Paralleling TFS764HG power trains

Posted by: ko3a4ok on 11/02/2011

I am using multiple TFS764 power trains in an active current sharing configuration, and I am having a transient response issue. At no load, one ("master") power train overdrives the others, and the "slave's" opto-feedback is at its max FB current. When the load is applied, the master hits its current limit, and the slave's feedbacks recover into zero optocoupler current within a few milliseconds. Nevertheless, the output produces a voltage sag ~70ms long (!!!). Can someone explain?

Each individual power train by itself is fine, stepping no load to full load.

I appreciate your comments.

Comments

Sounds like there is an issue with your sharing circuit --- if one supply is over-riding the other then the sharing circuit is failing to balance. You need to find out why you sharing circuit is not programming the right ajustment in error amp feedback references. Typically the sharing circuit should be low frequency in response but in this case it seems more like you have an issue with the basic DC function of the sharing circuit.

Thank you, PI-Surak! I am afraid this is not it. Indeed, the current share loop is slow. However, no current share circuit can balance out at no load. As soon as there is some 2% loading on the output, the problem goes away. No issue stepping from 2% to 100% load.

But this wasn't my question. Even at no load, I can bias my feedback such that it can recover from the lower rail fast, within a millisecond or two. The optocoupler shuts off then. But the power train still takes 70ms to recover. Is there soft start circuit that is activated? I fail to explain.

Thanks for your support. I have found a solution.