Clean PMT Power Supply

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Clean PMT Power Supply

Post by guest »

I've been working on a high voltage power supply for PMT applications using a cold cathode fluorescent tube inverter as a front end. The output of the inverter is doubled, yielding about 1200VDC. This is fed to a shunt regulator consisting of a string of four TIP50 transistors, which regulate the output to 600-1100VDC, a good range for working with PMTs. Right now, my breadboard supply is pretty stable, and the output ripple is on the order of 0.25V. I suspect that this can be improved, since the inverter front end is unshielded and there is a lot of garbage floating around at the switching frequency. I will post a schematic as soon as I am a bit more sure about the performance. CCFL inverters are available from quite a few sources for a modest sum, and relieve one of the need to design and wind your own HF, HV transformer. The shunt regulator circuit could be used with a variety of other high voltage sources in the 1000-1500V range.
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Richard Hull
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Re: Clean PMT Power Supply

Post by Richard Hull »

I would certainly be interested in seeing your schematic.

For a PMT used as a simple detector, as in a neutron counter, a very sloppy regulation and ripple factor is allowed. If the PMT is to be used specifically for Gamma spectroscopy, the most important characteristic is long term drift stability of the precise DC output voltage. This is, of course, due to the setup of the energy channels in the SCA and ADC devices.

+/- 0.5 volt drift out of a kilovolt is a big no-no over 2 hours or more. Normal regulations are held at .001% to .005% though .01% is acceptable for short analysis. This is far beyond the normal precision reguation found in most all HV supplies.

NIM module supplies of the 0-3kv type with led readouts, (two or more NIM spaces wide), are all setup for this precision task. I have seen them go for $600.00 each used!! I have bought two off of E-bay for $240.00 and $160.00, respectively. I just picked one up at a hamfest last weekend for $40.00. It is a crap shoot, but those supplies are the finest I have ever seen and used and how they pack such precision in a small package is beyond me. I wish I had a manual to study the circuitry. The Tennelec that I just bought is packed to the gills with circuit cards of the plug in type.

Richard Hull
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DaveC
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Re: Clean PMT Power Supply

Post by DaveC »

Richard is correct about the PMT supplies. Industrial units typically have ripple and drift in the 10's of PPM (0.001%) range. Because of the large gain of the PMT, any ripple will be intensified by roughly the current gain figure of the PMT.

Dave Cooper

You can fairly easily clean up the output of a lightly filtered supply by adding a 100 to 1000 mfd of output capacitance. A good alternative to a complex HV regulator is to put a low voltage 3 terminal linear regulator chip in the ground side of the HV PS. The regulator's input comes from the PMT and since the regulator is typically good for 30 to 40 volts input, it should be quite capable to handling the minor PS swings. The " voltage sense" signal for the regulator is taken from a voltage divider across the entire HV output. Normally a well filtered regulator will give you about 100 ppm stability.
guest

Re: Clean PMT Power Supply

Post by guest »

It's possible to run an LM317HV at higher voltages in floating mode. I'm playing with one at the moment at 1080 volts output. I believe the application notes have the circuit which involves a high voltage bypass transistor.

Mark Harriss
guest

Re: Clean PMT Power Supply

Post by guest »

Of course, 1000 uF at 1kV is a fair amount of stored energy (500J), as well as being fairly bulky. I'd hate to see that dissipated in an inadvertent flashover inside the tube or wiring.

I suspect that some form of LC filter might be a "lower stored energy" solution.

Some form of linear regulator might be the best approach. A simple darlington emitter follower with a smallish cap on the input can give you pretty good "stiffness", although you'll need to make sure that on startup and shutdown that you don't exceed the voltage rating on your pass device. With careful design, you don't need HV transistors, since all the transistor has to hold off is the "voltage drop" across the regulator.
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Re: Clean PMT Power Supply

Post by DaveC »

On further reflection, Jim is clearly right about the impracticability of a large output capacitance at more than 1KV - to say nothing about the problems of finding such capacitance.

Actually you probably dont' need even 1 ufd. A number of the multiplier circuits that I have seen use about 1nF per stage to handle 10 - 30 watt outputs. Because they operate at 20 - 40 kHz, or even higher freqs, the ripple is easily supported by small output caps.

What also seems to work well, is the use of voltage feedback from an output voltage divider to regulate the LV drive current. This can nearly eliminate ripple, by boosting the low voltage input to offset the HV output droop - on a per cycle basis. Makes a multiplier circuit into a very high voltage opamp.

Dave Cooper
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