JPH09135156A - Electric load driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a driving device whereby a switching element is surely protected from short-circuited current by instantaneous interruption of the switching element at the time of load short-circuit. SOLUTION: General purpose FET 14 and FET 12 with protection having an over-heat protecting function are serially provided on a path for supplying power from an external power source to a load 6 and also a thyristor Thy 1 which becomes conductive and turns off general purpose FET 14 when the both end voltages ((a)-point voltage) of FET 12 with protection become more than a prescribed voltage is provided. Then, at the time of driving the load 6, respective FETs are turned on by a driving signal from a control circuit 8 in order FET 12 with protection and general purpose FET 14. As a result at the time of short-circuit of the load 6, the (a)-point voltage is raised, the thyristor Thy 1 is turned on and general purpose FET 14 is quickly turned off. When over current is permitted to flow, loaded current is interrupted by the protection function of FET 12 with protection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive device for an electric load that drives an electric load by turning on / off a switching element provided in the power supply route of the electric load, and more particularly to a drive device for the electric load. The present invention relates to a drive device for an electric load suitable for protecting a switching element and a load from an electric current.

[0002]

2. Description of the Related Art Conventionally, in a programmable controller (hereinafter, also simply referred to as a PC) or the like, in a drive device (so-called output circuit) for driving an electric load, it is possible to avoid continuation of an overcurrent state and to In order to protect the controlled device from overcurrent, a fuse is provided in the power supply path to the electrical load, or in order to protect the switching element that connects and disconnects the power supply path from overcurrent, load as a switching element. An insulated gate transistor that can divide the current into a main current and a branch current is used, the branch current is detected using a resistor, etc., and the switching element is forced when the detected current exceeds a predetermined level. It is configured to be turned off (for example, Japanese Patent Publication No. 6-14281),
Is being done.

[0003]

However, when a fusing fuse is provided in the power supply path, the fusing fuse itself blows due to overcurrent and shuts off the power supply path, so that the electric load is short-circuited. In this case, there is a problem in that the power supply path cannot be momentarily cut off and the switching element cannot be protected from a short-circuit current.

On the other hand, when an insulated gate transistor is used as the switching element and an overcurrent is detected from the branch current of the switching element to turn off the switching element, a short-circuit current can be detected / interrupted. Although the element can be protected from the short-circuit current, a special element is used as the switching element, which causes a problem that the driving device becomes expensive.

The present invention has been made in view of these problems, and when the load is short-circuited, the switching element is momentarily disconnected,
An object of the present invention is to provide a drive device for an electric load that can reliably protect a switching element from a short-circuit current and can be realized at low cost.

[0006]

In order to achieve the above object, in the drive device for an electric load according to claim 1, a first switching element and a second switching element are provided on a power supply path of the electric load. Are connected in series, and the second
When the voltage across the switching element becomes equal to or higher than the predetermined voltage, the protection means turns off the first switching. The first switching element is turned on / off by a drive signal input from the outside, but at least the ON state is turned on.
At the time, that is, when the electric load is energized, the second switching element is turned on before the first switching element.

Therefore, in the drive device of the present invention,
When the first switching element is turned on by the drive signal, the power supply path of the electric load becomes conductive and the electric load is energized. At that time, if the load current is normal, the second The voltage across the switching element becomes approximately 0 V due to the ON operation of the second switching element, and the protection means does not operate.

On the other hand, when the electric load is short-circuited during energization, the power supply voltage is directly applied to the series circuit of the first and second switching elements, so that the voltage across the second switching element is the first and second. ON of the switching element of
It becomes a divided voltage obtained by dividing the power supply voltage by the voltage division ratio determined by the resistance, which is much higher than usual. Then, when the voltage across the second switching element rises in this way,
The protection means detects a short circuit of the electric load based on the voltage across the protection means and turns off the first switching element.

That is, according to the present invention, a short circuit of an electric load is
Instead of detecting from the load current, the voltage across the second switching element is detected, and by turning off the first switching element at the time of detection, a short circuit occurs between the first and second switching elements. The power supply path is momentarily cut off when a current flows.

Therefore, according to the present invention, the first and second
It is possible to surely protect the switching element of from the short-circuit current. Further, in order to protect the switching element from the short-circuit current as described above, it is not necessary to detect the current flowing through the switching element, and therefore, it is not necessary to use a special element capable of detecting the current for the switching element, and thus it can be realized at low cost.

In the present invention, the second switching element is configured to be turned on before the first switching element. This is because the first switching element is the second switching element. This is because, if it is configured to be turned on earlier than, the voltage across the second switching element becomes the power supply voltage immediately after the first switching element is turned on, and the protection means operates.

Therefore, the second switching element is the first switching element.
It is sufficient that the switching element is already in the ON state when the switching element is turned ON. For example, the second switching element may be kept in the ON state at the time of operation of the drive device, or when the electric load is energized. The second switching element and the first switching element may be sequentially turned on by the operation of the control device that outputs the drive signal.

By the way, as described above, a short circuit of the electric load is detected from the voltage across the second switching element,
When the first switching element is turned off,
Before the switching element is damaged, the power supply path can be shut off to protect the first and second switching elements from short-circuit current. However, when the electric load is not short-circuited, the second switching element The voltage between both ends does not increase, and therefore, even if an overcurrent larger than usual flows in the power supply path, it cannot be detected.

That is, when the electric load is not short-circuited, the voltage across the second switching element is ON.
It is determined by the resistance and the current value (both-end voltage = ON resistance × current value), and since the ON resistance of the switching element is very small, even if an overcurrent flows through the power supply path, the both-end voltage of the second switching element Is very small, and it is difficult to detect an overcurrent from the voltage across it because it requires a great deal of precision.

For this reason, in the drive unit according to the above-mentioned claim 1, it is necessary to separately take measures for overcurrent protection. For this measure, the above-mentioned conventional overcurrent protection measures,
In other words, it is possible to use overcurrent protection measures such as providing a fuse that blows due to overcurrent in the power supply path, or providing an overcurrent protection circuit that detects the load current and shuts off the switching element when the detected current is large. it can.

However, in order to carry out such conventional overcurrent protection, a separate component is required, which leads to an increase in cost. Therefore, in order to perform overcurrent protection more easily, as in the driving device according to claim 2, FET is used for the first switching element and the second switching element is
It is preferable to use a protected FET having a protection function of detecting overcurrent or overheat and forcibly releasing the conductive state.

That is, a FET with protection is conventionally known as a switching element having a function of protecting itself from overcurrent and overheat. However, such protected FE
Since T has a low withstand voltage, it can be used as a so-called power element for controlling energization of an electric load,
It can only be used in low withstand voltage fields where the voltage to be handled is small.

Therefore, in the drive device according to the second aspect,
Use a general-purpose FET that can realize a high breakdown voltage for the main power element (first switching element) that connects and disconnects the power supply path, and use a protected FET for the second switching element. Thus, it is possible to protect the first and second switching elements from overcurrent without separately providing a dedicated circuit (or element) for overcurrent protection. Therefore, according to the invention described in claim 2, it is possible to protect the first and second switching elements from a short circuit current and an overcurrent with a simple circuit configuration.

The FETs with protection have an overcurrent protection function of detecting an overcurrent flowing in the element and forcibly turning off the element, and those having an overheat of the element and forcibly turning off the element. One having an overheat protection function and one having both functions are known, and any one of them can be used as the second switching element as long as it can protect the first switching element from overcurrent. FET with protection
However, it is more preferable to use a protected FET having an overheat protection function for the second switching element, and to ensure close thermal coupling between the first switching element and the second switching element. It is good to arrange so that.

That is, when a protection FET having an overcurrent protection function is used for the second switching element, the protection function can surely protect the second switching element from overcurrent. Depending on the characteristics of the switching element, the current value at which the first switching element can normally be energized may be smaller than the current value at which the protection function of the second switching element operates. In this case, The switching element may overheat and may deteriorate or be damaged.

Therefore, in order to more surely protect the first switching element from overcurrent, a protected FET having an overheat protection function is used for the second switching element, and the second switching element itself is overheated. Even if it is not, the thermal coupling between the first switching element and the second switching element becomes dense so that the overheating protection function of the second switching element works due to the overheating of the first switching element. It is preferable to arrange them.

On the other hand, the protection means only needs to be able to forcibly turn off the first switching element when the voltage across the second switching element becomes equal to or higher than the predetermined voltage. It is more preferable to use a switching circuit in which the first switching element is turned off when the above conditions are turned on so that the first switching element can be turned off without a response delay. In this case, for example, as the switching circuit, the second circuit
If a thyristor that switches the gate voltage of the first switching element (FET) to a predetermined voltage at which the switching element (FET) turns OFF when the voltage across the switching element (FET with protection) becomes equal to or higher than a predetermined voltage, is used as a protection means. It is easier to configure.

In the case where the protection means is constituted by the thyristor as described above, by turning off the first switching element, the ON state of the thyristor is maintained even if the voltage across the second switching element drops. Until the thyristor is reset, the first switching element O
Since the FF state continues, it is possible to prevent the first switching element from being repeatedly turned on and off when the electric load is short-circuited.

The electric load driving device of the present invention can be used as an output circuit of a programmable controller for controlling energization of an electric load, as described in claim 4, for example.

[0025]

Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 1 is an electric circuit diagram showing an output circuit 10 of a programmable controller (PC) 2 to which the present invention is applied and its peripheral devices.

As shown in FIG. 1, the output circuit 10 is connected to both ends of a signal line forming a power supply path between the external power source (DC) 4 and the load 6. In the output circuit 10, the drain of the FET 14 is connected to the positive electrode side signal line drawn from the positive side of the external power source 4 through the load 6, and the negative side signal line drawn from the negative side of the external power source 4 is connected. The source of the FET 12 is connected, and the source of the FET 14 and the drain of the FET 12 are connected to each other. In FIG. 1, the diodes D1 and D2 connected between the drain and the source of the FET 12 and the FET 14 are parasitic diodes that allow the flow of current from the source side to the drain side.

Next, the FET 12 corresponds to the second switching element of the present invention, and as shown in FIG.
A protection having an overheat protection function, which includes an overheat detection circuit 12b for detecting overheat of the element 12a and an internal logic 12c forcibly turning off the element 12a when the overheat detection circuit 12b detects overheat of the element 12a. It is composed of an attached FET. The gate of the FET 12 with protection is connected to the opening / closing switches SW1 and SW2 via the resistor R1.
It is connected to the connection point with and via the resistor R2,
It is grounded to the ground line GND connected to the negative signal line.

The open / close switches SW1 and SW2 are set to P
It is composed of a semiconductor switch that is turned on / off in response to a drive signal from a control circuit 8 including a microcomputer built in C2, and is built in PC2 at the end of the open / close switch SW1 opposite to the open / close switch SW2. A power supply voltage (constant voltage) Vcc generated by a power supply circuit (not shown) is applied. Therefore, when the open / close switch SW1 is turned on, the gate of the protection FET 12 has a power supply voltage V
A predetermined drive voltage obtained by dividing cc by resistors R1 and R2 is applied. That is, the protection FET 12 is turned on when the opening / closing switch SW1 is turned on.

On the other hand, the FET 14 corresponds to the first switching element of the present invention and is composed of a general-purpose FET. The general-purpose FET 14 and the protected FET 12 are in close contact with each other through a member having an insulating property and a high thermal conductivity such as silicon rubber so that the thermal coupling is tight.

The gate of the general-purpose FET 14 is connected to the end of the open / close switch SW2 opposite to the open / close switch SW1 via resistors R4 and R3, and to the ground line GND via the resistor R5. It is grounded. Therefore, a predetermined drive voltage obtained by dividing the power supply voltage Vcc by the resistors R3, R4 and the resistor R5 is applied to the gate of the general-purpose FET 14 when the open / close switches SW1 and SW2 are both in the ON state. That is, the general-purpose FET 14
Is in the ON state when both the open / close switches SW1 and SW2 are in the ON state.

Next, the anode of the thyristor Thy1 whose cathode is grounded to the ground line GND is connected to the connection point between the resistor R3 and the resistor R4 for applying the drive voltage to the gate of the general-purpose FET 14. The gate of this thyristor Thy1 is connected via the resistor R6,
It is connected to the connection point a between the general-purpose FET 14 and the protected FET 12, and also through the resistor R7 to the ground line G.
Grounded to ND.

Therefore, the gate of the thyristor Thy1 is
A voltage obtained by dividing the voltage across the protection FET 12 (specifically, the drain-source voltage) VDS1 by the resistors R6 and R7 is input, and this voltage (the voltage across the protection FET 12 is extended. ) Is above a certain voltage,
Thyristor Thy1 is turned on, and resistors R3 and R4
A current flows from the connection point to the ground line GND, the driving voltage (gate voltage VG2) of the general-purpose FET 14 decreases, and the general-purpose FET 14 is turned off.

Next, the operation of the output circuit 10 of the PC 2 of the present embodiment configured as described above will be described with reference to FIG. When driving the load 6, the control circuit 8 first outputs a drive signal to the open / close switch SW1 to turn on the open / close switch SW1, and thus the protected FET 12.
After that, a drive signal is output to the open / close switch SW2 to turn on the open / close switch SW2, and thus the general-purpose FET 14.
N. This is to prevent the general-purpose FET 14 from turning on first and the thyristor Thy1 from becoming conductive before the driving of the load 6 is started. However, in this embodiment, the general-purpose F
Since the open / close switch SW2 for turning on the ET14 is configured to receive the power supply voltage Vcc via the open / close switch SW1, even if the open / close switch SW2 is mistakenly turned on before the open / close switch SW1, FET14
Does not turn on before the FET 12 with protection, and this configuration makes it possible to prevent the malfunction of the thyristor Thy1 more reliably.

As described above, the FET 12 with protection and the general-purpose FET
When 14 is sequentially turned on, the power supply path of the load 6 is closed, the load current ID flows through these FETs 12 and 14, and the load 6 is activated. At this time, if the load current ID is normal, the FET with protection 1
2 drain-source voltage VDS1 and general-purpose FET 14
Since the drain-source voltage VDS2 of each is approximately 0 V, the thyristor Thy1 does not operate and the FET 12 with protection does not overheat, so that the load 6 can be continuously driven.

On the other hand, as shown in FIG.
And the general-purpose FET 14 are both turned on (time point t1), the load 6 is started to drive, and then the load 6 is short-circuited (time point t).
2), the power supply voltage VL from the external power supply 4 is the general-purpose FET1
4 is directly applied between the drain of the FET 4 and the source of the FET 12 with protection.
The voltage at the connection point a with 12 (the drain-source voltage VDS1 of the FET 12) is about 0V, and the power supply voltage VL is F
The voltage divided by the ON resistances of the ETs 14 and 12 rises rapidly as an upper limit. At this time, when the voltage VDS1 reaches a predetermined voltage (ON voltage) at which the thyristor Thy1 turns on (time point t3), the thyristor Thy1 changes Conduct. Then, through the thyristor Thy1, the resistor R3 and the resistor R3
A current flows from the connection point with 4 to the ground line GND side, the gate voltage VG2 of the general-purpose FET 14 decreases, and the general-purpose F
ET14 turns off.

As a result, when the load 6 is short-circuited, the general-purpose FE is
The load current ID flowing through T14 and the FET 12 with protection is
Although it rises sharply, the route will be cut off immediately. That is, according to the present embodiment, even if a short-circuit current flows through the general-purpose FET 14 or the protected FET 12 when the load 6 is short-circuited, the short-circuit current is interrupted before the general-purpose FET 14 or the protected FET 12 is damaged by the short-circuit current. FET
14 and the FET 12 with protection can be protected from a short circuit current.

Since the thyristor Thy1 maintains the conductive state until the current becomes 0 or substantially 0 (that is, until it is reset) once it is conductive, the general-purpose FET 14
Is turned off and the drain-source voltage of the protected FET 12 becomes 0 V, the general-purpose FET 14 does not turn on again, and then the open / close switch SW2 is turned off.
Until F, the thyristor Thy1 is reset, that state is held.

Next, as shown in FIG.
Both T12 and general-purpose FET 14 are turned on (at time t
4) After the driving of the load 6 is started, the load current ID rises above the steady current and an overcurrent begins to flow into the power supply circuit (time point t5).
4 and the amount of heat generated by the FET 12 with protection increases.

The general-purpose FET 14 and the FET with protection
When the overheat detection circuit 12b in the FET 12 with protection detects abnormal heat generation of the element 12a (time point t6) due to an increase in the amount of heat generated by the element 12, the element 12a (that is, FET 12 with protection).
Is forcibly turned off, the power supply path of the load is cut off.

Further, as described above, the FET 12 with protection is turned off.
Then, since the general-purpose FET 14 is in the ON state, the drain-source voltage VDS1 of the FET 12 with protection rises sharply from about 0 V with the power supply voltage VL as the upper limit, but as in the case of the short circuit, the voltage VDS1 is the same as the thyristor Thy1. O
When the N voltage is reached (time point t7), the thyristor Thy1 becomes conductive, so that the general-purpose FET 14 is also turned off, and then the open / close switch SW2 is turned off and the thyristor Thy is turned on.
This state is retained until you reset 1.

Therefore, according to this embodiment, even if an overcurrent flows through the load 6 due to a decrease in the electric resistance of the load 6 or the like, the overcurrent causes the general-purpose FET 14 and the protection FET 12 to overheat and deteriorate. Or, it can be prevented from being damaged. Further, in the present embodiment, since such overcurrent protection is realized by the overheat protection function of the FET 12 with protection, it is not necessary to separately provide a special protection circuit for overcurrent protection, and a relatively simple circuit is used. The output circuit 10 can be protected from a short circuit of the load 6 and an overcurrent.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned embodiment and can take various modes. For example, in the above embodiment, the present invention is applied to the output circuit 10 of the PC 2, but the present invention turns on / off the switching element provided in the power supply path of the electric load to energize the electric load. If it is a drive device that controls, it can be applied in the same manner as the above-mentioned embodiment and the same effect can be obtained.

In the above embodiment, the DC external power source 4 is used.
Although the case where the load 6 is driven by direct current from the power source described above has been described, the present invention is also applicable to an apparatus that drives the load by alternating current by supplying power from an alternating current power source. In this case, in order to configure the first and second switching elements with the general-purpose FET and the protected FET, it is necessary to configure each switching element with a pair of FETs in order to flow the load current in both directions. However, the circuit configuration is slightly complicated, but basically, similar to the above-described embodiment, the short circuit of the load is detected from the voltage across the second switching element configured by the protected FET, Since it is only necessary to turn off the general-purpose FET that constitutes the switching element of No. 1, it can be easily realized.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing a configuration of an output circuit of a programmable controller according to an embodiment.

FIG. 2 is an explanatory diagram illustrating a configuration of a protected FET.

FIG. 3 is an explanatory diagram illustrating a protection operation of the output circuit according to the embodiment.

[Explanation of symbols]

2 ... PC (programmable controller) 4 ... External power supply 6 ... Load 8 ... Control circuit 10 ... Output circuit 12 ... FE with protection
T 12a ... Element 12b ... Overheat detection circuit 12c ... Internal logic 14 ... General-purpose FET SW1, SW2 ... Open / close switch Thy1 ... Thyristor R1 to R7 ... Resistor

Claims (4)

[Claims] 1. A first switching element which is provided in a power supply path for supplying power from an external power supply to an electric load and which is turned on / off by a drive signal input from the outside to connect / disconnect the power supply path. In a drive device for an electric load provided, a second switching element is connected in series to the first switching element, and the first switching element is turned off when the voltage across the second switching element exceeds a predetermined voltage. And a protection means for cutting off the power supply path is provided, and the second switching element is turned on before the first switching element at least when the electric load is energized. Drive device for electric load. 2. The switching element is composed of a FET, and the second switching element is composed of a protected FET having a protection function of detecting an overcurrent or overheat and forcibly releasing the conductive state. The drive device for an electric load according to claim 1. 3. The protection means comprises a thyristor which conducts when the voltage across the FET with protection reaches a predetermined voltage or higher and switches the gate voltage of the FET to a predetermined voltage at which the FET is turned off. The drive device for the electric load according to claim 2. 4. The drive device for an electric load according to claim 1, wherein the drive device is an output circuit of a programmable controller that drives and controls the electric load according to a predetermined control program.