Making ultrastable high voltage supply

[Marco]

I will go with a passive solution because if I add enough inductance (hopefully not a mega henry) and enough capacitance (nanoFarads, if I go high frequency) then I won't have to measure, and I can assume 10% instability, and correct for it with the passive bank of filters!

You do realize you are going from asking for ppm specs to building something to ppk specs right?

Rectifying the output from a transformer, filtering it a little and relying solely on transformer action to regulate the output is not a precision power supply.

[T3sl4co1l]

For a project, I need a power supply which can do 10-20kv at 300microAmps...

This is juist what a color TV HV power supply did....!!!! With shunt regulation using a PD510 tube.
Reduce voltage from 24KV to 20KV ajusting polarisation of the shunt regulator.

You can improve voltage regulation by modification of the grid control of the PD510 tube.

Indeed, color sets were regulated -- such was necessary to ensure correct picture size, color, brightness and (most of all) convergence.

I think stability was a few percent.

Originally achieved with a 30W 30kV 1mA shunt regulator triode -- watch out for x-rays on that one!

Tim

[David Hess]

CRT deflection is largely dependent on cathode voltage so on oscilloscopes, this potential of about -3 kV has to be stable to better than about 1%.  In practice it is usually much more stable than that.  The reference, error amplifier, and high voltage output divider limit performance but there is considerable improvement which could be made with better parts.  I think the limiting factor will be the high voltage DC divider; high voltage thick film resistors just are not good enough for the proposed performance but a series string of precision resistors should work.  Watch out for voltage derating, self heating, and voltage coefficient of resistance though.

Oscilloscope high voltage inverters operate between 20 and 50 kHz making filtering relatively easy but the transformers are not trivial to wind.  Getting to a higher output voltage will be difficult; I would first try using transformers intended for LCD CFL backlight inverters with the primaries connected in parallel and the secondaries connected in series.  I would plan on vacuum potting the entire high voltage assembly.

An alternative would be to use one LCD CFL backlight transformer with a capacitor-diode voltage multiplier on the output but the 100 uA current requirement may be a little too much.  With care this could work without vacuum potting.

[oldway]

CRT deflection is largely dependent on cathode voltage so on oscilloscopes, this potential of about -3 kV has to be stable to better than about 1%.  In practice it is usually much more stable than that.  The reference, error amplifier, and high voltage output divider limit performance but there is considerable improvement which could be made with better parts. HV power supply of analog oscilloscope can't be used because current is too low (100 to 150µA) and also output voltage. (generaly between 10 and 16KV...Only Tektronix 7000 series have higher accelerating voltage) I think the limiting factor will be the high voltage DC divider; high voltage thick film resistors just are not good enough for the proposed performance but a series string of precision resistors should work. With 20 KV, this seems difficult. Buy a cheap High voltage probe of 30KV and use the high voltage resistors of this probe Watch out for voltage derating, self heating, and voltage coefficient of resistance though.

Oscilloscope high voltage inverters operate between 20 and 50 kHz making filtering relatively easy Filtering is made by the capacitance between internal graphite coating and external one. You need a 20KV condenser ! but the transformers are not trivial to wind.  Getting to a higher output voltage will be difficult; I would first try using transformers intended for LCD CFL backlight inverters with the primaries connected in parallel and the secondaries connected in series.  You can't do that because insulation between primary and secundary is not intended to witstand such a high voltage I would plan on vacuum potting the entire high voltage assembly.

An alternative would be to use one LCD CFL backlight transformer with a capacitor-diode voltage multiplier on the output but the 100 uA current requirement may be a little too much.  With care this could work without vacuum potting.

[David Hess]

HV power supply of analog oscilloscope can't be used because current is too low (100 to 150µA) and also output voltage. (generaly between 10 and 16KV...Only Tektronix 7000 series have higher accelerating voltage)

They used x7 multipliers (sometimes Tektronix marked this x14 in their documentation) to produce 21kV post deflection acceleration from the 3kV peak-to-peak transformer output starting with the 150 MHz 7704 which was available starting in 1969.  I am not suggesting to use that many voltage multiplier stages for the reason you identify but that means raising the secondary voltage.

With 20 KV, this seems difficult. Buy a cheap High voltage probe of 30KV and use the high voltage resistors of this probe.

The thick film resistors used in these probes are pretty good but will be less stable then an assembled precision resistor.

Filtering is made by the capacitance between internal graphite coating and external one. You need a 20KV condenser!

Oscilloscope CRTs do not have an external coating or at least none of the "modern" ones since the 1960s I have seen do.  The oldest ones used wall bands or a continuous coating and later ones used a wound helix terminated at the grounded field forming electrode.

The series stacked capacitance itself in the voltage multiplier provides filtering but so what?  High voltage capacitors are readily available.  It is too bad these guys do not sell the needed transformer:

http://hvstuff.com/high-voltage-capacitors

You can't do that because insulation between primary and secundary is not intended to witstand such a high voltage

Which is why I would plan on vacuum potting the entire high voltage assembly as I mentioned in my post.  Two transformers in a balanced configuration to produce a grounded center tap would be better (my neon sign transformer has a grounded center tapped secondary) but I doubt the application will allow it.

I was thinking of something along the lines of a higher voltage version of the laser power supply Jim Williams showed in figure 11 on page 7 of Linear Technology application note 61:

http://cds.linear.com/docs/en/application-note/an61fa.pdf

[botcrusher]

Thanks for bringing up the environmental aspects of achieving stability- can I just immerse all of the electronics in a bath of mineral oil to reduce said effects? A pool of mineral oil with 4 wires sticking out- input 12V and output 10kV?

Not just mineral oil: low dielectric oils, the most common being transformer oils.
http://www.electrical4u.com/transformer-insulating-oil-and-types-of-transformer-oil/

However there are vegetable oils substitutes too, Google will find them.

Time to crack out that old sealed vat of PCBs

[Marco]

With a HV350REG available for 250$ it hardly seems worth the trouble.

[David Hess]

With a HV350REG available for 250$ it hardly seems worth the trouble.

The HV350REG is not going to have even close to the required stability but a separate DC regulator could be added to its output.  I know a easy design such a regulator could take but this post is too small to contain it.

I am kidding.  Instead of trying to design a 10+ kilovolt voltage regulator, float the HV350REG with a regulated low voltage and use the high voltage resistor divider on the combined output.  I am dubious however that the HV350REG output is quiet enough however.

[Alex Eisenhut]

https://www.matsusada.com/pdf/sem.pdf

[oldway]

In an old scrap CRT color TV or CRT monitor, you can salvage all the components needed to make your own 20KV power supply project for nothing or very cheap. (ferrite flyback transformer, high voltage power transistor, voltage multiplier, high voltage capacitors, ...)

As this kind of power supply is current limited with high internal resistance, a shunt regulation would be very simple and should have a great stability.

[David Hess]

As this kind of power supply is current limited with high internal resistance, a shunt regulation would be very simple and should have a great stability.

What form does a 10 to 20 kilovolt shunt regulator take?  Could it use a series string of high voltage avalanche diodes for level shifting?

[oldway]

Triode Svetlana GP5, ED500, ED510, PD500, PD510, 6BK4,....

http://www.jogis-roehrenbude.de/Russian/GP5/GP5.pdf

http://www.ebay.fr/itm/3-x-Russian-GP5-GP-5-6BK4-Vintage-Vacuum-Triode-Tube-NOS-ED500-5-/181703232728

[saturation]

Spellman's MPS high stability modules could meet your specs.

http://www.spellmanhv.com/en/Products/Modules/3W-15W/MPS.aspx

[Marco]

Instead of trying to design a 10+ kilovolt voltage regulator, float the HV350REG with a regulated low voltage and use the high voltage resistor divider on the combined output.  I am dubious however that the HV350REG output is quiet enough however.

You're going to need a lot of passive filtering and output capacitance for that to work ... high voltage dividers tend to have poor high frequency response, unless you want to build one with RC segments (not trivial to keep the response flat).

[David Hess]

Instead of trying to design a 10+ kilovolt voltage regulator, float the HV350REG with a regulated low voltage and use the high voltage resistor divider on the combined output.  I am dubious however that the HV350REG output is quiet enough however.

You're going to need a lot of passive filtering and output capacitance for that to work ... high voltage dividers tend to have poor high frequency response, unless you want to build one with RC segments (not trivial to keep the response flat).

It is an interesting problem; use a fixed parallel high voltage capacitance across the series resistor and trim the capacitance on the low voltage shunt resistor for best response.  Flat response is not required and most designs will perform better with a little bit of lead compensation.

Running a transient response test on a power supply like this though is probably better imagined than performed or at least admired from a safe distance.  The last time I worked on something like this, the observation was made that it is amazing what becomes a conductor at high voltages.

I would worry that the high voltage power supply has so much noise and drift that the error amplifier cannot remove it.  That Spellman supply linked by saturation looks like just the thing to avoid designing and building something that will likely fail spectacularly a few times.

[Brumby]

.... it is amazing what becomes a conductor at high voltages.

 

[oldway]

Shunt regulator act as a filter....
Flyback (current generator) + shunt regulator.
How the control works ?
Angle of the flyback power supply is regulated to maintain 500µA as average cathode current of the high voltage triode.
Polarisation of the triode is regulated by opamp to obtain the required output HV voltage.
NB: at power up, a delay is needed for the warm up of the triode.

[Marco]

It is an interesting problem; use a fixed parallel high voltage capacitance across the series resistor and trim the capacitance on the low voltage shunt resistor for best response.  Flat response is not required and most designs will perform better with a little bit of lead compensation.

Actually it was the reverse of I was thinking, you don't want capacitance at all on the lower leg.

At a 1:1000 attenuation you'd need GHz+ GBW to get good ripple rejection at 10+ Khz, and it's not necessary. Just let attenuation approach 1x at frequencies above a couple Hz.

Funny, I've never understood how shunt regulators work it seems :/

[oldway]

If needed, there is a cheap solution to make a 20KV capacitor: the Leyden jar. 

[David Hess]

It is an interesting problem; use a fixed parallel high voltage capacitance across the series resistor and trim the capacitance on the low voltage shunt resistor for best response.  Flat response is not required and most designs will perform better with a little bit of lead compensation.

Actually it was the reverse of I was thinking, you don't want capacitance at all on the lower leg.

At a 1:1000 attenuation you'd need GHz+ GBW to get good ripple rejection at 10+ Khz, and it's not necessary. Just let attenuation approach 1x at frequencies above a couple Hz.

The reason I would do it this way is that it is easier and safer to have a grounded trimmer capacitor on the lower leg of the resistive divider and it allows trimming of the phase lead.  The fixed high voltage capacitor across the high voltage part of the divider swamps other parasitic effects which may change over time.

The other design I would consider uses a very high voltage level shift on the output of the low voltage error amplifier so the gain within the loop is 1.  This effectively becomes a high voltage emitter follower but getting this to work at 10+ kilovolts would be more difficult.

Funny, I've never understood how shunt regulators work it seems :/

I could design a high voltage shunt regulator using solid state devices but I was interested in what other solutions there are.  Somewhere I have a several amp 50+ kilovolt rectifier assembly which is interesting to study.  It uses fiberglass supports to hold an array of individual stud rectifiers mounted on printed circuit boards.

[Marco]

The reason I would do it this way is that it is easier and safer to have a grounded trimmer capacitor on the lower leg of the resistive divider and it allows trimming of the phase lead.

The design isn't sensitive to it at all. It's not a question of phase lead, the phase goes from 0 to near 90 and back to near 0 regardless. The upper leg capacitance sets the -3dB point and the combination of the upper and lower leg capacitances set the high frequency attenuation. Whether the -3 db point of the attenuator is at 2 Hz or 3 Hz doesn't matter. Whether the high frequency attenuation is 0.99 or 0.98 and the phase is 1 degree or 2 degree doesn't matter, that's just screwing around in the margins, has little effect on loop regulation.

So say a 1G string for the upper leg with 100 pF worth of high voltage capacitor and a lower leg of 1M is all that's necessary.