Loop gain measurement set-up

I'm not familiar with how to perform direct control loop measurement in a no-opto flyback system, but you can get some useful information by measuring the output impedance.

Hi NDH,

Due to an issue with EZ I did not receive notifications for this post. My apologies. I understand this reply is likely too late to be useful for you and hopefully RDSmith's answer provided some assistance. In case anyone else has a similar request, I will answer your question.

As you mentioned, the feedback winding is approximately Vout *Nfb/Nsec while the primary gate is low, then negative while the primary gate is high. The IC is sampling the reflected voltage only after tFBDLY and only for tFB.

However, this waveform is not what the analyzer is looking for during loop measurements. So, for the Bode plot measurement we switch to a non-isolated feedback scheme. We achieve the same regulation point by connecting primary and secondary grounds then moving the feedback divider to connect to VOUT. VOUT may be a different voltage than the feedback winding so the top resistor may need tuning. The injection resistor is then added to this directly connected feedback divider and the loop response can be measured.

Best Regards,

Eric

Hi Eric,

Thanks for sharing more details on your measurement setup, interesting to know how you have solved this in this topology. As you mentioned I'm past the verification step at this point, so I will be missing out on the fun of trying this out for myself. In my case I have opted for verification of stability by means of load steps.

Kind Regards,

NDH