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 PW for autogating?
RickFinsta  [Team Member]
4/30/2012 2:31:25 PM EST
I was trying to do some research and couldn't find much; anyone here care to expound on how the pulse width modulation in autogated power supplies functions?
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XCQTR  [Member]
4/30/2012 4:44:48 PM EST
It works like an LED or DC motor speed controller. In most semiconductor, you cannot just lower the voltage like light bulb as they have minimum working voltage (in LED, this is Forward Voltage). So instead, you give it full voltage and turn off - equivalent to turn on and off the switch quickly....and you change (modulate) the time on/off. This ratio is called the "duty cycle" where 100% duty cycle is simply DC voltage. So it's simply changing the duty cycle of the square wave. See Illustration Here

Pulse Width Modulation is exactly what it says. The pulse width changes as whatever "modulates". In LED dimmer, it's the dimmer knob. IIRC, in I^2, it's the GaAs (photocathode) current.
cj7hawk  [Team Member]
4/30/2012 10:41:09 PM EST

Originally Posted By XCQTR:
It works like an LED or DC motor speed controller. In most semiconductor, you cannot just lower the voltage like light bulb as they have minimum working voltage (in LED, this is Forward Voltage). So instead, you give it full voltage and turn off - equivalent to turn on and off the switch quickly....and you change (modulate) the time on/off. This ratio is called the "duty cycle" where 100% duty cycle is simply DC voltage. So it's simply changing the duty cycle of the square wave. See Illustration Here

Pulse Width Modulation is exactly what it says. The pulse width changes as whatever "modulates". In LED dimmer, it's the dimmer knob. IIRC, in I^2, it's the GaAs (photocathode) current.

An excellent description - the only thing I would add is what "gating" is -
Gating is when the photocathode voltage is made momentarily higher than the MCP input voltage - this has the effect of stopping the electrons from migrating to the photocathode and so "gates" or switches the tube off.

Tubes can gate in the order of nanoseconds or faster - quick enough to catch light moving. Gating is used for other purposes, however autogating is where the gating is pulse-width modulated to control how much light is converted to photoelectrons.

Also, filmless tubes cannot run at DC without taking significant damage - in Gen4 and filmless designs for GaAs photocathodes, the gating is controlled to cause positive ions to "unburrow" from the photocathode, hence allowing for longer tube life.

Regards
David

RickFinsta  [Team Member]
5/1/2012 3:09:18 AM EST
Thanks; I'm familiar with PWM as I have built and tuned an EFI system for one of my cars, but I was curious as to the use in NV devices. I had read people complaining about whining and figured that was the trick to the AG system. That's a great explanation, especially when coupled with the "Sticky" thread about the inner workings of these units.