JP6506020B2 - Step-down chopper - Google Patents

Description

  The present invention relates to a step-down chopper, and more particularly to a step-down chopper provided with a protection circuit.

  Patent Document 1 discloses an electric power system for a vehicle which determines that an overcurrent flows when a time when an output voltage of a current sensor exceeds an overcurrent determination threshold exceeds an overcurrent determination time. In this electric power system for a vehicle, when the time when the output voltage of the current sensor exceeds the sensor abnormality determination threshold exceeds the sensor abnormality determination time, it is determined that the current sensor has failed, and the sensor abnormality determination time is an overcurrent By making the time shorter than the determination time, the occurrence of the overcurrent and the failure of the current sensor are identified.

  Further, according to Patent Document 2, a bypass circuit is formed in parallel in the main path for supplying the power supply voltage to the load, stopping the overcurrent detection function for a certain time after power-on, and for a certain time when the load is inserted and removed. An over current protection circuit is disclosed which reduces the current flowing through a current sensor provided in a main path to suppress an over current detection function. In this overcurrent protection circuit, it is possible to prevent an inrush current generated at the time of power on or at the time of insertion and removal of a load from being erroneously detected as an overcurrent due to an abnormality occurrence.

JP, 2009-213219, A JP, 2010-148262, A

  However, Patent Literatures 1 and 2 do not disclose the malfunction of the step-down chopper that occurs when the output terminals are short-circuited and the method for preventing the malfunction.

  Therefore, a main object of the present invention is to provide a step-down chopper capable of preventing malfunction when the output terminals are short-circuited.

  The step-down chopper according to the present invention is a step-down chopper which steps down a DC voltage applied between the first and second input terminals and outputs the voltage between the first and second output terminals. A switching element connected to the input terminal of the switch, a first diode having an anode connected to the second input terminal and the second output terminal, and a cathode connected to the other electrode of the switching element, and the other electrode of the switching element A reactor connected between the first output terminal and a capacitor, a capacitor connected between the first and second output terminals, a current detector for detecting a current flowing through the reactor, and detection values of the current detector Protection circuit that outputs a gate block signal when the current exceeds the first current value, and when the detection value of the current detector exceeds the second current value larger than the first current value. Current check The switching element is turned on / off so that the voltage between the second protection circuit that outputs a signal and the first and second output terminals becomes a target voltage, and a gate block signal is output from the first protection circuit. When the switching element is turned off, and the overcurrent detection signal is output from the second protection circuit, a control circuit for fixing the switching element in the off state, the anode is connected to the second output terminal, and the cathode is And a second diode connected to the first output terminal.

  Another step-down chopper according to the present invention is a step-down chopper which steps down a DC voltage applied between first and second input terminals and outputs the voltage between first and second output terminals, one electrode of which is one of the electrodes. A switching element connected to the first input terminal, a diode having an anode connected to the second input terminal and the second output terminal, and a cathode connected to the other electrode of the switching element, and the other electrode of the switching element A reactor connected between the first output terminal, a capacitor connected between the first and second output terminals, a current detector for detecting a current flowing through the reactor, and detection values of the current detector The first protection circuit that outputs a gate block signal when the first current value is exceeded, the gate block signal is not output from the first protection circuit, and the detection value of the current detector is Switching so that the voltage between the first and second output terminals becomes the target voltage, and a second protection circuit that outputs an overcurrent detection signal when the second current value larger than the current value of 1 is exceeded The element is turned on / off, the switching element is turned off when the gate block signal is outputted from the first protection circuit, and the switching element is turned off when the overcurrent detection signal is outputted from the second protection circuit. And a control circuit fixed to the

  Another step-down chopper according to the present invention is a step-down chopper which steps down a DC voltage applied between the first and second input terminals and outputs the voltage between the first and second output terminals. A switching element connected to the first input terminal, a diode having an anode connected to the second input terminal and the second output terminal, and a cathode connected to the other electrode of the switching element, and the other electrode of the switching element A reactor connected between the first output terminal and a capacitor, a capacitor connected between the first and second output terminals, a current detector for detecting a current flowing through the reactor, and detection values of the current detector The gate block signal is output when the first current value is exceeded, and the number of times the gate block signal is output is counted, and the count value is within a predetermined time. The first protection circuit that outputs a failure detection signal when exceeding a predetermined value, and the switching element turned on / off so that the voltage between the first and second output terminals becomes a target voltage, A control circuit is provided to turn off the switching element when the gate block signal is output from the protection circuit 1 and to fix the switching element in the off state when the failure detection signal is output from the first protection circuit. It is a thing.

  In the step-down chopper according to the present invention, the second diode is provided, the anode being connected to the second output terminal and the cathode being connected to the first output terminal. Therefore, when the first and second output terminals are short-circuited and the switching element is turned off, a resonant circuit including a capacitor can be formed to prevent an overcurrent from flowing in the reactor. Malfunction can be prevented.

  Further, in another step-down chopper according to the present invention, when the gate block signal is output and the switching element is turned off, the output of the overcurrent detection signal is stopped. Therefore, when the first and second output terminals are short-circuited and the switching element is turned off, the overcurrent detection signal can be prevented from being output, and the malfunction of the step-down chopper can be prevented. .

  Further, in still another step-down chopper according to the present invention, the number of times the gate block signal is output is counted, and a fault detection signal is output when the count value exceeds a predetermined value within a predetermined time. Do. Therefore, when the first and second output terminals are short-circuited, it is possible to prevent the gate block signal from being repeatedly output, and to prevent the malfunction of the step-down chopper.

FIG. 1 is a circuit block diagram showing a configuration of a step-down chopper according to Embodiment 1 of the present invention. It is a circuit block diagram which shows the structure of the protective circuit 9 shown in FIG. FIG. 2 is a circuit block diagram showing a configuration of a protection circuit 10 shown in FIG. It is a circuit diagram for demonstrating the problem which generate | occur | produces when the diode 7 shown in FIG. 1 is not provided. It is a circuit block diagram which shows the structure of the step-down chopper by Embodiment 2 of this invention. It is a circuit block diagram which shows the structure of the protective circuit 30 shown in FIG. It is a circuit block diagram which shows the structure of the step-down chopper by Embodiment 3 of this invention. It is a circuit block diagram which shows the structure of the protective circuit 35 shown in FIG. It is a circuit block diagram which shows the structure of the step-down chopper by Embodiment 4 of this invention. It is a circuit block diagram which shows the structure of the protective circuit 40 shown in FIG.

First Embodiment
1 is a circuit block diagram showing a configuration of a step-down chopper according to Embodiment 1 of the present invention. In FIG. 1, the step-down chopper includes input terminals T1 and T2, output terminals T3 and T4, a switch 1, IGBTs (Insulated Gate Bipolar Transistors) 2, diodes 3 and 7, current detectors 4, reactors 5, A capacitor 6, a voltage detector 8, protection circuits 9 and 10, an operation unit 11, and a control circuit 12 are provided.

  Input terminals T1 and T2 are connected to the positive electrode and the negative electrode of DC power supply 15, respectively, and receive DC voltage from DC power supply 15. A load 16 is connected between the output terminals T3 and T4. The step-down chopper steps down the DC voltage supplied from the DC power supply 15 and applies it to the load 16. The load 16 is driven by the output voltage of the step-down chopper.

  One electrode of the switch 1 is connected to the input terminal T1. The switch 1 is controlled by the control circuit 12 and is normally turned on, and is turned off when an overcurrent flows in the reactor 5 to protect the DC power supply 15, the step-down chopper, and the load 16.

  The collector of the IGBT (switching element) 2 is connected to the other terminal of the switch 1. The IGBT 2 is turned on / off by the control circuit 12. The anode of the diode 3 is connected to the input terminal T2 and the output terminal T4, and the cathode is connected to the emitter (node N1) of the IGBT2. Reactor 5 is connected between node N1 and output terminal T3.

  The current detector 4 is provided between the node N1 and the reactor 5, detects the current flowing through the reactor 5, and applies a signal indicating the detected value I to the protection circuits 9 and 10. The capacitor 6 is connected between the output terminals T3 and T4 and smoothes the voltage between the output terminals T3 and T4. The anode of the diode 7 is connected to the output terminal T4, and the cathode is connected to the output terminal T3. The voltage detector 8 detects the voltage VO between the output terminals T3 and T4 and gives a signal indicating the detected value to the control circuit 12.

  Protection circuit 9 activates gate block signal GB from the inactive level "L" level when detected value I of the current flowing through reactor 5 exceeds a predetermined upper limit current value (first current value) I1. Switch to "H" level of

  The protection circuit 9 includes an absolute value detection circuit (ABS) 20 and a comparison circuit 21 as shown in FIG. The absolute value detection circuit 20 outputs a signal indicating the absolute value of the detection value I of the current flowing through the reactor 5. The comparison circuit 21 receives the signal indicating the absolute value of the detection value I of the current flowing through the reactor 5 and the signal indicating the upper limit current value I1, and outputs the gate block signal GB. When the absolute value of detected value I of the current flowing through reactor 5 is smaller than upper limit current value I1, gate block signal GB is set to the "L" level of the inactive level, and absolute value of detected value I of the current flowing through reactor 5 If the value exceeds upper limit current value I1, gate block signal GB is set to the "H" level of the activation level.

  In the case where detection value I of the current flowing through reactor 5 exceeds predetermined overcurrent value I2, protection circuit 10 causes overcurrent detection signal OD to be switched from the inactive level "L" level to the activation level "H". Launch to the level. The overcurrent value I2 is set to a value larger than the upper limit current value I1.

  The protection circuit 10 includes an absolute value detection circuit (ABS) 25 and a comparison circuit 26, as shown in FIG. The absolute value detection circuit 25 outputs a signal indicating the absolute value of the detection value I of the current flowing through the reactor 5. The comparison circuit 26 receives the signal indicating the absolute value of the detection value I of the current flowing through the reactor 5 and the signal indicating the overcurrent value I2, and outputs the overcurrent detection signal OD. When the absolute value of detection value I of the current flowing through reactor 5 is smaller than overcurrent value I2, overcurrent detection signal OD is set to the “L” level of the inactivation level, and detection value I of current flowing through reactor 5 is When the absolute value exceeds the overcurrent value I2, the overcurrent detection signal OD is brought to the "H" level of the activation level.

  Returning to FIG. 1, the operation unit 11 includes a plurality of buttons operated by the user of the step-down chopper. By operating the operation unit 11, the user of the step-down chopper can instruct power on / off, reset of the step-down chopper, and the like.

  The control circuit 12 turns on the switch 1 when an instruction to turn on the power is given by the signal from the operation unit 11. The control circuit 12 turns on / off the IGBT 2 in a predetermined cycle so that the output voltage VO matches the target voltage VOT when the gate block signal GB is at the inactive level "L" level. The control circuit 12 turns off the IGBT 2 when the gate block signal GB is at the activation "H" level. When the gate block signal GB is changed from the "H" level of the activation level to the "L" level of the inactivation level, the control circuit 12 turns the IGBT 2 on and off again.

  The control circuit 12 fixes the switch 1 and the IGBT 2 in the off state when the overcurrent detection signal OD is set to the “H” level of the activation level, and stops the operation of the step-down chopper. Even when the overcurrent detection signal OD is changed from the "H" level to the "L" level of the activation level, the control circuit 12 operates the step-down chopper unless a signal indicating reset and power on is given from the operation unit 11. Not resume. The control circuit 12 fixes the switch 1 and the IGBT 2 in the off state when power off is instructed by the signal from the operation unit 11, and stops the operation of the step-down chopper.

  Next, the operation of this step-down chopper will be described. First, the case of driving a normal load 16 will be described. When the user of the step-down chopper operates the operation unit 11 to give a signal instructing power on to the control circuit 12, the control circuit 12 turns on the switch 1 and the IGBT 2 based on the detection value of the voltage detector 8. Is turned on / off in a predetermined cycle.

  When IGBT 2 is turned on, current flows from the positive electrode of DC power supply 15 through the parallel connection of switch 1, IGBT 2, reactor 5, capacitor 6 and load 16 to the negative electrode of DC power supply 15, and capacitor 6 is charged The reactor 5 stores electromagnetic energy.

  When the IGBT 2 is turned off, one terminal of the reactor 5 (terminal on the output terminal T3 side) reaches the other terminal (terminal on the node N1 side) of the reactor 5 via the parallel connection of the capacitor 6 and the load 16 and the diode 3 A current flows in the path to charge the capacitor 6 and release the electromagnetic energy of the reactor 5.

  When the on time of the IGBT 2 in each cycle is increased, the output voltage VO rises, and when the on time of the IGBT 2 in each cycle is shortened, the output voltage VO decreases. The control circuit 12 adjusts the on time of the IGBT 2 based on the detection value of the voltage detector 8 so that the output voltage VO matches the target voltage VOT. Thus, the output voltage VO is maintained at the target voltage VOT, and the load 16 is driven by the output voltage VO.

  When load 16 is normally operated, the current flowing through reactor 5 is maintained at a value smaller than upper limit current value I1 and overcurrent value I2, and both gate block signal GB and overcurrent detection signal OD are inactivated. It is maintained at the level "L".

  Next, the case where the output terminals T3 and T4 are shorted by a metal piece, for example, will be described. In this case, the current flowing to the reactor 5 is increased. When the detection value I of the current detector 4 is larger than the upper limit current value I1 and smaller than the overcurrent value I2, the gate block signal GB is set to the "H" level of the activation level, and the IGBT 2 is turned off. When the metal pieces fall or melt within the period when the IGBT 2 is turned off and the short circuit between the output terminals T3 and T4 is eliminated, the detection value I of the current detector 4 falls below the upper limit current value I1 and the gate Block signal GB is set to the inactive "L" level, and on / off of IGBT 2 is resumed.

  When the detection value I of the current detector 4 exceeds the over current value I2, the over current detection signal OD is set to the "H" level of the activation level, the switch 1 and IGBT 2 are turned off, and the operation of the step-down chopper is stopped. Be done. When the metal piece is removed while the operation of the step-down chopper is stopped and the short circuit between the output terminals T3 and T4 is eliminated, the switch 1 is turned on when the power on is instructed by operating the operation unit 11 The operation of the step-down chopper is resumed.

  Next, the reason why the diode 7 is provided will be described. FIGS. 4A to 4C are circuit diagrams for describing problems that occur when the diode 7 is not provided. In FIGS. 4A to 4C, the switch 1, the voltage detector 8, the protection circuits 9 and 10, the operation unit 11, the control circuit 12, and the like are omitted for simplification of the drawings and the description. .

  Further, the diode 7 is not provided, and the output terminals T3 and T4 are shorted by the short circuit 27. The short circuit 27 is, for example, a wiring and has a reactance. For this reason, in FIG. 4 (a)-(c), the short circuit 27 is represented by the symbol which shows a reactor.

  As shown in FIG. 4A, when the output terminals T3 and T4 are shorted by the short circuit 27 when the IGBT 2 is turned on, the IGBT 2, the reactor 5, and the short circuit 27 are connected from the positive electrode of the DC power supply 15. A current flows in a route RT1 leading to the negative electrode of the DC power supply 15 and from the first electrode of the capacitor 6 (electrode on the output terminal T3 side) to the second electrode of the capacitor 6 (output terminal T4 side) via the short circuit 27. A current flows in the route RT2 leading to the electrode). At this time, electromagnetic energy is stored in the reactor 5.

  When the current flowing through the route RT1 exceeds the upper limit current value I1 and the gate block signal GB is set to the “H” level by the protection circuit 9 and the IGBT 2 is turned off, as shown in FIG. Energy is released, and a current flows in a route RT3 from the one terminal of the reactor 5 to the other terminal of the reactor 5 via the short circuit 27 and the diode 3. The capacitor 6 and the short circuit 27 constitute a resonant circuit, and the capacitor 6 is charged to a negative voltage. That is, positive charge is stored in the second electrode of the capacitor 6 and negative charge is stored in the first electrode of the capacitor 6.

  Next, as shown in FIG. 4C, the electric charge of the capacitor 6 is discharged, and a current flows in a route RT4 from the second electrode of the capacitor 6 to the first electrode of the capacitor 6 through the diode 3 and the reactor 5. A current flows in a path RT5 from the second electrode of the capacitor 6 to the first electrode of the capacitor 6 via the short circuit 27.

  At this time, when the current flowing in the two paths RT3 and RT5 is superimposed and the current flowing in the reactor 5 exceeds the overcurrent value I2, although the IGBT 2 is turned off, the overcurrent detection signal OD is activated. The operation of the step-down chopper is stopped and the operation of the load 16 is stopped. Therefore, even if the short circuit 27 between the output terminals T3 and T4 is dropped or melted while the IGBT 2 is turned off, the operation of the step-down chopper is not automatically resumed, and the operation unit 11 is operated. Need to be turned on again.

  On the other hand, in the first embodiment, since the diode 7 is turned on when the voltage of the output terminal T4 becomes higher than the voltage of the output terminal T3, positive charges are stored in the second electrode of the capacitor 6 and the first electrode Negative charges are never stored. Therefore, even when the output terminals T3 and T4 are short-circuited by the short circuit 27, it is possible to prevent the operation of the step-down chopper from being erroneously stopped by the capacitor 6 being discharged after being charged to a negative voltage.

Second Embodiment
5 is a circuit block diagram showing a configuration of a step-down chopper according to a second embodiment of the present invention, which is to be compared with FIG. Referring to FIG. 5, this step-down chopper differs from the step-down chopper of FIG. 1 in that protection circuit 10 is replaced by protection circuit 30 and diode 7 is removed. The protection circuit 30 generates an overcurrent detection signal OD based on the output signal of the current detector 4 and the gate block signal GB.

  FIG. 6 is a circuit block diagram showing a configuration of protection circuit 30, which is to be compared with FIG. Referring to FIG. 6, protection circuit 30 is obtained by adding inverter 31 and AND gate 32 to protection circuit 10 of FIG. The output signal of comparison circuit 26 is applied to the first input node of AND gate 32, and gate block signal GB is inverted by inverter 31 and applied to the second input node 31 of the AND gate. The output signal of the AND gate 32 becomes the overcurrent detection signal OD.

  When the gate block signal GB is at "L" level, the output signal of the comparison circuit 26 passes through the AND gate 32 to become the overcurrent detection signal OD. When gate block signal GB is at "H" level, overcurrent detection signal OD is fixed at "L" level.

  Therefore, as shown in FIGS. 4A to 4C, the output terminals T3 and T4 are shorted by the short circuit 27, the gate block signal GB is set to the “H” level, and the IGBT 2 is turned off. Is discharged after being charged to a negative voltage, and the overcurrent detection signal OD is fixed at the “L” level even when the current flowing through the reactor 5 exceeds the overcurrent value I2. Therefore, the malfunction as described in FIGS. 4A to 4C can be prevented.

Third Embodiment
In the second embodiment, since upper limit current value I1 is smaller than overcurrent value I2, gate block signal GB attains "H" level whenever the current flowing through reactor 5 exceeds upper limit current value I1 and the overcurrent is also generated. The detection signal OD goes to "L" level. Therefore, even when the current flowing through reactor 5 exceeds overcurrent value I2, the operation of the step-down chopper is continued, so that overcurrent can not be interrupted, for example, when IGBT 2 is short-circuited. The third embodiment solves this problem.

  7 is a circuit block diagram showing a configuration of a step-down chopper according to a third embodiment of the present invention, which is to be compared with FIG. Referring to FIG. 7, this step-down chopper is different from the step-down chopper of FIG. 5 in that protection circuit 30 is replaced by protection circuit 35.

  FIG. 8 is a circuit block diagram showing a configuration of protection circuit 35, which is to be compared with FIG. Referring to FIG. 8, protection circuit 35 is obtained by adding timer 36 and OR gate 37 to protection circuit 30 of FIG.

  Timer 36 is set when the output signal of comparison circuit 26 is raised from the inactive "L" level to the active "H" level to start timing and start timing. When the predetermined time Tc1 has elapsed, the output signal φ36 is raised from the “L” level to the “H” level.

  Timer 36 is reset when the output signal of comparison circuit 26 is set to the “L” level to return to the initial state capable of clocking, and resets output signal φ 36 to the “L” level. OR gate 37 applies a logical sum signal of output signal φ 36 of timer 36 and an output signal of inverter 31 to a second input node of AND gate 32.

  When output signal φ 36 of timer 36 is at “L” level, the output signal of inverter 31 passes through OR gate 37 and is applied to the second input node of AND gate 32. In this case, the configuration of the protection circuit 35 is the same as the configuration of the protection circuit 30.

  When the current flowing through reactor 5 exceeds upper limit current value I1 and overcurrent value I2, protection circuit 9 raises gate block signal GB to "H" level of activation level, and protection circuit 35 compares comparison circuit 26. The output signal is raised to "H" level and timer 36 is set.

  In response to the gate block signal GB being brought to the “H” level, the IGBT 2 is turned off, and when the phenomenon shown in FIGS. 4A to 4C occurs, it is excessive within a time shorter than the predetermined time Tc1. Since the current is lost, the output signal φ 36 of the timer 36 does not go to the “H” level, and the overcurrent detection signal OD is maintained at the “L” level.

  When the IGBT 2 has a short circuit failure, the IGBT 2 is not turned off even if the gate block signal GB is set to the “H” level, and a current exceeding the overcurrent value I 2 continues to flow in the reactor 5. When the time when the current flowing through reactor 5 exceeds overcurrent value I2 exceeds predetermined time Tc1, output signal φ 36 of timer 36 is raised to the “H” level. Thereby, the output signal of the comparison circuit 26 passes through the AND gate 32 to become the overcurrent detection signal OC, the switch 1 and the IGBT 2 are fixed in the off state, and the operation of the step-down chopper is stopped. The other configuration and operation are the same as in the second embodiment, and the description thereof will not be repeated.

  In the third embodiment, the same effect as the second embodiment can be obtained. In addition, the operation of the step-down chopper can be stopped when an overcurrent flows through the reactor 5 for a predetermined time Tc1 or more.

Fourth Embodiment
In the second and third embodiments, for example, when capacitor 6 has a short circuit failure, when IGBT 2 is turned on, the current flowing through reactor 5 exceeds upper limit current value I1 and gate block signal GB attains "H" level. Are turned off to reduce the current flowing to the reactor 5. When the current flowing through the reactor falls below upper limit current value I1 and gate block signal GB is set to the “L” level, IGBT 2 is turned on again, and the above-described operation is repeated. The fourth embodiment solves this problem.

  9 is a circuit block diagram showing a configuration of a step-down chopper according to a fourth embodiment of the present invention, which is to be compared with FIG. Referring to FIG. 9, this step-down chopper is different from the step-down chopper of FIG. 7 in that protection circuit 9 is replaced by protection circuit 40.

  FIG. 10 is a circuit block diagram showing a configuration of protection circuit 40, which is to be compared with FIG. Referring to FIG. 10, this protection circuit 40 is obtained by adding a counter 41 and a timer 42 to the protection circuit 9 of FIG.

  The timer 42 provides the counter 41 with information indicating time. The counter 41 refers to the information indicating the time given from the timer 42 and counts the number of times the gate block signal GB which is the output signal of the comparison circuit 21 is raised from the “L” level to the “H” level. Then, counter 41 raises failure detection signal FD from the "L" level of the inactivation level to the "H" level of the activation level when the count value exceeds the predetermined value within predetermined time Tc2.

  When failure detection signal FD is raised to the “H” level, control circuit 12 fixes switch 1 and IGBT 2 in the off state to stop the operation of the step-down chopper. The counter 41 is reset when the operation of the step-down chopper is stopped, returns to the initial state, and resets the failure detection signal FD to the “L” level. The other configuration and operation are the same as in the second embodiment, and the description thereof will not be repeated.

  In the fourth embodiment, in addition to the same effect as the third embodiment, the operation of the step-down chopper is performed when the gate block signal GB is changed from the “L” level to the “H” level a predetermined number of times or more within a predetermined time Tc2. Can be stopped.

  It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is indicated not by the above description but by the claims, and is intended to include all the modifications within the meaning and scope equivalent to the claims.

  T1, T2 input terminals, T3, T4 output terminals, 1 switch, 2 IGBTs, 3, 7 diodes, 4 current detectors, 5 reactors, 6 capacitors, 8 voltage detectors, 9, 10, 30, 35, 40 Protection circuits , 11 operation units, 12 control circuits, 15 DC power supplies, 16 loads, 20, 25 absolute value detection circuits, 21, 26 comparison circuits, 31 inverters, 32 AND gates, 36, 42 timers, 37 OR gates, 41 counters.

Claims (2)

A step-down chopper for stepping down a DC voltage applied between first and second input terminals and outputting the voltage between the first and second output terminals,
A switching element whose one electrode is connected to the first input terminal;
A first diode having an anode connected to the second input terminal and the second output terminal, and having a cathode connected to the other electrode of the switching element;
A reactor connected between the other electrode of the switching element and the first output terminal;
A capacitor connected between the first and second output terminals;
A current detector that detects a current flowing through the reactor;
A first protection circuit that outputs a gate block signal when the detection value of the current detector exceeds a first current value;
A second protection circuit that outputs an overcurrent detection signal when the detection value of the current detector exceeds a second current value larger than the first current value;
The switching element is turned on / off so that the voltage between the first and second output terminals becomes a target voltage, and the switching element is turned on when the gate block signal is output from the first protection circuit. A control circuit which turns off and fixes the switching element in an off state when the overcurrent detection signal is output from the second protection circuit;
A step-down chopper comprising a second diode connected at its anode to said second output terminal and at its cathode to said first output terminal.   The second diode includes the capacitor when the first and second output terminals are short-circuited and the gate block signal is output from the first protection circuit to turn off the switching element. The step-down chopper according to claim 1, wherein a resonant circuit is formed to prevent a current exceeding the second current value from flowing to the reactor.

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