JPS6218921A - Over load protection circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overload protection circuit for a power supply device equipped with a switching element that switches on and off power output to a load.

[Prior art]

2. Description of the Related Art Conventionally, in a power supply device in which output is switched on and off using a switching element made of a semiconductor such as a transistor, measures have been taken to protect the switching element from thermal destruction in the event of an overload due to a load short circuit or the like. In order to suppress the heat generation of the switching element, there is a constant current circuit that limits the load current in advance, or a constant current circuit that detects the heat generation of the switching element that occurs when the load is overloaded due to a short circuit, etc., and reduces the power supply to the load. A thermal shutdown circuit or the like was used to stop the supply.

However, with the above-mentioned conventional configuration, it was not possible to avoid abnormal heat generation of the switching element during overload due to short-circuiting of the load or the like. For this reason, it is necessary to attach a heat sink to the switching element, and in order to incorporate the power supply device with the protection means described above into the box, the structure must take into account the temperature rise inside the box. However, it has had drawbacks such as posing difficulties in the design of the device.

[Purpose of the invention]

The present invention has been made in consideration of the above conventional problems, and includes a power supply device including a switching element.
The object of the present invention is to provide an overload protection circuit that can facilitate the heat dissipation design of the switching element and the heat dissipation design of a device incorporating the power supply device, etc. by accurately suppressing the heat generation of the switching element. be.

[Structure of the invention]

The overload protection circuit of the present invention includes voltage comparison means for comparing a voltage obtained by a load current flowing through the switching element with a reference voltage, in an overload protection circuit for a power supply device equipped with a switching element that performs intermittent power output. , means for receiving the output of the comparing means when the switching element is overloaded and setting the on/off time of the switching element so as to suppress the temperature rise of the switching element within an allowable temperature. , is characterized in that it is configured to be able to suppress a rise in temperature of the switching element.

[Example] An example of the present invention will be described below based on FIGS. 1 and 2.

A voltage comparator circuit 2 for detecting an overload due to a short circuit or the like in the load 1 is provided with a voltage comparator 3 to which a reference voltage is set. The input terminal of this voltage comparator 3 is
A resistor 5 and a resistor 6 are connected to a power supply line 4 that supplies DC power (Vcc). Above voltage comparator 3
The input terminal e is grounded via a resistor 7, and a resistor 8 is provided between the output terminal and the power supply path 4. The output terminal of the voltage comparator 3 is connected via a coupling capacitor 10 of the one-shot multi-circuit 9 to the e input terminal of a voltage comparator 11 to which a predetermined reference voltage is set. The one-shot multi-circuit 9 is used to set the time period for which the power supply to the load 1 is interrupted when the load 1 becomes overloaded. Resistors 12 and 13 and a diode 14 for preventing backflow are connected to the e input terminal of the voltage comparator 11, and a resistor 15 and a diode 16 for preventing backflow are connected to the input terminal (2). A capacitor 17 is interposed between the input terminal and the output terminal of the voltage comparator 11, and a resistor 18 is interposed between the output terminal and the power supply path 4.

The output terminal of the voltage comparator 11 is a switching circuit 19
It is connected to the base of an npn type transistor 21 via a resistor 20. The base and collector of the transistor 21 are grounded via a resistor 22 and a resistor 23, respectively, and the emitter is directly grounded. The collector of the transistor 21 is connected to the base of an npn type transistor 24, the emitter of this transistor 24 is grounded, and the collector is connected via a resistor 25 to the base of a pnp type transistor 26. The emitter of the transistor 26 is connected to the power supply path 4, and a resistor 27 is interposed between the power supply path 4 and the base. The collector of the transistor 26 is connected to the load 1, which is connected to the e input terminal of the voltage comparator 3 of the voltage comparison circuit 2.

Further, the voltage comparator 11 of the one-shot multi-circuit 9
An overload detection circuit 28 is connected to the output terminal of
This overload detection circuit 28 is connected to a power supply instruction circuit 29. The power supply instruction circuit 29 is connected to the base of the transistor 24 provided in the switching circuit 19 via a resistor 30.

In the above configuration, when starting up this circuit, a power supply instruction signal for supplying power to the load 1 is sent to the 0 point on the output side of the power supply instruction circuit 29, as shown in FIG. . This power supply instruction signal is input to the base of the transistor 24 via the resistor 30, and
is turned on. As a result, the transistor 26, which is a switching element that directly controls the supply of power to the load 1, is turned on, and power is supplied to the load 1. At this time,
Transistor 21 is turned off. Here, if the load 1 is not an overload for the transistor 26, the potential at the 0 point of the e input terminal of the voltage comparator 3 will be It becomes lower than the potential at the point. At the above 0 point, the load 1 is the transistor 26
A potential equal to the potential generated at the zero point is set by the resistors 5 and 6 at a critical current value that does not cause an overload. Therefore, when the load 1 is not overloaded, the potential at the 0 point of the output terminal of the voltage comparator 3 is H.
level (high level). As a result, the potential of the 0 point of the one-shot multi-circuit 9 is L level (low level)
Therefore, the transistor 21 is turned off, the transistor 24 is turned on, and the transistor 26 is turned on, and power is supplied to the load 1. However, when the load 1 becomes overloaded to the transistor 26 due to a short circuit or the like, the potential at the 0 point becomes higher than the potential at the 0 point, and the response operation at the 0 point becomes higher than the potential at the 0 point.
The potential at the point becomes L level. In this way, when the potential at the 0 point becomes L level, the next-stage one-shot multi-circuit 9 is activated, and the output terminal of the voltage comparator 11 has a pulse width t at the 0 point, as shown by the response operation ■. . An H level pulse that becomes an FF is output. The above pulse width t. FF is determined by resistors 15 and 18 and capacitor 17. The above pulse turns on the transistor 21 of the switching circuit 19, turns off the transistor 24, and turns off the transistor 26. As a result, the supply of power to the load 1 is cut off. Due to this operation, the potential at the 0 point becomes lower than the potential at the 0 point, as shown by the response action ■.
As shown by the response operation (■), the potential at the 0 point is returned to the H level again. The potential at the 0 point of the one-shot multi-circuit 9 is at H level during the pulse width tOFF, but becomes L level after this period has elapsed. As a result, the transistor 21 of the switching circuit 19 is turned off, and the transistor 21 of the switching circuit 19 is turned off.
4 is turned on, transistor 26 is turned on, and power is supplied to the load 1.

However, since the above load 1 is still an overload, 0
The potential at the point becomes higher than the potential at the 0 point as shown in response operation (2), and the potential at the 0 point becomes L level as in response operation (2). As a result, the potential at the 0 point of the one-shot multi-circuit 9 is instantaneously returned to the H level after the toN period. This causes the transistor 26 to operate instantaneously, i.e. t
. It is turned on during the H period, and power is supplied to the load 1 only during this period.

t of the transistor 26. FF period off, instantaneous tO
The operation in which the load 1 is turned on for an N period is that the load 1 is connected to the transistor 2.
6 continues for a period of overload, and the heat generated in the transistor 26 during the period tQN is t. By dissipating heat during the FF period, the transistor 26 is protected from thermal destruction due to excessive temperature rise.

Incidentally, the overload detection circuit 28 detects the 0 point signal of the one-shot multi-circuit 9 during overload.
It sends an overload signal to the power supply instruction circuit 29 and stops sending out the power supply signal from the power supply instruction circuit 29. This turns off the transistors 24 and 26, providing double protection means.

〔Effect of the invention〕

As described above, the overload protection circuit of the present invention allows the temperature rise of the switching element that performs intermittent output of the power supply when the load connected to the power supply becomes overloaded. The on/off time of the switching element is set so as to keep the temperature within a certain range. This suppresses the temperature rise of the switching element during overload, reliably protects the switching element from thermal destruction, and eliminates the need for attaching a heat sink. Furthermore, it is possible to easily design the heat dissipation of a device serving as a load in which the switching element itself and a power supply device using the switching element are incorporated, which may be negligible depending on the load conditions.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention, and FIG. 2 is a time chart of signals of each part shown in FIG. 1. 1 is a load, 2 is a voltage comparison circuit, 3 and 11 are voltage comparators,
9 is a one-shot multi-circuit, 19 is a switching circuit, 21, 24, and 26 are transistors, 28 is an overload detection circuit, and 29 is a power supply instruction circuit.

Claims (1)

[Claims] 1. In an overload protection circuit for a power supply device equipped with a switching element that performs intermittent power output, a voltage comparison means for comparing a voltage obtained by a load current flowing through the switching element with a reference voltage; and means for receiving the output of the comparison circuit when an overload occurs and setting the on/off time of the switching element so as to suppress the temperature rise of the switching element within an allowable temperature range. Load protection circuit.