High voltage PSU - Page 1
Forum section: electronics / projects

author: Maister
author_rank: Contributor
timestamp: May 04, 2013, 05:29:13 pm
content: Hello ...I'm working on a high voltage PSU within my bachelor thesis.These are the specs:12V in20kV out100µA max output currentThe unit has to be able to do both, source and sink current, because it will be used with other supplies within an ion-mobility spectrometer.I want to use a resonant switch mode converter with a transformer and a voltage doubler / cascade and the end.The thing is: when current is sunken, output voltage will increase, because the output capacitors are charging up.So here is my question:How can I include the ability to sink current without changing the output voltage ?I did already think about a chopper circuit with resistors and MOSFET to drain the current off the output, but there are no regular MOSFETs that can handle 20kV when fully closed.Hopefully someone can help me on that issue.Patrick

author: c4757p
author_rank: Super Contributor
timestamp: May 04, 2013, 05:35:39 pm
content: To use the MOSFET idea, you could use a cascode of thirty or so 1kV MOSFETs, but that's going to be quite large. Perhaps being large isn't unexpected though, being a 20 kV power supply. You're going to need the isolation distance anyway.

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2013, 05:37:38 pm
content: I'm not sure what the highest voltage regulating element might be that is practically available, but a typical possibility would be to put many small voltages in series. So instead of trying to sink current from a single capacitor charged to 20 kV, can you arrange a scheme where you sink current from twenty 1 kV capacitors in series using twenty current sink elements? It may be possible to find something (a MOSFET) that will withstand 1 kV?(Oh, double post after c4757p)

author: G7PSK
author_rank: Super Contributor
timestamp: May 04, 2013, 05:47:53 pm
content: Thyratron is the first thing that springs to mind and the only thing my limited knowledge comes up with for 20 Kv and above, not sure if it could be pulsed fast enough.

author: Neilm
author_rank: Super Contributor
timestamp: May 04, 2013, 06:03:47 pm
content: Depends how quick you want it to discharge and how efficient you want the circuit to be. One option would be just to have some resistors connected all the time and use them to discharge. Increase the output power of the HV circuit to adjust for the extra load.Adding 20 or so 1kV transistors sounds easy, but correctly biasing the gates to drive them would be interesting.Depending on how much you want to spend I know that there are some "solid state relays" that allow pulse discharge options. They are rated for amps of current at these sorts of voltages. Unfortunately, I can't remember the name of the company I saw that makes them. (German I think) Used "Mosfet" switching with a minimum on time in microseconds.Neil

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 04, 2013, 06:32:38 pm
content: It sound like you need a what we call a stepper. We design steppers for the Space community (NASA). However, our steppers only go up to 15 KV. Once you to up that high of a voltage you start having corona issues and partial discharge where ion start to degrade your insulation. We see this effect even at 10KV.What you need is a bulk supply of 20 KV. You then need to use opto isolators. In the Space community we use the HV801s. I think they are rated up to 8KV. So we use four in series. The top two are used to pull up to 15KV and the bottom two are used to discharge to ground or to a negative voltage if desired. However, the HV801 cost about 3 thousand each and you would need to add more in series to meet derating. Also to meet derating and increase reliability, you would need to add capacitor and high value resistor across each HV801 to insure the voltage across each HV801 does not exceed its rated value.Second source of opto is available from VMI diode.. They have an opto that can go up to 20KV. It is not space rated so I think it is much more affortable than the HV801. The only drawback is that its CTR (current transfer ratio is low).I hope helpsRobert Bolanos

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2013, 09:46:53 pm
content: Quote from: Neilm on May 04, 2013, 06:03:47 pmI think this is a really interesting idea.Instead of having an output capacitor bank, have an output resistor bank. Arrange the total resistance value so that when it is sinking the maximum expected current the voltage drop is still less than 20 kV. Then place a regulated supply across the resistor bank so that it supplies enough additional current to regulate the voltage to 20 kV. If the external load is drawing current the power supply will need to supply both the external load and also the internal resistor bank. So it will have to supply more than 100 µA. Maybe 200 µA?It does not look as if the power requirements are too large. For instance, 100 µA at 20 kV is only 2 W dissipated in the resistor bank.

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2013, 10:26:48 pm
content: I wonder how well a car ignition coil would work for this. They do 12V to ~ 10-20kV and you wouldn't need to mess around with a doubler / cascade.They're not intended to work continuously but you only need 2W and they're rated for like 50-100W.Shouldnt be a problem.If the response time for sinking current isn't very critical you could expand on the resistor bank idea and use a high voltage potentiometer mechanically connected to a servo motor.This way you could monitor the voltage/current on the high voltage side and adjust the sink current in realtime.Finding a pot that can handle 20kV maybe tricky though, I'd probably build my own and have it under oil.

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2013, 10:55:05 pm
content: Any IGBTs that work at those voltages wont be cheap unfortunately.

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2013, 11:35:02 pm
content: If using the output resistor idea you wouldn't need to switch at high voltages. You could use a resistor divider for the voltage sense in the feedback loop and then do the switching and regulation on the low side of the transformer.Depending on the required speed of regulation, it might even be possible to use an electrostatic machine in an electro-mechanical system and achieve voltage regulation by varying the motor speed?

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 05, 2013, 12:50:31 am
content: Go to the Voltage multiplier inc website and look for the OC250 optocoupler. This is rated to 25kv. You can use one to source and the other to sink. This should work for you if you want to sink and source high voltage.Rb

author: Maister
author_rank: Contributor
timestamp: May 08, 2013, 10:32:13 am
content: Thank you all a lot, guys!I also had the idea with Transistors in series, already, but as Neilm said it would be a pain to bias the gates correctly. I would have to use gate drive transformers for each MOSFET, and they would have to withstand the high voltage to ground as well. Or some kind of tricky bootstrap circuit, timed perfectly for not having one MOSFET delayed and getting the full voltage. Sounds to complicated to me.Static resistors at the output sounds pretty well, but the idea with an opto device is geat! Especially the optocoupler that rbola35618 posted, which seems to be great for that purpose.@Psi: I know it would be easier with a ready-to-go transformer like flyback from CRTV, or an ignition coil, but the task is to make my own one.So that you all a lot!You will hear from me how the project will turn out.

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 08, 2013, 04:42:45 pm
content: Hi Maister,If you need help with the opto design (VMI opto), you can email me at rbola35618@aol.com.Robert

author: SeanB
author_rank: Super Contributor
timestamp: May 08, 2013, 05:55:45 pm
content: There are some high voltage FET drivers that use a photocell stack and a discharge resistor to charge the gat of a high side MOSFET without any galvanic connection, just using light to drive it. You might be able to drive a series stack of the highest voltage MOSFET's you can get using a few of them, though you might have to do some interesting corrections for temperature.

author: babysitter
author_rank: Frequent Contributor
timestamp: May 08, 2013, 06:03:07 pm
content: May I suggest a large stack of cheap 400V photoresistors ?

author: Maister
author_rank: Contributor
timestamp: July 10, 2013, 01:28:45 pm
content: Hello guys..I'm still designing the resonant converter and i am stuck at the point of designing the transformer.I made a HF-transformer with ferrite core (ETD39) and i made sure to meet all the things like winding ratio, minimal turns on primary side, voltage isolation with Teflon between layers and i even grouted the whole transformer in epoxy resin.As i first tested it it with a full MOSFET bridge the signal coming out was looking good, but it lasted only one minute or so and got bad. One could hear a sizzling noise coming out of it. Inductance on both, primary and secondary side, is now much lower than before and winding ratio is much lower as well (calculated with sqr(L1/L2) ). However, DC resistance of secondary remains the same as before.Does anyone has a suggestion on what could have happend ?Patrick

author: WarSim
author_rank: Frequent Contributor
timestamp: July 10, 2013, 02:21:31 pm
content: Sound like not enough thermal dissipation and the insulation started to breakdown. Varnish, laptop, ptfe what ever you used.Enough to effect AC but not enough to alter low level DC yet.Can you use a thicker primary insulation to reduce primary winding density?Can you use a higher gauge primary to conduct heat out?Can you use silver primary conduction.Do you have an IR imager to help determine if core heating is a problem.Not knowing more details it is very hard to pin down your specific problem.Hopefully you don't have to use the toxic pottings.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 10, 2013, 03:18:11 pm
content: High voltage transformers for an application like this needs to be vacuum impregnated with a suitable, HV capable potting compound. The vacuum procedure is needed to ensure that absolutely no air bubbles is left trapped in between the copper windings, meaning that the whole thing has to literally be one solid block of potting compound, insulating material and copper wire.What you experienced was corona discharges forming in the trapped air bubbles due to the extreme electric field gradients near the copper wire. The corona discharges quickly nibbles away at the copper wire insulation, causing cavities and cracks to form. This in turn causes the wire insulation to arc over at voltages way below their rated operating voltage, yet everything seems fine when tested at low DC voltages, as you experience.Corona discharge due to trapped air pockets in HV applications has been the bane of many a piece of prototype equipment. Given time the corona discharges will for instance eat through even quite substantial insulating membranes made from Teflon and similar materials.The actual, mechanical design of the transformer has to be designed, such that you can guarantee that no air pockets remains within after the vacuum procedure. This is not always easy to do...

author: WarSim
author_rank: Frequent Contributor
timestamp: July 10, 2013, 03:32:30 pm
content: Yes that is the best way, but not real accessible to allot of people.Need to be old school when modern methods are not available.I see now that you said 20kv not 2kv. OopsUsing the old school methods in one transformer it is going to be huge and low efficiency just to compensate for all the issues.I have seen this done in multiple smaller transformer units but that adds a lot of complexity.I agree for that much input power it would be best and safer to have it made.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 10, 2013, 03:58:36 pm
content: Quote from: WarSim on July 10, 2013, 03:32:30 pmWill have to agree here. Admittedly I *can* think of a brute force solution to this problem, which at first blush one should be able to build on the proverbial kitchen desktop. But it would be neither pretty nor elegant, so I hesitate to even suggest it.If the OP's project is supposed to be a demonstration for a commercial product of sorts, then my idea isn't workable at all. Personally I would only consider doing this myself, if absolutely no commercial solution exist.

author: cosmos
author_rank: Regular Contributor
timestamp: July 10, 2013, 04:40:08 pm
content: Generating the 20kV sounds like a job well suited for a TV HV transformer/tripler combo, finding one in a dumpster should be possible.One part contains all the HV parts you need nicely potted with a 25kV capable cable exiting.Driving the low voltage side of the transformer takes a 1000V transistor or so, maybe less if you find a part from a 12V TV.Sinking power?Long line of resistors sinking about 100uA (resistors doubles as voltage readout)Feed 100uA to keep the voltage steady, 200uA to source 100uA and nothing to sink 100uA. (if memory serves the TV tripler where rated at a millimap or so so delivering 200uA should be easy)Use an adjustable voltage for driving the low voltage side of the transformer to control the output current/voltage?

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 10, 2013, 05:37:10 pm
content: Maister,What are the turns ratio of the transformer. Generally, you should keep the secondary voltage below 500 volts. This will keep corona issues down. then you use the a series of voltage multiplier like the one that David explained in his fundamental Fridays.Use the Cockcroft-Watson multiplier. This is the best way to generate a high voltage and most reliable way to generate high voltage.Robert

author: Maister
author_rank: Contributor
timestamp: July 10, 2013, 07:10:15 pm
content: @WarSim: I used ptfe between they layers and as epoxy i used some like thishttp://de.farnell.com/electrolube/eer1448rp250g/epoxy-resin-low-viscosity-250g/dp/1764826which is toxic.Idea sounds plausible that the insulation got too hot.@ElectroIrradiator: Sounds pretty plausible as well that little discharges damaged the insulation during operation.@cosmos: I know that it would be easy to use one of these CRTV potted transformers, but it is my task to build my own and be smaller.@Robert: I was going to use this multiplier anyway.My Plan was to have a Transformer with 10V going in at 8 primary windings, giving me 3400V at secondary with about 2700 turns on secondary site. Then having a three stage Cockcroft-Watson multiplier so give me my disired 20kVSo the Transformer only has to insulate 3400volts across the secondary side and between primary and secondary site, right ?I read a doctoral thesis about a resonant PSU which also had to deliver 20kV and there was the same method used, without having the transformer even potted. So it should work even without potting it.Maybe the major thing was (what i did not mention before) that i drove the core heavily into saturation, because i used a too high voltage at my bridge on the primary side. With the core and primary copper dissipating a lot of heat due to a very high current. So maybe the insulation died because of too high temperature. Or is it possible to kill the core when it gets heavily into saturation?

author: IanB
author_rank: Super Contributor
timestamp: July 10, 2013, 07:53:36 pm
content: If you saturate the core your inductor will start to look like a resistor. So those 8 turns in the primary will become essentially a short circuit. Nothing will limit the peak current anymore except the resistance of the wire.Did you monitor the temperature of your transformer during the tests at all? Did it seem like it got hot?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 10, 2013, 08:25:27 pm
content: Quote from: Maister on July 10, 2013, 07:10:15 pmThis won't work.- First, your transformer will only be 100% efficient in its transformation ratio when there is no load attached.- Secondly, your voltage step-up in your case is 6, not 3, as 20000 / 3400 ~ 6.Your required power is no longer peanut power, which is usually what the C-W voltage multiplier is used for. If the idle current is 150uA, allowing your PSU to sink 100uA while the active regulator still has 50uA to work with, then the idle power will be 3W. You really ought to develop some form of SPICE model for the voltage multiplier, to check if that is even remotely possible, given realistic values for the internal resistance of the transformer/oscillator. A voltage tripler, OK, those can be used for 'power applications', though with caution. Times six: I wouldn't bet on it.Depending on the load and the transformer resistances, then there comes a point, where further voltage multiplier stages won't actually increase the output voltage to any noticeable degree.It is a very iffy proposition to draw any kind of power through a 6 times voltage step up, even if you happen to use the more efficient full wave voltage doubler. The C-W voltage multiplier is simple, yet not the best way of doing this.- Thirdly, then you will almost certainly need some form of filtering of your output voltage at 20KV, or have to live with a very high ripple voltage. All your calculations are based on peak voltages, not RMS, which means the RMS output voltage will be 20KV / sqrt(2) *if* your voltage multiplier and transformer both are 100% efficient. [Really crude estimate.] Which they won't be. You probably cannot get around needing at least a CRC filter at 20KV, which will allow a high ripple voltage across the first reservoir capacitor, yet will still be able to deliver a fairly clean DC output.So the actually required no-load peak output voltage from the multiplier depends on the losses due to finite transformer/oscillator impedance, the finite capacitor sizes in the voltage multiplier and finally the voltage drop across the CRC filter resistor. All these while taking your worst case 250uA load into account.Quote from: Maister on July 10, 2013, 07:10:15 pmCorona discharge depends on the local electric field, not on the voltages involved. So whether it would work depends on the actual, physical design of the transformer. Do you have a reference to the article you mention?And yes, you can damage a ferrite core's magnetic properties by overheating it.

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 10, 2013, 09:30:07 pm
content: With a turn ratio of 2700 in the secondary and 8 on the primary, you will have a turn ratio of 337. You will have a lot of leakage inductance that will give you problems unless you know to make a resonant flyback.Can you measure the leakage inductance. Check out this video where I demonstrate how to measure leakage inductance.Watch at 14:20 of video if not the entire video.3000 volts is way too much voltage on the secondary. Corona will eventually destroy the insulation of the transformer. One of our competitor only goes up to 1000V maximum and pots the transformer. Lowering the turn ration will decrease your leakage inductance.Robert BolanosRB

author: IanB
author_rank: Super Contributor
timestamp: July 10, 2013, 09:36:47 pm
content: This may seem like a crazy suggestion (indeed it may even be a crazy suggestion), but have you considered using an electrostatic machine to generate your high voltage instead of an electronic solution? You will obtain kilovolts with ease, guaranteed smooth DC potentials, and as much current as you need just by making the machine big enough.The downside, of course, is that it won't be compact or quiet or easily transportable.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 11, 2013, 01:51:44 am
content: I am thinking crux of the problem is the understanding that with higher power more factors become important.The rules of thumb used all the time for lower powers disregard some factors because their effect is considered inconsequential.Just like in the video supplied by RB. The person in the video is going to be limited by how much power he can work with because he is also disregarding too many factors that will cause problems at higher power levels you are trying to achieve,Before people jump on soap boxes I am using the term power because using 1000A at 1v has increased factors just like 1000v at 1A.Higher power harder it is, but not impossible.In my other post I was trying to remind people that although the new tech like vapour disposition potting help allot in efficiency and size it is not the only solution. It is a fact that there are transformers that are rated over 1000kW that are not potted.Once I saw that you are asking for more than 2kW, even more factors now become important. And the 500W problems get even bigger.You need to make a real requirements list. Voltage output and power. surge current and power. And remember to specify peek rms, avg etc.Then we can figure out the many factors you need to consider and the effects it will have on your design.For an extreme example.An un-potted 3000kW high voltage transformer is about the size of a freezer.The potted version is about the size of the freezer compartment of a fridge.

author: IanB
author_rank: Super Contributor
timestamp: July 11, 2013, 02:02:38 am
content: Quote from: WarSim on July 11, 2013, 01:51:44 amWait a minute now, the first post says 20 kV output, maximum current 100 µA. On my calculator that is 2 W, not 2 kW! That's less by three orders of magnitude. You don't need a lot of power for electrophoresis, which is the application here.

author: IanB
author_rank: Super Contributor
timestamp: July 11, 2013, 02:08:47 am
content: The application needs a stable high voltage at negligible current to produce an electric field. Something along the lines of an air purifying ion generator would probably be the kind of circuit that is needed. It may even be cheaper to buy one of those and and reverse engineer it or cannibalize it.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 11, 2013, 02:18:06 am
content: Hmmm yes it does.Well that makes things simpler.Ok now the question is what is your multiplier going to use?I find the 100uA requirement extremely small.You are going to have more losses than output.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 11, 2013, 02:39:06 am
content: So to start losses areLeakageHigh ratio requiring primary distribution and the losses in efficiency, and no ordinary winder can do the job. Hand winging an option?About 300mA primary plus safety 500mA shouldn't require flat core but it is on the edge.I don't know what core insulator is required for the primary at this power level at the moment.A mp core should work depending on frequency. Letting a high ratio transformer core resonate will kill it.After having the heat problem due to saturation or maybe super saturation it is likely damaged anyway.I am sure I forgot some things.Allot depends on you energizer, AC, pulse.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 11, 2013, 02:45:22 am
content: ElectroIrradiator, I am not trying to cut you off, you know your stuff.I am just trying to translate all the can'ts into cans, so the author can make an informed decision of make or buy.

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 11, 2013, 02:50:46 am
content: Maister,Steve Battel is our competitor. He posted a video on High Voltage engineering on his website. He discusses about resonant flyback starting at 4.21 and mentions a little bit about our transformer (Southwest Research Institute) at 4:26. But I would view the entire 8 hours because he explains his methodology in space rated design work. This video is a comprehensive on High Voltage engineering.http://mediaex-server.larc.nasa.gov/Academy/SilverlightPlayer/Default.aspx?peid=a2bdec72cb224785a122b27b630e543e1dRobert

author: WarSim
author_rank: Frequent Contributor
timestamp: July 11, 2013, 03:23:18 am
content: I just watched the linked video.All I wanted to say is in the video he skipped over a few things that will cause the thread authors transformer to self destruct.I think you mentioned he pots over 1000v I can understand why he choses that option.For example in this transformer level wind is not a possibility due to the high ration a output voltage.So the options are gas displacement potted core or distributed primary and varnish wire is out because of distributed primary.Because I think this is DIY Gas displacement potted core is not an option.With extreme care a thicker varnish wire could be used but wrap cross linking is a danger due to high ratio.Primary insulator will need to be a high temp/high tensile, maybe PTFE and more that 8 wraps, meaning more secondaries.Depending on the math I would guess at 12, 4 zones of 3.And this will make the transformer at least 4 times the size as a potted transformer.Once the added core cost exceeds the price of a custom transformer the custom transformer is the better choice.I just don't like giving up before it is a fact.I have seen many applications where up to 12000v un-potted transformers where cheeper than the potted versions. Not to mention DIY saves the winding charges.What we have done is figure out all the math and then ask a transformer company to check the numbers with their computer.Of course it will be advisable to buy their core, as compensation.

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 11, 2013, 04:05:53 am
content: Maister,To design a resonant flyback, you first need to determine what will be your secondary voltage. At where I work, we generally do not go over 500 volts at the secondary. The voltages that we design are from 7.5 KV up to 15KV. We use the Cockcraft-Walton multiplier. For example, if I am designing a 10KV supply, then I would use 20 stages of the multiplier. Why is this important. Because if you are designing a resonant flyback, you have to take into account the capacitance that the secondary transformer sees from the multiplier and load. The capacitance of the high voltage diodes play a big role in the resonant frequency.The rule of thumb to calculate the capacitance at the secondary of the transformer is (diode_capacitance*numbers of multiplier stages= secondary capacitance. You have to calculate before you do anything else. Then you take that capacitance and translate it to the primary by taking the turn ratio (Nsec/Npri)^2*secondary capacitance=primarycapacitance. Now you are at the primary and need to take into account the capacitance of the MOSFET. The total capacitance will now resonate with the primary inductance. Once you have the resonant frequency, you need to set the switching frequency which needs to be set to a longer time period. I think I may have a Mathcad script that simplifies the design. I will see if I can find it and maybe post it here.Robert

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2013, 04:14:11 am
content: If you're OK with getting some custom glass made (a neon sign shop might be able to do it), it's possible to make a high voltage switch that is merely two electrodes sealed in a vacuum tube, one of which (negative side) is made of a photoelectric material. Simply illuminate that with a LED to close the switch.

author: Maister
author_rank: Contributor
timestamp: July 11, 2013, 07:06:32 am
content: Thank you all for so many answers!--> Cockcraft-Walton multiplier output voltage is input pk voltage * 2 * #stages. So I think I have done that correctly. 3 Stages will give me times 6.--> Yes, my output power will only be 2W! Not 2kW. The doctoral thesis, in which i use to read in for this, is written in german (ISBN: 978-3-941298-36-1, title: "Resonanzwandler für die Erzeugung hoher Gleichspannungen" author: "Martin Rentzsch"). He is getting 20kV with a 3 stage Cockcraft-Walton multiplier as well and an output power of 800W! However, his ripple is 200V pk-pkMy expected current will only be 10 uA at 20kV, because it is only used to accelerate ions in an e-field. I chose 100uA as max output current to be on save side. So it is 2W output power.--> I wound my primary quintofilar (if this word existsso 5 wires in parallel per winding) in order to fill the whole lengh of the core to have a good coupling and less leakage inductance.--> Of course I will watch the videos and I will measure the leakage inductance at the new transformer (as i wrote, the old one has already shorted turns or something going on, which decreases the inductance drastically).--> I did not want to design a resonant flayback, because I read that you will not be able to set your output voltage, but just have a fixed voltage.I wanted to do it like that, with only difference, that i am using a full bridge:--> I will also think about having just one of these ion generators--> Let's say I will get away with 500 V at the output of the transformer, so input of cascade. I will need 20 stages, because 500V*2*20 = 20,000VIs it possible to do that? With so many stages u will get nothing with load at the output, like Dave demonstrated in his video. Or am I gettig that wrong ?Patrick

author: WarSim
author_rank: Frequent Contributor
timestamp: July 11, 2013, 07:51:29 am
content: The schematic you supplied show a resonant input (I forget the technical name) which will help with transfer to the primary. I hope the paper gave the formula for selecting the two balancing caps. If selected correctly you can expect AC like input efficiency.This will remove allot of the requirements I mentioned before.I read they your primary is a pentuplit this is the next best thing to flat wire.I am still concerned about your not zoning your primary, but I haven't done any math to say for sure it is required. One thing I heard help with this type is segmenting the primary, but never used it personally. Wrap with six primaries the remove one for a uniform gap between the primary sets. This will indirectly help a little with the likely corona issueOf you are going to wind it traditionally be wary of core heating.Make sure you couple your secondary before you go anywhere near you intended output, or you may have the same problem as the first time. This will also indirectly help a little with the likely corona issue.You will likely need to wrap the core for protection and gaping. Loss of coupling due to secondary miss match is a hazard with any high ration transformers.Hope this transformer is a success.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 11, 2013, 10:05:07 am
content: Quote from: Maister on July 11, 2013, 07:06:32 amMy point was that - by whatever name - you are multiplying your transformer voltage 6 times, which makes the voltage multiplier extremely dependent on the load and generator/transformer impedances.Quote from: Maister on July 11, 2013, 07:06:32 amNo easy yes or no answer to that. What we really need are the exact specifications of your assignment, whether you had to determine these yourself, or if they have been given to you.I have never worked with an IMS myself, only 'real' particle/ion accelerators.However, my suspicion is that they share a common trait in that the accelerator and control voltages can frequently behystericallycritical. For instance your beam/drift current may only be 10 uA, yet what happens when you introduce the ions to be measured at the start of a new measurement? Does the load current from the PSU suddenly jump, causing the 20KV voltage to sag? If it does, is that a problem? Do you wish to separate Iron from Lithium in the drift line, or different isotopes of Iron? The requirements demanded of the PSU would seem to be hugely different, depending on the exact use case.If your PSU is just needed for setting up a new experiment, just so you can get to the 'real' work of your thesis, then buy or borrow a commercial module, which does what you want. You are at one of the major universities in Germany. Ask around, bring cold beer.If you have been given this assignment by a professor of physics, who didn't want to pay the cost of a proper replacement PSU for an useful piece of machinery, then you may have a problem. He might not know much about electronics, and perhaps didn't like the price tag of a commercial PSU, which has the proper specs to run the machine with the required degree of accuracy. Hey, let's give this job to one of the EE students, how hard can it be? (Yep, this happens).if this is intended to be (a demonstration for) part of a commercial product, then you must know the exact specifications before you can even start. Physical size limits, rough idea of final cost, power consumption, specs for ripple, output voltage, noise, load current ranges, allowed overshoot/sag, drift, etc. For a research machine many of these specs may very well be "don't care", yet even knowing that much would be quite useful. As an EE your job will frequently include attempting to wring the product specifications out of your customers, who may not even know exactly what they need.In the last case, once you know what you are aiming for, then the next step could be an afternoon spent in the company of LTSpice. This would allow you to get a rough idea of what you are up against, and what for instance your transformer/oscillator needs to be capable of with respect to output current and more. Simulating a rough circuit is not a substitute for a proper analysis, yet it could quickly answer many of your question. For me this step also tend to bring much enlightenment, as it gives me an opportunity to find out where my theoretical knowledge might need refreshing.

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 11, 2013, 01:42:37 pm
content: This is not a resonant flyback. It is more of a LCR resonator. Fortuanatly, I designed something very similar. I would suggest to move Cs in front to the left side of Lr. This will make probing the primany side of the transformer easier. If I recall, there will be two resonant frequencies. The first one will be at a lower frequency and it is due to the primary leakage inductance resonating with the total primary inductanace. You must must damp this first frequency. That is the purpose of R284 and C233. The main resonance frequency will be the Total primary inductance with the parallel combination of the primary inductance in parallel with the series inductor. In my schematic it will be (L parallel to L1). You will still have to account for the capacitance of the Cockcroft diode and translate that capacitance to the primary and add that to C1 in my schematic.You will end up with a nice sinewave and will be very efficient. In this example, it was used as a RF generator to generate a 300 vpp sinewave close to the 300KHz range. Howver, I don't see why it can be modified to generate high voltage.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 11, 2013, 01:44:01 pm
content: Here are some early simulations

author: Marco
author_rank: Super Contributor
timestamp: July 11, 2013, 08:20:42 pm
content: Quote from: Maister on May 04, 2013, 05:29:13 pmAs was said earlier, if 100 uA sink ability is enough you can just make a 200 uA power supply and sink 100 uA through a resistor.For a greater sink ability I'd suggest a string of avalanche diodes in parallel with the load (also you can add a current sense resistor at the bottom and use it's current to regulate the power supply). Won't exactly give a super steady voltage, but you can sink a lot of power.

author: TerraHertz
author_rank: Super Contributor
timestamp: July 12, 2013, 02:53:43 am
content: To sense the output voltage for feedback control, you already need a resistor to provide a sense current. As you show in your diagram. So (as others have suggested) simply make that resistor sink 100uA and have your supply able to provide 200uA. Total supply power, 4W.I've a schematic of a Spellman 200KV variable supply somewhere, if you want to see how they did it.Can also post photos of the insides of that supply, if you want to see how it's built.

author: edavid
author_rank: Super Contributor
timestamp: July 12, 2013, 10:29:43 pm
content: Quote from: rbola35618 on July 10, 2013, 09:30:07 pmI don't understand this at all. Obviously there are many, many transformers out there with higher secondary voltages, that don't get destroyed by corona. Most scopes use about 3kV, neon sign transformers 5kV up, CRT TVs 6-7kV, lab power supplies 5-20kV, etc. Where does your 500V or 1000V limit come from? Is it something specific to your applications? I don't see why it's relevant to the OP's situation.

author: edavid
author_rank: Super Contributor
timestamp: July 12, 2013, 10:37:18 pm
content: Quote from: cosmos on July 10, 2013, 04:40:08 pmThis is a really good solution. OP, why do you think you can make something smaller? The size is determined by the voltage and current.

author: rbola35618
author_rank: Frequent Contributor
timestamp: July 12, 2013, 10:46:24 pm
content: If you do not pot the transformer you will start to have corona which ionizes the air and will eventually attack the insulation. If you pot the transformer, the you need to do it in a vacuumed chamer to insure you don't pockets of air where the trap air with again start to ionize and start to attack the transformer. Once it is properly potted, then you can go to much highervoltages but since most people don't have a vacume chamber, then it is recommented that you use a lower secondary voltage. How long will it take to damage the transformer, I don't know. It is better to lower the stresses on the transformer and thenmake the the difference using multipliers. That has benn my experience.Robert

author: Odysseus
author_rank: Regular Contributor
timestamp: July 13, 2013, 04:42:55 am
content: I've been looking into building a HV DC supply similar to your needs, aside from active output regulation because I just want something to play around with.The design I've settled on is relatively efficient, simple, and doesn't use a transformer at all. Instead, it just uses the resonant rise in an LC tank circuit to reach a ~500V amplitude, ~100KHz output sine wave which drives a chain of voltage multipliers made of relatively cheap and common 1nF, 1KV ceramic caps and UF4007 diodes. The screen shot below should help explain the circuit more clearly.What isn't shown in the schematic is the high-side drive circuitry, and the fact that I'm currently using 10 multiplier stages, not just one. To drive the fets I'm actually using a half-bridge mosfet driver chip between the comparator and mosfets. To be precise, It's an NCP5106 although it may be easier to use a chip with a single input pin, like the NCP5104. The comparator I'm using is an LM319, which has a respone time of ~100ns. Anything reasonably fast will work. And don't quote me on the proper polarity of the comparator, I just use the one that worksI've had good results with it, but watch out for losses in the LC tank, which will reduce it's Q and output voltage. Just make sure that the tank inductor is designed for high frequency/smps use and a polypropylene film or C0G/NP0 tank capacitor. The caps I'm using in the multiplier are cheaper X7R ceramics, which are too lossy for to be used in the tank. When I tried using one in the tank, the Q was much worse and the capacitor was too hot to touch.Also, for the clamp diode, I'm actually using two 300V TVS diodes in series, instead of the single 510V one shown in the schematic, because that's all I have on hand. No problems with failure yet.And it may not be obvious, but there is actually very little power dissipated in the clamp diode. It only takes a small amount of power loss to dampen the high Q tank circuit.As far as regulation goes, it would be possible to use a PLL chip to dynamically set the phase offset between the drive signal and the sensed tank current, which would increase/decrease the tank's Q based on the output voltage. It would be like pushing somebody on a swing at slightly the wrong time, instead of harder or softer.Hope this wall of text helps.EDIT: And here are some pictures of the circuit on a breadboard lighting up a xenon flash tube. You'll notice I've actually got a full H-bridge built up, but I'm only using half of it. The current reading is from the 30V supply for the half-bridge. It rises to 120mA when unloaded. The majority of the power consumption is in the inductior. Despite being designed for this kind use it still gets quite hot.

author: Maister
author_rank: Contributor
timestamp: September 25, 2013, 03:37:00 pm
content: Thank you for all of your posts!I decided to use proper grade 2 enameled copper wire, which should isulate up to about 3.8kV which I bought recently. The wire used before was old stock and only grade 1. The insulation on that was very poor, I think.I lowered the output voltage of the transformer as well by adding more stages to the voltage multiplier.I hope this will prevent the windings from arcing again.The whole design with the resonant circuit is now scrapped because of the complexity of the design / the high part count / complex control methods that come along with this kind of circuit. On top of all I only want 3 watts out of the conrter, so I went for a design with only two low-side MOSFETs, which are applying the voltage to two primary windings. The primary windings are wound in opposite direction to get the AC on the output. (see schematic)So L1 and L2 are my primary windings.The issue i am getting with this setup (I am using my new transformer) is a huge ringing at the output. (see screenshot)Blue: output voltage at secondary with no load and a cheap 1:100 probe.Red: current in one the primary windings (scale is right, 2A per div, which is odd... it should only go up to 270mA linearly, because it is an inductor) (measured with a proper current probe from Tektronics).Yellow: logic signal for one of the FETs.Is this because of the capacitance of the secondary winding? What can I do to avoid the ringing?

author: Neilm
author_rank: Super Contributor
timestamp: September 25, 2013, 06:35:45 pm
content: My first thought is that it is a result of interwinding capacitance and the capacitance between the primaries and the secondaries. If your simulation does not show it, try putting a small capacitance (say 10pF) from the output to the input. Also try reducing the coupling factor for the inductors from 1 to say 0.95

author: IanB
author_rank: Super Contributor
timestamp: September 26, 2013, 02:40:48 am
content: You have sharp step changes in the square wave on the primary side. Step changes will induce ringing in any circuit that has any resonant properties. Is there a way you can soften the square wave so it has softer rising and falling edges, getting it closer to a sine wave or possibly a sawtooth wave?

author: Maister
author_rank: Contributor
timestamp: September 26, 2013, 07:53:44 am
content: Yes, I cant get the ringing in simulation as well, when I add some capacitors and reduce the coupling factor.I am experimenting with some higher-value gate resistors to slow down the rise time, but it gave me a huge current peak. Since my FET driving IC has a fixed dead-time, I realized that there is a time when both MOSFETs are turned on or half way turned on and since the primary windings are wound in opposite direction with same amount of turns, there is no inductance any more and the current is limited by nothing any more, except a little leakage inductance.I will try to slow down the rise time while not having them overlap now. I also used logic level MOSFETs and turned on their gates with 10V, which might also causes a quick rise time. I will change them to some IRF840 or so.

author: Maister
author_rank: Contributor
timestamp: September 26, 2013, 10:45:42 am
content: I changed the MOSFETs from IRL3803 to IRF830 and now it looks way better (see the screenshot as attachment).Gate resistor is 110Ohms to charge the gate and 55 Ohms to discharge it (with fast diode).So there is no ringing any more, but I can't explain the huge current peak, which remains. This peak does not vanish, when I use only one of the primary windings.By the way: I have a SB 340 Shottky diode across each MOSFET to compensate for the bad body diodes.

author: Maister
author_rank: Contributor
timestamp: September 26, 2013, 10:46:36 am
content: I forgot to attach the attachment

author: Neilm
author_rank: Super Contributor
timestamp: September 26, 2013, 06:13:01 pm
content: Have you looked at adding snubbers across the primary windings?

author: jimmc
author_rank: Frequent Contributor
timestamp: September 26, 2013, 06:44:00 pm
content: How about using vacuum rather than silicon?The PD500 (http://www.mif.pg.gda.pl/homepages/frank/sheets/010/p/PD500.pdf) or PD510 (http://www.mif.pg.gda.pl/homepages/frank/sheets/010/p/PD510.pdf) were designed as EHT shunt regulators at up to 25kV for colour TVs and will easily sink 0 - 1mA.Long obsolete of course but still available on ebay (http://www.ebay.co.uk/itm/1-PC-PD510-Philips-Shunt-Stabilizer-Triode-Tube-Valve-/220502123346) etc.Jim

author: Maister
author_rank: Contributor
timestamp: September 27, 2013, 05:11:40 pm
content: I measured inductance, leakage inductance and capacitance of the transformer, like rbola35618 showed in his video, which he postet on page 2.Capacitance turns out to be about 3uF and coupling factor is 0.997. With these values I am getting the same current peak in LTSpice, as in the real curciut. So that made it clear: such a high capacitance is charged when the MOSFET turns on and therefore the high initial current.Even with a very slow gate charging (I used 1kOhm gate resistance and added 10nF to the gate capacity) the current peak is still there.A snubber is a capacitor and a resistor in series, right ? That will increase the capacitance even more. Or am I wrong?Thanks jimmc, thats a great idea. But I am not allowed to use obsolete parts.

author: Maister
author_rank: Contributor
timestamp: October 03, 2013, 10:22:58 pm
content: I read about this current peak. It seems to be a common problem of transformers with a high with a high winding ratio like mine has (7 turns primary and 1470 secondary), because the winding capacity is very high.There are winding techniques to minimize the interwinding capacity, like back winding or z-winding. I combined both of them back winding and z-winding to build a new transformer and this one has also 1.8uF, which is better than 3uF, but still too high. The current peak occurs with the same height.Maybe I will put a transistor in a current-limiting configuration in series to cut the peak at some current and live with that loss.

author: rbola35618
author_rank: Frequent Contributor
timestamp: October 04, 2013, 01:41:58 am
content: You may be Abe to use the capacitance to your advantage and design a resonant flyback. That is what we do at work with high turn ratio transformers. You might to do a google and if there any design that you can use.

author: Neilm
author_rank: Super Contributor
timestamp: October 04, 2013, 08:19:54 am
content: Quote from: Maister on September 27, 2013, 05:11:40 pmThe most basic snubber would be a diode, cap and resistor. They are placed in parallel with the primary. When the voltage starts to rise, the diode conducts and charges the capacitor. The resistor is there to discharge the cap ready for the next one.There are several app notes on the web that show how to calculate the values.

author: Marco
author_rank: Super Contributor
timestamp: October 04, 2013, 03:27:21 pm
content: A snubber does him no good ... he doesn't have a flyback converter.

author: Maister
author_rank: Contributor
timestamp: January 23, 2014, 12:49:23 pm
content: Thank you all for your great comments and ideas!My Thesis is now done.I attached an image of the final circuit used in my design, just for your interest."Greinacher-Kaskade" is german for "Cockcroft–Walton generator"The design is able to deliver an output voltage between 1kV and 20 kV, adjustable. Input is 24V. All DC. Output ripple is about 8V pk-pk, measured @ 15kV output with a 100Meg load.Thank you all for your good advise.

author: oldway
author_rank: Super Contributor
timestamp: January 23, 2014, 02:51:07 pm
content: I did not read all the posts of this topic, but i think that what you are looking for is something who does allready exist.In color TV, we needed a stable 25KV voltage for feeding de CRT anode.This was done by shunt regulation of a flyback HV. (a flyback power supply is a current source.)Whe had than to sink part of the current in the shunt regulator.For this application, it has been developped a high voltage triode with high anode dissipation: that is the valve/tube : PD500.Specifications of HV triode PD500:Heater: 7.3V 0.3AAnode supply: 25KVMax anode power: 30WMax anode current: 1.6mABase: B9AUsing this triode as a shunt regulator, you can sink up to 1.6mA at 20kV, as needed.On the marked, there are a lot of high voltage power supplies (20kV or more), but they are not able to sink current.With one of those commercial HV supplies + a shunt regulator with PD500, you do what you wanted to do of a very simple and clever manner.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2014, 02:59:47 pm
content: I don't think this was mentioned before....... why not use an off-the-shelf or salvaged flyback transformer? Preferably from a portable TV, which will have a 12V primary. Or the kind with an accessible core, wind your own primary, and do the dumb 2N3055 driver (single or push-pull) that everyone does. Integral diode, fully potted construction, no worries about high voltage breakdown ever.As for sinking, an HV tube would do exactly that (over here, we had 6BK4C and etc., octals), though that would require an additional ~200V bias supply. Just as well draw enough current through the voltage sense resistor divider to account for it, and double the supply capacity. Another two watts, in the lab, who cares?Tim