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HIGH VOLTAGE isolated feedback.

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bowman1710

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Hi Guys,

I have an isolated power supply which is taking a lower voltage up to 350V DC @ 200W (push-pull topology). At the moment I don't want to rely on a aux winding to power the secondary side circuitry. There isnt a great deal out there with HV isolated feedback to go by, so I thought i'd ask and draw on other peoples experience with this. I would like a self powered configuration that can be run on the HV line (with low loss). Ive seen a few examples or suggestions online with the TL431, if anyone has any suggestion with TL431, that would be great.
,
Thanks in advance
 

BradtheRad

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The TL431 behaves very much like a zener diode, except that you can adjust its threshold voltage. It can only tolerate a certain amount of watt dissipation.

Is it possible for you to tap into your secondary at some spot on the windings? It will provide a lower voltage. However it's probably as much trouble to do that, as to add an auxiliary winding.
 

Warpspeed

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My first thoughts would be to simply use a commercial isolated power module, perhaps 12vdc to 5vdc or similar, driven from the regulated dc voltage available on the input side.

Getting enough current to reliably drive an opto direct from the 350v dc rail is going to be either lossy or complicated.

But how about a capacitive charge pump driven direct off the high voltage secondary ?

Lets see, 100pF charged to 350v contains 6.125 microjoules.

If you can discharge it every cycle down into a low voltage electrolytic, at say 20 Khz, that should be able to pump 125mW with excellent efficiency.
At 5V, that works out to 24.5mA.
That should easily be enough for any opto isolator.

It should work well over extremes of duty cycle.
I have never tried this myself, or seen it anywhere else, but I can see no reason why it should not work very well.
 

bowman1710

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The TL431 behaves very much like a zener diode, except that you can adjust its threshold voltage. It can only tolerate a certain amount of watt dissipation
I wasn't sure if there was some discrete way of getting around the wattage/voltage issue? That possible someone else had exprerince with.

Is it possible for you to tap into your secondary at some spot on the windings? It will provide a lower voltage. However it's probably as much trouble to do that, as to add an auxiliary winding.
Unfortunately no, the transformer in question is a planar transformer and being able to add/modify it is not an option for me

I have never tried this myself, or seen it anywhere else, but I can see no reason why it should not work very well.
I was hoping for something with a more "tried and tested" aspect to this as I wouldn't want the option of having to deal with unseen reliability issues in the long term.

Power integrations tiny switch will work fine off the 350VDC
Can they be used on the secondary at that sort of power? Is this something you have used for isolated feedback from a secondary yourself? The design needs to work to feed an opto, to then go to a controller as with the TL431 works with lower output voltages.
 

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I was hoping for something with a more "tried and tested" aspect to this as I wouldn't want the option of having to deal with unseen reliability issues in the long term.
.
Charge pumps and voltage doublers are by far the oldest and the simplest switched energy topology, well known and used constantly for over over a hundred years.
Its not some unproven tin foil hat theory.

Unseen long term reliability issues ?
If you spec the voltage ratings of both capacitors and suitably fast diodes, what other unseen issues could there possibly be.

It really does not get any simpler, or more flexible.
 

bowman1710

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Charge pumps and voltage doublers are by far the oldest and the simplest switched energy topology
Yes I agree I didn't mean that that type of design wasn't "tried and tested", i just meant that being used for isolated feedback purely. I just wonder if there is a reason behind why:

I have never tried this myself, or seen it anywhere else
maybe there is some reason why it isn't used or people haven't tried it, but it's worth looking into. I would have preferred if given the option to use something that other people have had experience with, that's all then there is something to go by or can help with troubleshooting.
 

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