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LT3748 minimum load?

Question asked by consumererik on May 17, 2018
Latest reply on May 18, 2018 by Feiguo

I'm working on a 24V to 340V flyback. The application is to charge and keep charged some really big electrolytics, with power enough to charge the 14x1.5mF = 21mF of caps within, say, 3 minutes.

We used the LT8304 with reasonable success, but have moved to the LT3848, as it gives more flexibility with an external FET and current sense resistor. 

The problem I am experiencing is due to the minimum load requirement. To start, I'm using a resistive load. 150kOhm on the bench. In LTSpice and on the bench, I observe that the part doesn't regulate without a substantial load. In LTSpice, this is < 10kOhm. 

Another goal is to have the standby power to be quite small (< 5 W seems realistic), as this application is in a energy storage system. So, when the caps are charged, that's essentially no load. But since the caps are rated to 350V, the output can't go higher than that ever. 

I've attached my real life schematic, and I realize the compensations R and C are not optimal. Nothing is snubbed yet, except for a zener to protect the switch node. On the scope I have no observed this to be a problem. 

What confuses me is the Rfb pin isn't regulated to the 1.223 reference, and instead is like 1.35V. 

Is it possible to use this part as I intend? 

Also attached is a spice schematic and xcel design spreadsheet. 

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