JP2004364367A - Protective circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a protective circuit capable of preventing the application of an overvoltage to a circuit for controlling the driving of a switching element even when the switching element is damaged. <P>SOLUTION: The protective circuit 10 divides the voltage at point A between an MOSFET 30 and a bipolar transistor 12 by means of resistors 17 and 18 and the divided voltage is applied to the gate of an FET 19. The FET 19 is turned on/off depending on the fact whether the applying voltage is higher than the threshold voltage of the FET 19 or not. When the FET 19 is turned on, an FET 16 is turned off and the bipolar transistor 12 is turned off thus forcibly interrupting a current flowing from the MOSFET 30 to the bipolar transistor 12. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a protection circuit that prevents an overvoltage from being applied to a drive circuit that controls on / off of a switching element.
[0002]
[Prior art]
Usually, for example, as shown in FIG. 3, a MOSFET (Metal-Oxide Semiconductor Field Effect Transistor) 30 is directly connected to a drive circuit 31 for controlling ON / OFF of the MOSFET 30.
[0003]
Further, a configuration including a protection circuit for protecting the switching element so that the switching element such as the MOSFET 30 is not damaged by an overvoltage applied to the switching element has been conventionally considered (for example, Patent Document 1). The protection circuit (gate circuit 2) detects a voltage value of a voltage applied between the collector and the emitter of the switching element (IGBT (Insulated Gate Bipolar Transistor) 1), and when the voltage value is an overvoltage, a gate resistance value. Is to increase. Thus, it is possible to prevent the switching element from being damaged due to the application of an overvoltage to the switching element.
[0004]
[Patent Document 1]
JP-A-5-336732 (pages 2-3, FIG. 1)
[0005]
[Problems to be solved by the invention]
However, as shown in FIG. 3, when the MOSFET 30 and the drive circuit 31 are directly connected, for example, when the drain D and the gate G of the MOSFET 30 are short-circuited, the overcurrent flows from the MOSFET 30 to the drive circuit 31. When I flows, an overvoltage is applied to the drive circuit 31, and there is a problem that the drive circuit 31 may be damaged. That is, there is a problem that not only the MOSFET 30 but also the drive circuit 31 may be damaged due to the damage of the MOSFET 30.
[0006]
Further, when the MOSFET 30 is damaged and an overcurrent I flows through the drive circuit 31, there is a problem that the entire circuit board on which the drive circuit 31 is mounted may be adversely affected.
Further, the gate circuit 2 (protection circuit) described in Patent Literature 1 can prevent the IGBT 1 (switching element) from being damaged by overvoltage, but when the IGBT 1 is damaged due to some influence. Again, an overcurrent may flow from the IGBT 1 to the gate-off transistor 23 and other elements via the resistors 201 and 200. Therefore, there is a problem that an overvoltage is applied to the gate-off transistor 23 and other elements, and the gate-off transistor 23 and other elements may be damaged. That is, there is a problem in that the IGBT 1 may be damaged, so that not only the IGBT 1 but also the gate-off transistor 23 and other elements may be damaged.
[0007]
In view of the above, in the present invention, a protection that can prevent an overvoltage from being applied to the drive circuit that controls the driving of the switching element even when the switching element is damaged is considered. It is intended to provide a circuit.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the present invention employs the following configuration.
That is, the protection circuit of the present invention is disposed between the control terminal of the switching element and the drive circuit that controls the switching element to turn on and off, and an overvoltage is applied to the drive circuit from the control terminal of the switching element. A switch for switching connection and disconnection of a current path between a control terminal of the switching element and the drive circuit in synchronization with on / off control of the switching element by the drive circuit. Means, voltage detection means for detecting a voltage applied to the drive circuit from a control terminal of the switching element, and, when an overvoltage is detected by the voltage detection means, the switch means is forcibly turned off and the current is Switch control means for interrupting the path.
[0009]
For example, consider the case where the switching element is a MOSFET, the drain and the gate of the MOSFET are short-circuited, and an overcurrent is about to flow from the gate of the MOSFET to the drive circuit. According to the protection circuit, the voltage detection means detects the voltage applied to the drive circuit from the gate of the MOSFET, and when the voltage is an overvoltage, forcibly turns off the switch means to switch the gate from the MOSFET to the drive circuit. Current path can be interrupted. As a result, even when the switching element is damaged, it is possible to prevent an overcurrent from flowing into the drive circuit, so that it is possible to prevent an overvoltage from being applied to the drive circuit.
[0010]
Further, the switch control means of the protection circuit has a comparison means for comparing the voltage detected by the voltage detection means with a predetermined threshold, and when the detection voltage of the voltage detection means exceeds the threshold, the switch control means The means may be forcibly turned off.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram illustrating a protection circuit 10 according to an embodiment of the present invention. In addition, about the structure similar to FIG. 3, the same code | symbol is attached | subjected.
[0012]
As shown in FIG. 1, the protection circuit 10 has a cathode connected to the gate (control terminal) of the MOSFET 30 (switching element) and an anode connected to the drive circuit 31, and outputs a drive control signal S 1 for controlling the drive of the MOSFET 30. It has a diode 11 for transmitting from the drive circuit 31 to the MOSFET 30, and a bipolar transistor 12 having an emitter connected to the gate of the MOSFET 30 and a collector connected to the drive circuit 31, and for extracting charges accumulated in the MOSFET 30 to the drive circuit 31 side. .
[0013]
The protection circuit 10 includes an FET 16 having a drain connected to the base of the bipolar transistor 12 via the resistor 13 and a gate connected to the control circuit 15 via the resistor 14, and a control circuit having a drain connected via the resistor 14. An FET 19 (switch control means), which is connected to the gate 15 and the gate of the FET 16 and whose gate is connected between the resistor 17 and the resistor 18. Further, one end of the resistor 17 is connected to the point A in FIG. 1, the other end is connected to one end of the resistor 18, and the other end of the resistor 18 is connected to the ground. The source of the FET 16 and the source of the FET 19 are connected to each other and to the ground. The diode 11, the bipolar transistor 12, the resistor 13, and the FET 16 constitute a switch means (broken line frame B in FIG. 1) described in the claims. Further, the resistor 17 and the resistor 18 constitute a voltage detecting means (broken line frame C in FIG. 1) described in the claims.
[0014]
The MOSFET 30 is used, for example, to configure each phase of an inverter circuit for supplying AC power to a three-phase AC motor. The control circuit 15 may include, for example, one inverter as a logic circuit that inverts the signal level of the drive control command signal S0 input to the drive circuit 31.
[0015]
For example, when the drive control command signal S0 is at a high level, the drive control signal S1 increases the gate voltage of the MOSFET 30 via the diode 11 and turns on the MOSFET 30. On the other hand, when the drive control command signal S0 is at a low level, the bipolar transistor 12 is turned on and a predetermined current flows from the MOSFET 30 to the drive circuit 31 to lower the gate voltage. A control signal S2 for controlling on / off of the bipolar transistor 12 is output from the control circuit 15. First, the on / off of the FET 16 is controlled based on the control signal S2, so that the current flowing to the base of the bipolar transistor 12 is controlled, and the bipolar transistor 12 is turned on and off.
[0016]
The protection circuit 10 detects the voltage at the point A between the MOSFET 30 and the bipolar transistor 12 via the resistor 17 and divides the voltage by the resistors 17 and 18, and the divided voltage is applied to the gate of the FET 19. The FET 19 is turned on and off based on whether or not the voltage value of the applied voltage (the voltage value at the point D) is larger than the threshold voltage value of the FET 19.
[0017]
The resistance value of each of the resistor 17 and the resistor 18 may be, for example, a voltage (gate-source voltage) between the gate and the ground of the MOSFET 30 that is the maximum allowable voltage (exceeding the maximum voltage). Is set so that the voltage at the point D when the threshold voltage becomes equal to the threshold voltage of the FET 19.
[0018]
2A shows a signal waveform of the drive control signal S1, FIG. 2B shows a signal waveform of the control signal S2, FIG. 2C shows a timing chart of the FET 16, and FIG. 2 (e) shows a timing chart of the bipolar transistor 12, FIG. 2 (e) shows a relationship between the voltage value at point D and the threshold voltage value (threshold) of the FET 19, and FIG. ing. The horizontal axes in FIGS. 2A to 2F have the same time scale.
[0019]
As shown in FIGS. 2A and 2B, for example, the control signal S2 is generated so as to be the same as the inverted drive control command signal S0. That is, when the drive control command signal S0 is at a high level, the drive control command signal S0 is set to a low level, and when the drive control command signal S0 is at a low level, the drive control command signal S0 is set to a high level. Alternatively, the high level or the low level of the control signal 2 is controlled in synchronization with the low level.
[0020]
As shown in FIG. 2C, the FET 16 turns on and off based on the control signal S2 shown in FIG. That is, for example, the FET 16 turns on when the control signal S2 is at a high level, and turns off when the control signal S2 is at a low level.
[0021]
Further, as shown in FIG. 2D, the bipolar transistor 12 turns on and off based on the on and off of the FET 16 shown in FIG. 2C. That is, for example, the bipolar transistor 12 turns on when the FET 16 turns on, and turns off when the FET 16 turns off.
[0022]
Then, as shown in FIG. 2E, for example, when the drain and gate of the MOSFET 30 are short-circuited at a certain point and the voltage value at the point D becomes larger than the threshold voltage value of the FET 19, as shown in FIG. Then, the FET 19 is turned on. Then, since the control signal S2 is not input to the FET 16, the FET 16 is turned off as shown in FIG. 2C, and the bipolar transistor 12 is also turned off as shown in FIG. 2D.
[0023]
Thus, when the voltage value at the point D becomes larger than the threshold voltage value of the FET 19, the bipolar transistor 12 is forcibly turned off, so that the current path between the gate of the MOSFET 30 and the drive circuit 31 is cut off. Thus, even when the drain and the gate of the MOSFET 30 are short-circuited, it is possible to prevent an overcurrent from flowing from the gate of the MOSFET 30 to the drive circuit 31.
[0024]
As described above, since an overcurrent does not flow from the gate of the MOSFET 30 to the drive circuit 31, it is possible to prevent an overvoltage from being applied to the drive circuit 31, and to damage not only the MOSFET 30 but also the drive circuit 31. Can be prevented.
[0025]
Further, it is possible to prevent an overvoltage from being applied to other elements on the substrate on which the drive circuit 31 is mounted.
<Other embodiments>
The present invention is not limited to the above embodiments, and various configurations can be adopted within the scope described in the claims. For example, the following configuration changes are possible.
[0026]
(1) In the above embodiment, the MOSFET 30 is used as a switching element whose drive is controlled by the drive control signal S1 generated by the drive circuit 31, but the switching element is, for example, an IGBT or a GTO (Gate Turn Off). thyristor) is not particularly limited.
[0027]
(2) In the above-described embodiment, the FET 19 is used to prevent the control signal S2 from being input to the FET 16 when the voltage value at the point D also increases the threshold voltage value (threshold) of the FET 19. , D is not limited to the FET 19 as long as it is possible to compare the voltage value at the point D with a predetermined threshold value and prevent the control signal S2 from being input to the FET 16 based on the comparison result.
[0028]
(3) In the above embodiment, the diode 11, the bipolar transistor 12, the resistor 13, and the FET 16 are combined as switch means for controlling the current flowing from the MOSFET 30 to the drive circuit 31. The configuration of the switch means can be switched between connection and cutoff of the current path between the gate of the MOSFET 30 and the drive circuit 31 based on the comparison result between the voltage value at the point D and a predetermined threshold value. It may be configured by an element other than the combination of the diode 11, the bipolar transistor 12, the resistor 13, and the FET 16.
[0029]
(4) Further, in the above embodiment, the control signal S2 in FIG. 2B is synchronized with the drive control command signal S0 in FIG. 2A and becomes high when the drive control command signal S0 is low. However, the control signal S2 may be controlled to be always at a high level in a normal state, and may be controlled to be at a low level only when an overvoltage is applied from the gate of the MOSFET 30 to the drive circuit 31. . Note that, in the normal operation of the MOSFET 30 in this case, when the low-level drive control signal S1 is input to the gate of the MOSFET 30, a configuration for extracting the charges accumulated in the MOSFET 30 is required.
[0030]
(5) In addition, instead of the circuit in which the diode 11 and the bipolar transistor 12 are connected in parallel in the above-described embodiment, for example, a triac is provided, and a current path in both directions is provided only when an overvoltage is applied to the drive circuit 31. You may comprise so that it may forcibly cut off.
[0031]
【The invention's effect】
As described above, according to the present invention, the voltage detection means detects the voltage applied to the drive circuit from the control terminal of the switching element, and when the voltage is an overvoltage, forcibly turns off the switching means to control the switching element. The current path from the terminal to the drive circuit can be cut off. As a result, even if the switching element is damaged, it is possible to prevent an overcurrent from flowing into the drive circuit, so that it is possible to prevent an overvoltage from being applied to the drive circuit.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a protection circuit according to an embodiment of the present invention.
FIG. 2 is a diagram showing a timing chart.
FIG. 3 is a diagram showing a conventional switching element and drive circuit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Protection circuit 11 Diode 12 Bipolar transistor 13, 14 Resistance 15 Control circuit 16 FET
17, 18 resistance 19 FET
30 switching element 31 drive circuit

Claims (2)

A protection circuit that is disposed between a control terminal of the switching element and a drive circuit that controls on / off of the switching element, and that prevents an overvoltage from being applied to the drive circuit from the control terminal of the switching element. ,
Switch means for switching connection and disconnection of a current path between a control terminal of the switching element and the drive circuit in synchronization with on / off control of the switching element by the drive circuit;
Voltage detection means for detecting a voltage applied to the drive circuit from a control terminal of the switching element;
A switch control means for forcibly turning off the switch means to cut off the current path when an overvoltage is detected by the voltage detection means;
A protection circuit comprising: The switch control means has a comparison means for comparing the voltage detected by the voltage detection means with a predetermined threshold, and when the detection voltage of the voltage detection means exceeds the threshold, forcibly switches the switch means The protection circuit according to claim 1, wherein the protection circuit is turned off.

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