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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I'm about to start my first foray into high voltage drivers, and I have no idea where to start. The driver is an electrostatic driver, I need at least 25 fully parallel channels. A XMOS (perhaps FPGA) will control the channels, and adjust output voltages from 0-300v. This will be operating at a high rate- I want ~150khz update rate on each channel, or a bit under 4mhz frame update rate. Where do I start with amplifying a low voltage signal from a DAC (Probably 16 bit) to the 0-300v range? What things should I look for in design and saftey (besides the obvious voltage limiting)?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I would start with tightening the requirements specifications for the amplifiers;
Input input voltage range e.g. 0v to 3v2678
Output 150 kHz update but what is the required output signal bandwidth? e.g. 0 to 20 kHz or settling time = 1 us etc.
output voltage range e.g. 0v to 300v
load impedance e.g. (real power) + (reactive power) or (dynamic resistance) + capacitance
suppose the load is equivalent to 1 kOhm (capacitively coupled) in parallel with 1 nF 'real' output current would be +/- 150v/1k = +/- 150 mA peak reactive current = (peak voltage 'slew' rate) x (load capacitance) = (dV/dt) x C = 2 x pi x f x Vpk x C = 2 x pi x 20E3 x 150 x 1E-9 = 18.85 mA peak
since reactive and real current are 90 degrees phase difference the amplifier peak output current is just whichever is larger. if the load capacitance is 10nF then the peak load current would be 185 ma etc.
if the bandwidth includes 0 Hz (d.c.) then a supply of >300V dc is required if the low end frequency response does not include dc (e.g. 20 Hz to 20 kHz) then a lower (safer) supply voltage could be used with an output transformer.
Conversely if you wanted an amplifier that could switch between min and max output within 1 us with 1 nF load the amplifier would need to supply at least 300 mA peak and would need a >300 Vdc supply.
With 25 parallel channels that would potentially require a 2.5 kW min. supply !!
so before designing the amplifier tighter specifications for required output signal bandwidth, slew rate and load impedance are needed.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
If you need DC the amplifiers are going to be a bit of a headache unless you use class A ... in theory there are opamps to make an easy bridge driver, but they are over 150$ a piece.
Without spending that money you get circuits like this :
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I was planning on using this chip: It is serial though, so that would not be preferable. I would imagine if I did use this, I would just need to amplify the current.
That's alot to take in, i'm going to need some time to digest that. If I am understanding this correctly, here are some more specifics: I see a lot of DAC's with a 0-4.096v range, so I think that is a good baseline. The load capacitance is 50pf. It is electrostatic so I imagine this is the big thing. I think a Megahertz bandwidth or a 1us slew rate will be good for the application So say if I wanted to switch from min (0v) to max (300v) in 1us into 50pf (or 80pf to accommodate for wiring and traces), what would the current be? I tried following your calculations but I either get really large numbers or small numbers. Could you point me to a page with more information?
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
How is the serial-parallel converter supposed to help you amplify DAC output to 300 Vpp?
For 1 us risetime with a DAC you will need a 1 MHz DAC and a 1 MHz bandwidth analog amplifier ... substantially harder than 150 kHz.
Wait maybe I misunderstood ... do you want 25 digital channels and do you want to "amplify" DAC output simply to set the upper rail voltage for the channel push/pull stages? I doubt you would need to modulate it fast enough to need an amplifier, you could simply use the DAC output as a reference for a switching power supply.
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Designing a High Speed High Voltage Driver
