Re: SSTC DC blocking caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 

Hi Chris,

On Fri, 24 Oct 2003, Tesla list wrote:
 > am I right in thinking the DC blocking caps are only there so the coil will
 > only draw power at Fres? I wonder what would happen if I took them out and
 > drove the coils direct. I imagine as long as the frequency is near Fres
 > then it wont draw any more power and short out the outstage, comments ?

They're there to do what the name says, block DC. They don't change the
way the secondary or primary draws current, nor the amount of AC current,
at or away from f_res.

So why use the cap? 1) DC on a inductor is equal to DC on a short circuit
=> huge current draw and mosfets blow up. Typically you don't get DC from
the SSTC powerdriver so you can leave the cap out.

But: 2) for more exotic things like when using an interrupter or doing
audio modulation, or when mosfet switching can be expected not to be 100%
equal, there's a risk of a DC level building up in the output. To
prevent failure due to this small risk of DC, you'd use the cap in series,
and the primary coil then sees just the AC and all is fine again.

Only criteria for the capacitance of the cap is: large enough :) A few
100nF, maybe even one uF. Not resonant with the primary! Unless you want
really large primary coil currents, and use correspondingly high-rated
capacitors.

cheers,
   - jfw

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