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UCC28951: Clamping diode broken

Part Number: UCC28951

Tool/software:

Hi Mike,

Hope you are well. 

I have a question about the primary clamping diode of PSFB, namely Db and Dc in the slua560 design. 

As per your previous answer, the link is shown below. I am confused after the calculation.

UCC28951-Q1: significance of primary side diodes D4 and D5 - Power management forum - Power management - TI E2E support forums

For example, In your calculation,  Iclamp_diode = (ILout peak)*Ns/Np - ((1-D)*fs)*Vout/Lout, I think it should be Iclamp_diode = (ILout peak)*Ns/Np - ((1-D)*Ts)*Vout/Lout, which is V=L*di/dt right?  In the equation, I think (ILout peak)*Np/Ns is the current of Tx pri-peak value. ((1-D)*Ts)*Vout/Lout is the start transient current of the output side. Please correct me if I am wrong. Based on the EVM (600W 12V/50A output board), Vout is 12V, Lout is 2uH, Fs is 100KHz, Ns/Np is 1/21= 0.0476 (or use Np/Ns = 21). The result is about -17A! Therefore, I am confused how to select a good diode for it.

In my case, I use the 1200V 2A diode for 700Vin 48Vout/5A 250W board, which is getting very hot, and then blow-up. Can you please help, why it is blow-up? Besides, is that diodes can work in the hard switching mode or not? Because the lagging leg is not easy to achieve ZVS. 

My Lik is 23uH, Ls is 47uH (and tried 150uH, 330uH, and 470uH also). switching freq is 100KHz (and tried 50KHz also). Nothing improved. Can you please give guidance on it? I am very appreciative.  Many thanks.

Kind regards,

Zoe