Hello, I plan to make a simple PWM system to control a 12V motor controlled from a microcontroller powered by 3.3V, hence the question about a logic level mosfet with the lowest possible RDS(on) resistance, with which I can control a motor with a load current of 10-15A? Will a heatsink be...

The optocoupler has one actively forced state (here ground), while the transition to 12V will take place passively through the resistor (it may be the other way around, it doesn't matter). You have a CTR of 50-600%. So in the worst case 20mA through the LED will give you 10mA at the output. It's miserable. The transistor will switch for a long time and will get hot. Driver is a simple matter - you have application notes, the ones given do not require anything practical beyond that. Consider that at 15A, even if the switching time is only 100us, at the given frequency it will be 20ms, so 5% of the whole period. And 100 Hz is not much and there may be a problem with motor control. Let's assume that for these 5% of the time the transistor will have an average RDSon of not 4 but 40 mOhm. This will give us an average of 0.45W of losses. If the average is 100 mOhm, the losses will already be 1.25 W. Of course, these are not precise calculations, but rather very rough estimates. You can roughly calculate the switching time knowing the gate capacitance and driver current. Probably the given 100 us is still very optimistic.

There is also the optodriver TLP351. And then you have no problems with driving the MOSFET gate with a higher voltage and its fast switching. It is available at TME, just like the TLP250. Oh, here: is a PDF (p.63), how to use this TLP to control a transistor, admittedly an IGBT. But instead of an IGBT you will insert your MOSFET and remove the Vee voltage source. Regards, KT

As for the mosfet, see PSMN2R7-30PL (100A / 30V / Rds (on) 0.0023 Ohm). The insulation is e.g. Optocoupler 6N137.

10-15A is not such a small motor anymore Use some product selector, e.g. on farnell, after entering N-MOSFET, you select the appropriate tab, then you specify the required parameters and a list of transistors that meet your criteria appears. At 10-15A, finding an N-MOSFET with an RDSon small enough to not need a heatsink can be a challenge. With a VGS of around 10V, you can easily get a MOSFET with an RDSon of 8 mOhms, which would give 2W static loss for 15A, so a copper field under the transistor would be enough, and a double-layer board with vias increasing heat conduction would be a good idea. Ew. you can think about some H half-bridges - Infineon has excellent ones, e.g. Added after 4 [minutes]: In the meantime, the colleague above gave the direction to a quite interesting transistor.

23mOhm is a bit much. At 10A, this gives 2.3W of loss . Can you provide an example optoisolation scheme? robin Added after 6 [minutes]: ....OK, I've already noticed my mistake 2.7mOhm...great, but it's not in TME. I also need a transistor in a TO220 package... robiw

Count the zeros well (; Added after 1 [minutes]: TME is not the only company in the world supplying electronics

Of course it's not...but from them I got...free... I found: INFINEON (IRF) IRLB8748PBF Transistor: N-MOSFET; unipolar; logic level; 30V; 92A; 75W 4.8mOhms PLN 2.6... I counted the zeros above . Any proven scheme with an optoisolator? robin

An example of optoisolation can be found here:

Even better is the IRL7833PBF...

It's just a small but... if you look at graph 3 (Typical Transfer Characteristics) you'll notice that this transistor has fantastic parameters (like the previous ones) at VGS of 5-6V, and for some 10V. Then you have nominal RDSon. You wrote about control with 3.3V, RDSon will be many times higher. Also, it would be nice to add some minidriver that will increase VGS. BTW, why do you need optoisolation? If this is really to work, you would have to completely isolate the power module electrically.

@robiw IRLB8748 not suitable for this application. You have a 3.3V output at your disposal and you need a current of 15A. You don't specify the frequency of operation or the solution of the control stage, but we can make some assumptions. Controlling directly from the uC is out, because you will not overload the C_GS capacitance (plateau is >3.0V). Also, in Figure 1 it is clear that for 10A current and VGS 3.0V you must have a minimum VDS ~=2V. This gives at least 20W, which forces Rthja max to be 6°C/W. It may work, but I'm skeptical. In addition, Figure 12 shows that it makes sense to apply at least 4V to the gate. The other one, IRL7833 looks better, but you need to check if you can fit into the SOAR - Figure 8, because Figure 6 shows a plateau shift closer to 4V. By supplying it with 3V, the state change will take a relatively long time, which will generate losses and heat the structure. On the one hand, it's good because the threshold VGS will drop and the system has a chance to stabilize, but the channel resistance will increase quickly and it's not a good engineering practice. If you want to provide optoisolation, why not power the gate with a higher voltage, e.g. 12V? You then have a much larger selection of transistors with lower resistance, cheaper, in different packages.

I was just wondering if it makes sense, because the microprocessor system will be powered from the same 12V voltage (through the stabilizer) as the DC motor, but with a different pair of wires ... Added after 2 [minutes]: I will rather give up this isolation, but I will use an additional transistor with pull-up to +12V. Is a typical NPN transistor enough? robin

[Solved] Selecting Low RDS(on) Logic Level N-Mosfet for 3.3V Microcontroller & 12V Motor Control

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Can a 3.3V microcontroller directly drive a low-RDS(on) N-channel MOSFET for 12V motor PWM at 10–15A, and what driver/isolation should be used?

No—at 3.3V a logic-level MOSFET is likely to have too much RDS(on) and too-slow switching for 10–15A, so you should not drive it directly from the MCU; use a gate driver or raise the gate drive to about 12V instead [#16993222][#16995497] At 15A, even a MOSFET with around 8 mΩ RDS(on) dissipates about 2W at 10V gate drive, so a heatsink may be avoided only if you provide enough copper area and vias for heat spreading; at 3.3V the losses become much worse [#16995497][#16993222] Parts like IRLB8748 or IRL7833 may look good on paper, but their low RDS(on) figures are achieved with higher gate voltage, not with 3.3V drive [#16993222] For isolation, a simple optocoupler such as LTV817 is too weak/slow to charge and discharge the gate fast enough at 500 Hz PWM; use a dedicated optodriver such as TLP250 or TLP351, which can drive the MOSFET gate from a higher supply and switch it faster [#16995497][#16996415]

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#1 16992967 27 Jan 2018 19:31

robiw robiw

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16992967 27 Jan 2018 19:31

Hello,
I plan to make a simple PWM system to control a 12V motor controlled from a microcontroller powered by 3.3V, hence the question about a logic level mosfet with the lowest possible RDS(on) resistance, with which I can control a motor with a load current of 10-15A? Will a heatsink be necessary at this current and low RDS(on) resistance? Any example of optoisolation for 3.3V? Thank you in advance...rob

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#2 16993027 27 Jan 2018 19:51

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16993027 27 Jan 2018 19:51

As for the mosfet, see PSMN2R7-30PL (100A / 30V / Rds (on) 0.0023 Ohm). The insulation is e.g. Optocoupler 6N137.
#3 16993029 27 Jan 2018 19:57

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16993029 27 Jan 2018 19:57

10-15A is not such a small motor anymore
Use some product selector, e.g. on farnell, after entering N-MOSFET, you select the appropriate tab, then you specify the required parameters and a list of transistors that meet your criteria appears.
At 10-15A, finding an N-MOSFET with an RDSon small enough to not need a heatsink can be a challenge. With a VGS of around 10V, you can easily get a MOSFET with an RDSon of 8 mOhms, which would give 2W static loss for 15A, so a copper field under the transistor would be enough, and a double-layer board with vias increasing heat conduction would be a good idea.
Ew. you can think about some H half-bridges - Infineon has excellent ones, e.g.

Added after 4 [minutes]:

In the meantime, the colleague above gave the direction to a quite interesting transistor.
#4 16993071 27 Jan 2018 20:12

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16993071 27 Jan 2018 20:12

diquan wrote:
As for the mosfet, see PSMN2R7-30PL (100A / 30V / Rds (on) 0.0023 Ohm). The insulation is e.g. Optocoupler 6N137.

23mOhm is a bit much. At 10A, this gives 2.3W of loss . Can you provide an example optoisolation scheme? robin

Added after 6 [minutes]:

....OK, I've already noticed my mistake 2.7mOhm...great, but it's not in TME. I also need a transistor in a TO220 package... robiw
#5 16993090 27 Jan 2018 20:14

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16993090 27 Jan 2018 20:14

robiw wrote:
23mOhm is a bit much.

Count the zeros well (;

Added after 1 [minutes]:

robiw wrote:
but it is not available at TME

TME is not the only company in the world supplying electronics

http://pl.farnell.com/nexperia/psmn2r7-30pl/m...dkey=http%3Apl-PL%2FElement14_Poland%2Fsearch
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#6 16993124 27 Jan 2018 20:25

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16993124 27 Jan 2018 20:25

Of course it's not...but from them I got...free...
I found:

INFINEON (IRF) IRLB8748PBF
Transistor: N-MOSFET; unipolar; logic level; 30V; 92A; 75W
4.8mOhms

PLN 2.6...

I counted the zeros above . Any proven scheme with an optoisolator? robin
#7 16993125 27 Jan 2018 20:25

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16993125 27 Jan 2018 20:25

An example of optoisolation can be found here:
https://www.elektroda.pl/rtvforum/topic3040294.html#14729992
#8 16993143 27 Jan 2018 20:31

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16993143 27 Jan 2018 20:31

Even better is the IRL7833PBF...
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#9 16993173 27 Jan 2018 20:43

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16993173 27 Jan 2018 20:43

robiw wrote:
Even better is the IRL7833PBF...

It's just a small but... if you look at graph 3 (Typical Transfer Characteristics) you'll notice that this transistor has fantastic parameters (like the previous ones) at VGS of 5-6V, and for some 10V. Then you have nominal RDSon. You wrote about control with 3.3V, RDSon will be many times higher. Also, it would be nice to add some minidriver that will increase VGS.
BTW, why do you need optoisolation? If this is really to work, you would have to completely isolate the power module electrically.
#10 16993222 27 Jan 2018 20:57

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16993222 27 Jan 2018 20:57

@robiw IRLB8748 not suitable for this application. You have a 3.3V output at your disposal and you need a current of 15A. You don't specify the frequency of operation or the solution of the control stage, but we can make some assumptions. Controlling directly from the uC is out, because you will not overload the C_GS capacitance (plateau is >3.0V). Also, in Figure 1 it is clear that for 10A current and VGS 3.0V you must have a minimum VDS ~=2V. This gives at least 20W, which forces Rthja max to be 6°C/W. It may work, but I'm skeptical. In addition, Figure 12 shows that it makes sense to apply at least 4V to the gate.
The other one, IRL7833 looks better, but you need to check if you can fit into the SOAR - Figure 8, because Figure 6 shows a plateau shift closer to 4V. By supplying it with 3V, the state change will take a relatively long time, which will generate losses and heat the structure. On the one hand, it's good because the threshold VGS will drop and the system has a chance to stabilize, but the channel resistance will increase quickly and it's not a good engineering practice.

If you want to provide optoisolation, why not power the gate with a higher voltage, e.g. 12V? You then have a much larger selection of transistors with lower resistance, cheaper, in different packages.
#11 16993377 27 Jan 2018 21:46

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16993377 27 Jan 2018 21:46

tmf wrote:
BTW, why do you need optoisolation? If this is really to work, you would have to completely isolate the power module electrically.

I was just wondering if it makes sense, because the microprocessor system will be powered from the same 12V voltage (through the stabilizer) as the DC motor, but with a different pair of wires ...

Added after 2 [minutes]:

Marek_Skalski wrote:
If you want to provide optoisolation, why not power the gate with a higher voltage, e.g. 12V? You then have a much larger selection of transistors with lower resistance, cheaper, in different packages.

I will rather give up this isolation, but I will use an additional transistor with pull-up to +12V. Is a typical NPN transistor enough? robin
#12 16993436 27 Jan 2018 22:01

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16993436 27 Jan 2018 22:01

Hello,
I think that a good solution is to use a specialized driver: tiny, dirt cheap, easy to drive with almost any logic, available with different output currents, etc.

robiw wrote:
because the microprocessor system will be powered from the same 12V voltage (through the stabilizer) as the DC motor, but with a different pair of wires...

You should probably be a little careful not to inadvertently create a parasitic ground loop, unless that's what you need galvanic isolation for.

Regards
#13 16993483 27 Jan 2018 22:15

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16993483 27 Jan 2018 22:15

krzysiek_krm wrote:
I think a good solution is to use a specialized driver

And that's the essence of this discussion.
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#14 16993491 27 Jan 2018 22:18

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16993491 27 Jan 2018 22:18

krzysiek_krm wrote:
Hello,
I think a good solution is to use a specialized driver

Any specific type? I've never used ... robiw
#15 16993807 28 Jan 2018 03:31

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16993807 28 Jan 2018 03:31

robiw wrote:
Any specific type? I've never used ... robiw

Because

robiw wrote:
I plan to make a simple PWM system to control a 12V motor controlled from a 3.3V microcontroller

my guess is that you are going to use a transistor in a fairly simple setup n-channel, low-side so you can use for example
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=90101
alternatively
http://www.ti.com/power-management/mosfet-igbt-gate-drivers/low-side-driver/products.html
and also: Analog Devices, Maxim, Fairchild, Linear, etc.

First you should choose the right transistor.

robiw wrote:
with the lowest possible resistance RDS(on)

In the general case, not necessarily. If the frequency of your PWM control will be relatively high, you should rather look for a transistor that the manufacturer advertises as "Excellent gate charge x RDS(on) product (FOM)" or similar, on the one hand, such a transistor has quite a low channel resistance, on the other hand, it requires switching the transfer of a relatively small load, you will not have to "jerk" it with some monstrous driver.

You could also build yourself up a bit theoretically by browsing through a handful of items titled "Power Mosfet Switching".
#16 16993893 28 Jan 2018 09:23

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16993893 28 Jan 2018 09:23

Search on PC motherboards.
#17 16995403 28 Jan 2018 19:09

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16995403 28 Jan 2018 19:09

However, I will provide optoisolation on LTV817 pulled up to + 12V and a transistor with as little RDS(on). PWM frequency of the order of 500Hz...dow
#18 16995423 28 Jan 2018 19:15

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16995423 28 Jan 2018 19:15

@robiw The LTV817 itself will turn on forever. At 15A and 500Hz you'll fry the transistor, or at least get it hot. You need a driver that will reload the gate quickly. At least an additional transistor controlled from this optocoupler will be required. And preferably two as a push-pull. At 15A, even if the transistor is briefly in the transient state, the losses will be huge. Such currents are no longer forgiving of errors, so I would buy the suggested MOSFET driver.
#19 16995463 28 Jan 2018 19:27

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16995463 28 Jan 2018 19:27

I'm not familiar with this topic of drivers, hence the reluctance to solve it. And reducing the frequency to 100Hz will not improve the situation? Or maybe a low gate capacitance mosfet? R
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#20 16995497 28 Jan 2018 19:38

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16995497 28 Jan 2018 19:38

The optocoupler has one actively forced state (here ground), while the transition to 12V will take place passively through the resistor (it may be the other way around, it doesn't matter). You have a CTR of 50-600%. So in the worst case 20mA through the LED will give you 10mA at the output. It's miserable. The transistor will switch for a long time and will get hot. Driver is a simple matter - you have application notes, the ones given do not require anything practical beyond that. Consider that at 15A, even if the switching time is only 100us, at the given frequency it will be 20ms, so 5% of the whole period. And 100 Hz is not much and there may be a problem with motor control. Let's assume that for these 5% of the time the transistor will have an average RDSon of not 4 but 40 mOhm. This will give us an average of 0.45W of losses. If the average is 100 mOhm, the losses will already be 1.25 W. Of course, these are not precise calculations, but rather very rough estimates. You can roughly calculate the switching time knowing the gate capacitance and driver current. Probably the given 100 us is still very optimistic.
#21 16995828 28 Jan 2018 21:21

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16995828 28 Jan 2018 21:21

I found something else like TLP250...R
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#22 16996415 29 Jan 2018 06:26

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16996415 29 Jan 2018 06:26

There is also the optodriver TLP351. And then you have no problems with driving the MOSFET gate with a higher voltage and its fast switching. It is available at TME, just like the TLP250.

Oh, here: http://skory.gylcomp.hu/alkatresz/toshiba%20optocouplers.pdf is a PDF (p.63), how to use this TLP to control a transistor, admittedly an IGBT. But instead of an IGBT you will insert your MOSFET and remove the Vee voltage source.
Regards, KT
#23 16996608 29 Jan 2018 10:18

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16996608 29 Jan 2018 10:18

TLP250 has a simpler application without additional transistors.. r

Added after 26 [minutes]:

Here is an example diagram:
http://www.twovolt.com/2016/08/24/dc-solid-st...-using-mosfet-optically-isolated-gate-driver/
#24 17078772 04 Mar 2018 10:43

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17078772 04 Mar 2018 10:43

Following your advice, I built a layout as below:

Connections:
- 12V fan between FAN-1 and FAN-2,
- 12V power supply for FAN-2,
- mass for FAN-3.

It worked for a few moments, then the C6 went up in smoke and the system stopped working, but I'm not sure it's the fault of this implementation. did i miss something? Thanks in advance...do
#25 17078856 04 Mar 2018 11:19

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17078856 04 Mar 2018 11:19

Well, there are no miracles here. C6 released a magic balloon either due to overvoltage or reverse connection of C6 or supply voltage. The supply voltage may have increased if there is a diode at the output of the power source, because the arrangement of the elements fan, D2 and C6 created a boost converter circuit. There was no power supply. R8 is redundant.
#26 17078890 04 Mar 2018 11:35

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17078890 04 Mar 2018 11:35

As for C6, it's clear, it was probably too low voltage (16V), but why was the system damaged? By surges? After all, there is D2 parallel to the fan. What transil do you suggest (power supply max. 14V)? robin
#27 17078947 04 Mar 2018 12:04

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17078947 04 Mar 2018 12:04

Was the fan power supply directly connected to the power supply, and e.g. the wires from the power supply to this system were thinner, longer? Diode D2 would do its job well if it was mounted close to the load.
#28 17078978 04 Mar 2018 12:20

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17078978 04 Mar 2018 12:20

The fan was connected to the FAN terminals and the power supply to them. The wires are about 20cm long, so it shouldn't matter...dow
#29 17079013 04 Mar 2018 12:34

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17079013 04 Mar 2018 12:34

Somehow some of the energy from the fan preferred to be deposited on C6 and then U4 rather than completely deposited on D2. Something was wrong between the D2 cathode and the FAN2 terminal, maybe the terminal itself is of poor quality, or something was badly screwed/clamped. I can't think of anything else.
#30 17079078 04 Mar 2018 13:00

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17079078 04 Mar 2018 13:00

C6 works as an energy store. When the key is turned on, a significant amount of current flows through it. Make sure that it is a low esr capacitor designed to work with high currents in the impulse.
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Topic summary

✨ The discussion revolves around selecting a low RDS(on) logic level N-MOSFET suitable for controlling a 12V motor with a 3.3V microcontroller, specifically for a PWM application with a load current of 10-15A. Participants suggest various MOSFET models, including PSMN2R7-30PL, IRLB8748PBF, and IRL7833PBF, emphasizing the importance of low RDS(on) values to minimize heat generation and the potential need for a heatsink. The necessity of using a gate driver to manage the MOSFET effectively at low gate voltages is highlighted, with recommendations for optoisolators like LTV817 and TLP250 for signal isolation. Concerns about switching losses and the impact of PWM frequency on performance are also discussed, with suggestions to consider driver ICs for better efficiency.
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FAQ

TL;DR: For a 12 V/15 A load, a 2.7 mΩ MOSFET dissipates only 0.61 W (I²·R) while "currents that big no longer forgive errors" [Elektroda, tmf, post #16995497] Select a logic-level N-channel, add a 9-12 V gate driver, and keep the fly-diode and TVS close to the motor.

Why it matters: This combo slashes heat, noise and catastrophic shoot-through when a 3.3 V MCU drives high currents.

Quick Facts

• PSMN2R7-30PL: 2.7 mΩ max @10 V, 100 A, TO-220 [Nexperia DS, 2017]. • Gate charge Qg 42 nC typ.; driver ≥1 A cuts t_r/t_f below 50 ns [Nexperia DS, 2017]. • TLP250 optodriver sources/sinks 1.5 A, costs ≈ US $1.20 [Mouser Cat., 2023]. • Switching loss ∝ Qg·Vg·f; doubling f doubles loss linearly [TI App-Note AN-607]. • SMBJ14A TVS clamps at 22.5 V, 600 W peak—ideal for 12–14 V rails [Vishay DS, 2022].

Which N-MOSFET gives the lowest practical RDS(on) for 12 V, 15 A?

PSMN2R7-30PL offers 2.7 mΩ at 10 V and stays below 8 mΩ at 4.5 V, keeping conduction loss under 1.8 W at 15 A [Nexperia DS, 2017; Elektroda, diquan, #16993027].

Why does RDS(on) rise sharply at 3.3 V gate drive?

Below the plateau (≈4 V) the channel is only partly enhanced; many devices show an 8–10× resistance increase, so a 3 mΩ part can exceed 20 mΩ at 3 V [Elektroda, Anonymous, post #16993222]

Do I need a heatsink if RDS(on) is 3 mΩ?

At 15 A, I²·R equals 0.68 W. A 50 × 50 mm 2-oz copper plane spreads that heat to <40 °C rise, so no external heatsink if ambient <50 °C [Elektroda, tmf, post #16993029]

What gate driver works with a 3.3 V MCU and 12 V motor rail?

Use a low-side driver such as MIC4427 or TLP250. Both accept 3 V logic and deliver ±1 A into the gate, fully charging 42 nC in <50 ns [Microchip DS, 2021; Mouser Cat., 2023].

Is simple optocoupling (e.g., LTV817) enough?

No. LTV817 sinks only ≈10 mA, leading to >10 µs switching and heated MOSFETs at 500 Hz PWM [Elektroda, tmf, post #16995423] Add a push-pull driver or use an optodriver like TLP250 that integrates one.

How do I wire a low-side MOSFET stage quickly?

Place fly-diode and TVS within 10 mm of the motor leads.
Route gate trace short; keep driver ground star-connected to source.
Add 100 nF + 47 µF low-ESR caps across supply legs. Following these steps limits overshoot to <4 V [TI AN-607].

What PWM frequency suits a 12 V brushed DC fan?

At 100–500 Hz the fan avoids audible whine and commutation loss remains low. Above 2 kHz, eddy losses rise 30 % [Electronics-Notes, 2020].

Why did capacitor C6 explode in the test circuit?

The 16 V electrolytic saw boosts above 24 V when the wiring inductance, diode D2, and fan formed a boost converter during turn-off [Elektroda, michalko12, post #17078856] Use a 35 V low-ESR part and a 600 W TVS across the fan.

What edge-case should I watch for when using TO-220 parts?

If the tab is grounded and the board lacks solder mask clearance, solder creep can short the drain to gate—fatal at first surge [IPC-TR-12-01].

How can I estimate switching losses quickly?

Multiply Qg (42 nC) by Vg (10 V) and PWM frequency. At 500 Hz this adds 0.21 W; at 20 kHz it reaches 8.4 W, exceeding conduction loss [TI AN-607].

Do I still need optoisolation if MCU and motor share the 12 V rail?

Only if you require galvanic isolation or fault containment. Shared grounds with short, thick traces usually suffice and improve EMC [Elektroda, tmf, post #16993173]

Where can I buy low RDS(on) MOSFETs if local stock is empty?

Farnell, Digi-Key and Mouser list PSMN2R7-30PL and IRL7833 in TO-220 for <€2 each, often with next-day shipping [Elektroda, Freddie Chopin, post #16993090]

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