PMOS gate protection for overvoltage with zener, help needed - Page 1
Forum section: electronics / beginners

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 10:59:16 am
content: Hi there, I am trying to protect P-Channel mosfet gate but i am not sure how. Mosfet Vgs is max -20V and my power supply (VSS) can be 12V or 24V .. so with 12V i don't need protection, but for 24V i would probably burn mosfet. I am driving mosfet gate to ground with driver.Driver spreadsheet:https://www.lcsc.com/datasheet/lcsc_datasheet_2412091609_JSMSEMI-IX4426NTR-JSM_C7466362.pdfMosfet spreadsheet:https://www.lcsc.com/datasheet/lcsc_datasheet_2410122037_HL-120P03_C20071222.pdfI am not sure how this zener will work with this parallel resistor of 10K, but if i leave without it then 12V will not work to charge the gate since zener will block it.By default gate is pulled high to VSS voltage so mosfet is off, when i set IN1 high, driver will pull OUTA to ground, creating a negative gate voltage, but if it is 24V, this will burn the mosfet or i am doing everything wrong here ?D1 is 18V zener, and Q2 is the same circuit as Q1 (it is 2 gate driver)... thank you for the help

author: awesomechapro
author_rank: Contributor
timestamp: March 16, 2025, 01:44:03 pm
content: I would recommend removing or significantly reducing the value of resistors R1 and R3, as a high value doesn't allow the zener to clamp the voltage properly. Apart from that it should be fine.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 16, 2025, 01:54:25 pm
content: Also need to increase the value of R7 and R8 to limit current through the Zeners. This will also slow down the turn-on time of the FETs, but there's probably a reasonable compromise to be found.Honestly given how many discrete components you're going to need, the gate driver IC is basically pointless. Might as well go for an all-discrete driver.

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 02:23:37 pm
content: Quote from: mtwieg on March 16, 2025, 01:54:25 pmarent r7 and r8 at the and together with 10K, basically i think that gate is powered directly from 24V over 10K resistor and driver is only to pull it down to ground (open it). I am using driver because i need logic level to control the gate (ESP32) is on board (inverted logic .. high signal = low gate) .. can you explain or give some example of what you mean by all-discrete driver ?

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 02:26:57 pm
content: Quote from: awesomechapro on March 16, 2025, 01:44:03 pmI am still beginer in this so i am not sure how to calculate those values .. when you said significantly reduce the resistor .. you got any idea what values should that be? .. i was thinking of high resistor because when mosfet is on (and mostly it ill be constantly on) to have less current pulled to the ground, driver will pul OUTA to the ground and there will be constant current here, or i am wrong ?

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 16, 2025, 02:52:31 pm
content: Quote from: Cvijo on March 16, 2025, 02:23:37 pmMy suggestion was in addition to awesomechapro's.In order for the zener diodes to limit the Vgs on the MOSFETs, the value of R7/R8 must be much larger than the value of R1/R3.The circuit as shown in your first post would not provide any reduction in Vgs, since R1/R3 are much greater than R7/R8.If you were to only remove R7/R8, then this would limit the Vgs. But this would also result in very large currents flowing through the diodes and R7/R8, likely damaging them. Increasing R7/R8 (maybe 1-10K) will reduce this current to more reasonable levels.

author: Peabody
author_rank: Super Contributor
timestamp: March 16, 2025, 02:55:58 pm
content: I don't understand how the zener circuit and the parallel resistor have any effect on the gate. The driver is high-current push/pull though a series 22R resistor, so how does that other stuff have any material effect on the gate voltage? When the driver output goes high at either 12V or 24V, the gate will be very close to that, and when it goes low, the gate will be very close to ground.It seems if you connect the gate to the midpoint of a resistor divider between V+ and the driver output, then the gate would be at V+ when the output is high, and it would be at V+/2 when the output is low. So at 12V, the gate voltage would drop to 6V, which is still more than enough drop to fully turn on the mosfet. Of course the resistors would slow it down a bit, so the driver becomes just a voltage translator for the control lines.Edit: Or maybe you could just drive the gate directly through a series 6V zener. When driver output is high, the gate would be at V+ - 0.7V, which is still off. And when it's low, the G/S voltage is at 18V (barely ok) or 6V, but the mosfet is still fully on. And you wouldn't lose any speed, or waste any power in resistors.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 16, 2025, 03:35:43 pm
content: Quote from: Cvijo on March 16, 2025, 02:26:57 pmTo simplify let us ignore R2 for a moment and now (without it) calculate a voltage at Q1 gate.We have 24V at connected in serie R7 (22ohm), D1 (18V), R1 (4k7).As 18V is drop along D1 then 24-18=6V left to be drop along 22ohm+4k7. It is typical voltage divider so you can calculate voltage along R1 as 6V * 4k7/(4k7+22ohm) = 6*4700/4722 = 5.97V.At gate you have voltage that is drop at D1 and R1 so 18V+5.97V=23.97V.We cannot say that you didn't reduced 24V but the reduction is certainly not enough.When you understand this calculations read once more all answers you got.Quote from: Cvijo on March 16, 2025, 02:23:37 pmTypical small signal npn transistor can have Vce like 50..70V and need only 0.7V to switch it on. So it can easily be driven from digital circuit output and can have its collector switching 50V.

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 04:09:51 pm
content: Quote from: PGPG on March 16, 2025, 03:35:43 pmWell i still need to understand this, as i said i am beginner and i did read and watch about zener, but i guess i got it wrong, i thought it will have only 6V after zener not 18V+6V .. so my calculation was that on the gate i will have 6V, so 6V(gate)-24(source) = -18 Vgs.R2 was added when VCC is 12V battery but that again i don't think will work. Trying to understand all this and now i am even more confused about your answer and 23.9V on the gate

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 16, 2025, 04:41:59 pm
content: Let's start from the beginning. What is the purpose of this circuit? Just to switch two DC loads from +12 ... +24V supply line? Does it need to be a very fast switching? If not, you don't need a gate driver IC, a few small signal BJTs will do the trick and will allow you to protect the gates etc. In the circuit as shown you have painted yourself into a tight corner, as the driver chip can output over 1A current and will just blow up any zener protection unless you put a sensible series resistor on the driver output (increase R7, R8 to 1K and remove R1, R2 for example). This will however slow the switching speed considerably and if you switch heavy loads (Amps) frequently, could easily overheat MOSFETs as the energy dissipated during longer switching transients might be substantial.CheersAlex

author: ledtester
author_rank: Super Contributor
timestamp: March 16, 2025, 04:52:54 pm
content: There is discussion of a similar problem (supply can be up to 32V, max Vgs is 20V) here:https://www.eevblog.com/forum/projects/searching-a-high-side-driver-ic-for-a-p-channel-mosfet-do-you-know-any/

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 05:54:53 pm
content: Quote from: Alex Nikitin on March 16, 2025, 04:41:59 pmHi Alex, circuit is for my boat switching load with display (picture in attachment) .. this is my first PCB and project so i am learning this on the go.For switching speed it is usually 80% will be ON or OFF state (like fridge, pump, sonar, radar, autopilot, horn, anchor up/down, etc ...), some of the load will require PWM, usually only LED lights for dimming, so i guess 20KHz is enough for that.There is 16 mosfets (channels) and each should be able to handle up to 10A, usually 2A (i tried to find the lowest Rds PMOS 2mOhm). When i did a calculation for a temp of the mosfet it was good without the heatsink.My idea was to be able to connect this circuit to 12V service batteries or 24V service batteries, depending on the boat.This all has to be high side switching (because of how electricity is installed on the boat, only positive wires going to console) so i am not able to use N-channel or at least it will complicate things with bootstrap and 100% duty cycle problem so i decided to go P-channel.I could use only the driver without the pullup resistor to the VCC, but then there is a 1ms delay when you turn on the power all mosfets are ON until driver start to work (until my microcontroller doesn't set input driver signals to low) and I guess i would like to keep the mosfet OFF by default not to leave the gate floating until driver kicks in.So what you are suggesting is to remove the pull up resistors (R2 and R1) and only change R7 to 1K (to limit current from driver to the gate, and to protect zener in normal mode not reverse bias), and for 20Khz switching for LED dimming this would still be Ok i think.And what about a problem with the power on and initial state of the mosfets or this is something i shouldn't worried (when i tested it all the led lights (16 of them) turn on for short period and i was thinking that will not be good on the boat to turn on all the loads).Thank you for your help

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 16, 2025, 06:33:54 pm
content: And you have built it already!OK, let's see what can we do in this situation. 20kHz PWM complicates things however I did a quick simulation of the circuit attached using a similar size MOSFET model and it appears OK-ish. You would need the Schottky diode (D2 on my circuit) for PWM, though you might get away without it for an occasional switching.To clarify - V2 source is your driver chip output, R1 is load for 12A .CheersAlex

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 07:49:30 pm
content: Quote from: Alex Nikitin on March 16, 2025, 06:33:54 pmwell i know .. i wanted to learn how to solder too so i said, let me order that PCB I made and solder that.. all solder joint worked too (and again i never solder anything, especially not smd components), and tbh, everything is working atm with the first try and that was shocking for me(except that overvoltage protection for the gate, when i put 21V supply nothing is clamped, it is 21V between gate and sourceQuote from: Alex Nikitin on March 16, 2025, 06:33:54 pm20KHz might even be lower, i must check on what PWM led lights should work without blinking, but i think i read somewhere it is much lower than 20Khz for eyes to see the blinking.Question about that Schottky diode, what is the purpose of it, to charge the gate faster, bypassing the resistor?So if i switch my components with those, and with your values i should be good?Thank you very much for helping, and everyone that replied, i was doing search on all those topics and it's too much information for me that I've never done anything in electronics. Jumping to this forum and how you people reply fast and put in effort to help is outstanding, thank you all very much.P.S .. i tried to replicate your schematics in LTSplice i think it is called, download it few weeks ago, how can i set-up that mosfet options, i managed to set up the pulse from your screenshot, but when i pick from components i only got some fields to populate like (length, width, drain area, source area ...), and 22Ohm resistor is just to limit the current for zener, and if i lower that 470 Ohm resistor wouldn't that result in better switching or the problem is the zener that will get hot?

author: Cvijo
author_rank: Contributor
timestamp: March 16, 2025, 08:01:02 pm
content: Quote from: ledtester on March 16, 2025, 04:52:54 pmTy very much for this link, i was looking at that Ian circuit but at my level it is science fictionway too advanced for me atm, but did learn a few things to consider, ty.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 16, 2025, 08:09:22 pm
content: Quote from: Cvijo on March 16, 2025, 07:49:30 pmNot to charge but to discharge the gate, without this diode the MOSFET will close too slowly.Quote from: Cvijo on March 16, 2025, 07:49:30 pmMight be. It should protect the FETs. One thing to watch for is a potential heat dissipation on PWM devices. Lowering the PWM frequency will help.Quote from: Cvijo on March 16, 2025, 07:49:30 pm22 Ohm is an anti-parasitic here, it lessens the chance of self-oscillations on the gate which can be deadly.I attach the file for LTSpice simulation, all models are standard from the package, should work as is.CheersAlex

author: PGPG
author_rank: Frequent Contributor
timestamp: March 17, 2025, 01:01:46 am
content: Quote from: Cvijo on March 16, 2025, 04:09:51 pmYou are confused because you understand all voltages as measured in relation to GND and I was writing about voltage at MOS gate pin related to its source pin (it is this voltage that is limited in datasheet) and in addition when writing I was ignoring the sign of voltage. I thought that it is obvious from calculations what I am calculating so also about what voltages I'm writing. PMOS transistor "doesn't know" how far it is from GND so doesn't cares about voltages related to GND. Only voltages related to its source pin cares for it.Quote from: Cvijo on March 16, 2025, 04:09:51 pmMy calculation says that you will have 0.03V (related to GND) at gate so Vgs will be 24-0.03 or rather 0.03-24. I never think with negative values. Calculating (and imaging) in head difference between two negative values is more complicated that difference between two positive values.I just know if the patient is NMOS/npn or PMOS/pnp and always think with positive numbers (only sometimes 'my voltages' rises toward real positive supply and sometimes rises toward negative supply.When I started to be interested in electronic there were only pnp germanium transistors available and all circuits had (+) of battery connected to GND and (-) as a supply. Instead of using '-' in front of any calculated voltage (as + is at GND than everything in circuit is at negative voltages) I was always thinking that base is at 2V (really -2V) so at emitter I have 1.75V (really -1.75V) because Vbe=0.25 (really -0.25V) and so on. It is much simpler thinking that Vbe = 2-1.75=0.25 than thinking -2-(-1.75)=-2+1.75=1.75-2=-0.25.When npn came I thought about them exactly the same (something like using a different frame of reference).So I always think in positive numbers but just know what they mean and than for PMOS/pnp they just have the arrow pointing in opposite way.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2025, 10:39:44 am
content: Quote from: Cvijo on March 16, 2025, 10:59:16 amThe IX4426 is only specified to work properly up to 20V. 25V is the absolute maximum rating, i.e. exceeding that voltage might damage it, but it doesn't mean it will work properly. Refer to the recommended operating conditions on the data sheet.Perhaps a few transistors is a better solution.

author: Cvijo
author_rank: Contributor
timestamp: March 17, 2025, 01:08:50 pm
content: Quote from: Zero999 on March 17, 2025, 10:39:44 amHi Zero and thank you for the suggestion and point out IC max voltage problem and i would like to simplify the curcuit but aren't transistors bad at PWM, I will need 20Khz PWM will that work with that too ?

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2025, 01:41:40 pm
content: Quote from: Cvijo on March 17, 2025, 01:08:50 pmThere is no reason why transistors can't be used for PWM. A MOSFET is a type of transistor after all.The circuit I've posted above isn't the fastest solution, but it's more than adequate to drive your MOSFET with 20kHz PWM.It's designed for 5V logic. I missed the ESP32. Fortunately it's easy to modify for 3.3V logic, just swap R2 for 1k to 1k5.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 17, 2025, 01:54:58 pm
content: Quote from: Zero999 on March 17, 2025, 10:39:44 amWell, thereal IX4426 made by IXYSis rated for Vcc up to 30V (35V absolute max), but the one Cvijo linked to is apparently a knockoff. Just another reason to not use the IX4426 (either version) and do an all-discrete driver.Quote from: Zero999 on March 17, 2025, 10:39:44 amThat should work if the resistor values are chosen properly.Here's another option I've used many times in the past (different part numbers, but overall same thing). The base and emitter resistors of Q2 can be used to adjust the pull-down strength.

author: Peabody
author_rank: Super Contributor
timestamp: March 17, 2025, 02:03:44 pm
content: You might need 20KHz for LED dimming for photography or video lighting. Or if the switching is going to be audible, 20KHz or even higher might be needed to take it above human hearing. But otherwise, I don't think you will actually see any flicker if it's 100Hz or higher.

author: Peabody
author_rank: Super Contributor
timestamp: March 17, 2025, 02:09:46 pm
content: @Zero999, in the first transistor circuit, what prevents both transistors from being on at the same time for a while during each transition, producing a short?

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 17, 2025, 02:27:18 pm
content: For a beginner’s project, yours looks extremely nice, congratulations!It is unfortunate that you have already laid out the PWB, because I would have used smart high-side power switch, which are extensively used in the automotive sector.For instance, the Infineon BTS723GW provides two independent switches in a single package, is capable of operating up to 58 volts, has a diagnostic feedback, and has a host of protective features like short circuit, overcurrent limit, thermal, over voltage, reverse battery, under voltage lockout and others. Capable of 2.9 amps per channel which can be paralleled.This is only one example. There are many more options available from a dozen different manufacturers.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2025, 03:03:51 pm
content: Quote from: Peabody on March 17, 2025, 02:09:46 pmThe dead band of 2VBEwhen one transistor turns on and the other turns off.

author: Cvijo
author_rank: Contributor
timestamp: March 17, 2025, 07:20:51 pm
content: Quote from: schmitt trigger on March 17, 2025, 02:27:18 pmThank you, it is a prototype and first attempt and I already made a new design with some improvements and added esp32 integrated on the board, as you can see now on my picture i made a dev board pins just to plug the board. I could make a change that you suggested and i have already looked for some power switches. The problem i think with your example i would need 3 in parallel and that chip is expensive, multiply all that by 16 (that's how many i need outputs) its adds up a lot. Protection is nice to have what they offer but i would say it is overkill, i already changed my design and put 2 P-channel MOSFETs for reverse battery protection (I haven't test that yet. I added few screenshots of the PCB and battery protect circuit so someone can maybe give me additional suggestions for improvements, but i should open a new thread for thatI found that this one BTS6143D (https://www.lcsc.com/datasheet/lcsc_datasheet_2410121621_Infineon-Technologies-BTS6143D_C95232.pdf) is a much higher current power switch so switching to that one might be worth it, i guess i don't need any of that other things like gate driver, zener diodes, resistors etc.) but i am not sure will i need more in parallel to get 10A per output without overheating.Again thank you all very much for helping me out, help here is way over my expectations, and for a beginner like me this means a lot !

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2025, 07:25:28 pm
content: Have you built the board? If so, here's a way to make it work, just by moving parts around. It involves putting the zener in series with the gate, as suggested by peabody. The downside is gate voltage is dependant on the supply voltage.

author: Cvijo
author_rank: Contributor
timestamp: March 17, 2025, 07:53:40 pm
content: Quote from: Zero999 on March 17, 2025, 07:25:28 pmHi Zero, thank you again .. i have built the first version just out of curiosity, but this new version i am still tunning and trying to make it better and listen to all suggestions you guys have.Can you explain a bit if you don't mind what you mean by gate voltage is dependent on the supply voltage, i see that you removed 4.7K resistor and put Zenner 6.2V on the gate.I attached a new schematic that i did with Alex help on this thread (changing some resistors and putting 40V 3A 500mV Schottky diode) for faster discharging as he mentioned.Edit: ahh i see, so if my VCC is 12V i will have like 6V difference at the gate, might not be to fully open the mosfet, but with 24V it will be at 18V, am i right ? (this thing for working with 12 and 24 is giving me headachei should built 2 boards depending on the battery voltage)

author: Cvijo
author_rank: Contributor
timestamp: March 17, 2025, 08:02:19 pm
content: Quote from: Zero999 on March 17, 2025, 01:41:40 pmI have tried to make your circuit in the LTSpice but i am doing something wrong here i think .. my gate voltage is not switching.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 17, 2025, 08:27:58 pm
content: The STMicro VN5025AJ-E is 40 amp, 41 volt high side switch. With a 3 volt compatible logic input.At US$2.48 in single units, I don’t think you could match the price using discretes while implementing all the protection and diagnostic features.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2025, 08:37:44 pm
content: Quote from: Cvijo on March 17, 2025, 07:53:40 pmI agree it's not ideal, but your MOSFET will turn on fine with a gate voltage of 6V. It's specified down to 4.5V, but the on resistance is a little higher, than at 10V. Whether this is acceptable or not, depends on the drain current.https://www.lcsc.com/datasheet/lcsc_datasheet_2410122037_HL-120P03_C20071222.pdfBy the way, I know I'm being a bit picky, but please flip the MOSFETs over in the drawing, like I did. It makes it easier to follow, without the additional lines crossing.Quote from: Cvijo on March 17, 2025, 08:02:19 pmYou forgot to connect R2 to 0V.

author: Cvijo
author_rank: Contributor
timestamp: March 17, 2025, 08:59:46 pm
content: Quote from: Zero999 on March 17, 2025, 08:37:44 pmYes you are correct, i was trying to somehow rotate it but forgot about the flipping.. and yes it is a disaster to follow like this i agree completely(fixed it)Its not picky, I should follow some guidelines and do that correctly, when i was drawing it i was .. damn this is uglyQuote from: Zero999 on March 17, 2025, 08:37:44 pm