Project 'HELLFIRE' (Single phase multi-purpose HF transformer driver) - Page 1
Forum section: electronics / projects

author: CM800
author_rank: Frequent Contributor
timestamp: September 07, 2016, 10:59:31 pm
content: Welcome toProject:HELLFIRE.Enjoy your stay.This has been a project I have been working on and off for some time now. I have a large X-ray transformer, and plans to wind some similarly large ones with some monstrous ferrite cores, a dear friend of mine also has a large number of such transformers he wants to operate and is dying for a Full-Bridge Driver. (high1voltage1rules)It is my intention to develop an off-line full bridge 'universal' driver for the higher power X-ray transformers. I intend to develop this as two sub projects.Project 1 (HELL): A Regulated Buck/Boost converter. (Possibly PFC?) to allow 0-600VDC output at up to 24KwThis will function similar to a solid state Variac, providing me with additional abilities to operate in constant voltage mode with a current limit control. The output is to be ‘clean’ DC (5-20V ripple max at full current / full voltage). This will not be an isolated unit, however I imagine generally, below 32A supply, I will be using my high power isolation transformer before this unit. Considering this is being used with 70kV X-ray transformers, a floating DC bus should really be the least of my concern.Main Concerns of Project 1 (for now):- Should I implement a PFC, how difficult will it be on this power scale? While not a 3 Phase PFC by any means the current load suggests it should be interleaved at least?Project 2 (FIRE): A full-bridge driver for high voltage, high current loads with an interface to the former project.This unit’s goal is to drive Unknown transformers with manual control. I intend to have the user control:- Frequency (At least 15kHz to 30kHz, I will build a separate unit for 30kHz -> 100kHz possibly)- Voltage (Controlled through a comms link to HELL)- Current Limit (Set the maximum current you want to allow your transformer to draw on a peak-by-peak basis)- Average Current Limit (Average of the maximum current, running average I assume, sample length would need to be worked out for optimal safety of the transformer)My main concerns for project 2 focus around (for now):- Unknown transformer inductance / resistance.- Sudden changes from no load to full load (Arcing duty) (and how to current limit with such an application.ComponentsI already have for this project consist of:- More then suitable IGBTs ( 600V, 600A | 600V, 300A | 1200V, 300A | 1700V, 800A )- Plenty of large capacitors (500V, 450V, all 2500uF++ )-AC/DC current transformer for prototypingTools:- 32A Isolation transformer- Multimeters- DS1104Z Oscilloscope- access to a 400MHz LeCroy if absolutely required at work.- Basic power supply- 80V / 20A SMPS variable supply- Suitably large Variac with basic safety E-Stop & Air Pressure foot switch system)Any input would be greatly appreciated, suggestions on implementation and building something ‘plug and play’ friendly to a degree. I know these are a significant undertaking. I have already had a pretty functional full-bridge up and running with just PWM / Variac control with good success (until the fuse blew on my 13A plug when I was drawing 32A from it…)I have made starts in some areas of Project 2 mostly, without touching Project 1.I would like advice on how best to managed this project and go about it in an organized way. While I want to do all the footwork (it is my project) Guidance on what areas / directions I should focus is what I really hope to benefit from you all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2016, 12:43:05 am
content: Unless you want nothing butHELLFIRE, first priority is protecting those transistors. Especially for more than 1kW power levels!Step 1. Read up on desat detection.Usually, you'll have an isolated gate driver* with an onboard latch, which disables its output when a desat event is detected (an input-low state resets the latch).*Find three reasons why isolation is important, even critical, to operation: (treat this as Step 2)Step 3. Come up with a control scheme. I suppose if you'll be doing diverse applications, you'll want a universal controller: H-bridge, on-off PWM, phase shift (always-on) PWM, and resonant (frequency tracking) capability. Or if not a full universal controller, then a set of option boards that can provide these functions.Step 4. Come up with an interface and safety scheme. The most basic is an on/off latch (with push-to-start and push-to-stop buttons/logic inputs), with feedback from whatever fault signals apply (the gate drive desat signals for one; inverter overcurrent, or output overvoltage, faults are excellent additions).Avoid the temptation to use an MCU as a substitute for critical analog circuitry. An FPGA/CPLD is marginal, and must be carefully programmed. An MCU is almost impossible to write good code for. Discrete logic is the easiest to build and the easiest to prove. Always use as few internal states as you can get away with. Two reasons:1. It's not an intractable problem to analyze all possible states. When you have only 4 bits of storage (say, a CD4043), you only have a modest 2^4 = 16 states, which can be enumerated with pen and paper!2. You have fewer (ideally zero) unexpected states. NEVER make the assumption: "oh, but that input parameter will never happen, so the CPU will never divide by zero..."Outside of the critical section, you can put as much buggy software as you like! Use an Arduino for front panel interface and communications, who cares! It's the high power core that needs to be right. One misstep at 20kW, and IGBTs grenade, sending shrapnel and arc flash everywhere.If nothing else, it is absolutely critical to use a desat protection circuit, and to do it in hardware. With this in place, you can literally through your entire collection of box wrenches at the machine, in operation; it will barely leave a scratch*, and the transistors will be safe for another day.*In the time it takes for the desat to kick in, current will build to a few kA, and the voltage will have dropped from whatever supply was (325V? 650V?) to a short circuit, causing a small spark. Which will leave a small pit or scorch on the wrench, but because it turns off within microseconds rather than exploding over tens of milliseconds, there's no arc flash, no extended vaporization of metals, no shrapnel ejected, and no transistors failed.Given that the short circuit current is nonetheless quite large, of course -- you need to design your metalwork for minimal inductance (between a given transistor and whatever's clamping its turn-off voltage: preferably a Vpeak or dV/dt snubber, or if not, then the opposing transistor's internal antiparallel diode), so that the transistor doesn't explodeanywayfrom overvoltage.I've designed and built these systems before (as you might guess). If you'd like more engineering assistance on this industrial project, I'm currently available during evening hours.Tim

author: CM800
author_rank: Frequent Contributor
timestamp: September 08, 2016, 07:23:43 am
content: Quote from: T3sl4co1l on September 08, 2016, 12:43:05 amI've heard of desat before. I haven't read up on it too much. (I've generally never seen people implement it for hobbiest projects such as teslacoils / xrt drivers.However the idea of IGBTs not blowing up in my face is something that pleases me very much.... Let's have a look.Quote from: T3sl4co1l on September 08, 2016, 12:43:05 am1. Massive common-mode voltages (I think that's the term?) the gate drive to the upper IGBT of your half-bridge is moving between 0V and 600V or even more as the negative rail.2. Logic safety. If your working on your control logic, ideally you want to be able to poke around with your scope and multi meter without accidentally shorting out a 600V 1000J + Mains line in your face.3. User safety. If you are having buttons on the unit, you don't want them following around with a HVDC potential to earth, live and neutral wire.4. Design safety. It's less easy to explode everything if you isolate all your sensing (Current, Voltage) and all your control (IGBTs) so that there is no chance of high currents flowing through your PCB and vaporizing hours of soldering itty bitty SMT components.My current gate drivers are 4A opto-isolated gate driver ICs (no gate resistor, the poor things)I've had some good success with them. Yesterday I took the PWM controlled drive to full voltage (380VDC bus) and full PWM, drawing 31A... I was surprised it was so low and didn't blow up in my face considering that the transformer is rated for 20kW @ 400V momentary current. I was at the right switching frequency. I'm curious why it didn't draw upwards of 60-80A.Quote from: T3sl4co1l on September 08, 2016, 12:43:05 amThis is an area I am painfully weak at. While I can do a good PWM control easily enough, things like phase shift and resonant are areas I'm not familiar with at all. I shall look to change this. Time to go datasheet / Appsnote digging.Quote from: T3sl4co1l on September 08, 2016, 12:43:05 amI'd imagine the best way would be taking them all to independent latches with LEDs on them for identification, along with a single latch that breaks the power stage enable line? (and link all these to micro-controller inputs for referencing / error code display as well)Quote from: T3sl4co1l on September 08, 2016, 12:43:05 amThis was something I was pretty sketchy about. I've never been good with analog circuitry... but no time like the present I suppose. I can agree with MCUs being outside of core protection. I kind of like CPLDs as they are my latest tool to my box (and when you have a new hammer, everything looks like a nail)Quote from: T3sl4co1l on September 08, 2016, 12:43:05 amReally? That's pretty damn awesome. I shall refrain from trying that however anywayQuote from: T3sl4co1l on September 08, 2016, 12:43:05 amI'm already familiar with low inductance designs from reading / talking on 4hv.org forums and their work over there.I figure either the standard two bus bars, or a nice copper laminated bus.Quote from: T3sl4co1l on September 08, 2016, 12:43:05 amThanks Tim, Really appreciate the input you have given me on this. I would greatly appreciate assistance on planning the control scheme, that is the area I feel most weak at / concerned about getting -right-- If anyone wants to find me later I'll be under a mountain of application note and datasheets

author: jbb
author_rank: Super Contributor
timestamp: September 08, 2016, 08:29:11 am
content: Hi CM800This sort of project could be a lot of fun and I hope you get to have some of it.At risk of being pushy, I've played this game before and have a couple of suggestions for safe and more productive engineering:A Residual Current Device (RCD) / Ground Fault Circuit Interrupter (GFCI) is probably a good idea.Use an enclosure! Preferably a metal one with earthed sides so that a loose wire will give you a nice fault to earth. And trip the RCD / GFCI mentioned above.Keep your hands out of it. An acyrlic lid will make you think before you stick your hands in. It doesn't have to be very thickSaid acrylic lid will also keep shrapnel out of your eyes. Youwillblow stuff up in this project.Put a drain/discharge resistor on all electrolytic caps > 50V. Otherwise they can retain lethal charge for days or longer.At the power levels you're talking about, you'll have quite a lot of stored energy (unless you're pursuing some unusual design practices). Do not short out your large capacitor banks with a screwdriver - it will wreck the screwdriver and damage the caps. See point above.Ideally you should keep the lid shut whenever there's mains power inside. That means that in order to move your probes you would have to turn the power off, safe the unit, open the lid, move the leads, close the lid, restart the unit.If the above sounds like too much of a drag, you can drillmodestholes in the lid to allow you to poke probes through. It's better than nothing.Absolutely use isolation between any user interface and any power components.Isolation between your power stage and you control stage is highly recommended. Not only will it reduce the damage when things go wrong, it will also reduce the impact of electrical noise a bit.Consider attaching a dedicate DMM (or maybe a good old-fashioned panel meter) to your DC link at all times. This way you can check it's down before you stick your fingers into it.Power and VoltageI'm assuming you have 3 phase, 400V line-to-line, 32A supply. For power levels > 2.3kW (i.e. single phase 10A) you want to go straight to 3-phase designs.Available voltage:Generally, a rectifier will give you sqrt(2) times the line-to-line voltage. Thus Vnom = 565V.Allow +10% for high line conditions: Vmax = Vnom * 1.10 = 622V.Allow -10% for low line conditions and -10% for filter inductors etc.: Vmin = Vnom * 0.9 * 0.9 = 457V.Available power:Max apparent power: sqrt(3)*400 V*32 A = 22170 VAAllow for 10% drop in line voltage. 22170 * (1-0.10) = 19953 VAIf using a PFC stage, assume power factor is 1 and efficiency is 95%: 19553 VA * 1 * 0.95 = 18955 W (i.e. 19kW)If using a diode bridge, efficiency will be good, but power factor will be poor (say 0.75) and efficiency is 97%: 19553 VA * 0.75 * 0.97 = 14515 WGiven that your output is expected to be a transformer, there doesn't seem to be much point in building an isolated input stage. The DC link is still going to be dangerous, and 20kW of isolating transformer will be large and heavy.A standard PFC stage (i.e. voltage sourced, IGBT based), is aboost-type converter; your output voltage is higher than the peak line-to-line. A typical output voltage would be 650 - 700 V (use 1200V IGBTs). As a plus, it's going to be regulated & stable. It won't have much ripple as long as your 3-phase supply is good.A 6-switch thyristor controlled rectifier is also a possibility. They are a bit old-school but very robust. They arebuck-type converters; your output voltage is lower than the peak line-to-line voltage. You could design one of these to produce, say 0 - 450V DC. It can be regulated, but it will have output ripple at 6 times (that's the number of thyristors, see) the line frequency, i.e. 300 or 360 Hz. It will probably be cheaper than the standard PFC stage and will be more robust. However, it will have poor power quality (i.e. low power factor) on the AC line side. You will therefore not be able to draw as much current from your 32A supply. It could also pollute the power on your premises.A current-fed topology is also an option. This is where you use a 6-switch thyristor to make a controlledcurrentoutput. (Yes, an H-bridge can work with a current source rather than a voltage source). However, the second stage design will have to be quite different. These days, current-fed topologies tend to only be used for monstrous (over 30 Megawatt) drives.CoolingHere are some very rough guesses as to the losses you should expect.Input power: 20kWPFC stage: 95% efficient: 19kW out,1kW losses as heat.Bridge stage: 95% efficient (30kHz is actually quite high for this power level): 18kW out,1kW losses as heat.Transformer: 98% efficient: 17.6kW out,0.4kW losses as heat.As you can see, that's a lot of heat. You'll have to use air or water. Air cooling will need big (loud) blowers. Water cooling will need water blocks, pumps, filters and radiators. At this power level I would suggest air cooling as it requires less mechanical integration and can't leak everywhere.ControlT3sl4co1l is dead right about having analogue protection. Youwillscrew up the code at some point. Or hit 'pause' on the debugger.I suggest the following:Current sensors on all 3 phases incoming.Comparators trip the converter if any current is too high or low.Voltage sensing on DC bus.Comparator trips the converter if DC bus is too high.Current sensing on the output terminals to the transformer.Comparators trip the converter if any transformer current is too high or low.Watchdog timer chip on control MCU. Walk the watchdog timer with code in your most important control loop. If the code gets stuck, the timer runs out and trips the converter.OR all of these together (note: many comparators have open-drain outputs which can be bussed together) and feed them into a single S-R flip flop. This flip-flop is responsible for tripping the converter.Controlling a 3-phase PFC stage is a job for a micro controller. Use a big one, like an ARM Cortex M3 or more. I like the Texas Instruments C2000 series (quite powerful, excellent PWM peripherals, good AD peripheral) but they are hard to get started with (typical time for going from blank folder to first compile is 1-2 weeks for us hairy-knuckled electrical engineers due to the ####ing linker command files. Suggest you start with a sample project...). (Floating point can make your life easier here.) Texas Instruments also have reference designs, schematics & code you might want to look at.Controlling the test transformer is probably a job for a second micro. That way you won't have to mess about making sure your two control tasks (PFC and transformer) don't run into each other.ConstructionIf you're using a voltage-sourced converter, you need minimal inductance. In this case laminate busbars are preferable. If you try to make your own with sheet copper, I suggest using moderately thick polymer sheet (polystyrene, teflon, ??) between the +ve and -ve. Kapton tape might be attractive (it's so thin!) but it only takes one little burr to rub through it.The unit will be large and heavy, so don't skimp on the box.Good luck!jbb

author: CJay
author_rank: Super Contributor
timestamp: September 08, 2016, 08:48:44 am
content: Could I add one simple thing?This sort of electronics is 'scary' for a hobbyist, so along with that dedicated multimeter on the DC link, add a very simple indicator (like a LED) to it as well, something that won't fall foul of EM fields, RFI, that sort of thing.That way if anything happens to the multimeter you have a belt to go with the braces and can be reasonably certain the DC is off before placing hands in danger's way.

author: CM800
author_rank: Frequent Contributor
timestamp: September 08, 2016, 09:27:39 am
content: Thanks Jbb for your response, I agree with most of your 11 point list, a little ‘eh’ about enclosing everything with mains power while testing, But that’s probably because I’m pretty comfortable working with it. Probably more then I should be. (Same goes for me waving around my earth stick at the 70kV transformer)….Fine, I shall enclose it.I’m afraid I do not have a 3 phase line anywhere but work, where they would never let me take this in. I only have access to single phase at home. While I want the ability to pull a full line from a single phase point if I want, I can imagine generally using only single phase 32A most of the time as that’s what I will be running up to my ‘workshop’. I also want to run it of a 13A socket with a current limit control setting.Considering the breadth of power levels in that. Might it be worth making 2 converter projects? Maybe start off with a ‘prototype unit’ designed for up to 3kW power level, then a second one that can take single or 3 phase input at up to 20kW or more.I can see cooling being done as air or water. I will have to evaluate form factor, but it’s safe to say I have some heatsinks large enough to sit on without touching the ground.Laminated busbars seem to be a go then. I intend to use rackmount cases, 1 for the PFC / Digital Variac, the other for the Driver.You can see a few of my exploits with basic PWM control, open loop here:Might be worth, rather then going directly, to the biggest most complex bastard to design, to make a smaller one for a home wound transformer at 3kW max power level, with a single phase PFC.I've got something functional for the higher power which is plug and play with a high explodyness factor, but enough to get some fun out of these big XRTs for now.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2016, 07:35:49 pm
content: Quote from: CM800 on September 08, 2016, 07:23:43 amI've been using it since my first experiments with ~1kW and up. The transistors I have burned throughout my careercan be held in one hand.Quote2, 3 and 4 are pretty much the same thing (from different perspectives, which is excellent). The third I was looking for was along the lines of -- why can't you use an HVIC driver, for instance?The reason is, when the high side transistor turns off, the low side transistor will experience ground bounce. HVIC drivers are made to tolerate very little of this, usually 6V (I think I've seen up to 30V in more recent datasheets; they won't communicate during such excursions though, they just maintain whatever state they were left in).And yes, having Kelvin terminals helps, but one will always over/undershoot the other, and vice versa. So you need, at the very least, a very robust driver; for the other reasons as well, you need an isolated one.QuoteOh and, since we're talking industrial modules here -- +/-15V drive becomes a necessary option. The negative bias helps prevent dV/dt and ground bounce effects from causing accidental turn-on, which is more important for the bigger, lumbering modules (relative to discrete transistors, which nonetheless still benefit from using -5 to -10V 'off' bias).I've had excellent success with e.g. IXDD630CI and similar. Logic input, handles the voltage range, plenty of current. For supply, RECOM makes 3W and 5W converters that should deliver about as much as you need, with +/-15V supplies. Or you can roll your own.On one project, I also used ADUM2xxx isolators, which have more dV/dt rating and lower delay and skew than any opto. That was necessary because that used MOSFETs. There are many alternatives on the market today (of the monolithic, magnetic or electric (capacitive) isolator type), or you can use optos (IGBTs are usually slow enough).The ADUM isolators are kind of low on isolation ratings, but they'll deliver good lifetime. The isolation barrier (polyimide film) incurs damage proportional to voltage: at 100V, it's good "forever"; at 250V, it's decades; at 5kV, it's seconds. So, they'll withstand line transients, which cause wear disproportionately but are infrequent, and mains voltage, which is modest (nominally, FWB rectified 480VAC is no more than +/-350VDC with respect to safety ground, and therefore the same amount of Vpp on the high side isolators). Probably the wear rate contribution is similar from both causes, so that a 10 year lifetime is reasonable.Beware that some datasheets lie, like Fairchild's 6N136 datasheet: it's rated for 1000V/us at a 10V step. Probably not even enough step height to deliver forward-bias charge into the phototransistor, so it's a pretty "duh" spec. Other mfg rate theirs at only 100V/us, for a more typical 100 or 1000V step. (Hint: use SFH6345 instead, or the many internally-shielded types, which offer 10kV/us or greater immunity for a 1000V step.)QuoteQuoteNice thing about CPLDs is they're EEPROM/Flash (at least, the older generation was; I'm not entirely sure about today's), so the logic is always-on.There are NV FPGAs, but beware the startup time. The ones that have onboard Flash and self-configure will be in an open-circuit state until it's complete. I think there are a few that are Flash-per-LE or something like that, thus effectively replacing CPLDs in high-rel, complex-logic applications. Most have external configuration. Needless to say, if for any reason, the FPGA is momentarily unconfigured, or in reset or debug, you need to make sure the circuit is safe against pins freezing in place (you may want to use a hardware missing-pulse detector, and deadtime enforcement circuitry, to prevent transistors turning on simultaneously), or going open circuit (use pull-ups/downs to set default state).Cheers,Tim

author: CM800
author_rank: Frequent Contributor
timestamp: September 08, 2016, 07:51:00 pm
content: Quote from: T3sl4co1l on September 08, 2016, 07:35:49 pm

author: jbb
author_rank: Super Contributor
timestamp: September 08, 2016, 08:05:04 pm
content: Hi CM800If you've got single phase, that's fine. I think you're right about starting medium power and going to high power later if you want. The back of the envelope changes a bit...Assuming 230V 13A:Apparent power: 230V * 13A: 2990WAllow -10% low line voltage: 2691WAssume PFC stage is 93% efficient: 2502WAt these power levels you can get pre-cooked PFC control chips.You could also handle the required currents on a PCB, which will save you a lot of trouble.On DC output control:A standard single-phase PFC boost rectifier will typically output around 380V and use 600-650V rated power semiconductors. You can then put a buck converter on the output to yield an adjustable 0 - 350V output (note: leave some headroom so that the buck converter controls can hide some of the DC buds capacitor ripple variation).If you tweak the design, and use 1200V devices, you could adjust the PFC output bus up to 650V DC (efficiency will drop a bit), and have a buck output from 0 - 600V. For extra credit, adjust the PFC stage output voltage in tandem with your output voltage setpoint for maximum efficiency.A plus of having a buck output stage (with a small filter capacitor) is that it can be current limited and short-circuit protected.On inductors:They're going to be quite big (think fist sized).Standard laminated silicon steel is not very good above 10kHz switching frequency.Iron powder cores are likewise not very good.Amorphous / nano crystalline iron cores can be quite good in the 10 - 30 kHz range.'Distributed-gap' materials (Sendust, MPP, HighFlux, KoolMu) can be quite good in the 10 - 30 kHz range.Ferrites will probably be your best bet when availability is considered. A large range is available. E65 cores are generally available and quite large.On enclosures:Rack mount cases are probably a good option.On gate drive:Once again, T3sl4co1l is dead right: Use of negative gate drive voltages really helps to improve reliability. For a smaller 3kW system youmightbe able to do without, but I suggest that for a lab tool you want high reliability. +/-15V is quite acceptable. Some people use +15/-8, which leads to slightly quicker turn-ons but is a bit more fiddly to drive.A number of vendors (Texas Instruments, Avago Tech and others) have isolated gate driver chips with dessert protection built in.