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What we need for testing HV tube supplies
What we need for testing HV tube supplies
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Old 8th January 2018, 03:48 PM   #1
jackinnj is offline jackinnj  United States
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What we need for testing HV tube supplies
Default What we need for testing HV tube supplies

The issue came up in the discussion of the LT3080 as a high voltage floating regulator.

There are four basic tests that you have to perform on all power supplies. Power supply rejection ratio (PSRR), noise, impedance and stability. (From impedance you can examine stability.)

Easily done with the tools at hand for opamps and ss power amplifiers. It gets really tricky to do so inexpensively for high voltage power supplies.

Take PSRR -- you could use a transformer to modulate the voltage input of a high voltage supply -- if you can find one with bandwidth of 10 Hz to 10 MHz that can sustain 100mA or more. Cost $300 and up.

Build a modulated DC power supply -- like the Kepco BOP -- the bandwidth of the Kepco is only 20kHz. Maybe a VDMOS amplifier using ham radio techniques? (The beloved F5 amplifier I built has bandwidth to around 1MHz with IRF devices) There's are several designs in ARRL handbooks which could be modified.

Well, I am open to ideas.
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Old 8th January 2018, 09:58 PM   #2
Elvee is offline Elvee  Belgium
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I don't think anything is insurmountable or even costly: take the PSRR, if you feed a floating supply from the ground side using a wideband buffer or amplifier, you can easily arrive at decent results provided the capacitance of the DC supply wrt. ambient is reasonable.
Here is an outline, the LT1210 only has a 66MHz GBW, but if you accept lower output currents (or if you shop around), wider bandwidths are possible
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Old 9th January 2018, 06:11 PM   #3
jackinnj is offline jackinnj  United States
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What we need for testing HV tube supplies
Z measurement. A loop to induce current into the output of a power supply? I just show a Tek current probe and amplifier as I've had one for a while About the most output I can expect from the VNA is 10 dBm
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Old 10th January 2018, 07:42 PM   #4
Elvee is offline Elvee  Belgium
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Here is the way I would do it. Modern VNA's have a usable range >100dB, meaning the setup will be able to resolve ~0.5mΩ.
If that's not sufficient, a wideband buffer followed by a 5Ω resistor could be inserted in port 1. This would better the floor by 20dB
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Old 12th January 2018, 09:24 PM   #5
DualTriode is offline DualTriode  United States
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I am thinking a Voltage Controlled Resistor.

https://www.vishay.com/docs/70598/70598.pdf

For high voltage.

http://web.cecs.pdx.edu/~jmorris/ece...ert%20p152.pdf

The circuit in post #2 makes me fear for the series current passing through the AC voltage source.

Place the VCR at the DUT input to monitor the DUT output for PSRR.

Place the VCR at the DUT output to monitor the DUT input for reverse PSRR.

DT
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Old 12th January 2018, 11:47 PM   #6
trobbins is offline trobbins  Australia
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In practise, does PSRR measurement need to extend to 10MHz? The LT3080 datasheet extends to 1Mhz, and shows a consistent loss of PSRR above 100kHz.

Is that extent based on rectification diode turn-off resonance parasitic voltage levels that could be present at the input to such a HV DC regulator? Or is the regulator being driven by a mains switchmode supply that has a HV DC output, and the concern is that the switchmode output has content that extends to 10MHz?

Last edited by trobbins; 12th January 2018 at 11:54 PM.
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Old 13th January 2018, 01:14 AM   #7
jackinnj is offline jackinnj  United States
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What we need for testing HV tube supplies
PSRR to1MHz would be satisfactory. For measuring Zout of the power supply, perhaps (I think) a decade more as the gremlins seem to manifest themselves any from 10s of kHz to 10MHz.

If you had a sound-card analyzer, you would not be able to see the Jung-Didden super-regulator in oscillation mode with the AD797 error amplifier. With the remarkable AD797 you can get the Zout to under 10 micro-Ohms, but... With the AD797 I was quite able to see New York's AM station WABC on the HP3577 spec analyzer.

As suggested by one of the European engineers from ADI, a simple a.m. radio can be your best sensor for power supply instability.
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Old 14th January 2018, 12:37 AM   #8
jackinnj is offline jackinnj  United States
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What we need for testing HV tube supplies
Quote:
Originally Posted by Elvee View Post
Here is the way I would do it. Modern VNA's have a usable range >100dB, meaning the setup will be able to resolve ~0.5mΩ.
If that's not sufficient, a wideband buffer followed by a 5Ω resistor could be inserted in port 1. This would better the floor by 20dB
Looking at Semtech and Littlefuse TVS on Digikey's site. (I've used small incandescent bulbs as current suppressors. They do have DC resistance which contributes to noise.)

TVS have pretty significant capacitance, 100's to 1,000's of pF's, or not in this application?
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Old 16th January 2018, 01:58 AM   #9
DualTriode is offline DualTriode  United States
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Please tell me more about instability.

This is what I am thinking about stability.

The power supplies that we are speaking of will be used to power tube amplifiers.

What is the issue with connecting the known load, the amplifier, to the power supply under test and using a function generator to sweep, pulse or whatever at the input of the amplifier?

While sweeping pulsing or whatever at the amplifier input connect the test probes to the HV output of the regulator under test. Trigger the bode plot from the function generator.

If the power supply + audio amplifier assembly is stable under test conditions it will also be stable functioning as an audio amplifier.

DT
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Old 16th January 2018, 08:58 AM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by jackinnj View Post
TVS have pretty significant capacitance, 100's to 1,000's of pF's, or not in this application?
The ones intended for the protection of high speed signal lines have a much lower capacitance, typically <1pF, sometimes much smaller.

Some examples:
Low capacitance ESD protection for high-speed interfaces | Nexperia

https://www.google.com/url?sa=t&rct=...qAGuGWEU5Qp6ZA

https://www.infineon.com/dgdl/Infine...3e9d6619a20002
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