| 摘要 |
PROBLEM TO BE SOLVED: To reduce the number of overvoltage protection circuits by preventing the occurrence of overvoltages. SOLUTION: A switching power unit is provided with an FET 40, which converts a DC voltage into an AC voltage supplied to the primary winding of a transformer 32 by switching the DC voltage. A diode 42 and a filter capacitor 44 convert the AC voltage induced in one secondary winding 36 of the transformer 32 into a DC voltage. Another diode 48 and a filter capacitor 50 convert the AC voltage induced in the other secondary winding 38 of the transformer 32 into a DC voltage. Both terminal voltage of a resistor 54, indicating the voltage across the filter capacitor 50, is supplied between the gate and anode of a shunt regulator 56. The cathode of the regulator 56 is connected to a photocoupler 58, and the output of the photocoupler 58 is supplied to a control circuit 60 which controls the FET 40. To the serial circuit of the regulator 56 and photocoupler 58, a DC voltage is supplied from the filter capacitor 33 via a resistor 62, and in addition, another voltage is supplied from the filter capacitor 50 via a resistor 64 also.
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