MOV is rated at too low voltage?

[treez]

The MOV in DER-356 is rated to break down at 354V.....the DER-356 circuit is to 265VAC ..this is 375V peak..that means the MOV will be breaking over due to the mains......surely this is not right....I thought the MOV break over voltage had to be above the mains peak voltage?...(but below the DC bus capacitor max voltage)?

V250LA4P MOV
http://www.farnell.com/datasheets/482376.pdf

schematic page 7 shows mov in circuit:
http://www.powerint.com/sites/default/files/PDFFiles/der356.pdf

 

[betwixt]

I gave up trying to download the der356.pdf, at my internet speed I left it about 10 minutes and it was still downloading! MOVs are normally rated at their maximum operational voltage, not their clamping voltage though. So one rated at 250V like the V250LA4P would sit quite happily with 250V RMS across it without becoming conductive. In other words, it would not clamp until some voltage in excess of 354 appeared across it. Without seeing the schematic, I would agree that it looks to be a borderline rating.

Brian.

 

[treez]

To be honest, I think its absolutely bizarre. Betwixt you appear to agree with me that the MOV’s clamping voltage of 354V is well under the peak voltage of 256VAC mains, which is 375V.
This MOV is utterly & clearly not rated to do the job for which it is intended, do readers agree?

It isn't just powerint.com that do this, but many others too...

Take this 15W offline LED driver schematic by ti.com...
**broken link removed**

The MOV, "VDR1" is S07K300E2 by EPCOS
**broken link removed**

This MOV has a clamping voltage of 385V +/-10%. The tolerance means that the clamping voltage could be as low as 346V. ..346V is well below the peak of 265VAC mains which is 375V.
This is just absolutely bizarre….there are countless application note examples where the MOV clamping voltage is well below the peak of 265VAC mains.
Can any reader fathom this out?..are they just hoping that the mains will never go outside of 230VAC +/-2%?

 

[betwixt]

The MOV rating in the TI design looks OK to me, the design is supposed to run at 220V and the MOV is good to 300V.
Don't forget that MOVs of this type are not used as regulators but as surge protectors. One rated at say 250V should not conduct at (250 * sqr(2)) = 354V, it should be able to withstand that indefinitely and only become conductive at some voltage higher than that. The trade-off in design is the risk of false conduction versus the risk of failing to conduct when needed, it is quite ordinary to see spikes on the mains AC and you wouldn't want them to blow the fuses each time. Exactly how much overhead should be accomodated depends on the ability of the circuit to survive under slight overvoltage. It might be easier to think of the MOV rating as being the normal operating RMS value rather than the voltage it is to protect.

Brian.

 

[WimRFP]

An MOV has a relative smooth current versus voltage characteristic. I think you are mixing the clamping voltage with V(1mA), and the voltage rating is mostly given as RMS. So V(1mA) can be 360V+/-10% for a 230Vrms device (info from Epcos datasheet), but the actual high current clamping voltage can be in the 500..600V range.

When using this device at 230V+10%, it is subjected to 358Vp. The peak leakage may exceed 1mA, as V(1mA) for a 230Vrms device can be 324. Therefore you normally select a varistor with a higher RMS voltage rating.

If you expect an unstable and/or polluted mains supply, you select 1 or even two steps above the maximum long term mains voltage. When selecting a higher voltage MOV, the circuit/device to be protected "receives" larger transients, and this should be taken into account.

In your example (S07K300E2 in the TI circuit diagram) they use a 300Vrms device, according to the Epcos datasheet V(1mA) = 470V, so the leakage current due to the mains peak will be very low.

 

[treez]

I see what you are saying Betwixt , but that circuit runs on 220VAC mains, and as you know, these can vary up to 265VAC (i'm not talking about transients). The peak of 265VAC is 375V. The clamping voltage of the S07K300E2 MOV by EPCOS is 385V +/-10%. That is 346.5V to 423.5V. So in other words, low_tolerance_band MOVs will conduct at 346.5V. These MOVs will break over at the peak of 265VAC mains. That is not supposed to happen. Not by any means. This is a "ship-sinker", and I cannot understand why they've used that MOV?

wimRFP: I am considering the "385Vdc" value in the datasheet for the S07K300E2 MOV. This must be the breakover voltage , and not V(1mA). This must be the breakover voltage, because having a MOV datasheet and not stating that MOV's breakover voltage wouldn't appear to make sense?..therefore, the "385VDc" must be the breakover voltage? unless I have missed something big here?

EDIT:
ah, OK, I see what you both mean now....the data for that MOV is spread between page 5 and page 6, and I was just looking at page 5. Even so, neither pages gives the breakover voltage, so it seems a little unusual. It seems strange that the Vdc of "385V" is below the peak of 300VAC mains.

 

[dick_freebird]

There's conduction enough to fail some leakage, quality
type spec, and there's conduction enough to actually
matter to the product, or component reliability (back
in the day, MOVs weren't expected to tolerate repeated
hard breakover; I haven't kept track, however). Just
because something conducts, is not necessarily a killer -
zener diodes have substantial below-knee conduction
and nothing bad comes of it, unless you make your
circuit sensitive to leakage current. What the impact is
to whatever is the design in question, wants some thought.

 

[WimRFP]

@treez: when I look to my Epcos datasheet for the S07K300E2 device, it mentions 470V under the Vv 1mA column. The value under Vdc (in your case 385V) is the maximum operating voltage when using a DC supply, it is not a breakdown value. Please scroll through the datasheet to vind the Vv (1mA) value.

 

[FvM]

A V250 MOV is guaranteed to have below 1 mA leakage at 250 VAC. At 265 VAC, the worst case leakage may be around 10 mA, which is clearly beyond the continuous power dissipation rating of a 7 mm MOV disc. V275 would be the correct MOV voltage for 265 VAC.