Comment by londons_explore
1 day ago
> HVDC lines can have up to 10% ripple
That's exactly why one uses a high switching frequency, MOSFETs and has a tiny ripple (perhaps 0.1%). This can be obtained cheaply with multiphase convertors.
Mosfets are now cheaper than IGBT's where you are paying for power losses and plan to run at full load for more than a few days to months. That's why nearly all EV's use MOSFETs - (and will use GAN MOSFETs at MHz switching rates when the patents run out)
Remember that the cable acts like a capacitor/inductor pair to ground. Ripple currents that are lost through it are not wasted money - merely wasted capacity and resistive losses in the cable. At these currents, you can assume earth is a perfect conductor, so no losses there either.
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.
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