Can't deliver full power and poor load regulation
Hi, This design is with the TOP267 with universal input and 12V 2.9A output. Attached is the schematic. The series resistor at the bias winding R6 is 100R. It was added because the bias voltage was so high due to the high voltage spikes at the bias winding. As part of the safety standards that this product needs to comply with, extra earth screens need to be added between the primary and secondary layers in the transformer which made the leakage inductance high. I believe that it is this high leakage inductance that has caused the high voltage spikes at the bias winding. The power supply performance is acceptable at low line. With 85Vac input, it gives 16V at no load and 11.95 at 3.3A just before going into auto-restart. However at high line, with 260Vac, it has poor load regulation and can't deliver full power. The maximum output current before going into auto-restart is 2A and the output voltage is 11.6V. How can I solve these problems? Thank you.
Comments
Hi Surak,
The drain current leading edge current spikes is way shorted than LEB time of 220ns.
I added a series resistor of 6.8R with the snubber diode D4 and that has increased the load delivery. However, I just noticed that the no load standby voltage is very high, about 14V. How can I reduce this?
I found that reducing the value of the optocoupler resistor will reduce the Vout at no load. Using 120R, the resistor heats up at no load and gives 12.3V. Why is that so? Do you have other suggestion to reduce Vout at no load?
Re load regulation, I found that reducing the resistor that is in parallel with the optocoupler LED from 1K to 100R improves the load regulation a lot. Now Vout is 12V at 2.5A. Do you see any problem having this resistor so low in value?
Thanks.
Please send voltage and current of drain at full load. I cannot say what is wrong from what you describe.
Hi Surak,
I have some waveforms which were captured at Vin = 265V and at 12V 2.3A output. The power supply can't go higher than that without going into auto-restart. We need a maximum of 2.9A.
The power supply went into auto-restart soon after the waveforms were captured. Usually it will resume if the load is disconnect and reconnect again. But this time it was in ocsillation even when I recycled the power. We need to go to production very soon. Please help.
Hi Surak,
I have some waveforms which were captured at Vin = 265V and at 12V 2.3A output. The power supply can't go higher than that without going into auto-restart. We need a maximum of 2.9A.
The power supply went into auto-restart soon after the waveforms were captured. Usually it will resume if the load is disconnect and reconnect again. But this time it was in ocsillation even when I recycled the power. We need to go to production very soon. Please help.
Hi Surak,
I have some waveforms which were captured at Vin = 265V and at 12V 2.3A output. The power supply can't go higher than that without going into auto-restart. We need a maximum of 2.9A.
The power supply went into auto-restart soon after the waveforms were captured. Usually it will resume if the load is disconnect and reconnect again. But this time it was in ocsillation even when I recycled the power. We need to go to production very soon. Please help.
Drain voltage spike is very high and seems to suggest transformer design with excessive leakage inductance. If you have primary OVP protection function in your design then this is probably what is causing auto-restart.
Hi Surak, indeed I have too come to that conclusion in my original post. I also mentioned that the high leakage inductance is perhaps unavoidable due to the double layer earth screen that are needed for the safety standards requirement.
Thanks for your response. But it has not been helpful so far.
Look at drain current and see if there are excessively high leading edge current spikeswhich may be causing early shutdown.