Capacitors in series across 177.5V

[cupoftea]

Hi,
We are doing an 180Vout SMPS with 2v5 reference.
The upper divider resistor will have 177.5V across it...we want some compensation capacitance across this.......it appears the cheapest way to do this is with four 50V, same value, 0805 ceramic caps in series.
But will their different leakage currents mean unequal voltages across the three capacitors?

 

[barry]

Treez, I don't know if you’re ignorant, lazy, or have chosen the wrong career path, but the answers to your questions are readily available using the internet and common sense.

1) putting 4 caps in series will give you 1/4 the capacitance.
2) if you can accomodate four 0805’s, why not just use one larger cap?
3) How expensive do you think a 250V 0805 cap is? (Hint, it’s not. A 0.1 uF 250V part is about $0.29 USD.)

 

[cupoftea]

...thanks, thats what i thought......until...i am seeing, on the component sites, you have to buy at least 5 or 10 of each value anyway.....so i want a few different overall capacitance levels.......and [10 x 50v] caps is cheaper than [10 x 250v] caps............and we can do stuff like three 2n7's in series with 1 x 3n3, etc etc
....its only for hobby prototyping at the moment.....50v and below and they deffo get mucho cheaper.

100nF , 250V is indeed cheap...100nF, i noticed, is a "cheap value"...probz because its commonly used.

Treez, I don't know if you’re ignorant, lazy, or have chosen the wrong career path

...yes, all of the above

 

[FvM]

Beyond usefulness of your solution, if you are series connecting capacitors with DC bias above the individual rating, you always need parallel DC balancing resistors.

 

[cupoftea]

Thanks, from what you say, and looking at Farnell etc, it appears the cheapest way to do this is to split the upper divider resistor into four series 0805's...and then also have an 50v 0805 cap across each resistor.

 

[KlausST]

Hi

it appears the cheapest way to do this is

Is it?
Maybe you could give the part numbers you´ve chosen.
Did you also calculate increassed part cout, assembling cost, board space...?

Klaus

 

[mtwieg]

Leaving aside the question of how many pennies one might save....

If you want to break the compensation cap into series caps, best to also break its parallel resistor into equal number of equal-value resistors. Then connect each series C in parallel with each series R. Thus you will get DC balancing and the desired transfer function.