How to choose Mosfet resistor values for biasing and gate protection? - Page 1
Forum section: electronics / beginners

author: jnz
author_rank: Frequent Contributor
timestamp: January 01, 2019, 09:03:30 pm
content: I found a really long stackexchange post on this, and some other data but got lost in the weeds.See attached pic. I'm trying to hold that 5V DEVICE pin low all the time except when the micro decides otherwise, including the scenario the micro is not powered or has no code. I have up to 100mS to switch when it needs to happen. Something like this small transistor:https://www.mouser.com/datasheet/2/348/ru1j002yn-e-1018335.pdf]https://www.mouser.com/datasheet/2/348/ru1j002yn-e-1018335.pdfGate resistor:I think I need a gate resistor on the right side of the pullup closer to the mosfet. For gate protection, but as I understand this it's more micro protection because an unpowered mosfet will appear like ground to the micro for a short moment and the gate resistor is to slow this process down. So if I have a 5V micro that can source 20mA, I want something like a 250ohm resistor here to stay at that limit so long as the switching speed is acceptableSo... On that... I won't ever see the mosfet as "ground" because it'll never be unpowered, the biasing weak pullup is there.How do I determine an appropriate value for the gate resistor?Bias resistor:Next, when I turn this mosfet off by grounding the gate, I'll be leaking 50uA into the ground keeping the weak pullup down. This is more than 1/2 the "sleep" current I'm shooting for. Just how weak can I make this pullup? Can I "get away" with 200k? I don't know the line capacitance so I assume I can't really determine this effectively.Is there a general rule for acceptable mosfet gate bias resistor values?

author: spec
author_rank: Frequent Contributor
timestamp: January 02, 2019, 01:47:22 am
content: Hi jnz,The value for the pull up resistor is limited by the gate leakage current, which for the RU1J002YN small signal NMOSFET is quite high at +-10uA. So a 100k pull-up resistor may only pull up to 4V rather than 5V which would still be OK. But I would be inclined to find another NMOSFET with lower gate/substrate leakage current if you want to use a higher value of pull up resistor to reduce static current consumption. If I get time, I will have a look too. If you really want to reduce static drain current there are techniques to do this but the circuit would be more complicated.In slow applications like this, the gate resistor (gate stopper) prevents the NMOSFET from oscillating due to the NMOSFET's high frequency response and relatively large parasitic capacitances. A good starting point for gate stoppers in slow applications and with low parasitic capacitances (for a MOSFET) is from 10R to 500R, with 50R being a good choice. For high power MOSFETs, with parasitic capacitances of nF, a gate stopper of 1R to 22R would be more appropriate for slow applications. Note that the gate stopper must be physically mounted on the MOSFET gate terminal (or within a couple of mm) using as short a leads as possible.Just a cautionary note. The higher the resistance value of the pull up resistor, the higher will be the impedance of the MOSFET gate node which will become more susceptible to electrostatic pick-up, so a good layout is essential and, possibly, screening may be required. A decoupling capacitor from the MOSFET source to the 5V supply close to the MOSFET would also be advisable.https://www.mouser.com/datasheet/2/348/ru1j002yn-e-1018335.pdf(there is a bit more about gate stoppers in this thread:https://www.eevblog.com/forum/projects/circuit-for-mosfets-in-parallel-for-extra-current-capacity/msg2082850/#msg2082850)

author: spec
author_rank: Frequent Contributor
timestamp: January 02, 2019, 02:25:37 am
content: Hi again jnzThere are quite a few small-signal NMOSFETs with gate threshold voltages around 1V and gate leakage currents of around 100nA (as opposed to 10uA). A good example is the Diodes DMN2990UFZ. With this NMOSFET you could have a 1M pull up resistor and a recommended gate stopper of 47R, 50R, or 56R, so the static drain current would be reduced from 50uA to 5uA.Note though that the above is dependent on the microcontroller input/output pin leakage current being no more than 1uA. This will need to be checked from the microcontroller's datasheet.https://www.diodes.com/assets/Datasheets/DMN2990UFZ.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 05:26:29 am
content: Depends on the MCU pin characteristics. Some are clamped to VCC with diodes, others are clamped with zeners from ground. The latter is usually a "5V tolerant" type on lower-supply MCUs.Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2019, 12:10:17 pm
content: Quote from: jnz on January 01, 2019, 09:03:30 pmThe gate of an unpowered MOSFET always looks open with its normal input capacitance.I select the gate series resistor to limit the peak current through the I/O pin so somewhere between 1mA and 5mA or 4.7k to 1k but 220 or 270 ohms will work fine also.QuoteThat is always a problem. The issue is leakage from the MOSFET and leakage from the circuit construction so the environment is important.The gate leakage specified in the datasheet reflects how the power MOSFET was tested and not its actual performance so it will certainly be much lower. That makes 200k or even 1M feasible in most cases.But if you want to be sure and maintain the lowest power draw, then some kind of active pull-up which can be disabled is called for. This might be as simple as an external CMOS gate which is always powered.

author: jnz
author_rank: Frequent Contributor
timestamp: January 02, 2019, 06:31:59 pm
content: Quote from: spec on January 02, 2019, 02:25:37 amSpec,Thanks a ton for the advice and taking the time to look up a part! That's a little smaller part than I need, like seriously tiny, but I'll see if the package makes sense. If not I'll find a common SOT sized part I know I'll never get stuck on lead times with.Can you do me a quick favor. I get some of the issues with leakage current, butneed to clarifythat if the leakage is 10uA, that's "like" a 500k resistor to ground? Which is why 100k pull up and 500k pull down is a voltage divider to get 4.16V the gate would actually see - did I get that right?I'll try and find a cheap, in stock, common sized, smaller than SOT-23 part with low leakage current.

author: jnz
author_rank: Frequent Contributor
timestamp: January 02, 2019, 06:40:17 pm
content: I moved the gate resistor as close as possible to the gate pin.However... just a sanity check, a pull down resistor to bias the NMOSFET could be on either side of the gate resistor. Doesn't seem like it matters. But, a pull up resistor should to be on the micro side of the gate resistor, not the mosfet side, correct?uC---Rpullup---Rgate----GATEvsuC---Rgate---Rpullup----GATEThe first would just increase the pullup resistance to Rpullup+Rgate. But the latter would be a voltage divider when the uC is grounding, right? This doesn't matter at 100k up and 100gate, but would much more matter at 10kgate. Is there a practical issue here or does it not matter?For consistency is would this be an issue?uC---Rpulldown---Rgate----GATE

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2019, 10:50:35 pm
content: The gate resistor is such a low value compared to the pull-up resistor that the voltage division does not matter.Both should be located close to the power MOSFET for safety reasons and the pull-up resistor should be closest. If the connection between the driver and power MOSFET opens, then the pull-up resistor holds the MOSFET off or on. The gate resistor decouples the MOSFET from the capacitance of the connection between the driver and MOSFET preventing destructive oscillation. In extreme designs the gate resistor might be split in two with half at the MOSFET and half at the output of the driver.

author: jnz
author_rank: Frequent Contributor
timestamp: January 02, 2019, 11:39:24 pm
content: Quote from: David Hess on January 02, 2019, 10:50:35 pm100% opposite of what I thought. Really figured the gate resistor should be closer than the pullup.As to if the driver/gate opens, wouldn't the pullup prevent oscillations, or because it's so weak is this what you're saying? That because the gate acts like a capacitor this is where the oscillation would come from... but at that point I still don't see why it matters for oscillation if it's 100k pullup tapping in before the gate resistor or 100k+50ohm to at the gate after it. But do get why it would matter for division (but yes, inconsequentially). Guess I'm looking for best practice here.I get what you're saying about an extreme of two resistors. Makes sense, like SPI/I2C ringing.Not sure if it matters, but my original picture above showing a 1K pullup, is wrong, it's way closer to 4V and 250k pullup. I'm not sure that matters or not.

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2019, 01:45:04 am
content: Quote from: jnz on January 02, 2019, 11:39:24 pmIn most cases it will never matter.QuoteThe point is if the MOSFET becomes disconnected from the driver and the pullup is at the driver. In that case, it is disconnected from the pullup also.QuoteIf the pullup is that low then other considerations become more important. Either the gate resistor needs to be lower value which is usually not a problem or they need to swap positions to place the gate resistor closer to the MOSFET so no voltage division occurs.

author: spec
author_rank: Frequent Contributor
timestamp: January 03, 2019, 04:45:43 am
content: Quote from: jnz on January 02, 2019, 06:31:59 pmMy pleasureYes, that is a tiny NMOSFET- I was in a rush when suggesting it. I will find a decent sized NMOSFET for you.Quote from: jnz on January 02, 2019, 06:31:59 pmIt is indeterminate really, because from the datasheet the gate can source or sink 10uA worst case. What you can determine though is that if the gate is sinking 10uA there will be 10uA flowing through the 100K pull-up resistor, so there will be a 10uA * 100k = 1V drop across the 100K resistor. This means that the maximum voltage that the gate will ever see is 5V - 1V = 4V. If 4VG/S is sufficient to turn the NMOSFET on, the circuit will work OK. But if you increased the resistor to 500k the 10uA leakage current would cause a voltage drop of 5V so the gate could never get any higher than 0V, so it would never turn on.But, in real life, the NMOSFET would probably work OK with a higher value pull-up resistor. But the thing is why mess about with an NMOSFET that has a worst case leakage current of 10uA, when there are many other suitable NMOSFETs that have worst case leakage currents of only +-100nA. From a personal point of view, unless there were some other overriding factor, I would not consider a small signal MOSFET with a gate leakage current of 10uA for any design, even if the leakage current had no effect on the application whatsoever- why would you?Just a word about selecting components. There are three main approaches:Worst caseAverage valuesIt will be all right on the dayI always use worst case design whenever possible (not quite true, but a bit involved to describe here). So, for example, a particular BJT may have an average current gain (HFE) of 500, but a worst case HFE of only 100. Using the average HFE you may be able to get away with just one transistor for a particular circuit function, but using worst case you may need two BJTs in a Darlington configuration.The data sheet normally specifies average values and worst case values and you can design using average values, but then there is the risk that the very component that you buy is worst case and your circuit may not function. Average value design is used in some commercial products to reduce costs. On the production line, one in a hundred, say, of a particular PCB card may not work and would simply be trashed (for any fault).All right on the day is fairly self explanatory, I would suggest. A typical all right on the day design approach would be, I will be dissipating around 50W in this transistor so if I pop in a 50W transistor- it will be all right (on the day).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2019, 10:37:36 am
content: Quote from: jnz on January 02, 2019, 11:39:24 pmWe're not worried about oscillation once it's hard on (or off) -- there's not enough (small-signal) gain left to muck things up.Also, a pull(up/down) is no substitute for a gate resistor, in that the gate circuit oscillation has a low impedance. This can be found by taking Zo = sqrt(Lstray / Cg). Lstray in turn will be in the ballpark of mu_0 * length / 4, where length is the length of the stray inductance path (i.e., cables, traces, pins..). It's typically in the low ohms range, hence why modest value gate resistors are adequate, and why we wouldn't expect any contribution from a ~kohms resistor in parallel.If the driver is active and becomes disconnected and the pull resistor changes the transistor's state, then it's going through the linear range (on to off, off to on, whatever the case) where it can oscillate. In that case, there's a stub hanging off the gate (the wire/trace the driver used to be connected to), which can have interesting reactance at special frequencies -- but if we've still got the resistor there (or if the resistor burns out somehow, but it was placed close to the transistor, disconnecting the stub), that swamps the reactance and prevents oscillation.We can still screw things up in perhaps unexpected ways. A zener diode from source to gate makes a good gate voltage protector, but it's also a capacitance on a short lead inductance, and that combination has just the right impedance that it can oscillate -- typically in the 100s MHz, for fast transistors that can oscillate that high (which includes most high voltage switching (SuperJunction) transistors). In that case, simply putting the gate resistor closest, or using a ferrite bead or whatnot, does the trick.Tim

author: spec
author_rank: Frequent Contributor
timestamp: January 04, 2019, 03:56:05 am
content: + jnzAttached, as promised, is a list of a few NMOSFETs for you to consider.

author: spec
author_rank: Frequent Contributor
timestamp: January 04, 2019, 04:15:33 am
content: About the gate stopper resistor.The main function of the gate stopper resistor, is to stop parasitic oscillations by the MOSFET of its own accord. To be effective the gate stopper must be placed close to the gate and not be too higher resistance. The oscillations are due to the parasitic L,C & R of both the MOSFET and the circuit layout. It is quite right that a hard pull up or pull down can prevent oscillations, but this is bad practice, especially for a non continually switching application like this, and, sooner or later, it will bite you. A hard pull up or pull down may not prevent oscillations though- it all depends on the layout, including decoupling. Bear in mind that when a MOSFET oscillates it is normally in the 1MHz to 20Mz range (always 4MHz with me) and at that frequency range the physical circuit and components are nothing like the schematic. And with the newer low voltage, low gate threshold, high conductance (and massive parasitic capacitance) MOSFETs the situation is much more critical.But anyway, in this application, there is not a hard pull down and the pull up is so weak (aiming for 1MR) that in terms of parasitic oscillations it is practically an open circuit or worse, due to the parasitics of a high value resistor.About the gate stopper protecting the driving circuit (MCU). This is rather an odd one, because what are you protecting the driving circuit from? Sure, if you are driving the MOSFET in a fire and brimstone situation, say a SMPS, where there are all sorts of nasty things going on at the MOSFET gate, but then there is no way you could be driving the MOSFET direct under those circumstances anyway. Any gate stopper that had a high enough resistance to protect the driving element would slug the MOSFET to hell and it would not be able to switch at even 50kHz let alone 4MHz used in some SMPS circuits. Instead you would have to use a gate driver chip between the MOSFET and the controlling element (MCU). In that situation the gate stopper, in conjunction with other components, would be used for shaping the gate waveform rather than stopping parasitic oscillations.Just a footnote about parasitic oscillations and protecting the driving element.There are a few options that can be used to tame an oscillating MOSFET, in addition to gate stoppers. David Hess mentioned an effective one: fit a lossy elements on the leads of the MOSFET. This could simply be a lossy ferrite bead placed on the MOSFET self leads, as you often see on commercial equipment, especially SMPS and TVs.If you do have a concern about protecting the input of a MOSFET driving element, there are simple techniques to achieve this without increasing the value of the gate stopper resistor: a couple of schottky catching diodes, for example.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2019, 04:02:34 pm
content: An application of driver protection -- industrial modules. When an IGBT fails, well -- when any transistor fails with significant power available, it fails as a three-way short, connected by the expanding plasma ball that used to be the die and wire bonds. In an offline SMPS, or an industrial converter, say, that means hundreds of volts peak that appears at the gate resistor.I've designed a +/-15V, 15A peak gate driver with desat protection and isolation; when the desat protection wasn't enough and the IGBT failed, the driver always ended up partially damaged. I did add schottky diodes to clamp the driver output voltage to the supplies, but enough current still got into the driver to damage (but not explode) it.A somewhat more heroic effort would allow protection (bigger diodes, staged gate resistors?). Otherwise, figure it's going to die.A typical case in SMPSs is the controller chip (with internal driver) ends up poofed, so you have to replace half a dozen components (resistors, IC, transistor, fuse) to fix one.Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 04, 2019, 05:36:02 pm
content: I have occasionally had to split the gate resistor in two with half at the gate and half at the driver not to protect the driver from physical damage but to limit di/dt when driving the capacitance of a long line which otherwise caused excessive ground and/or supply bounce. Few applications are this sensitive.A high value pull-up resistor at the gate makes sure that the gate is held in a known state and the MOSFET is off if the gate driver becomes disconnected. A low value of pull-up resistor is used with an open collector or open drain driver when higher speed is required.I can think of a couple ways to protect the driver from a worst case power transistor short but none are as economical as just replacing the driver. A power transistor short is a complete failure anyway so it is not like reliability would be improved. The extra parts would actually reduce reliability, increase cost, and decrease performance.