Mosfet paralleling reliability in real life - more smaller ones vs fewer bigger  - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 20, 2019, 01:30:41 pm
content: Hi folksI have a question about reliability of power supply with higher number of paralleled transistorsFor example 2 Switch forward converter with planned 2x 5 in parallel switched at moderate frequency 200kHzIf current will be equally shared losses are calculated ~10W per device 4.5:5.5 Conductive:Switching losses ratioQuestion is if then I can trust that this many devices have enough similar parameters to work reasonably and over longer time periodYes then here is also some layout issue with switching parallel at this speed, but there little extra trace inductance at Drain side can be beneficial

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 20, 2019, 04:42:35 pm
content: Five MOSFETs in parallel doesn't strike me as terribly economical unless you are relying on passive convection cooling and need to get the heat flux (ie - W/cm2) down.Paralleling MOSFETs isn't usually a big deal as long as you increasingly derate the current for each additional switch and use an individual gate resistor for each one, however at 200kHz you might have a problem with differing loop impedances which will lead to uneven dynamic current distribution. Adding a bit of inductive impedance in series with the source of each switch can be helpful here; putting the L in series with the drain is less effective because you don't get the benefit of negative feedback from degeneration.That said, it is always preferable to minimize the number of switches in parallel, and (5) seems a bit much unless this is a step-up application and the input voltage is low (e.g. - 12V to 340VDC to supply the H-bridge in an inverter).

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 20, 2019, 06:10:27 pm
content: But five small is actually cheaper than one big and I can use fullpak ones with simpler and cheaper mountingPlus I need to switch relative high current at short duty, about 20-30A it makes problem with single device and high switching losses

author: rx8pilot
author_rank: Super Contributor
timestamp: September 20, 2019, 06:29:03 pm
content: This YT channel is gold.https://youtu.be/wjSM2oYY3OY

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 20, 2019, 07:00:33 pm
content: Quote from: Miyuki on September 20, 2019, 06:10:27 pm30A isn't particularly challenging for a single device or, perhaps, two at most. What is the maximum supply voltage?

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 20, 2019, 07:05:52 pm
content: Supply voltage is 500-650V what calls for at least 800V transistors where choice is limited

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 20, 2019, 08:04:03 pm
content: In the end, like every thing else in engineering, there will be tradeoffs.My pragmatic advice, is if this is a high volume, cost sensitive product with a proper design budget, then by all means optimize the design. This may mean that a few iterations may be required to satisfy your cost, performance and reliability goals.In the other hand, if the design is for very few units and you are on a tight design timeline, go for the simplest approach. Use a minimum of larger Mosfets.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 20, 2019, 09:37:29 pm
content: Quote from: Miyuki on September 20, 2019, 07:05:52 pmWell that certainly changes things... You need to limit transition time to around 25ns to keep switching losses manageable but that results in something like 26V/ns dV/dt which is way too much for hard-switched operation, especially when body diode conduction is involved (which it will be in the two-switch forward). Also, 19.5kVA is a bit much for a two-switch forward... like 10x too much. Granted, there's not as much of a penalty in transformer utilization between a single ended and bridge converter at this frequency, but there still is one. At this power level (and input voltage range) the full bridge converter is pretty much exclusively used, a shaving a few $ off the switch cost is not even a consideration.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 21, 2019, 08:36:18 am
content: Transmited power is "just" about 1500W, but it is constant current / power and need high possible output voltage at low currentWhy would body diode conduct any bigger portion of time, it is unwanted and just some nasty osculations do it.With small parallel transistors current in each shall be enough small to allow reasonable switching speed and losses.With one device I will need to go with SiC device or accept high losses and problem with cooling

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 21, 2019, 09:46:16 am
content: Quote from: Miyuki on September 21, 2019, 08:36:18 amThe energy stored in the transformer magnetizing and leakage inductances (and any other stray inductance in the loop) is returned to the supply via the body diodes each switching cycle. Even in fast 800V-900V SuperJunction MOSFETs these diodes will have a reverse recovery time in the >300ns range, which is way too slow for operation at 200kHz.The body diodes in SiC MOSFETs are faster, but they are PN type, not Schottky, so still have a reverse recovery time (30ns - which is fast enough - to 60ns - which is marginal) and a really high forward voltage drop (>4V in most cases) because of SiC's wide bandgap.As a result, handling this much power in a single converter at 200kHz practically begs for a (quasi-)resonant topology.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2019, 10:25:54 am
content: Why not a pair of IGBTs? Should be just workable at that frequency, or fine a bit lower. If you're going for lower switching losses in a compact package (hence the higher frequency) I guess that wouldn't work out; instead I would recommend SiC MOSFETs.Paralleling MOSFETs is generally fine, but don't forget to consider stray inductance. In the Sam Ben-Yaakov video, he considers the current divider circuit formed by Rds(on)s. Add stray inductances in series with these, and change the current to a step or ramp source, and you will have a representation of current sharing in the switching circuit. Current equalizes after a few L/R time constants, so besides reducing stray inductance, it can actually help to have some additional resistance.Note that the stray inductance is, always, the loop inductance, from one switch to its opposing switch. For the two-switch forward, that's one transistor, and its catch diode, through the local bypass capacitor. The other transistor and diode act in an independent loop, and that loop needs to have the correct inductance as well. (The bypass can be shared, that's fine -- as long as both inductances work out.)A typical layout, with transistors lined up on a heatsink, you would need to alternate transistor and diode, and put bypass caps in front of the row.It's not much more trouble to simply use fully independent inverters. Use a single transistor and diode per leg, and a pair of legs, with a transformer and all the support components (input bypass cap, driver, output rectifier..) per channel or phase. Use N phases, with a phase shift between each, to reach the desired total capacity.This is a bit annoying to do at 1500W, so I don't know that I would bother to do it (probably, I would choose a single phase, or two phase interleaved, full wave, forward converter). It is the only practical way to scale up arbitrarily.The fundamental problem with scaling, is what ultimately underlies this argument. It becomes much more apparent at low voltages: where stray inductances hit so much harder. Say you're doing a 12V to 1.0V Vcore supply on a motherboard. You need 100A out, so you're switching pulses of about 100A / N at the input, for N phases. A hundred amperes in a single stage is just preposterous: that's a 0.12 ohm switching load impedance to begin with, and even doing it at 100kHz, you have to contend with the reactance of even a very compact loop of 5nH being 3mohm, i.e., drawing a reactive power of 2.6% of the total power, give or take. And that's just at Fsw; basically all the harmonics are going into it as well. That reactive power is just going to be burned as switching loss, unless you go to lengths to conserve it (quasi-resonant snubber?), and even then, you can't snub very much of it because the snubber itself is going to have about as much stray inductance!So you need to divide and conquer, and 10A per phase is far more manageable, indeed well enough that Fsw can be pushing 1MHz while switching losses stay quite comfortable.Finally, the other side is this: why not just build a bunch of inverters and run them in parallel? Why phase interleave? The best part is, when phases are interleaved, their ripple currents interfere and partially cancel out. You can save on total capacitance and filtering this way.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2019, 10:30:45 am
content: Quote from: MagicSmoker on September 21, 2019, 09:46:16 amReverse recovery is only relevant in hard switching (CCM).Flyback is typically operated in DCM, so this doesn't matter (for certain degrees of "matter" -- in the two-switch, the diodes act in series and one will inevitably recover before the other, leaving some recovery charge left in the other one).Although I suppose you wouldn't be doing DCM at this power level, but you shouldn't be forcing a flyback design at this power level, either.On a related note, in ZVS (inductive load) configurations (including resonant), body diode recovery occurs during normal load current, while the transistor is on -- this probably lengthens the recovery time (because the recovery voltage drop is small?) but it means most MOSFETs are adequate to a MHz or more, even high voltage Si being adequate at half a MHz or more.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 21, 2019, 11:03:59 am
content: Quote from: T3sl4co1l on September 21, 2019, 10:30:45 amErr... the OP first said this is a two-switch forward, not a flyback.Quote from: T3sl4co1l on September 21, 2019, 10:30:45 amWell, someone hasn't had their coffee yet this morning... Re-read my previous post.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2019, 11:27:59 am
content: Quote from: MagicSmoker on September 21, 2019, 11:03:59 amNote sure how I internalized it as flyback, but it works out the same on the primary side at least.Transformer utilization is half, relative to full wave; I wouldn't call it insignificant at scale. There may be many costs much greater than the transformer alone, but in an absolute sense, no.QuoteJust reiterating it in case it hasn't become crystal clear to the OPTim

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 21, 2019, 11:44:05 am
content: Quote from: T3sl4co1l on September 21, 2019, 11:27:59 amOh, I totally agree that the (hard-switched) two-switch forward is not even remotely the topology of choice here (despite immunity from cross-conduction failure, which is otherwise very compelling), but the penalty in power throughput at >100kHz is not nearly as bad as you might think because max flux swing has to be greatly curtailed to keep core losses under control, anyway. Practically speaking, the advantage in transformer utilization of bipolar vs. unipolar operation at fsw >100kHz is more like 20%, rather than a doubling. Still, one shouldn't be hard-switching 650V at 200kHz, anyway, so rather a moot point.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2019, 12:16:11 pm
content: Quote from: MagicSmoker on September 21, 2019, 11:44:05 amAh yeah -- that's one of those crusty old things that I've manged to internalize without consideration! Thanks for bringing that up.Back in the day down at like 20-50kHz, you'd be able to push Bsat, but it's still hard today to find materials that will handle that much at 200kHz (even 100kHz in modest sizes). Modern materials like N97 and 3F46 are quite good, but not quite that good. And that explains the relative prevalence of otherwise-kinda-shitty topologies -- like two-switch forward -- in modern application.So put that on the bonfire of obsolete design information as well.Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 21, 2019, 12:54:22 pm
content: I would like to use ETD shape core, it have relative big area to volume ratio so I can push flux swing high (about 200mT at 200kHz) with sustainable temperature riseBut it is far far from saturation even at single ended topologyAlso switching is just half hard, at turn off since turn on is covered by leakage inductance at almost zero current and have just Coss losses//edit:wonder what will do to switching losses driving mosfets off very hard with like -10V with minimal gate resistance and rely on diodes to clamp that spike

author: mzzj
author_rank: Super Contributor
timestamp: September 21, 2019, 06:30:42 pm
content: Quote from: Miyuki on September 21, 2019, 08:36:18 amThis sounds pretty much like welding machine..15 years ago Kemppi was using matched 3xparallel IGBT's and 2-switch Forward converter topology on smaller welding machines that I'm familiar with. 160kHz switching frequency.BUT suitable IGBT's for 650v supply voltage operation (ie 800 to 1200v rating ) are lot less common than 600v rated ones.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 21, 2019, 06:59:26 pm
content: Quote from: mzzj on September 21, 2019, 06:30:42 pmYes at 400V It will be easy to use Infineons H5 and they can switch even at this frequencyBut 800-900V ones are very limited choiceAnd 1200V ones are slow

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 21, 2019, 09:58:24 pm
content: Quote from: Miyuki on September 21, 2019, 12:54:22 pmYou sure about that? I'm working on a transformer design right now that uses a state-of-the-art ferrite similar to TDK/EPCOS N97 and at 200mT/200kHz it wpuld have a loss ratio of 1.06kW/m3, or somewhere between 2x and 5x what is tolerable. In fact, the ideal flux swing is more like 100mT, at which point the loss ratio drops to 165W/m3(the power of the B exponent at work), but that still is enough to cause a temperature rise of 67C over ambient (which happens to be ideal, as there is a loss minimum at 100C, similar to many other power ferrites).Quote from: Miyuki on September 21, 2019, 12:54:22 pmEh? Turn-on is very lossy in a MOSFET-based hard-switched converter due to the energy stored in Coss. You really will need to either use SiC MOSFETs or else go with a ZVS or above-resonance topology.Quote from: Miyuki on September 21, 2019, 12:54:22 pmMakes no difference with MOSFETs. Or IGBTs, for that matter. The only reason to bring the gate negative during turn-off is to prevent shoot-through from Miller capacitance of the opposite leg turning on, but that is not a consideration in the two-switch forward.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 22, 2019, 02:44:27 am
content: Quote from: MagicSmoker on September 21, 2019, 09:58:24 pmA note, there's very little energy stored in Coss as such, but because the opposite side capacitor is in series with the supply, that voltage stacks with its Coss energy, flipping the curve: instead of delivering little power in the high-C region, it delivers maximum power there. The super-bottom-heavy C(V) curve makes it look like a drawn-out diode recovery (it happens over 10s of volts, not ~1V), and the low side switch dissipates similar power as a result (or the switching loop inductance absorbs the energy and turns it into voltage spike, or a snubber consumes it).Which is also a good point, with SuperJunction transistors you need to take special care of the switching loop at low current conditions. It's not hard switching into diode recovery, but it's almost as rough.QuoteWell, you can get more current through the driver+gate resistances -- this is basically why datasheet t_f figures are so much longer than t_r, there's less drop from Miller plateau voltage to driver output low voltage. It's not usually necessary, but it can still help you get a little bit more performance. (More often, you're burning switching speed with additional gate resistance anyway.)Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 22, 2019, 10:59:09 am
content: Quote from: T3sl4co1l on September 22, 2019, 02:44:27 amYes, this is an underappreciated 2nd order effect - and, to be honest, one I hadn't considered here - but I mainly referred to Coss because the OP did. That said, whether Coss stores enough energy to be a problem very much depends on the type of MOSFET and, especially, the thermal resistance from junction to case/sink/ambient, etc. Those TO-220FP devices tend to have terrible Rth-jc (typically 3C/W), so even 4-5W of additional loss can be a real headache.Quote from: T3sl4co1l on September 22, 2019, 02:44:27 amTechnically correct, but not remotely practical - or even desirable - in the context of what the OP wants; namely, to drive (5) switches in parallel. It would take heroic efforts just to get the transition times to 50ns, much the less the <20ns that could be achieved with a single MOSFET and an appropriately sized bipolar driver in a tight loop.

author: jonpaul
author_rank: Super Contributor
timestamp: September 22, 2019, 11:22:52 am
content: Hello from Paris:Paralleled MOSFETs are subject to parasitics inthe VHF range, use gate damper resistors and/or beads.Best topology for high power designs nowadays is NOT FWD, etc but soft switching, resonant, etc.You can read numerous papers and books on these topics. This greatly lowers switching loss and can run at 500k-20 MHz. Thus caps and transformers and inductors shrink.Just the ramblings of a retired EE, worked since 1960s in analog and power design.Bon journee,Jon

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 23, 2019, 11:51:47 am
content: MagicSmoker: You are right, I was blindly using some table from TDK and it is way off at this frequenciesupper limit is much lower, more like 150mT at max dutyjonpaul: Resonant converter with constant current output and wide output voltage? For constant voltage and steady currents they are nice, but for this application it little too exotic. I dont even find much papers about this use. And have no idea how it can be controlled.I might try to use one big Si MOSFET like IXFH40N85X with high allowed gate voltage and drive it really hardIt have Ugss +-30V and 40V transientsSo driving it with something like +20 and -10 can get reasonable small switching lossesAnd keep layout really tight to limit transients

author: Kleinstein
author_rank: Super Contributor
timestamp: September 23, 2019, 12:20:34 pm
content: If the power is to high for a single switch, one could consider a multi phase design. It needs a 2nd / 3 rd transformer / inductor, but could reduce parasitic inductance as smaller units are used. I am not sure if there are suitable controllers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2019, 12:50:10 pm
content: Resonant has a surprisingly wide output range, though I'm not sure how effective it is over a given range. I haven't tested it much. The maximum gain does depend on Q factor, and Q factor depends on loading, so the current limit must fall as the output voltage goes up (which is really to say, you're tapping a constant fraction of VARs from the tank, at a different impedance Z_L ~ Vout / Iout). VARs would actually rise with Q, except that there's an inverter current limit (often implemented as a differentiator on tank capacitor voltage; same thing), preventing it from exploding under unloaded conditions.What's your preoccupation with fast gate drive? It's not like you can handle, in any meaningful way, the parasitics of a TO-247 at that kind of current and dI/dt. You can't put a snubber any more than about 10nH away from the switch.What are you doing, CC/CV supply, full variable range?Tim

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2019, 12:52:16 pm
content: Could be also worthwhile to use some sort of two stage design or low/high voltage converters combined if power need is only ~1500W but 30A peak at 650v rail is needed.That would imply over 1:10 output voltage range adjust-ability or sumthing like that.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 23, 2019, 01:49:07 pm
content: Quote from: T3sl4co1l on September 23, 2019, 12:50:10 pmWith today available powerful and fast drivers like IXD_614 what will give me Fall time about 10ns with Cl 4nF and Output resistance about 200-300 mOhmAnd I need to get Turn off about 20ns @ 30A, gate drive shall not be a big deal with enough negative biasWith some smd diodes to catch voltage rise from transformer and power rails at opposite sides of board to get low inductance path back to capacitorsIt might work (at least in my head, it can produce big bang in lab)Output envelope is more or less limited to constant Power, with limits at about 100A and 80V, with actual output characteristic controlled by software to anything from CC to CV and weird shapes

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2019, 05:38:27 pm
content: Sure, they exist. But that's orthogonal to my statement, it's not a counter-argument it's just another fact?Hmm, come to think of it, as low as 10nH shouldn't be necessary, as long as the peak current really isn't much over 30A. <60nH should be adequate. You still have to figure out some kind of clever gate drive -- ideally, using a transistor with Kelvin drive terminals. Otherwise, source pin ESL degenerates gain at high frequencies, even if the switching loop is tight, and even if the gate drive is extra strong.I wonder if you'll be able to get away with Si PN diodes for the catch diodes, or need to use 1200V SiC.If you're obsessing over speed because of efficiency, consider lowering the frequency, or using a resonant converter front-end, powering a synchronous buck (probably multi-phase) output. The resonant could still provide pre-regulation over a modest range, but I don't know that that would really help you much; the buck still needs to handle the full range.More bother, certainly, but more likely to meet such requirements I think.Note that 80V and 100A is a full 8kVA operating range, so you need to dimension the inverter for that much capacity. Even if you're only using 1.5kW of it at any given condition. Switching loss goes as a fraction of the total 8kVA range -- all the capacitance and inductance and voltage and current swing are available whether you're using them or not. That's why resonant is so attractive here, the switch loop reactances are stirred into the supply every cycle, no need for switching loss as such.(There are resonant buck configurations as well, but they're similarly limited on input or output range.)Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 23, 2019, 06:22:31 pm
content: Primary current is peaking about that 30A for 100A outputI dont see any Si high voltage transistors with Kelvin lead, just SiC ones and it is not way I want, so just push drive voltage higherI dont need maximal efficiency, just have coolable semiconductors and have transformer at temperature it wont "melt" (up to 120°C)And keep reasonably simple control algorithmI will run some simulations about what waveforms will look with parasitic

author: splin
author_rank: Frequent Contributor
timestamp: September 23, 2019, 08:33:11 pm
content: Quote from: T3sl4co1l on September 23, 2019, 05:38:27 pmNot if you have paralled units, interleaved or not, powering up only as many as required for the given load. That also allows you to optimize the convertor efficiency as they only need to operate at a fixed output current, voltage or power (apart from one of them of course). Control could be interesting though - especially for more dynamic loads.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2019, 05:21:57 am
content: Dropping cylinders on a lightly loaded engine, nice.Good for multi-channel (usually phased), in parallel so variable over a range of current; harder to do over a range of voltage (outputs stack in series), but can be arranged that way as well.You could even make a grid of both, say 4 or 6 or 9 or however many channels wired back and forth like this, and remove channels that are unused due to the constant power curve!In the large-NxM limit, each stage could be a fixed V/I converter of whatever sort (probably resonant), and it would be a two-dimensional power DAC. Just turn channels on and off as needed. Probably a binary series though; well, maybe not strict binary, but a unary coded n-ary may be cheaper due to parts reuse (that is, use say base 3 instead, where each digit is coded as 00, 01, 11).And you could still, after all that, have a continuous section, which could even be linear, to fill in the remaining gaps while costing very little overall efficiency.Probably, the all around ideal N and M are very small, due to sheer parts count increasing cost and size, versus possible efficiency savings, or efficiency losses due to repetition.Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 24, 2019, 03:19:41 pm
content: So I put it in Spice and when expecting tight layout to keep inductances lowSimulated results are promising, turn on is very soft and at turn off it is still manageable (I know real diodes will be somewhat worse than LTspice ones), secondary side should also be fine with 200V schottkyesTransistor models are from Infineons library driven with +-15VEven calculated snubber losses arent something hugeSo I thing it worth try to build prototype

author: mzzj
author_rank: Super Contributor
timestamp: September 25, 2019, 10:57:09 am
content: Your sim model doesn't have any shared source inductance?