SWITCHING POWER SUPPLY

PURPOSE:To facilitate size reduction and cost savings while magnetic saturation of a main transformer is avoided by a method wherein, from the time of start and while a switching transistor is in an off-state, the capacitor of a snubber circuits is charged in accordance with the instruction of a timing part. CONSTITUTION:While the capacitor 2 of a starting timing circuit 12 is charged after an input power supply 1 is closed, a transistor 4 is turned on, a relay contact 14 is switched to a make contact side and a snubber capacitor 15 is charged through the primary winding of a main transformer 19 and a snubber diode 17. After a predetermined time passes, the snubber capacitor 15 is connected in parallel with a snubber resistor 16. After that, a switching operation is started and, if a switching transistor 20 is turned off, an exciting energy stored in the main transformer 19 is released into the snubber capacitor 15. As the snubber capacitor 15 is charged at the time of the start of the switching power supply, the resetting operation wherein the exciting energy of the main transformer 19 is released is finished in a short time.