UCC28700-Q1: unstable switching

Hello Tetsuro,

I think the reason why RCS= 270mohm makes SW stable because looking at the datasheet the equation actually calculates Rcs= 281mOhm for your application. 

Can you please share the schematic.

Regards,

Sonal

Hello Tetsuro,

In additional to Sonal's comment, you may try to reduce your Na turn ration one turn at a time.
The reason your circuit unstable is because your current loop signal too small to trigger voltage loop feed back.
Lower voltage loop feedback voltage might mitigate the issue.

If the circuit still have issue please feel free to let me know.

Best Regards
Kevin

Sonal

Thanks for your reply.

According to a customer they cannot increase the Rsc value.

Best regards,

Tetsuro

Hello Kevin,

The customer replaced the IC from UCC28700  to UCC28730 and unstable switching was solved. The circuit differece is only HV connection and RSTR removal.  Then he changed Vblk voltage from min to max by manually and found sometime the device did not operate. Please see attached ppt.

His questions are;

1. Reason why UCC28730 solve the unstable switching?  (I think due to switching frequesncy difference)

2. Reason why the device gets operation fail? 

Could you help me?

If you need more information please let me know.

UCC28730Q1 start up issue_28Mar18.pptx

Hello Kevin,

Thanks for your reply.

Regarding the answer for second one, could you teach me about any documant which describes reason why RS requires or the direct connection may cause issue?  because the datasheet dosen't describe about RS requirment.

Best regards,

Tetsuro