Multiple LL N-Channel MOSFET's in Series - Page 1
Forum section: electronics / beginners

author: TheDood
author_rank: Regular Contributor
timestamp: August 03, 2019, 05:47:09 pm
content: Hey guys, I was wondering if is it possible to reduce power dissipation by using multiple LL MOSFET's (VDSmax ~55V) in series to replace a single higher ΩDSon power MOSFET (VDSmax ~200V)? Are there special considerations to design for or take into account?As an example:(4) IRLZ44N : total RDSon 0.088Ω(1) IRF640N : total RDSon 0.150ΩThanks!

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: August 04, 2019, 12:02:55 am
content: Quote from: TheDood on August 03, 2019, 05:47:09 pmYou could put the mosfets in parallel to reduce power in each but together they would still generate same amount of heat.If its just a switching mosfet application why not use a lower RDSon mosfet ?

author: Circlotron
author_rank: Super Contributor
timestamp: August 04, 2019, 12:13:22 am
content: If it is a fast switching application like in a power supply then the total energy stored in the drain-source capacitance and dissipated every switching cycle might be a whole lot greater with series low voltage mosfets. Driving all the non ground referenced gates will be a nuisance. So will making sure the max voltage across each mosfet is not exceeded. IRF640 is not a modern mosfet. There are much better ones available today.

author: digsys
author_rank: Unknown
timestamp: August 04, 2019, 12:34:11 am
content: Without taking into account as to what it is used for - the answer is YES of course - BUT there are considerations to address, some have been pointed out -IF it is just a switch, and you're worried about losses, and multiple parallel is not an option (which it is) - then you have 2 major questions -1/ Protection of each FET, should one FET go O/C or short, leaving the other(s) with too high voltage2/ Extra complexity of drive circuits, as these are all at different levels and protection for 1/

author: Whales
author_rank: Super Contributor
timestamp: August 04, 2019, 12:44:03 am
content: > 2/ Extra complexity of drive circuitsThis would be my main point here. The lower mosfet can be driven normally, but the upper mosfet must be driven relative to its source pin. This requires much more complex driving circuitry.If at all possible: try and get a better mosfet, it will make things a lot simpler. Are there any reasons you would prefer to do otherwise?

author: Jwillis
author_rank: Super Contributor
timestamp: August 04, 2019, 01:02:58 am
content: Having two MOSFETs with different RDS(on) can cause current unbalance.This means that one of the MOSFETs would carry more load than the other creating unbalanced heat distribution between the 2.Edit: I should also mention that Mosfets in series can have unequal voltage sharing and with different RDS(on) can exasperate this problem even further.This can destroy the components.

author: Circlotron
author_rank: Super Contributor
timestamp: August 04, 2019, 01:06:38 am
content: https://au.element14.com/infineon/ipp110n20n3gxksa1/mosfet-n-ch-200v-88a-to220-3/dp/2212835

author: TheDood
author_rank: Regular Contributor
timestamp: August 04, 2019, 05:03:16 am
content: Thanks for the responses. The question is asked with SMPS in mind. Was curious what complications would arise. It seems there's easier ways to achieve a lower RDSon then using LL FETs in series.Feel free to list any low RDSon power FETs you guys use. That infineon is more what I'm after, thanks @circlotron, do you know of any that are a bit cheaper but still low RDSon like that?**I've been trying to understand Xc as well. I set up a 47nF film cap in series with an LED pair (1 LED faces 1 way the other LED is in parallel & faces the other way), 1k POT, and a 60Hz 120VAC potential. I calculate a Xc of ~56.4kΩ, this means only ~2mA would flow through LED pair? When I turn the POT to 1kΩ I get a 2.0V drop across it (though LEDs are uneffected regardless of POT rotation, LEDs seem to shine same intensity??), this correlates to 2mA. The LEDs read a 2.2V drop across them and the film cap reads 120V across it. All I've heard and read says that a capacitor in series with AC load drops V, but it seems like really it's current limiting rather than V dropping? Impedence deals with opposition to current flow, right? Is it capacitive resistance or capacitive impedence, and are they the same thing? (Pic is showing only testing ccts that I'm using to confirm an understanding, not using for anything in reality, bottom loop is Xc cct previously described)Thanks!EDIT:I'd throw the multimeter in series with the Xc cct to determine amperage, but my multimeter doesnt measure AC current :/ I have a single channel scope with a max sensing input of 50V, so I may try to throw that on there but its my first and only scope and Im pretty green so just double checking everything before I try to use it on the unisolated mains Xc cct. Was going to attach to either side of the LED pair and see what the wave form looked like and then see if messing with the POT is effecting LEDs or not.

author: Circlotron
author_rank: Super Contributor
timestamp: August 04, 2019, 05:33:35 am
content: https://www.digikey.com.au/products/en/discrete-semiconductor-products/transistors-fets-mosfets-single/278?FV=9780001%2C142c0051%2Cmu200V%7C2068%2Cffe00116&quantity=0&ColumnSort=612&page=1&pageSize=500

author: Jwillis
author_rank: Super Contributor
timestamp: August 04, 2019, 06:02:36 am
content: IRF3205http://www.irf.com/product-info/datasheets/data/irf3205.pdfGood all around for most switching applications up to 55 volts

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2019, 09:08:40 am
content: STW70N60M2, better Rds(on) than the examples you've brought up.Qg(tot), Coss, package size, cost? What are those?Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 04, 2019, 09:35:04 am
content: Thanks Tim, MOSFET Coss is something I did not know about and will need to study up on, Ill add it to "ferrororesonant transformer." Very informative.Whales, nope just curious if it was possible and what special design characteristics would be nessecary. I was thinking of trying to reduce power dissipation. I will take your advice.**EDIT:If I used N MOSFETS, to replace schottkys in a full bridge, wouldnt my gate V need to be greater than the load V?

author: TheDood
author_rank: Regular Contributor
timestamp: August 04, 2019, 09:47:41 am
content: Quote from: Jwillis on August 04, 2019, 06:02:36 amCan eBay be trusted on MOSFETS? Any components?Looks like the Vishay 200V FET (see attached .pdf thanks Ctron) @ $2.50/ is my cheapest low RDSon higher VDS option thus far. I'm liking the irf3205 for my LL goto, thanks JdubEDIT:I forgot to add the .pdf I was referencing, but it's there now, oops

author: Circlotron
author_rank: Super Contributor
timestamp: August 04, 2019, 10:03:12 am
content: Quote from: TheDood on August 04, 2019, 09:47:41 amThat's 55V not 200V.

author: TheDood
author_rank: Regular Contributor
timestamp: August 04, 2019, 10:33:34 am
content: Also, looking for a rail to rail comparator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2019, 11:50:37 am
content: What voltage, speed?Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 04, 2019, 07:55:00 pm
content: Quote from: T3sl4co1l on August 04, 2019, 11:50:37 amWas thinking single shottky for half wave rectification and a cap in parallel across comparator so that it had charge to output when supply V was 0. So I guess up to 120V? Also not sure on speed, 20kHz?I've some lm358s and was looking at the lm339 for true comparator but both opamps have lower supply V's than I was expecting. Trying to get acquainted with a few different "goto" generic choices with maybe some higher V supply maxes

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2019, 01:11:22 am
content: What?What, exactly, overall, are you doing? This appears to go well beyond single MOSFETs and comparators...Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 05, 2019, 03:37:16 am
content: Quote from: T3sl4co1l on August 05, 2019, 01:11:22 amI'm just tinkering trying to develop an understanding so that I can build XYZ in the future. When I say "Trying to get acquainted with a few different "goto" generic choices with maybe some higher V supply maxes," that is exactly what I'm after. I'm really big into effeciency and just starting out into electronics so looking for parts that are currently used (ie the IRF640N is outdated) that offer lower power dissipation than what I've been exposed to. Idk brands or who to buy from or anything, so just trying to learn.I'd like to make a variable CC/CV PS (what I had in mind when talking about the cap in parallel) that I can use for my bench, and also interested in audio amplification. I'd like to design a drone at some point and effeciency would be very important to me then as well. All projects seem like comparators would be nessecary, the higher voltage comparator more so in the PS, but I'm not even sure how accurate that is lol. I'd like to increase my comprehension so that I can modify cheap projectors to increase lumen count, make effecienct small wattage AC/DC PS's (95%+), create a drone + receiver, ec, just typical nerd stuff haha

author: BrianHG
author_rank: Super Contributor
timestamp: August 05, 2019, 05:25:01 am
content: Ok, 2-3$us in qty 10 parts:0.015ohm 200vhttps://www.digikey.com/product-detail/en/vishay-siliconix/SQP90142E_GE3/SQP90142E_GE3-ND/73263180.024ohm 200vhttps://www.digikey.com/product-detail/en/infineon-technologies/IRFB4227PBF/IRFB4227PBF-ND/1928083

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2019, 08:57:30 am
content: Okay, yeah... comparators and other control logic stuff, do it at low voltage. Add level shifters into and out of the block. So, a 100V buck converter made with a discrete controller, use a bootstrap gate driver.I tend to reach for LM393 (5-30V) or MCP6562 (2.7-5V) for jellybean comparators.Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 18, 2019, 08:17:05 pm
content: Hey guys back with more questionsCan someone look over this block diagram and help me confirm my understanding or correct it? Thanks in advance..Trying to reduce power dissipation in the rectification part of PS cct, so I can isolate effeciency loss to the regulation part.In terms of total efficiency is the power required by the op amp offsetting any localized effeciency gain noticed in a reduced diode turn on time?If I'm using opamps to slam diode on fast, am I using less power? Or am I just turning diode on faster, or both? There's still a V drop across diode, its just that the op amp is facilitating the V needed so that the load V isn't disturbed?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2019, 09:26:52 pm
content: What's the opamp powered from?What about the voltage drop in the opamp output stage?Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 19, 2019, 04:41:58 am
content: Thanks for the reply Tim, you've been very helpful.C1Depending on the load, I would have to add or remove R's, my thought was perhaps a 5V-10V max op amp output?The V drop across the diode? Or I'm not sure I'm comprehending? To me it seems the OP amp turns the diode on, but that there's still a 0.7V drop across it, what I'm trying to understand, is, P=V*I, so really, I'm not gaining any in effeciency turning the diode on quicker because the load current is still flowing through a 0.7V drop regardless the origin of the voltage (turned on diode is like (loadV + 0.7V), still flowing same current just a bit more V to calculate power dissipation from)? Also, because barely no current flows in an OP amp (right?), how much power are they consuming, generating? Is this offsetting any effeciency (if I happen to not be comprehending the power dissipation across diode correctly)?If I were to switch schottkys out for FETs, so that the load current doesn't get dropped by 0.7V, just by the RDSon of the FET (assuming current produces less V drop than 0.7V), would that increase effeciency?For calculations let's assume a 30VAC supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2019, 07:41:43 am
content: Just that the op-amp powered from a "sky hook" is the electronic version of the mechanical beginner'sshaft passer.The crux of it is this: it's easy to forget that the op-amp is itself made of transistors. It cannot generate more voltage than supplied with; the full load current (plus a little extra!) is drawn from those supplies; and the full voltage difference between supply and output is burned as power dissipation.Op-amp input pin current is pretty low, right, but the output current has to come from somewhere.So, making a synchronous or precision rectifier, with op-amps, and expecting any efficiency out of it, is a fairly early and simple mistake to make.Once this is realized, it's easy to see why this can never work, but also that one must explore circuits a bit more clever to actually implement it.So... not to beat you to death about it, just to be perfectly clear what it is, and that we've all been there at some point.Using transistors is indeed one such case; they must be controlled to turn on and off at the appropriate voltages/currents, but one should also be careful not to slam them on and off (e.g. using a comparator and gate driver), because that will lead to oscillation and ugly recovery* effects! A modest gain factor, between Vds and Vgs, is probably best.*Recovery is effectively the time a diode needs to "decide" when being turned on or off. There is forward and reverse recovery. Forward recovery is a relatively high forward voltage drop, for the first however many, usually nanoseconds, as the forward current rises to reach its nominal value; this manifests effectively as a series inductance (higher dI/dt --> shorter t_fr, higher peak V_fr). Forward recovery usually isn't very substantial.Reverse recovery is the time taken for the current to go through zero, to some negative peak I_rr, during which time the voltage drop remains near Vf; this manifests effectively as a parallel capacitance (higher dV/dt --> shorter t_rr, higher peak I_rr).Diagrams of reverse recovery and more discussion can be found here:https://www.allaboutcircuits.com/technical-articles/switching-losses-effects-on-semiconductors/In an emulated or active diode like we're discussing here, the recovery is determined by the speed of the op-amp and transistors.Here's a fairly simple example as a single diode, although, I guess I don't know how explanatory it will be since it uses a handful of BJTs -- I may be inviting something more complicated than you're prepared for..? Explanation below:The leftmost transistor is used as an inverted* diode (base strapped to emitter). This is done for two reasons: one, the forward voltage drop matches that of the next transistor; two, the breakdown voltage is higher (about Vceo, say 60V for 2N3904). The second transistor is also inverted, in a common-emitter configuration (except the collector is common, because inverted; but again, it works the same way!), so if the "Cathode" voltage is high, the first diode is reverse biased and the 470 ohm resistor turns on this transistor, pulling its output (the schematic emitter) down towards ground.*BJTs are always introduced like a Tetris piece, a stack of N-P-N or P-N-P blocks. But this is really pretty disingenuous as to how they work, or are made. Well, most BJTs aren't symmetrical, one end is made stronger than the other, giving that side has a lower voltage rating (typically 2-10V) than the other (typically 20-100V+), but also better "emission" of electrons/holes into the transistor, hence it's called the "emitter" and is typically used as the common (grounded / reference) terminal. It still works just transistorly if we swap E and C -- but, the performance is very different, hFE(inv) typically being quite low, under 5 say. So that will be one drawback that applies here.The PNP and three other resistors is a simple current source, about 0.6mA. This biases the second transistor, giving it voltage gain and a working range of about 0-5V.Finally the two rightmost transistors are a complementary emitter follower; they boost the gain node's output current from ~0.6mA to >60mA. This is able to drive the MOSFET gate reasonably quickly. The follower is not biased, which is kind of unfortunate (this introduces another nonlinearity, probably making this circuit awkward to use on arbitrary AC waveforms, say), but probably not worth addressing when used with most power circuits (like mains or switching supply rectification).So, conversely, when the "Cathode" voltage dips negative, the first transistor turns on, shunting current away from the second one, turning it off, and allowing the gate voltage to rise. This limits the FET voltage drop (Vds) to perhaps -60mV at low currents (the "decision range" is driven by the BJT's exponential gain, so a few multiples of 26mV goes from reasonably-fully-on to reasonably-fully-off), and I_load * Rds(on) at higher currents.Two or three major downsides to this circuit are: the supply requirement (5V referenced to "Anode"); the relatively high bias requirement (mainly the 470 ohm resistor, or ~11mA; this is due to the low hFE(inv)); and probably the switching speed, say for switching supply application (this will take some 100s of ns, which is worse than a high speed PN diode, and despite the lower voltage drop, it may not turn out to perform better than a schottky diode). For mains rectification it would be okay, but the first two issues are still unfortunate.Note another characteristic of this: the transistors act as an amplifier,with a limited output voltage range-- 0-5V. The input might be 40V. It's not enough to have a gain function here: there also must be a limiting function. If we used an ideal op-amp (one with a true skyhook for supplies!) to set Vgs = Vds * -100 say, we'd end up applying a tremendous voltage to the gate, blowing it in no time. We need a limited output, so Vgs remains in a reasonable range.Also, since it's a 5V supply, we would want to use a logic-level FET, which is fine, they're plentiful.The supply can of course be increased to use others, but the bias current will be even more annoying then, and a better solution would be attractive.There are of course ICs for this; LT for instance makes one, if you don't mind paying for it of course:https://www.analog.com/media/en/technical-documentation/data-sheets/4320fb.pdfWhich, despite the multi-dollar price tag, is really a pretty good deal when you're tight on space and can't afford the heatsinks or the temp rise (let alone the raw efficiency points) of a passive (diode) solution. The knock-on savings are heatsinks, mounting hardware, fans, all those sorts of things.Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 20, 2019, 12:50:59 am
content: Quote from: T3sl4co1l on August 19, 2019, 07:41:43 amThat was phenomenal! Its posts like this that allow people to really begin to understand and comprehend. Thanks. A. Ton! I've given it a once over but it will honestly take me a min to fully digest & comprehend. Appreciate the insight and methodical explanation. No offense taken.(I'm only trying to acquire knowledge so to me it would be well worth it regardless, but also, thanks for going easy lol)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2019, 02:54:07 am
content: Cheers!Tim

author: mzzj
author_rank: Super Contributor
timestamp: August 21, 2019, 01:44:05 pm
content: Quote from: TheDood on August 04, 2019, 09:47:41 amNo.Ebay is riddled with fake mosfets.Some other components are actually worth the gambling but high performance mosfets from china/ebay are not.

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 21, 2019, 02:10:45 pm
content: Quote from: TheDood on August 04, 2019, 09:47:41 amNo.Quote from: nigelwright7557 on August 04, 2019, 12:02:55 amIn case someone stumbles on this thread later: that's not true. Provided current is perfectly balanced, power loss would be half.

author: Circlotron
author_rank: Super Contributor
timestamp: August 22, 2019, 02:35:03 am
content: Quote from: Ice-Tea on August 21, 2019, 02:10:45 pmI'll second that.Each mosfet would have half the current and the voltage drop would be halved, so each mosfet would have one quarter the dissipation. Times 2 = half.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2019, 03:40:24 am
content: Total Rds(on) goes down by half, so for the same load current, voltage drop is half, and power is half.Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 24, 2019, 04:35:07 am
content: @Tim (and others)So I've tried to comprehend what you're telling me but still not quite there.What I've got so far:Op-Amp output is limited to OP-Amp PS.OP-Amp dissipation = deltaV between input & outputCurrent flow through a diode when turning off is somewhat like an inelastic collision, its almost like the current bounces off the diode, these elastic type of reactions are described as forward and reverse recovery characteristics?The duration of the recovery times is dependent on the amount of charge stored in the depletion zone and/or N-P boundary?Some more questions:If the op amp is in an essentially open loop gain configuration, the only power loss is the Vf of the diode between output pin and inverting input pin, right(?), but at what current? Is the current dependent on the diode or the op-amp? When you said the current has to come from somewhere were you saying that a decent portion of the load current flows through the OP-Amp output pin, or is it only the corresponding current of the diode at its Vf (if op-amp PS were limited to diode Vf)?Trying to understand where the greatest power losses come from, attached are (2) MOSFET recovery charts, 1 on, 1 off. Are we mainly concerned with the time duration it takes to complete the switching action (refer to chart)?Also, it looks like the efficiency plateau's around 92%? This is a generalized graph? The efficiency would be dependent on RDSon and the speed that Vds is changed? Not all FETs would plateau here? At 1kHz there'd be an even greater efficiency, albeit not much greater?As far as the schematic, I get the inverted diode config, but that's about it. Trying to understand it more but having difficulty visualizing the current path when cathode is +V and when cathode is -V.Also, why not just use 1 transistor with a 1kΩ on the base instead of the cascading triplet?When cathode is +5V, the MOSFET is turned on with ~5V @ gate, but when cathode <5V, the inverted diodes begin to turn on and bypass of the MOSFET gate which begins to close MOSFET. It seems that this cct doesn't slam MOSFET on & off? It almost seems like the inverted diodes or load line feedback is buffering the speed of the switching? Gate V goes from 0V to 5V but dependent on how fast load V is transitioning between 0V and 5V?

author: TheDood
author_rank: Regular Contributor
timestamp: August 24, 2019, 11:21:52 pm
content: Actually after further review, I don't understand the inverted diode configuration.When does T1 ever conduct?Is T2 emitter current = to its base current?Is there significance in T3 being PNP? Could you use an NPN there instead, and get same type of action?Is T5 redundant? Ie, does it ever flow current?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2019, 11:52:14 pm
content: If you need help visualizing the operation you can put it in SPICE. The inverted hFE of SPICE models is not usually very realistic, but it should still be adequate to show operation.For a testing circuit I would recommend a SINE voltage source and series load resistor, with the "anode" staying common ground. Try it without the active circuit (just the MOSFET, gate grounded), and with.Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 25, 2019, 07:44:13 am
content: Whats a good SPICE simulator to use? Tried EAGLE and partsim. EAGLE required more know-how than I was anticipating, are there better options?

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 25, 2019, 06:58:20 pm
content: LTSpice

author: TheDood
author_rank: Regular Contributor
timestamp: August 26, 2019, 05:44:08 am
content: Thanks @Ice-Tea I'll check it outCan you or someone help me find the ESR for the 2.2μF capacitor on this data sheet?

author: mzzj
author_rank: Super Contributor
timestamp: August 26, 2019, 03:41:03 pm
content: Quote from: TheDood on August 26, 2019, 05:44:08 am15-20 milliohms guessed from impedance graphs.

author: TheDood
author_rank: Regular Contributor
timestamp: August 26, 2019, 07:22:06 pm
content: Quote from: mzzj on August 26, 2019, 03:41:03 pmThanks, could you help me understand how you are calculating/guesstimating?I've been finding the Xc per Hz but when I plug into a 305V cct I get pretty much the exact figures that are plotted on the Irms charts.I was trying to note the perceived minor differences in the calculated ideal current and the plotted current, then solving for R in each scenario, and chalking up any differences in R to ESR, but I'm nowhere near precise enough. It seems I'm getting whole R values not 10s of mΩ values and not really sure this is what you were meaning?Example:2.2μF, @ 800HzXc = 1 / ( 2×3.14×800×0.0000022)Xc = 90.42ΩI = 305 / XcI = 3.37AChart looks maybe closer to only 3.3A???(305V / 3.3A) - (305V / 3.37A)=2Ω

author: mzzj
author_rank: Super Contributor
timestamp: August 27, 2019, 02:04:35 pm
content: Quote from: TheDood on August 26, 2019, 07:22:06 pmLowest dip in the impedance graph is indication of the ESR, ignore frequency and ignore the Irms charts. (page 12 Impedance Z versus frequency f)Not by any means 100% accurate method but better than nothing.

author: TheDood
author_rank: Regular Contributor
timestamp: September 24, 2019, 02:11:31 am
content: I've been working with LT Spice, but it takes really long to run only a 5s simulation (about an hr). Any ways of making it faster? The task manager says it's only under a ~25% CPU load and it's an old laptop. I've changed the tolerances to 0.01 hoping it would speed it up, but if it did I sure couldn't tell lol. I see a "Simulation Speed" at the bottom, is there a way to bump it up?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2019, 05:14:09 am
content: Five seconds? What are you trying to find that can't be learned in 1ms?Tim

author: Zero999
author_rank: Super Contributor
timestamp: September 24, 2019, 08:39:06 am
content: To answer the original question: yes, MOSFETs can be connected in series, in an arrangement known as a cascode, but it's a bit more tricky, than simply connecting them in parallel.https://en.wikipedia.org/wiki/CascodeFor help, regarding LTSpice: please post the .asc file.