Poor light load regulation on multi output supply
Hi, I’m having a poor light load regulation issue on the 15vdc line of our 4 output supply. The problem occurs when the supply is installed in our machine and when the machine is at idle. At idle, the 5vdc regulates well at 5vdc with .6A. However the 15vdc is about 18.9vdc with .17A. When the machine is running the 15vdc regulates well @15vdc with 2.3A. As a temporary bandaid, I connected a 50ohm power resistor on the output which brings the 18.9v down to about 16.6vdc.
Is it unreasonable to expect the supply to regulate the 15vdc @ only .17A? I was planning to modify the feedback loop and move from the 5 volt output to the 15 volt output, but I thought I would check with Power Integrations for any other suggestions.
Please see the attached schematics, bd layout and transformer drawing for reference.
Thank you for your time.
Comments
Thank you for taking a look at my P.S. issue.
I need to keep the 5V and 15V outputs isolated from each other. If I would use your first suggestion, I believe I would need to tie the grounds together. Is that correct? If so, I'll need to use suggestion #2 and use a regulator.
Thank you.
You are right. For your application, you really need post stage regulator to solve that problem.
regards
Thanks for your help.
Looks like your power supply suffers from cross regulation problem, which is normal or reasonable for multiple output design. The power supply is controlled and regulated by the 5V design, and thus the 15V is actually open loop. Cross regulaation is very hard to solve, transformer design is very important for the cross regulation performance. But I think your transformer looks good enough,you can not anticipate much with your transformer design. and definitely, 5V regulation will be worse if the power supply is regulated by 15V output. There are two possible solutions for this problem:
1. if 5V and 15V regulation not have to be very tight, you can try weight control. The primary side switch will be controlled by the 15V and 5V together. so the 5V regualtion can not be that good, but cross regulation can be a lot better.
2. Linear regulator for post stage if you really want tight regulation for both outputs. The output with high current output is controlled directly by feedback, and regulation of another output is maintained by linear regulator. The thermal is always a issue for the linear switch. Fortunately,voltage drop on the linear switch drps a lot at high load.
regards