I am currently designing a Flyback-based offline AC/DC converter similar to the following schematic (with a few changes e.g. sync rectifier at the output). Everything works nicely but the custom-made transformer is slightly "overheating" (~75°C) at full load (12V - 39W). Since this device will be used in a container where convective cooling is rather limited and the external ambient temperature can reach up to 60°C, the goal is to keep the temperature rise as low as possible.
After double checking, the temperature rise is indeed coming from the transformer itself rather than the peripheral components.
I am aware that a transformer can suffer from overheating e.g. if it is over-driven / saturated, however, it does not seem to be the case here.
Transformer main parameters:
- \$L_P=320\mu H\$
- \$L_{P(leak)}\approx 6\mu H\$
- \$N_{PS}=6.6\$
- \$R_{P}=414m\Omega\$
- \$R_{S}=13m\Omega\$
- \$I_{OUT}=4A\$
System parameters:
- Supply Voltage: \$90VAC-264VAC\$
The questions I have are the following:
- Can you say that the above temperature is really excessive for this operating condition (12V@39W)?
- Are there any other possible reasons why it would heat up considerably?
- Provided that the peripheral components are kept cool (e.g. RCD Snubber), can they affect at all the thermal aspect of the transformer?
- What mechanism could possibly cause the losses in a transformer to increase at a higher supply voltage? I was expecting it to be larger at a lower supply voltage, due to the higher primary's current.
Since the question revolves mostly around the reasons why a transformer in such a topology would heat up, I decided to keep the details out.
EDIT#1
Below are two measurements for \$90VAC\$ and \$264VAc\$ respectively, with both the \$V_{DS}\$(orange) of the primary's mosfet and its current sense voltage drop \$V_{CS}=I_{CS}\cdot 200m\Omega\$ (blue).
Due to technical limitations, the whole switching amplitude range could not be displayed in the osci for the higher supply voltage.
I appreciate any feedback.
Thanks.