J
Joerg
- Jan 1, 1970
- 0
P said:wrote in message
Fred, thanks for the idea with the diode clamp. I had tried something
like that before, but it seems to work quite well. So I have added a
linear current limiter to the battery, set at about 80 amps, but I also
changed the output capacitors to what they actually are, 3300 uF each,
with 0.016 ohms ESR. I found that it will take about 300 mSec for the
output voltage to come up to 300V, and during that time the capacitors
seem to be dissipating about 300 watts each!
Actually, it will work without those capacitors, since the square wave
has only about 1 uSec during which the transformer is not being driven.
I needed them previously when I had them connected in a doubler
configuration to work on 12 VDC. So I should be able to use something
much smaller, and/or use the capacitors in the VF drive. But I think I
may need an inductor to ease the peak current in them as well.
In some ways it's amazing that the components lasted as long as they
did. The capacitors have 0.5*300*300*6600= 297 W-Sec so at 24V that
would be 12 amp-seconds, but since the original simulation showed the
peak voltage being reached at about 50 mSec it would be 240 amp-seconds.
If I limit the current to 80 amps it should reach the desired voltage in
about 297/(24*80) = 154 mSec. But if the capacitors are also dissipating
300 watts each during charge, that would be added, and there would be
more like 900 watt-seconds, so the peak might be reached at 450 mSec. Of
course real components do not always match the simulator (or
vice-versa), and the more accurate the simulation the longer it takes.
I'm running it now and it seems to be stabilizing at about 270 mSec, but
it took probably 10-15 minutes. I'll see what it looks like when it
reaches 350 mSec at which point the PWM stops.
Wow, the RAW file is 1 GB! Looking at the last 20 mSec, the input power
is 783W, output is 276W, the MOSFETs are 9.7W, the two series pass are
23W each, and there is still 214W in each of the output capacitors! At
this point the input current has dropped to 33A.
"Where has all the power gone, long time passing..." Bob Dylan, PP&M
Actually, though, this is still "reactive" power, because the capacitors
are still charging. The actual ripple current is about 1 amp, so the
real power is minimal. I needed to continue the simulation until full
charge was reached. Looking at the first 20 mSec of startup, the
capacitor current is just 3.7A. The series pass current limiters are
dissipating about 900W, so that might be a problem. But they will only
do so for a short time, and should gradually drop to the 23 watts at 250
mSec or so.
250msec is not short in the eyes of a transistor. When it comes to the
SOA that counts almost as "DC". You aren't really planning on those li'l
ZTX849, are you? That would be one quick *PHUT* and they'll turn into a
puff of black smoke
At least the linear regulator reduces the transients during turn-on,
since the MOSFETs only see about 3 volts at first. It may be better to
design a switching current limiter, but I'm trying to reduce complexity
and enhance reliability, so a brute force linear circuit might be
acceptable.
The way to handle this is via your controller. It should issue very
short pulses during start-up, slow repetition rate. If you can't do
short pulses then leave out lots of cycles until the caps are at nominal
voltage, then throw the "virtual clutch" and go full throttle. This
requires no extra parts.
Also, C10 could gradually charge up despite R6. Might want to zener that.
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