Using the on-resistance of a power MOSFET to control a DC-DC converter

Most converter designs assume that a closed power switch has zero volts across it. In general this is a valid assumption that reduces the design complexity. However, the fact that a power switch does have a finite resistance means that there will be a non-zero voltage across it during its on time. This voltage can be taken advantage of. This paper proposes a simple control technique that utilizes the variable parasitic on-resistance of a power MOSFET. A 48-5 V 20 W forward converter is implemented to confirm the theory and demonstrate its practicality. The proposed technique provides a converter with self-oscillation, self-overload protection, wide load regulation, zero voltage switching throughout the load range and a reduced component count and cost