I was looking at HVDC systems out of curiosity and I have a question that I have not exactly found an answer for.
In HVDC converter, because of the high voltage (Tens/Hundreds of kV) they have to use so-called IGBT valves, which is multiple devices in series in order to increase the voltage blocking capability. My question is why is there a limit to how many devices can be used in series? Is it because the conduction losses are too great? Is this also why they can't use let's say 10 MOSFETs in series instead of a big IGBT?
I would also like to know how the parallelism is affected in this case. I know that for MOSFETs this isn't a big problem, as they conduct less at a higher temperature, thus no positive feedback leading to the destruction of the device. I'm not necessarily looking for a straight answer, I am happy to research this myself if I am pointed in the right direction.
Best Answer
HVDC design engineer by trade here. You don't want to use MOSFETs for the simple reason of cost (chip area) per A of current and since MLC concept was published, everyone is switching at low speed. Paralleling both IGBTs and MOSFETs becomes a challange at kA levels due to parasitics and fault cases. Series connecting is a major challange. Don't do it unless you absolutley have to and be prepared for several issues with high losses in snubbers, avalanche and so on.
Losses in snubbers, timing requirement on your gate drive, avalanche capability or ns response time in local gate drives with collector voltage slope regulation.
Your simple small MOSFET SMPS on your bench will usually work fine with parallelled MOSFETs as long as you have individual gate resistors. Moving up into kV and kA territory, parasitics will have far more to say and all work against you. Matched chips from the same wafer is something you won't be able to do at home and the kV of withstand will make sure you have a long way to go from your gate driver to each chip too, so you are in for a challange.
Bottom line: Don't series nor parallel any transistors unless you absolutley have to!