Ref: my Dec 05 post, "transformers for cheap low current flyback supply,
+/- 45V ? "After holidays I was able to devote some time to this and I have a pair of independent supplies built. They basically work but there are two problems and they are likely related.
1) Low efficiency, GIANT current spike at turn on, immediately followed by a sag in the sense resistor current, then it rises back up at a more expected ramp.I have a theory this is due to the transformer not having good inter-winding coupling. It is a cheap, small, versatile transformer, the J.W. Miller PM620-07, 14 uH inductance per winding, with six windings. My hookup has 3 of the windings in series as primary, and 3 in series for the secondary.
The odd thing is that there is a lot of wasted power but nothing is getting hot.
2) the calculated feedback resistors do not give the expected output.I have used the PWM chip, UCC3801, and similar parts a few times before but have never seen this: the resistors I put into the feedback network do not cause the supply to run at the correct voltage. I had to resort to putting in a pot, adjusting it to cause the output voltage to decrease to the right voltage, then measuring the pot to find out the right value of resistance. Before doing this I was getting output voltages of 112V or so, not too safe when using 63V electrolytics.
It could be that the big current spike is somehow confusing the chip's cuurent sense. Maybe there is a minimum pulse width that always causes the chip to run with too high of a duty cycle.