quick question about TVS diode

[3roomlab]

i am looking to add a TVS diode post common mode filters and fuse, however after reading some other posts about "softening" the energy spike into a TBS, does it make sense to have a larger array of TVS (say 3pcs?) to absorb spikes (say a 342V BZW06-342B for 220Vac) together with an array of 10R (60J) NTC in series?

[Circlotron]

If you are going to use several TVSs, put several low voltage ones in series to guarantee load sharing. If identical full-voltage ones are paralleled the one with the lowest actual clamping voltage will hog any clamping current and the others will just go along for the ride.

[3roomlab]

thanks for the tip

[T3sl4co1l]

Paralleling TVS isn't so bad.  Up in the surge range, their ESR is a relatively large part of the voltage drop.

Small surges (low currents, modest voltages, long time periods) will experience imbalance, though given that the tempco is positive, it need not be destructive anyway.  Avalanche breakdown is different from forward bias.

NTCs wouldn't seem to be useful for a suppression system.  Nor PTCs; they take an extreme amount of energy to open.  Use a good old fashioned fuse.

Some details about what application you're considering would be helpful.  If this is for mains, TVS aren't used there; lightning strikes deliver too much energy for that to be safe or practical.  If you need strong clamping in that type of environment, a multistage suppressor would be desirable: perhaps using some very large MOVs to take the brunt of it, then resistors and TVSs to attenuate the remaining overshoot, and RLC filtering to dampen the transients due to stray inductance.

If this is for automotive, common mode isn't a big concern.  You can always just shunt stuff to ground, and I don't think you have to worry much about "-V" being other than GND, as long as it's bonded to the engine or chassis at some point -- though that can potentially open up as well, to be fair.  The nature of surges is different: lower voltage, modest current, very long duration (e.g., load dump).  Everything else is small electrical switching noise type pulses, easily filtered or clamped.

Tim

[3roomlab]

yep its AC mains. mainly to protect just a power bar, which will power test equipment
it is likely i will  just mod the protection thingys into the power bar

in another forum i read that they just size the bi-directional TVS with just 339vac (slightly above 311V peak), then they also have a 642V MOV and then GDT @ over 800V. the theory is to allow the TVS capture small spikes, MOV the medium, GDT the really huge ass ones.

it sounds logical, but i wonder what are EEV forumners opinion on those?

[T3sl4co1l]

It sounds like a fire hazard to me.

[SeanB]

yep its AC mains. mainly to protect just a power bar, which will power test equipment
it is likely i will  just mod the protection thingys into the power bar

in another forum i read that they just size the bi-directional TVS with just 339vac (slightly above 311V peak), then they also have a 642V MOV and then GDT @ over 800V. the theory is to allow the TVS capture small spikes, MOV the medium, GDT the really huge ass ones.

it sounds logical, but i wonder what are EEV forumners opinion on those?

You forgot the one important thing of a thermal fuse in close thermal contact with the VDR and the TVS, in series with the incoming mains. Place the GDT after the mains fuse direct across the mains, where it will blow the fuse when it operates, then a common mode choke before the thermal fuse and the MOV, then another common mode choke and the TVS. Each of the MOV and TVS has it's own thermal fuse in the supply line, so that if either fail as high current draw the thermal fuse will open and not allow a fire. GDT only needs a fused inlet side, it will never generate enough heat without a massive current flow to blow the fuse.

Note that a fuse is recommended as it fails safe, unlike a breaker which can arc over and weld itself shut and never trip, though you can put a larger value fuse before the breaker for final protection. This is done in the power system where you have breakers in the home, and a supply side fuse which is there for major fault clearing only like a shorted cable.