Power Loss and Efficiency Calculation of Resonant LLC Converter

Philip Adlhoch

Hello,

I am currently trying to rebuild the LLC converter according to the application note "Power Loss and Efficiency Calculation of Resonant LLC Converter". Unfortunately, I do not understand how the values for R_case_ambient are calculated. Is it obtained by varying the value until the temperature at the mosfet or diode is 125°C?

Furthermore I don't understand how to get the values in figure 10. The efficiency is given by efficiency=(Uo*Io)/(Uin*Iin). If I simulate this, however, I come to other values. I have simulated the date of PSIM myself, but I also do not get the exact values from the application note. 

Albert_Dunford

Hi Philip,

The thermal devices in PSIM contain a thermal equivalent circuit that connects the junction to the case. This thermal equivalent is RC values that describe the traansient thermal network as you move throught the various layers of material. The Gan System datasheets have a nice description, goto, www.gansystems.com to have a look, I screenshot the relevant material.

 

 

So this information is used, in the thermal model here:

you can see how R1, R2, C1, etc match.

 

For R_case_ambient this is the thermal impedance of the path connecting the "case" to the ambient air. This will be the heatsink. You need to look this up from the heatsink datasheets. Below is a typical graph from a heatsink I found on digikey. You need the dC/W number for the airflow you are using.

 

 

For the values in the graph of figure 10. It could be that the thermal model has updated or there has been some other change. I will confirm the values.

 

Albert