Comment by Workaccount2
1 day ago
400V electric vehicles and 400,000V transmission lines play by different rules.
There are no MOSFETS anywhere in HV applications. IGBTs, but no MOSFETS. Most converters use thyristors and newer ones use IGBTs. No matter what, PN-junctions are king for HV silicon applications.
Also ripple is a function of filtering not switching. The reason higher switching frequencies generally have better ripple characteristics is because smaller capacitors can filter them and/or larger capacitors filter them better. So in a cost constrained/size constrained product you get more filtering for the same buck same size.
I also can't figure out what you are saying in your last line, apologies.
Well, SiC MOSFET do get used, but yeah. SiC JFETs are indeed better, lower lower with the same wafer technology, avalanche proof, high heat proof (the polyimide passivation hurts beyond ~220 C).
Much easier to drive when you stack them for HV.
That said, GaN is there for capacitive converters due to being able to run very efficient at >10 MHz switching frequency.
These converters in principle fit in very compact phase change coolant/insulator vessels, for example with propane. The capacitors at those frequencies get to be tiny, like, smaller than the transistor package by volume.
> 400V electric vehicles and 400,000V transmission lines play by different rules.
When stacked, they don't. Plenty of research on stacking both MOSFETs and entire power converters.
With stacking, the figure of merit (ie. Kilowatts per dollar, loss percentage) isn't a function of voltage (although the fact that you have to have an integer number in series and parallel could influence the design if you want to use off the shelf components)
Today's HV converter stations use IGBT's mostly because they used to be the best thing to use back in the 2010's when the design process for them started.
The reasons for using IGBTs is not only because BJTs withstand higher voltages, but also because their Vce(sat) can provide much lower loss than Rds(on) at high currents. I x V vs I^2/R.
Vce never really goes below 2 volts... Which for a 1000 amps means the running costs of the converter are 2000 watts * number of stages (~2800). 5.6MW of heat. That quickly dwarfs the purchase cost of those IGBTs.
Whereas the same calculation for MOSFETs [1] gives 4242 stages and an Rdson of 1.9 milliohms... = 8 Megawatts! Which sounds worse... But you can parallel the MOSFETs by spending double the money on them, reducing the loss to 4 megawatts... Or you can double it again to reduce the loss to 2 megawatts, etc.
When you run something 24*7, energy losses cost way more than capital costs - and MOSFETs let you make that tradeoff, whereas IGBTs do not.
[1]: https://www.infineon.com/cms/en/product/power/mosfet/silicon...