nagle
Junior Member level 1
So the start of your waveform matches up fairly well, but at 10ms instead of plateauing around 0.6V (0.27A), the input current decreases and starts ringing until stopping completely at 22ms. Try probing other points in the circuit and see if you see any odd behaviors correlating with that.
L3 is probably saturating. Its saturation current is 270mA. LTSpice's default inductor model doesn't model saturation at all. When the current drops low enough that the inductor comes out of saturation, the ringing starts.
I tried shorting out R18 and L3. That's just noise filtering to prevent switch noise from getting back into the USB power source. That eliminates L3 saturation as a factor, and increases final output a bit, but it still takes about 45ms to charge up the 2uF caps. With R18 shorted out, there's no good place to measure a voltage drop and calculate current.
(This is all on a surface mount board, so it's hard to tap in for current. Voltage, no problem,)
Is that a possibility? Do I need a diode somewhere to drain out the transformer primary current on each cycle?Now, if the core is not fully resetting during the "off" time
(and I see nothing explicitly making that so) maybe there
is flux walk / staircasing that leads to eventual saturation.
But flyback converters are kinda outside my scope. Just
a thing to consider.
The default transformer model in LTSpice is oversimplified for this sort of thing. No saturation, and no hysteresis. If the model diverges badly from the real world, that's the most likely reason. Everything else has a reasonable model. Especially with L3 out of the picture, as noted above.