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.
