Power Supply With Controlled Demagnetization Time
Original Publication Date: 1985-Nov-01
Included in the Prior Art Database: 2005-Feb-19
In a forward converter switch mode power supply, the normal duty cycle is extended by switching on an additional demagnetization winding to alter the demagnetization time when a certain input mains voltage threshold is reached. This accommodates a wide range of input voltage in a universal input power supply without the need of selective voltage doubling. Fig. 1 shows the normal configuration for a forward converter topology switch mode power supply. It can be shown that Vo = Vbus X w where w is the switch transistor Q1 conduction period expressed as a fraction of the operating period T. Fig. 2 shows a typical transformer waveform, and in order for the core fully to demagnetize the theoretical maximum duty cycle, w is 0.5. If this is exceeded, the core will saturate.