Criticize my HV supply design

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 11:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Criticize my HV supply design

Post by Silviu Tamasdan » Sat Nov 11, 2017 4:07 pm

LOL the question about mylar was about a completely different project - one that uses gases but not for fusion but rather for floating things up in the air. :p
There _is_ madness to my method.

Nnnnnnn
Posts: 58
Joined: Tue Jun 13, 2017 10:25 pm
Real name: Niels Geerits

Re: Criticize my HV supply design

Post by Nnnnnnn » Sun Nov 12, 2017 6:35 pm

Hi Silviu,

Great post! I might be repeating what others have already said. I also built my own working HVPS using a half bridge, a ferrite toroid and a CW Multiplier (40kV. 10kV @ 50mA is the most I have used for now). Now I am going to change the Inverter to a push pull type circuit with a center tapped Primary. This allows me to use only N-FETs in addition I will be lowering the primary voltage from 250V and 4A to 30V at 20A (these FETs are cheaper). Drain Source ringing on my P-FET (half bridge) was a pain. It is good you are taking more time to think of a smarter way to switch those FETs.

I think your power size estimations might be off. My supply uses a smaller core than what you are proposing. In addition commercial low voltage supplies I own have smaller core sizes (e.g. Computer PSUs, Welder supply). What did you base your estimations on?

Also save yourself some money and start with low voltage and low current so you can use cheap FETs. You don't want a bucket of dead power FETs like me.

Don't forget flyback diodes. I was told the diodes built into mosfets aren't that great for this purpose, but this may be wrong.

To measure the voltage you really don't need any special resistors. I got 100 0.25W 10Mohm resistors and put them in series (this takes time). They don't have to have a high precision, because the high amount of resistors averages out errors. After the 10Mohm resistors I put a single precise 100k ohm resistor. I measure the voltage across this resistor. Cost 15€ IIRC.

One last thing. Now that you know you can probably get away with a smaller core you will probably need more primary windings. Winding the secondary of a toroid is a pain (takes very long). You may want to bump up the frequency a bit to save on windings. (I am at 50khz and 10 primary windings). It is up to you though higher frequencies come with other problems (switching losses, skin effect)

Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 11:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Criticize my HV supply design

Post by Silviu Tamasdan » Sun Nov 12, 2017 7:04 pm

Niels,

Thanks for the advice. It's the first time I design a SMPS, so I used some formulas I found that were used in other similar projects. One of them was P=(Ae(cm2)/1.152)^2 which I have seen used in a similar circumstance. I probably need to do more reading. However, I am OK with using a larger core area, it makes much less likely that I will overload it and gives me the potential for in-place upgrades. The cores I'm using for the transformer aren't toroidal but E+E so much easier to wind for. The toroid in the pictures is just for the initial tests. I'll re-calculate the transformer when I have some time to do some serious reading, but I'm still planning to use E-cores and leave a large power buffer in the calculations. No magic smoke should be lost here. In addition it will allow me to not complicate the primary and secondary windings too much.

I already got my HV resistors, that's 3 pieces of 1.5Gohm each, about 17cm long each, rated for 30+kV. I won't push each that far obviously. They cost me about $20 together, so the same ballpark as your setup, and will save me soldering 100 resistors in series. The nice thing about these resistors is that they screw end-to-end into each other so placing them in series doesn't even require any soldering.

Cost of the power MOSFET/IGBT is no concern. I have a source for obtaining them fairly cheaply. My current version of the design uses 600V/60A IGBTs which cost me about $1 a piece. And in fact I have received yesterday a few 1200V/40A IGBTs which had cost me about the same.
There _is_ madness to my method.

Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 11:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Criticize my HV supply design

Post by Silviu Tamasdan » Thu Dec 21, 2017 5:00 pm

I had very little time to get stuff done lately, but here are the latest developments.

I've read a lot more about SMPS topologies and practical implementations, had help from some people on 4HV.org, and realized a fundamental error I was making in the calculations above.

Using the information I got now I will be adapting a known working design to my parameters. I will use proper gate drivers for the IGBTs, and a comparator-based overcurrent protection scheme. Since the OCP will use only half of the 4 comparators in a LM339, one of the unused ones will become the oscillator instead of a separate 555. I have increased the design frequency from 20kHz to 40kHz, to get more efficiency out of the ferrite transformer and save me a lot of wire in the secondary. The oscillator will actually generate double that (80kHz) and the signal will be divided by 2 by a T flipflop made with a CD4027 to achieve near-perfect 50% duty cycle. In series with the primary of the transformer I will have an inductor to provide enough reactance to briefly absorb any current spikes (shorts or arcing in the secondary) until the regular overcurrent protection kicks in. The protection inductor will be made on an identical ferrite core and same primary as the main transformer, but without a secondary.I've also reduced the secondary voltage to 9000V to avoid overstressing the CWM components, and increased the CWM stages to 2.

For the transformer itself I will use a U+I ferrite with a core area of 8.4cm2 which should have ample reserve for a design power of 1000W. The primary will be 4 turns of litz (28 strands of 22AWG enameled wire, should support 25A current and the calculated peak current needed is 23.57A; I may make it 30 strands which will support up to 28A). The secondary is 450 turns of high-voltage 28AWG wire with 0.5mm of silicone insulation. This accounts for approx 10% core losses (hysteresis, flux leakage etc). Max secondary current based on wire gauge is 200mA, but will not go over 100mA because that's what my HV diodes are rated for (30kV, 100mA, 100ns) and the primary driver is dimensioned for that. The secondary geometry, based on the core window area available minus space needed for the primary, will be 25 turns per layer, 18 layers total. Layers separated by 2x0.1mm mylar sheets, one above and one below the connections between layers.This will give me a core window utilization of about 92%.

The above followed by a 2-stage CWM should reach 50kV at 18mA.

Below picture of some HV parts I got in the meantime.
Top row, 30kV and 20kV caps for use in the CWM.
Middle row, large E+E cores for a later, higher power version of the SMPS. The core area for these is 16cm2. Next some high voltage resistors (20x2.2Mohm and 3x1.5Gohm) to be used in voltage dividers for measuring. And a roll of 28AWG HV wire.
Bottom row polypropylene caps, the U+I core for the transformer discussed above and caps for filtering the rectified mains voltage for the primary.

For size reference, the length of one of the E cores is 14cm (5.5 inches). Each half weighs almost 3 pounds.
Attachments
20171221_105813.jpg
My new HV friends
There _is_ madness to my method.

Rex Allers
Posts: 364
Joined: Sun Dec 30, 2012 8:39 am
Real name:
Location: San Jose CA

Re: Criticize my HV supply design

Post by Rex Allers » Fri Dec 22, 2017 10:06 am

Are those resistors from Russia? I bought some that look like that a good while back from a Russian seller. Mine are all 200 Mohm.

If they are like the ones I got, they have a threaded screw projecting from one end and a threaded hole in the other, so you can screw them together end to end.

If it helps, what I worked out is the threads are metric 4-.70. I wanted several resistors in parallel for a load so made two aluminum bars where I drilled holes in one of them and threaded 4-.70, and drilled slightly bigger holes in the other bar to accept 4-.70 screws.

par-res.jpg
parallel resistors
This doesn't help your supply project, but maybe a bit of help in future work.
Rex Allers

Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 11:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Criticize my HV supply design

Post by Silviu Tamasdan » Fri Dec 22, 2017 1:03 pm

Yes, they are exactly the same Russian resistors. They do screw into each other and the thread is M4 indeed. I bought them largely due to this feature. It makes it very practical to construct all sorts of dividers easily.

I'm not sure of the power rating for these but the seller states that they are all rated for 35kV. While for the 1.5Gohm that may be true (the dissipated power would be 0.9W), I seriously doubt it for the 2.2Mohm as that would make a resistor dissipate 560W. I however guess a realistic rating would be 20-30W maximum each.

I don't plan to make a power load out of them. For that purpose I have a whole bunch of 100k/10W and 20k/20W resistors on the way.
I plan to put 100 of the 100k resistors in series for a 10M/1000W load, with a midpoint tap (so it can be used as 10M/1000W end-to-end or 5M/500W one-half or with the 2 sections in parallel for 2.5M/1000W) and 100 of the 20k in series for a 2M/2000W load. These would allow me to test the PS with various loads from 5 to 25mA at 50kV.

I actually have 29 of the Russian 2.2M resistors. I guess I could make a power load out of them, if I connected 25 of them in a 5x5 series/parallel grid; if my guess of 30W for each is correct then the load would be 2.2M/750W max, so it could be used at up to 40kV/18mA or so. It wouldn't be useful for testing at 50kV though. Perhaps the power dissipated could be increased by submerging in oil, but I'm not sure how would the lacquer they're covered in resist the mineral oil.
There _is_ madness to my method.

User avatar
Richard Hull
Moderator
Posts: 11473
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Criticize my HV supply design

Post by Richard Hull » Fri Dec 22, 2017 4:46 pm

I doubt the resistors are more than 5 watt unless they are ceramic. The question is....Are they composite or spiral wound resist under that coating? I am fairly sure they are not wire wound.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 11:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Criticize my HV supply design

Post by Silviu Tamasdan » Fri Dec 22, 2017 8:57 pm

You can't tell how they are made, the red coat is very thick and you can't guess what's under it. You can't peek inside either because both ends are covered with metal caps.

I changed the primary wire to 60x24AWG wire, will support up to 30A and increases the usable frequency to above 60kHz should I decide to try it. I made 90cm of it, picture below. The end loops will be cleaned of enamel (molten aspirin works great for that, hope tomorrow is a warm day because I have to do it outside, aspirin releases very irritating fumes) and soldered to flat 0.7mm copper terminals. I will also place it inside a piece of PVC-based heat-shrink tubing and after the primary is wound that will get shrunk in place.
Attachments
20171222_094321.jpg
There _is_ madness to my method.

Rex Allers
Posts: 364
Joined: Sun Dec 30, 2012 8:39 am
Real name:
Location: San Jose CA

Re: Criticize my HV supply design

Post by Rex Allers » Sat Dec 23, 2017 11:04 am

A little bit more on those Russian resistors.

I forgot to mention that the length on the ones I have is not real consistent. If you look at the thin bar in the picture I posted earlier, it's a little wavy and there is a bit of a gap between the end of one and the bar.

Per Richard's question, I looked at the ones I have and there is a small imprint of the traces visible in the outer coating. I can see that the resistor is a helix pattern. The width of the resistive stripes is about .055" and the spacing is about .115". The length of the resistor in between the end caps is about 5".

They have a nice shiny brown surface that looked to me like ceramic, but looking closely I see small amounts of the brown on the metal end caps, so maybe more likely something like epoxy paint.

I also have some EBG 200 Mohm resistors that I found specs for. They are rated at 11.7 watts. Those are smaller in diameter and shorter. Just roughly I'd estimate 2/3 the size of the Russian ones. So my best guess for the Russian ones would be about 20 W.
Rex Allers

Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 11:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Criticize my HV supply design

Post by Silviu Tamasdan » Sat Dec 23, 2017 12:29 pm

I asked the Russian seller about power rating. He replied that the official power rating of these resistors is 5W. So definitely my 2.2M ones wouldn't work at more than 3kV each. Your 200M should be able to take 30kV.

At this point I only plan to use them for a HV divider. 3*1.5G +2*2.2M in series i.e. 4.5G + 4.4M. Not a perfect 1000:1 but close (1024:1). Or if I'm able to select 2 which add up to 4.5M that would give 1001:1
There _is_ madness to my method.

Post Reply