LM25184-Q1: Question on reducing ringing on RCD circuit

Hi Ethan,

Thank you for posting.  It appears that the snubber is not adequate. Do you know the leakage inductance of the transformer?  

Anyway,  I would try to increase C1740 to 47nF, and at the same time reduce the resistors from 3.9k down to 2k, and see if the ringing is more attenuated.

Another approach is to use a zener clamp instead of the RCD snubber like shown in our example circuit

Please let me know the results.

Thanks,

Youhao

Hi Youhao, 

They tried the suggestions you mentioned here. The results show no much improvement. 

Could you please help check other solutions here? If this cannot be solved, they will consider to replace it. 

1. R=3.3k, C=40nF, peak voltage of ringing is 45.2V

2. R=1.5k, C=20nF, peak voltage of ringing is 43V

3. Use zener clamp, SMAJ17A, 17V, took from our EVM. Peak voltage of ringing is 49.2V

Thanks!

Ethan Wen

Hi Youxi, 

One more update from customer. They tried with below method to reduce the ringing. 

Rclp=3.3k,Cclp=20nF, Rdamp=5.1,Cdamp=470pf

Below is the waveform for this method. The result shows better, but still need to further reduce. 

Besides, could you please help provide the parasitic capacitance of internal MOSET in LM25184-Q1? 

Thanks!

Ethan Wen

Hi Ethan,

Thank you for the info. Actually their additional RC snubber would be my next suggestion if the first two did not work well.

Their use of TVS is not correct.  It should be a Zener other than a TVS.  These are different product.  A 27V Zener would conduct at about 27V but a 27V TVS would at a much higher voltage, pushing the MOSFET peak voltage to a higher level.  

Anyway, use the additional RC is also a good choice. The waveform already looks very good.  Do you mean the customer wants to further suppress the ringing?  In that approach, could they try to increase the 5.1 Ohm to ~15 Ohm, and keep the 425pF in the additional RC snubber?  They may see a little higher in the first peak but better damped ringing afterwards.

Thanks,

Youhao

Hi Youhao, 

Customer feedback that overall they think 33V peak voltage should be reliable for LM25184-Q1. The ABS of LM25184-Q1 internal FET is 70V. Do you think 33V is sufficient in full temperature range? 

They are trying to use Zener to see if any improvement. 

The last thing is that please help provide the parasitic capacitance(Cds) of internal MOSET in LM25184-Q1. 

Thanks!

Ethan Wen

Hi Ethan,

Yes 33V is sufficient.  The MOSFET's 70V rating is for the entire operating temperature range, if you refer to the datasheet abs max rating table.

Regarding the internal FET's parasitic capacitances, they are not included in the datasheet for they are not separately characterized.  I can ask our design team to take a look and email to you.

Thank you for promoting the LM518x devices.

Best Regards,

Youhao