Electronics > Projects, Designs, and Technical Stuff
High voltage feedback
timh2870:
I've been working on this push-pull converter to go from 24 volts to a fully isolated 370ish volts dc. The feedback circuit should have given me 373 volts with the actual 1meg/6800 ohm resistors chosen for the voltage divider, but I got 396 volts when I cranked up the input. I changed the 6.8k for a 10k, and tried again. I dialed back my bench power supply when the output went over 500v. Any suggestions?
Tim
Neilm:
As all I have to go by is your picture, I would guess your divider load is a bit too much for the transformer. Try increasing the values by 10 times or so.
floobydust:
Um, when the TL431 is off it experiences very high voltage. It's only rated 37V max. so likely it is damaged. Think what happens when you have say 200VDC at your voltage sense circuit.
T3sl4co1l:
Why is compensation on the receiver, not the error amp?
Where's the choke? There has to be a choke, either the transformer itself (flyback), on the secondary side (forward), or on the primary side (current-fed forward).
As mentioned, to use a TL431 with high voltage, needs something like a zener shunt regulator to limit its off-state voltage.
You may also find it's worthwhile to use a TLV431 or other improved version, that requires less minimum current flow.
Tim
timh2870:
A push pull converter "shouldn't" need a choke. I've had this thing up to 260 watts with no ringing on the fets, but I was probably going to put a common-mode choke on the output anyway.
It looks like the best solution is to add a zener (12v or higher) around the tl431 and opto... might need some additional components.
As for compensation around the tl431, I don't know how to calculate those and can't seem to find what would be reasonable values to start with.
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