# Half bridge converter capacitor selection

Discussion in 'Electronic Design' started by R zaman, Jun 4, 2004.

1. ### R zamanGuest

I would like to know the exact way to choose the value of the
capacitors C1 and C2 in a half-bridge DC-DC converter. Should it be
selected based on the ripple-current?

2. ### Tam/WB2TTGuest

Are you talking about the voltage doubler capacitors, or the ones in series
with the transformer?

Tam

3. ### Tim WescottGuest

Schematic please. I assume you're looking at someone's schematic that
has two capacitors, which happen to be labeled C1 and C2, but there's a
lot of different ways to make anything.

Give a text-only schematic, or a link to a website, or post something
readable (jpg, gif or pdf) on alt.binaries.schematic.electronic.

4. ### R zamanGuest

I am talking about the basic hard-switching half-bridge converter. The
diagram can be viewed in:
http://www.coremaster.com/appnotes/an108.pdf

The half-bridge converter will be used for the (2nd stage) dc-dc stage
(in buck mode) of a 2-stage universal PFC converter. Electrolytic
condenser can be used for c1 and c2. But the problem with the
electrolytic condenser is that the it has very high ESR compare to the
film condenser and consequently the loss will be very high. SO we
would like to use film condenser. It would be great if anyone
enlighten me the procedure for calculating the value of C1 and C2.

Thanks

5. ### CBarn24050Guest

The half-bridge converter will be used for the (2nd stage) dc-dc stage
The primary current charges/discharges the caps each cycle, so it depends on
the frequency, the pri current and how much voltage ripple at the mid point you
are prepared to tolerate. The usual it/c formula applies. Typical values for an
off line bridge would be 0.47uf to 2uf. most designs have a series capacitor as
well of same size.

6. ### Harry DellamanoGuest

If you have a PFC front end you must have a large bulk cap. Either split
that in half and use the center point as the transformer return or put a
single cap in series with the transformer to ground. C1 and C2 are not
necessary just shown for clarity. If you use a single film cap in series
with the transformer to ground, the voltages and currents thru said cap are
now easy to calculate because you do not have it in parallel with the bulk
cap.
cheers
harry

7. ### Jan PanteltjeGuest

Yes C1 and C2 set the ripple current (primary).
They also need to be suitable for the switching frequency of cause.
Note that the regulator feedback look will decrease ripple in the output too.
Anyways, you could have electrolitics and an other type of cap in parallel.
JP

8. ### Tam/WB2TTGuest

Harry,
I have seen circuits with a voltage doubler front end, which would have a
split filter capacitor. Rather than connect the transformer to their center
tap, they used a pair if 1 ufd caps going from - to + with the center tap of
the 1 ufd going to the transformer. Not sure why, unless the main filters