JPH07154224A - Overvoltage protection device - Google Patents

Abstract

PURPOSE:To prevent destruction of a power MOSFET by surely turning on the power MOSFET without operation of a driver as an oscillation circuit when the N-channel power MOSFET provided to a current path from a DC power supply to a load with the two-terminal driver and an overvoltage is produced in the DC power supply. CONSTITUTION:When a power supply voltage Vc of the two-terminal driver reaches an upper limit voltage VHH depending on a breakdown voltage of Zener diodes ZD1-ZD3, it is discriminated that an overvoltage is generated in the DC power supply connecting to a power MOSFET 18, the power MOSFET 18 is forcibly turned on, the Zener diode ZD1 is short-circuited after the power MOSFET 18 is turned on to selects a discrimination voltage for the overvoltage to be a lower limit voltage VHL depending on the breakdown voltage of the Zener diodes ZD2, ZD3. As a result, the on-time of the power MOSFET 18 is extended to prevent thermal destruction due to the oscillation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage protection device for protecting an N-channel power MOSFET provided as a high-side switch in a current path from a DC power supply to a load from overvoltage, and particularly, a power MOSFET having a two-terminal type. The present invention relates to an overvoltage protection device suitable when driven by the above driving device.

[0002]

2. Description of the Related Art Conventionally, in a control device using a semiconductor element, an overvoltage protection circuit for preventing the output semiconductor element connected to the power source from being destroyed when an overvoltage occurs in the power source is disclosed in, for example, Japanese Patent Laid-Open Publication No. As disclosed in Japanese Patent Laid-Open No. 50-36942, there is known one in which a breakdown current of a Zener diode forcibly causes an output semiconductor element to conduct, and a load of the output semiconductor element absorbs an overvoltage. ing.

[0003]

However, such a conventional overvoltage protection circuit is a two-terminal type drive for turning on / off an N-channel power MOSFET provided as a so-called high side switch in a current path from a DC power supply to a load. When applied to a device, a power MOS
FET is turned on and off at a predetermined frequency, power MOS
There is a problem that the FET is overheated and the power MOSFET is thermally destroyed in some cases. Hereinafter, this problem will be described in detail.

First, for example, as shown in FIG.
N-channel power MOSFET 8 connected in series with the main switch 84 in the current path from 0 to the load 82
The two-terminal type driving device that drives 6 is the main switch 8
4 is in the ON state and the power MOSFET 86 is in the OFF state, both ends of the power MOSFET 86 (drain-
The power supply capacitor C10 is charged through the diode D10 by the voltage generated between the sources), and the charge accumulated in the power supply capacitor C10 and the voltage across the power MOSFET 86 cause the internal control circuit 88, the drive circuit 90, etc. An auxiliary power source 92 for supplying power is provided.

Therefore, when the above-mentioned conventional overvoltage protection circuit is applied to such a two-terminal type driving device, the overvoltage protection circuit is provided with an auxiliary power supply 92 as shown by reference numeral 94 in FIG.
Zener diodes ZD11 to ZD13 which are turned on when the power supply voltage Vc output from the device reaches a predetermined upper limit voltage,
The protection transistor TR11 for forcibly turning off the drive transistor TR10 provided in the drive circuit 90 to turn on the power MOSFET 86 and the protection transistor TR11 by the breakdown current flowing when the Zener diodes ZD11 to ZD13 are turned on. Resistors R11 and R12 that turn on (that is, turn on the power MOSFET)
It can be configured by

In the drive unit of FIG. 5, power M
The drive circuit 90 for turning on / off the OSFET 86 is
When the driving transistor TR10 which is turned on / off by the control signal from the control circuit 88 is in the off state, the power supply voltage Vc is supplied to the power MOSF via the resistors R13 and R14.
Power MOS by applying to the gate of ET86
The FET 86 is configured to be turned on, and further, when the power supply voltage Vc is abnormal (overvoltage), the power MOSFET is turned on.
Zener diodes ZD14 and ZD15 for overvoltage protection that limit the gate voltage of T86 to a predetermined voltage or less are provided.

Overvoltage protection circuit 9 configured as described above
4, when the main switch 84 is in the ON state and the power MOSFET 86 is in the OFF state, the battery voltage VB becomes an overvoltage and the terminal voltage (power source voltage V10) of the power source capacitor C10 which is supplied with power from the battery 80.
When (c) exceeds the break voltage of the Zener diodes ZD11 to ZD13, the protection transistor TR11 is turned on, and the power MOSFET 86 is forcibly turned on.

By the way, when the power MOSFET 86 is turned on at the time of occurrence of such an overvoltage, from the battery 80 via the power MOSFET 86 and the main switch 84.
A current flows through the load 82, the terminal voltage of the load 82, in other words, the ground potential KG of the drive device becomes substantially equal to the voltage obtained by subtracting the ON voltage of the power MOSFET 86 from the battery VB, and the power supply voltage in the drive device at this time. Vc is a voltage across the power supply capacitor C10 of the auxiliary power supply 92 with reference to the ground potential KG. In this state, the positive electrode side potential of the power supply capacitor C10 becomes higher than the positive electrode side potential of the battery 80, so that the power supply capacitor C10 is not charged from the battery 80, and the power supply capacitor C10 controls the control circuit 88 and the overvoltage protection circuit 94. The power supply voltage Vc immediately drops due to the discharge to the like, and as the power supply voltage Vc drops, the protection transistor TR11 in the overvoltage protection circuit 94.
Is turned off, and the power MOSFET 86 is also turned off. Then, in this way, the power MOSFET 86 is OF
Then, the battery voltage VB, which is an overvoltage, is applied to the power supply capacitor C10 again, so that the power supply voltage Vc increases and the overvoltage protection circuit 94 operates, causing the power MOSFE to operate.
T86 is turned on.

That is, when the conventional overvoltage protection circuit is applied to the two-terminal type driving device as described above, the driving device operates as an oscillating circuit and the power MOSFET is periodically turned on and off. It will be. The oscillation frequency at this time is zener diodes ZD11 to ZD.
It depends on the junction capacitance of D13, the capacitance of the power supply capacitor C10, etc., but usually reaches several hundred KHz, and if overvoltage occurs for a long time due to, for example, a load dump, during that time,
The drive device repeats the above oscillation.

On the other hand, since the operating frequency of the drive circuit 90 shown in FIG. 5 is determined by the resistors R13 and R14 and the gate capacitance of the power MOSFET 86, when the protection transistor TR11 is turned on / off at the above oscillation frequency. The gate voltage stabilizes at an intermediate potential between when the protection transistor TR11 is on and when it is off. As a result, the power MOSFET 8
6 cannot be driven in a completely saturated region where its internal resistance is minimum, and the power MOSFET 86
Is fixed in the saturated region where it is in a half ON state.

Therefore, if the overvoltage is generated for a long time, the loss with respect to the load current in the power MOSFET 86 increases, the heat generation amount of the power MOSFET 86 increases, and the power MOSFET 86 is thermally destroyed in some cases. If the gate capacitance of the power MOSFET 86 is reduced and the operating frequency of the drive circuit 90 is increased, the heat generation of the power MOSFET 86 due to the oscillation can be suppressed.
Since the allowable current of 86 becomes small, this time, the power MOSFET 86 is driven by the high-speed switching operation at the time of overvoltage.
Overcurrent flows in the power MOS, and as a result, power MOS
The FET will cause thermal breakdown.

The present invention has been made in view of these problems, and an N-channel power MOSFE provided as a high side switch in a current path from a DC power supply to a load.
When T is driven by a two-terminal drive device, when an overvoltage occurs in the DC power supply, the power MOSFET is reliably turned on without operating the drive device as an oscillation circuit to prevent the power MOSFET from being destroyed. It is an object of the present invention to provide an overvoltage protection circuit that can be used.

[0013]

SUMMARY OF THE INVENTION The present invention made to achieve the above object is to provide an N-channel power MOSFET provided in a current path from a DC power supply to a load, and an N-channel power MOSFET provided in parallel with the power MOSFET. The power MOS
An auxiliary power supply that accumulates electric charges in the power supply capacitor by the voltage across the FET and also generates a power supply voltage by the charges accumulated in the power supply capacitor and the voltage across the switching element, and operates by receiving power supply from the auxiliary power supply. ,
The power MOSF is controlled by a control signal input from the outside.
A drive circuit provided with a drive circuit for driving the load by turning on / off the ET, and when the overvoltage occurs in the DC power supply, turns on the power MOSFET to protect the power MOSFET from the overvoltage. An overvoltage judging device for judging that an overvoltage has occurred in the DC power supply when the power supply voltage supplied from the auxiliary power supply exceeds a predetermined judging voltage, and the overvoltage judging device If the determination voltage is not determined, the determination voltage is set to the first determination voltage, and when the overvoltage determination means determines the overvoltage, the determination voltage is set to the first determination voltage until the overvoltage determination means stops determining the overvoltage. The determination voltage setting means for setting a second determination voltage lower by a predetermined voltage and the power MOS when the overvoltage determination means determines the overvoltage. And protection means for turning ON the ET, is characterized by comprising a.

[0014]

In the overvoltage protection device of the present invention configured as described above, when the overvoltage determination means has the power supply voltage supplied from the auxiliary power supply exceeding the predetermined determination voltage, the DC power supply is used. To determine that an overvoltage has occurred,
When the overvoltage is determined by the overvoltage determination means, the protection means turns on the power MOSFET. Further, the determination voltage in the overvoltage determination means is the first voltage if the overvoltage determination means has not determined the overvoltage by the determination voltage setting means.
When the overvoltage determining unit determines the overvoltage, the second determining voltage lower than the first determining voltage by the predetermined voltage is set until the overvoltage determining unit stops determining the overvoltage.

That is, in the present invention, a so-called hysteresis is provided to the judgment voltage in the overvoltage judgment means, and when the overvoltage judgment means judges the overvoltage of the DC power supply, the voltage for the overvoltage judgment is thereafter changed to the normal judgment voltage (first judgment voltage). ) By changing to a lower second determination voltage, the power MOSF
After turning on ET, the time until the power supply voltage from the auxiliary power supply drops and the power MOSFET is turned off is lengthened, and when an overvoltage occurs in the DC power supply, the power MOSFE
This prevents T from turning on and off at high speed.

As a result, according to the present invention, as in the conventional device, when an overvoltage occurs in the DC power supply, the power MOS
FET is turned on and off at high speed, power MOSFET
T does not overheat, power MOSFE
It becomes possible to reliably protect T.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 2 is a schematic configuration diagram showing the overall configuration of a vehicular turning indicator to which the present invention is applied, and FIG. 1 is an electric circuit diagram showing the configuration of a power MOSFET drive circuit and an overvoltage protection circuit in this device. Is.

As shown in FIG. 2, the vehicular turning indicator according to the present embodiment includes turning indicators 1 provided on the left and right sides of the vehicle, respectively.
0L, 10R and a direction indicator switch 12 operated by the vehicle driver as necessary to turn on one of the direction indicator lamps 10L, 10R, and a direction indicator via the direction indicator switch 12 and the fuse 16. It is provided in a current path for turning on the direction indicator lamp, which is composed of a battery 14 for supplying power to the lamp 10L or 10R, and when the direction indicator switch 12 is turned on, the current path is periodically turned on and off, thereby To turn on or off any of the turn signal lamps 10L or 10R connected by the instruction switch 12, in order to connect or disconnect the current path,
The drain is connected to the battery 14 via the terminal TB,
The direction indicating switch 12 is provided with an N-channel power MOSFET 18 whose source is connected via a terminal TL.

Between the terminals TB and TL to which the power MOSFET 18 is connected, a resistor R1, a diode D1, and a power supply capacitor C1 are connected in parallel with the power MOSFET 18.
Auxiliary power source consisting of is connected. This auxiliary power supply
Similar to the auxiliary power supply 92 shown in FIG. 5, the direction indicating switch 1
2 is ON, and the power MOSFET 18 is OF
In the F state, the power supply capacitor C1 is charged by the voltage generated between the terminals TB and TL, and the charge accumulated in the power supply capacitor C10 and the power MOSFET 86 are charged.
The power is supplied to the inside of the device by the voltage across both terminals.

The power supply capacitor C1 includes a constant current circuit IC, a Zener diode D2 and a transistor TR.
1 is connected to the constant voltage circuit, and the constant voltage VT output from the constant voltage circuit causes the power MOSF
Various reference voltages used to control ON / OFF of the ET 18 can be generated.

On the other hand, in the vehicular turning indicator of the present embodiment, a driving circuit 20 for driving the power MOSFET 18, an integrating circuit composed of a charging / discharging capacitor C0 and a resistor R0, and a turning indicator 12 are provided. When ON, a charging / discharging control voltage is applied to the integrating circuit via the buffer B1 to charge / discharge the charging / discharging capacitor C0, thereby generating a blinking signal S1 as the control signal for blinking the direction indicator lamps 10L, 10R. A blinking circuit 22 for generation, a rapid discharge circuit 24 for detecting that the direction indicating switch 12 is turned off, turning on the transistor TR2 and rapidly discharging the charging / discharging capacitor C0, and the power MOSFET 18
From the current path to the direction indicators 10L and 10R
Detecting disconnection, turn signal 10 by blinking circuit 22
A short circuit / disconnection detection circuit 26 which prevents an overcurrent from flowing to the power MOSFET 18 due to a short circuit or disconnection of its current path by prohibiting the blinking operation of L and 10R.
An abnormality (overvoltage) of the battery voltage VB is detected from the power supply voltage Vc supplied from the reference power supply 28 and the reference power supply 28 that generates the determination voltage for detecting the short circuit / wire breakage in the short circuit / wire breakage detection circuit 26, An overvoltage protection circuit 30 for forcibly turning on the power MOSFET 18 is provided.

Here, the blinking circuit 22 divides the constant voltage VT from the constant voltage circuit to generate the lower limit voltage and the upper limit voltage of the charging / discharging capacitor C0, and the voltage across the charging / discharging capacitor C0 is the upper limit voltage. Until the output of the buffer B1 is set to High level to charge the charging / discharging capacitor C0 with a predetermined time constant, and when the voltage across the charging / discharging capacitor C0 reaches the upper limit voltage, the voltage across Until the lower limit voltage is reached, the output of the buffer B1 is set to Low level, and the charging / discharging capacitor C0 is discharged with a predetermined time constant. The charging / discharging capacitor C0 is repeatedly charged / discharged, and the charging is performed. It is well known in the art to output a low level blinking signal S1 for turning on the power MOSFET 18 at times.

The rapid discharge circuit 24 is a power MOS.
By detecting that the direction indicating switch 12 is turned off from the battery voltage VB supplied to the device when the FET 18 is turned off, the transistor TR2 is turned on and the charging / discharging capacitor C0 is rapidly discharged. When the instruction switch 12 is turned on, charging of the charging / discharging capacitor C0 is started from the initial state of zero charge,
This is for blinking the direction indicator lamp 10L or 10R at a constant cycle immediately after the direction indicator switch 12 is turned on.
This is the circuit previously proposed by the applicant in Japanese Patent Application No. 5-240036. The rapid discharge circuit 24 is configured to rapidly discharge the charging / discharging capacitor C0 when the constant voltage VT output from the constant voltage circuit becomes equal to or lower than a predetermined voltage and the device cannot operate normally. May be.

Next, the reference power source 28 synthesizes the constant voltage VT from the constant voltage circuit and the power source voltage Vc proportional to the battery voltage supplied from the auxiliary power source to synthesize the power MOSFET.
When the power MOSFET 18 is turned on, a voltage corresponding to the load current flowing in the direction indicators 10L and 10R is generated, and the voltage is corrected by a diode or the like to a voltage that matches the temperature characteristics of the power MOSFET 18, and the overcurrent flowing when the power MOSFET 18 is turned on. To generate a judgment voltage corresponding to
When the power MOSFET 18 is turned on, the short-circuit / disconnection detection circuit 26 has a battery voltage VB based on the ground potential KG, in other words, a drain of the power MOSFET 18.
When the voltage between the sources is taken in and the voltage value exceeds the overcurrent determination voltage, it is determined that an abnormality such as a short circuit or disconnection has occurred in the current path from the power MOSFET 18 to the direction indicator lamps 10L and 10R, and the LED blinks. The blinking operation of the power MOSFET 18 by the circuit 22 is prohibited. The short circuit / disconnection detection circuit 26 and the reference power supply 28 are the circuits previously proposed by the applicant of the present application in Japanese Patent Application No. 5-262580.

Among the above-mentioned parts constituting the vehicular turning indicator of the present embodiment, the parts surrounded by the one-dot chain line in FIG.
The parts other than the power MOSFET 18, the resistor R1, the integrating circuit including the resistor R0 and the charging / discharging capacitor C0, and the power supply capacitor C1 are housed in one IC package. Further, in FIG. 2, VR is a direction indicator lamp 10.
R represents the voltage across R, and VL represents the voltage across the turn signal lamp 10L.

Next, the drive circuit 20 and the overvoltage protection circuit 3
The configuration of 0 will be described in detail with reference to FIG. Figure 1
As shown in FIG. 5, the drive circuit 20 of the present embodiment is similar to the conventional device illustrated in FIG. 5, and the drive transistor T is turned on / off by the blink signal S1 output from the blink circuit 22.
When the driving transistor TR3 is in the OFF state (that is, when the blinking signal S1 is at the low level), the power MOSFET 1 is provided via the resistors R3 and R4.
By applying the power supply voltage Vc to the gate of the power MOSFET 8, the power MOSFET 18 is turned on.
Furthermore, when the power supply voltage Vc is abnormal (overvoltage), the power M
The Zener diodes ZD4 and ZD5 for overvoltage protection that limit the gate voltage of the OSFET 18 to a predetermined voltage or less are provided.

Next, the overvoltage protection circuit 30 of this embodiment is
An abnormality (overvoltage) of the battery voltage VB is determined based on the power supply voltage Vc supplied from the auxiliary power supply, and the power MOSF
The ET86 is forcibly turned on, and like the conventional device illustrated in FIG. 5, the power supply voltage Vc is a predetermined upper limit voltage V.
Zener diode ZD1 that conducts when reaching HH1
ZD3 and the driving transistor TR3 in the driving circuit 20
Is forcibly turned off and the power MOSFET 86 is turned on
A bias voltage is applied to the protection transistor TR5 by the breakdown current that flows when the protection transistor TR5 that causes it and the Zener diodes ZD1 to ZD3 are conductive, and the protection transistor TR5 turns on (that is, the power MOSFET turns on). R5, R6
It has and.

Further, in the overvoltage protection circuit 30 of the present embodiment, the base has a resistor R9 and a resistor R5 and a resistor R5.
NPN connected to the connection point with 6, the emitter connected to the ground line (ground potential KG) of the device, and the collector connected to the power supply line (power supply voltage VC) of the device through resistors R7 and R8. Type transistor TR6
, The base is connected to the connection point between the resistor R7 and the resistor R8, and the emitter and collector are the Zener diode ZD.
PNP type transistor TR connected to both ends of 1
7 and are provided.

Therefore, in the overvoltage protection circuit 30 of this embodiment, when the power supply voltage Vc reaches the upper limit voltage VHH, the Zener diodes ZD1 to ZD3 are turned on, and the protection transistor TR5 is turned on, the transistor T is simultaneously turned on.
R6, and eventually the transistor TR7 is turned on, and both ends of the Zener diode ZD1 are short-circuited by the transistor TR7. After that, the power supply voltage Vc is the upper limit voltage VHH determined by the breakdown voltage of the Zener diodes ZD2 and ZD3. Until the lower limit voltage VHL, which is lower by the breakdown voltage of the Zener diode ZD1, becomes equal to or lower than the lower limit voltage VHL, the protection transistor TR remains in the ON state, and by extension, the power MOSFE.
The ON state of T will be maintained.

Next, the operation and effect of the vehicular turning indicator of the present embodiment constructed as described above will be described with reference to FIG.
A description will be given along the time chart shown in. First, when the direction indicator switch 12 is connected to the direction indicator lamp 10L or 10R when the power supply voltage Vc is normal, as in the area A shown in FIG. 3, the blinking circuit 22 operates, and the blinking circuit 22 charges and discharges. A blinking signal S1 that changes periodically is output in synchronization with the charging / discharging time of the capacitor C0. If the blinking signal S1 is at low level, the power MOSFET 1
8 is turned on and the power MOSFET 18 is turned off when the blinking signal S1 is at a high level, power is periodically supplied to the direction indicator lamp 10R or 10L to which the direction indicator switch 12 is connected, and the direction indicator lamp 10R or 10L blinks.

Next, as in the area B shown in FIG. 3, when an overvoltage occurs when the direction indicating switch 12 is in the OFF state, the device is open to the battery 14, so that the overvoltage occurs. The power MOSFET 18 and the internal circuit are not destroyed. On the other hand, as in the area C shown in FIG. 3, when the direction indicating switch 12 is ON, the blinking signal S
1 is High level, that is, the power MOSFET 18 is OF
In the F state, an overvoltage is generated and the power supply voltage Vc
When the voltage reaches the upper limit voltage VHH, the Zener diodes ZD1 to ZD3 in the overvoltage protection circuit 30 break, the protection transistor TR5 turns on, and the power MOSFET 1
8 is forcibly turned on. The moment the overvoltage occurs,
Between terminals TB and TL (in other words, power MOSFET 18
Overvoltage is applied between the drain and source of the power MOSFET, but after this voltage rises to approximately the upper limit voltage VHH, the power MOSFET
Since 18 is turned on, it immediately drops.

Then, in this way, the power MOSFET 1
When 8 is turned on, the power supply voltage Vc gradually decreases from the upper limit voltage VHH according to the capacity of the power supply capacitor C1 and the current consumption of the internal circuit. On the other hand, power M
When the OSFET 18 is turned on, the overvoltage protection circuit 3 is simultaneously released.
Since the transistors TR6 and TR7 in 0 are turned on and both ends of the Zener diode ZD1 are short-circuited, the power MOSFET 1 is kept until the power supply voltage Vc reaches the lower limit voltage VHL.
The ON state of 8 continues, and the power supply voltage Vc is the lower limit voltage VHL.
The normal ON / OFF control of the power MOSFET 18 is restarted at the point of time when the power MOSFET 18 has fallen.

During this time, since the overvoltage exceeding the upper limit voltage VHH is not applied to the power MOSFET 18 and the internal circuit, the power MOSFET 18 and the semiconductor in the circuit do not cause breakdown voltage breakdown. Also, power MO
Since the gate of the SFET 18 is biased with a voltage determined by the breakdown voltage of the Zener diodes ZD4 and ZD5, the power MOSFET 18 is turned on very well.
State, and power MOSF like the conventional device
Since the ET 18 is not turned on / off at a high frequency, the power MOSFET 18 is not thermally destroyed.

Next, as in the area D shown in FIG. 3, when the power MOSFET 18 is in the ON state by the blinking signal S1 output from the blinking circuit 22, when the overvoltage occurs, the overvoltage is the power. Since the MOSFET 18 is discharged through the direction indicator lamp 10L or 10R which is a load, no overvoltage is applied to the power MOSFET 18 and the internal circuit, and there is no problem.

As described above in detail, in the vehicular turning indicator of this embodiment, the Zener diodes ZD1 to ZD3 are used as the overvoltage judging means for judging the overvoltage, and the power supply voltage Vc is the Zener diodes ZD1 to ZD.
When the upper limit voltage VHH determined by the breakdown voltage of D3 is reached, it is considered that an overvoltage has occurred, and the power MOSFET
18 is forcibly turned on, and power MOSFET 18
After turning on, the both ends of the Zener diode ZD1 are short-circuited, and the overvoltage determination voltage is switched to the lower limit voltage VHL determined by the breakdown voltage of the Zener diodes ZD2 and ZD3. After the power MOSFET 18 is turned on, the power supply voltage Vc drops. Then, the time until the power MOSFET 18 is turned off is lengthened. Therefore, according to this embodiment, the power M is generated when an overvoltage occurs, as in the conventional device.
OSFET18 turns on and off at high speed, power MO
SFET can be prevented from overheating, power MOSFET
Will be able to reliably protect.

Here, in the above embodiment, the overvoltage protection circuit 30 is configured to determine the overvoltage by using the Zener diodes ZD1 to ZD3.
For example, as shown in FIG. 4, the power source voltage Vc is divided using the voltage dividing resistors R21 and R22, and the constant voltage VT from the constant voltage circuit is divided using the voltage dividing resistors R31 to R33. A power supply voltage V generated by generating a reference voltage Vref (= VHH) for overvoltage determination and divided by voltage dividing resistors R21 and R22.
The comparator 32 compares the divided voltage value Vcc of c with the reference voltage Vref.
By comparing the magnitudes by using, the comparator 32 outputs an overvoltage determination signal (High level) when the divided voltage value Vcc becomes equal to or higher than the reference voltage Vref, and the protection transistor TR5 is turned on via the resistor R5. Turn it on,
Further, at the time of overvoltage judgment, the judgment signal from the comparator 32 turns on the transistor TR30 via the resistor R35.
N, the voltage dividing resistor R33 for generating the reference voltage Vref is short-circuited, and the reference voltage Vref is set to a lower reference voltage Vref than usual.
Even if it is configured to switch to (= VHL), it is possible to obtain the same effect as in the above embodiment.

If the overvoltage protection circuit 30 is constructed in this way, the overvoltage detection voltage (VHH), the overvoltage protection release voltage (VHL),
Since the overvoltage determination time and the like can be freely set and there is no temperature characteristic unlike a Zener diode, overvoltage protection can be performed with high accuracy.

Further, in the above embodiment, the case where the overvoltage protection device of the present invention is applied to the vehicular turning indicator device has been described. However, the present invention uses an N channel power MOS used as a high side switch. Any device that is driven by a terminal type drive circuit can be applied, and the same effect as that of the above-described embodiment can be obtained.

[Brief description of drawings]

FIG. 1 is an overvoltage protection circuit and power MOSFE according to an embodiment.
It is an electric circuit diagram showing the drive circuit of T.

FIG. 2 is a schematic configuration diagram showing a configuration of an entire vehicular turning indicator according to an embodiment.

FIG. 3 is a time chart showing the operation of the vehicular turning indicator according to the embodiment.

FIG. 4 is an electric circuit diagram showing another configuration example of an overvoltage protection circuit.

FIG. 5 is an electric circuit diagram showing a configuration of a conventional overvoltage protection circuit.

[Explanation of symbols]

10L, 10R ... Direction indicator lamp 12 ... Direction indicator switch 14 ... Battery 18 ... Power MOSFET 20
Drive circuit 22 Flashing circuit 30 Overvoltage protection circuit 32 Comparator C1 Power supply capacitors ZD1 to ZD5 Zener diode TR3 Drive transistor TR5 Protection transistor

Front page continuation (72) Inventor Katsumi Nakamura 1-1, Showa-cho, Kariya city, Aichi prefecture, Nihon Denso Co., Ltd. (72) Inventor Hideyuki Ikemoto 3-in Iyama, Shinome-cho, Anjo city, Aichi prefecture

Claims (1)

[Claims] 1. An N-channel power MOSFET provided in a current path from a DC power supply to a load, and the power MO provided in parallel with the power MOSFET.
An auxiliary power supply that accumulates charges in the power supply capacitor by the voltage across the SFET and also generates a power supply voltage by the charges accumulated in the power supply capacitor and the voltage across the switching element, and operates by receiving power supply from the auxiliary power supply. The power MOSFET is turned on by a control signal input from the outside.
An overvoltage protection device, which is provided in a driving device including: a drive circuit which is turned off to drive the load; and which turns on the power MOSFET when an overvoltage occurs in the DC power supply to protect the power MOSFET from the overvoltage. The overvoltage determination means for determining that an overvoltage has occurred in the DC power supply when the power supply voltage supplied from the auxiliary power supply exceeds a predetermined determination voltage, and the overvoltage determination means determines the overvoltage. Otherwise, the determination voltage is set to the first determination voltage, and when the overvoltage determination means determines the overvoltage, the determination voltage is lower than the first determination voltage by a predetermined voltage until the overvoltage determination means stops determining the overvoltage. When the determination voltage setting means for setting the second determination voltage and the overvoltage determination means are determining the overvoltage, the power MOSFET is turned on. An overvoltage protection device, comprising: