450V rated Linear regulator suffers high voltage spike to its input

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:
https://www.microchip.com/wwwproducts/en/LR8

Do you think there could be long term damage due to these overvoltages?

Presumably the lowest voltage is the better.

When you switch on you get a resonant rise in volts on the input capacitance of your ckt due to inductance in the line, a lot of power supplies use an inrush resistor to limit the over-volts this produces - e.g. 10E ntc power inrush limiter

Any unloaded or lightly loaded LC circuit will do this for a step input voltage, max over volts = 2x input step.

- - - Updated - - -

450V worth of zener on the input is a good idea too, after a fuse. ( 6 x 68V zener + 39V, 1W each)

Hi,

Presumably the lowest voltage is the better.

Click to expand...

I don´t agree - but I can´t saay it is "wrong".

Maybe the lower voltage - but combined with low impedance and the stored energy in the capacitor is more harmful than a short pulse. I don´t know.

@OP:
Any overvoltage may cause damage. It may be a long term failure or an immediate failure. It may cause higher input currents, higher supply currents, increased power dissipation, overheat, fire.
Thus I´d absolutely avoid this situation.

Klaus

is more harmful than a short pulse. I don´t know

Click to expand...

the last bit above is correct, the higher volts will eventually kill the device - statistics has proven it for semis over many years...

Neither should be acceptable, both will kill the thing eventually. Add a TVS on the input (or perhaps connect it from the LDO's Vin to Vout).

How low a voltage do you need this to start up at? Add a zener or resistor in series to add another 50-100V. The resistor may better limit avalanche energy and because I'm sure your steady state current is low and voltage is high you could have quite a large resistor.

A quick search on digikey shows no regulators above 450V however for similar applications I've used a depletion mosfet with a zener or TL431 to easily form 500V+ voltage regulators.

Figure 5 at this link is the zener topology
https://www.electrical4u.com/applications-of-mosfet/

Which is less harmful to the LR8...

Click to expand...

It is the total energy (integrated power) that is damaging...

if the total energy is delivered in such a way that it can be safely dissipated (over a long time) then I will say it has no serious effect. But if the same energy is dumped over 10us or 100us, the effect (damage) will be practically the same. The capacitor is absorbing about 10mJ (at 500V) and if this is dumped over 1us, the power will be ...

If the regulator is rated at 450V, and you decide to be somewhat conservative, you should try to keep the voltage below 400V.

I am not wise enough, but much of these damages are cumulative.

you can see from the scope shots that the device is not absorbing or clamping the volt spike, thus there is a margin above 450V on the production, however at some point the production will change ... and then failures...

Thanks, We cannot add anything on the PCB itself , as there's no room.
We can only add stuff to the live and neutral wires that connect to the product. –Hence the X2 capacitor we added, as described in the top post.

The "500v peak" scope shot in the top post was with a 100nF X2 capacitor connected between live and neutral. There was also 25uH (10 metres) of coiled up mains cable ahead of that so that we could simulate line inductance.
When we changed the coiled up cable to 100metres of (coiled up) length, then the voltage peak goes up to 550V (as attached)…..youch! (100m of mains cable had an inductance of 73uH )
Obviously, the more “line inductance” we add, the higher the peak voltage to the LR8 is going to go….youch again!

So, the attached 550v peak scope shot is with 73uH of mains cable and then a 100nf x2 capacitor, then the product

We are wondering what is the typical maximum line inductance in the installation that our lamp will see?
It’s a lamp that gets put in pub gardens, outdoors.

the theoretical max peak is exactly 2x the peak input step, far better to use a "440" volt bi-directional TVS (littlefuse make them) and reduce the cap size... by having a larger cap you are lowering the Zo of the system - lowering the damping and allowing a peak closer to theoretical maximum. By raising the Lin you are increasing the energy stored in the Lin - not always a good thing ...