Issues with PMOS + NPN Switch Circuit – Gate Voltage Not Switching Properly - Page 1
Forum section: electronics / repair

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 12:47:08 pm
content: Hi everyone,I'm working on a circuit using a PMOS + NPN switch configuration where the NPN transistor pulls the PMOS gate to ground when its base is high, causing the PMOS to turn on. When the NPN transistor is low, the PMOS gate is pulled up to the source voltage (12V), switching the PMOS off.I have a setup with 5 of these circuits, and 3 of them are working fine, but 2 are not switching properly.Here’s the issue: When the circuit is supposed to be off (the gate should be at 12V), I’m measuring the gate voltage while the PMOS source voltage is at 12V. However, I find that the gate voltage is around 0.5V, and it is not stable—it keeps fluctuating, which causes the PMOS to stay on.I’ve already replaced both the PMOS (IRF4905) and the NPN (2N3904), but the issue persists. I also unsoldered the resistors used for measurement, and they are showing the expected values: 2x 100kΩ and 1x 220Ω. Additionally, I had cleaned all the residue with IPA to ensure there's no contamination.I even tried removing the newly replaced NPN transistor and measuring the gate voltage, but it still wasn't being pulled to 12V as expected.The NPN transistor is controlled by a digital Arduino pin, and I’ve even replaced the Arduino, but the problem still persists.I’ve attached both the schematic design and the PCB design of the circuit in case that helps with troubleshooting.Does anyone have any ideas why this might be happening? Could it be a leakage issue or some other factor affecting the gate voltage stability? Any suggestions on troubleshooting this would be greatly appreciated!Thanks in advance!

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2025, 12:55:39 pm
content: What's the: switching frequency and load?

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 01:00:26 pm
content: I’m not using a specific switching frequency since the circuit only switches when I manually trigger it via a button. The NPN transistor is controlled directly by the digital Arduino pin, which turns the circuit on or off when I press the button. So, it only switches on demand, not on a continuous frequency.Currently, there is no load connected. The circuit is connected to a terminal breaker and an XLR4 connector, but they are not connected to anything at the moment. The circuit is essentially in a no-load state.

author: tom66
author_rank: Super Contributor
timestamp: April 10, 2025, 01:04:41 pm
content: Is 'O7' the output of the circuit, or the input?

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 01:06:52 pm
content: The O7 is the terminal block which is the output yes. and the 12OUT_1 is the input.Apologies for the confusion of the names, I took different schematic to the one in the PCB design

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2025, 01:27:43 pm
content: Quote from: Wulfenlied on April 10, 2025, 01:00:26 pmWhat's the load current?No, of course this circuit won't work. Q1's drain and source are the wrong way round, so the body diode will bypass the transistor, making the output on all the time.

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 01:35:30 pm
content: Hello,Thanks for your response. The source is connected to the positive voltage (12OUT_1) WHICH IS +12v coming from a dcdc converted that outputs 12, hence, the net label (12OUT) , and the drain is connected to a terminal block, which is the output that is currentlynotconnected to anything, so again,no load

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2025, 02:11:01 pm
content: Quote from: Wulfenlied on April 10, 2025, 01:35:30 pmYour schematic is drawn the wrong way round. The inputs should be on the left and outputs on the right-hand side.Have you tried this circuit? The gate-source voltage is capped by the gain of the the common emitter amplifier stage (Q1, R1 and R2) making it less vulnerable to supply voltage spikes. V2 is the 3.3V output from the MCU.P-MOSFET driver slow.asc

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 02:29:07 pm
content: Hey Zoro,Thanks again for your response. If you pay attention to the schematic you will find that it is simply mirrored, the source and gate is subjective to the symbol placement.I will attach a screen capture from the datasheet of the P channel MOSFET that I am using in the design for more clarification.

author: Peabody
author_rank: Super Contributor
timestamp: April 10, 2025, 02:42:51 pm
content: If you remove the NPN, but the gate voltage is still not 12V, then I think the mosfet is leaking current from the gate to the drain. With the NPN still removed, use your meter to measure the voltage drop across the pullup resistor. Of course there shouldn't be any.

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 02:48:51 pm
content: Hello Peabody,Thank you so much for your response. I just did what you asked the voltage across R1(100k, the pull up resistor) is fluctuating between 11.1 and 11.3v, that's so weird..I am confused, what does it mean now ? the PMOS is the component that is leaking ?

author: Peabody
author_rank: Super Contributor
timestamp: April 10, 2025, 03:07:17 pm
content: Yes, it means the mosfet is bad. It could have been subjected to excessive gate voltage - a spike or something - or it could be a counterfeit mosfet, even one of a batch of counterfeit mosfets, or it could be marked wrongly so that it's actually a different part (maybe even a mosfet with an 8V maximum G/S voltage). Or I guess it could be that there is leakage on the PC board and the mosfet is fine.Edit: To check the board, you could remove the mosfet and the resistor, then check resistance from the gate pad to ground. It should be infinite.

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 03:19:18 pm
content: Hey Peabody, I appreciate your insight. I will be testing what you suggesting right now, and I will reply to you with the results.Just an FYI, I don't know whether it is possible to get counterfeit components but I got them from RS Components or Mouser. and the MOSFET that I am using is IRF4905

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2025, 03:24:50 pm
content: Quote from: Wulfenlied on April 10, 2025, 02:29:07 pmI know, but the convention, i.e. best practice is, to draw schematics with the inputs on the left and outputs on the right and not to use contradictory labels containing the word out, when it's power in. Please learn from this, to avoid confusion in the future.

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 04:02:23 pm
content: Hey Peabody,I have removed the PMOS and the 100k Pull-up resistor from the PC board, and have tested the resistance between the gate pad and ground and it was around 1.7M ohms.I have also tested the resistor again after removing it it was very precise 100k ohmI have tested the P-MOS after removing it, and I could turn it on and off using the diode mode in the multimeter.What do you think ?@Zoro, thanks for the advice, I appreciate it

author: Peabody
author_rank: Super Contributor
timestamp: April 10, 2025, 05:18:15 pm
content: Quote from: Wulfenlied on April 10, 2025, 04:02:23 pmAnd that's with the 2N3904 still removed from the board? If it was removed, then there should be no connection from the gate terminal to anywhere else. So resistance should be infinite. I don't know what the 1.7Meg resistance would be.QuoteI'm not sure how you tested the mosfet. On a breadboard, connect the gate and the source with a jumper (pins 1 and 3). Then with the meter in diode mode, measuring the drain and the source (pins 2 and 3), you should get something like 0.5V in one direction, which would be the body diode forward biased, and nothing in the other direction, which would be the body diode reverse biased with the mosfet off. If that all checks out, then the mosfet may be good. If you got them from RS or Mouser, then they aren't couterfeits. But that still doesn't explain the 11.1V drop across the pullup resistor, which tells you current is flowing through it. And if the NPN isn't there, there should be no current at all through that resistor because there should be no DC gate current.

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 05:39:14 pm
content: Hey, yes I have tested the PMOS exactly how you mentioned. And it all checks out.The NPN, the pull-up resistor 100k and the PMOS were removed when I tested the resistance from the gate pad to the ground. It was giving me a value of 1.7M ohms, and earlier before as mentioned when i only had the NPN removed from the circuit, there was a voltage drop of 11.2v fluctuating. What on earth could the reason behind these? Where is the parallel path is coming from ? What is causing the leakage ?

author: Peabody
author_rank: Super Contributor
timestamp: April 10, 2025, 09:08:18 pm
content: Is the gate pad connected to anything else on the board besides the pullup and the NPN collector? If this is a 4-layer board, there may be something in the middle that you can't see. I really don't have an answer. 1.7Meg is a lot of resistance, but I don't think it should be there. Could there be some conducting flux on the board. Well, I just don't know what else to suggest.When you found the 11.1V drop across the pullup resistor, with the NPN removed, do you recall what the output voltage was on the drain?You know, this is getting close to being an impossibility, which is an indication that something you've told us, and which you believe to be true, isn't true. The schematic, the mosfet pinout, or something like that. Or that we are not correctly understanding something you're saying.

author: Wulfenlied
author_rank: Contributor
timestamp: April 10, 2025, 09:28:44 pm
content: Nothing is connected to the gate beside the NPN collector. And the voltage at the drain when only the NPN was disconnected was 12v stable, AKA the PMOS was switched on. While the gate was fluctuating around 0Everything I have mentioned was based on tests that were done and anchored on studies and trial and errors that I constantly done for 4 consecutive days.Bear in mind that I have fixed many switch circuits like these. The reason why I’m here to begin with is because two of the switch circuits in my current project are not working because of the same exact reason. I may be wrong I am only here to wish to know where am I wrong. Just point it out and I will double check.The last thing I want to conclude from all of this is that it is a PCB issue itself.Thanks for your prompt responses and help.

author: tom66
author_rank: Super Contributor
timestamp: April 10, 2025, 10:37:45 pm
content: 1Meg+ is normal for a power MOSFET gate to source resistance. It won't actually be infinite. The gates on these devices are quite large and, although insulated, no insulator is perfect, so you will get leakage over that area.

author: Peabody
author_rank: Super Contributor
timestamp: April 10, 2025, 10:59:21 pm
content: If the source was 12V and the drain was 12V, and the NPN had been removed, and the pullup resistor was actually connected to the source, and not to ground, then it's impossible for the gate to be at zero volts unless some trace is incorrectly grounded. So I was wrong about the gate leaking to the drain. There has to be some connection to the gate that's drawing it low, and if the NPN isn't there to do that, then I'm at a loss to explain it.Edit: the connection shown in red below should be a dead short. Is it?

author: Peabody
author_rank: Super Contributor
timestamp: April 10, 2025, 11:14:07 pm
content: Quote from: tom66 on April 10, 2025, 10:37:45 pmIf I remember correctly, he was checking the resistance to ground from the gate pad - with the mosfet, the NPN, and the pullup removed.

author: Wulfenlied
author_rank: Contributor
timestamp: April 11, 2025, 09:20:45 am
content: Quote from: Peabody on April 10, 2025, 10:59:21 pmThe source was 12V and the drain was 12V and the NPN was removed and the pull up resistor was connected to the source in one side and the other side Collector of NPN and the Gate. I know it doesn't make sense that's why I am desperate to know what's wrong.The connection shown in red IS a short, there is a pour that I hid so you can visualise it but I will attach how it looks like

author: Peabody
author_rank: Super Contributor
timestamp: April 11, 2025, 01:55:44 pm
content: Do you have another board that isn't populated yet? If so, you could check for continuity between the mosfet gate and every other pad on the board, to see if it's connected anywhere that it shouldn't be, anywhere but the NPN collector and one side of the pullup. I guess you could do the same with a populated board, but with the mosfet removed.Well, as I said before, I think we've reached an impossibility, and that's probably because something we think is true isn't true. Or, there's just something wrong with this board. I think you said you had other copies where this all works.

author: Andy Watson
author_rank: Super Contributor
timestamp: April 11, 2025, 02:22:27 pm
content: Quote from: Wulfenlied on April 10, 2025, 12:47:08 pmThis is very odd! Even if there is a fault the voltage should be stable. Can we assume that you are working on a non-conductive surface/desk/work bench and that you are not touching the probe with your fingers? Also, that the tab of the mosfet is isolated?

author: Wulfenlied
author_rank: Contributor
timestamp: April 11, 2025, 02:36:39 pm
content: Quote from: Andy Watson on April 11, 2025, 02:22:27 pmHello, I always use ESD Protection, Mats and wrist band while I am working. And no of course I am not touching the probes with my fingers while using the multimeter.

author: Wulfenlied
author_rank: Contributor
timestamp: April 11, 2025, 02:41:59 pm
content: Quote from: Peabody on April 11, 2025, 01:55:44 pmI do have more than one of the same PCB, empty and complete. and of course i didn't test for continuity between the gate pad and every oother pad in the board, I would test this but that I have another board with the same setup and same components and logic and it's working just fine.Quote from: Peabody on April 11, 2025, 01:55:44 pmIt may be the board after all of that.