How to drive a low Rdson mosfet for PWM - Page 1
Forum section: electronics / beginners

author: sahko123
author_rank: Frequent Contributor
timestamp: May 16, 2022, 11:30:16 am
content: Im trying to drive a heater element in a jbc t245 soldering iron and I'm having issues getting an Rdson of less than 2-3ohms which for a 2.5 ohm heater element, means that half of the power gets burned away into heat in the mosfet and the other half gets turned into heat on purpose. The mosfet I am using is a PSMN1R1-40BS mosfet with a quoted Rdson of 1.3mOhms and im trying to get at most 500mOhm. I've not used power mosfets too often so im definetely doing something wrong just not sure what it is.The circuit below is what im using. There is pwm coming from an arduino nano every.My main question is how do I go about driving the mosfet to be able to get as low an Rdson as possible?

author: rob77
author_rank: Super Contributor
timestamp: May 16, 2022, 11:35:05 am
content: why on earth are you using a N-channel mosfet as a high side switch ?

author: sahko123
author_rank: Frequent Contributor
timestamp: May 16, 2022, 11:36:31 am
content: Quote from: rob77 on May 16, 2022, 11:35:05 amBecause I am not familiar with mosfets. Why shouldnt i use it as a high side switch?

author: rob77
author_rank: Super Contributor
timestamp: May 16, 2022, 11:45:28 am
content: Quote from: sahko123 on May 16, 2022, 11:36:31 ambecause when you open it, the voltage at it's source will raise to supply voltage as the gate is and it will immediately close itself.. when N-chan is used as high side it has to have it's gate driven with voltage above supply voltage.furthermore 24V is not safe for the gate , Vgs max of your mnosfet is 20V, so it will not work (problem above) and most probably just kill your mosfet.best way is to move your load into the drain - use the mosfet as a low side switch and use a proper gate driver.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 16, 2022, 11:50:16 am
content: The schematic in the OP is not the best, here's why and a way to fix it:

author: CaptDon
author_rank: Super Contributor
timestamp: May 16, 2022, 01:59:08 pm
content: Put the source of the mosfet at the negative rail, put your heater between the +24 and the drain, and do something with the gate circuit to prevent the gate from going more then 10 volts positive above the source voltage. You will get the best published RDS that way.

author: eugene
author_rank: Frequent Contributor
timestamp: May 16, 2022, 02:42:04 pm
content: As CaptDon suggested, putting the N channel MOSFET on the low side of the load will make it easier to drive. You might be able to drive the PSMN1R1-40BS directly from 5V PWM but it might not turn on completely (you might not get down to the low advertised RDSon.) Switch to a HUF76639S3S and it should work fine.

author: Picuino
author_rank: Super Contributor
timestamp: May 16, 2022, 05:19:06 pm
content: You will also need to add an inverting diode in parallel with the heater, to prevent the mosfet from being burned out by overvoltages on disconnection.Freeweel diode:https://en.wikipedia.org/wiki/Flyback_diodeEdit: It would be better to use a mosfet that switches at low voltages (logic level MOSFET), for example this one:https://eu.mouser.com/datasheet/2/916/BUK9M24_40E-1539865.pdfThis way you can directly connect the 5volt signal to the gate through a small resistor.Edit 2:Design the layout well to minimize inductances and resistances because with such high currents (10A) it is easy for parasitic inductances to affect the operation of the circuit.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 16, 2022, 05:57:48 pm
content: This article shows the effects of capacitances on transistors and mosfet's. A dedicated drive to the gate seems to be the most assertive option.http://prosje.be/Projects/BelastingSchakelen.html

author: Picuino
author_rank: Super Contributor
timestamp: May 16, 2022, 06:40:45 pm
content: In some cases the driver can be dispensed with, if very fast switching is not necessary.Switching a resistor is a slow system, which does not require a high switching speed.With the mosfet I propose, BUK9M24-40E, and a 5Volt digital output with 20mA current, the 8 nCoulomb gate charge of the mosfet is loaded in 400ns, which will be approximately the switching time. To switch a resistor, it is not necessary to be faster.

author: Picuino
author_rank: Super Contributor
timestamp: May 16, 2022, 06:53:25 pm
content: I have calculated the heating of the mosfet and it is better to use one with lower Rds_on resistance to limit its dissipation.Another better option:https://eu.mouser.com/datasheet/2/916/BUK9M11_40E-2938156.pdfIn this case the switching time will be close to (less than) 600ns.The power dissipated by conduction will be a maximum of 2W at Id=10A Vgs=5V and Temp_junction=175ºC.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 16, 2022, 08:58:51 pm
content: Mosfet are driven by voltage, that's why it won't work, needs 12-15V between gate and source for lowest Rds.When you apply 24V to the gate, it conducts... then you have 24V at the source, the voltage difference between gate and source would be gone (0).Ok reality it won't turn off, but self-regulate in the ohmic region due the gate threshold voltage, it will find the equilibrium for the load current, RDS voltage drop and VGs.If you measure Vgs, you probably see around 3-4V.You need a P-ch MOSFET, a high-side driver or bootstraping the mosfet.The bootstrap circuit is easy, but will only work if the PWM never runs at 100% (DC).24V is too much for the gate! You must add a 12/15V tener.

author: free_electron
author_rank: Super Contributor
timestamp: May 16, 2022, 09:39:36 pm
content: Quote from: rob77 on May 16, 2022, 11:35:05 amThese days everybody uses N-mos. P-mos is just not as good.Use a boost pump driver and done.

author: langwadt
author_rank: Super Contributor
timestamp: May 16, 2022, 09:42:52 pm
content: Quote from: free_electron on May 16, 2022, 09:39:36 pmor use a high side switch and get everything in one package, including thermal and short circuit protection

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2022, 09:55:32 pm
content: Yes, bootstrapping can be used to drive a high side N-MOSFET. The crude circuit posted above will work, but the 1k resistor slows down the on time. Another transistor and diode can be added to speed it up.There are special ICs which also offer undervoltage protection, which deactivates it, if there isn't a high enough voltage on the capacitor to turn the MOSFET fully on, which would cause overheating.

author: rob77
author_rank: Super Contributor
timestamp: May 17, 2022, 04:40:24 pm
content: Quote from: free_electron on May 16, 2022, 09:39:36 pmmy point was "why not using a n-channel as a low side" ? i also suggested to make it low side switch and use a proper gate driver...

author: rob77
author_rank: Super Contributor
timestamp: May 17, 2022, 04:46:31 pm
content: i think everyone forgets that a load in the OP is a HEATER... there is a very high probability that the heater will need to run at 100% duty cycle at least during startup, with 100% duty cycle the voltage boost will not work...the simple solution is to use the mosfet as a low side switch, no need to over-complicate things.

author: Picuino
author_rank: Super Contributor
timestamp: May 17, 2022, 05:22:42 pm
content: I did not forget it.Also, a heater does not need to have a PWM signal at 31kHz. Switching the gate to 15Hz is enough. Such a low switching frequency produces no audible noise and makes the driver unnecessary at the speeds that a MOSFET switches, commanded by a low current (20mA) logic output.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2022, 05:45:50 pm
content: Quote from: rob77 on May 17, 2022, 04:46:31 pmI doubt it has to run at 100% duty. 99% would probably be more than enough.

author: sahko123
author_rank: Frequent Contributor
timestamp: May 17, 2022, 07:33:53 pm
content: Yeah max probably 45-50% otherwise I'll dump like 240W or so into it when at most I want to dump 100W

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2022, 08:39:47 pm
content: Here's a bootstrap circuit which will work from 24V and drive the MOSFET gate with between 12V and 8V. The on time is 49.95ms with an off time of 50µs.The voltage across C1 starts near 24V, so the voltage on C1's positive swings from 24V to 48V.R1 & R2 form a potential divider, to cut the voltage delivered to M1's gate by half. Q2 acts as an emitter follower to reduce the output impedance, so the gate charges very quickly.If the MOSFET is kept on too ling C1 will discharge, causing the gate voltage to drop too low to keep it hard on, resulting in excessive power dissipation.Assuming you're using a microcontroller, set the maximum on time so the voltage on C1 is still high enough to keep the MOSFET on, followed by an off which is long enough to charge C1.Note thatthis circuit is inverting. The the input is high, the MOSFET is off. Add another transistor to make it non-inverting if necessary.Warning don't rely on this circuit, or any other to protect against fire. If there's a risk it can get dangerously hot, if the controller fails on, add a bimetal strip and/or thermal fuse to cut the power if it overheats.

author: Picuino
author_rank: Super Contributor
timestamp: May 18, 2022, 07:33:04 am
content: The gate of the mosfet should not exceed the maximum 20 volts it supports.

author: eugene
author_rank: Frequent Contributor
timestamp: May 19, 2022, 12:59:30 am
content: I really don't understand why people continue to suggest gate bootstrap circuits. Is it because we think the circuits are interesting and we want to show off our favorite version? The poor OP cannot be anything but confused.It's been noted repeatedly that putting the N channel MOSFET in the low side is the only solution that makes sense.Further, it's been shown than MOSFETs are available (even during chipageddon) that have adequate specs and can be driven directly from a microcontroller pin or other logic gate. Rise time might be faster if a beefy gate driver is used, but it likely doesn't matter, especially if the PWM frequency can be slowed down.So what does anyone have against the super simple solution of a single transistor in the low side being driven directly by some logic device? It's just a friggen resistive heater!

author: Zero999
author_rank: Super Contributor
timestamp: May 19, 2022, 04:55:45 am
content: Quote from: Picuino on May 18, 2022, 07:33:04 am20V relative to the source. In this case the source is at 24V, when on, so the gate voltage will be around 32V to 36V.Quote from: eugene on May 19, 2022, 12:59:30 amYes, I find gate bootstrap circuits fun and interesting, but I posted it because the first bootstrap circuit shown wasn't suitable because it would drive the gate at too higher voltage. The original poster can't have been that confused, otherwise he would have asked more questions and wouldn't have thanked the post.QuoteYes, low side drive is much more convenient and easier.

author: Picuino
author_rank: Super Contributor
timestamp: May 19, 2022, 09:39:09 am
content: Quote from: Zero999 on May 17, 2022, 08:39:47 pmR2 can be exchanged for a 12 volt zener with advantage, since the voltage of Vgs remains stable longer.

author: Zero999
author_rank: Super Contributor
timestamp: May 19, 2022, 10:20:56 am
content: Quote from: Picuino on May 19, 2022, 09:39:09 amThat will help a bit, or or replace R1 with a 1.2mA current source.Neither the current source, nor the zener will make any difference because the gate voltage falling from 12V to 8V will make very little difference to MOSFET's on resistance.EDIT: I see what you mean, regulating the voltage will allow longer on times. Increasing the resistor values would have more effect though. Adding 100pF bypass capacitor would prevent the higher resistance from slowing it down.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 19, 2022, 11:25:24 am
content: Correct me if I'm wrong, but higher VGS, while not providing a significative RDS reduction, does enhance quite a lot the switching time, as the RC constant is the same, so the optimum VGS is reached faster.Ex. if for 8Vcc it takes 1us for 7VGS, with 12V it lowers to 400ns, about 60% faster.Although using a transistor driver doesn't make a RC circuit, it should improve the switching anyways.

author: Zero999
author_rank: Super Contributor
timestamp: May 19, 2022, 01:41:16 pm
content: Quote from: DavidAlfa on May 19, 2022, 11:25:24 amWhat goes up, must come down. In other words, a higher voltage will improve the turn-on time, at expense of the turn-off time.

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2022, 07:50:29 pm
content: Quote from: Picuino on May 19, 2022, 09:39:09 amHere's an example of using the zener to get a 99.999% duty cycle.

author: sahko123
author_rank: Frequent Contributor
timestamp: May 22, 2022, 10:47:03 pm
content: So i prototyped a direct drive pwm circuit with the arduino (and low side switch) and I realised my main issue will be trying to get the thermocouple reading with any sort of repeatability. The pwm turns off occasionally to take a TC reading but if I use a low side switch I end up with a 24V common mode voltage (which is also lifting the circuit away from mains earth but this wont be an issue since the circuit is isolated).I'm using an OPA189 as a diff amp to read the tc which can handle the common mode voltage across the inputs but I'm not sure I'll be able to extract the actual reading properly. So a high side switch will bring the common mode to ground when sampling the tc instead of 24V with the low side switch. With the differential being the tc reading (though its the same in the other case just I seem to be struggling to read the tc properly).

author: CMTan
author_rank: Contributor
timestamp: May 23, 2022, 05:55:42 am
content: Hi,I am wondering why are you using NMOS as a high side switch?Typically you should be using the NMOS as a low side switch. You circuit will work with some simple modification.1. Move the load to the drain side (high side) of the MOSFET2. Reduce the VGS to 12V instead of 24V since VGS max = 20V3. Add a diode (any high switching diode will work even IN4148) at the gate to quicken the off discharge for the NMOS.4. With that, the PWM duty cycle will control the power to the soldering iron.Please refer to attached modified circuit.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2022, 06:29:53 am
content: Quote from: sahko123 on May 22, 2022, 10:47:03 pmThe thermocouple shouldn't share its 0V with the heater. It must be completely isolated to work properly.

author: langwadt
author_rank: Super Contributor
timestamp: May 23, 2022, 07:00:01 am
content: Quote from: Zero999 on May 23, 2022, 06:29:53 amwith somerihng like a T12 soldering iron where the thermocouple is in series with the heater and you only have two wires, that's kinda hard

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2022, 08:00:01 am
content: Quote from: langwadt on May 23, 2022, 07:00:01 amThat sounds like a bad, cheap and nasty design to me. Adding extra wires wouldn't cost that much.High side drive is the only way then.A bootstrap could work. The heater would need to be turned off often enough to take a temperature reading anyway. It will need to be off for long enough for the capacitor to charge, before a temperature reading is taken. The time RC constant with a 10µF capacitor and a 2R4 heater would be 24µs so 0.5ms or so would be long enough.

author: langwadt
author_rank: Super Contributor
timestamp: May 23, 2022, 09:42:20 am
content: Quote from: Zero999 on May 23, 2022, 08:00:01 amsimple, efficient, failsafehigh side can be simpler, but lowside is possible with a diff amp that can handle the common mode of the heater supply

author: sahko123
author_rank: Frequent Contributor
timestamp: May 23, 2022, 10:14:09 am
content: I'm going to try and divide the input by 1.5 or so to step it below the rail slightly and then it should be good

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2022, 10:26:18 am
content: Quote from: sahko123 on May 23, 2022, 10:14:09 amA differential amplifier run off 24V would do.Why not use a relay to switch the heater? That way the high side can be switched, without having to mess around with a bootstrap.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 23, 2022, 10:35:56 am
content: Assuming the temperature need to be read several times per second, does a relay switch make any sense to you?It'll die in no time.The series TC is pretty common even for big brands like JBC.Just get a cheap P-ch mosfet like the AOD403, make your life easier!

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2022, 10:48:10 am
content: Quote from: DavidAlfa on May 23, 2022, 10:35:56 amI agree. A P-MOSFET is easier.Here's an example circuit. It will work with a 24V supply and 5V MCU signal, butthe AO6407 must be replaced with a MOSFET with a drain-source voltage rating of at least 30V. The AOD403 is a decent suggestion.

author: langwadt
author_rank: Super Contributor
timestamp: May 23, 2022, 07:48:42 pm
content: Quote from: Zero999 on May 23, 2022, 10:48:10 am~2.5mA * 2.2K = ~5.5Vstick a zener across R1 you you want to be sure

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2022, 08:26:11 pm
content: Quote from: langwadt on May 23, 2022, 07:48:42 pmIf you go for the zener, why bother with the resistor?The OP is running the MCU at 5V, so the gate-source voltage will be about -9.5V. Another possibility is adding an LED in series with Q1's collector, to act as an indicator.

author: langwadt
author_rank: Super Contributor
timestamp: May 23, 2022, 09:36:13 pm
content: Quote from: Zero999 on May 23, 2022, 08:26:11 pmyou need something to turn off M1

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2022, 09:39:47 pm
content: Quote from: langwadt on May 23, 2022, 09:36:13 pmYou're right.I wouldn't bother with the zener, then.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 23, 2022, 11:32:27 pm
content: Why keep going with this and the silly overcomplicated thing?Switching a heater at 36KHz is plain stupid, a normal implementation will be 10-100Hz, so the turn-off resistor will do the job fine, neither a zener is required, just a resistor divider.

author: CMTan
author_rank: Contributor
timestamp: May 24, 2022, 03:48:35 am
content: Quote from: DavidAlfa on May 23, 2022, 11:32:27 pmAgree totally.

author: sahko123
author_rank: Frequent Contributor
timestamp: May 24, 2022, 09:20:23 am
content: I can always lower the frequency I just want to keep it as high as possible so that I dont need as large a capacitor bank for decoupling.On a seperate note I've read that thermocouples have a low source impedance which akes sense so shouldnt it be easy to extract a reasonably precise differential mode signal from a large common mode voltage? It seems that I can amplify the signal when no common mode voltage is present but once applied the output gets tied to close to 0V (with Vos). The op-amp is driven off of 24V ad the common mode voltage is also ~24V with a max of 24.5V. Would this common mode maximum be before damage occurs or should the cmmr also work at this voltage?

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2022, 12:01:21 pm
content: Quote from: DavidAlfa on May 23, 2022, 11:32:27 pmAgreed. Note the schematic I posted doesn't contain a potential divider. Attached is it with a supply voltage of 24V and the base at 5V. VGS= 16.5-24 = -9.5V. The voltage across R2 is 0.7V below the base voltage, giving 4.3V. The collector current is roughly equal to the emitter current and R2 = 1k, giving 4.3mA, which drops 9.5V across R1, a 2k2 resistor.Quote from: sahko123 on May 24, 2022, 09:20:23 amThe frequency makes no difference to the decoupling capacitance requirement. If anything too higher frequencies pose a risk of interference. It's not so much the frequency, but the switching speed, which can be slow if it's operated at a low frequency.QuoteA thermocouple has a low impedance, but the voltage is tiny, which makes it more difficult to read it, when there's a large common mode voltage.The absolute maximum input voltage rating of an op-amp which is safe limit, before damage can occur, is often different to the common mode voltage range, which is the operational range. For example the LM358's maximum input voltage rating is 32V, but its input voltage range is V+−1.5. This means, when run off 24V, its inputs can tolerate 32V, without being damaged, but it will only work properly with its inputs biased up to 22.5V.https://www.ti.com/lit/ds/snosbt3j/snosbt3j.pdf?ts=1653328133496&ref_url=https%253A%252F%252Fwww.google.com%252FNo one but you knows how the op-amp is configured. Post a schematic.By the way, go for the P-MOSFET, high side drive. It's much easier. Refer to the schematic at the start of this post.