JP2001218361A - Protective circuit against overheating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the heating state of a PTC thermistor from continuing, when the PTC thermistor is kept in a high-resistance condition due to over current, using a simple structure. SOLUTION: When an operating switch 7 is turned on, the input signal of an signal inputting part 52 is at a high level, and FET2 is turned on. When a motor 4 is short-circuited or a current supply line 63 is in a ground fault condition to flow over current and the temperature of the PTC thermistor 3 rises to becomes the high-resistance condition, a potential VA is lowered. As a result, a transistor Q1 is turned off and a transistor Q2 is turned on, so that the signal level of the signal inputting part 52 of drive circuit 5 drops to a low level. The application of drive voltage to a the gate of the FET2 from a voltage output part 53 of the drive circuit 5 is stopped, thereby turning off the FET2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current supply circuit in which a switch means and a positive temperature coefficient thermistor are connected in series between a power supply unit and a load. The present invention relates to an overheat protection circuit for preventing the overheat.

[0002]

2. Description of the Related Art Conventionally, fuses have been known to protect circuit components from overcurrent. However, once this fuse is blown off due to overcurrent, it must be re-energized unless it is replaced with a new fuse. There is a drawback that you can not. Therefore, a circuit has been proposed in which a positive temperature coefficient thermistor is connected in series between the power supply unit and the load instead of the fuse, and other circuit components are protected by the PTC thermistor (Japanese Patent Application Laid-Open No. 9-222448). reference).

[0003] Positive Temperature Coefficie
nt) Thermistor (hereinafter referred to as “PTC thermistor”)
Has a characteristic that the resistance value increases as the temperature rises. Therefore, if an overcurrent flows through the PTC thermistor due to a short circuit in the wiring or the like, the temperature of the PTC thermistor rises to a high resistance state, thereby reducing the level of the current flowing in the wiring and protecting other circuit components. Becomes

[0004]

However, in the circuit described in Japanese Patent Application Laid-Open No. Hei 9-222448, even when an overcurrent flows and the temperature of the PTC thermistor rises to a high resistance state, it flows through the PTC thermistor. Although the current level is greatly reduced, the current does not completely stop flowing. When a state where an overcurrent flows, for example, a short-circuit state continues, PT
When the temperature of the C thermistor decreases, the current level increases again due to the decrease in the resistance value. Therefore, an equilibrium state is established at a certain current level, and the heat generation state of the PTC thermistor continues. For example, when a PTC thermistor manufactured by kneading a polyolefin-based resin and conductive carbon to form a sheet is used, the temperature in a high resistance state is about 120 ° C. Therefore, when a PTC thermistor is used as an overcurrent protection component, there has been a problem that a circuit must be designed in consideration of heat resistance so as not to adversely affect peripheral electronic components.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problem, and it is possible to prevent the PTC thermistor from continuing to generate heat when the PTC thermistor enters a high resistance state due to an overcurrent with a simple configuration. An object is to provide an overheat protection circuit.

[0006]

The present invention comprises switch means and a positive temperature coefficient thermistor connected in series between a power supply unit and a load, and drive means for turning on and off the switch means. In a current supply circuit that supplies a current from the power supply unit to the load by turning on, when the temperature of the positive temperature coefficient thermistor rises to a predetermined level or more when the switch means is in an on state,
A switch control means for forcibly turning off the switch means is provided.

According to this configuration, when the temperature of the positive temperature coefficient thermistor rises to a predetermined level or more while the switch means is on, the switch means is turned off. The heat generation state does not continue, and the surrounding components are properly protected.

In general, a positive temperature coefficient thermistor enters a high resistance state when the temperature rises, thereby lowering the level of current to protect the circuit components. Therefore, the switch control means forcibly turns off the switch means when the load-side potential of the positive-characteristic thermistor falls below a predetermined level, whereby the positive-characteristic thermistor enters a high-resistance state. Therefore, it is detected with a simple configuration that the temperature of the positive temperature coefficient thermistor has risen above a predetermined level.

The switch control means is configured not to turn off the switch means until a predetermined time has elapsed from the time when the switch means is turned on, even if the detection means detects the decrease. In addition, it is possible to avoid an erroneous operation in which the switch is erroneously turned off immediately after the switch is turned on and before the potential on the load side of the positive temperature coefficient thermistor rises to a predetermined level or more.

When the positive temperature coefficient thermistor is electrically connected between the switch means and the load,
When a disconnection occurs between the switch means and the positive characteristic thermistor and the switch means side is short-circuited to the ground, an overcurrent flows through the switch means, but no overcurrent flows through the positive characteristic thermistor. However, by connecting the positive temperature coefficient thermistor to the switch means thermally, the switch means is turned off when the temperature of the positive temperature coefficient thermistor rises to a predetermined level or more due to a rise in the temperature of the switch means. Will be protected. That is, the switch control means not only prevents the positive characteristic thermistor from generating heat, but also
It can also function as means for protecting the switch means in an emergency when the positive temperature coefficient thermistor does not operate.

[0011] Further, an operating means which is turned on by an external operation may be provided, and the driving means may turn on the switch means when the operating means is turned on. According to this configuration, for example, when the operation unit is turned on by the operator, the switch unit is turned on, and the current is suitably supplied from the power supply unit to the load.

When the operating means is turned on, a high-level signal is inputted to the driving means, and the switch control means controls the temperature of the positive temperature coefficient thermistor to rise above a predetermined level. The high-level signal is changed to a low-level signal.
When the high level signal is input, a drive signal is output to the switch means to turn on the switch means, and when the high level signal is changed to a low level signal, the output of the drive signal is stopped. It can be.

In this case, when the operating means is turned on, a high-level signal is input to the driving means, whereby a driving signal is output to the switching means and the switching means is turned on. When the temperature of the PTC thermistor rises above a predetermined level, the high-level signal is changed to a low-level signal, and the output of the drive signal to the switch is stopped. It will be turned off.

[0014]

FIG. 1 is a circuit diagram of a current supply circuit provided with an embodiment of an overheat protection circuit according to the present invention. This current supply circuit K1 is connected between the power supply unit 1 and the ground by N
A channel MOS-FET (hereinafter simply referred to as “FET”) 2, a PTC thermistor 3, and a motor 4 are connected in series.
When T2 is turned on, a current supply line 61 connecting the power supply unit 1 and the drain of the FET2, a current supply line 62 connecting the source of the FET2 and the PTC thermistor 3,
In addition, a current is supplied from the power supply unit 1 to the motor 4 via a current supply line 6 including a current supply line 63 connecting the PTC thermistor 3 and the motor 4.

The power supply unit 1 comprises a vehicle-mounted battery and a power supply circuit. The vehicle-mounted battery outputs a battery voltage V _B (for example, V _B = 12 V in the present embodiment). The power supply voltage V _CC (in this embodiment, for example, V _CC = 5 V) is generated for the operation of each circuit by using the battery voltage V _B. Hereinafter, a terminal of the power supply unit 1 that outputs the power supply voltage V _CC is referred to as a power supply V _CC .

The PTC thermistor 3, for example, enters a high resistance state when an overcurrent flows and the temperature of the thermistor itself rises, and the high resistance state reduces the level of current flowing through the current supply line 6. Thus, circuit components such as the FET 2, the motor 4 and the current supply line 6 are protected from overcurrent.

The PTC thermistor 3 includes both an inorganic type mainly composed of a ceramic such as barium titanate and a resin type obtained by kneading, molding and cross-linking a mixture of an insulating polymer such as polyethylene with conductive carbon. Is applicable.

Here, as means for performing the above-mentioned crosslinking treatment, a method of irradiating a polymer material with ionizing radiation to form a bond between radicals generated, or a method of blending a peroxide with the polymer material and applying A method of generating a chemical bond by generating a radical in an atmosphere is also effective. By performing such a cross-linking process, when returning from the state of overcurrent abnormality or overheating abnormality to the normal state, the conductivity of the PTC thermistor can return to the same level as before the occurrence of the abnormal state.

When barium titanate is used, a part of barium atoms is replaced by strontium atoms or lead atoms, so that the temperature at which the resistance starts to increase is at least 50%.
It can be controlled in the range of ~ 200 ° C. On the other hand, in the resin type, the temperature at which the resistance starts to increase is governed by the melting point of the base resin. The melting point of the base resin can be adjusted slightly by controlling the crystallinity of the resin, but its range is considerably smaller than that of the inorganic type. In the case of polyethylene, the melting point range is 100-135 ° C depending on its crystallinity.
It changes to the extent. When manufacturing a PTC thermistor using this, the resistance increase start temperature is slightly lower than the above-mentioned melting point due to kneading of conductive carbon, but the current value can be controlled by the amount, area and thickness of kneaded carbon. The control of thickness, shape and shape is easier than that of the inorganic type. Other than polyethylene, use of polyvinylidene fluoride, polyolefin, or the like is also conceivable. In this case, the melting point changes to 140 to 150 ° C. due to the crystallinity of polyvinylidene fluoride.
Therefore, by kneading carbon with this, it is possible to manufacture a PTC thermistor having a resistance increase start temperature of 130 to 140 ° C.

The motor 4 is for driving, for example, an electric door mirror (not shown) of the vehicle. Drive circuit 5
Are a power input unit 51, a signal input unit 52, a voltage output unit 53
The provided, consisting of a charge pump circuit for generating a drive voltage by amplifying the battery voltage V _B to be taken from the power input unit 51, for example, a double voltage. Operation switch 7
Is for instructing driving of the electric door mirror, that is, the motor 4, and a series circuit including the operation switch 7 and the resistor R11 is connected between the power supply _VCC and the ground.

When the operation switch 7 is off,
The input signal of the signal input section 52 of the drive circuit 5 is a resistor R12,
When the operation switch 7 is turned on, the input signal of the signal input section 52 is connected to the power supply V _CC via the resistor R12 and is set to the high level.

When the operation switch 7 is turned on and a high-level signal is input to the signal input section 52, the drive circuit 5 applies a drive voltage from the voltage output section 53 to the gate of the FET 2 to turn on the FET 2. I do. Further, the driving circuit 5
As will be described later, when the transistor Q2 of the overheat protection circuit 8 is turned on and the signal input section 52 is set to low level, F
Stop application of the drive voltage to the gate of ET2
Turn off.

The overheat protection circuit 8 includes resistors R1 to R7 each having a predetermined resistance value, transistors Q1, Q2, and Q4 formed of NPN transistors, a transistor Q3 formed of a PNP transistor, and capacitors C1 and C2.

A series circuit comprising resistors R1 and R2 is connected between the current supply line 63 on the load side of the PTC thermistor 3 and the ground, and the base of the transistor Q1 is connected to the connection point between the resistors R1 and R2. . That is, the base of the transistor Q1 is connected to the current supply line 63 via the resistor R1.

The emitter of the transistor Q1 is grounded,
The collector is connected to the base of the transistor Q2, connected to the collector of the transistor Q3 via the resistor R3, and further grounded via the capacitor C1.

The emitter of the transistor Q2 is grounded,
The collector is connected to the signal input section 52 of the drive circuit 5. The signal input section 52 is grounded via the resistor R4.

The emitter of the transistor Q3 is connected to the current supply line 61, and the base is connected to the collector of the transistor Q4. A resistor R5 is connected between the base and the emitter of the transistor Q3.

The emitter of the transistor Q4 is grounded,
The base is grounded via a capacitor C2,
It is connected to a connection point between the operation switch 7 and the resistor R11 via the resistor R6. A resistor R7 is connected in parallel to the capacitor C2.

The ratio of the resistance values of the resistors R1 and R2 is
When the potential _VA on the load side (current supply line 63) of the PTC thermistor 3 becomes higher than a predetermined level, the voltage across the resistor R2 becomes higher than the threshold value of the transistor Q1 and the transistor Q1 is turned on. ing. Also,
The resistors R3 and R12 have a relatively small resistance value for current limiting, and the resistors R11 and R4 have a relatively large resistance value for securing a required voltage level. The resistor R6 and the capacitor C2 constitute a delay circuit having an RC time constant determined by the resistance value and the capacitance value.

Next, the operation of the current supply circuit K1 will be described. When the operation switch 7 is turned on, the drive circuit 5
When a high-level signal is input to the signal input unit 52, a drive voltage is applied to the gate of the FET 2 from the voltage output unit 53 to turn on the FET 2, and the power supply unit 1 supplies the motor 4 with the current via the current supply line 6. Current is supplied.

When the operation switch 7 is turned on,
When the base potential of the transistor Q4 rises at a rate of increase determined by the RC time constant of the resistor R6 and the capacitor C2, the transistor Q4 is turned on when the threshold voltage becomes equal to or higher than a predetermined time. When the transistor Q4 is turned on, the resistance R
5, a voltage across the resistor R5 generated by the current is applied between the base and the emitter of the transistor Q3 to turn on the transistor Q3.

Here, when the transistor Q4 is turned on a predetermined time after the operation switch 7 is turned on, the FE
As the on-resistance of T2 is sufficiently small, RC
A time constant has been set.

Therefore, when the current is normally supplied to the motor 4, the potential _VA is equal to or higher than a predetermined level due to the resistance of the motor 4, and as a result, the transistor Q
1 turns on. As a result, the resistor R3 becomes the transistor Q
1, and the transistor Q2 is turned off. Thereby, the signal input section 52 of the drive circuit 5
Is maintained at a high level, and the ON state of the FET 2 is continued.

On the other hand, when the motor 4 is short-circuited or the current supply line 63 is grounded, an overcurrent flows and the PTC
When the temperature of the thermistor 3 rises and the PTC thermistor 3 enters the high resistance state, the potential _VA decreases. As a result, the transistor Q1 is turned off, so that the capacitor C1 is charged, a voltage equal to or higher than the threshold is applied between the base and the emitter of the transistor Q2, and the transistor Q2 is turned on.

When the transistor Q2 is turned on, the signal input section 52 of the drive circuit 5 is grounded via the transistor Q2, whereby the signal level of the signal input section 52 drops to a low level. As a result, application of the drive voltage from the voltage output unit 53 of the drive circuit 5 to the gate of the FET 2 is stopped, and the FET 2 is turned off.

When the FET 2 is turned off, the potential _VA goes to a low level.
Continue as long as T2 is off. Therefore, even if the operation switch 7 is turned on, the FET 2 remains off.

As described above, according to the present embodiment, the PTC
The thermistor 3 enters a high resistance state, and the potential V on the load side
_{When A} becomes equal to or lower than a predetermined level, the transistor Q1 is turned off, thereby turning on the transistor Q2, setting the signal level of the signal input section 52 of the drive circuit 5 to low level and turning off the FET2. It is possible to prevent the current from continuously flowing through the thermistor 3, thereby stopping the overheating state of the PTC thermistor 3. Therefore, it is possible to prevent adverse effects of the high temperature on other circuit components of the current supply circuit K1.

Further, a delay circuit comprising a resistor R6 and a capacitor C2 is provided, and the transistor Q3 is turned on after a predetermined time elapses from the transition region immediately after the FET2 is turned on and the ON resistance of the FET2 is sufficiently reduced. Load side of PTC thermistor 3 (current supply line 6
The transistor Q2 does not turn on until the potential _{VA of} 3) sufficiently rises to turn on the transistor Q1. Therefore, it is possible to prevent a malfunction such as turning off the FET 2 when the PTC thermistor 3 is not in the high resistance state.

The circuit configuration of the overheat protection circuit 8 is shown in FIG.
However, the present invention is not limited to the above. If the load-side potential of the PTC thermistor 3 lowers to a predetermined level or less, the signal level of the signal input unit 52 of the drive circuit 5 may be reduced to a low level. Good. Further, the above operation is performed after the on-resistance of the FET 2 is sufficiently reduced from the time when the operation switch 7 is turned on, thereby preventing a malfunction such as turning off the FET 2 even though the current is normally supplied. can do.

The present invention is not limited to the above-described embodiment, but may employ the following modifications.

In the above embodiment, an example in which an N-channel MOS-FET is used as the switch means has been described. However, the present invention is not limited to this, and semiconductor switches such as P-channel MOS-FETs, bipolar transistors, IGBTs, and solid state relays are used. Alternatively, a relay such as an electromagnetic relay or a photo MOS relay may be used.

In the above embodiment, the PTC thermistor 3
Is disposed between the FET 2 and the motor 4, and the base of the transistor Q1 is connected to the load side of the PTC thermistor 3 (between the PTC thermistor 3 and the motor 4) via the resistor R1. .

For example, the PTC thermistor 3 is disposed between the power supply unit 1 and the FET 2, and the connection position of the base of the transistor Q1 is the same, that is, connected between the FET 2 and the motor 4 via the resistor R1. You may. Also in this case, when the PTC thermistor 3 enters the high resistance state, the potential of the current detection line 63 drops below a predetermined level and the transistor Q1 turns off from on, and operates in the same manner as in the above embodiment.

The PTC thermistor 3 is connected to the power supply 1 and the F
ET2, the base of transistor Q1 is connected to P
Load side of TC thermistor 3 (PTC thermistor 3 and FE
T2) may be connected via a resistor R1. In this case, the potential at the connection point of the resistor R1 when the current is normally supplied increases by the ON resistance of the FET2. However, when the PTC thermistor 3 enters the high resistance state, the potential at the connection point increases. , And operates in the same manner as in the above embodiment.

In FIG. 1, when the current supply line 62 is short-circuited to the ground between the FET 2 and the PTC thermistor 3, no overcurrent flows through the PTC thermistor 3, so that the PTC thermistor 3 is in a high resistance state. No. Therefore, it is preferable to connect the PTC thermistor 3 close to the FET 2 to reduce the size of the current supply line 62.

In FIG. 1, when the current supply line 62 is disconnected between the FET 2 and the PTC thermistor 3 and the power supply 1 is short-circuited to the ground, an overcurrent flows through the FET 2 while an overcurrent flows through the PTC thermistor 3. Does not flow. Therefore, the PTC thermistor 3
Thermally connected to T2, eg, PTC thermistor 3 and F
It is preferable to attach ET2 to the same heat sink.

In this case, for example, FE
When the temperature of T2 rises, the temperature of the PTC thermistor 3 also rises, so that the FET2 can be turned off in the same manner as in the above-described embodiment, and the overcurrent can be prevented from continuing to flow. The PTC thermistor 3 can be protected from overheating.

In the above embodiment, the PTC thermistor 3
It is detected that the temperature of the PTC thermistor 3 has risen by detecting that the potential on the load side of the PTC has dropped below a predetermined level. However, the present invention is not limited to this. For example, a temperature sensor is disposed near the PTC thermistor 3. Set up, PTC thermistor 3
May be directly detected.

In the above embodiment, the motor 4 is used as the load.
However, the present invention is not limited to this and can be applied to lamps and other general loads.

[0050]

As described above, according to the present invention,
When the temperature of the positive temperature coefficient thermistor rises to a predetermined level or more when the switch means is on, the switch means is forcibly turned off. Does not continue, and surrounding components can be properly protected.

Further, the switch control means forcibly turns off the switch means when the load-side potential of the PTC thermistor drops below a predetermined level. The fact that the resistance state has been reached can be detected, whereby the rise of the temperature of the positive temperature coefficient thermistor to a predetermined level or more can be detected with a simple configuration.

Also, the switch control means does not turn off the switch means until a predetermined time has elapsed from the time when the switch means is turned on, even if the detection means detects the decrease. In addition, it is possible to avoid an erroneous operation in which the switch is erroneously turned off immediately after the switch is turned on and before the potential on the load side of the positive temperature coefficient thermistor rises to a predetermined level or more.

By connecting the positive temperature coefficient thermistor to the switch means thermally, when the temperature of the positive temperature coefficient thermistor rises to a predetermined level or more due to the temperature rise of the switch means, the switch means is turned off. The means can be suitably protected.

Further, it is provided with operating means which is turned on by an external operation, and the driving means turns on the switch means when the operating means is turned on. When the operating means is turned on, the switch means is turned on, so that current can be suitably supplied from the power supply unit to the load.

When the operating means is turned on, a high-level signal is input to the driving means. The switch control means is adapted to switch the temperature of the positive temperature coefficient thermistor to a predetermined level or more. The high-level signal is changed to a low-level signal.
When the high level signal is input, a drive signal is output to the switch means to turn on the switch means, and when the high level signal is changed to a low level signal, the output of the drive signal is stopped. By doing
The switch means can be reliably turned off with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a current supply circuit including one embodiment of an overheat protection circuit according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Power supply part 2 FET (switch means) 3 Positive characteristic (PTC) thermistor 4 Motor (load) 5 Drive circuit (drive means) 6, 61, 62, 63 Current supply line 7 Operation switch (operation means) 8 Overheat protection circuit

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Fumiaki Mizuno 1-7-10 Kikuzumi, Minami-ku, Nagoya-shi, Aichi F-term in Harness Research Institute, Inc. (reference) 2F056 RG03 RG06 5E034 AA07 AB01 AC10 DA02 DA10 5G013 AA02 BA01 CA02 CA07

Claims (6)

[Claims] 1. A power supply unit comprising: switch means and a positive temperature coefficient thermistor connected in series between a power supply unit and a load; and drive means for turning on and off the switch means. A current supply circuit for supplying a current to the load from a switch control means for forcibly turning off the switch means when the temperature of the positive temperature coefficient thermistor rises to a predetermined level or more while the switch means is on. An overheat protection circuit comprising: 2. The overheat protection circuit according to claim 1,
The overheat protection circuit, wherein the switch control means forcibly turns off the switch means when a load-side potential of the positive temperature coefficient thermistor falls below a predetermined level. 3. The overheat protection circuit according to claim 2,
The switch control means is configured not to turn off the switch means until a predetermined time elapses from the time when the switch means is turned on, even if the decrease is detected by the detection means. And overheat protection circuit. 4. The overheat protection circuit according to claim 1,
An overheat protection circuit, wherein the positive temperature coefficient thermistor is electrically connected between the switch means and the load, and is thermally connected to the switch means. 5. The overheat protection circuit according to claim 1, further comprising an operation unit that is turned on by an external operation, wherein the driving unit is configured to turn on the operation unit when the operation unit is turned on. An overheat protection circuit characterized by turning on a switch means. 6. The overheat protection circuit according to claim 5,
When the operating means is turned on, a high-level signal is input to the driving means, and the switch control means lowers the high-level signal when the temperature of the positive temperature coefficient thermistor rises above a predetermined level. When the high level signal is input, the driving means outputs a driving signal to the switching means to turn on the switching means, and the high level signal is changed to a low level signal. Then, the output of the drive signal is stopped.