Author Topic: How to drive a low Rdson mosfet for PWM  (Read 2283 times)

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Offline sahko123

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How to drive a low Rdson mosfet for PWM
« on: May 16, 2022, 11:30:16 am »
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.
[attach=1]

My main question is how do I go about driving the mosfet to be able to get as low an Rdson as possible?
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Online rob77

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Re: How to drive a low Rdson mosfet for PWM
« Reply #1 on: May 16, 2022, 11:35:05 am »
why on earth are you using a N-channel mosfet as a high side switch ?
 

Offline sahko123

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Re: How to drive a low Rdson mosfet for PWM
« Reply #2 on: May 16, 2022, 11:36:31 am »
why on earth are you using a N-channel mosfet as a high side switch ?

Because I am not familiar with mosfets. Why shouldnt i use it as a high side switch?
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Online rob77

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Re: How to drive a low Rdson mosfet for PWM
« Reply #3 on: May 16, 2022, 11:45:28 am »
why on earth are you using a N-channel mosfet as a high side switch ?

Because I am not familiar with mosfets. Why shouldnt i use it as a high side switch?

because 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.
 
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Online RoGeorge

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Re: How to drive a low Rdson mosfet for PWM
« Reply #4 on: May 16, 2022, 11:50:16 am »
The schematic in the OP is not the best, here's why and a way to fix it:



 
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Offline CaptDon

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Re: How to drive a low Rdson mosfet for PWM
« Reply #5 on: May 16, 2022, 01:59:08 pm »
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.
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Offline eugene

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Re: How to drive a low Rdson mosfet for PWM
« Reply #6 on: May 16, 2022, 02:42:04 pm »
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.
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Offline Picuino

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Re: How to drive a low Rdson mosfet for PWM
« Reply #7 on: May 16, 2022, 05:19:06 pm »
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_diode


Edit: 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.pdf
This 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.
« Last Edit: May 16, 2022, 05:28:58 pm by Picuino »
 

Online Avelino Sampaio

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Re: How to drive a low Rdson mosfet for PWM
« Reply #8 on: May 16, 2022, 05:57:48 pm »
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
 

Offline Picuino

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Re: How to drive a low Rdson mosfet for PWM
« Reply #9 on: May 16, 2022, 06:40:45 pm »
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.
 
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Offline Picuino

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Re: How to drive a low Rdson mosfet for PWM
« Reply #10 on: May 16, 2022, 06:53:25 pm »
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.pdf

In 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.
« Last Edit: May 16, 2022, 07:02:44 pm by Picuino »
 

Offline DavidAlfa

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Re: How to drive a low Rdson mosfet for PWM
« Reply #11 on: May 16, 2022, 08:58:51 pm »
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.
« Last Edit: May 17, 2022, 11:25:12 pm by DavidAlfa »
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Offline free_electron

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Re: How to drive a low Rdson mosfet for PWM
« Reply #12 on: May 16, 2022, 09:39:36 pm »
why on earth are you using a N-channel mosfet as a high side switch ?
These days everybody uses N-mos. P-mos is just not as good.
Use a boost pump driver and done.
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Offline langwadt

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Re: How to drive a low Rdson mosfet for PWM
« Reply #13 on: May 16, 2022, 09:42:52 pm »
why on earth are you using a N-channel mosfet as a high side switch ?
These days everybody uses N-mos. P-mos is just not as good.
Use a boost pump driver and done.

or use a high side switch and get everything in one package, including thermal and short circuit protection
 
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Online Zero999

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Re: How to drive a low Rdson mosfet for PWM
« Reply #14 on: May 16, 2022, 09:55:32 pm »
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.
« Last Edit: May 16, 2022, 09:57:17 pm by Zero999 »
 

Online rob77

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Re: How to drive a low Rdson mosfet for PWM
« Reply #15 on: May 17, 2022, 04:40:24 pm »
why on earth are you using a N-channel mosfet as a high side switch ?
These days everybody uses N-mos. P-mos is just not as good.
Use a boost pump driver and done.

my 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...  ;)

 

Online rob77

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Re: How to drive a low Rdson mosfet for PWM
« Reply #16 on: May 17, 2022, 04:46:31 pm »
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.
 

Offline Picuino

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Re: How to drive a low Rdson mosfet for PWM
« Reply #17 on: May 17, 2022, 05:22:42 pm »
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.
 
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Online Zero999

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Re: How to drive a low Rdson mosfet for PWM
« Reply #18 on: May 17, 2022, 05:45:50 pm »
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.
I doubt it has to run at 100% duty. 99% would probably be more than enough.
 
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Offline sahko123

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Re: How to drive a low Rdson mosfet for PWM
« Reply #19 on: May 17, 2022, 07:33:53 pm »
Yeah max probably 45-50% otherwise I'll dump like 240W or so into it when at most I want to dump 100W
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Online Zero999

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Re: How to drive a low Rdson mosfet for PWM
« Reply #20 on: May 17, 2022, 08:39:47 pm »
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 that this 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.


 
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Offline Picuino

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Re: How to drive a low Rdson mosfet for PWM
« Reply #21 on: May 18, 2022, 07:33:04 am »
The gate of the mosfet should not exceed the maximum 20 volts it supports.
 

Offline eugene

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Re: How to drive a low Rdson mosfet for PWM
« Reply #22 on: May 19, 2022, 12:59:30 am »
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!
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Online Zero999

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Re: How to drive a low Rdson mosfet for PWM
« Reply #23 on: May 19, 2022, 04:55:45 am »
The gate of the mosfet should not exceed the maximum 20 volts it supports.
20V relative to the source. In this case the source is at 24V, when on, so the gate voltage will be around 32V to 36V.

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.
Yes, 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.

Quote
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!
Yes, low side drive is much more convenient and easier.
 

Offline Picuino

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Re: How to drive a low Rdson mosfet for PWM
« Reply #24 on: May 19, 2022, 09:39:09 am »
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 that this 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.



R2 can be exchanged for a 12 volt zener with advantage, since the voltage of Vgs remains stable longer.
 


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