| 摘要 |
PROBLEM TO BE SOLVED: To reduce loss in a snubber circuit of a switching power supply unit. SOLUTION: A series circuit of a switching device 3 and a primary coil 8 of a transformer 2 is connected to a DC power supply 1. An output rectifying and smoothing circuit 4 is connected to the transformer 2. A control circuit 5 is provided to turn on and off the switching device 3. The snubber circuit 6c is connected to the primary coil 8 in parallel. The snubber circuit 6c is constituted of a series circuit consisting of a diode 16 and a capacitor 17. An FET 20 is connected to the capacitor 17 in parallel as a variable impedance device. A constant voltage diode D1 is connected between one end of the capacitor 17 and a gate. A time constant resistor R1 is connected between the gate and the other end of the capacitor 17. A negative feedback resistor R2 is connected between a source and the other end of the capacitor 17. A smoothing capacitor Cf is connected to the resistor R1 in parallel.
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