| Abstract |
Forward with active clamp is an excellent converter topology for a low voltage high current power
supply. Efficiency, size and EMI behavior are important design parameters in such a converter.
Efficiency and EMI behavior can be improved, for example, by applying soft switching, which
reduces switching losses and dv/dt-values in the circuit. Better efficiency allows the designer to design
a smaller overall size for the product. It is, therefore, beneficial to turn on switches of the converter so
that there is as low voltage over them as possible during the turn-on instant. This paper contains a
study of the requirements for low-voltage on switching for the primary switch of a Forward with
active clamp. Analytical equations to determine the voltage over the switch prior to turning on and to
determine crucial time intervals that are required to lower the voltage to a desired level prior to turning
on are provided. It is proposed in the paper that the voltage level prior to turning on can be lowered,
for example, by adding an inductor in parallel with the primary winding of the transformer. A number
of simulated and some measured results from a 3.4 V 30 A prototype confirm the analysis and verify
that the additional component can be used to alter the voltage level over the switch prior to turning on
the device. |
