Electronics > Electronics
Motor driver circuit - suggestions for improvements
sys 49152:
Hello.
Attached is a schematic of the motor driver I am using on my robots. This circuit drives 2 NMB PM55L stepper motors and on one robot it drives 4 of them.
I am thinking of revising the boards and wanted a bit of input.
Should I add suppression diodes? The reference diagrams I have seen so far have been equivocal on the matter though I'm aware that, on balance, it's probably advisable.
Is there any inherent reason why it's a bad idea to run 2 motors from each stage? I'm using this circuit with a 4x4 skid-steer chassis with the motors connected in parallel. Are there any engineers shuddering at the thought or is it not an issue?
Regards
Tim
jwatte:
Beacause you use MOSFETs, snubbing diodes are not needed. The body diodes of the MOSFETs are sufficient. Just make sure the MOSFETs really are rated for 2x the voltage you will be providing the motors.
Running multiple motors from a single switch is not something I've done. I'd assume electrically that it's similar to just running a single, bigger motor (two small inductors in parallel.)
I assume those are unipolar motors, as you're not doing an H-bridge? The biggest change I'd make would be to switch to unipolar motors, if you need more torque or lower weight. But if unipolar is good enough, that's not needed.
You may also want EMI capacitors across the motor leads to ground -- 10 nF each, perhaps?
Finally, depending on the gate charge of those MOSFETs, you may be temporarily exceeding the rated current on the outputs of your microcontroller each time you switch them on or off. A current limiting resistor on each output pin of the controller would be safer -- 100 ohms might be sufficient, even if a dead short through 100 ohms would still exceed the rated current of an output pin.
Soeren:
Hi,
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---Beacause you use MOSFETs, snubbing diodes are not needed. The body diodes of the MOSFETs are sufficient.
--- End quote ---
The intrinsic diodes are not ment for dampening the kick back, they're merely a byproduct of any mosfet - and they are slow acting!
External diodes should be used with inductive loads. A (real) snubber circuit can be added to lower the switching losses of the device itself.
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---Running multiple motors from a single switch is not something I've done. I'd assume electrically that it's similar to just running a single, bigger motor (two small inductors in parallel.)
--- End quote ---
The parallel coupling of Inductors results in a lower inductance, just like resistors!
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---I assume those are unipolar motors, as you're not doing an H-bridge? The biggest change I'd make would be to switch to unipolar motors [...]
--- End quote ---
Perhaps proof read before posting an answer? ;D
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---Finally, depending on the gate charge of those MOSFETs, you may be temporarily exceeding the rated current on the outputs of your microcontroller each time you switch them on or off. A current limiting resistor on each output pin of the controller would be safer -- 100 ohms might be sufficient, even if a dead short through 100 ohms would still exceed the rated current of an output pin.
--- End quote ---
PIC's won't get hurt by this, and if anything, I'd go with a 10 Ohm resistor, but for efficient switching, a driver peaking at around 4A is the way to smash the Miller Wall.
jwatte:
--- Quote from: Soeren on December 04, 2012, 05:32:20 PM ---
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---Beacause you use MOSFETs, snubbing diodes are not needed. The body diodes of the MOSFETs are sufficient.
--- End quote ---
The intrinsic diodes are not ment for dampening the kick back, they're merely a byproduct of any mosfet - and they are slow acting!
--- End quote ---
I've seen several seemingly robust solutions that rely on the body diodes for snubbing. If you want fast diodes, Schottkys are kind-of hard to find in high voltage grades. Although if we're talking 250mA/12V unipolar stepper motors, this probably isn't really a problem. I've driven such things with BS-170s on breadboards without mishaps...
--- Quote ---
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---Running multiple motors from a single switch is not something I've done. I'd assume electrically that it's similar to just running a single, bigger motor (two small inductors in parallel.)
--- End quote ---
The parallel coupling of Inductors results in a lower inductance, just like resistors!
--- End quote ---
Agreed! That's why I said a bigger _motor_, as in bigger current draw.
--- Quote ---
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---I assume those are unipolar motors, as you're not doing an H-bridge? The biggest change I'd make would be to switch to unipolar motors [...]
--- End quote ---
Perhaps proof read before posting an answer? ;D
--- End quote ---
Yes! The first thing I would do would be to proof-read my posts :-) I meant to suggest considering bipolar motors, if more power is needed. Which it might not be.
--- Quote from: jwatte on December 04, 2012, 12:48:06 PM ---Finally, depending on the gate charge of those MOSFETs, you may be temporarily exceeding the rated current on the outputs of your microcontroller each time you switch them on or off. A current limiting resistor on each output pin of the controller would be safer -- 100 ohms might be sufficient, even if a dead short through 100 ohms would still exceed the rated current of an output pin.
--- End quote ---
PIC's won't get hurt by this, and if anything, I'd go with a 10 Ohm resistor, but for efficient switching, a driver peaking at around 4A is the way to smash the Miller Wall.
[/quote]
Yep, the main point was to make sure you check the output of the controller, whatever you're using.
Given that we're talking unipolar steppers here, though, chances are that the kinds of high-gate-charge MOSFETs that need separate drivers aren't involved.
Speaking of which: Is there a good, through-hole, 5V gate driver that works with 100% duty cycles? I previously used some decent 5V drivers from International Rectifier, but they use a bootstrap that needs a lower duty cycle -- so, only for PWM and continual stepping, not for standing still.
Soeren:
Hi,
--- Quote from: jwatte on December 04, 2012, 09:22:07 PM ---I've seen several seemingly robust solutions that rely on the body diodes for snubbing.
--- End quote ---
The operative term here is "seemingly robust". How did you judge?
The only way to make it robust with the intrinsic diodes is, to use eg. a 10A mosfet where 100mA is needed and that would be absolutely crazy, as the gate charge goes up with current handling.
And please don't call a free wheeling diode a snubber. A snubber is an RC circuit and the electrical function is different between the two.
--- Quote from: jwatte on December 04, 2012, 09:22:07 PM ---If you want fast diodes, Schottkys are kind-of hard to find in high voltage grades.
--- End quote ---
You're the one mentioning Schottky diodes here. Schottkys are usually not the answer and a lot of them are a bit on the slow side as well.
But what do you consider high voltage in that respect?
A Schottky like 1N5822 is 40V/3A and besides its slowness 40V caters for the majority of hobby 'bots.
Go with Fast Recovery diodes or Superfast Recovery diodes if you want speedy devices with high voltage and current ratings, or, if the current is within its capabilities, the 1N4148 is blindingly fast compared to Schottkys.
--- Quote from: jwatte on December 04, 2012, 09:22:07 PM --- Although if we're talking 250mA/12V unipolar stepper motors, this probably isn't really a problem. I've driven such things with BS-170s on breadboards without mishaps...
--- End quote ---
BS170 is good for driving LEDs (providing a high enough voltage), as they need current limiting anyway and they're good for fast driving due to their switching times, but I wouldn't use it for a motor, as it has a higher voltage drop than a bipolar.
The reason that you didn't have any casualties is probably a mix of the time you actually ran it, the pulse current abilities of 1.2A and pure luck.
I have had several devices (30V/3.4A static) give up the ghost trying to drive a small 16 Ohm speaker. A simple 1N4148 cured it :)
--- Quote from: jwatte on December 04, 2012, 09:22:07 PM ---Given that we're talking unipolar steppers here, though, chances are that the kinds of high-gate-charge MOSFETs that need separate drivers aren't involved.
--- End quote ---
It's a common misunderstanding that MOSFETs can be properly driven by a microcontroller pin.
If you want the fast switching it's capable of (which is what gives it a high efficiency), you need to poor buckets of current at it when it hits the Miller Plateau. If you don't, a bipolar solution will be better.
--- Quote from: jwatte on December 04, 2012, 09:22:07 PM ---Speaking of which: Is there a good, through-hole, 5V gate driver that works with 100% duty cycles? I previously used some decent 5V drivers from International Rectifier, but they use a bootstrap that needs a lower duty cycle -- so, only for PWM and continual stepping, not for standing still.
--- End quote ---
MOSFET drivers are usually made to be able to provifde a high peak current for switching where the high current is needed. For holding the device in conduction (after the switch) it only takes a very modest current compared.
TI makes the UCC2742x (where x is 3, 4 or 5) dual 4A drivers in 8-PDIP, the 3, 4 and 5 covers Dual Inverting, Dual Non-inverting and one of each respectively.
Microchip makes TC4421 (Inverting) and TC4422 (Non-inverting) 9A single drivers, both comes in 8-PDIP TO220-5.
The 4A and 9A of the above drivers are their peak ratings of course, the continous currents are 300mA and 2A respectively.
I'm sure there are many more out there, but the mentioned drivers are some I know - I usually make my drivers from passives, as I like to have full control over all aspects.
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