High Current PWM ?

[pwssr]

I need to control the speed of 2 high current draw (<=30a @ 12-14V) motors with the highest current at startup and steady state of less than 15a. I am thinking using MOSFETs to drive the motors should do the job. I do have a couple of IRLZ44's which look plenty healthy for this job.

Looking for suggestions or cautions.

TIA,
Paul

 

[phalanx]

What is the current draw when the rotor is locked?

Also, do you need speed control or is this just an on/off application. Do you need to reverse either of the motors?

 

[pwssr]

What is the current draw when the rotor is locked?

It won't be - but peak (startup) is under 30A (and fused)

Also, do you need speed control or is this just an on/off application. Do you need to reverse either of the motors?

Speed control = yes
Reverse = no

 

[phalanx]

You may think that the motor will never stall (and that may very well be the case) but a proper design takes into account that absolute extremes that the system could experience.

If we take 30A to be the max current in the system then the IRLZ44 will work but it will need some thermal control help. Looking at the datasheet, you can see that that the Rds(on) for it with a 5V gate/source voltage is about 0.033 ohms. At 30 amps that translates to 30^2 * 0.033 = 29.7 watts. 30 watts means it is going to get very hot and will require a heat sink. The good news is this falls inside the Safe Operating Area of the FET for DC loads. If you can increase the gate voltage, you can lower Rds(on) which translates into a cooler running FET.

With 30A circulating, if you quickly shut off the FET which is very easy to do with a microcontroller, you are going to generate a large voltage spike from the back EMF of the motor. You will need a protection diode around the FET to protect it from the spike. Without one, you will put considerable stress on the FET and it will likely give up the ghost rather quickly.

 

[pwssr]

You may think that the motor will never stall (and that may very well be the case) but a proper design takes into account that absolute extremes that the system could experience.

If we take 30A to be the max current in the system then the IRLZ44 will work but it will need some thermal control help. Looking at the datasheet, you can see that that the Rds(on) for it with a 5V gate/source voltage is about 0.033 ohms. At 30 amps that translates to 30^2 * 0.033 = 29.7 watts. 30 watts means it is going to get very hot and will require a heat sink. The good news is this falls inside the Safe Operating Area of the FET for DC loads. If you can increase the gate voltage, you can lower Rds(on) which translates into a cooler running FET.

With 30A circulating, if you quickly shut off the FET which is very easy to do with a microcontroller, you are going to generate a large voltage spike from the back EMF of the motor. You will need a protection diode around the FET to protect it from the spike. Without one, you will put considerable stress on the FET and it will likely give up the ghost rather quickly.

Great info - thanks a lot!

Paul

 

[Oznog]

I would not use these devices by choice. I would use a "logic level" MOSFET that has a fantastically stronger current drive and lower rds-on at 5v. Note that it's not just a matter of rds-on, there's a matter of having enough gate voltage to ensure the specified Id is greater than the current used or it will stop being a switch. At 5v, the IRFZ44NIR data sheet I pulled (there are a number of part variations) can drive up to 25A as a switch but one, you want a lot of margin to get a lot rds-on, and two, temp and part variations (particularly in the threshold voltage) can substantially change that chart. Vth is specified to be anywhere 2v-4v. Assuming that chart was for a 3v middle-of-the-road Vth, you get a Vth=4v device and the Vgs-Id relationship shifts down by 1v. Now that part has an Id of only 9amps when cold, though it does go up to about 17 amps when it's at its maximum junction temp. Need more margin!

Take a look at "hexfets" and stuff in a 8-SOP pkg. Like this:
**broken link removed**

Vgs/Id curve shows Id at 5v is insanely more than you will use. Rds =0.003 ohms. You will not need a heatsink, nor can one even be used.

Or
https://www.fairchildsemi.com/ds/FD/FDS7088N7.pdf

Has similar stats. Very strong drive on 5V! Rds-on at 5v, 15 amps is only 1.1mOhms!

The thing to note is these switches have little heat dissipation ability and you can't use a heat sink. With a 1.1 milliohm resistance at Vgs=5v at 15 amps Id, heat is only 1/4 watt. However, switching losses on PWM are going to be VERY important. Losses will basically be proportional to the frequency.

Paralleling the MOSFETs will improve their dissipation and these things are quite cheap and small. The only problem is that the extra capacitance of multiple gates on one PWM pin may slow down the pin's transition due to the capacitive loading. These devices do have a higher gate capacitance at 5v than the IRFZ44.

Many amateurs are initially hesitant to use a SOP-8 pkg since it "feels weak" not to have a heatsink. The only real truth is that the SOP-8 devices will burn out quite fast in some sort of fault condition due to the lower dissipation of the "problem's" heat whereas a big TO-220 with a big heatsink might take it for several seconds or even indefinitely. These SOP-8 devices are plain far superior switches for logic-level job- relatively few TO-220 devices have very strong drive on Vgs=5v.

In a way, TO-220 devices are on the way out as the most common solution, the industry trend is for these far cheaper SMD pkgs, a desire to avoid heatsinks that add many dollars (even hundreds) to the cost, and consumers want smaller devices. Most of the neat new stuff that works so much better is in ultra-small devices.

 

[chiba]

**broken link removed**
I use this as a speed controller for my homemade milling machine, with some big fat heatsinks on the fets it's survived lots of motor stalls over 57A.