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Hi:

I know it's hardly a hardware guys' forum, forgive me for being desperate.

I designed a non-power factor corrected flyback converter type battery ch=
arger before, it works not bad, about 80% efficiency, 90-240VAC in, 24VDC=
, 2.5A out. 0.8W no-load loss.

Recently I added a PFC boost stage in the front of the flyback converter =
with the same transformer, the new charger also works, gives the same loa=
d, power factor as high as 0.99, total efficiency 78%.
Its no-load behavior is sort of weird, the no-load loss is about 0.8W at =
first.then after a few seconds, it's oscillating between 0.2W to a few wa=
tts, I cannot explain, but still not harmful since I don't need to meet t=
he energy star standard.

Then I designed a new transformer with higher primary turns, ( to cut dow=
n on core loss). So with the old transformer, the circuit works only at 1=
7% duty cycle, while with the new one, the duty cycle can be as high as 5=
0%. What strange is: both the transformer winding and flyback FET tempera=
ture are lowered by 20-30C, but the total charger efficiency is still sim=
ilar. That's unknown #1.

The only changes I made to the board other than the transformer are: chan=
ge current sense resistor values, add a low pass filter at the primary fe=
edback path (since with higher primary inductance, the feedback voltage s=
pike is higher, thus it will force the charger into overvoltage protectio=
n), also a higher voltage FET was used.

The no-load behavior is totally weird this time. The no-load loss is as h=
igh as 10W! If it sees load within this period then the charger will work=
 as normal. However, if after a few seconds without seeing the load, the =
input power can suddenly shoot to 40-50W, which you may assume the transf=
ormer soon gets saturated and adding load to it won't pull it back.
Then if you turn off the input and turn on again, the charger repeats thi=
s process, it's not dead at all, it always work if a load is applied to t=
he output in time.

I was confused a lot, how to explain that?

PS:

I'm using a UC2843B chip from Onsemi for the flyback stage, with only a p=
rimary feedback.


Regards

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