Loss Calculation LLC Converter 3400W

Philip Adlhoch
Philip Adlhoch Altair Community Member
edited June 2023 in Community Q&A

Hello,

I have designed a LLC converter with PSIM. As optimal values I have chosen Q=0.6 and K=4. 
Boundary data:
Vin_rated=400V
Vin_min=300V
Vin_max=500V

Vout=50V
Po=3400W

f=200kHz

Now I would like to create a thermal model for this and determine the efficiency of the converter. For the mosfets I use the STW45NM50 and for the diodes the STPS200170TV1. 
Now I have simulated the model again, with the appropriate thermal resistors for the different loads (300V - 500V, and load of 0.2 - 1).

The losses in the mosfets or diodes show that they are lowest at 300V, and highest at 500V, although the overall efficiency is lowest at 300V and highest at 500V. These values cannot be plausible. So I wanted to ask if my model is correct at all, and if not, what I am doing wrong. 
Attached is the PSIM model and the Excel file with the determined values. 

 

 

I have already read through the note application of PSIM (POWER LOSS AND EFFICIENCY CALCULATION OF RESONANT LLC CONVERTER), however I cannot find the error.

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Answers

  • Rhonda_20369
    Rhonda_20369 New Altair Community Member
    edited June 2023

    Hi, Phillip,

    Sorry for the late response.

    Your question is posted in "Training and Learning" where only the training related questions are answered. 

    For technical questions, please post your question in the product category. In this case, the product is PSIM.

     

    How did you calculate the efficiency?

    The losses from the switching devices maybe lower at 300V and higher at 500V. However, you have other components in your system. Are the losses from other components lower at 300V and higher at 500V?

    In your system, the power consumed by the DC load can be calculated as "Pout = Vout * Iout" because the waveforms are pretty flat.

    However, the DC input power cannot be expected to be "Pin = Vin * Iin". Due to the high level of noise at the input, you need to determine whether to use DC (average) values or to use the RMS values of Vin and Iin. 

    In addition, in PSIM simulation with thermal model, the switching losses are not represented in the voltage and current waveforms. Therefore, Pin=Vin*Iin should not be used. The reason for not representing switching transient in waveforms is to save the simulation time. The nonlinearity of the switching devices during switching transient consumes large amount of computation. 

    PSIM uses thermal models to calculate the switching losses, and hence, avoid the long computation time.

    The correct way to calculate the efficiency for your circuit is: 

    Eff = Pout / (Pout + sum(Ptotal_loss of all devices) + sum(Ploss from other passive components in the circuit) )

     

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