UCC28951: Clamping diode broken

ZY W

Prodigy 50 points

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

7 months ago

0 Mike O' 7 months ago

TI__Guru* 86266 points

Hello,

Your inquiry will be reviewed in the order it was received.

Regards,

0 Mike O' 7 months ago in reply to Mike O'

TI__Guru* 86266 points

Hello,

I am doing well. Thanks for asking.

I am glad that posts from 4 years ago are still being read

You are correct the equation is in error. Instead of (*fs) it should have been (/fs) or replace by Ts.

I corrected that old post. Thanks for pointing out the error/typo.

In regards to sizing the diode one you have the peak clamp current (Iclamp_diode) these diodes need to dissipate the energy stored in the leakage inductance and shim inductance.

E = (1/2)*(Llk+Ls)*Iclamp_diode^2

You could estimate that the power dissipated by this diode is energy times switching frequency (fsw) of the primary

Pclamp_diode = E*fsw

So, based on your design what is your peak current and how much power does it dissipate based on Lllk and Ls? Is your diode rated for this peak current and power?

Regards,

0 ZY W 6 months ago in reply to Mike O'

Prodigy 50 points

Hi Mike,

Thanks for your reply. The diode issue has been addressed.

One more question, it is about RCD circuit. In the EVM design, D4 D6 R6 C1 are consisted a RCD circuit, right? I am wondering what is the purpose of R7 and R8?

I used to design the circuit by using webench. The RCD is shown below. The problem is, I got the ZVS on secondary MOSFET. But the power loss is still high. I think it is because of the ringing, which is really bad. Can you please suggest how to solve this issue. Many thanks.

0 Mike O' 6 months ago in reply to ZY W

TI__Guru* 86266 points

Hello,

Your inquiry will be looked into in the order it was received.

Regards,

0 Mike O' 6 months ago in reply to Mike O'

TI__Guru* 86266 points

Hello,

The following thread discusses the circuit in question and the information in this thread should be able to answer your questions regarding this clamp.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1132532/ucc28950-ucc28950-senkron-rectf-snubber-question/4202839?tisearch=e2e-sitesearch&keymatch=UCC28950%2520Clamp#4202839

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Regards,