Back to Back MOSFET What Happens to Output Capacitance? - Page 1
Forum section: electronics / projects

author: hal9001
author_rank: Regular Contributor
timestamp: November 05, 2023, 10:46:49 am
content: If the output capacitance for a 2n7002 is 20pF then what does the total output capacitance become for back to back mosfets? Series caps should half capacitance. Is that what happens here? I want to pass data through these mosfets and want to know how output capacitance will effect the data.

author: CaptDon
author_rank: Super Contributor
timestamp: November 05, 2023, 02:21:43 pm
content: Please extend your drawing a bit. Where is your data output taken from? The center point between the two mosfets? Is this being used as a push pull current / voltage amplifier? How are you driving the fets to provide a gate delay so as to avoid both fets being on at the same time causing shoot through? Anyway, assuming the drain of the upper fet and the source of the lower fet are tied to power supply rails with basically zero impedance then in reality you will be charging and discharging the pair of capacitances in parallel and therefore it will act as double the value and not half as you propose. What is your intended data rate? The capacitance probably isn't even a factor until you hit maybe a 100KHz or higher switching speed.

author: temperance
author_rank: Super Contributor
timestamp: November 05, 2023, 03:04:54 pm
content: QuoteYou didn't read what the OP is asking. It's two back to back MOSFET's acting as a bidirectional switch. As such no special gate drive sequence or dead time is needed.To answer the OP question. You can calculate the capacitance but Coss changes with drain source voltage and manufacturer.https://www.onsemi.com/pdf/datasheet/nds7002a-d.pdfPage 5 bottom left: Cosshttps://assets.nexperia.com/documents/data-sheet/2N7002.pdfFig. 12Indeed capacitors in series 1/c= 1/c1 + 1/c2...The same applies for Cgs and Cgs in parallel with CdsAre you sure about the use of N-channel FET's?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2023, 04:15:21 pm
content: 10 to 100+ pF depending on Vds's.You will have to define the circuit better to get a more specific answer. More to the point, you may be disturbed that this answer isn't single-valued in the first place. The direct consequence is signal distortion as Vds varies with the signal itself. For digital signals (as a transmission gate), the average capacitance during an edge determines bandwidth (maximum bitrate).Due to the large capacitance, and awkwardness of driving Vgs, it's strongly recommended to just go get a (74HC)4066 or something like that, with far less capacitance and charge injection, and reasonable Rds(on) for logic signal purposes.Tim

author: CaptDon
author_rank: Super Contributor
timestamp: November 06, 2023, 03:40:44 am
content: You are correct that I didn't understand the O.P.'s use of the term 'back-to-back' and didn't derive a clue from the incomplete schematic. I wonder what the O.P.'s intended purpose and voltage, current and frequency will be 'gated' by this circuit.

author: hal9001
author_rank: Regular Contributor
timestamp: November 06, 2023, 09:34:04 am
content: I want to mux a connector pin that can be RS232 (+- 15V) or ttl data (3.3V) depending on hardware on the other side of the connector. Data speeds up to low MHZ. I hope this diagram helps.Quote from: T3sl4co1l on November 05, 2023, 04:15:21 pmThanks Tim. The bilateral switch like 74HC4066 is almost perfect but cant block higher than VCC (6V). Is there another device like bilateral switches that can handle up to 15V? MOSFET transmission gates look right for the job but Im only just learning more about them, hence this post.

author: hal9001
author_rank: Regular Contributor
timestamp: November 06, 2023, 09:42:13 am
content: Quote from: temperance on November 05, 2023, 03:04:54 pmCheers! Is it better to make a bidirectional switch with P-channel MOSFETs? Can you kindly explain the advantages of P-channel for this use?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2023, 10:43:14 am
content: Quote from: hal9001 on November 06, 2023, 09:34:04 amAh, interesting. And this must be electronically switched for some reason? (The most likely use-case I would think: the hardware interface varies with, well, the hardware, so a hardware solution should be more than sufficient: a switch or jumper. Or just not wiring them to the same connector at all.)There is a more expedient solution, I think:RS-232 drivers with disable are available. So you can trivially shut that off by just not enabling it. Good, got that done.If you had a 3.3V output driver with wide voltage range when disabled, you'd be set, just wire them together and be done.But how would you do that?Well, you could construct your own by making a tristate / bus driver gate, using diodes in series with the pull-up and pull-down transistors so that, when off, the transistor body diodes don't clamp the voltage to VDD/GND.But if we had a device with these already integrated, wouldn't that be handy?Consider the RS-485 bus driver. Yes, it's a differential pair, but we don'tneed touse it that way. Fundamentally, each output is a TTL driver with tri-state and wide output voltage range (the same series-diodes gimmick, or equivalent). It's differential when we use both outputs. But we could use just one, and leave the other open circuit. And we can ignore the receiver if it's a transceiver type chip.You'll want to check between 3.3 and 5V rated types, and see which ones have suitable outputs. Possibly among the 3.3V ones, there are high-level drivers where instead of a series PN diode, it's back-to-back MOSFETs, so it's actually switching it as such,Bonus, or liability as the case may be: the drive strength is huge, 100mA+. If this is uncomfortably high, simply put a resistor in series with it. Otherwise... maybe strong drive is useful even? Will certainly drive a long line a hell of a lot harder than RS-232 can. Not that a long line will have anywhere near the noise immunity needed for reliable use...QuoteThere are 15-30V parts, like the DG4xx series, and relatives. e.g.https://www.digikey.com/short/1hzqz82tNotice these need supplies exceeding the signal range, and you can either provide a +/-15V supply externally, or leech off the charge pump in the RS232 chip most likely -- check the internal diagram, but usually there's +V and -V pins, which normally just have a bypass cap and absolutely nothing else on them, but in fact they're just the not-so-internal supply rails of the device('s output drivers), and as long as you aren't using much, you can tap off some of this just fine. The datasheet may say not to draw current from these pins, which is a concern (you're on your own at that point*), but it's probably fine, for just a switch (very little load current).*This just means, it'll probably work, but the manufacturer may decide to change the design at some later date, and maybe it's really unhappy with current drawn from these pins, and now your design fails when made with new parts. Or that that's an unsupported configuration and if you ever go to them for support, they'll likely just say "well there's your problem" and not elaborate any further until you "fix" that discrepancy. If you don't care about either of these, and like it's a one-off or whatever, yeah, it doesn't matter.Alternately, use a super-old-school (ancient, really) driver like SN75155, which requires said supplies (no onboard charge pump), with the analog switch.There is the matter of transient protection. Since you say it's going to a connector, it's probably liable to ESD discharge, or errant screwdrivers, or cable-discharge events, or etc. You should probably get an analog switch rated for ESD, or add clamping or TVS diodes to the connector and some series resistance between diode and switch to further limit surge current into the switch.With the alternate en/dis-abled RS-232/485 drivers from above, you should be fine as long as both devices are rated to desired/required ESD levels (usually 8/15kV per IEC 61000-4-2). Possibly a disabled RS-232 driver doesn't appreciate being back-driven by something else; check that it's truly tri-state (allowed voltages while disabled includes the 485 driver's range).Oh, probably choose a 485 driver with fairly slow slew rate limiting, or controlled by pin and set it nice and slow. Comparable to the 232 driver will be good, to keep EMI down. Don't want a balls-out 50Mbps driver spewing trash across your bench, or cables or wherever this thing is going.Anyway, using proper devices -- interfaces, or switches, and TVS or other protection as needed -- keeps things consistent and simple and reliable, and means you don't have to solve awkward questions about exceeding Vgs due to transient conditions on the cable, or all the level shifting, that you'd need for the bare MOSFET analog switch circuit. (The most obvious problem with your diagram as shown: the logic is only, say, 5V or whatever, so it can only enhance an N-MOS to about 3V, and above that, it doesn't conduct. And for say -15V on the line, Vgs is now at the maximum 20V with no headroom for transient protection at all, and that thing is toast as soon as someone zaps it. There are many ways we can address that -- which are interesting analog design questions themselves -- but those questions will not help you get a switched driver any time soon, and are better explored for their own curiosity.)Tim

author: temperance
author_rank: Super Contributor
timestamp: November 06, 2023, 04:12:03 pm
content: If I understand you correctly, you want to use one UART to receive either a signal coming from an RS232 port or a signal coming from an other micro controller.In that case, a signal MUX is the wrong approach.What you want to do is convert both signals into the logic levels required for the receiving UART and use AND or NAND gates to select one or the other signal.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 06, 2023, 10:51:22 pm
content: Quote from: hal9001 on November 06, 2023, 09:34:04 amYour drawing will not work as hoped, as wired.The classic dual MOSFET switch is driven by a floating photovoltaic gate driver, very different from what you have drawn, as their gates can float.seehttps://toshiba.semicon-storage.com/info/catalog_20230823_ALQ00029.pdfyou want a low Coff, especially at higher baud rates, and some care to ensure RS232 does not feed into TTL.Something like a CPC1017 will do, or you may like a default-on choice, and use a CPC2317N.You can make your own MOS-relay, with a photovoltaic output part, like TLP3906 and your choice of FETsWith level shifting, vanilla MOSFETs might even be usable, as isolation is not critical here.What voltage supplies do you already have ? If you create the rs232, you may have +/-12 for example ?Addit: If size matters more than price, the MAX14777 claims-15V/+35V Beyond-the-Rails™ Signal Range from a Single 3.0V to 5.5V SupplySelectable -15V/+15V Signal Range for Lower Supply Currenthttps://www.analog.com/en/products/max14777.html

author: temperance
author_rank: Super Contributor
timestamp: November 07, 2023, 12:03:49 am
content: @ PCB.WizDid you read what I wrote?Solving this in the "mixed" signal domain is difficult. Solving this on the receiving micro controller side is easier and doesn't require any special components.-An RS232 receiver-One buffer and appropriate filtering for the TTL signal-AND or NAND gates to select which signal you want to receive.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 12:19:43 am
content: Quote from: temperance on November 07, 2023, 12:03:49 amYes, but the OP is actually wanting to dual purpose a pin.They want to outputeitherTTLorRS232 - rather like Apple phones use USB connectors for doing Audio or USB.

author: temperance
author_rank: Super Contributor
timestamp: November 07, 2023, 12:23:49 am
content: Me stupid, you are correct. I thought he wanted things the other way around.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 12:26:38 am
content: Quote from: hal9001 on November 06, 2023, 09:34:04 amThe issue there is gate drive.A simple remedy could be to add level shifters,assuming you have a +15V rail available.A part like PSSI2021SAY (Constant current source in SOT353 package) can give light gate drive, non isolated, that is 30V swing compliant.By default it sources 15uA, so a G-S resistance of 470k, gives you ~7V of gate drive. The TTL and RS232 drivers will not notice the 15uAPick a dual-N package with low Coss.

author: hal9001
author_rank: Regular Contributor
timestamp: November 07, 2023, 11:41:14 am
content: Thanks for the replies.Quote from: T3sl4co1l on November 06, 2023, 10:43:14 amThanks Tim you answered questions I was thinkng! RS485 line drivers have direction control. Do you think bidirectional data like SMBUS might even work with this method?Quote from: PCB.Wiz on November 06, 2023, 10:51:22 pmNever seen "Beyond the rails" switches, I will look at MAX14777 and similar parts, Cheers! Does Cout or Cio matter for baud rate for solid state relays like CPC1017? CPC1017 has both low but when searching for other parts, I wonder which to look for.Quote from: PCB.Wiz on November 07, 2023, 12:26:38 amIs the PSSI2021SAY to apply +15V to VS and control the gate of the MOSETs as in the example ?To answer some question, yes it must be electronically switched. I have +15 and -15V rails but I can change them lower or higher if it needs to be.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 08:22:50 pm
content: Quote from: hal9001 on November 07, 2023, 11:41:14 amCio is the cap across the device, between the LED and photo elements, it's usually fixed pF region.Cout is the OFF-STATE leakage cap, so that determines the high speed isolation.Note Cout varies with bias, and some parts elevate the test V to spec a lower CoutBigger FETs have lower RDS on, but the larger area gives higher Cout.Quote from: hal9001 on November 07, 2023, 11:41:14 amYes, the PSSI2021SAY sources 15uA, (no Rext) so you swap the LED with your MOSFET gates, and choose a gate-source resistor on the MOSFET pair of say 470k.Drive as shown with a Biased transistor, or a small mosfet, to shift logic to +15VWith +15V, the RS232 high swing static will be Vgs threshold below 15V, so choose a lower threshold FET. (still well above RS232 thresholds)There is a possible 30V max swing, but with the low 15uA I think its fine to use ESD protected FET that may clamp VGS.ie you are free to focus select on a low Coss FET, with low threshold : the Nexperia NX138AKS looks a candidate worth testing.