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Guru
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Pulsed High Voltage Current Sources for 600V and 1200V

03/21/2014 7:52 AM

Friends,

This is an exercise to make high voltage electrical and electronics device testing somewhat easier to some extent.

I just thought of this a little difficult area to be tried - Pulsed High Voltage Current Sources for 600V and 1200V. Both current source and current sink designs in the 1-20 mA are considered but can also be lower down to 1nA to 20mA. we can discuss here to see if we can have some good ideas.

Purpose of this exercise to have some good control over current source and sink while testing other devices (DUT). If for example we test a capacitor and we find linear or non-linear response in the ramp voltage gradient then we will know how best a capacitor will work at high voltage and can we trust its dielectric or not.

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Guru
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#1

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/21/2014 12:02 PM

I'm a little bit confused. Are you looking for a function generator in the voltage range of 600 and 1200 Volts? Perhaps you just need a fast semiconductor that can switch on and off a DC supply of 600 or 1200 volts?

And, are you using specific current probes to measure the response of your DUT?

Perhaps you are looking for a wide band current transformer?

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Guru
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#2
In reply to #1

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/21/2014 1:03 PM

No, this is about pulsed or switched current sources from high voltage power supplies. We often come across current limiting devices but rarely a constant programmable current sources that use such a high voltage up to 600V and even 1200V.

Many MOSFET test circuit employ current limiting scheme to prevent damage to DUT. However there is no general use scheme available for direct implementation as constant current source above 100V.

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Guru
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#3

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/21/2014 5:51 PM

Pulsed power is a very specialized field of electronics that still relies on vacuum tube technology. The most common tubes used are thyratrons. Very often a specialized capacitor bank with a very low ESR is slowly charged with the thyratron quickly discharging the capacitor across a load. This is the common topology used for the kickers used in synchrotrons.

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Guru
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#4
In reply to #3

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/21/2014 6:46 PM

That is true. However, for low cost and simpler test set up solid state devices are to be explored. At higher currents there is this pop-corn problem. Linear response is another serious problem. There has been a set back with GaAs devices and SiC devices are the options on hand.

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#5

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 8:57 AM

You will do well to obtain a copy of "High Peed Pulse Technology" by F. Frungel published by Academic Press. We have used some of this information to construct low impedance welding equipment that operates above 200,000 amps.

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#6
In reply to #5

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 9:15 AM

welderman:

Very interesting one.

200KA is sure a huge current. What is the duration for this and what is the operating voltage or peak power you expect to use? This perhaps may be just Tens of Volts like spot welders. Larger currents are required if surface area to be welded is high.

Circuit inductance may be of serious consideration and also RF generated while switching such a high current forming some tuned network having high frequency harmonics.

I think IGBT will be the right choice for such applications with kA capacity and an array of them fired simultaneously on a bus bar.

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#7
In reply to #6

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 10:11 AM

This is tens of volts, specifically for spot welding. We have dealt with circuit inductance by developing and producing our own low impedance pulse transformers. However, the recommendation to study Frungel's work, is good for many applications.

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Guru
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#10
In reply to #7

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 4:22 PM

For welding SCR / TRIAC fired AC into step down transformer is enough.

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#11
In reply to #10

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 7:04 PM

You are correct; SCR/TRIAC fired into a step down transformer is enough. However, we are looking for more than enough.

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Guru
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#12
In reply to #11

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 11:27 PM

welderman:

If electrical impulse energy is applied in multiples of about 20ms then 50Hz / 60Hz direct fired pulse should do else one can go for 400Hz option also. However for electronic chip wire bonding like application microseconds time domain is the only feasible option. Such applications fall in the range I am discussing here.

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#13
In reply to #12

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/23/2014 8:05 AM

Very unlikely that we would use 200,000 amps of current for chip wire bonding applications. Our attention is instead focused on the impedance of resistance welding equipment relative to the impedance of welds.

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Guru
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#14
In reply to #13

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/23/2014 9:13 AM

welderman:

I have seen such welding in gas cylinders and reactor vessels where resistance welding is used. They often end up with bad quality welds in many parts and these are inspected using Radiography. Defect rate is pretty high.

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#15
In reply to #14

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/23/2014 10:34 AM

This is why anecdotal evidence fails miserably.

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Guru
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#16
In reply to #15

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/23/2014 8:46 PM

redfred:


This company manufactures about 400 gas cylinders per month and then they getthese tested through Radiography by another NDT Company. About 30% on an average of their cylinder joints show bad welding at some point so they redo them from fresh.


They use arc welding electrode, preset current and set the speed of welding on fresh cut V-grove to fill the gap. I think most of the low investment manufacturers may be using same procedure.


If resistance welding current profile is analyzed then it may give some details of why there was a failure during welding. I can offer this help to them but their equipments look very old type.

How to prevent such failure is another area of interest.

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#17
In reply to #16

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/23/2014 9:32 PM

I do not dispute the anecdote that a company gets a 30% failure rate using electric arc welding in any application. I do dispute that this incident implies that electric arc welding is itself a faulty technique for all welding applications. I am mildly perplexed but accepting the idea that any company would continue to perform any process with a 30% failure rate. Hopefully this company is exploring if this is the wrong technique, training, or other reason for such a high failure rate.

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#18
In reply to #17

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/23/2014 11:33 PM

redfred:

Basically cost is added in NDT testing, redoing and retesting and again ending up with similar failure rate.

Company owners are friends so I can perhaps help them do the tests for free. However, I may get some idea if any improvement can be done on the basis of information or not.

There is a possibility that electrodes are not identical and their material is not uniform, but there must be some standard for that also as those material come from other supplier. This part also to be evaluated as that part sure will affect the statistics.

I am also considering monitoring the weld temperature or arc spectrum if that can also supplement information.
I don't think this company will changed the setup unless they are very sure of some gain.

What are the other parameters that can be considered?

I have just now spoken to the factory owner and I will conduct tests soon. I can also share information here for the benefit for those who may have similar problems.

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#8

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 10:32 AM

There are many uses one can make for high-voltage switches, current sources and high-impedance probes. Fortunately most high-voltage power MOSFETs have low leakage currents and work well as linear devices at low operating currents. What's more, thankfully, when operated at fixed currents in the subthreshold region, their gate-source bias voltage doesn't change significantly with drain voltage.

One thing that's generally useful with high-output-impedance current sources is a way to also monitor the voltage across the current source, as part of the circuit. When you're working at high voltages this is especially important, because there are no commercially-available high-impedance (i.e., infinite impedance) high-voltage probes.

One idea for making such a probe is to use a MOSFET as a source follower. But there are two problems with the simple circuit at right. First, we don't know what the FET's VGS(on) gate-source voltage is, creating a big error in the measurement. Second, the current though the 10M divider varies with probe voltage, so the MOSFET's drain current ID changes over a huge range, further changing VGS and introducing more error.

A third issue is that we'd like to establish a minimum current through the MOSFET follower, to help speed it's falltime response. (The risetime is very fast, because it can marshal high currents to handle a high +dV/dt = +I/C, charging the MOSFET's COSS capacitance. It's discharging, the -I for -dV/dt, that's an issue.)

My circuit design at left solves these issues, by adding a fixed precision current sink, with a correction for the varying current taken by the divider resistor.

It also adds an offset correction for the unknown VGS value. This adjustment is made with the probe's input grounded.

The MOSFET part shown, made by IXYS, is an IXTP02N120P or IXTY02N120P. This is an n-channel enhancement-mode type, available in either a TO-220 or D-Pak SMT package. The '02' means it can handle 200mA, and the '120' means it's rated at 1200 volts. It's a small-die part with a COSS capacitance of only 9pF. At 50uA and 1.1kV it dissipates only 55mW.

The circuit works properly down to below -5V and up to within 5 to 15 volts of the positive supply.

The negative-going slew rate, taking into account Q1 and Q2, is 2.8 V/us. One could use a higher current-sink value. A rather cute enhancement I've made to the circuit (not shown) is to add a dynamic increase in the current sink, proportional to the rate of fall. This allows for a fast high -dV/dt, without increasing the quiescent power dissipation.

This circuit appears in our forthcoming book, The Art of Electronics, x Chapters, and is discussed in more detail there.

Now to return to Shyam's original topic, a high-voltage current source. Yes, the circuit above contains a precision high-voltage current sink. MOSFET transistors are available to extend its operating range all the way to 4500 volts, and cascode techniques can go further. The current sink is used to bias Q1, but one could simply eliminate Q1 and keep the rest as a current sink with output voltage monitor. As for pulsing the current, there are many ways to do that, depending on one's goals.

There's a different, but similar, well-established way to make a current source / sink: use a precision voltage follower with a bootstrapped voltage source on its output and and a current-setting resistor to the input. If you make the MOSFET source-follower with a depletion-mode rather than enhancement-mode transistor, you can get a convenient floating voltage to power an op-amp, and create a precision local tracking high-voltage, for circuits of this type. But that's a subject for another post.

I love designing current sources. There's an entirely different approach one can take, that's well-suited for making programmable bipolar high-voltage current sources, e.g., with a compliance of +/-320 volts, etc. But Shyam asked about 600 to 1200-volt sources, and a bipolar design would add unnecessary complexity.

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#9
In reply to #8

Re: Pulsed High Voltage Current Sources for 600V and 1200V

03/22/2014 4:10 PM

Winfield Hill:

Very interesting design.

It requires control voltage -5V to +1000V sounds a bit difficult thing as it may need another programmable voltage source which may not go to negative voltage at all, but sure this design idea is good one.


I do have few thousands of IXYS MOSFETs of 1kV 12A so I can try this. I think what I have have rise time and fall time of about 10ns to 12ns range.

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#19
In reply to #8

Re: Pulsed High Voltage Current Sources for 600V and 1200V

04/03/2014 11:38 PM

Winfield Hill,

I am thinking to use for testing Solar Panel I-V measurement using automated variable pulsed current sink. Up to 100V Panel and up to 10A current to start with may be ideal.

This idea can be extended to 600V and 1200V applications but may never be required for solar panels as they are often wired for lower safe operating voltages.

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#20

Re: Pulsed High Voltage Current Sources for 600V and 1200V

04/30/2014 8:58 AM

The schematic with the MOSFET and 1100V is very interesting. It should be possible to do something about the HV drive requirement. There is where a small pentode or beam power tube might stand the voltage and be reasonably linear as a voltage amp for the MOSFET.

I still use vacuum tubes for the output stages in things like that because I end up with a transient during some experiment due to an inductance and that's not good for MOSFETs. All my stuff is low volume though. 8-)

If testing PV arrays in the field, tubes may be a no-go, but if in a facility it could be economical depending on how many equipments need to be made and how long they have to last.

A very cheap alternative would be a seldom used 'sweep tube' like a 12BQ6. The 12V heater makes it unpopular for most audio/ham hobbyists and so there are 100K's of them lying around NOS in warehouses for $4-6 single quantity. It's not rated 1100VDC as a supply voltage but it will hold it off forever as it has a 5KV pulse rating. Another good one is the 6CD6 but it is much larger.

Back to the driver voltage, I have trouble getting a MOSFET to be reasonably linear as a HV amplifier due to the issues around Vth. They can vary a bit from one to the other because most of them were made for switching rather than linear low current use and there's not a lot of attention given to perfection around Vth, not a consideration for switching.

If you solve the voltage driver issue with a MOSFET, would you please post the circuit back?

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#21
In reply to #20

Re: Pulsed High Voltage Current Sources for 600V and 1200V

04/30/2014 10:21 AM

If you like my post, give me a GA. :-) Anyway, as you can see my circuit solves the linear use of a power MOSFET's Vgs problem, for its purpose, which was high-impedance measuring of high voltages. As part of the circuit, there's a high-voltage MOSFET current source (actually, a current sink), which also has solved the Vgs problem, this time using the cascode connection with an op-amp BJT CS feedback loop.

But you can just as easily place the MOSFET where the BJT is, and it works fine, except then the op-amp has to drive the MOSFET's higher Ciss capacitance. I've done this with high current MOSFETs using a G=1 driver amp IC after the op-amp, and using the standard op-amp-gate-capacitance isolation trick.

Although the currents are small in my probe example circuit, the technique works well at high currents, if the MOSFET can handle the heating. You can get up to 4500V MOSFETs, so the motivation to do the job with vacuum tubes is reduced. However, there are convenient low-cost tubes (radar switches) one can use up to the 25kV region.

If you need medium high voltages and really high power, IGBTs and IGBT modules are the way to go. I made an instrument that did 1.2kV and 1200A with a large IGBT module, purchased on eBay. :-) This was with a medium-sized IGBT module, they go up to some pretty impressive voltage and current ratings. E.g., 4.5kV and 6kA.

Ahem, I assume we're talking pulsed.

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#22
In reply to #21

Re: Pulsed High Voltage Current Sources for 600V and 1200V

04/30/2014 10:40 AM

Here's how to use an op-amp to drive a power MOSFET.

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#23
In reply to #22

Re: Pulsed High Voltage Current Sources for 600V and 1200V

05/05/2014 1:59 PM

Right.

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