Transformer design to achieve soft switching in low power Flyback converters

Soft switching in DC-DC converters is a well accepted technique and its advantages are widely discussed. Undoubtedly, the soft switching reduces the power loss in the Main switch while improving the overall EMI in the converters. This facilitates the increase of switching frequency of converters. This paper highlights a design methodology for the transformer in a fly back converter, which automatically accomplishes the complete soft switching of the main switch. No extra circuit or the control scheme is needed to attain Zero Current Turn On and total Zero Voltage Switching (both during turn ON and Turn OFF) of the Main MOSFET switch. Also, it eliminates the Dissipative snubber which is an essential component in fly back converter. However, this scheme is wholly applicable to fixed Input and constant load applications. Nevertheless, most of the Industry scenarios are similar at low power range of about 50 to 100 watts, because invariably the DC-DC converters are operated from a Power factor correction module (Whose output is fairly regulated to around 400V DC) and the output load is constant. The design concept is demonstrated on a 60 watt fly back converter operating from a 400V DC bus and operated at 100 KHz.