Viper35L entering normal mode

[Bnx]

Hello, community,

I try to  develop power supply  based on VIPER35L, but  it  runs only in burst mode never reaching steady regulation. I have made almost a copy  of  STEVAL-ISA171V1, except of transformer, which was made by myself. According application note  AN4812.

  What might be the culprit?  Are  transformer data  correct in it ?  I mean ratio  8:1:1. Isn’t  it too  small to power  self?

 Several oscilloscope  snapshots are taken.  Any advice appreciated.

 

Bnx  

[Peter BENSCH]

Welcome ​@Bnx, to the community!

The most likely issue is not just the 8:1:1 ratio, but that the auxiliary winding is not providing enough stable VDD supply. The controller then hits UVLO, falls into burst mode, and never reaches steady regulation.

Please check first:

• does the VDD waveform clearly stay above the UVLO threshold (8.5V)?
• Aux winding polarity and rectifier orientation
• Transformer primary inductance and demagnetisation timing
• ZCD waveform on the drain

So, 8:1:1 is not automatically too small, but the transformer design and polarity are critical.

Hope that helps?

Regards
/Peter