JP2002354788A - Power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply circuit wherein its switching element is never latched in an interrupted condition even if the input power supply is temporarily interrupted. SOLUTION: The control power supply circuit 5 comprising a step-down chopper circuit is provided with a switching element Q2, an inductor L5, a filter capacitor C5, and a diode D5. The switching element Q2, in conjunction with a controller 51 using the filter capacitor C5 as its power source, composes an integrated circuit IC. The controller 51 contains a function to break and latch the switching element Q2 when a current flows into the switching element Q2 in the direction opposite to the one in ON status. A reverse current blocking diode Dc blocking the above reverse direction current for the switching element Q2 is connected to the drain side of the switching element Q2, preventing the latching function activated by the controller 51.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-down type chopper circuit in which a controller for controlling ON / OFF of a switching element has a protection function.

[0002]

2. Description of the Related Art Conventionally, an inverter circuit is used for a power supply unit in various devices for lighting and power. A switching element is used in this type of inverter circuit, and power for a control circuit that controls on / off of the switching element is supplied from a control power supply circuit.

Incidentally, as this kind of control power supply circuit, for example, a step-down chopper circuit having a configuration shown in FIG. 3 is sometimes used. The illustrated chopper circuit includes a series circuit in which a switching element Q2 composed of a MOSFET and a smoothing capacitor C5 are connected in series via an inductor L5, and a diode D5 is connected in parallel to the series circuit of the inductor L5 and the smoothing capacitor C5. . ON / OFF of the switching element Q2 is controlled by the controller 51. A DC voltage is applied to a series circuit of the switching element Q2, the inductor L5, and the smoothing capacitor C5. Of the diode D5 is set so as to be emitted in a path passing through.

As described above, since this kind of chopper circuit is widely used in various devices, an integrated circuit IC in which the switching element Q2 and the controller 51 are integrated into one chip in order to simplify the configuration of the control power supply circuit. (Eg, Power Intelligent MIP0222). This type of integrated circuit IC has three terminals, and a DC power supply for applying a DC voltage to a series circuit of the switching element Q2, the inductor L5, and the smoothing capacitor C5 during a start-up period immediately after power-on. Is supplied to the controller 51, and the power is supplied from the smoothing capacitor C5 to the controller 51 when the voltage across the smoothing capacitor C5 increases. Here, it is assumed that the illustrated chopper circuit is used as a control power supply circuit of a control circuit used for the inverter circuit, and the DC power supply also supplies power to the inverter circuit. The controller 51 has various protection functions. For example, when a current flows in the switching element Q2 in a direction opposite to that in the ON state, the controller 51 cuts off the switching element Q2 and switches the switching element Q2.
The function of latching the cut-off state of the switch is provided.

[0005]

Since the switching element Q2 is a MOSFET, it has a body diode. When the DC voltage applied as an input falls below the voltage across the smoothing capacitor C5, the switching element Q2 is switched through the body diode. There is a possibility that a current flows in a direction opposite to that when the element Q2 is turned on. The input DC voltage becomes lower than the voltage across the smoothing capacitor C5, for example, when the input DC voltage is momentarily stopped, and when the input DC voltage is obtained by rectifying the commercial power supply. Such a phenomenon may occur due to a momentary power failure of a commercial power supply.

As described above, when a current flows in the switching element Q2 in a direction opposite to that when the switching element Q2 is turned on, the switching element Q2 is cut off and the cutoff state is latched. Even so, the controller 51 does not release the latch because the voltage across the smoothing capacitor C5 has not decreased, and as a result, the voltage across the smoothing capacitor C5 decreases to such an extent that the operation of the controller 51 cannot be maintained. The chopper circuit cannot be restarted. In short, when the switching element Q2 is cut off due to a temporary stop of the input DC voltage or the like, a relatively long time is required until the operation of the chopper circuit is restarted.

If the above-mentioned chopper circuit is used as a control power supply circuit for supplying power to a control circuit for controlling the inverter circuit, the power supply to the control circuit is not performed normally, and the inverter circuit is relatively long. There is a case where it is inconvenient to use because it is stopped for a long time.

The present invention has been made in view of the above circumstances, and an object thereof is to prevent the switching element from being latched in a state where the switching element is cut off even when the supply of power to the input side is temporarily stopped. It is an object of the present invention to provide a power supply circuit which can be restarted immediately by restoring the power supply.

[0009]

According to the first aspect of the present invention, a series circuit in which a switching element and a smoothing capacitor are connected in series via an inductor is connected between both ends of a DC power supply,
A controller that controls the switching element to repeat on and off at a high frequency is provided, a diode is connected in parallel to a series circuit including the inductor and the smoothing capacitor, and the energy stored in the inductor when the switching element is turned on is calculated. A power supply circuit for discharging to the smoothing capacitor through a path passing through the diode when the switching element is turned off, the power supply circuit having a function of flowing a current in a direction opposite to a direction of a current flowing when the switching element is turned on, the controller However, while power is supplied from the smoothing capacitor except for a start-up period immediately after power-on, when a current flows in the switching element in a direction opposite to that when the switching element is turned on, the power is not supplied from the smoothing capacitor until the power supply is stopped. Machine for latching the switching element in a cut-off state Having a reverse blocking diode for blocking a current in a direction opposite to that when the switching element is turned on, on a path from one end of the DC power supply to one end of the inductor through the switching element. Features.

A second aspect of the present invention is characterized in that, in the first aspect of the present invention, the reverse blocking diode is inserted between the DC power supply and the switching element.

According to a third aspect of the present invention, in the first aspect, the reverse blocking diode is inserted between the switching element and the inductor.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) In this embodiment, a control power supply circuit used in a discharge lamp lighting device for lighting a high pressure discharge lamp used as a light source of a projector is illustrated as an embodiment of the present invention. I do. As the high-pressure discharge lamp, a metal halide lamp or an ultra-high-pressure mercury lamp having a short distance between electrodes so as to be regarded as a point light source is used.

As shown in FIG. 1, a step-down type chopper circuit 1 to which a DC voltage is input from a DC power supply (not shown) is provided as a DC power supply circuit, and the DC voltage output from the chopper circuit 1 is alternately changed by a polarity inversion circuit 2. It has a configuration in which the voltage is converted to a voltage and applied to a high-pressure discharge lamp (hereinafter, simply referred to as “discharge lamp”) DL. An igniter 3 for generating a high voltage for lighting the discharge lamp DL is inserted between the polarity inversion circuit 2 and the discharge lamp DL. That is, the chopper circuit 1
Supplies power to a load circuit including the polarity inversion circuit 2, the igniter 3, and the discharge lamp DL.

The operations of the chopper circuit 1 and the polarity inversion circuit 2 are controlled by a control circuit 4. In the control circuit 4,
Power is supplied from a control power supply circuit 5 provided separately from the chopper circuit 1 and to which a DC voltage is input from a DC power supply common to the chopper circuit 1. That is, the inputs of the chopper circuit 1 and the control power supply circuit 5 are common, connected to a DC power supply (not shown) via the low-pass filter LF, and a DC voltage of, for example, 370 V is input from the DC power supply. The low-pass filter LF blocks a high frequency higher than the switching frequency of the chopper circuit 1 and the control power supply circuit 5.

As the DC power supply, for example, a commercial power supply which is rectified by a rectifier circuit comprising a diode bridge and boosted by a booster type chopper circuit is used. The boost chopper circuit can be designed so that the input current from the commercial power supply does not cause a pause.
By setting the switching frequency of the switching element in the step-up chopper circuit relatively high and inserting a low-pass filter between the commercial power supply and the low-pass filter that blocks high frequencies above the switching frequency, the input current waveform from the commercial power supply is almost smooth. , And a waveform with little distortion can be obtained. In addition, since the boost chopper circuit can make the waveform of the envelope of the input current substantially proportional to the output voltage waveform of the rectifier circuit, the phase of the input current from the commercial power supply almost matches the phase of the input voltage, and the high input Power factor can be obtained. In short, the step-up chopper circuit is used as a power factor improving circuit for reducing generation of external noise and obtaining a high power factor. Note that a DC power supply having another configuration such as a battery power supply can be used.

The switching element Q1 of the chopper circuit 1 is composed of a MOSFET, and includes an inductor L1 connected to a DC power supply via the switching element Q1. That is, a series circuit of the switching element Q1 and the inductor L1 is inserted between the positive electrode of the DC power supply and the polarity inversion circuit 2. The connection point between the switching element Q1 and the inductor L1 is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the negative electrode of the DC power supply. Further, a series circuit of a capacitor C1 and a current detecting resistor R1 is connected in parallel to a series circuit of the inductor L1 and the diode D1. The voltage across the capacitor C1 is applied to the polarity inversion circuit 2 as an input voltage. The gate and source of the switching element Q1 are connected to the secondary winding of the pulse transformer PT1, and the on / off of the switching element Q1 is controlled by a control signal given from the control circuit 4 via the pulse transformer PT1. The ON / OFF switching frequency of the switching element Q1 is set relatively high.

With this configuration, energy is stored in the inductor L1 from the DC power supply through the switching element Q1 during the ON period of the switching element Q1, and the energy of the inductor L1 passes through the capacitor C1 and the diode D1 during the OFF period of the switching element Q1. Released in the route,
Charge is stored in the capacitor C1. In other words, the diode D1 is provided to regenerate energy stored in the inductor L1 through a path passing through the capacitor C1 when the switching element Q1 is turned on. By such an operation, the voltage between both ends of the capacitor C1 is reduced with respect to the input DC voltage. The ratio of the output voltage to the input voltage (step-down ratio = output voltage / input voltage) is determined by the on / off duty ratio of switching element Q1. That is, the output voltage of the chopper circuit 1 is made variable by changing the on / off duty ratio of the switching element Q1 by the control circuit 4.

The output voltage of the chopper circuit 1 is
The voltage across the resistor Rs corresponding to the output current of the chopper circuit 1 is input to the current monitoring unit 43 provided in the control circuit 4 via the determination unit 41 provided in the control circuit 4. Here, the output voltage of the chopper circuit 1 is substituted for the lamp voltage of the discharge lamp DL, and the output current of the chopper circuit 1 is substituted for the lamp current of the discharge lamp DL. As described below,
The determination unit 41 inputs a voltage proportional to the output voltage to the power monitoring unit 42 during a period when the output voltage of the chopper circuit 1 is within the constant power control range set for the lamp voltage, and outputs a voltage that deviates from the constant power control range. Is configured to input a constant voltage to the power monitoring unit 42. Since the output voltage of the chopper circuit 1 reflects the lamp voltage, the output voltage of the chopper circuit 1 is taken out separately from the path for inputting the output voltage to the judging section 41, and inputted to the lighting judgment section 44 of the control circuit 4, The lighting determination unit 44 determines the lighting state of the discharge lamp DL. The lighting determination unit 44 determines the start (dielectric breakdown), lighting (start of arc discharge), transition to a stable lighting state, extinguishing, and end of life (wear of electrodes) of the discharge lamp DL.

The output current of the chopper circuit 1 is determined by the current monitor 4
The current value monitored by the current monitoring unit 3 and detected by the current monitoring unit 43 is also input to the power monitoring unit 42. The power monitoring unit 42 obtains the output power of the chopper circuit 1 by multiplying the input current value and the input voltage value, and gives an instruction to the control signal generation unit 46 so that the output power is maintained at a constant value. The on / off duty ratio of the switching element Q1 is controlled. That is, constant power is supplied from the chopper circuit 1 to the polarity inversion circuit 2. In addition, since the power and the voltage are constant while the determination unit 41 outputs the constant voltage, the chopper circuit 1 outputs a constant current. That is, the output control unit includes the determination unit 41, the power monitoring unit 42, the current monitoring unit 43, and the control signal generation unit 46.

The polarity inversion circuit 2 is a bridge circuit composed of four switching elements Q3 to Q6. The switching elements Q3 and Q4 are connected in series to form one arm of the bridge circuit. Are connected in series to form the other arm of the bridge circuit. MOSFETs are used for the switching elements Q3 to Q6. Each arm of the bridge circuit is connected in parallel to the capacitor C1. Further, each of the switching elements Q3 to Q6 is connected to the drive signal generation unit 45 of the control circuit 4 via the drive circuits DV3 to DV6, respectively. The drive signal generation section 45 generates a binary drive signal, and turns on and off the set of the switching elements Q3 and Q6 and the set of the switching elements Q4 and Q5 alternately by the drive signal.

Between the connection point of switching elements Q3 and Q4 forming one arm of polarity inversion circuit 2 and the connection point of switching elements Q5 and Q6 forming the other arm, an inductor L2 and a capacitor C2 are connected. A series circuit is connected, and a series circuit of the secondary winding of the output transformer T1 provided in the igniter 3 and the discharge lamp DL is connected in parallel between both ends of the capacitor C2. Therefore, the polarity inversion circuit 2
By the above-mentioned operation, the polarity of the voltage applied to both ends of the discharge lamp DL is alternately inverted, and the alternating voltage is applied to the discharge lamp DL. Here, by using a lamp having a relatively low rated voltage as the discharge lamp DL, it is possible to use a small switching element Q3 to Q6 having a low capacity. Since the on-resistance is reduced, the amount of heat generated can be suppressed. In other words, it is possible to provide a discharge lamp lighting device which has a high energy use efficiency and a relatively small amount of heat generation, which leads to a reduction in energy wasted as heat loss without being used for optical output among the input electric energy. Will be possible.

The igniter 3 operates upon receiving the output of the pulse generating circuit 6 connected between the output terminals of the chopper circuit 1,
When energy corresponding to a plurality of pulses generated from the pulse generation circuit 6 is accumulated, a high-voltage pulse is generated in the secondary winding of the output transformer T1 at an appropriate timing. The pulse generation circuit 6 includes, for example, a resistor R6 and a capacitor C connected between the output terminals of the chopper circuit 1.
And a series circuit of the trigger element QT and the primary winding of the pulse transformer PT2 is connected between both ends of the capacitor C6. Therefore, the pulse generation circuit 6 performs a period in which the voltage across the capacitor C6 charged from the chopper circuit 1 via the resistor R6 can reach the breakover voltage of the trigger element QT (that is, the output voltage of the chopper circuit 1 is relatively low). A pulse is generated during a high period (in other words, a period when the input impedance of the polarity inversion circuit 2 is relatively high).

As the igniter 3, for example, a capacitor C3 charged by a secondary output of a pulse transformer PT2 having a primary winding connected to the trigger element QT of the pulse generation circuit 6 is provided, and a primary winding of the output transformer T1 is provided. A circuit in which a series circuit of the line and the discharge gap SG is connected between both ends of the capacitor C3 can be employed. In this configuration, when the voltage across the capacitor C3 reaches the breakover voltage of the discharge gap SG, the charge of the capacitor C3 is released through the primary winding of the output transformer T1, and the high-voltage pulse is applied to the secondary winding of the output transformer T1. Occurs.

The control power supply circuit 5 for supplying power to the control circuit 4 is a step-down chopper circuit having the same configuration as that of the chopper circuit 1, and similarly to the conventional configuration, the controller 51 is used as the switching element Q2. In addition, those constituting an integrated circuit IC are used. That is, the operation of the integrated circuit IC is the same as that of the conventional configuration, and the controller 51 has a protection function and the switching element Q
2 is configured to shut off the switching element Q2 when a current flows in a direction opposite to the ON state, and to latch a state in which the switching element Q2 is shut off while power is supplied to the controller 51. Switching element Q2
Is connected to the positive electrode of the DC power supply via a low-pass filter LF, and the other end of the switching element Q2 is connected to an inductor L5. Further, a cathode of a diode D5 is connected to a connection point between the switching element Q2 and the inductor L5, and an anode of the diode D5 is connected to a negative electrode of the DC power supply via a low-pass filter LF. A series circuit of an inductor L5 and a smoothing capacitor C5 is connected in parallel between both ends of the diode D5. Here, a capacitor having a relatively large capacity is used as the smoothing capacitor C5. The voltage across the smoothing capacitor C5 is used not only for the power supply of the control circuit 4 but also for the power supply of the controller 51 except for the startup period immediately after the power is turned on.

Therefore, for example, when the load of the chopper circuit 1 increases and the input voltage of the control power supply circuit 5 suddenly drops, the protection function is activated and the cutoff state of the switching element Q2 is latched. become. That is, since the smoothing capacitor C5 having a relatively large capacity is provided on the output side of the control power supply circuit 5, the output voltage is maintained even if the input voltage decreases, and the output voltage is higher than the input voltage. Condition occurs. In order to release the cut-off state of the switching element Q2, it is necessary to wait for the voltage across the smoothing capacitor C5 to fall to such an extent that the operation of the controller 51 cannot be maintained, and while the latched state continues, Even if the input voltage is restored, the protection function is kept activated, and the switching element Q
2 cannot be restarted.

Therefore, in the present embodiment, in order to prevent the protection function of the controller 51 from operating when the input voltage becomes lower than the output voltage, a backflow preventing device is provided between the DC power supply and the drain of the switching element Q2. Is inserted. With this configuration, even if the input voltage is lower than the output voltage of the control power supply circuit 5, the current that tends to flow backward through the switching element Q2 is blocked by the reverse blocking diode Dc.
Protection function does not operate, and the switching element Q
2 can be prevented from being latched in the cutoff state.

Hereinafter, the control circuit 4 will be described in detail. The control circuit 4 includes a control signal generation unit 46 that generates a control signal for turning on and off the switching element Q1 of the chopper circuit 1. The control signal generation unit 46 is supplied with DC power to the chopper circuit 1 (hereinafter, “ After power-on, the duty ratio in switching of the switching element Q1 is changed in accordance with the state monitored by the power monitoring section 42 and the current monitoring section 43, and the polarity inversion circuit 2 is turned on.
Is controlled so that an appropriate voltage is applied to

Immediately after the power is turned on, when the output voltage of the control power supply circuit 5 rises, the control circuit 4 starts operating, and a control signal for controlling the switching element Q1 of the chopper circuit 1 is output from the control circuit 4; A drive signal for controlling the switching elements Q3 to Q6 of the polarity inversion circuit 2 is output. Therefore, an alternating rectangular wave voltage is applied to the discharge lamp DL, and a high-voltage pulse is applied to the discharge lamp DL by the operation of the igniter 3. If normal when the high voltage pulse is generated, the discharge lamp DL is turned on.

By the way, in the discharge lamp DL used in the present embodiment, the lamp voltage becomes low immediately after lighting (immediately after the shift to arc discharge) because the vapor pressure of mercury is low. The lamp voltage increases. Therefore, in order to reach the stable lighting state (rated lighting state) in a short time, the applied current may be increased. It is necessary to limit the current value to a slightly larger value (about 1.5 times). Therefore, the output current of the chopper circuit 1 is set to a constant current until a transition to the stable lighting state, and after reaching the stable lighting state, the output power of the chopper circuit 1 is set to a constant power in order to stabilize the light output. Control.
That is, the period in which the output power of the chopper circuit 1 is controlled to be constant power is a period in which the output voltage of the chopper circuit 1 is within the constant power control range set based on the rated voltage of the discharge lamp DL. Thus, while the output voltage of the chopper circuit 1 is within the constant power control range, constant power is supplied to the discharge lamp DL, and the discharge lamp DL lights up in a stable lighting state.

(Second Embodiment) In the first embodiment, the reverse blocking diode Dc of the control power supply circuit 5 is
The reverse blocking diode Dc prevents the current flowing in the switching element Q2 from flowing in the switching element Q2 in a direction opposite to that when the switching element Q2 is turned on. 2 and not on the drain side of the switching element Q2.
As shown in (5), the same operation is performed even if a reverse blocking diode Dc is inserted on the path from the source of the switching element Q2 to the connection point between the diode D5 and the inductor L5. Other configurations and operations are the same as those of the first embodiment.

[0031]

According to the first aspect of the present invention, a series circuit in which a switching element and a smoothing capacitor are connected in series via an inductor is connected between both ends of a DC power supply so that the switching element repeats on and off at a high frequency. Providing a controller for controlling, a diode is connected in parallel to a series circuit consisting of the inductor and the smoothing capacitor, and the energy stored in the inductor when the switching element is turned on is passed through the diode when the switching element is turned off. A power supply circuit that discharges to the smoothing capacitor, the switching element having a function of flowing a current in a direction opposite to a direction of a current flowing when the switching element is turned off, and the controller excluding a startup period immediately after power-on. Power is supplied from the smoothing capacitor and the switching is performed. When a current flows in a direction opposite to that of the ON state, the switching element has a function of latching the switching element in a cut-off state until power supply from the smoothing capacitor is stopped. A reverse blocking diode for blocking a current in a direction opposite to that when the switching element is turned on is inserted on a path extending through the inductor to one end of the inductor.

According to this configuration, the current in the opposite direction to the time when the switching element is turned on does not pass through the switching element, and the protection function of the controller is prevented from being activated by the current flowing back to the switching element. Can be. As a result, the DC power supply is temporarily shut off,
Even when the voltage of the DC power supply is lower than the voltage across the smoothing capacitor, the controller is not latched in a state where the switching element is shut off, and the controller operates when the voltage of the DC power supply returns. Thus, the switching element can operate normally. In other words, even if the input-side power supply (DC power supply voltage) is temporarily stopped, the switching element is not latched in the cut-off state and can be restarted immediately after the DC power supply voltage is restored. become.

According to a second aspect of the present invention, in the first aspect of the present invention, the reverse blocking diode is inserted between the DC power supply and the switching element. In the invention according to the first aspect, the reverse blocking diode is inserted between the switching element and the inductor, and in any configuration, a current flows backward from the smoothing capacitor toward the DC power supply through the switching element. In order to prevent this, it is possible to cope with a simple configuration using only one reverse blocking diode.

[Brief description of the drawings]

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

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

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 5 Control power supply circuit 51 Controller C5 Smoothing capacitor D5 Diode Dc Reverse blocking diode L5 Inductor Q2 Switching element

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toru Niwa 1048 Kadoma Kadoma, Osaka Pref.Matsushita Electric Works, Ltd. (72) Inventor Junichi Hasegawa 1048 Odaka Kadoma, Kadoma, Osaka Pref.Matsushita Electric Works Co., Ltd. (72) Inventor Katsuka Nakata 1048 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Works, Ltd. (72) Inventor Shigeho Ikeda 3-9-114 Shindaka, Yodogawa-ku, Osaka Meiji National Industrial Co., Ltd. (72) Inventor Shigeru Ohara Noryo 3-9-114 Shintaka, Yodogawa-ku, Osaka Meiji National Industrial Co., Ltd. (72) Inventor Yuji Sasaki 3-9-1-14 Shintaka, Yodogawa-ku, Osaka Meiji National Industrial Co., Ltd. (72) Invention Person Hitaro Nagao 3-9-14-1 Shindaka, Yodogawa-ku, Osaka Meiji National Industrial Co., Ltd. (72) Inventor Atsushi Koseki Osaka City 3-9-1, Shindaka, Yodogawa-ku Meiji National Industrial Co., Ltd. (72) Inventor Junichi Umayaya 3-9-1-14 Shindaka, Yodogawa-ku, Osaka Meiji National Industrial Co., Ltd. (72) Inventor Noriaki Nishida Osaka 3-9-14 Shindaka, Yodogawa-ku, Tokyo Meiji National Industrial Co., Ltd. F-term (reference) 5H007 AA17 BB03 CA02 CB02 CB05 CC07 CC12 DA04 DA05 DC02 DC05 FA02 FA12 FA19 GA05 5H730 AS05 AS11 BB13 BB15 DD04 EE07 EE08 FD01 VFD31FG05 VV06 XX02 XX13 XX22 XX29 XX33 XX45

Claims (3)

[Claims] A controller that connects a series circuit in which a switching element and a smoothing capacitor are connected in series via an inductor between both ends of a DC power supply, and controls the switching element to turn on and off at a high frequency;
A diode is connected in parallel to a series circuit including the inductor and the smoothing capacitor, and energy stored in the inductor when the switching element is turned on is released to the smoothing capacitor via a path passing through the diode when the switching element is turned off. A power supply circuit, having a function of flowing a current in a direction opposite to a direction of a current flowing when the switching element is turned on when the controller is off, and the controller supplies power from the smoothing capacitor except for a start-up period immediately after power-on. And a function of latching the switching element in a cut-off state until a power supply from the smoothing capacitor is stopped when a current flows in the switching element in a direction opposite to that when the switching element is turned on. From one end of the power supply to one end of the inductor through the switching element Power supply circuit wherein the when the switching element on a path, characterized by comprising inserting a reverse blocking diode that prevents current opposite that. 2. The power supply circuit according to claim 1, wherein said reverse blocking diode is inserted between said DC power supply and said switching element. 3. The power supply circuit according to claim 1, wherein said reverse blocking diode is inserted between said switching element and said inductor.