JPH0590928A - Gate controller for switching element - Google Patents

Abstract

PURPOSE:To allow the gate controller for switching element to be contributed to prevent the defect of a switching element by suppressing a surge voltage generated at gate interruption of the switching element low. CONSTITUTION:A gate voltage is set to a nonsaturation region operating voltage by turning on a 1st auxiliary switching element 33 at first and turning off a 2nd auxiliary switching element 34, the result of detection by a current detection means is compared with a fault deciding reference value and when the result of detection is a faulty decision reference value or below, the 1st auxiliary switching element 33 to set the gate voltage to the saturation region operating voltage, and when the result of detection is the fault decision reference value or over, the setting of the saturation resion operating voltage is inhibited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching element gate control device improved to protect a voltage control type switching element such as a MOSFET.

[0002]

2. Description of the Related Art Conventionally, some voltage-controlled switching elements such as MOSFETs have been used in inverters for driving electric motors, for example. This is shown in FIG. AC power is rectified and smoothed by the rectifier circuit 1 and the smoothing capacitor 2 into DC. The DC output is given to the inverter 3, and the inverter 3 converts the DC voltage into the AC voltage to drive the electric motor 4.

The inverter 3 is a voltage-controlled switching element 5U, 5V, 5W and 5X, 5 such as a MOSFET.
It is configured by connecting W and 5Z in a bridge. The inverter 3 is provided with an overcurrent detector 6. The switching elements are driven and controlled by a drive circuit 7, and the drive circuit 7 is controlled by a control circuit 8.

Now, referring to FIG. 6, a driving circuit for one of the switching elements (for convenience, the reference numeral "5" is attached) will be described. The negative side of the gate drive power supply 9 has a diode 10 connected in series with the source side of the switching element 5.
Connected to the cathode. The positive side of the power source 9 is connected to the gate of the switching element 5 via the resistor 11.

An auxiliary switching element 12 composed of an NPN transistor is connected between the gate of the switching element 5 and the cathode of the diode 10. When the auxiliary switching element 12 is turned off, the switching element 5 is turned on and the auxiliary switching element 12 is turned on. When turned on, the forward drop of the diode 10 is reverse-biased between the gate and source of the switching element 5 and the switching element 5
Is supposed to turn off. The auxiliary switching element 12 is turned on / off by controlling the photocoupler 13 by a signal from the control circuit 8.

The electric motor 4 is driven by turning on / off each switching element of the inverter 3 appropriately. In this case, if the switching element 5U is damaged and the drain and source are short-circuited, the switching element is switched. 5X
When is turned on, an excessive short circuit current from the smoothing capacitor 2 flows from the switching element 5U short circuit portion to the switching element 5X. This overcurrent is detected by the overcurrent detector 6, the detection signal from the overcurrent detector 6 is given to the control circuit 8, and the control circuit 8 cuts off the gate signal of each switching element.

At this time, a large surge voltage is generated between the drain and source of the switching element 5X, and the switching element 5X is damaged. As a countermeasure against this, a snubber circuit including a snubber diode, a snubber capacitor, and a snubber capacitor discharge resistor is connected to suppress the surge voltage.

[0008]

The surge voltage generated when the above-mentioned short-circuit current is interrupted is extremely large as compared with the surge voltage generated when the switching element is turned off during normal inverter operation. Therefore, from the viewpoint of protection of the switching element, the snubber circuit has to be upsized, which is uneconomical. There was also a problem that the inverter main circuit configuration became complicated.

The present invention has been made in view of the above circumstances, and an object thereof is to suppress a surge voltage generated when the gate of a switching element is cut off to a low level and to contribute to preventing damage to the switching element. It is in the area of providing a control device.

[0010]

A gate control device for a switching element according to the present invention controls the operation of a voltage-controlled switching element such as a MOSFET, and a current detecting means for detecting a current flowing through the switching element. A gate voltage switching means for switching the gate voltage, a comparing means for setting the first gate voltage to an operation voltage for the non-saturated region and comparing the detection result by the current detecting means with an abnormality determination reference value, and the detection result is abnormal. And a gate voltage control means for setting the gate voltage to the saturation region operation voltage when it is equal to or lower than the judgment reference value, and prohibiting the setting of the saturation region operation voltage when the detection result exceeds the abnormality judgment reference value. It

[0011]

When the gate voltage is first set to the non-saturation region operation voltage and the switching element is driven, the drain-source current becomes smaller than when the gate voltage is set to the saturation region operation voltage. Become. In this case, even if an overcurrent such as a short-circuit current flows through the switching element,
The current is smaller than when the gate voltage is set to the saturation region operation voltage. The current flowing through such a switching element is detected by the current detecting means, and the detection result is compared with the reference value.

When the detection result is less than the abnormality determination reference value, it can be determined that no overcurrent is flowing, and in this case, the gate voltage is set to the saturation region operating voltage. After that, when the gate of the switching element is cut off, a large surge voltage does not occur. Further, when the detection result exceeds the abnormality determination reference value, it can be determined that an overcurrent is flowing, and in this case, setting of the saturation region operating voltage is prohibited. Therefore, the energizing current is not so large even if it is an overcurrent because the energizing current corresponding to the non-saturation operation region is the base. Therefore, even if the gate of the switching element is cut off thereafter, a large surge voltage does not occur.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 2 shows the main circuit configuration of the inverter. The inverter 21 includes a rectifier circuit 2
2 and the smoothing capacitor 23 provide a DC voltage. The inverter 21 converts a DC voltage into an AC voltage and drives the electric motor 24.

The inverter 21 is constructed by bridge-connecting voltage-controlled switching elements 25U, 25V, 25W and 25X, 25W, 25Z such as MOSFETs. The inverter main circuit 21 is provided with a current detector 26 as current detecting means. Each of the switching elements is driven and controlled by a gate drive circuit 27, and the gate drive circuit 27 is controlled by a control circuit 28.

FIG. 1 shows switching elements 25U and 25U.
The gate drive circuit for one of V, 25W, 25X, 25W, and 25Z (this is called switching element 25 for convenience) is shown. The negative side of the gate drive power supply 29 is connected to the cathode of a diode 30 connected in series with the source side of the switching element 25. The positive side of the power source 29 is connected to the gate of the switching element 25 via the gate resistor 31.

A voltage dividing resistor 32 and an NPN are provided between the gate of the switching element 25 and the cathode of the diode 30.
First auxiliary switching element 33 composed of a transistor
Are connected in series, and the second auxiliary switching element 34 is connected to the series circuit of the voltage dividing resistor 32 and the auxiliary switching element 33. The gate resistor 31, the voltage dividing resistor 32, and the auxiliary switching elements 33 and 34 constitute a switching circuit 35 as a gate voltage switching means. The auxiliary switching element 34 is turned on / off by controlling the photocoupler 36 by a signal from the control circuit 28.

Incidentally, FIG. 3 shows the output characteristics of the power MOSFET. This figure shows the relationship between the drain-source voltage V _DS and the drain current I _{D using} the gate voltage (V _GS ) of the power MOSFET as a parameter. That is, since the drain current I _D also changes in accordance with the change in the gate voltage V _GS , the gate voltage is initially applied low by utilizing this characteristic, and the conduction current of the power MOSFET at this time, in other words, the drain current I _D. Is low, and even if an overcurrent occurs, the surge voltage at the time of gate cutoff can be kept low.

The control circuit 28 has a microcomputer, for example, and functions as a comparison means and a gate voltage control means. The operation including its function will be described below.

The control circuit 28 gives an OFF instruction signal to the photocoupler 36 and an ON instruction signal to the first auxiliary switching element 33 by an operation start input. This turns off the photocoupler 36, which turns off the auxiliary switching element 34. As a result, the divided voltage determined by the gate resistor 31 and the voltage dividing resistor 32 is applied to the gate of the switching element 25 as the gate voltage. This divided voltage is set to the voltage for the unsaturated operation region. As a result, the switching element 25 operates in the unsaturated operation region.

At this time, the current flowing through the switching element 25 is detected by the current detector 26 with the main circuit current, and the detection result is given to the control circuit 28.
The control circuit 28 compares the detection result with the abnormality determination reference value. The reference value is set in advance to a value such that it is possible to determine whether the energization current in the non-saturation region is normal or overcurrent.

The control circuit 28 turns off the first auxiliary switching element 33 when the detection result is equal to or less than the abnormality determination reference value, that is, when it is determined that the overcurrent is not generated, and the switching element 25 is turned off. The gate voltage is raised and set to the saturation region operation voltage. As a result, the switching element 25 operates in the saturation region.

Even if the second auxiliary switching element 25 is turned on and the gate of the switching element 25 is cut off in the situation where such an overcurrent does not occur, a large surge voltage does not occur.

On the other hand, when the switching element 25 is first operated in the non-saturation region, when the current detection result in the control circuit 28 exceeds the abnormality determination reference value, that is, the overcurrent occurrence state. When it is determined that the first auxiliary switching element 33 is turned on. In other words, the gate voltage is not set to the saturation region voltage. Note that the energizing current at this time is not so large even if it is an overcurrent because the energizing current corresponding to the non-saturated operation region is the base.

Therefore, the second auxiliary switching element 34
When the switch is turned off to cut off the gate of the switching element 25, the energizing current of the switching element 25 is not so large, so that a large surge voltage does not occur.

FIG. 4 shows the waveform of each part of the circuit. As can be seen from the figure, when the gate voltage V _{GS is set} to the non-saturation region operating voltage, even if an overcurrent occurs, the conduction current (drain current _ID ) does not become so high. Therefore, even if the first auxiliary switching element 34 is turned off and the gate is cut off, the surge voltage (indicated by reference symbol V _E ) does not increase.

Although the present invention is applied to the switching element used in the inverter in the above-described embodiments, the present invention can be widely applied to other switching elements in general, and various modifications can be made without departing from the scope of the invention. Is.

[0027]

As is apparent from the above description, the present invention compares the detection result by the current detection means with the abnormality determination reference value by setting the initial gate voltage to the non-saturation region operation voltage. Is less than or equal to the abnormality determination reference value, the gate voltage is set to the saturation region operation voltage, and when the detection result exceeds the abnormality determination reference value, the saturation region operation voltage is prohibited from being set. The surge voltage generated when the gate is cut off can be suppressed to a low level, which contributes to the prevention of damage to the switching element and also contributes to downsizing of the snubber circuit.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of a gate drive circuit showing an embodiment of the present invention.

FIG. 2 is an electric circuit diagram showing an inverter device.

FIG. 3 is a diagram showing output characteristics of a power MOSFET.

FIG. 4 is a diagram showing the operation status of each part.

FIG. 5 is an electric circuit diagram showing an inverter device showing a conventional example.

FIG. 6 is an electric circuit diagram of a drive circuit.

[Explanation of symbols]

21 is an inverter, 25U, 25V, 25W, 25X,
25W and 25Z are switching elements, 26 is a current detector (current detecting means), 27 is a gate drive circuit, 28 is a control circuit (comparing means, gate voltage control means), 31 is a gate resistor, 32 is a voltage dividing resistor, 33 Is a first auxiliary switching element, 34 is a second auxiliary switching element, and 35 is a switching circuit (gate voltage switching means).

Claims (1)

[Claims] 1. A device for controlling the operation of a voltage-controlled switching element such as a MOSFET, wherein a current detecting means for detecting a current flowing through the switching element, a gate voltage switching means for switching a gate voltage, and a first gate voltage. Is set to a non-saturation region operation voltage and the detection result by the current detection unit is compared with an abnormality determination reference value; and when the detection result is equal to or less than the abnormality determination reference value, the gate voltage is set to the saturation region operation voltage And a gate voltage control means for prohibiting the setting of the saturation region operation voltage when the detection result exceeds the abnormality determination reference value.