Switching power source apparatus and control method thereof

Abstract

An image forming apparatus includes a converting unit to convert a voltage level of power; a first switch to switch power supplied to the converting unit; a second switch connected in series with the first switch and turned on after the first switch has been turned on; and a voltage clamp unit to clamp a level of a voltage applied across the series connection of the first switch and the second switch to a predetermined voltage or less.

1 . An image forming apparatus, comprising:
 a converting unit to convert a voltage level of power;   a first switch to switch power supplied to the converting unit;   a second switch connected in series with the first switch and turned on after the first switch has been turned on; and   a voltage clamp unit to clamp a level of a voltage applied across the series connection of the first switch and the second switch to a predetermined voltage or less.   
     
     
         2 . The image forming apparatus of  claim 1 , wherein the second switch has a withstanding voltage equal to or greater than a predetermined value that is high enough to withstand a maximum voltage applied across the series connection of the first switch and the second switch during operation of the image forming apparatus. 
     
     
         3 . The image forming apparatus of  claim 2 , wherein the first switch comprises a metal-oxide-semiconductor field-effect transistor (MOSFET). 
     
     
         4 . The image forming apparatus of  claim 3 , wherein the second switch comprises a bipolar junction transistor (BJT). 
     
     
         5 . The image forming apparatus according to  claim 4 , wherein:
 the MOSFET comprises a drain terminal;   the BJT comprises an emitter terminal; and   the drain terminal of the MOSFET is connected to the emitter terminal of the BJT.   
     
     
         6 . The image forming apparatus of  claim 1 , wherein the voltage clamp unit comprises a diode connected to the series connection of the first switch and the second switch to conduct a current in a direction opposite to a direction of a current that flows from the series connection of the first switch and the second switch to the converting unit when the first switch and the second switch are turned on. 
     
     
         7 . The image forming apparatus of  claim 6 , wherein the diode is connected in parallel with the series connection of the first switch and the second switch. 
     
     
         8 . The image forming apparatus of  claim 1 , wherein turning off either the first switch or the second switch turns off the image forming apparatus. 
     
     
         9 . The image forming apparatus of  claim 1 , wherein:
 the first switch comprises a first capacitor;   the second switch comprises a second capacitor; and   the first capacitor and the second capacitor form a resonant circuit with the converting unit .   
     
     
         10 . The image forming apparatus of  claim 9 , wherein:
 the converting unit comprises a transformer having an inductance; and   the inductance of the transformer is the inductance of the converting unit that forms the resonant circuit with the first capacitor and the second capacitor.