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Can a low side driver control three N-type MOSFETs connected in series?

Discussion in 'Power Electronics' started by Scott McKie, Feb 5, 2019.

  1. Scott McKie

    Scott McKie

    Jun 20, 2014
    Can three series connected N-type MOSFETs be turned on and off in unison by one TC4420 MOSFET driver chip at 50/50 duty cycle / 65 kHz?
    They are all in the DC Ground line to a resistive LOAD which has it's other end connected to V+.
    Thanks in advance
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Why would you connect the MOSFETs in series?
    That sounds more like a parallel connection.

    Please show us a circuit diagram and give us some more background information to help us understand what you are trying to achieve.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    On behalf of Scott:
    Hello Harold,
    Thank you fro your interest in my question.
    The question is properly stated in that the three MOSFETs are connected in series with each other, with the resistive LOAD's ground connection beign with the Drain of the top MOSFET. The other connection of the resistive LOAD is truly connected to V+, in this case 24 VDC.
    FYI --my electronics background is being assigned to and graduating from the US Navy Class A Electronics School , Class A Radar School. I served in the CIC (Combat Information Center) aboard the USS MIDWAY CVA 41. I've been involved with a decades long electronics research project that produced my first US Patent in 1992 that was the basis for a new patent section.
    I'm pretty sure what a series circuit and what a parallel circuit is.
    I posted the question on this forum in hopes that I could get an answer to my question as no n seems to be available during a standard Googdle search on the Net.

    I hope that you can help me with this, as I know of no valid reason why it can't be accomplished.
    I'm hoping that you, or other forum members might be able to help me with this question.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The issue here is that the lowest MOSFET will need only Vg1 = Vgs1 >= Vgsth (assuming identical MOSFETS with Vgsth1 = Vgsth2 etc. = Vgsth)
    The second MOSFET will need Vg2 = Vgs2 >= Vgsth+Vds1.
    The third MOSFET will need Vg3 = Vgs3 >= Vgsth+Vds1+Vds2.

    For small values of Vds and a sufficiently high Vg such that the third condition above is met, a single driver may suffice. It is certainly not the best way but may be feasible depending on a lot of different parameters. You'll have to ensure that the driver circuit can deliver (sink and source) three times the gate charge of a single MOSFET in a short time. I also recommen you use separate decoupling circuits (resistors, diodes, whatever is used in a MOSFET driver circuit) to decouple the gate drive between the transistors.
    Note that very fast switching is of the essence. If one transistor were to pinch off faster than the other two transistors, this one will "see" the full V+ which is likely too much for it (I guess that's the reason for series connecting the MOSFETs in the first place, right?).
    It were imho a better solution to find a single MOSFET that can handle your power requirements.

    This app note for example doesn't deal with series connected MOSFETs but gives a lot of general background information on driving MOSFETs.
    Tha fios agaibh likes this.
  5. davenn

    davenn Moderator

    Sep 5, 2009
    Before this thread goes any further .....
    This would be a really excellent time to see your circuit so that we all can see what you are playing with
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Series connected mosfets are occasionally used to switch voltages higher than a single mosfet is capable of.

    In some cases there are resistors placed across the mosfets to help to equalize the voltage across them, other times the small about of leakage is used for the same purpose.

    A naive approach is to connect the driver to each of the gates via separate resistors. As you increase the voltage, the bottom mosfet turns on, which increases the gate voltage on the second (that then turns on) which finally reduces the source voltage of the top mosfet, increasing it's Vgs, allowing it to turn on too. The problem is that there is some delay between the mosfets turning on. This potentially results in one or more mosfets exceeding their maximum Vds. This will increase their dissipation while they are switching on and you need to be mindful of their avalanche limits.

    The reverse happens during switch off, however to have the additional problem that except for the bottom mosfet the others will have a negative Vgs. If the sum of the Vds of the mosfets below any given mosfet exceed the limit on Vgs then you will kill your mosfets.

    In your case, you specify a Vdd of 24V. I can't imagine you're planning mosfets in series to withstand 24V, but in any case, if your mosfets have a Vgs(max) of 30V you should be good to go.

    However it seems an odd thing to do for such a low voltage.
    Tha fios agaibh likes this.
  7. Alec_t


    Jul 7, 2015
    Do you have the MOSFETS configured like that to perform an AND function? If so, there's surely a simpler solution.
  8. Scott McKie

    Scott McKie

    Jun 20, 2014
    Hello to all who have answered, and "thank you",
    I am going to try as best I can under the circumstances to describe what my request for help is for, which hopefully will possibly clear things up a bit.
    First -- there are two sets of 3 series connected MOSFETs, a common resistive LOAD, and 2 more series connected MOSFETs between the two circuits "Ground" connection and 24 VDC "V+" connection.
    These two "sets" of MOSFETs are positioned within a circuit that also includes a common LOAD to them both, and also a common full wave bridge rectifier topology common to both MOSFET sets.
    Each complete MOSFET "set" is activated sequencially, at a specific frequency just under 65 kHz, controlled via the dual 12VDC, 50/50 duty cycle, square wave output signals from a 14528 Dual Adjustable Monostable signal generator. .
    The reason for using MOSFETs connected in series in each "set" is to be able to control the direction of voltage movement through common circuitry to both MOSFET sets in an opposite / reverse direction in some areas such as is common in a full wave bridge rectifier.
    The full wave bridge rectifier, in this case, is not diode based due to the voltage drops incurred, but are all N-Type MOSFETs, based on higher effeciency and controllable "on" / "off" times per MOSFET set separate from the input input frequency .
    This circuitry is not for any type of voltage control, as the MOSFETs being used are 600 VDC / 30 Amp / TO220 MOSFETs and the circuit "V+ value is only 24 VDC.
    Both series sets of MOSFETs sequentially supply voltage to the "common" resistive load, and the "routing" through the common sections of the circuitry is controlled on the "Ground" section by the three low side MOSFETs effectively wired in series, with the two high side MOSFETs, again effectively wired in series, completing the circuit.
    Each of the two sets of series connected high side MOSFETs are driven by a NCP5104 driver, with both low and high side driven in unison.
    I would post the schematic as requested here if the circuitry was under patent protection; but it is not as yet covered, so I cannot post it.
    I realize that this makes the job of understanding that much harder, but my hands are tied at the moment and I hope that this is not a "game buster" for all of you.
    I ask that you "center" specifically on my question / description and not leap to preconceived notions as to the circuitry purpose -- this will only make things more difficult.
    I can tell you that the circuitry has worked when rectifier diodes were used, but their drawbacks have necessitated this design change to MOSFETs, mainly because this is a low voltage trial and when successfully completed, the V+ value will be raised to several hundreds of volts.
    Because MOSFETs are not normally connected in series for "back and forth" voltage direction control in a common circuit -- I have come to you for help.
    I will endeavor to do my best to describe the circuitry in question to you.
    Thanks in advance for any more help you may offer.
  9. Alec_t


    Jul 7, 2015
    LTspice suggests it's possible, but unpredictable, if the FETs aren't perfectly matched, but the FETs would need to be logic-level types and the individual Vds experienced by the FETs will be quite different.
    Last edited: Feb 6, 2019
  10. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    It is too bad that you have somehow wandered into an open forum of skilled hobbyists, some of whom are also professionals in their field. We exchanges ideas and solutions freely here, although some members may make other arrangements in private messages that include some sort of recompense for services rendered. Perhaps you should seek their advice after completing appropriate non-disclosure and non-compete agreements.

    Since retiring to Florida after thirty-six years as a degreed electrical engineer, and many years before that as an electronics technician and hobbyist, I discovered that I am not allowed to practice electrical engineering in Florida without a Professional Engineer license. Because of Florida law, I am very cautious in my responses in this forum, lest someone with "esq" initials after their name decides to drag me into court for "practicing engineering without a license."

    My entire career has been spent doing work for the United States of America, first as an enlisted Airman from 1963 to 1967 performing maintenance on a sophisticated dual-radar controlled 20mm Gatling Gun fire control system; then as a civilian defense contractor from 1967 to 2014 with involvement in laser weapon systems in the 1970s, the intelligence and reconnaissance community in the 1980s, network and personal computer support for a major directorate at Wright-Patterson AFB in the 1990s, and operation, maintenance, and test configuration of a 1.7 MeV tandem particle accelerator from 1996 to retirement in 2014. Major customers for the particle accelerator include Northrop Grumman, Oak Ridge National Laboratories, and Homeland Security's military arm, Defense Threat Reduction Agency or DTRA.

    Not a single employer ever asked if I had PE credentials, so I never bothered to obtain an Ohio PE License, nor join the IEEE or any other professional organizations because of certain non-disclosure documents that Uncle Sam had me sign to obtain security clearances "above Top Secret". I don't regret any of this, but if I had done more due diligence I might have tested for the PE exam after graduating with a BEE degree from the University of Dayton in 1978. However, other things seemed to be more important at the time.

    Good luck on whatever your circuit is supposed to do. MOSFETs are fascinating creatures and are just now reaching their stride in pulsed power managed circuits. Patents look good on a resume, and may be necessary to obtain venture-capital funding for new product development, but I have always considered them to be a "license to steal" by corporate giants with deep pockets full of cash and a large stable of attorneys on hand ready to pounce on the "little guy" who dares to try to present any competition. Much better IMHO is the trade secret. You can't steal what you don't know about.

    Hop - AC8NS
  11. Scott McKie

    Scott McKie

    Jun 20, 2014
    Hi Hevans1944
    Could you have been born in 1944? If so, same here.
    Thanks for the kind thoughts
    The Patent was more for forcing the US Patent Office to deal with a topic it had been treating like the plague for decades.
    My project has been privately financed for over 10 years now, and we have exceedingly mega-deep financial pockets, along with some of the biggest and best legal sharks already on our side.
    Enjoy your retirement!

    Scott McKie -- [mod edit: E-Mail removed, use PM instead]
    Last edited by a moderator: Feb 7, 2019
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You may also want to have a look at this somewhat older thread.
    A picture, as requested, is worth a 1000 words...
    If you fear disclosure, then a verbal description is as problematic as a schematic. A cryptic verbal description on t h eother hans, to prevent disclosure, doesn't allow us to understand your problem and therefore we are a bit at a loss when it comes to helping you.

    Sorry, doesn't jibe. Voltage doesn't move (not normally anyway). Electrons move which is called a currrent.

    A set of series connected switches (that's seemingly what the MOSFETs are used for, supplies power (voltage, current) at the end of the series connection only if all switches are ON. If any of the switches is OFF, the end of the series connection is without power. As each single one of your MOSFETs seem to be well able to switch the power to the load, I can't see a need for a series connection.
  13. duke37


    Jan 9, 2011
    If you are looking to switch high voltage and current eventually, you should look up the way that the France/England interconnector changes DC to AC.
    hevans1944 likes this.
  14. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Or, in the United States of America, the Pacific Intertie Network.
  15. BobK


    Jan 5, 2010
    So, free energy?

    bushtech, hevans1944 and (*steve*) like this.
  16. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    This is a pretty deep rabbit hole to dive into unprepared, which I did a few years ago, thanks to the Internet, out of curiosity rather than conviction. I am still unconvinced, but like the folks from Missouri, "show me" and I could change my mind. The head honcho behind the zero-point energy, vacuum energy, free energy and other over-unity woo-woo "science" is Tom Bearden, who has a website with lots of links to branching passages in the rabbit hole. A reader could spend weeks or even years exploring, but in the end there is nothing substantial to find IMO. To balance out the woo woo stuff that Bearden churns out, and to warn weary explorers of the rabbit hole, there is this contrarian website.

    Well, it could be that we are wrong and this thread has nothing at all to do with woo woo stuff, that it is all about how to stack MOSFETs in series for greater voltage handling capability and how to make them all turn on and turn off in unison without self-destructing. Lots of stuff available in the open literature to explore that, but probably no one here with any actual experience (lately) to give worthwhile advice. Suggest the thread be closed until such expertise presents itself to a moderator.

    Hop - AC8NS
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