Re: Half bridge converters: Why two caps?
- From: legg <legg@xxxxxxxxxxxxxxx>
- Date: Wed, 24 Sep 2008 16:40:11 -0400
On Wed, 24 Sep 2008 08:15:17 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
Mook Johnson wrote:
"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:0cgCk.1383$YU2.167@xxxxxxxxxxxxxxxxxxxxxxx
Most schematics for half bridges contain two caps, like this one:
http://schmidt-walter.eit.h-da.de/smps_e/hgw_smps_e.html
What's the point? If you leave off one of the caps it works just as well.
Unless the bridge can coast which it usually can't there also won't be any
advantage in avoiding a DC run-up.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
How about startup transient before the single cap charged up. The
transformer must be designed to see full DC voltage for a few cycles without
saturating. Also the that full voltage will be reflected across the
transformer to the secondaries and might either cause problems or require
the peak voltage rating to be somewhat higher.
Doesn't work in either case. If the half-bridge output sits at GND as it
usually does when idle and then start you'd have the same issues. At the
end of the day the circuit just has to go through a phase where some
saturation is inevitable. That's why mine all have a soft-start by means
of a current limiter.
Conventionally, an 'idling' half bridge is off - high-z. The
appearance of the power rail produces 1/2 the supply rail at the
junction of the two-cap circuit, through KCL, with no control or power
circuit involvement.
A single cap obtains charge equalization only through the active
bridge and magnetic structure being driven, not always with benign
start-up effects.
Originally, the availability of rectified-line-rated film capacitors
with ripple current ratings in the ampere region was limited, and the
cost of larger parts was prohibitive. To reduce cost at lower power
levels, a single cap was often used - sometimes coupled to he
center-tap of the electrolytics in the input voltage doubler, when
present.
Current through either configuration must still pass between the power
rails, through whatever decoupling is present - typically the bulk
electrolytics and a (usually overwhelmed) film cap.
The circuit you use will either work or not, depending on the
circumstances, and the care you take in their anticipation. Until you
appreciate the full ramifications of a component's function in the
circuit, prudence would suggest that simple accuracy is required, in
attempting to copy someone else's design.
RL
.
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