gardenyu
Newbie level 4
no load loss
I designed a non-power factor corrected flyback type charger before, works fine, 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 works fine, gives the same load, power factor as high as 0.99, total efficiency 78%.
Its no-load behavior is sort of weird, about 0.8W at first.then after a few seconds, it's oscillating between 0.2W to a few watts, I cannot explain, but still not harmful.
Then I designed a new transformer with higher primary turns, ( to cut down on core loss). What strange is: both the transformer winding and flyback FET temperature are lower by 20-30C, but the total charger efficiency is still similar. That's unknown 1.
The only difference I made to the board other than the transformer are: change current sense resistor values, add a low pass filter at the primary feedback path (since with higher primary inductance, the feedback spike is higher, thus force the charger into overvoltage protection), also a higher voltage FET was used.
The no-load behavior is totally weird this time. The no-load loss is as high 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 spike to 40-50W, which you may assume the transformer 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 this process, it's not really dead, will often work if the load is added to the output in time.
I was confused a lot, how to explain that?
Added after 1 minutes:
I'm using a Uc2843B chip from Onsemi for the flyback stage, with only a primary feedback.
I designed a non-power factor corrected flyback type charger before, works fine, 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 works fine, gives the same load, power factor as high as 0.99, total efficiency 78%.
Its no-load behavior is sort of weird, about 0.8W at first.then after a few seconds, it's oscillating between 0.2W to a few watts, I cannot explain, but still not harmful.
Then I designed a new transformer with higher primary turns, ( to cut down on core loss). What strange is: both the transformer winding and flyback FET temperature are lower by 20-30C, but the total charger efficiency is still similar. That's unknown 1.
The only difference I made to the board other than the transformer are: change current sense resistor values, add a low pass filter at the primary feedback path (since with higher primary inductance, the feedback spike is higher, thus force the charger into overvoltage protection), also a higher voltage FET was used.
The no-load behavior is totally weird this time. The no-load loss is as high 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 spike to 40-50W, which you may assume the transformer 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 this process, it's not really dead, will often work if the load is added to the output in time.
I was confused a lot, how to explain that?
Added after 1 minutes:
I'm using a Uc2843B chip from Onsemi for the flyback stage, with only a primary feedback.
