Hi,
I'm trying to create a MOSFET PWM driver for DC motors/solenoids. I eventually plan to create an H bridge, but this is my first attempt.
The DC motor I've tested this with has a coil resistance of about 22 ohms, so at 12 v this is drawing around 550mA. At 100% duty, the MOSFET is quite cool, equally at 0% duty. At any other duty cycle, the MOSFET quickly heats up. I want to drive some fairly big currents (automotive throttle could peak at 8A or so) so I'm worried that at higher loads this will be a problem.
What am I missing? Why is there so much more heat when switching the MOSFET?
That MOSFET needs 10V at the gate, you are not giving it enough.
It also needs a reasonably high current to switch fast, though that
depends on the PWM frequency.
Try changing the 6k8 to 680 ohms in the first instance, at least you'll
get it turning on fully. Decreasing the 4k7 to 1k5 might help with the
turn off speed, but it will slow down turn on...
A much better way to drive a MOSFET from 3.3V is to use a MOSFET
driver chip, such as MIC4420, which will switch even a large mosfet in
200ns or so. Then you can PWM at 50kHz even.
Thanks for your help. I was reading the threshold voltage on the data sheet, but after a little extra research, I see I need more than that. I see that most of the graphs reference 10v Vgs so i guess that's what it's best to use. It'll certainly give me a lower Rds. Still don't understand why i get the heat when using PWM, but don't at 100% PWM.
I'll check out the MOSFET driver module too, thanks for the tip.
I just wanted to ask a few questions about these MOSFET drivers, hopefully someone can help me out.
Supply voltage - 4.5 - 18v on the datasheet. Does this have significant affect on the output? The 12v supply in my application will actually be automotive Vbattery (11-15 or so) - should I give it a regulated 10V or something?
Gate pull down - I can't find any example circuits - Do people pull down the output of these drivers or not?
Can I use this component to drive a P-Channel and N-Channel from the same logic output? I'm hoping to create a full H-Bridge eventually
Is the peak output current an indicator of how fast it'll manage to switch the MOSFET? I've noticed that at 6A, this is the highest current driver I can find.
Thanks for the tip on the diode. Done a little research....I will be switching inductive loads so I need something in there.
Why not buy an H-bridge. Well, they seem expensive to me, and I wanted to try and make one to be honest. I don't know if mine will be any cheaper in the end.
I just wanted to ask a few questions about these MOSFET drivers, hopefully someone can help me out.
Supply voltage - 4.5 - 18v on the datasheet. Does this have significant affect on the output? The 12v supply in my application will actually be automotive Vbattery (11-15 or so) - should I give it a regulated 10V or something?
It will deliver more current at higher supply voltage, you'll need to ensure it never
gets more than 18V which might mean some filtering on a vehicle 12V supply.
Gate pull down - I can't find any example circuits - Do people pull down the output of these drivers or not?
The driver has its own pull down on the input I think - the datasheet should tell
you all these details. The datasheet must have an example circuit.
Can I use this component to drive a P-Channel and N-Channel from the same logic output? I'm hoping to create a full H-Bridge eventually
on a 12V supply yes. But you must prevent shoot-through so you cannot
switch the top and bottom switches in the same arm of the H-bridge simultaneously
due to the turn-off time being more than zero.
Is the peak output current an indicator of how fast it'll manage to switch the MOSFET? I've noticed that at 6A, this is the highest current driver I can find.
Yes, its overkill, add a 10 ohm resistor on the output to keep things sane. 6A is only
for 18V supply though. You must add adequate decoupling to the driver chip, ceramic
and right next to it. 10uF + 0.1uF would be reasonable.
The reason you see heat on PWM is because you have large switching losses
due to the sluggish switching - switching losses depend on switching time, switching
frequency and load power.
One option is to use a "logic level" MOSFET which eliminates the need for 10v at gate. These devices also tend to reduce the need for a driver chip. I've run PWM at 5A directly off Arduino logic with no significant heating. Higher currents require a heat sink. I only needed PWM at 1KHz. Much higher switching rates may require a driver to overcome gate capacitance.
I may also mention the an Hbridge can be replaced by a DPDT relay. This assumes that motor direction switching only occurs "occasionally" as in a robot. I have achieved high currents at much lower cost than a hi amp Hbridge controller. Note that switching the relay a 0 current eliminates electrical stress on the contacts.
The optocoupler also adds to the heating.
You want the MOSFET to quickly switch from full-on to full-off and vice versa.
The opto drags out the transition time so the MOSFET spends more time in its higher resistance range where more heat is generated.
A logic level device like this one
can fully switch on & off from 4.5V. Its Rds at 20A is quite low - see page 2 of the data sheet:
Rds at VGS=4.5V, ID=20A, typical 0.007 ohm.
Thus with your 8A load, power dissipated = IIR = 88.007 = 0.448W.
But you have to switch it quick - the opto will just kill it.
The MOSFET is also providing the isolation from the 12V source, so the opto isn't even needed.
