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Registered Member #1571
Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44
Hi There,
This is a very simple question but I need clarification.
I have two types - 1.5KE440A and 1.5KE440CA.
I need to add these to a full bridge I am constructing. One is uni directional and the other series is bidirectional. How do I use these?
I understand I need to use them between the top of the bridge and either side of the primary connection. Question is do they work like zeners - cathode pointing to (+) top of the bridge... and how would I use the bidirectional TVS's?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, what Sulaiman said is only true if the stray inductance of the bridge layout is negligible, so that 300V at the DC bus capacitor is still 300V across the devices, even under high di/dt. Most people don't seem to understand the concept of stray inductance, so TVS are a good Band-aid. (If your stray inductance is too high they will let you know by getting stinking hot.)
However, most modern MOSFETs and IGBTs are avalanche rated anyway, which essentially means that they have a TVS built in. Overall, I prefer to not use any TVS, and just try to keep the bridge layout as compact as possible, with minimal loop area on the DC bus, and use the slowest gate drive consistent with good efficiency, to minimize di/dt. (People who sweat over ultrafast gate driver designs are actually making life harder for themselves in this respect.)
I do always use a pair of back-to-back zeners between gate and source, though. (Not TVS, on account of their high capacitance.)
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