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555 alternative / need 500V DC square wave 1mAmp

E

Erwin Maes

Jan 1, 1970
0
Hi,
are there alternatives for the 555?
I need 500VCD at 1milliAmp (square wave 150kHz)

thanks for any suggestions!
 
T

Tim Wescott

Jan 1, 1970
0
Erwin said:
Hi,
are there alternatives for the 555?
I need 500VCD at 1milliAmp (square wave 150kHz)

thanks for any suggestions!

I'd consider a 555 and a high-voltage amplifier, if the 555 were precise
enough.
 
M

Mark Jones

Jan 1, 1970
0
Tim said:
I'd consider a 555 and a high-voltage amplifier, if the 555 were precise
enough.


* Hmm, how "square" is "square?"
* Is 500vDC being driven into a Z=500k load (1mA actual peak current?)
At this V and F, stray capacitance and inductance may distort the
waveform, depending on the source impedance.
* Also, what is the limit on supply current? Higher allowances can
yield better performances.

Pick any two: High Speed, High Voltage, High Resolution...
 
E

Erwin Maes

Jan 1, 1970
0
No, it has rise and fall times of 100nSec. I need 10nSec....
* Hmm, how "square" is "square?" 10nSec up and 10nsec down...
* Is 500vDC being driven into a Z=500k load (1mA actual peak current?)
At this V and F, stray capacitance and inductance may distort the
waveform, depending on the source impedance.
* Also, what is the limit on supply current? Higher allowances can yield
better performances.
It's fead by 230VAC, so plenty of that I suppose ;.)
 
W

Winfield Hill

Jan 1, 1970
0
Erwin Maes wrote...
Hi,
are there alternatives for the 555?
I need 500VCD at 1milliAmp (square wave 150kHz)

and also
I need 10nSec rise and fall times. (edited response)

Hah! So you imagine the required switching current is 1mA because
you envision a 500k load resistor to +500V? Sorry, Erwin, whenever
you talk fast risetimes, you MUST consider the capacitance that's
present. For example, let's say you can do a very good job and
keep the total node capacitance, including the switch, to just 20pF.
To get a 10ns falltime you'll need a switch current to ground of
i = C dV/dt = 20pF 500V / 10ns = 1A, which is much more than 1mA!

I mention falltime, because this is something you can achieve with
a n-channel MOSFET pulling to ground. However, risetime is another
matter, because when you turn off the MOSFET, you'll get a slow
risetime of 10us from your 500k pullup resistor with our postulated
20pF of node capacitance. Of course you can use a smaller pullup
resistor, but you can see that a 10ns risetime isn't very practical
that way. Consider, for a 10ns time constant you'd need a 500-ohm
pullup resistor, dissipating 250W, and now you'd be switching 2A to
ground. What's more, a 10ns RC time constant is slower than your
desired 10ns risetime. :>)

I've made several 500V pulse generators with 10ns rise and falltime,
but they required considerable knowledge and effort to design. One
used a transformer-coupled n-type pullup FET, with 0.5A gate drive.
It had 5ns 500mA gate drive for both ON and OFF, which required some
interesting circuitry. Dissipation is another issue; in the particle
accelerator where my design is used as an on-demand beam switch, it
generates up to 30k pulses/sec, and 150kHz cycling might be too much.
 
E

Erwin Maes

Jan 1, 1970
0
Winfield,

thanks for the update and info!
I'll check out the design and see if I can first make a scaled down
version that only needs about 24V at 100nsec rise and fall times.
 
W

Winfield Hill

Jan 1, 1970
0
Erwin Maes wrote...
I'll check out the design and see if I can first make a scaled down
version that only needs about 24V at 100nsec rise and fall times.

What are you trying to make? You can use 600V half-bridge FET
drivers to get a 40ns rise-fall performance with a 500V swing.
 
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