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Re: Resonant charging and quench time
Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> I am designing a
> DC tank supply employing resonant charging. My target is to have a bang
power
> (primary capacitor charging voltage) stable and repetible from bang to bang.
> Playing with a MicroSim model of it I bumped into a sad thing: when the
spark
> gap OPENS there is, of course, some power still left in the primary
> capacitor/coil circuit. The current left in the primary coil will make up a
> high voltage spike that, in turn, will charge back the primary capacitor
to a
> certain potential. The polarity and magnitude of this voltage DEPENDS ON THE
> QUENCH TIME, i.e. on the exact moment in time the spark gap opens! If this
> voltage is positive, the following resonant charging cycle will not charge
> anymore the primary capacitor to a voltage equal to about twice the tank
supply
> voltage, but to a lower value. Sad thing, because the next bang will have
thus
> LOWER energy. If this voltage is negative, the following resonant charging
> cycle will not charge anymore the primary capacitor to a voltage equal to
about
> twice the tank supply voltage, but to a HIGHER value. Again, bad news
because
> you'll possibly break the capacitor or, at least, trigger the safety gap, if
> you are lucky to have one. This voltage left on the primary capacitor
when the
> spark gap opens is almost random, as it is random the exact value of the
quench
> time from bang to bang. Also those not using resonant charging will have
their
> calculations and setups messed up by this factor.
The residual energy left in the primary capacitor after quenching
is usually not a factor.
Typically, the residuals are around a few percent or so, since the
gap (in reality) cannot quench until the circulating current is very
close to zero.
If variations of a few percent are too great for experimental
purposes, then a 'De-Qing' circuit is required in order to
precisely dump the remaining energy in the charging reactor once
the desired primary voltage is attained.
A spark gap across the cap bank is definitely NOT recommended in
a DC resonant charging scheme, since the gap will not auto-clear
after a fault event (as in an AC system), and the sustained fault
currents will damage the rectifier stacks.
You wouldn't put a spark gap across the output of your car's
charging system to protect the electronics, would you?
The operation of a DC resonant charger is predictable, and the
voltages developed by the circuit can (and should) be calculated.
The capacitor should be simply rated to handle the maximum possible
voltage.
--
-GL
www.lod-dot-org