Power supply topology for 150kV

[BootstrapBill]

This is the coolest project I've seen in awhile... then again, I don't get out much.

So, a CT scanner x-ray source that needs 100kW at 120kV. I don't have any direct experience with this sort of thing, but my understanding is that x-ray tubes are fairly benign loads, with the caveat that when used in a CT scanner they need pretty good regulation of their anode voltage and current to prevent density artifacts in the resulting image slices. That argues against using a 5+ stage multiplier, as regulation tends to decline with every stage and you need close to 1A of current (Cockcroft-Walton multipliers are usually used for less than 10mA of current). Thus, I would use appropriate construction techniques to allow for a 30-40kV secondary so that only a 3-4 stage multiplier is needed. This page has some useful information on selecting/sizing a multiplier: http://www.voltagemultipliers.com/html/multdesign.html.

One other thing to consider is that you really don't want a choke on the secondary side, as it will need to be designed to withstand the full secondary voltage as well, so a good topology to consider that isn't too hard to get working is a PWM'ed buck converter feeding a full bridge with the bridge switches running at a fixed 50% duty cycle. The late Abraham Pressman covers this topology at length in his book on switching power supply design.

I was thinking exactly what you are thinking with the buck feeding the full bridge. The only thing of concern would not being using an inductor on either side of the HV transformer. I've never seen just a bridge and a transformer. Can this be done at this power level? High frequency noise? It would have a lot of losses but I would not need to worry about the voltage control with varying load like with a resonant design.

I'm tempted to use several transformers with secondaries in series as some x-ray heads do. This would make isolation more reasonable.

[IanB]

Thinking crazy thoughts here, but wouldn't it be easier just to take a direct feed from the power company at their distribution voltage (~ 10 kV?), step it up and rectify it? For the different voltages you could use different output taps on the transformer.

Yeah, OK, it's gonna be wildly expensive, but so surely is building this thing from the ground up...

I know, not really practical 

[Marco]

I'm tempted to use several transformers with secondaries in series as some x-ray heads do. This would make isolation more reasonable.

How does that make life easier for the transformer maker compared to a segmented secondary?

The MOSFETs I linked simply allow you to care a little less about switching losses, fast and very low reverse recovery, so if you really wanted to use hard switching you could. Although in retrospect I guess even with those switches you wouldn't want to at that power level. Still, they just switch just a tiny bit faster than your IGBT bricks and you don't have to worry about reverse recovery or dV/dt so much.

[BootstrapBill]

Segmented secondaries with doublers on each winding is how some x ray generators work. I haven't seen an off-the-shelf core that can handle this power.

[LaserSteve]

Former CT instructor here. CT tubes get babied. I would not call them benign.

The cathode to anode distance is very short, making them vulnerable to small amounts of gas being liberated from the anode and tube wall. Thus arc diagnostics need to  decide in a few milliseconds either to keep scanning or abort based on the severity and duration of the arc.

What I worked on had a scan electrode as well,  which gave the system a beam shift on the anode for interpolation.

The power supplies were beefed up much more then you might expect, for "conditioning" the tube prior to use on a PT. CONDITIONING was a 10 to 20 minute sequence to getter any outgassing, evenly expand the rotating anode, prevent thermal shock from cracking the vacuum seals, and store a known amount of heat  units  in the anode. A rather sophisticated HU counter kept track of all this. If PT treatment did not occur soon enough, a cooldown was mandated, followed by more CONDITIONING as needed.

It's been a while, but the PSU module was two sixty plus KV units with independent current and voltage regulation for 120 KV differential, and 850  Ma total. All in an amazingly small box powered by 440/3PH on the sliprings. Split supplies allowed for some tube management  tricks that I was never briefed on.

Said supply could adjust voltage on the order of 500-1000 times per revolution around the PT. This was used to reduce PT exposure by "creative" modulation patterns.

Full voltage full current[~~450 mA was only for warmups and emergency imaging of certain kinds of accident trauma when the radiologist decided saving an adult  life outweighed the consequences in terms of probability of tissue damage.

There was a set of conditions where you could skip CONDITIONING, if total HU to be used was short and use was immediate.
 

Steve

[BootstrapBill]

Nice to hear from another FSE. They pretty much are the same these days. Some use grounded-anode single supplies. It couldn't have been that long ago if adaptive dose modulation was in play.. They spend a lot of effort on dose modulation. They should have gone with the math guys.. the new iterative reconstruction algorithm that is replacing single back projection has cut dose in half.

[jbb]

Hi all

On 11kV utility feed: just no.  You'd then be exposed to prospective faults on the order of 11kV * 10,000A = approx 200MVA.  You don't want that. Huge expensive switchgear, and you'd need a special license just to turn it on or off.

On available cores: it's easy to just stack multiple coils next to each other to increase the core area (but not the winding area).  Leave a small gap (e.g. a sheet of mica paper) between each cores to allow some wiggle room as large cores can have poor mechanical tolerances.

On using multiple transformers:

• Reduces secondary output voltage, and therefore turns ratio +
• Allows use of more, lower-voltage, rectifier stacks.  Might also avoid the direct series connection of diodes ++
• You need primary-secondary isolation that can handle the full stand-off voltage --
• If you already have oil in the system somewhere, oil-insulated transformers are well understood and give good lifetime.
• High primary-secondary isolation means air/oil/whatever gaps.  This leads to leakage inductance which some topologies can't handle.

Like I said, you'll need to work out your insulation scheme.  Will you run either the anode or cathode at/near ground?  Grounded cathode makes heater control easier.  Grounded anode makes ... err ... anode-stuff easier?  Or you could run a split-rail system, with ground in the middle (easier transformer isolation).

[BootstrapBill]

Thanks for quantifying some of the things I was thinking in regards to the multiple transformers. My high voltage "tank" as they say in the industry will definitely be an oil bath. The filament current (heater) may be a thread in itself haha.. it would be floating on that -120kv

[jbb]

It's probably good that you've already crossed the oil Rubicon.  Once you've accepted the (small!) risk of an oil spill / fire for the "tank," adding a bit more oil probably isn't going to violate customer specs.  Oil cooling could also help with your peaking requirements (it adds specific heat to your system).

I'm not sure what your physical layout will be, but even if the transformer and "tank" (rectifiers, capacitors, bleed resistors, sense resistors??) are right next to each other, I suggest you take the trouble to separate their oil baths with a bulkhead & proper bushings.  That way when you cook something the damage will be limited :-)

Sounds like your cathode end will be on potential.  Isolating the filament to -120kV will be a challenge.  It is definitely possible to do so.  You might want to look at a classic AC transformer connected directly to the filament, with filament measurements (V, I etc.) on the primary side; that approach should be quite reliable.  This assumes that you don't need super-accurate sensing, or true DC excitation.

[MagicSmoker]

I was thinking exactly what you are thinking with the buck feeding the full bridge. The only thing of concern would not being using an inductor on either side of the HV transformer.
...
I'm tempted to use several transformers with secondaries in series as some x-ray heads do. This would make isolation more reasonable.

The inductor required by the buck converter takes over the (current) averaging function of the secondary side inductor in a conventional forward-type converter (of which the full bridge is one variant). You can also delete the buck output capacitor (aka - bridge input capacitor) if you run the bridge legs with a slight overlap; this makes the bridge current-fed, instead of voltage fed, and has some significant benefits as far as switching losses and robustness to short circuits. Some overlap of the bridge leg conduction time is critical for the current-fed variation because you don't want to interrupt current through an inductor, but since the bridge isn't width-modulated you can usually rely on the IGBTs turn-off delay to ensure a bit of overlap when driven at 50% duty cycle.

As for secondaries in series, yes, that is one of the many "appropriate construction techniques" I was alluding to (along with vacuum varnish impregnation, quad insulated wire, etc.). Each secondary only needs to withstand the voltage difference across it, then. And as others have mentioned, you won't be able to process this amount of power with a single core, anyway. Which reminds me, you probably want to use UU cores to achieve maximum separation of the windings (at the expense of increased leakage inductance).

[Marco]

vacuum varnish impregnation

For an oil submerged transformer?

[BootstrapBill]

Thanks for the Abraham Pressman recommendation.  I just looked over the 3rd edition- looks like it's all there.

So for this transformer- I would ideally like to build it myself under the supervision of a consultant for learning/cost purposes but understand most companies would want to do the building themselves. It's funny you can contact Mag-inc and tell them what you need and they give you core/turn/gauge. I'm pretty sure it's just a goon plugging a few numbers into an online calculator. They won't let you talk to the engineer on the phone so I will never know.

Anyone have any transformer specialist to recommend?

[Marco]

Here's a transformer in the right ballpark. Indeed a remarkably small box and only 23 kg ... good luck getting near that.

[MagicSmoker]

vacuum varnish impregnation

For an oil submerged transformer?

No, vacuum or vacuum/pressure varnish impregnation are alternatives to oil immersion, and possibly superior for high frequency/high voltage transformers. The OP is encouraged to do his own research into the matter.

[BootstrapBill]

So I am looking for a good buck converter IC. Haven't been able to find any info on converters in the kW range. I'm looking to go voltage controlled. I suppose voltage control for my adjustable "rail voltage" could be done using a voltage divider from output with trim pot to send feedback to error amplifier in the pwm chip? Would a TL494 be ok or is there a more modern go-to these days?

[Vtile]

What about 12500 lead acid batteries in series to get on the right voltage.

Jokes aside, cool project.

PS. Don't blow up your siliscope.

[BrianHG]

So I am looking for a good buck converter IC. Haven't been able to find any info on converters in the kW range. I'm looking to go voltage controlled. I suppose voltage control for my adjustable "rail voltage" could be done using a voltage divider from output with trim pot to send feedback to error amplifier in the pwm chip? Would a TL494 be ok or is there a more modern go-to these days?

I know MCUs might not be your territory, but I would probably use a dsPIC with it's PWM output and flash ADC inputs.  You would be able to software control frequency, using 2 x PWM channels, one for the high side and another for the low to software control your gate drive overlap & with the multiple ACD inputs, you can both resistor divide input and monitor you transformer drive & monitor current consumption for diagnostic & emergency shutdown.

The last project my friend did was a 50kva 600v, 3 phase BLDC PWM driven motor designed to test snow-mobile transmission units, torture-testing them to their breaking point to make sure the mechanics don't explode, with the IGBT driven by one dsPIC, with monitor, control and torque readouts through the RS-232 port.

[jbb]

I agree with BrianHG that using a microcontroller is a good step.

I know MCUs might not be your territory, but I would probably use a dsPIC with it's PWM output and flash ADC inputs.  You would be able to software control frequency, using 2 x PWM channels, one for the high side and another for the low to software control your gate drive overlap & with the multiple ACD inputs, you can both resistor divide input and monitor you transformer drive & monitor current consumption for diagnostic & emergency shutdown.

Let's look at the functions you'll need:

• Incoming supply monitoring
• Soft charge control & monitoring
• "Adjustable rail" control
• System monitoring (e.g. temperature)
• Communications with interlocks / logging systems.

For your reference, the Magna-Power brand is generally considered quite robust and capable of handling challenging loads.  They use a current-fed approach http://www.magna-power.com/support/technical-notes/overview-current-fed-power-processing.  If you want to use a topology like this, the buck stage and H bridge stage need to be PWM'd synchronously.  It's much easier to make this happen with a suitable micro.

There are many options for the micro.  Don't stress about the cost - it will be dwarfed by the power components.  Here are some considerations:

• High clock frequency (>10MHz) to yield fine PWM resolution.
• Good PWM peripherals - you should be able to synchronise many PWM units to run in lock-step.
• Good ADC - you should be able to a) set up a list of channels to sample in hardware and b) trigger this sample from the PWM units.  Minimum 12b resolution.
• Lots of memory.  I suggest min 64kB program and 16kB RAM.
• >= 16 bit core.  8 bit micros will require more programming effort to make your arythmetic happen.
• Floating point is not required. But you can use it if you want.  If using fixed-point, the general suggestion is to do everything in per-unit scaling.

Some possbilibites:

• dsPIC
• Texas Instruments C2000
• ARM Cortex M
• Maybe MSP430

On switching frequency & materials: if using IGBTs, you'll probably end up in the 1 - 10kHz range and may produce a lot of accoustic noise.  Amorphous/nanocrystalline iron cores are particularly susceptible to magnetostriction and have been known to make unacceptably loud products.

On connecting the MCU to the power stage:

• Isolate the controls from your PC! You will blow the converter up at least once and don't want to send your PC with it.
• I suggest you isolate the controls from the power stage as well.  This will reduce the impact of electrical noise and improve safety.
• At least in the initial development phase, I suggest you do hardware (opamps, comparators, flip-flop) protection of the hardware stage as well as software protection.  This  means that your converter doesn't blow when your software hangs.
• LEM current sensors are very well regarded. http://www.lem.com/ They provide isolation for free.
• LEM voltage sensors are not great (actually current sensor + shut resistor) but do you really want to mess about?
• Typically voltage sensors consist of a resistor divider + isolating amplifier + isolated PSU
• Use isolated gate drivers.  You can usually just but an industrial module (but they're expensive!) is you don't want to have trouble.  Rolling your own for such a high power job will be hard if you haven't done it before.
• Use gate drivers with desaturation protection.  This is a last-ditch short-circuit protection method and it will help.

[T3sl4co1l]

If the OP is struggling to complete a design with such an antiquated and basic controller as a TL494, I don't see MCUs being any help here.

The OP needs professional help.  And I mean that in the best way possible, of course -- he should hire an experienced designer, before he begins work on something powerful enough to be quite dangerous!

Tim

[BrianHG]

If the OP is struggling to complete a design with such an antiquated and basic controller as a TL494, I don't see MCUs being any help here.

The OP needs professional help.  And I mean that in the best way possible, of course -- he should hire an experienced designer, before he begins work on something powerful enough to be quite dangerous!

Tim

Yes, I agree.  My friend who worked on the snow-mobile transmission test jig is a serious electrical and Newtonian engineer who did all the math / research beforehand with experience of many years in motor control and robotics, working his way up in power through the years and with the power factor I mentioned above, the power source could have easily killed through both the voltages involved, or created a deadly explosion as I saw all the IGBTs and all the other high power components were in a thick steel explosion proof housing with gigantic fuses.

[razberik]

What about 12500 lead acid batteries in series to get on the right voltage.
Jokes aside, cool project.

Actually not a joke. My colleague told me about some ionizing detector for SEM microscope they bought 25-30yrs ago from some Japanese company. They supplied the +1kV potential and floating electronics by a lot of batteries in series. It was a few plastic tubes filled with chains of batteries. Replacement of batteries was "service personal only", so these Jap guys had to travel around the world.
This colleague got actually inspired for another type of detector. Kilovolt supply was SMPS and floating electronics was supplied by NiMH battery. Communication through some Avago optocouplers available that time. Floating electronic had ON/OFF mechanical switch to save battery life.
Never got to serial production.

[BootstrapBill]

I actually like the MCU approach and thank you for the links/suggestions. I was planning on controlling soft start, contactors, interlocks etc with the arduino since I'm familiar with it and use it regularly. I starting thinking I would go the analog chip route because the arduino only does 490Hz pwm. I did not explore more professional MCU options.

Regarding switching frequency I am going for 40kHz. Reason is to be out of the audio range and make it more reasonable for my transformer designer.

I have hired professional help for the magnetics and multiplier and may be taking the next step soon and hire a designer. Problem is no one wants to get involved with 100+kV. Anyway after consulting with a company that does large mulitipliers we decided on a 10kV dry transformer to feed the multiplier. They claim it should be stable.

[james_s]

What about 12500 lead acid batteries in series to get on the right voltage.
Jokes aside, cool project.

Actually not a joke. My colleague told me about some ionizing detector for SEM microscope they bought 25-30yrs ago from some Japanese company. They supplied the +1kV potential and floating electronics by a lot of batteries in series. It was a few plastic tubes filled with chains of batteries. Replacement of batteries was "service personal only", so these Jap guys had to travel around the world.
This colleague got actually inspired for another type of detector. Kilovolt supply was SMPS and floating electronics was supplied by NiMH battery. Communication through some Avago optocouplers available that time. Floating electronic had ON/OFF mechanical switch to save battery life.
Never got to serial production.

Several years ago I was given a box of roughly 400 lightly used 9V batteries that came from the required annual replacement in the primarily line powered smoke alarms in a retirement home. I was real tempted to chain up a couple hundred of them in series but after seeing the arc I could draw from 15 of them I was always too nervous. They can deliver a few Amps into a short circuit for a brief period.

[oldway]

http://www.fug-elektronik.de/en/products/high-voltage/hch.html
HCH 50000-150000    150 kV    300 mA    50 kW    600 mm    2000 mm    800 mm    930 kg

https://www.alibaba.com/product-detail/50kv-100kv-150kv-200kv-high-voltage_60398045736.html

https://www.google.com/patents/US6967559

[BootstrapBill]

Picked up an arduino DUE which has the arm cortex M3.

Much to the dismay of T3sl4co1l I now have 10 bit variable duty PWM at 40kHz   

int Feedback = A0;   // feedback connected to pin 0
int val = 0;         // variable to store the read value

void setup() {
  // PWM Set-up on pin: DAC1
   
  REG_PMC_PCER1 |= PMC_PCER1_PID36;                     // Enable PWM
  REG_PIOB_ABSR |= PIO_ABSR_P16;                        // Set PWM pin perhipheral type A or B, in this case B
  REG_PIOB_PDR |= PIO_PDR_P16;                          // Set PWM pin to an output
  REG_PWM_CLK = PWM_CLK_PREA(0) | PWM_CLK_DIVA(1);      // Set the PWM clock rate to 84MHz (84MHz/1)
  REG_PWM_CMR0 = PWM_CMR_CPRE_CLKA;                     // Enable single slope PWM and set the clock source as CLKA
  REG_PWM_CPRD0 = 2100;                                  // Set the PWM frequency 84MHz/40kHz = 2100                                   // Set the PWM duty cycle 50% (2100/2=1050)
  REG_PWM_ENA = PWM_ENA_CHID0;                          // Enable the PWM channel     
}

void loop()
{
val = analogRead(Feedback);
REG_PWM_CDTY0 = val * 2;
}

Just need to dig deeper into the atmel ADC so I can get past the arduino IDE limited 10bit and get the 11bit to match my output.

I figure 11 bit should be good enough?