Hi Jeremy,
On Sun, Jul 13, 2014 at 9:45 PM, Jeremy Gassmann <jeremyee78@xxxxxxxxx>
wrote:
> Hey Steve,
> Thanks so much for the detailed response! I have a couple more
> questions:
>
> If you are just using the line voltage as an input (say 240 VAC), do you
> need a 1:1 isolation transformer or can you connect it directly?
>
There is no need for isolation, but it can be quite useful when attempting
to use an oscilloscope to verify the operation of the H-bridge. Please be
aware of the short circuit that exists if you attempt to hook the scope
ground reference to the output of the DC supply, it will short through the
rectifier diodes to ground and likely blow up connections in the scope.
You will, however, need some way of charging up the big storage caps. Most
people just use a variac, but this can be prohibitively expensive for a
high power coil, so another alternative is a resistor in series with the AC
line, bypassed with a relay/contactor. The relay closes after the caps are
charged.
>
> To properly size the current rating of the power supply diodes, do you need
> to run some simulations with SPICE or are there some design equations
> compiled somewhere that you can start with (I'm a little rusty on my
> electronics design courses)?
>
Well, it really just depends on how much power you want your coil to handle
or use. Its rather difficult to provide design equations... most designers
just seem to have some gut instinct on how to size the coil and its
components for a given power level. My generalization was basically to
pick a rectifier with a current rating of at least 2X the "RMS" line
current (really, the average line current is the better number to use as it
relates more directly to heat dissipation in a semiconductor, but RMS is
close enough). So for a 10kW coil running on 240V, the RMS line current
would be *at least* 41.7A, however its likely to be about 1.5X greater than
this due to the high harmonic content of the line current when feeding this
type of supply, so figure the line current to be 63A. Id pick a >120A
diode, you can often find dual-diode modules on ebay in the 10-30 dollar
range.
The 1.5X comes from the typical power factor of 0.6 to 0.7 for this type of
supply. The actual power factor will depend on the AC line impedance and
the size of the capacitors used, and how much the voltage is sagging on the
caps. I used LTspice (its freeware, and one of the best simulator tools
ive ever used) to model the supply and to calculate the power factor based
on actual output KW vs RMS line current and voltage.
>
> Thanks again for all the info (and patience) as I try to understand DRSSTC
> design. I have a lot of research to do!
>
Just in case you missed it (some stuff is a little out of date, but
hopefully still useful)
http://stevehv.4hv.org/SSTCindex.htm
Steve
>
> Jeremy
>
>
>
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