LM5185: The transformer whistling for LM5185

Hi Liang,

Thank  you for designing with the LM5185. I looked at your design calculator.  5:1 turns ratio is recommended but you used 1:1 turns ratio.  

This may leads to very high peak current on the primary side and hence the SW ringing.  And, your 22V Zener clamp on the primary side is too high with this 1:1 turns ratio.  I would suggest you to first adjust the transformer turns ratio to 5:1 and do necessary changes in the Zener and RFB. 

If you don't want to change the transformer and would like to operate the circuit below mostly 100kHz, you may consider to reduce the Zener voltage to ~8V, and see if it improves.

Regards,

Youhao 

Hi Youhao,

We have modify the LM5185 transformer to 5:1 TURNS RATIO and change the Zener clamp to 8V Zener voltage,

there are no improve for the transformer whistling issue;

At present, the jitter of the switch waveform should be caused by whistling, but adjusting the loop compensation parameters does not help much with this jitter.

I have tried reverse adjustment to make the system more unstable (phase margin close to 0), and the waveform has not changed significantly. 

I have also tried positive adjustment, and the phase margin reached 80. The problem is consistent. The crossing frequency has also been adjusted,

Under different load A numbers, the switching frequency and duty cycle vary, and the jitter remains the same;

Please also help to review our schematic and LM5185-DESIGN-CALC form and provide your suggesting how to solve the transformer whistling issue, thanks.

Hi Liang,

After you change the transformer turns ratio to 5:1, what is your primary inductance? Did you check your new transformer design with our design calculator?  May I bother you to share you updated schematic, with component values accurately labeled?  

Could you also capture the waveforms under light load, and heavy load:

• Vin, Vds, COMP, CS
• Vin, Vds, COMP, Vout

When sharing the waveforms, please tell me the load conditions, and mark the traces.  You may zoom in and zoom out of the mode transitions (where you see switching frequency changes radically).

Thanks,

Youhao