[SOLVED] Problem with Forward Converter

[coates]

Hi guys,

I'm trying to improve my understanding of converters and HF transformers by prototyping a single transistor forward converter using a pulse transformer. However the waveforms are not as I expected.

The MOSFET is a logic type and driven through R1 by PWM. The pulse transformer is the Murata 77208C. According to the datasheet all windings are the same. The diode is UF4005.

Here are the waveforms I'm getting for MOSFET drain voltage (top) and gate voltage (bottom). Time base is 5us/div. Top trace is 10V/div, bottom trace is 5V/div.





The top photo is for low duty and the bottom is approaching 50% duty.

I expect the drain voltage to spike to some arbitrary voltage as the leakage inductance on the primary winding discharges, then settle to +24V (12V * 2) as the reset winding L1 discharges through D1 and back to +12V. Then when the core is fully discharged the leakage inductance will start ringing about +12V with the drain capacitance.

The top waveform confuses me, in particular. It leads me to believe that the reset winding isn't working at all because the drain voltage doesn't return to +12V. However, I can't see why. Please help!

Many thanks.

 

[FvM]

You should probably start with low duty cycle and lower pulse frequency. The transformers main ("magnetizing") inductance is quite high, respectively the freewheeling pulse has low current and results in an only slowly increasing drain voltage. Due to this delay, the transformer doesn't fully desaturate before the next pulse.

The same happens apparently with 50 % duty cycle, but at a considerably higher current level.

 

[coates]

You should probably start with low duty cycle and lower pulse frequency. The transformers main ("magnetizing") inductance is quite high, respectively the freewheeling pulse has low current and results in an only slowly increasing drain voltage. Due to this delay, the transformer doesn't fully desaturate before the next pulse.

The same happens apparently with 50 % duty cycle, but at a considerably higher current level.

Thanks. Lowered the frequency and the waveforms were as predicted.

So the slow edge is caused by the slow reversal of the core field due to low current? How does one know without experimentation whether the current will be large enough to reverse the field in time?

 

[FvM]

Waveforms at low magnetizing current are almost meaningless. The second waveform is more problematic because not-return to zero magnetization at a high current level can lead to transformer saturation. You possibly should stay away from 50 % duty cycle with 1:1 demagnetizing winding ratio.

I would primarly look for loaded waveforms and watch the absolute drain current.