JP2722591B2 - Switching type power supply - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device used for various electronic devices.

2. Description of the Related Art In a switching type power supply device, there is a conventional example using a temperature fuse shown in FIG. 3 as overcurrent protection at the time of output overload and short circuit. In FIG. 3, reference numeral 21 denotes a switching transformer, 22 denotes a secondary side rectifying / smoothing circuit, 23 denotes a main switching element, 27 denotes a primary side control circuit, and 28 denotes a constant voltage control circuit. Resistance on anode side
24 and thermal fuse 25 are connected in series and contact each other,
The other end of the thermal fuse 25 is connected to the anode 26a of the output terminal, and the cathode 26b of the output terminal is connected to the cathode of the secondary-side rectifying / smoothing circuit 22. Detected, the output circuit is cut off when the thermal fuse 25 is blown, and abnormal heat generation and smoke generation of the component or the printed circuit board at the time of overload and short circuit are prevented. In this conventional example, the method of detecting the temperature of the resistor 24 by the temperature fuse 25 is used. However, when the other components generate the most heat when the output is overloaded and the temperature of the component is easily detected, the temperature of the component is reduced. Temperature may be detected.

Problems to be Solved by the Invention In the conventional circuit example as described above, in the output overload state, the temperature rise of the heat-generating component is detected by the temperature fuse 25 contacted with the component, and the contact state with the component is detected. In some cases, the change in the thermal resistance is large and the detection level varies greatly, and the detection time also changes greatly depending on the environmental temperature. In addition, there are problems in that selection of the thermal fuse 25 requires many experiments and time, and that once the thermal fuse 25 is blown, the thermal fuse 25 needs to be replaced and repaired.

The present invention has been made in view of the above circumstances, and automatically returns to a normal operation by improving detection accuracy of an overload state, reducing stress on other components during overload, and removing an overload state of output. The purpose is to do so.

Means for Solving the Problems In order to solve the above problems, the present invention relates to a switching type power supply device, wherein when the output current becomes a certain value or more, the switching type power supply device is connected to a secondary side rectification smoothing circuit and output from an output current detection circuit. And a function of outputting an ON / OFF signal at a predetermined interval after a predetermined time after the overload state based on a signal from the timer function and a determination function for determining an overload state of the output as an input. It is composed of an output on / off control circuit and an output switch element operated by the output signal.

Operation Since the output current is directly detected by adopting the above configuration, the accuracy of detecting the overload condition is improved, the stress on other parts during overload is reduced, and the overload condition of the output is removed to automatically return to normal operation. It is possible to

Embodiment FIG. 1 shows an embodiment of the switching type power supply of the present invention. 1 is a switching transformer, 1a is a primary winding,
1b is a secondary winding, 2 is a secondary side rectifying and smoothing circuit, 3 is a main switching element, 6 is an output switching element made of a field effect transistor, and 5 is connected to the cathode side of the secondary side rectifying and smoothing circuit 2. And an output current detection circuit connected to the source side of the output switch element 6.
Is a resistor, 13 is a transistor, 5a and 5b are current detection terminals, 5c
Is the output terminal of the current detection circuit, 7 is the current limiting resistor, 8a, 8b
Is an anode and a cathode of an output terminal; 14 is a primary side control circuit for controlling the main switching element 3; 15 is a constant voltage control circuit;
Has the function of receiving the output signals of the output current detection terminals 5a and 5b as inputs, and having a function of judging an overload state, a timer function, and a function of outputting on / off waveforms at a fixed interval after a fixed time. Output on / off for controlling switch element 6 /
An off control circuit 4 is a bias constant voltage circuit (6V) for supplying a stable bias from the output of the secondary rectifier circuit 2 to the output current detection circuit 5 and the output on / off control circuit 16.
It is. The primary control circuit 14 and the constant voltage control circuit 15 are connected by a photocoupler 17. The operation includes an output current detection circuit 5 in proportion to the overload state when a current higher than the rated output current flows on the output side or when a large current flows to the output circuit when the output side is short-circuited. The voltage drop between the current detection terminals 5a and 5b also increases. When the voltage exceeds a certain specified value, an overload signal is output from the output terminal 5c of the current detection circuit 5. The output on / off control circuit 16 generates a signal which repeats an on / off waveform arbitrarily set as shown in FIG. 2 (A) by a built-in timer function after a predetermined time using the previous signal as an input signal. The output switch element 6 is controlled via the current limiting resistor 7 in synchronism with the on / off signal to turn on / off the output current at regular intervals as shown in FIG. Keep the average value low. When the overload condition on the output side is removed, normal operation automatically returns to normal operation, causing abnormal heat generation of components and printed circuit boards, etc.
Smoke emission can be prevented.

In this embodiment, a microcomputer is used as the output on / off control circuit 16.

Advantageous Effects of the Invention As described above, according to the present invention, the output current is directly and electrically detected, so that the detection of an overload state is less affected by the environmental temperature, and the detection accuracy and reliability are improved. In addition, the normal operation can be automatically restored by removing the overload of the output, which facilitates the inspection and improves the user service because there is no replacement of parts. When a large peak current flows as a load of the power supply, for example, in the case of a motor load, the normal use current is within the rated current of the power supply. In this case, it is sufficient to perform overload protection by considering the thermal design of each part at or below the rated current, and it is possible to flow even a momentary peak current,
Unlike the conventional case, there is no need to perform a thermal design in consideration of the peak current, and the product is excellent in miniaturization and economy.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of the switching type power supply device of the present invention, FIG.
FIG. 1 is an electric circuit diagram of a conventional switching type power supply device. 1. Switching transformer, 1a ... Primary winding, 1b ...
Secondary winding, 2 ... Secondary side rectifying / smoothing circuit, 3 ... Main switching element, 4 ... Bias constant voltage circuit, 5 ...
Output current detection circuit, 5a, 5b ... current detection terminal, 5c ... output terminal of current detection circuit, 6 ... output switch element, 7 ...
... current limiting resistors, 8a, 8b ... output terminals, 9-12 ... resistors, 13 ... transistors, 14 ... primary control circuit, 15 ...
Constant voltage control circuit, 16 ... Output on / off control circuit, 17 ...
... Photo coupler.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-53-67849 (JP, A) JP-A-63-164477 (JP, U) JP-A-60-108195 (JP, U)

Claims (1)

(57) [Claims] 1. A primary circuit including at least a primary control circuit and a main switching element with a DC power supply as an input;
It comprises a switching transformer, a secondary rectifying / smoothing circuit, a constant voltage control circuit, an output current detecting circuit, and an output switch element. The anode of the secondary rectifying / smoothing circuit is connected to the output terminal anode, and the output terminal cathode is output. Connected to the drain side of the field effect transistor as a switching element, connected from the source side of the field effect transistor to the cathode of the secondary side rectifying / smoothing circuit via the output current detection circuit, and the signal output of the output current detection circuit The terminal is connected to one end of the output on / off control circuit, and the other end of the output on / off control circuit is connected to the gate side of the field-effect transistor via a resistor via a resistor, so that the output current is equal to or greater than a certain value. In the case of (1), a switching type power supply device having an intermittent function of turning on / off an output at a fixed interval after a fixed time.