Re: Capacitor sizing on half-bridge SSTCs

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Original poster: "Jan Wagner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

On Thu, 29 May 2003, Tesla list wrote:
 > Original poster: "Mccauley, Daniel H by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
 > What kind of capacitor sizing are you typically using on half-bridge driven
 > SSTCs???  I tried to calculate the size, but i can't get a reasonable model
 > for the resonator coil to get a good idea of load.  These are the capacitors
 > going from positive / negative rails to one end of the primary winding.
 > Basically, when the h-bridge switches, the current flowing from these
 > capacitors is what goes in the primary winding.  I think I saw a few people
 > using 1uF with 1Meg equalizing resistors, but just wanted to get a good
 > ballpark.

You can use upper and lower cap banks with equal capacitance as the series
cap in a full-bridge topology.

If you think about it, the load applied voltage of the half-bridge is half
of the full-bridge, so you need twice the capacitance as in the
full-bridge. In the AC model of the half-bridge circuit, the upper and
lower cap banks parallel up, so: upper bank capacitance = lower bank
capacitance = any full-bridge series cap capacitance previously "found good".

~10uF 350V electrolytic and a one or two of 470nF 400V foil pulse caps,
all in parallel, used as the upper and lower banks, is one example which
works just fine (tried at >80kHz and about 500W or there, and 230VAC mains).

cheers,
  - jfw

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