Why 400V in PFC circuits ?

[mundachakka]

Almost every PFC circuits that I came across, outputs 400v before being coupled to an secondary dc-dc converter. What is the reason behind this ?

[Siwastaja]

Well, they are boost converters, and if you look at boost schematic, it has this diode which connects the input voltage to output directly - in other words, output voltage cannot be smaller than input voltage; boost can only increase the voltage.

Biggest 1-phase mains voltage commonly available is 240VAC (e.g. traditionally in the UK). They changed to 230VAC nominal but changed the tolerances so that +10% is allowed. This means maximum mains voltage one wants to design power supplies for is 230VAC * 110% = 253VAC, at very least.

But the boost converter will see the maximum PEAK of the input voltage. The output voltage hence has to be more than the peak.

For 253VAC RMS, peak voltage is sqrt(2) * 253V = 358V.

Add some safety margin and round to a nice round number, and there you have it, 400VDC.

[bidrohini]

You have already got a good answer here. You can find some more in the links below:

https://electronics.stackexchange.com/questions/308837/why-does-the-voltage-across-the-dc-link-capacitor-in-a-boost-pfc-circuit-usually

https://www.edaboard.com/threads/hi-why-pfc-output-voltage-is-400v.219093/

[mundachakka]

thank you

[T3sl4co1l]

It could be higher of course, but then the boost ratio is also higher, leading to lower efficiency at low Vin.  Or it could be somewhat arbitrary (preferably in the middle, say 200-300V, but 100-400 wouldn't be much of a stretch) using a SEPIC or flyback type converter instead of boost (with flyback offering the distinction of isolation too) -- but this also adds expense and/or losses, and these features (other voltages, isolation) are never(?) needed in practice.

As it happens, 400V is an excellent supply for subsequent stages, especially half-bridge or two-switch forward converter, or LLC resonant, using 600-700V MOSFETs.  (Which was still pretty true in the days before SuperJunction MOSFETs were introduced, and has only been improved since then.)

(To explain that last point: prior generation MOSFETs scaled as Rds(on) ~ Vds(max)^2, so high voltage parts performed especially poorly in comparison.  They were acceptable in the 400-600V range, but 900V+ parts were used more grudgingly, or best avoided altogether.  Which was kind of annoying for industrial applications, where 400-480VAC input (560-750V bus) is common, needing 800V+ rated devices.  This also made IGBTs more attractive than MOSFETs, despite their slower operation or limited voltage drop.  (Industrial applications tend be more tolerant of slow switching, too.)  SJ technology has brought significant improvements, for both types really, but in particular, MOSFETs now scale as Rds(on) ~ Vds(max), so they are competitive in any voltage rating.  Nowadays, MOSFETs are preferred for high efficiency and low to medium power designs, and IGBTs for high power or cost-effective designs.)

Tim

[Terry Bites]

Its worth noting that the PFC is operating as a boost converter in its own right.
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