VIPER06 no-isolated can't start (over current protection ?)

[simon_12542]

Hello,
I have designed the same schematics of the website : 

The output should be 18V and no significative current (1-5mA output because MCU off). 
I have tested the input voltage at [80 ; 200] V and I have the same problem. The chip try to start during 400ns and stop during 1sec.
I can confirm there is no problem on the output because I have checked the output.
The transformer is a wurth : 7508111324
The transformer parameter is : 

The VCOM is always 3.3V during the start-up and it doesn't "move".
The yellow is ILIM pin and blue is DRAIN : 

do you have any idea ?

 

[Peter BENSCH]

Apart from possible layout errors, which cannot yet be recognised due to the lack of pictures of the layout, the transformer may be somewhat oversized for the output power of only 18...90mW and may have too high a primary inductance, for example. You can try increasing the output load from 90mW to e.g. 0.5-1W using a load resistor, which gives the transformer the opportunity to discharge the primary inductance.

However, it may also be due to the dimensioning of the compensation network, which leads to an unfavourable phase margin. For the low output power, Cp with 1nF appears too large (please test with 470-820pF), Rc with 18k also too large (please test with 11k).

Regards
/Peter

[simon_12542]

Dear Peter,

Thank you for your reply.

I have tried the following tests that you proposed:

• Added 330 Ohm 1W on the 18V output => no improvement
• Changed Cp to 680pF and Rc to 18k => no improvement

I will try to change the transformer with one that has a lower inductance. However, I don't see any recommendations on the datasheet.

For your information, my output is 5 mA at startup, but after all peripherals are activated, it can go up to 2 W.

[simon_12542]

For update, I have tested with the VIPER06HS without any improvement.