p-channel mosfet always on when VCC over 9.5 V - Page 1
Forum section: electronics / beginners

author: fagiano
author_rank: Regular Contributor
timestamp: March 01, 2014, 06:46:24 pm
content: Hi, I made a board with a p-channel switch (see schematic) it works fine when VCC is up to 9.5V when I go above that it starts conducting also when it should be off(I'd like it to work at 12V). I tried playing around with resistor values but doesn't seem to make much difference. Interestingly enough it worked for a little while when poked it with a finger, but I can't reproduce that.My scope tells me the voltage at the gate does indeed go very near ground so for what I understand it should be off. In fact, other than staying "on", all voltages look the same as when is working correctly at ~9.5/10V.my actual mosfet is a STB80PF55T4I originally had R1 = 1 meg and R4 = 47kthank you for your helpciaoAlberto

author: fcb
author_rank: Super Contributor
timestamp: March 01, 2014, 06:58:10 pm
content: If your Gate is near gnd, the P-channel (in your config) will be ON.

author: fagiano
author_rank: Regular Contributor
timestamp: March 01, 2014, 07:05:59 pm
content: Ops!, brain fart. I meant the gate is at 8.6V with VCC 12V (wont turn off), gate 8.4V with VCC 9.5V it works fine.ciaoAlberto

author: fcb
author_rank: Super Contributor
timestamp: March 01, 2014, 07:18:04 pm
content: Try disconnecting Q1, the 47K pull-up should definitely keep that P-chan off.Also, make sure you have the S & D connected correctly and you haven't transposed them (or anything else).Lastly, is your signal generator definitively switching off Q1?

author: fagiano
author_rank: Regular Contributor
timestamp: March 01, 2014, 07:43:36 pm
content: Ok, I tried to connect a 3A load and it started to work... until now I was using a led. Why is so?thank you for your helpciaoAlberto

author: fagiano
author_rank: Regular Contributor
timestamp: March 01, 2014, 08:01:02 pm
content: Is not really working, I think was my bench psu fluctuating when the relays kicked in at 3 amp load. With a 3A load and a 12V wall wart I can see more voltage dropping but is not completely turning off. I'm confused.Alberto

author: fagiano
author_rank: Regular Contributor
timestamp: March 01, 2014, 08:42:10 pm
content: Ok, I managed to reproduce something. If the mosfet is ON and the load is connected when the sig gen goes low the mosfet stays on. If i disconnect the load(I leave just a led), wait for the sig gen to go low, then connect the load the mosfet turns off but not completely(the moment the load is connected). Does it say something?Alberto

author: fagiano
author_rank: Regular Contributor
timestamp: March 01, 2014, 09:05:28 pm
content: I think is is doing that because the shitty wallwart drops it's voltage from 12 to 11 when the load is attached. It still doesn't turn off with a 12V supply. Argh!

author: Neverther
author_rank: Regular Contributor
timestamp: March 01, 2014, 09:39:04 pm
content: Something wrong with base current, or is the transistor used just leaking really bad?Try slapping a diode where R4 is in your schematic. It should offset the voltage needed at base to cause current to flow trough it.Also lower value R1 will help with switchingtime if the fet gets hot during switching (not an ideal driving method, but works good enough).

author: sleemanj
author_rank: Super Contributor
timestamp: March 01, 2014, 10:11:56 pm
content: Quote from: fagiano on March 01, 2014, 07:05:59 pmLike an N channel a P channel needs a potential difference between gate and source, in the case of P the gate must be lower than source to turn on, and close to (or above) source to turn off.The threshold on the datasheet is between 2 and 4 volts, so with 8.6v on the gate, and 12v at source, it's got about 3.4v difference and likely above threshold so your FET will be conducting a little (in the linear region). With 9.5 at source and gate at 8.4 you have 1.1v which is well below threshold so the fet won't be conducting.So in other words, I don't see anything unusual in your results, it's working how I would expect.Edit to add: ok just looked at your schematic, so I revice that to "So in other words, you need to look at why your gate voltage is not being pulled high enough, or rather, why your transistor is conducting when it shouldn't be."

author: fagiano
author_rank: Regular Contributor
timestamp: March 02, 2014, 09:53:45 am
content: You are right, it has to be the BJT. I removed it(and lifted a pad in the process) and if I poke with a resistor where emitter and collector were, everything switches nice and hard. I'll try to replace it with another device. I have a number of SOT-23 transistors it's just going to be a pain to test them.

author: fagiano
author_rank: Regular Contributor
timestamp: March 02, 2014, 10:15:08 am
content: I replaced the BC817-25 with a MMBT3904 and now the gate sits at 10V and it works well enough. So out of ~20 different BJT models I have in my components drawers I got one that didn't work...I still don't understand why the gate is not ~12V as LTspice claims. am I still missing something?thank youciaoAlberto

author: sleemanj
author_rank: Super Contributor
timestamp: March 02, 2014, 01:10:09 pm
content: Quote from: fagiano on March 02, 2014, 10:15:08 amWell, for the gate to not be 12v, current is necessarily flowing through the resistor causing a voltage drop across it.You have 2v drop across your 47k resistor, right, so that's .00004A, 40 microamps which by definition is flowing through that resistor and transistor. So are you sure you are really grounding out the base when you want to switch that bjt off, or is that base resistor being lifted a little bit above ground (or rather above the diode-drop).Assuming of course that you are grounding the base (resistor) to turn it off, and not leaving it floating.

author: fagiano
author_rank: Regular Contributor
timestamp: March 02, 2014, 02:50:39 pm
content: Ok, I found the problem, I'm an idiot. I had the footprint with emitter and collector swapped. I guess that's what you get when you don't make your own footprint. Anyway, I hacked a TO-92 on the board and ir works perfectly. 11.983V at the gate. You say I shouldn't leave the base floating, do you mean I should have a pull down on the base?Anyway, the thing that amaze me is that kinda worked, to the point that was almost usable. How is that possible?thank you a lo,t I've learned quite few things. I understand when Dave says "I hope your boards don't work".ciaoAlberto

author: w2aew
author_rank: Super Contributor
timestamp: March 02, 2014, 05:08:56 pm
content: Quote from: fagiano on March 02, 2014, 02:50:39 pmThe reason is that BJTs will work in a limited way with their collector and emitter terminals reversed, but they will operate very poorly.

author: fagiano
author_rank: Regular Contributor
timestamp: March 02, 2014, 06:56:42 pm
content: Thank you, that explains a lot. It is quite frightening, however the next time it wont take me 2 days to figure it out.ciaoAlbertoPS: w2aew, I'm a big fan of your channel

author: w2aew
author_rank: Super Contributor
timestamp: March 02, 2014, 10:06:03 pm
content: Quote from: fagiano on March 02, 2014, 06:56:42 pmThanks!It all points out that you learn more from your failures than from your successes. Like the old saying, "Good judgement comes from experience, experience comes from bad judgement."