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450V linear regulator suffers overvoltage spike when switched on at mains peak

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Flyback

Well-Known Member
Hello,
We are using a 450V rated LR8 linear regulator in our offline product. If the product happens to get switched on at mains peak then the LR8 gets a voltage of 600V peak at its input (as in the scope shot attached).

However, if we put a 100nF X2 capacitor across live/neutral, then the peak voltage applied to the LR8 is 500V. …However, the peak duration is longer even though its less voltage (as shown in attached scope shot).

Which is less harmful to the LR8?

LR8 Linear regulator datasheet:
http://www.microchip.com/wwwproducts/en/LR8
 

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crutschow

Well-Known Member
Most Helpful Member
The higher voltage is likely more damaging, buth either could zap the device.

Why don't you add a transient suppression diode (TVS) to protect the device?

Or use a zero-crossing start-up circuit?
 

dknguyen

Well-Known Member
Most Helpful Member
Crazy. I only learned there were linear regulators that could work at 250V a few weeks ago, and now here is one that works at 450V.

Is this the kind of thing you can solve by putting an RC snubber at the input (well, I guess you putting a cap across it it very similar)? It looks like it, but I've never heard of an RC snubber being used for something like startup ringing. What would cause that behaviour?
 

ChrisP58

Well-Known Member
How much current are you drawing from the regulators output?

If it's small, you might consider adding a resistor in series with that 100nF X2 cap to slow it's rise time. And probably get rid of the spike entirely.
 
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