JPH0779545B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a switching power supply, and more particularly to a protection circuit against a DC output side abnormality.

B. Outline of the Invention The present invention is a protection type switching power supply that stops switching of a switching transformer by detecting an abnormality in a DC output, and detects a DC output abnormality and a state in which a switching element current exceeds a certain value for a certain time. By stopping the operation of the switching element when the above operation continues, it is possible to prevent an erroneous protection operation due to a short-time overload or the like.

C. Conventional technology Fig. 2 shows a circuit diagram of a conventional switching power supply. The DC power supplied from the DC power supply between the terminals PN is provided with the smoothing capacitor 1 and the primary winding of the switching transformer 2.
It is supplied to a series circuit of 2 ₁ and switching-element 3 and the current detecting resistor 4. The switching element 3 is driven under the ON / OFF ratio control by the switching control circuit 5. By this drive, AC power having a predetermined transformation ratio is taken out to the secondary windings 2 ₂ , 2 ₃ , 2 ₄ of the switching transformer 2, and the AC outputs are rectified and smoothed circuits 6 ₁ , 6 ₂ , respectively. 6 ₃ converts into DC power. Auxiliary DC power supply circuit 6 ₁ and 6 ₂
The output of is stabilized by the three-terminal regulators 7 ₁ and 7 ₂ , respectively, and taken out to terminals C and B. The output of the rectifying and smoothing circuit 6 ₃ comprising the main DC power source is directly taken to the terminal A,
The stabilization of the voltage is performed by controlling the on / off ratio of the switching element 3 by the switching control circuit 5. That is, the output voltage of the circuit 6 is taken into the constant voltage control circuit 8, and the circuit 8 takes out the pulse signal whose on / off ratio is controlled by the error amplification by comparison with the set voltage, and the pulse signal is isolated. Is supplied to the switching control circuit 5 via the photo coupler 9 for use, and the switching control circuit 5 for amplifying this pulse is stabilized by driving the switching element 3.

Next, the protection circuit will be described. An abnormal current of the switching element 3 due to a short circuit of a load on the DC output side is taken into the switching control circuit 5 as a detection signal of the resistor 4, and this control circuit 5 shortens the ON period of the switching element 3 and further reduces it to zero. An overcurrent protection circuit is provided to limit the output current. Further, in the case of voltage abnormality such as overvoltage or undervoltage of DC output, the abnormal voltage detection circuit 10 detects the abnormality, and a photocoupler using this detection signal as a photothyristor output element.
A voltage protection circuit is provided, which is insulated by 11 and applied to the switching control circuit 5, and the control circuit 5 stops the operation of the switching element 3.

D. Problems to be solved by the invention In the conventional circuit, the photo coupler is detected by the abnormal voltage detection.
When a signal is given to 11, the photothyristor which becomes an output element of the photocoupler 11 turns on, and the photothyristor maintains the turn-on state even when a detection input signal is input or disconnected. For this reason, when the abnormal voltage is once detected, recovery means is provided such that the control power supply of the switching control circuit 5 is once turned off or turned off, and the photothyristor is turned off to resume the operation thereafter.

In the conventional circuit having such a protection operation, the power supply function is lost even if the DC output voltage is instantaneously abnormal, and a power failure occurs from the viewpoint of the load. For example, in the case of an electric motor in which a large starting current flows through the load of the switching power supply, the DC output voltage may temporarily drop due to the starting current of the load. At this time, the operation of the switching power supply is stopped and the starting of the electric motor fails.

Therefore, the conventional circuit is designed to have a current capacity such that a voltage drop does not occur in the starting current for a load such as an electric motor in which the starting current flows. Therefore, there is a problem that the power source itself becomes expensive.

E. Means and Actions for Solving Problems The present invention has been made in view of the above problems, and a series circuit of a switching transformer and a switching element is provided in parallel with a DC power supply, and a switching transformer is provided. In the switching power supply provided with a DC conversion circuit for rectifying and smoothing the next output to obtain a DC output, and a voltage control circuit for driving the switching element at an ON / OFF ratio for making the DC output constant, a DC output abnormality is detected. An abnormal voltage detection circuit for detecting, and a protection circuit for stopping the switching operation of the switching element when the current of the switching element becomes a certain value or more and the state where the abnormal voltage detection circuit has a detection output continues for a certain time. The switching operation is stopped only when the DC voltage error and the state of the switching element current above a certain value continue for a certain time. , The temporary overload, etc. to continue the supply operation.

F. Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention. 2 is different from FIG. 2 in that a photocoupler 12 having a phototransistor as an output element is provided in place of the photocoupler 11 to measure the time when an abnormal current flows through the resistor 4 for a certain period of time and the photocoupler 12 has a detection output. The point is that a timer circuit 13 is provided to give a command for protection against a voltage abnormality to the switching control circuit 5 when both detection outputs are started and continue for a predetermined time or longer.

In such a configuration, when a load such as an electric motor which is temporarily in an overloaded state is connected, the detection output of the photocoupler 12 is obtained by the detection of the abnormal voltage detection circuit 10. At this time, the switching control circuit 5 starts the control of the switching element 3 in the maximum ON period or the OFF period, and continues the switching operation. With this control, the voltage of the resistor 4 is also in the abnormal detection state, and the timer circuit 13 starts the time measurement when both conditions of the detection signal from the photocoupler 12 are satisfied. Then, when both of the above conditions continue beyond the time limit of the timer circuit 13, the timer circuit 13 commands the switching control circuit 5 to stop the switching operation.

Therefore, even if there is a temporary excessive current such as a starting current in the load of the electric motor or the like, the operation of the switching element is continued when it is within the time limit of the timer circuit 13 for that time,
It is possible to return to normal operation when the overcurrent ends. At this time, the output capacity of the switching power supply may be one that can withstand overload with a short-time rating, and it is possible to start normal operation without failing to start or the like while designing a relatively small capacity.

G. Effect of the Invention As described above, according to the present invention, the switching operation is stopped when both conditions of the detection of the DC output abnormality and the abnormality of the switching element current are satisfied and it continues for a certain time or more. It is possible to obtain a reliable operation and protection operation against a load such as an electric motor, etc., while having a circuit configuration having a relatively low output capacity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a conventional circuit diagram. 2 ... Switching transformer, 3 ... Switching element, 4 ... Resistor for current detection, 5 ... Switching control circuit, 8 ... Voltage control circuit, 10 ... Abnormal voltage detection circuit, 12
...... Photo coupler, 13 ...... Timer circuit.

Claims (1)

[Claims] 1. A series circuit of a switching transformer and a switching element is provided in parallel with a DC power source, and a DC circuit is provided for rectifying and smoothing the secondary output of the switching transformer to obtain a DC output. In a switching power supply provided with a voltage control circuit that drives the switching element at an on / off ratio, an abnormal voltage detection circuit that detects an abnormality in the DC output, and the current of the switching element exceeds a certain value A switching power supply comprising a protection circuit for stopping the switching operation of the switching element when a state in which the voltage detection circuit has a detection output continues for a certain period of time.