Oznog
Active Member
Well, it looks like my plans for a xenon strobe I could control with a PIC for rapid, attenuated pulses appear to be trashed. I was planning to use a FET to switch the power to the tube and cut it off after a portion of the discharge.
The scope showed the cap voltage went from:
0 450v
200usec 375v
400usec 310v
600usec 260v
800usec 225v
1msec 200v
Now at the beginning of the pulse the voltage sees dv of 75v in 200usec, or 375kV/sec. That's a 330 uF cap so that seems to be an initial current of 123 amps! Somebody check my math- is that right? (dV/dt)*C?
Is that right?? I'm amazed at the magnitude here! I can't switch anything like that... my high voltage FET (not installed yet) was good for around 15 amps at the 5v gate voltage I used and the data sheet says an absolute max surge of 48 amps. The cap was a "very high ripple" rating of several amps rms but this is way outside that too.
I know there's a strategy of a small flash cap separated from a bigger power cap with enough resistance between them that the flash cap will drain and the tube's arc quenches, then the cap refills pretty quickly off the main cap. This isn't going to give me the variable pulse widths I was looking for though, more caps won't really fit into an enclosure of the desired size, and it's too pricey overall.
Any ideas? I looked up bipolars and didn't see anything in that voltage range that could switch >100amps. Some HV FETs could handle over 100amp surges but they're like $10 parts and need a Vgs of 20v to do it, which my circuit don't have (it runs off of an unregulated 12v battery). The gate driver would have to be a bit complicated too.
Maybe I'm missing something? Did I get those calculations on the current right?
The scope showed the cap voltage went from:
0 450v
200usec 375v
400usec 310v
600usec 260v
800usec 225v
1msec 200v
Now at the beginning of the pulse the voltage sees dv of 75v in 200usec, or 375kV/sec. That's a 330 uF cap so that seems to be an initial current of 123 amps! Somebody check my math- is that right? (dV/dt)*C?
Is that right?? I'm amazed at the magnitude here! I can't switch anything like that... my high voltage FET (not installed yet) was good for around 15 amps at the 5v gate voltage I used and the data sheet says an absolute max surge of 48 amps. The cap was a "very high ripple" rating of several amps rms but this is way outside that too.
I know there's a strategy of a small flash cap separated from a bigger power cap with enough resistance between them that the flash cap will drain and the tube's arc quenches, then the cap refills pretty quickly off the main cap. This isn't going to give me the variable pulse widths I was looking for though, more caps won't really fit into an enclosure of the desired size, and it's too pricey overall.
Any ideas? I looked up bipolars and didn't see anything in that voltage range that could switch >100amps. Some HV FETs could handle over 100amp surges but they're like $10 parts and need a Vgs of 20v to do it, which my circuit don't have (it runs off of an unregulated 12v battery). The gate driver would have to be a bit complicated too.
Maybe I'm missing something? Did I get those calculations on the current right?
