I'm trying to produce a DC isolated output voltage of approximately 2.5V at 100mA from a ~55V DC supply. The purpose is to charge individual cells in a lead-acid battery from the power of the battery as a whole (it's a 48V system, 2V cells).
I've been playing with a PoE pulse transformer (Coilcraft POE13F-25LB).
The idea is to control pulses in the primary via PWM.
I'm switching one side of the primary to GND via an N-channel MOSFTET (Fairchild FQP45N15V2) which is driven by a MOSFET driver (Microchip MCP1415).
I'm switching at 250KHz to match the design frequency of the transformer.
I'm not sure this is the correct way to drive the pulse transformer. Here's a scope grab showing the pulse from the driver and the voltage on the MOSFET side of the transformer.
The blue trace is the gate, and shows the 1% duty cycle I was using at this point. The yellow trace shows the voltage drop when the MOSFET turns on, and then when it closes the voltage climbs again and oscillates.
This second grab shows the same situation at a 40% duty cycle: It's this oscillation when the MOSFET turns off that's causing problems. Do I need to do something else to drive the transformer?