Parallel power mosfets - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: August 15, 2018, 10:35:48 am
content: I was designing a DC load to test power supplies of up to 600 watt ( 20 amp at 30 volts ) and realistically there is no way to use a single mosfet for that kind of power draw. I was wondering if there is a simple way to balance the current of paralleled mosfets without actively monitoring the current through each one and adjusting the gate voltage as it would take way too many pins on the micro controller and a lot of PCB space. I will be using a VRM heat sink to cool the mosfets and smaller individual heat sinks for other components, so they are electrically isolated from the rest of the board, but connected together. I think that will reduce a lot of the voltage drops and the losses on the PCB itself.

author: 741
author_rank: Frequent Contributor
timestamp: August 15, 2018, 11:10:07 am
content: Horwitz & Hill, 3rd ed sayfor fully on/off (saturated) MOSFETS, just parallel them, no worries.For linear-region operation (not fully on or off) you need load-balancing resistors.I can look up the detail if you like.

author: mzzj
author_rank: Super Contributor
timestamp: August 15, 2018, 11:39:52 am
content: Load balancing resistors or active circuit with opamp between MCU and mosfets.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 11:48:17 am
content: To avoid pyrotechnic effects:- use LINEAR MOSFETs with a DC specified SOAR (e.g., IXYS) in parallel- stay FAR away from newest-generation switching MOSFETs.- to be on the safe side, go up to half the specified power rating max.- use load balancing resistors- if you can, provide separate gate driversMuch luck !

author: JohnG
author_rank: Frequent Contributor
timestamp: August 15, 2018, 12:17:47 pm
content: Quote from: Wolfgang on August 15, 2018, 11:48:17 amThis!If you can't get some of the aforementioned linear IXYS parts or similar, best to look for some old IR parts on ebay or similar.John

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 12:55:31 pm
content: if you have trouble getting linear MOSFET devices or budget restrictions, and you have strictly less than, say 30 or 40 Volts, you may use a bank of 2N3055s with 2A each.With proper fan cooling and equalizer resistors, this should work fine. If you need a low level drive, you could run those with a MOSFET driver in a "Darlington" configuration.

author: wraper
author_rank: Unknown
timestamp: August 15, 2018, 01:04:00 pm
content: Quote from: JohnG on August 15, 2018, 12:17:47 pmCertainly not on ebay, unless you want fake junk.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 15, 2018, 01:04:10 pm
content: I want to possibly make this a commercial product ... so I can't really source out older mosfets. I was recommended the BUK956R1-E100 but it's no longer manufactured. The fets will be in thei linear region for the most part, so they need balancing, but as I said, it cannot be done using the micro controller ... I.e sensing current through each one and providing a different gate drive voltage. I was recommended to put a gate resistor, a source resistor and a NPN transistor at the gate to actively balance the fets, something similar to this:Does this work with the newer mosfets you mentioned? otherwise can you recommend mosfets that can be purchased in bulk and still work fine with this method? or if there's a better method which works with the newer mosfets?

author: OM222O
author_rank: Frequent Contributor
timestamp: August 15, 2018, 01:06:24 pm
content: Quote from: Wolfgang on August 15, 2018, 12:55:31 pmUnfortunately that is gonna take a massive amount of space ... budget is not a really big issue but space is way more important factor.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 01:13:29 pm
content: If its commercial, I would go for:- current linear MOSFETs (IXYS). I guess you need 6 with a big fan cooling block for 30V 20A- ONLY over a reliable distribution channel (DigiKey, Farnell, ...)I do not like your gate drive circuitry. This is why:- gate capacitance of linear MOSFETs is in the multi-nanofarad range. You need to drive them with a really low impedance, not some kOhm.If you do it like you did, oscillations due to the nonlinear feedback capacitance are likely.- To be on the safe side, drive every MOSFET with a speedy, capacitive load tolerant OpAmp (e.g. LT1363) with proper compensation separately.Almost all commercial loads do that, and for a good reason. There are a lot of examples on the web.

author: exe
author_rank: Unknown
timestamp: August 15, 2018, 01:22:17 pm
content: I'm very curios what mosfets to use for linear operation. I've seen some existing power supplies and electronic loads, they seems don't always use "proper" parts. But they tend to use a lot of very beefy high-voltage (200V+) fets for the job. So, it seems the common solution is to derate mosfet by a huge factor.I wonder if high-current BJTs would be a better solution for this. Like, 2sta1943 and similar.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 15, 2018, 01:30:26 pm
content: I had a look at IXYS linear fets ... the IXTP80N075L2 seems to be a good choice ... and it's available from mouser or digi key (e.g:https://www.mouser.co.uk/ProductDetail/IXYS/IXTP80N075L2?qs=sGAEpiMZZMshyDBzk1%2fWiw4EkXjINyPrw%252bbxieWKqs3%252bgsLWFujKwg%3d%3d).what do you think about that?

author: OM222O
author_rank: Frequent Contributor
timestamp: August 15, 2018, 01:31:03 pm
content: Quote from: Wolfgang on August 15, 2018, 01:13:29 pmCan you please provide an example or a source I can read?

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 01:43:24 pm
content: You are right, a lot of manufacturers try to get away with a larger number of switching MOSFETs operated at a fraction of their power level, even if their datasheet does not even have a DC rating. For older devices (with a large chip area) this seems to work somehow; for newer ones with small die sizes it gets more critical with every generation.So - the problem gets worse of over time as die sizes shrink, and this is why special made linear FETs entered the market for electronic loads, startup circuiits, current limiters, ...The spirito effect is less prominent with lower voltge FETs, so some manufacturers take a chance and try to get away with that. For higher voltages (> 50V) I would not recommend to use any FET without a DC rating.For linear MOSFETs (e.g., IXTK240N075L2, at 25$) you have a 75°C case temperature specified SOAR of 575W (75V, 7.7A). Bipolars like the 2STA1943 (about 3$) you mentioned have 150W. So ist about 1:4.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 01:48:01 pm
content: Its even in this forum,Topic: Dynamic Electronic Load Project (Read 60210 times)and dave did one as well.

author: exe
author_rank: Unknown
timestamp: August 15, 2018, 02:17:30 pm
content: Quote from: Wolfgang on August 15, 2018, 01:43:24 pmWhy so? I thought smaller die make temperature gradient less pronounces, this supposed to help the problem...Quote from: Wolfgang on August 15, 2018, 01:43:24 pmAbout a year ago or so I tried to find linear FETs, not much luck... Those I found were terribly expensive (IXYS), available only in US and had huge gate capacitance (=hard to drive fast). May be they a good for big loads, but I didn't find anything less beefy for smaller projects.Quote from: Wolfgang on August 15, 2018, 01:43:24 pmYeah, you see the price?. Power rating is impressive, but I wouldn't dare to try to pump 0.5kW through a single device (a common opinion is maximum power ratings for mosfets are unrealistic as they assume an "infinite" heatsink).

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 02:30:37 pm
content: Quote from: exe on August 15, 2018, 02:17:30 pmOf course not. But the safety margin for a BJT or switching MOSFET is several times larger than a linear MOSFET. When you account for the number of devices needed, the size of a whole block, the driver circuitry and reliability, the cost of a linear MOSFET seems to be well justified. And at really high voltage (close to 1kV) you dont have a choice anyway ...

author: exe
author_rank: Unknown
timestamp: August 15, 2018, 02:34:55 pm
content: Quote from: Wolfgang on August 15, 2018, 02:30:37 pmThat's true, marketing department seems did a great job on factoring in this, uhm, factors into the price. The price is very high, but not too high. Too bad there are virtually no low-power linear fets.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 02:41:35 pm
content: If you ask me (and if you look at the whopping gate capacitance) they use a large die size plus the other tricks. The price factor is more or less the increased cost for the larger chip.We should X-ray a linear and a switching MOSFET to see what die sizes they have for the same current specs. My bet: price factor is about the same as chip size factor.

author: JS
author_rank: Frequent Contributor
timestamp: August 15, 2018, 04:53:21 pm
content: Quote from: exe on August 15, 2018, 02:17:30 pmA small die is harder to cool but ends having lower on resistance, so thats why is good for switching and bad for linear.JS

author: mzzj
author_rank: Super Contributor
timestamp: August 15, 2018, 05:10:47 pm
content: Quote from: OM222O on August 15, 2018, 01:06:24 pmYou need pretty big heat sink for 600 watts unless its liquid cooled so its effectively dictating your sizing in any way.10x MJL3281 BJT would handle the task with 75c heat sink temperature and 125c junction temperature. = 30 usdIXTK-TX240N075L2 can handle 166 Watts with same heat sink and junction temperature, you need 4pcs = 120 usdBJT's are easier to balance with resistors (need lot less voltage drop)For higher voltages the ixys mosfet would be clear winner.

author: exe
author_rank: Unknown
timestamp: August 15, 2018, 05:17:43 pm
content: Quote from: JS on August 15, 2018, 04:53:21 pmI don't think smaller die gives lower resistance, I'd say the opposite (think of a big mosfet as an array of small ones). It's just the advance in technology makes it possible mosfets with the same Rds, but smaller.I think bigger dies increase chances to have local defects and non-uniform area which I believe contributes to Spirito effect (due to single local hot-spotting). But I can't really support my point of view.So, how big linear fets avoid this problem? AFAIK they deliberately made cell array very non-uniform (with different Vgs(th)) so to avoid a single hot spot. Instead there are multiple small hot spots evenly spread all over the die which is safe because the whole heats evenly. I think I read about this in an IXYS datasheet.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 05:21:59 pm
content: ... not wanting to be pedantic, but the MJL3281A has no DC curve, either (the longest SOAR exposure time in the diagram is 1s) Probably not the best for an electron load

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 15, 2018, 07:54:27 pm
content: HelloQuote from: OM222O on August 15, 2018, 01:04:10 pmThat kinda thing could work but not with the configuration and values shown above. The idea is that the gate bjt'samplify the effect of the balancing source resistor. There is no amplification provided here with the valuesshown (Av ~ Rc/Re) so these bjt's are ineffective, also they would be more suitable if configured in adifferential configuration (that is sharing a common emitter resistor like in a LTP) rather than individualcommon emitter amps as shown above which limit's the min size of Re and thus voltage gain and would also amplifysupply noise (if it had any voltage gain in the first place).Even then I would say considering the extra components this method requires the alternative method of providing anopamp (quad op's are inexpensive) to drive each MOSFET individually is preferred , It's not that much different incost/space and would also have a better performance since we are multiplying available drive current rather thandividing it.The two factors to consider which contribute to MOSFET imbalance are1/ initial Vgs(th) spread due to manufacture process2/ reducing Vgs(th) due to neg temp co.Max initial Vgs(t) variation would be the deciding factor in minimum dc voltage gain required if trying to usediff amps to balance FETS .Quote from: exe on August 15, 2018, 05:17:43 pmWhats impressive about these IXYS MOSFETS is that they achieved it whilst still keeping a relativley high gfs(because lower gfs MOSFETS are more thermally stable anyway) . I also read that article about how IXYS achievedit and 'by variation in cell density' I think they meant they reduced the cell density (or make the vgs(t) ofcells higher) at certain places on the die that are susceptible to overheating and thermal runaway (like at theedges of the die or maybe near the pin connections where current density is highest) these are natural hot spotsand failure points I would guess. Since large gfs values aren't really required for a current sink though thereare plenty of cheaper alternatives you can use in your Eload. Also some of these IXYS mosfets have very largeCgd (Crss) values particularly when operating at very low Vds, this makes them unsuitable for Eloads which haveto operate right down to 0V input.Regards

author: Circlotron
author_rank: Super Contributor
timestamp: August 15, 2018, 09:21:21 pm
content: SPW47N60C3 is fairly robust, widely available and not too expensive at the right places. I’ve actuallly used several thousand Chinese copies that were quite okay. DC SOAR rated 400W.http://pdf.datasheetcatalog.com/datasheet2/9/0ofis9h4ghr8x0p4ac10l25yyupy.pdf

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 10:08:42 pm
content: Hi,I've seen it but the DC SOAR is almost too good to be true. Its an old device (not recommended for new design), so maybe it works.

author: ogden
author_rank: Super Contributor
timestamp: August 15, 2018, 10:36:34 pm
content: Maynuo and similar loads uses IRFP250NDiscussion:https://www.eevblog.com/forum/blog/eevblog-1023-rigol-dl3021-electronic-load-teardown/Comment section:

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 10:47:08 pm
content: ... the RIGOL loads do the same. I dont know if they screen their parts for coming from the old days with the larger die sizes,or they just use some many in parallel so they are (hopefully) far enough away from the spirito borders (even if they have no specs) ...

author: exe
author_rank: Unknown
timestamp: August 15, 2018, 10:53:42 pm
content: Quote from: ogden on August 15, 2018, 10:36:34 pmI've seen IRFP150N (similar device) in some linear applications from "reputable" vendors. It seems to be popular, I wonder why. Do they know something not mentioned in the datasheet? Or just a cargo cult? Rigol DP832 (power supply) uses IRFP260N, which looks to me a device from the same series.

author: exe
author_rank: Unknown
timestamp: August 15, 2018, 11:06:30 pm
content: I think I have a clue. Infineon's site says it's made using planar technology.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 11:07:54 pm
content: Hi, the IRF510 is an ancient part with a good compromise between switching speed and SOAR robustness.When you look at Fairchild, e.g., they had a DC SOAR specified for their IRF510A, so linear operation for those is not illegal.Just dont load it too hard (because of its high RthjCase), and it will survive switching and linear operation.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 16, 2018, 08:53:30 am
content: Guys I still haven't gotten answer to my main question, how do you parallel mosfets with load balancing?Do you suggest using a quad op amp setup with a gain of x0.25 to drive each mosfet individually?I appreciate the recommendations for the mosfet, but let's focus on that question.

author: duak
author_rank: Super Contributor
timestamp: August 16, 2018, 09:43:13 pm
content: Here's what I've found with loads which seems to be corroborated above. It's also used in one hp load I found the schematic for. The better (least bad) approach is to use an op-amp with a source sense resistor for each FET. The gain, ie. transconductance or Iout/Vin should be 1/4 of the total. There should be another current sense for the overall current.As noted before, a higher voltage earlier technology FET seems to survive better here, I suspect because the channel resistance can be higher than a lower voltage FET and so there is some inherent current sharing between the cells in the FET.Cheers,