Ac/DC LinkSwitch Bias as Second Output
Posted by: ssleandro on 03/24/2023
I'd like to know if is possible and if is useful to use bias as second output?
My design requer 5V/200ma Isolated and a 15V/200mA Non-Isolated (To drive IGBT).
I try to simulate that in PiExpert but it was hard to converge to a solution.
I appreciate a help in that.
It is possible to use bias as a second non-isolated output although you must also consider that the cross-regulation between the 5V output and 15V output will be worse since there can be different loading conditions on those outputs. If you want to regulate using the primary 15V bias, the regulation for the 5V secondary output will suffer, and vice versa.
This cross-regulation will be affected by the coupling between your primary bias winding and secondary output winding and since you need isolation between those windings for safety reasons, it makes the regulation worse. Your application determines if this will still be acceptable.
Regarding the PIExpert design, using LinkSwitch-HP is not optimal since it is optimized for a higher power range than for this application. It has a higher primary current limit than what you would need so you will be forced to use a bigger core to not saturate while still having optimal operation.
We can use LinkSwitch-XT2 since it will be more appropriate for this power range. Since the PIExpert sheet is not configured with the given output configuration in mind, what we can do is start the design with the 15V output as the main output and set the current output based on the total effective output power. Since the 5V output contributes 1W output, this translates to 266mA effective output current for the 15V output and this will be the value that will be used for the inductance calculation.
You can then specify the individual outputs and their contribution in the Multiple Output Parameters section and take RMS current values from there to select the wires for your transformer.
Please see attached pixls file for more details.
The transformer is designed with a RM5/I core. The wire size is based on 8A/mm^2 current density for all the windings.
The stack-up of the windings is shown in [Transformer Stackup.png]. I've verified that this fits the bobbin with still some room to spare.
You can adjust the 5V output winding turns to see which will result in a better output.