Dual output supply: 15V 1A non isolated and 5V 1A isolated
Hi all,
I'm working on designing a compact ACDC power supply using InnoSwitch3 or similar. My input is universal mains and I need two outputs:
15V 1A non-isolated, referenced to primary
5V 1A isolated from mains and from the 15V rail
Can I generate the 15V non-isolated output from an auxiliary winding on the primary side of the transformer, and use that as my main 15V rail? Use same winding that is used for bias?
Since only the 5V output needs isolation, I plan to use it as the regulated output. Would this be the best approach?
What’s the best way to handle cross-regulation if the 5V output experiences large load changes? I’m concerned the 15V auxiliary winding may vary too much with load imbalance. Both rails will have varying loads.
Is there a better topology or approach for this setup? I'm considering building two separate PSU but I'm limited on space.
Hi,
Good day! Thank you for choosing Power Integrations and for your trust in our devices.
I've prepared a PI Xls design for your reference using InnoSwitch3-EP device. This is configured to use the bias winding to supply the non-isolated 15 V output, while the regulated main output is the isolated 5 V rail. This setup is feasible; however, as you have mentioned, the non-isolated output may vary significantly with load variations. Hence, to ensure output stability, a dedicated regulator is recommended to stabilize the 15 V non-isolated output.
For current-carrying capacity, I’ve verified that the conductor used in the bias winding/non-isolated output meets the requirement of >200 circular mils per ampere (CMA). I used the standard formula for the area of a circular conductor: A = pi (d/2)^2, then converted the result from mm² to circular mils using the following converter: Convert Square Millimeter to Circular Mil (unitconverters.net). Then, divided the converted area to the RMS current given by the PI Xls as shown in the attached screenshot. As a result, I've calculated a CMA equal to 282 Cmils/A.
Another thing, please design the bias resistor (RBP) using the formula provided in the application note (page 18). Since you're using the bias winding to power the 15 V output, the absence of a dedicated bias supply may result in increased no-load power consumption.
PI Xls: https://piexpertonline.power.com/fileManager/index?shared=18dab42903bbaee5df4d7e5baf646b873494c81deb9288f22b0aa1ca57b4898d
AN-72: AN-72 - InnoSwitch3 Family Design Guide | Power Integrations
Kindly let me know if you have any questions or clarifications.
Thank you.
Kind Regards,
Power Integrations, Inc.