Feedback for high voltage

[krish2487]

Hello all

I need an idea of how to sense a high voltage on the output and close a feedback loop

Specs:
I am trying to build a 3525 based isolated boost converter which boosts 36v to 380v DC

but i am lost as to how to sense the output voltage to close the feedback loop
The feedback has to be isolated

am i right in assuming that a classic Tl431 feedback can be used??
just attenuate the output through a appropriate network and use it to drive a opto

 

[FvM]

am i right in assuming that a classic Tl431 feedback can be used?

Sounds promising. If the opto coupler's current consumption is unwanted at the high voltage node, you may want to add an
auxilary low voltage supply to power it. But 1 or 2 mA should be sufficient.

 

[krish2487]

thank you...

optos current consumption is not an issue.

just an isolated high voltage feedback is...

thank you very much

 

[Mr.Cool]

when you place your resistors as part of your Vsense, calculate how much voltage drop is across each resistor. you will find that you may need 6 resistors in series.

i usually don't use optocouplers. instead i just go to a couple of op-amps in differential measure mode and that's good enough. used succesfullly up to 600V on the bus in high noise environments.

keep in mind.. what happens if you lose a resistor. will the voltage still be attenuated? or will you suddenly have high voltage on the input pin of your optocoupler? you need a way of beign able to lose any ONE resistor in the attentuation path which will cause a FAULT trip OFF event.. without putting super high voltage on the pin of sensative component (op-amp, optocoupler etc). think about it..

product reliability is what separates a hobby project from a professional product.

Mr.Cool

 

[FvM]

what happens if you lose a resistor

Usually, resistors are assumed reliable in contrast to semiconductor devices.
A suitable assembly technology and sufficient power rating is required, however.

 

[hakeen]

Hi Krish

If you need insulation for safety (especially reinforced), I would still use an insulator. Phototransistor optocoupler with TL431 would be a conveninet solution. There are many solution notes about that.

Cheers

 

[Mr.Cool]

Usually, resistors are assumed reliable

famous last words my friend...

 

[FvM]

If you're referring to safety risks, the feedback loop surely can't achieve a safe limitation of converter output voltage, you need independent means anyway. In so far, possible damage of parts by a disconnected resistor isn't particularly a safety risk. The important point with resistors is, that you don't need to fear a short circuit fault, as long as you keep the rating. In case of a fusing resistor, you can even expect a safe behaviour with overload.

 

[krish2487]

@above
thank you all.
it has been quite a help.

thank you very very much.