paralleling MOSFETs  - Page 1
Forum section: electronics / projects

author: Electr0nicus
author_rank: Regular Contributor
timestamp: May 30, 2014, 11:17:57 pm
content: HiI'm currently designing on a 100kHz full bridge converter, which has a power throughput of approx. 4500W. It is used to drive a high voltage load on the secondary side, so there is 5kV @ 0,8A on the secondary. On the primary, I have a relatively low DC voltage of 40-60V. This voltage has to be stepped up to that high voltage. So you can imagine that on the primary side of the transformer very large currents are flowing (130A peak).So far so good.The PWM and control circuitry chips and components are pretty much selected. The only thing which causes me quite a headache is the selection of the switching MOSFETs and their cooperation with the gate driver chips. As a requirement the MOSFETs shouldn't have more than 15W I²R losses. Which isn't very easy to archieve at that high currents. So I currently selected IPB019N08N3 MOSFETshttps://www.infineon.com/dgdl/IPB019N08N3_Rev2.3.pdf?folderId=db3a304313b8b5a60113cee8763b02d7&fileId=db3a30431add1d95011ae87fdf90569fThey are quite a beast of MOSFET, but have really high gate charge/capacitance, which is no wonder. To get the conduction losses low, I will need to connect 4-6 of these MOSFETs in parallel to make one "Bridge MOSFET". So the total MOSFET count of the full bridge will be 16-24pcs. To drive that high gate capacitances, gives me the headaches I previously mentioned.The IPB019N08N3 MOSFETs can handle 180A each, so every single MOSFET can handle the full operating current of 130A peak without going up into flames.They are only paralleled up for lower On- resistance. But my question is: Do I really need MOSFETs, where each one can handle the full operating current, or can I use f.e. five 30A MOSFETs and rely on that they share the current equally between each other. There shouldn't be a thermal runaway possible because of the positive TC. Using lower power MOSFETs will relax my gate drive requirements very much. But I don't know if that's good design practice.What do you think about it ?GreetingsGregor

author: dannyf
author_rank: Super Contributor
timestamp: May 31, 2014, 12:00:59 am
content: QuoteWith the kind of questions asked, you are better off starting at 45w first. As you gain more experience and confidence, move up the scale.QuoteRead the datasheet carefully and see how the 180a is quantified. Then ask yourself if those conditions apply to your application.QuoteMinimum pratical differences - low Rds mosfet is no different from higher Rds mosfets paralleled. You do get better thermal conduction with smaller devices.QuoteThink again,QuoteAt 100Khz and 4x+ 130A mosfets, your biggest problem is to drive them.

author: AlfBaz
author_rank: Super Contributor
timestamp: May 31, 2014, 12:17:55 am
content: Quote from: Electr0nicus on May 30, 2014, 11:17:57 pmHi GregorI'm not an EE so place what I say in contextI recently looked at paralleling MOSFET's but for use in the linear region, to see if I needed a series resistor with each one to force current sharing.During the course of my reading it was noted that for switching application the +ve temperature coefficient doesn't help much because of the temperature time constants involved and the short on bursts of the MOSFET's. I would imagine the problem would be greater on startup before the current bursts have had a chance to work their magic

author: retrolefty
author_rank: Super Contributor
timestamp: May 31, 2014, 12:23:48 am
content: Paralleling many mosfets (I've seen up to 8 used) is what those high current R/C ESC control modules all use. It does complicate the driving requirements and there are special purpose MOSFET driver chips for just that applications.Example:http://www.hobbyking.com/hobbyking/store/__28373__Hobbyking_YEP_180A_HV_4_14S_Brushless_Speed_Controller_OPTO_.html

author: max666
author_rank: Frequent Contributor
timestamp: May 31, 2014, 12:51:59 am
content: 130A Transformer running at 100kHzWhich transformer are you intending to use?Transformer losses like the following come to mind:Core lossesHysteresis lossesEddy current lossesWinding joule lossesSkin effectProximity effectThose are the ones getting worse with increased frequency. It looks to me your MOSFET On-Resistance might not be your biggest problem.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: May 31, 2014, 01:40:59 am
content: Thanks for your posts so far. I will supply you with some further Information.This project is a idea of a friend (mechanical engineer). I tend to get approached by people asking me for help with their crazy projects.He pretty much gave me his prameters and asked if I can figure some way out, how this whole thing can work. I had designed some switch- mode converters in the past. But their requirements were very low compared to that project. The biggest thing was a full bridge converter which outputs 800W at 100V from a 40V DC supply, to drive some huge LED panels with 100V per string. There you could pretty much use all off the shelf compontents, drivers, Mosfets etc. Still works perfectly after 3 yearsBut I digressQuoteI don't see a immediate problem looking at the MOSFET. The 180A a continous so that should be fine, cooling requirements should be met and exceeded. And MOSFETs have a positive TC, and every literature says that they can't run away thermally because of that. Power MOSFETs themselves consist out of thousands of single MOSFET cells on the die. So internally the are paralleled too. In my application they should be very well thermally coupled. The heatsinks will be made out of machined copper or aluminium and they will act as a heatsink and also as a conductor. They will be screwed on large copper pads which are connected to the Sources and Drains of the MOSFETs. When the MOSFETs are properly arranged in the layout four separate heatsinks will be used to build up the full bridge. Those heatsinks will then also carry the currents in the bridge at the same time. As the primary voltage is <60V isolation should be no problem.QuoteThat's the exact thing, about what I wanted to ask the forum for helpQuote from: max666 on May 31, 2014, 12:51:59 amThe current might look high, but for selecting a proper core the most important parameter is the core crosssection area. Look at the following artice, under "Leistung".http://www.mikrocontroller.net/articles/Transformatoren_und_Spulenin English:http://ludens.cl/Electron/Magnet.htmlThe transformer will be big for sure. A PM114 or UU141/78/30 will be used. Core material N87.The transformer shouldn't be the problem.I have a low %delta B so Hysteresis losses shouldn't be problematic.For wiring I will use copper foil (mutiple layers) on the primary and High Frequency wire on the secondary. So the skin effect should be minimizedJoule losses I reduce using abovementioned copper foilCore Losses should be <20W according to EPCOS magnetics design toolSure all those losses add up, and may be a issue in the end. But as the transformer likely has to be submerged in some sort of transformer oil (because of the high secondary voltage) a cooling oil flow can be established if heat generation will be a problem. But up to now It doesn't look like that, but I also have to admitt that I haven't calculated everything to the detail. As the project currently is in the "is it even possible" stage

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 31, 2014, 02:40:17 am
content: With such a high output voltage, stray capacitance is going to be a real problem at 100kHz. Not to mention that you'll need really fast high voltage diodes to rectify the output. 20kHz or so would be more reasonable.Exactly what's the application? I'm guessing some sort of high power microwave transmitter for telecommunications.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: May 31, 2014, 02:56:15 am
content: Quote from: NiHaoMike on May 31, 2014, 02:40:17 amIt will power a 3kW Magnetron (thats the real transmitted power, as the efficiency is 70% it will need approx. 4kW on the Input) to heat up stuff (in a industrial environment). Pretty much a big microwaveBut because only DC voltage can be used on the input, the easy way using only a MOT simply doesn't work. I need to recalculate the transformer, to see if 20kHz will work too, but I doubt it. I will use a few diodes in series on the HV- side. So I can get away with easy available 1,6kV diodes (SF1600http://www.farnell.com/datasheets/1349698.pdf) with reasonable swiching speed. Of course I will need some high value parallel voltage sharing resistors (approx. 10MOhm) on each diode, so the leakage current will be increased, but you can't have everything

author: electrophiliate
author_rank: Regular Contributor
timestamp: May 31, 2014, 05:21:26 am
content: I looked into this once before and came across the following tips in Application Note AN-941 from International Rectifier:http://www.irf.com/technical-info/appnotes/an-941.pdfPARALLELING POWER MOSFETs· Use individual gate resistors to eliminate the risk of parasitic oscillation.· Ensure that paralleled devices have a tight thermal coupling.· Equalize common source inductance and reduce it to a value that does not greatly impact the total switching lossesat the frequency of operation.· Reduce stray inductance to values that give acceptable overshoots at the maximum operating current.· Ensure the gate of the MOSFET is looking into a stiff (voltage) source with as little impedance as practical.· Zener diodes in gate drive circuits may cause oscillations. When needed, they should be placed on the driver side ofthe gate decoupling resistor(s).· Capacitors in gate drive circuits slow down switching, thereby increasing the switching unbalance between devicesand may cause oscillations.· Stray components are minimized by a tight layout and equalized by symmetrical position of components and routingof connections.

author: johansen
author_rank: Super Contributor
timestamp: May 31, 2014, 06:58:19 am
content: switching 100 amps at 50 volts isn't a problem, the problem is doing it at 100Khz in a hard rectified manner.if you want to build an LLC resonant converter, you could do that.there are folks who are running 500 amps at 300 volts on a 1% duty cycle, using slow IGBTS.. and they don't blow up.however, those are resonant conditions, 10% full load current or less at both turn on and turn off.the problem with switching 5000 watts at 100Khz is the leakage inductance around those mosfets. also, the leakage inductance of the transformer is going to be incredible, and its going to ring with the secondary at a very nasty frequency...to get around that, you can build 10 H bridges, 10 transformers, rectify them separately and run them in parallel, run the secondaries in series, wind them for 400-500 volts instead of 5000.It would be very difficult to build a single 5Kw transformer for 5Kv and 100Khz, but i suppose it could be done. the parasitic capacitance of the secondary will be a bitch.you might as well try to figure out how to incorporate it into the resonant frequency.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: May 31, 2014, 07:37:03 pm
content: Quote from: johansen on May 31, 2014, 06:58:19 amWell, you will only know if it's really that bad, if you built it up and test it. Or ditch the project completely. That's the problem. Although I know now, that that building up such a converter, will be extremely difficult, the research spirit in me want's to give it a tryDoes anyone know a way to simulate such parasitics properly. Of course I can add the lumped parasitic elements in my LTspice simulation, but I doubt that this simulation really tells me the true behaviour of the circuit in the real world.Btw. I use a phase shift PWM controller, with fixed 50% duty cycle, fixed switch on/off times, and ZVS (zero voltage switching). Currently I run the simulation with a LTC3722-1

author: max666
author_rank: Frequent Contributor
timestamp: June 01, 2014, 01:12:55 am
content: Quote from: Electr0nicus on May 31, 2014, 07:37:03 pmOf course! And please go ahead and proof us all wrong. That's what it's all about, if you can figure out how to do that and can tell us how it's done; then we all learn something.And i will be the happy to admit that i was wrong, thinking this might be impossible (or let's say impractical).But since you asked, we're just offering our opinion and cautiously try to tell you that you might be reaching too high.

author: dannyf
author_rank: Super Contributor
timestamp: June 01, 2014, 02:10:29 am
content: QuoteThe fancier approach you take, the less likely you will succeed.There is a reason the shortest distance between two points is a (simple!) straight line.

author: mzzj
author_rank: Super Contributor
timestamp: June 02, 2014, 03:51:34 am
content: Quote from: johansen on May 31, 2014, 06:58:19 amI would vote for 5-10 smaller briges and transformers also. Besides, transformer cores will be easier to buy and more selection of materials for them.For secondary rectifier diodes look for 1200v Silicon Carbide Schottky diodes from Cree, iehttp://www.digikey.com/product-detail/en/C4D02120A/C4D02120A-ND/2679410

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 02, 2014, 05:41:45 am
content: Elecrt0nicusthe problem with switching 5000 watts at 100Khz is the leakage inductance around those mosfets. also, the leakage inductance of the transformer is going to be incredible, and its going to ring with the secondary at a very nasty frequency...The statement above from Johansen sums it up. The leakage energy increses with the square of the current and becomes a HUGE problem at higher currents and frequencies.The phase modulated full bridge converter (UC3875 etc) is an excellent topology (quasi resonant) but for high voltage operation not the correct choice, in my opinion.The massive inter-winding capacitance of the high voltage secondary will result in a huge reflected capacitance that will ring with the leakage inductance.Also, the secondary high voltage diodes will have a poor reverse recovery time which will seriously limit the operating frequency.The huge parasitics of high voltage converters make resonant topologies much more attractive than hard switched ones.With resonant topologies one can usually absorb the parasitics into the resonant elements so they effectively disappear.I would look at something like an LLC topology or similar like Johansen suggested.DickThere is a very good reason why high power DC systems utilise many paralleled power supplies rather than a single huge supply.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: June 02, 2014, 07:10:10 am
content: Thank you for your help so far!I will look into LLC converters as a last chance.Building up 10 separate hard switched converters won't be a option, also space wise.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: June 05, 2014, 03:43:47 pm
content: Ok.I've looked into LLC converters and decided using them. Although they are more complex, it seems this topology is the right one for the problem.I think I'll go with 5 seperate 1000W 1000V Converters, connected in series to archieve the 5kV output voltage. 1kW modules are also convenient because I have the appropriate 1,5kW power supply in my lab to test them.I'll create a new topic in the projects forum, because I have some questions about the selection of the resonant components, and because this topic will be too much sidetracked if I start discussing LLC converters here. In theory the operation is clear, but I need to know what issues I'll have to solve in the real word.GreetingsGregor

author: diyaudio
author_rank: Frequent Contributor
timestamp: June 05, 2014, 04:51:12 pm
content: Quote from: Electr0nicus on June 05, 2014, 03:43:47 pm"if" you prototype 20% near what you described do show off your progress.