Overvoltage Protection Device

Abstract

To provide an excessive voltage prevention device capable of protecting not only a switching element but also a drive circuit from damage caused by excessive voltage.SOLUTION: An excessive voltage prevention device includes an IGBT 30; a Zener diode 32 electrically connected between a collector terminal 30c and an emitter terminal 30e of the IGBT 30; a light emitting diode 34 electrically connected in series with the Zener diode 32 and outputting an optical signal when an excessive voltage is applied to the IGBT 30; a drive circuit 8 for driving the IGBT 30; and a control circuit 10 that can receive the optical signal from the light emitting diode 34 and outputs a first control signal to the drive circuit 8 based on the received optical signal. The drive circuit 8 conducts the IGBT 30 by supplying a voltage to a gate terminal 30 g of the IGBT 30 based on the first control signal.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an overvoltage protection device for protecting a switching element from an overvoltage. [Background technology]

[0002] For example, an active clamp circuit is known for protecting a switching element such as an IGBT (Insulated Gate Bipolar Transistor) from overvoltage (see, for example, Patent Document 1). The active clamp circuit includes a Zener diode electrically connected between the collector terminal and gate terminal of the IGBT. A drive circuit for driving the IGBT is electrically connected to the gate terminal of the IGBT.

[0003] In this active clamp circuit, when the IGBT is turned off, an overvoltage (surge voltage) is applied to the collector terminal of the IGBT due to the inductance component of the power supply line. If this overvoltage exceeds the breakdown voltage of the Zener diode, the Zener diode becomes conductive, and voltage is input from the collector terminal of the IGBT to the gate terminal of the IGBT via the Zener diode. This forces the IGBT to turn on, and current flows between the collector and emitter terminals of the IGBT. As a result, the IGBT is released from the energy of the inductance component, protecting the IGBT from overvoltage. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-166734 Summary of the Invention [Problem to be solved by the invention]

[0005] However, although the above-described active clamp circuit protects the IGBT itself from overvoltage, a current may flow from the collector terminal of the IGBT through the Zener diode into the drive circuit, potentially damaging the drive circuit.

[0006] The present invention is intended to solve the above-mentioned problems, and its object is to provide an overvoltage protection device that can protect not only switching elements but also drive circuits from damage caused by overvoltage. [Means for solving the problem]

[0007] An overvoltage protection device according to one aspect of the present invention includes: a switching element having a first terminal, a second terminal, and a third terminal, the conduction between the first terminal and the second terminal being controlled in accordance with a voltage or current input to the third terminal; a constant voltage element electrically connected between the first terminal and the second terminal of the switching element; a light-emitting element electrically connected in series with the constant voltage element and outputting an optical signal when an overvoltage is applied to the switching element; a drive circuit for driving the switching element; and a control circuit capable of receiving the optical signal from the light-emitting element and outputting a first control signal to the drive circuit based on the received optical signal, wherein the drive circuit causes the switching element to be conductive by supplying a voltage or current to the third terminal of the switching element based on the first control signal.

[0008] According to this aspect, a constant voltage element and a light-emitting element are electrically connected in series between the first terminal and the second terminal of the switching element. When an overvoltage is applied to the switching element, the light-emitting element outputs an optical signal to the control circuit. The control circuit outputs a first control signal to the drive circuit based on the optical signal from the light-emitting element, thereby turning on the switching element. This allows the switching element to absorb the overvoltage and protect the switching element from the overvoltage. Furthermore, when an overvoltage is applied to the switching element, an excessive current does not flow into the drive circuit as in the above-mentioned active clamp circuit, so damage to the drive circuit can be avoided.

[0009] For example, the overvoltage protection device may further include an inverter circuit including a plurality of the switching elements and supplied with power from a power source, and a changeover switch that can be switched between a first state that allows the supply of power from the power source to the inverter circuit and a second state that blocks the supply of power from the power source to the inverter circuit, and the control circuit may be configured to output a second control signal to the changeover switch based on the optical signal from the light-emitting element, and the changeover switch may be configured to be switched from the first state to the second state based on the second control signal.

[0010] According to this aspect, the control circuit outputs a second control signal to the changeover switch based on the optical signal from the light-emitting element, thereby switching the changeover switch from the first state to the second state, thereby stopping the supply of current to the inverter circuit and more reliably protecting the switching element from overvoltage.

[0011] For example, the overvoltage protection device may further include an optical fiber that guides the optical signal from the light emitting element to the control circuit.

[0012] According to this aspect, even if the switching element and the control circuit are far apart, the optical signal from the light emitting element can be easily guided to the control circuit via the optical fiber.

[0013] For example, the constant voltage element may be configured as a Zener diode whose cathode side is electrically connected to the first terminal of the switching element and whose anode side is electrically connected to the second terminal of the switching element via the light-emitting element.

[0014] According to this aspect, a Zener diode can be used as the constant voltage element.

[0015] For example, the constant voltage element may be configured to be a varistor.

[0016] According to this aspect, a varistor can be used as the constant voltage element.

[0017] For example, the light-emitting element may be configured to be a light-emitting diode whose anode side is electrically connected to the first terminal of the switching element via the constant voltage element and whose cathode side is electrically connected to the second terminal of the switching element.

[0018] According to this aspect, a light emitting diode can be used as the light emitting element.

[0019] These comprehensive or specific aspects may be realized as a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM, or may be realized as any combination of a system, a method, an integrated circuit, a computer program, and a recording medium. [Effects of the Invention]

[0020] According to an overvoltage protection device according to an aspect of the present invention, it is possible to protect not only the switching elements but also the drive circuits from damage caused by overvoltage. [Brief explanation of the drawings]

[0021] [Figure 1]FIG. 2 is a diagram showing a circuit configuration of the induction heating device according to the embodiment. [Figure 2] 2 is a diagram showing an IGBT and its peripheral circuitry of the induction heating device according to the embodiment; FIG. [Figure 3] 4 is a flowchart showing the flow of operations of the induction heating device according to the embodiment. [Figure 4] FIG. 1 is a diagram illustrating an active clamp circuit according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0023] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the scope of the claims. Furthermore, among the components in the following embodiments, components that are not described in the independent claims that represent the highest concepts are described as optional components.

[0024] In addition, the drawings are not necessarily strict illustrations, and the same reference numerals are used to designate substantially the same components in the drawings, and redundant explanations are omitted or simplified.

[0025] (Embodiment) [1. Circuit configuration of induction heating device] First, the circuit configuration of an induction heating device 2 (an example of an overvoltage protection device) according to an embodiment will be described with reference to Figures 1 and 2. Figure 1 is a diagram showing the circuit configuration of the induction heating device 2 according to an embodiment. Figure 2 is a diagram showing an IGBT 30 according to an embodiment and its peripheral circuitry.

[0026] As shown in FIG. 1, the induction heating device 2 includes a main circuit 6 for induction heating an object 4 to be heated, a drive circuit 8 for driving an inverter circuit 26 (described later) of the main circuit 6, and a control circuit 10 for controlling the drive circuit 8 and the like.

[0027] The induction heating device 2 is a device for induction heating (so-called induction hardening) an object 4 to be heated with a high frequency of, for example, several kHz to several hundred kHz. Note that induction hardening is a heat treatment in which high frequency electromagnetic induction is generated to heat and harden each surface of the object 4 to be heated. The object 4 to be heated is, for example, a tubular metal part used in a vehicle, a machine tool, or the like.

[0028] As shown in FIG. 1, the main circuit 6 includes a changeover switch 12, a power supply circuit 14, a matching circuit 16, and an induction coil 17.

[0029] The changeover switch 12 is, for example, a magnetic switch, and is electrically connected between the power supply 18 and the power supply circuit 14. The changeover switch 12 is switchable between a first state in which the power supply 18 allows the supply of AC power to the power supply circuit 14, and a second state in which the supply of AC power from the power supply 18 to the power supply circuit 14 is blocked. The power supply 18 is, for example, a three-phase AC power supply such as a commercial power supply, and supplies three-phase 60 Hz (or 50 Hz) AC power.

[0030] The power supply circuit 14 is an oscillator for outputting a high-frequency current of a predetermined frequency, and includes a rectifier circuit 20, choke coils 22 and 24, and an inverter circuit 26.

[0031] The rectifier circuit 20 is a full-wave rectifier circuit using, for example, six thyristors 28, and converts AC power from the power source 18 into DC power.

[0032] The choke coils 22 and 24 are electrically connected to the output side (DC side) of the rectifier circuit 20 and smooth the DC power from the rectifier circuit 20.

[0033] The inverter circuit 26 is electrically connected to the output side of the rectifier circuit 20 via choke coils 22 and 24. The inverter circuit 26 is a single-phase full-bridge inverter circuit using, for example, four IGBTs 30 (an example of switching elements), and converts smoothed DC power into single-phase high-frequency power by turning the four IGBTs 30 on and off at high speed. This allows the power supply circuit 14 to generate a high-frequency current of a predetermined frequency.

[0034] Each IGBT 30 has a collector terminal 30c (an example of a first terminal), an emitter terminal 30e (an example of a second terminal), and a gate terminal 30g (an example of a third terminal), and conduction between the collector terminal 30c and the emitter terminal 30e is controlled according to the voltage input to the gate terminal 30g.

[0035] Between the collector terminal 30c and the emitter terminal 30e of each IGBT 30, a Zener diode 32 (an example of a constant voltage element) and a light emitting diode 34 (an example of a light emitting element) are electrically connected in series.

[0036] The cathode side of the Zener diode 32 is electrically connected to the collector terminal 30c of the IGBT 30, and the anode side of the Zener diode 32 is electrically connected to the emitter terminal 30e of the IGBT 30 via the light-emitting diode .

[0037] The anode side of the light-emitting diode 34 is electrically connected to the collector terminal of the IGBT 30 via the Zener diode 32, and the cathode side of the light-emitting diode 34 is electrically connected to the emitter terminal of the IGBT 30. When an overvoltage is applied to the collector terminal 30c of the IGBT 30 electrically connected to the light-emitting diode 34, the light-emitting diode 34 emits light to output an optical signal.

[0038] In this embodiment, an IGBT 30 is used as the switching element, but the present invention is not limited thereto. For example, a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or a bipolar transistor may be used. The MOSFET has a drain terminal (an example of a first terminal), a source terminal (an example of a second terminal), and a gate terminal (an example of a third terminal), and conduction between the drain terminal and the source terminal is controlled according to a voltage input to the gate terminal. The bipolar transistor has a collector terminal (an example of a first terminal), an emitter terminal (an example of a second terminal), and a base terminal (an example of a third terminal), and conduction between the collector terminal and the emitter terminal is controlled according to a current input to the base terminal.

[0039] The matching circuit 16 is used to supply the active power generated by the power supply circuit 14 to the induction coil 17 with high efficiency. The input side of the matching circuit 16 is electrically connected to the AC side of the inverter circuit 26. The matching circuit 16 is a parallel resonant circuit configured with a capacitor 38 and a matching transformer 40.

[0040] The induction coil 17 is electrically connected to the output side of the matching circuit 16. The induction coil 17 is used to inductively heat the object 4, and is arranged to surround the object 4. A high-frequency current from the power supply circuit 14 is supplied to the induction coil 17 via the matching circuit 16, thereby inductively heating the object 4.

[0041] The drive circuit 8 is a circuit for driving the inverter circuit 26 of the main circuit 6, and is mounted on a drive board. As shown in FIG. 2, the drive circuit 8 is electrically connected to the gate terminal 30g and the emitter terminal 30e of each IGBT 30, and drives each IGBT 30. Specifically, the drive circuit 8 supplies a voltage to the gate terminal 30g of each IGBT 30, thereby making each IGBT 30 conductive (turn-on). The drive circuit 8 also cuts off the supply of voltage to the gate terminal 30g of each IGBT 30, thereby making each IGBT 30 non-conductive (turn-off).

[0042] For convenience of explanation, in FIG. 2, the drive circuit 8 is electrically connected to the gate terminal 30g and the emitter terminal 30e of one IGBT 30, but in reality, it is also electrically connected to the gate terminals 30g and the emitter terminals 30e of the remaining three IGBTs 30.

[0043] The control circuit 10 is a circuit for controlling the drive circuit 8 and the changeover switch 12, and is mounted on a control board. As shown in Fig. 2, the control circuit 10 has a light receiving element 42 and a signal output unit 44. An optical fiber 46 is arranged between the light receiving element 42 and the light emitting diode 34 to guide the optical signal from the light emitting diode 34 to the light receiving element 42.

[0044] The light receiving element 42 is, for example, a photodiode, and is capable of receiving an optical signal from the light emitting diode 34 via an optical fiber 46. The light receiving element 42 outputs the received optical signal to a signal output unit 44.

[0045] The signal output unit 44 outputs a first control signal to the drive circuit 8 based on the optical signal from the light receiving element 42, and outputs a second control signal to the changeover switch 12. Based on the first control signal, the drive circuit 8 supplies a voltage to the gate terminal 30g of the IGBT 30 corresponding to the light emitting diode 34 that output the optical signal, thereby making the IGBT 30 conductive (turn on). Furthermore, the changeover switch 12 is switched from the first state to the second state based on the second control signal.

[0046] [2. Operation of induction heating device] Next, the operation of the induction heating device 2 according to the embodiment will be described with reference to Fig. 3. Fig. 3 is a flowchart showing the flow of the operation of the induction heating device 2 according to the embodiment.

[0047] When the object to be heated 4 is to be induction heated by the induction coil 17, the control circuit 10 controls the changeover switch 12 to switch to the first state, and also controls the drive circuit 8 to drive the inverter circuit .

[0048] As a result, AC power from power supply 18 is supplied to rectifier circuit 20 of power supply circuit 14 via selector switch 12. Rectifier circuit 20 converts the AC power from power supply 18 into DC power and outputs the DC power to inverter circuit 26 via choke coils 22 and 24. Inverter circuit 26 converts the DC power from rectifier circuit 20 into high-frequency power and outputs the high-frequency power to induction coil 17 via matching circuit 16. As a result, object 4 to be heated is induction heated.

[0049] Hereinafter, a case will be described in which one of the four IGBTs 30 unintentionally becomes non-conductive (turned off) while the object to be heated 4 is being induction-heated as described above.

[0050] In this case, an overvoltage (surge voltage) is generated due to an inductance component (e.g., choke coils 22, 34) of the power supply circuit 14, and this overvoltage is applied to the collector terminal 30c of the IGBT 30 that has become non-conductive (S101). When this overvoltage exceeds the breakdown voltage of the Zener diode 32, the Zener diode 32 becomes conductive (S102), and a reverse current begins to flow through the Zener diode 32. This causes a current to flow through the light-emitting diode 34 that is electrically connected in series with the Zener diode 32, and the light-emitting diode 34 outputs an optical signal (S103).

[0051] The optical signal from the light-emitting diode 34 is guided to the light-receiving element 42 of the control circuit 10 via the optical fiber 46. As a result, the light-receiving element 42 of the control circuit 10 receives the optical signal from the light-emitting diode 34 (S104) and outputs the received optical signal to the signal output unit 44. Based on the optical signal from the light-receiving element 42, the signal output unit 44 outputs a first control signal to the drive circuit 8 and outputs a second control signal to the change-over switch 12 (S105).

[0052] The drive circuit 8 turns on (turns on) the IGBT 30 to which an overvoltage is applied based on the first control signal from the signal output unit 44 (S106). As a result, a current flows between the collector terminal 30c and the emitter terminal 30e of the IGBT 30, and the IGBT 30 can absorb the energy of the inductance component, thereby protecting the IGBT 30 from overvoltage.

[0053] Also, the switching switch 12 is switched from the first state to the second state based on the second control signal from the signal output unit 44 (S107). As a result, the supply of AC power from the power source 18 to the power supply circuit 14 is interrupted, and the power supply to the inverter circuit 26 is stopped.

[0054] Note that there is a time lag (T1 < T2) between the response time T1 from when the signal output unit 44 outputs the first control signal until the drive circuit 8 turns on the IGBT 30, and the response time T2 from when the signal output unit 44 outputs the second control signal until the switching switch 12 is switched to the second state. Therefore, the switching switch 12 is switched to the second state with a slight delay after the drive circuit 8 turns on the IGBT 30.

[0055] [3. Effect] Here, while referring to FIG. 4, an active clamp circuit according to a comparative example will be described. FIG. 4 is a diagram showing an active clamp circuit according to a comparative example. [[ID=1)]5

[0056] <( As shown in FIG. 4, in the active clamp circuit according to the comparative example, a Zener diode 102 is electrically connected between the collector terminal 100c and the gate terminal 100g of the IGBT 100. Also, a drive circuit 104 for driving the IGBT 100 is electrically connected to the gate terminal 100g and the emitter terminal 100e of the IGBT 100.

[0057] In this active clamp circuit, when the IGBT 100 is turned off, an overvoltage is applied to the collector terminal 100c of the IGBT 100 due to the inductance component of the power supply line. If this overvoltage exceeds the breakdown voltage of the Zener diode 102, the Zener diode 102 becomes conductive, and a voltage is input from the collector terminal 100c of the IGBT 100 to the gate terminal 100g of the IGBT 100 via the Zener diode 102. This forces the IGBT 100 to turn on, and a current flows between the collector terminal 100c and the emitter terminal 100e of the IGBT 100. As a result, the IGBT 100 is released from the energy of the inductance component, and the IGBT 100 is protected from the overvoltage.

[0058] However, in the above-described active clamp circuit, although the IGBT 100 itself is protected from overvoltage, current flows from the collector terminal 100c of the IGBT 100 through the Zener diode 102 to the drive circuit 104, which may damage the drive circuit 104. Furthermore, because the drive circuit 104 does not sense that an overvoltage has been applied to the IGBT 100, there is a risk that it may continue to operate until it breaks down.

[0059] In contrast to this, in the present embodiment, a Zener diode 32 and a light-emitting diode 34 are electrically connected in series between the collector terminal 30c and the emitter terminal 30e of the IGBT 30. When an overvoltage is applied to the collector terminal 30c of the IGBT 30, the light-emitting diode 34 outputs an optical signal to the control circuit 10.

[0060] By receiving the optical signal from the light-emitting diode 34, the control circuit 10 determines that an overvoltage is being applied to the IGBT 30 and turns on the IGBT 30. This releases the IGBT 30 from the energy of the inductance component, thereby protecting the IGBT 30 from the overvoltage.

[0061] Furthermore, the control circuit 10 switches the changeover switch 12 from the first state to the second state, which stops the power supply to the inverter circuit 26, thereby more reliably protecting the IGBT 30 from overvoltage.

[0062] Furthermore, the Zener diode 32 and the light-emitting diode 34 are not electrically connected to the gate terminal 30g of the IGBT 30. Therefore, when an overvoltage is applied to the collector terminal 30c of the IGBT 30, a current does not flow into the drive circuit 8 as in the above-described active clamp circuit, and damage to the drive circuit 8 can be avoided.

[0063] (Variations, etc.) While the overvoltage protection device according to one or more aspects of the present invention has been described based on the above-mentioned embodiment, the present invention is not limited to the above-mentioned embodiment. As long as it does not deviate from the spirit of the present invention, various modifications conceivable by those skilled in the art to the present embodiment and configurations constructed by combining components of different embodiments may also be included within the scope of one or more aspects of the present invention.

[0064] In the above embodiment, a Zener diode is used as the constant voltage element, but the present invention is not limited to this, and a varistor, for example, may also be used.

[0065] Furthermore, in the above embodiment, the overvoltage protection device is applied to the induction heating device 2, but the invention is not limited to this and may be applied to various devices such as industrial machinery or home appliances.

[0066] The control circuit 10 in the above embodiment may be realized by a dedicated electronic circuit. The dedicated electronic circuit may be integrated on a single chip, or may be formed on multiple chips. Alternatively, the control circuit 10 may be realized by a general-purpose processor and a memory storing software programs or instructions. In this case, the processor functions as the control circuit 10 when the software programs or instructions are executed.

[0067] Another aspect of the present invention may be not only the induction heating device 2 but also a method including steps corresponding to characteristic components included in the induction heating device 2. Another aspect of the present invention may be a computer program that causes a computer to execute each of the characteristic steps included in the method. Another aspect of the present invention may be a computer-readable non-transitory recording medium having such a computer program recorded thereon. [Industrial Applicability]

[0068] The overvoltage protection device according to the present invention can be applied to, for example, an induction heating device for high-frequency hardening of metal parts. [Explanation of symbols]

[0069] 2 Induction heating device 4 Object to be heated 6 Main circuit 8,104 drive circuit 10 Control circuit 12. Selector switch 14 Power circuit 16 Matching circuit 17 Induction Coil 18 Power supply 20 Rectifier circuit 22,24 Choke coil 26 Inverter circuit 28 Thyristor 30,100 IGBT 30c,100c Collector terminal 30e,100e Emitter terminal 30g,100g gate terminal 32,102 Zener diode 34 Light-emitting diode 38 Capacitors 40 Matching transformer 42 Photodetector 44 Signal output section 46 Optical Fiber

Claims

1. a switching element having a first terminal, a second terminal, and a third terminal, and in which conduction between the first terminal and the second terminal is controlled in accordance with a voltage or a current input to the third terminal; a constant voltage element electrically connected between the first terminal and the second terminal of the switching element; a light-emitting element electrically connected in series with the constant voltage element, which outputs an optical signal when an overvoltage is applied to the switching element; a drive circuit for driving the switching element; an inverter circuit including a plurality of the switching elements and receiving power from a power source; a changeover switch that can be switched between a first state that allows the supply of power from the power source to the inverter circuit and a second state that cuts off the supply of power from the power source to the inverter circuit; a control circuit capable of receiving the optical signal from the light-emitting element, and outputting a first control signal to the drive circuit and a second control signal to the changeover switch based on the received optical signal; the drive circuit supplies a voltage or a current to the third terminal of the switching element based on the first control signal, thereby making the switching element conductive; the changeover switch is switched from the first state to the second state based on the second control signal, A response time from when the control circuit outputs the first control signal until when the drive circuit turns on the switching element is shorter than a response time from when the control circuit outputs the second control signal until when the changeover switch is switched to the second state. Overvoltage protection device.

2. The overvoltage protection device further includes an optical fiber that guides the optical signal from the light emitting element to the control circuit.

10. The overvoltage protection device of claim 1.

3. The constant voltage element is a Zener diode whose cathode side is electrically connected to the first terminal of the switching element and whose anode side is electrically connected to the second terminal of the switching element via the light emitting element.

3. An overvoltage protection device according to claim 1 or 2.

4. The constant voltage element is a varistor.

3. An overvoltage protection device according to claim 1 or 2.

5. The light-emitting element is a light-emitting diode whose anode side is electrically connected to the first terminal of the switching element via the constant voltage element and whose cathode side is electrically connected to the second terminal of the switching element. The overvoltage protection device according to any one of claims 1 to 4.

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