400V TVS should be fine for UK/European mains product?

[treez]

Hello,
Our contractor tells us that we cannot use a 400V TVS in our offline (216-265VAC) Power factor corrected LED driver.
He says that we will need to use the 440V TVS of the same family, since he says that the 400V TVS would be damaged by the leakage current that it would experience.
Here is the SMCJ family TVS datasheet:
https://m.littelfuse.com/~/media/el...s/littelfuse_tvs_diode_smcj_datasheet.pdf.pdf
Our contractor insists that we must use the SMCJ440A instead of the SMCJ400A. Surely the SMCJ400A conducts less than 1mA of leakage current all the way up to 447V, and so should be fine?
The schematic of where we use the TVS is as attached.

 

[Kajunbee]

Out of curiosity what is reason for choosing one over the other. Is it cost, physical size or unwanted effect on circuit.

 

[betwixt]

If it leaked 1mA there could be up to 250mW of dissipation but the data sheet says 1uA not 1mA so it shouldn't be a problem. The reference to 1mA is the test current not the leakage.

Brian.

 

[treez]

Thanks , our contractor argues that if the mains were to "swell" up to 300VAC , then the peak of that is 424V, which is above the Vr value for SMCJ400A of 400V, and so he claims that leakage current would be greater than 1uA , and he says that this is not good for TVS's.

 

[Kajunbee]

The 440a seems to be the more robust component so I figured it must be cost or physical size.

 

[treez]

The thing is we need protection for our Vdd pin of our LED driver which is 450V (abs max). The SMCJ440A doesnt offer as good protection for the 450V pin of the led driver.

 

[mtwieg]

Obviously using a higher voltage TVS will keep the TVS safer, but that misses the point which is that the TVS isn't meant to protect itself.

But even the 400V part won't offer much protection if your driver can only handle 450V. Have you checked reference designs for your driver?

 

[Kajunbee]

What effect does series connecting tvs diodes have on leakage current. Would leakage current of one 400 be the same as equivalent in series.

 

[treez]

Thanks, i beleieve seriesing TVS diodes is not advisable unless specifically the datasheet tells it is ok.
If you look at the littelfuse tvs range, it is only the expensive ones that say it is ok to series them.

 

[betwixt]

The problem with assessing leakage in series TVS is knowing if the voltage across them is proportional to their conduction threshold. They are not specified as a particular resistive (leakage) value at low voltage so the voltage distribution is unpredictable. The threshold should still remain the total of all TVS though.

For example:

400V ----- TVS1 (50V rating) ----- TVS2 (400V rating) ----- 0V

The total is 450V so it shouldn't conduct overall but can you be sure the 50V TVS isn't conducting (and therefore 100% leaky) and the 400V TVS isn't doing all the work?
In days of old, it was common to use several diodes in series to achieve the total PIV needed in rectifier stages and the trick was to add a high value resistor in parallel with each diode to equalize the voltage across them. I'm not sure if the same methodology applies in the case of TVS or whether the addition leakage current negates any benefit anyway.

Brian.

 

[Kajunbee]

I read a paper by Vishay titled " series stacking of tvs diodes" that says the Vr ratings should be equal to keep the Ipp rating. If not the Ipp would only be that of the lower rated tvs. The problem I see with this is if you use two 200 v and one fails the one remaining will be below your working voltage. I can think of several other reasons why it's not practical but no advantage
Also the vishay paper states that surface mount tvs are no longer available up to 440 v. Since there only available up to 170 volt series connecting may be the only option if higher rated tvs is needed.