Multiple output Ac-Dc converter design problem

Posted by: pbdavis73 on 01/31/2013

I have used PI expert to design a multiple output AC-Dc converter based on TOP254. The image of the schematics is attached. Following are the details of the design

Input : universal AC input

Outputs

1. 5V @ 1.5A

2. 12V@1A

3. -12V@0.2A

My problem is that I am not able to achieve the -12V output accurately. I have tried weighted feedback in the assumption that cross regulation is the problem. I have loaded all three outputs fully the 5V and 12V outputs remain with limits, but the -12V remains out of range. I only get from -10.7 to -10.1V. Would like some suggestions as to what I should do. The local PI support tells me that I can only expect this much accuracy from a multiple output power supply. Is it really so.

Comments

Can you tell me your regulation requirements for these three outputs?

Thanks

It is impossible to meet 5% regulation for all these three outputs with single feedback. As you know, the referemce, the feedback resistors all have tolerance, and thus 2% regulation is reasonable for output with feedback. without feedback, it is really impossible to maintain the 5% regulation.

I don't mean regulation. I am not getting the exact output at -12V. By the way, I even tried weighted feedback from all three outputs, but it only gets worse. So if you are saying I cannot get 5% regulation, then what is the regulation I can expect. Are the outputs I have mentioned the maximum I can expect?

I am not sure what is the reasonable expectation for regulations of mutiple power supply, it is decided by the coupling and dummy load. But I am sure that the 5% regulation is impossible. The -12V setpoint problem can be improved with very good coupling between the -12V channel and the channel with feedback. But the other channel will suffer from the setpoint/regulation problem again.

I really suggest you use post stage linear regulator to solve the regulation of the -12V, since the load is only 0.2A, and you can make sure that the input voltage of -12V is very close to -12V at 0.2A full load.

regards