24V 0.350A Power supply using TNY275
We have designed 24V 0.350A power supply using TNY275G + 1 device using PIExpert. Supply cuts out about 15% low of desired output.
I can get about 260mA at 85Vac, 170mA at 120Vac and 300mA at 240Vac input before voltage shuts off.
Changed input bulk DC filter cap from 10uF to 22uF no help.
Tried changing TNY275 to TNY276 no help.
Transformer was wound by a long time supplier of ours.
Checked polarity of transformer to verify proper connection to board.
What is wrong with the design? Could the transformer be saturating?
Thanks,
MarkM
Comments
I was all set to check the suggestions posted by PI-Surak. As a final "what if", I changed the TNY275 device to a TNY276 and now the design works. I can get way beyond the rated output of the transformer core and the voltage is stable. It had a 10uF cap across BP/M pin to common to bump the performance level up one tick to the next device.
The board was populated with all of the suggested components that didn't work with the TNY275. What is the issue with the circuit. Why doesn't it work with the TNY275 but works with the TNY276?
Can somebody please explain what is going on?
Thanks, MarkM
Hi,
One of the important assumptions that our tools (PIExpert) makes is the efficiency of the power supply. If the efficiency of your design does not match the design calculations, then you will run into power delivery problems.
Assuming that the inductance of your transformer and its construction matches the results of PIExpert,I believe that this is what is happening in your design since using a part with a higher current limit seems to resolve the issue.
The efficiency of the design depends on transformer design/construction, the loss in the snubber circuit used, the losses in the output diode and any secondary side components.
We recommend that you verify the assumptions made while making the design and any design calculations with the actual performance on the bench.
This includes power supply efficiency, reflected output voltage (or VOR), the minimum DC-Bus voltage, peak inverse voltage of the output diode, shape of drain current and magnitude of peak current of the Tiny Switch, drain -source voltage of the Tiny-Switch, operating temperature of the Tiny-Switch and any other components used etc.
I hope this helps you to resolve the issue.
Regards
PI-Sarek
This design should work. Are you sure you don't have primary and secondary transformer windings reversed? Check drain current and voltage waveforms. Look for saturation and/or very short on pulse. also look for leading edge spike of drain current.This would be a clue that there was either an error in transformer construction or tranformer is installed in board incorrectly. Also check priamry winding inductance with meter to see if is correct.