[SI-LIST] Re: Dielectric Strength of series capacitor_EMI

  • From: "Rick Brooks (ricbrook)" <ricbrook@xxxxxxxxx>
  • To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 27 Jun 2012 16:15:18 +0000

In theory, you can do anything.
In the real world, I would not expect series caps to share the total voltage 
equally.
I've seen designs for HV diode array and HV cap arrays where parallel elements 
in each leg are added to help balance the instantaneous voltage.
cheers
 Rick




-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Riley, Jonathan
Sent: Wednesday, June 27, 2012 7:16 AM
To: 'rohit.mishra@xxxxxx'; 'balaseven@xxxxxxxxx'; 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Dielectric Strength of series capacitor_EMI

Hi Bala

You also need to consider the difference in leakage currents too. This affects 
both of the capacitors and also the PCB. You can overcome both of these issues 
by putting a balancing resistor in parallel with each capacitor, the value of 
the resistor being fairly high (high enough to allow a significantly larger 
current to flow than the leakages, so this becomes the dominant current). Of 
course, if the equipment does not permit a DC path, you cannot do this. It is 
generally simpler and safer to use a single capacitor whose rating meet or 
exceed your requirements.

Regards
Jonathan

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Rohit MISHRA
Sent: 27 June 2012 06:05
To: balaseven@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Dielectric Strength of series capacitor_EMI

Bala,

Theoretically, you can connect two similar capacitors in series since both will 
share half of breakdown voltage and can serve your purpose but Remember, real 
capacitors value always comes with some tolerance !!!

Suppose you have two capacitors of 10 UF Value with 10 % tolerance and 
breakdown voltage of 2 KV So worst condition will be when one capacitor will 
have 9 UF and another will have 11 UF Value.

When you connect these two capacitors serially across 4 KV Source, The one with 
lesser value ( 9 UF) will charge fast and will also share more voltage and 
hence would break down much earlier than you would predict when simply dividing 
the voltage among them.

So you shouldn't expect capacitors to charge to the same voltage unless they 
are the same series by the same manufacturer and out of the same production run.

Hope that helps.

Rgds,
Rohit Mishra 



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of bala
Sent: Wednesday, June 27, 2012 3:07 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Dielectric Strength of series capacitor_EMI

Hi All,
I have to use 4KV line to ground capacitor for safety,and,due to some design 
restrictions,i planned to use two 2KV capacitor in series.

My question is,Can we add the Dielectric Strength of the capacitor connected in 
series and consider them as cumulative.?
We usually apply 'Kirchhoff Voltage Law' to calculate the total Voltage,can we 
apply the same for Dielectric strength?

--
bala


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