PFC Capacitors in series - how to protect from one of the capacitors shorts?

[Magnethicc]

Hello Everyone,
We are doing a PFC where the bus voltage is 800V. 2 capacitors are used in series to allow for 500V capacitors.
There may be an abnormal situation where one of the capacitors shorts and thus the entire 800V will appear on the 500V which will blow it up (after some non-zero time) and a lot of smoke will be released which is prohibited.

I was wondering if you guys have any neat ideas on how to protect the capacitor from blowing up?
Everything is allowed - blowing a fuse, blowing a VIA, disconnecting the capacitors, whatever you have in store I would greatly appreciate.

Thank you.

 

[KlausST]

Hi,

in my eyes the first thing you need is some kind of charge balancing ... otherwise you (sooner or later) will get a DC runaway.

Klaus

 

[Magnethicc]

Hi,

in my eyes the first thing you need is some kind of charge balancing ... otherwise you (sooner or later) will get a DC runaway.

Klaus

there is around 200k on each capacitor for this balancing

 

[cupoftea]

Have a zener/Resistor/Opto voltage sense across each cap....make it shut down the PWM. Say a few 100V zeners in series for each cap

 

[Magnethicc]

Have a zener/Resistor/Opto voltage sense across each cap....make it shut down the PWM. Say a few 100V zeners in series for each cap

I should've stated that the input voltage is 480Vrms.
Since now the peak of the voltage is at around 670V even with PWM off the 500V capacitor will still blow up.

 

[Easy peasy]

fuses are the normal protection, coupled with lots of 5W zeners at the rated voltage of each cap across each cap - that way the zeners take out the fuse if the volts get to rated.

 

[Magnethicc]

fuses are the normal protection, coupled with lots of 5W zeners at the rated voltage of each cap across each cap - that way the zeners take out the fuse if the volts get to rated.

Ordered alot of 160V 3W zeners and will connect 3 in series on each capacitor (Total Vz = 480V) and will test if zeners will hold until fuse opens.
I will come back and report once I would do this test.

*3W and not 5W because the 5W ones have a bit worse tolerances and we need the better torerance 3W.
Thank you.

 

[FvM]

What's the fuse rating and exact specification, e.g. total fusing I²t?
I doubt that the zeners will hold it.

 

[hiphei]

Are TAZ devices here any good in place of a regular zener..?

 

[FvM]

Transient suppressors are zeners with a different (specifically surge related) spec. For the present problem, you'll look at pulse power rating in the ms up to 100 ms range.