, which causes a problem.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a power supply circuit such as a step-down chopper circuit whose output voltage gradually rises after the power is turned on, and to provide a power supply. An object of the present invention is to provide a power supply device capable of avoiding a phase advance mode in a transient unstable state from the time of turning on to a stable state, and capable of reducing stress applied to a switching element.

[0017]

In order to solve the above problems, according to the invention of claim 1, a power supply circuit for converting an AC power supply into a DC voltage and outputting it to a smoothing capacitor, and at least a first power supply circuit. An inverter circuit that has a switching element and converts a DC voltage into an AC high-frequency voltage and supplies the load to the load; and an oscillation operation of the inverter circuit is controlled, and if the voltage across the first switching element is low, In the power supply device including the control unit capable of inputting the ON signal of the switching element of No. 1, the inverter circuit performs self-excited oscillation operation after the AC power is turned on and until the voltage across the smoothing capacitor reaches a constant value. After the voltage across the smoothing capacitor reaches a certain value, the separately-excited oscillation operation is performed.

According to a second aspect of the present invention, the power supply circuit converts an AC power supply into a DC voltage and outputs the DC voltage to a smoothing capacitor, and at least a first switching element. The DC voltage is converted into an AC high frequency voltage. An inverter circuit for supplying the load to the load, and a control unit for controlling the oscillating operation of the inverter circuit and capable of inputting an ON signal of the first switching element when the voltage across the first switching element is at a low level. In the power supply device, the inverter circuit performs self-oscillation until the voltage applied to the inverter circuit reaches a certain value after the AC power is turned on, and the voltage applied to the inverter circuit reaches a certain value. After that, it is characterized by performing separately excited oscillation.

According to the third aspect of the present invention, the power supply circuit converts the AC power supply into a DC voltage and outputs the DC voltage to the smoothing capacitor, and at least the first switching element. The DC voltage is converted into an AC high frequency voltage. An inverter circuit for supplying the load to the load, and a control unit for controlling the oscillating operation of the inverter circuit and capable of inputting an ON signal of the first switching element when the voltage across the first switching element is at a low level. In the power supply device including the above, the threshold value for determining the voltage level across the first switching element is changed according to the change in the voltage applied to the inverter circuit.

According to a fourth aspect of the present invention, when the voltage across the smoothing capacitor decreases, the threshold value for determining the voltage level across the first switching element decreases.

According to the fifth aspect of the present invention, there are a plurality of thresholds for determining the voltage level across the first switching element, and the thresholds are discontinuous in response to changes in the voltage applied to the inverter circuit. Characterize that it changes.

According to the sixth aspect of the present invention, there are a plurality of thresholds for determining the voltage level across the first switching element, and the thresholds continuously change according to the change in the voltage applied to the inverter circuit. What to do is characteristic.

According to a seventh aspect of the present invention, the power supply circuit is a step-down chopper circuit including at least a second switching element.

According to an eighth aspect of the invention, the power supply circuit is a step-up / down chopper circuit including at least a second switching element.

According to a ninth aspect of the present invention, the first switching element and the second switching element are the same.

According to the tenth aspect of the invention, the inverter circuit is a half-bridge type inverter circuit.

[0027]

[Embodiment]

(Embodiment 1) FIG. 1 shows a circuit diagram of a first embodiment according to the present invention.

The difference from the conventional example shown in FIG. 9 is that a capacitor C3 and a switching element Q are provided so that self-excited oscillation is possible.
An inductance element L1 having secondary windings n21 and n22 is provided between contacts of 1 and Q2, three-terminal switches SW1 and SW2 for switching between self-excited oscillation and other-excited oscillation are provided, and a starting circuit 4 is provided. Therefore, the same configurations as those of the other conventional examples are denoted by the same reference numerals and the description thereof will be omitted. Here, the secondary windings n21 and n22 of the inductance element L1 are connected to one terminal of the three-terminal switches SW1 and SW2.
Of the drive circuits 1a and 1b are connected to the second terminal, and the gate terminals of the switching elements Q1 and Q2 are connected to the third terminal through resistors R3 and R4. . The starting circuit 4 is for starting the oscillation of the first inverter circuit INV immediately after the power is turned on,
It does not operate after the inverter circuit INV starts oscillating.

When the inverter circuit INV starts oscillating, self-excited oscillation is performed using the secondary voltage generated in the secondary windings n21 and n22 of the inductance element L1 depending on the mode of the current flowing in the primary winding n1 of the inductance element L1. When the operation is performed and the phase is advanced, the direction in which the voltage is generated is different, and the ON signal is not input to the switching elements Q1 and Q2. That is, the self-excited oscillation operation is a control method that is unlikely to be in the phase advance mode, but its disadvantages are that its characteristics are easily affected by variations in the windings and that controllability is difficult because the frequency and duty are difficult to change. Bad things and so on. Therefore, as the control of the inverter circuit INV, the self-excited oscillation is used in addition to the separately excited oscillation as shown in the conventional example shown in FIG. 9, and the self-excited oscillation is performed from the power-on until the voltage V1 reaches a predetermined value. The phase advance mode is avoided by switching to the separately excited oscillation when the voltage V1 reaches a predetermined value. Self-oscillation is caused by switching elements Q1, Q
Since the timing at which 2 turns on is determined by the voltage of the inductance element L1 provided in the resonance circuit, it is difficult to enter the phase advance mode. In a steady state where the voltage V1 exceeds a predetermined value, control may be performed for power supply fluctuation and output adjustment. In this case, the separately excited oscillation has excellent controllability.

In this embodiment, the secondary winding n21,
Although the inductance element L1 having n22 is used, the inductance element L1 may be omitted if the current transformer is used as the transformer To to control the switching element. Further, instead of the MOSFET having a built-in diode, transistors Q1 and Q2 in which diodes D1 and D2 are connected in antiparallel are provided.

With this configuration, it is possible to determine whether the switching element voltage is Hi / Low, the regeneration mode can be determined, and the phase advance mode can be avoided.

(Embodiment 2) FIG. 2 shows an operation waveform diagram of the second embodiment according to the present invention.

The difference from the conventional example shown in FIG. 9 is that the voltage V1
Is detected by resistors R5 and R6, and a detection voltage Vy obtained by dividing the voltage VQ2 across the switching element Q2 by resistors R1 and R2 is input to the level correction unit 5. Depending on the magnitude of V1, the detection voltage Vy, that is, the voltage VQ across the switching element Q2.
The Hi / Low discriminating section 6 discriminates between the high level and the low level of the voltage VQ2 across the switching element Q2 by performing the level correction of 2, and the same components as those of the conventional example are designated by the same reference numerals. Therefore, the description is omitted.

With this configuration, it is possible to determine whether the switching element voltage is high level or low level (hereinafter referred to as H
This is called i / Low discrimination. ) Is changed according to the change of the voltage V1, that is, when the voltage V1 is low, the threshold for determination is lowered so that both ends of the switching element Q2 can be surely maintained even in a low voltage V1 period after the power is turned on. H of voltage VQ2
i / Low discrimination is possible, and thus regeneration mode can be discriminated and phase advance mode can be avoided.

(Third Embodiment) FIG. 3 shows a circuit diagram of a third embodiment according to the present invention.

The present embodiment has a more specific structure than the second embodiment shown in FIG. 2, and the Hi / Low discriminating unit 6 in the circuit shown in FIG. N3, and the level correction unit 5 includes resistors R11 to R17,
Comparator COMP1, COMP2, switching element Q1
1 and Q12, and the same configurations as those of the other second embodiment are denoted by the same reference numerals and the description thereof will be omitted.

Next, the operation will be briefly described. Resistors R5, R
The voltage V1 divided into the detection voltage Vx at 6 is supplied to the comparator CO
It is input to the non-inverting input terminals of MP1 and COMP2. On the other hand, the voltage Vcc divided into the voltages Vcc1 and Vcc2 by the resistors R11 to R13 is input to the inverting input terminal. While the voltage V1 is low immediately after the power is turned on, the detection voltage Vx becomes lower than both the voltages Vcc1 and Vcc2, so that the outputs of the comparators COMP1 and COMP2 both become low level, and the switching elements are connected via the resistors R14 and R15. Since Q11 and Q12 are turned off, the voltage VQ2 across the switching element Q2 is equal to the resistances R1 and R2.
The partial pressure is detected by. When the voltage V1 rises and the detection voltage Vx becomes higher than the voltage Vcc2 and lower than the voltage Vcc1, the output of the comparator COMP1 becomes low level, the output of the comparator COMP2 becomes high level, and the resistance R14,
Since the switching element Q11 is turned off and the switching element Q12 is turned on via R15, the resistor R17 is connected in parallel to both ends of the resistor R2, and the voltage VQ2 across the switching element Q2 is divided by the resistors R1, R2 and R17, and is detected. The detection voltage Vy decreases. When the voltage V1 further rises and the detection voltage Vx becomes higher than the voltages Vcc1 and Vcc2, the outputs of the comparators COMP1 and COMP2 both become high level, and the switching elements Q11 and Q12 are turned on via the resistors R14 and R15. So the resistance R
The resistors R16 and R17 are connected in parallel to both ends of the switching element 2 and the voltage VQ2 across the switching element Q2 is equal to the resistances R1, R2 and R2.
The divided voltage is detected by 16 and R17, and the detected voltage Vy further decreases.

In this way, the detection voltage Vy is reduced as the voltage V1 rises. In other words, the voltage Vy is reduced.
Since the detection voltage Vy is increased with the decrease of 1, even if the voltage V1 is low, the switching element voltage Hi /
It is possible to discriminate Low, and thus it is possible to discriminate the regeneration mode and avoid the phase advance mode. Although the detection voltage Vy is changed in three steps in the present embodiment, it may be changed in n steps (n> 2).
Further, it may be configured to change continuously.

(Fourth Embodiment) A circuit diagram of a fourth embodiment according to the present invention is shown in FIG. 4, and an operation waveform diagram thereof is shown in FIG.

The difference from the second embodiment shown in FIG. 2 is that the Hi / Low discriminating unit 6 uses a comparator COMP3 that compares the detection voltages Vx and Vy and outputs a comparison output Vo, and a level correcting unit. 5 and the Hi / Low discriminating unit 6 are also used. The same components as those of the second embodiment are denoted by the same reference numerals and the description thereof will be omitted.

Here, when the switching element Q2 is on, the voltage VQ2 across the switching element Q2 is substantially zero V, and when the switching element Q2 is off, the voltage VQ2 across the switching element Q2 is approximately equal to the voltage V1. Therefore, even if the value of the voltage V1 is low, by properly selecting the voltage division ratio of the resistors R5 and R6, the detection voltage Vx and the detection voltage Vy can be obtained.
Is as shown in FIG. 5 (a), and the comparison output Vo as shown in FIG. 5 (b) is obtained. Therefore, even if the voltage V1 is low, it is possible to determine whether the switching element voltage is Hi / Low. The mode can be discriminated and the phase advance mode can be avoided.

(Fifth Embodiment) FIG. 6 shows a circuit diagram of a fifth embodiment according to the present invention, and FIG. 7 shows an operation waveform diagram thereof.

The difference from the fourth embodiment shown in FIG. 4 is that a Zener diode ZD1 is connected in parallel to both ends of a resistor R6 and a Zener diode ZD2 is connected in parallel to both ends of a resistor R2. Therefore, the same configurations as those of the other fourth embodiment are denoted by the same reference numerals and the description thereof will be omitted.

When the voltage V1 is low, it is preferable that the resistance values of the resistors R2 and R6 are as large as possible in order to improve the detection sensitivity. However, if the resistance value is increased, when the voltage V1 rises to a high voltage, the voltage of the resistors R2 and R6 becomes high this time, and if the selection of the constant is wrong, the comparator COMP3 is selected.
The input terminal's maximum rating may be exceeded. To prevent this, Zener diodes ZD1 and ZD
2 is provided to protect the input terminal of the comparator COMP3.

With this configuration, the detection voltage Vx and the detection voltage Vy are set as shown in FIG. 7A by properly selecting the voltage division ratio of the resistors R5 and R6 even when the voltage V1 is low. Therefore, since the comparative output Vo as shown in FIG. 7B is obtained, even if the voltage V1 is low, the switching element voltage H
The i / Low discrimination can be performed, and thus the regeneration mode can be discriminated and the phase advance mode can be avoided.

(Sixth Embodiment) FIG. 8 shows a circuit diagram of a sixth embodiment according to the present invention.

This circuit is one in which the present invention is applied to another circuit configuration, and is different from the second embodiment shown in FIG. 2 in that capacitors C3, C4, transformer To, load Z1, and
Diodes D3 and D4 are omitted, and a series circuit of a capacitor C11 and a load Z2 is connected in parallel to both ends of the switching element Q1. An inductance element L2 is omitted and a diode D11 and an inductance element L4 are connected to both ends of the smoothing capacitor C1. Is connected in parallel, and the inductance element L4 is connected between the positive output terminal of the rectifier DB and one end of the smoothing capacitor C1. The same components as those of the second embodiment are denoted by the same reference numerals. The description is omitted by attaching. Here, the load Z2 is composed of a series circuit of an inductance element L10 and a capacitor C10, and a load LOAD connected in parallel with the capacitor C10. Further, instead of the MOSFET having a built-in diode, transistors Q1 and Q2 in which diodes D1 and D2 are connected in antiparallel are provided.

With this configuration, it is possible to discriminate the Hi / Low of the switching element voltage even when the voltage V1 is low, so that it is possible to discriminate the regeneration mode and avoid the phase advance mode.

In all the above embodiments,
The loads Z1 and Z2 may have any other configuration.

[0050]

EFFECTS OF THE INVENTION Claims 1 to 6 and Claim 10
According to the invention described in (1), it is possible to provide a power supply device capable of avoiding a phase advance mode in a transient unstable state from power-on to a stable state, and capable of reducing stress applied to a switching element.

According to the seventh and eighth aspects of the present invention, by providing the power supply circuit in which the output voltage gradually rises after the power is turned on, it is possible to suppress the inrush current when the power is turned on, and It is possible to provide a power supply device capable of avoiding a phase advance mode in a transient unstable state from power-on to a stable state, and capable of reducing stress applied to a switching element.

According to the ninth aspect of the present invention, by providing the power supply circuit in which the output voltage gradually increases after the power is turned on, it is possible to suppress the inrush current when the power is turned on, and it is possible to reduce the size. It is possible to provide a power supply device capable of avoiding a phase advance mode in a transient unstable state from power-on to a stable state, and capable of reducing stress applied to a switching element.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment according to the present invention.

FIG. 2 is a circuit diagram showing a second embodiment according to the present invention.

FIG. 3 is a circuit diagram showing a third embodiment according to the present invention.

FIG. 4 is a circuit diagram showing a fourth embodiment according to the present invention.

FIG. 5 is an operation waveform diagram according to the above embodiment.

FIG. 6 is a circuit diagram showing a fifth embodiment according to the present invention.

FIG. 7 is an operation waveform diagram according to the embodiment.

FIG. 8 is a circuit diagram showing a sixth embodiment according to the present invention.

FIG. 9 is a circuit diagram showing a conventional example according to the present invention.

[Explanation of symbols]

 1 Control part C Capacitor INV Inverter circuit POW power supply circuit Q Switching element Vs AC power supply Z Load

Claims (10)

[Claims] 1. An inverter having a power supply circuit for converting an AC power supply into a DC voltage and outputting it to a smoothing capacitor, and at least a first switching element, for converting the DC voltage into an AC high frequency voltage and supplying it to a load. Power supply device comprising a circuit and a control unit for controlling the oscillating operation of the inverter circuit and capable of inputting an ON signal of the first switching element when the voltage across the first switching element is at a low level. In the inverter circuit, after the AC power is turned on, the self-excited oscillation operation is performed until the voltage across the smoothing capacitor reaches a constant value, and after the voltage across the smoothing capacitor reaches a constant value, other A power supply device characterized by performing an excited oscillation operation. 2. An inverter having a power supply circuit for converting an AC power supply into a DC voltage and outputting it to a smoothing capacitor, and at least a first switching element, for converting the DC voltage into an AC high frequency voltage and supplying it to a load. Power supply device comprising a circuit and a control unit for controlling the oscillating operation of the inverter circuit and capable of inputting an ON signal of the first switching element when the voltage across the first switching element is at a low level. In the inverter circuit, the self-excited oscillation operation is performed until the voltage applied to the inverter circuit reaches a constant value after the AC power is turned on, and the voltage applied to the inverter circuit reaches a constant value. After that, the power supply device is characterized by performing separately excited oscillation operation. 3. An inverter having a power supply circuit for converting an AC power supply into a DC voltage and outputting it to a smoothing capacitor, and at least a first switching element, for converting the DC voltage into an AC high frequency voltage and supplying it to a load. Power supply device comprising a circuit and a control unit for controlling the oscillating operation of the inverter circuit and capable of inputting an ON signal of the first switching element when the voltage across the first switching element is at a low level. In the power supply device, the threshold value for determining the voltage level across the first switching element is changed according to the change in the voltage applied to the inverter circuit. 4. The power supply device according to claim 3, wherein when the voltage across the smoothing capacitor decreases, the threshold for determining the voltage level across the first switching element decreases. 5. A plurality of thresholds for determining the voltage level across the first switching element are provided, and the thresholds change discontinuously in response to changes in the voltage applied to the inverter circuit. The power supply device according to claim 3 or 4. 6. A plurality of thresholds for determining the voltage level across the first switching element are provided, and the thresholds are continuously changed according to a change in voltage applied to the inverter circuit. The power supply device according to claim 3 or 4. 7. The power supply device according to claim 1, wherein the power supply circuit is a step-down chopper circuit including at least a second switching element. 8. The power supply device according to claim 1, wherein the power supply circuit is a step-up / down chopper circuit including at least a second switching element. 9. The first switching element and the second switching element.
9. The power supply device according to claim 1, wherein the power supply device is the same as the switching element. 10. The power supply device according to claim 1, wherein the inverter circuit is a half-bridge type inverter circuit.