I am designing a SEPIC converter, powered by 3.7 V Li-Ion cell. Frequency 500kHz, Transformer 10uH per side, 1:1 ratio. Ferrite material #87. Output to be variable by adjusting duty cycle.
It has good efficiency over 80% at 7W. But when I push it to the design goal of 10W, efficiency drops too much. Tested with a load of 10 ohms and 39 ohms. When duty cycle adjusted to increase output using either load, the the transformer core and MOSFET heat up quickly going over 7W. Looking at the MOSFET drain with a current transformer, I was expecting to see a saturating core with current spiking, or rising drain-source voltage. But no. Observing the drain current wave on the scope, the inductive charge ramp rises as expected with increased output current. As the voltage to the load is increased by increasing the duty cycle, the inductor current ramp flattens out, and even the slope reverses! What is the cause?