N-channel MOSFET High Side Switch Circuit - Page 1
Forum section: electronics / beginners

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 07:15:47 am
content: First post. I've been an electronics hobbyist since I was 12 but not for the last 40+ years. Obviously things have changed a little, so I'm effectively a newbie again.Now, to the point; I've seen a lot of discussions and articles on n-channel mosfet high side switching and therefore seen a lot of different circuit ideas. What I've come up with is:The input PWM will be at about 980 Hz. I've simulated this within KiCad and at appears to work perfectly, although I've not breadboarded it yet. Given that I've not come across it before I wonder what I've missed. Can someone point out problems with this circuit to explain why others do not appear to be using it.

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2025, 07:34:12 am
content: If you cut the PWM signal the output will go +24V constantly.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 07:36:50 am
content: True but that is dealt with in software. The off state will output a high (~5V) to the NPN.

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2025, 07:38:51 am
content: R3 15K likely wont pass enough current to turn the FET on fast enough to keep 1khz switching loses low.But you are limited in that you cant go too low for R3 because you are burning off everything over 18V in the D1 zener

author: PGPG
author_rank: Frequent Contributor
timestamp: April 23, 2025, 07:41:43 am
content: It is not N-MOSFET high side switch.Like npn needs at base higher voltage than at emitter N-MOS needs at gate higher voltage than at source.Because of this your N-MOS will never be switched on (in the sense of having very little resistance between S and D with both being at 24V level).Your circuit works like npn 'emitter follower' but with probably bigger Vgs than npn's Vbe.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 07:53:37 am
content: Thanks for that. I believed the resistor pulling the gate to 24V would suffice. I see your point that D and S will be near enough at 24V once switched on but that's not the condition when it's off. Due to the short duration of the pulses, will that really matter? I can see that there may be an issue with the mosfet getting hot because the gate voltage will not be held at or above the threshhold for the entire pulse, although I would expect the mosfet's inherent capacitance may cause the gate to stay high enough.Maybe it's time I just breadboard this and test the real-world result. I'll try and do that in the next few days and report back.

author: PGPG
author_rank: Frequent Contributor
timestamp: April 23, 2025, 08:45:17 am
content: Quote from: OldBeard on April 23, 2025, 07:53:37 amVgs threshold is typically defined for very small Id current (when transistor starts to conduct and not when it really conducts).For working with high current the Vgs will be much bigger (there should be charts in datasheet).I don't know what transistor you plan to use but if Vgsth is for example 2..3V you can expect Vgs being about 4..5V when it is switched on (in your sense).So about getting hot:- at truly switched on N-MOSFET you can have for example 0.1V drop,- at N-MOSFET 'switched on your way' you can have 4V drop.And the power dissipation semi proportional to it.You have two following good solutions:- use enough big capacitor charged when N-MOSFET is switched off and used to drive gate with higher than 24V when it is switched on,- use P-MOSFET.Quote from: OldBeard on April 23, 2025, 07:15:47 amI have begun with electronic when in 1968 (being 10 years old) I have build my first detector radio receiver. Then I made fast progress and in 1982 I have build my oscilloscope and when with my brother we setup our very small electronic company for more than 5 years this oscilloscope was practically the only measurement equipment we had.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 08:53:11 am
content: Thanks for the feedback. So, although I specified a 4V gate threshold for the mosfet, it's sounding to me like the Spice simulation didn't produce a realistic result.What you describe as a possible solution is to use the charge pump circuit I've seen around a lot. From the little I've learned about p-channel mosfets, I'll steer clear of that idea, as it appears most others do as well.

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2025, 10:32:47 am
content: Quote from: OldBeard on April 23, 2025, 08:53:11 amDon't steer clear of P-Mos. They are awesome and the most common choice for high side switching at lower currents, except for a few situations.They have a worse Rdson for the same cost and switch a little slower, but a lot of the time that doesn't matter. Especially for hobby stuff.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 10:45:06 am
content: Ok, thanks. I'll look into them a bit further. Then maybe get a few and try them out.

author: Seekonk
author_rank: Super Contributor
timestamp: April 23, 2025, 11:16:23 am
content: Real men don't use P MOS. I buy these 12V 500ma line operated 100-140V AV power supplies off alli for less than a buck. They easily work with light loads down below 20V. Then you can do conventional drive circuits.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 11:19:12 am
content: I'm not seeing how any of that relates to my circuit, which runs on 24V and the load current at that voltage will be close to 3 amps.

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2025, 11:31:55 am
content: Quote from: OldBeard on April 23, 2025, 11:19:12 amThere's 100's of PMos spec'ed for like 20A 40VAnd they do make P-Fets up to crazy high power levels, like 40V 300A @ 1.8mR but you pay 2x-6x what you'd pay for the same rated N-Fet.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 11:37:36 am
content: Psi, I really should make it clear who I'm replying to. My last message was in reply to Seekonk. I just couldn't see the relevance of what was being described. I never had a doubt that there would be plenty of suitable p-channel devices. Right now (in between reading and typing here) I'm trying to learn why so many people shy away from them.

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2025, 11:55:16 am
content: When dealing with N-Fet for high side switching the usual method is to throw an isolated powersupply at the problem.Anything will work as long as its isolated so you can tie one side to the source so it moves up/down with the load.Little isolated DCDC SMT/TH bricks are common, but mains powered modules work too and are easy to wire up for prototyping

author: PGPG
author_rank: Frequent Contributor
timestamp: April 23, 2025, 12:00:30 pm
content: Quote from: OldBeard on April 23, 2025, 11:37:36 amStart replying from Quote key and delete from message everything except you want to Quote.I didn't found the better way here. At KiCad forum I just select a piece of text and select Quote and have it in message. Here didn't found how to do it simpler way than by quoting whole and deleting.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 12:05:09 pm
content: Quote from: PGPG on April 23, 2025, 12:00:30 pmThanks. I haven't spent much time on forums for a long time and need to get back into how they work.

author: Seekonk
author_rank: Super Contributor
timestamp: April 23, 2025, 01:17:02 pm
content: Any high side chip driver can be used with an isolated power supply or other drive circuit allowing N FET to be used. Also with a micro, a spare output pin can be driven PWM to create power thru a capacitor and rectifier to make high side DC. Everyone has extra pins.

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 01:23:29 pm
content: Quote from: Seekonk on April 23, 2025, 01:17:02 pmThanks. I'll keep those suggestion in mind, although none of the T12 controller designs I've seen (so far) use either. By far the most common solution I've seen is to use a charge pump with an NMOS. Clearly others know more about this stuff than I do.

author: JoeyG
author_rank: Regular Contributor
timestamp: April 23, 2025, 01:45:25 pm
content: Some has already covered the N channel solutions for high side switching.There are other Youtube videos on this subjectHowever+1 P channel MOS FET (and simple drive circuit)Just FYIIn automotive they also have a common ground and need to switch the high side of the 12v/24v VddAutomotive have high side FET drivers , where they use N channel MOS FETs with charge pumps to get the gate voltage higher then the Vdd to turn on.Because of the charge pumps PWM speed is not a fast as a P channel MOSFET but may switch fast enough for you.Buy high side switch rather than build circuithttps://www.infineon.com/cms/en/product/power/smart-power-switches/high-side-switches/classic-profet-24v-automotive-smart-high-side-switch/Seems to switch ~ 250uS on and off (check data sheets)Buy High High side driver(add your own N channel FET)https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/MIC5018-High-Side-MOSFET-Driver-DS20006631A.pdf

author: OldBeard
author_rank: Contributor
timestamp: April 23, 2025, 02:00:11 pm
content: Quote from: JoeyG on April 23, 2025, 01:45:25 pmThanks for the info. I have watched the first of those videos, which offers three alternatives, which I wondered about. Specifically, why have I not encountered the second and third options in my searches and only seen the first option once, where it was replaced with a charge pump in the next iteration of the circuit. I've not yet watched the second video.I'm aware there are high side switching devices I could use but that kind of defeats my purpose of creating a new soldering iron controller and learning to understand the innards in the process. I'm now using components and principles which didn't exist the last time I was active in electronics.