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LR8 linear regulator running off 510V when 480V is abs max?

treez

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Hi,
We seem to have this LR8 regulator working well off input of 510VDC input.
The load is only one milliamp.

It worked for several hours the other day.
Do you think it will be OK going forward?
Its 30V above abs max, but it hasnt blown up, so maybe the "abs max" is a little understated in truth?

LR8
 

Easy peasy

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Often the higher the o/p volts the higher the input volts that can be tolerated because the dV across the part is less than 480V

if you go off data sheet though - and it fails later in the field - the buck stops with you, why not a 39V or 47V zener in series with the input ?
--- Updated ---

actially 450 is listed as the max differential in the data sheet - are you exceeding that?
 

treez

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actially 450 is listed as the max differential in the data sheet - are you exceeding that?
Yes we are exceeding that, but we had it running for several hours the other day, and it worked throughout.
 

KlausST

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Hi
Yes we are exceeding that, but we had it running for several hours the other day, and it worked throughout.
You stress the isolation barrier. It may work for years, it may fail within hours. Don´t complain if it fails.
Isolation barrier damage may be long time fail. First nothing happens, just a bit increase in input_current or supply_current. The more it gets damaged the higher the current. After a while maybe function fail .. but maybe just further increase in current --> causing heat.
Maybe you can feel it gets warmer after a month, maybe hot after two months, maybe too hot after three months --> fail. Who knows...

Klaus
 

Easy peasy

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I'm not sure about isolation - it has an internal fet or BJT that drops the voltage - likely 450 or 500V rated.

Again a 47C zener in series with the input may cure all - assuming the input stays above 80V or so
 

mtwieg

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Absolute maximum ratings are typically derived according to HALT testing on a large sample of units to cover process variations. A single sample operating at room temperature for a few hours is a joke compared to that.

Why would you be willing to exceed the ratings if the manufacturer isn't?

Dropping some input voltage with an appropriate zener like Easy Peasy mentioned is an option, though it will increase dropout voltage. If that's not acceptable, then you'll likely need to use an external pass transistor.
 

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