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Mains Transient protection circuit looks very bad?

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treez

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Hello,
Our contractor has put forward a Mains Transient protection circuit (for our power factor corrected 40W offline LED driver) which looks so disastrous that please may I put it here for judgement? (it is as attached).

The idea is that any transient gets shunted by both the TVS and the controlled NFET , M3. The Current clamp based around M1 ends up taking some of the transient voltage, so that the LED driver and LEDs don’t get exposed to the transient overvoltage spike.
The problem I see is that the resistive divider which connects to the ZR431 has extremely high value resistors, and so noise tripping of the ZR431 is likely. Also, the NFET M3 has a 100k resistor connected Gate-Source. This is very high value, and so the NFET M3 may noise trip, and thus turn ON when it shouldn’t and disastrously shunt the high voltage DC Bus and get blown up.

What do you think? Should we kick this circuit out? In theory it seems fine, but in practice…ummmmm.

ZR431 datasheet:
https://www.diodes.com/assets/Datasheets/ZR431.pdf

(This PCB has very little room for components, and so we can’t fit more conventional Mains Transient protection circuitry on it. –This is why the attached circuit has been put forward.)
 

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  • Mains Transient protection.pdf
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What a crap ckt - for too many reasons to type here, ask if the std transient testing has been done on it...!!?

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The lower fet is always on for starters .... carrying 600mA max ( + or - ) and the idea is it turns off for overcurrents, and the 1A fet M3, turns on for overvoltage, if this results in >0.6A in the lower fet then the volts will suddenly go up again on M3, you have an oscillator ... also the energy rating of the dies in the fets is so low as to cast doubt ...

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for a worst case ZR431F the xtor Q2 will be always on at high ambient ...
 
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Please see end comment first
...........................
Thanks,
The idea was supposed to be that M3 clamps the voltage across itself to 413V, thus protecting the LED driver. As you say, the NFET M1 acts like a current clamp. Any voltage above 413V gets taken up across M1, which is 900V rated.
for a worst case ZR431F the xtor Q2 will be always on at high ambient ...
Thanks, though worst case cathode current for the ZR431 is about 40uA…..that would drop about 0.2V across the BE of the PNP….surely not enough to turn it ON?

One of the reasons that we put the M1/M3 based circuit in there, is because the blessed TVS voltage climbs above 450V as it trys to clamp a transient…so we put the M3 Based shunt in there in the hope that it would prevent the voltage across the TVS (and hence across the LED driver) from going above 450V.

If the shunt FET M3 needs to pass more than 0.6A in order to clamp its voltage to 413V, then we assumed that M3 would just simply short out the high voltage DC Bus and that voltage would then appear across the M1 NFET?

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There is, by the way, a littelfuse LSP05 MOV module at the mains input, but it "lets through" up to 900V.....therefore we needed the extra mains transient protection as shown here.

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1.5SMC400A datasheet
https://www.littelfuse.com/~/media/...littelfuse_tvs_diode_1_5smc_datasheet.pdf.pdf

..........................
Damned me, ive just realised i forgot to put the 3n3 capacitor across the 10V zener on the schem...now as attached...sorry.
 

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  • Mains Transient protection_1.pdf
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worst case is 50uA and o.4V Vbe = xtor on... at 50C or higher ... (look at worst case spec on xtor)

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... still an oscillator during transients ...
 
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Thanks, but 50uA * 5100R = 0.255V = Vbe
...means PNP not turned ON....surely?

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... still an oscillator during transients ...
We actually hit the circuit with the attached Mains Transient spike generator in order to test it, and it survived. (We put an isolated 10V supply across the 3n3 capacitor first, because that capacitor would be charged up to 10V in normal running).
The spike generator turns the back-to-back IGBTs on at mains peak, and thus a voltage spike goes into the circuit due to the LC circuit.

..By "survived", I mean the LED driver didn’t die and the M3 and M1 FETs did not die. The MOV module was not used for this spike generator test. Admittedly the spike only went up to some 600V. We found that the voltage across the LED driver stayed below 450V……….but when we removed the TVS, it does not always stay below 450V for some reason. In other words, the M3 obviously doesnt do a great job of clamping the rail to 413V.

However, I do share your concerns.
 

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  • Mains transient generator.pdf
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Please find attached the more complete version of the Mains Transient protection circuit, with the MOV and DC Rail capacitor included.

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************ Moisture on the 8MEG2 resistors *************

Thanks all, Do you think that the 8MEG2 resistors could have their resistance affected by atmospheric moisture settling on them? If so, then this could result in the circuit destroying itself….
The lamp enclosure is actually open to the elements, -it is not *totally* sealed. Well, there are gaskets around most of the mechanical interfaces…however, the wire entrance hole for live and neutral to get to the PCB, comes up through a hole in the heatsink on which the PCB rests……this hole is not sealed as it would be expensive to seal it. So moist external air can first come in through the gap between the lamp and the vertical mounting pole which is dug into the ground….this cannot be totally sealed since the pole belongs to the customer and is obvioulsy a separate part to the lamp itself.

So what I am saying is, moist air can make its insidious way in to and around the PCB, and could settle on the 8.2Meg resistors, and thereby reduce their resistance, and thereby make the ZR431 false trip…..which would mean M3 shunting the high voltage DC rail when it shouldn’t, and thereby blowing up M3 and indeed R4. The PCB hangs upside down, so water can't easily collect over the PCB, but a film of moisture from the atmosphere could still settle on the 8MEG2 resistors and reduce their resistance?
*****************************
 

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  • Mains Transient protection_2.pdf
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Hi Treez,

Happy New Year. Not sure if I get where the problem hole is, to be honest. How about a large bag of adequately-sized grommets to snugly fit the wire going into the box and hopefully making the interior less at risk of humidity entering? And/Or a can of the stuff council employees spray on PCBs periodically as part of the maintenance to keep PCBs dry and free from exposure to dust, moisture, etc.?
 
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.. still an oscillator during transients ...
Thanks, that wouldnt bother us, -all that we care about is that VDS of M3 stays below 450V.
Transients , we beleive, only last for typically 50us, so we wouldnt mind some wild behaviour in this time space.

By the way, do you think a power BJT with a zener fed base would make a better active-TVS than the M3 based circuit of the first post?..ie would it be more robust because of using a BJT?

Its really awkward trying to have an overvoltage protector which doesnt trip at high mains peak (404v in UK) and yet clamps the voltage below 450V.

A MOV salesman told us that once in every 20 years, the normal uk mains goes up to 404vpk (ie 285VAC) . He seemed to say he got this from EN50160
 

Incidentally, do you think the attached circuit would be worth doing for mains transient absorption?

We were thinking that if we use this, then we can avoid using the shunt fet in the top post, and just use the current clamp, but totally switch the clamp FET M1 off if the mains goes above 430V or so..................since the capacitor in the attached schem wouLD be there, this would provide the absorption of transient that we need?

The MOV lets thru 1100V.
The LED driver has a 450V Vdd pin.
 

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  • Capacitor transient absorber.pdf
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