I have been working on a
primary feedback coil for quite a while now. My first attempts all
failed which now appears to be due to non zero switchings
which was simply caused by driver latency.
The high current switching caused two problems, the first one being
that even
though the IRG4PF50WD IGBTs I use can conduct in excess of 700 amps,
if they are made to switch much more than their 204 amp max
rating the maximum allowable dissipation can be exceeded ( as Terry
Fritz has shown me). The second problem was that such high
current switching generated a heap of RF noise and caused all sorts of
problems with the
logic. The biggest of those was that the 555 timer used in my
interrupter would speed up.
The solution to all this was simply to get a lower Frequency secondary. I am now 100% happy with primary current feedback.
10.5" X 48" winding length. 2200 turns .5mm wire. 250uH
Topload
6" X 26" Al duct
Coupling
K = .15
The circuit is very much like
Terry Fritz's design using logic gates and a monostable to kick things
off reliably at the start of each chirp before feedback is strong
enough. The monostable pulse length is set to just less than one half
cycle of the coil or about 9us.
Gate drive is just a standard gate drive transformer setup using two
ferrite cores. The transformers themselves took a while to sort out as
the low 50kHz operating frequency meant they needed more turns than
usual.
Here the test setup can be seen.
It 's pretty messy, but I didn't want to make any more boards
until I had every thing working properly. If anything is going to go
wrong it will happen now !
Sparks, 61" worth. The coil
as seen was running 7.5 cycles (150us) , 110 BPS, 365V DC bus. Primary
current was around 300 amps. Input power is about 1350 watts. Arcs were
connecting to the target or roof every few seconds. I think there is
another foot of spark in this setup. All that needs to be done is
increase the tank cap to .4 or .5 uf. Later when I do the final build
with this coil I'll test that theory.
With the proof of concept out of
the way I'll build a new smaller 6" secondary in the 50 to 60kHz range
so I have a nice portable setup. The 10 X 48" secondary is still
destined to become part of a high power CM600 setup.
I'll add in some scope shots of various waveforms when I get a chance.
