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Offline sys 49152Topic starter

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Motor driver circuit - suggestions for improvements
« on: December 04, 2012, 04:42:43 AM »
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

Offline jwatte

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Re: Motor driver circuit - suggestions for improvements
« Reply #1 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. 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.

Offline Soeren

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Re: Motor driver circuit - suggestions for improvements
« Reply #2 on: December 04, 2012, 05:32:20 PM »
Hi,

Beacause you use MOSFETs, snubbing diodes are not needed. The body diodes of the MOSFETs are sufficient.
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.


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.)
The parallel coupling of Inductors results in a lower inductance, just like resistors!


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 [...]
Perhaps proof read before posting an answer? ;D


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.
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.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jwatte

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Re: Motor driver circuit - suggestions for improvements
« Reply #3 on: December 04, 2012, 09:22:07 PM »
Beacause you use MOSFETs, snubbing diodes are not needed. The body diodes of the MOSFETs are sufficient.
The intrinsic diodes are not ment for dampening the kick back, they're merely a byproduct of any mosfet -  and they are slow acting!

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
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.)
The parallel coupling of Inductors results in a lower inductance, just like resistors!

Agreed! That's why I said a bigger _motor_, as in bigger current draw.

Quote
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 [...]
Perhaps proof read before posting an answer? ;D

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.


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.
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.

Offline Soeren

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Re: Motor driver circuit - suggestions for improvements
« Reply #4 on: December 04, 2012, 11:24:46 PM »
Hi,

I've seen several seemingly robust solutions that rely on the body diodes for snubbing.
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.


If you want fast diodes, Schottkys are kind-of hard to find in high voltage grades.
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.


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...
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 :)


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.
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.


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.
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.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline sys 49152Topic starter

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Re: Motor driver circuit - suggestions for improvements
« Reply #5 on: December 05, 2012, 03:23:21 AM »
Thanks for your input.

I'll do some more work on the circuit before I build the next revision.

Tim.

Offline jwatte

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Re: Motor driver circuit - suggestions for improvements
« Reply #6 on: December 05, 2012, 02:41:31 PM »
If you want fast diodes, Schottkys are kind-of hard to find in high voltage grades.
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.
Quote
the 1N4148 is blindingly fast compared to Schottkys.

That's counter to all the design advice I've read until now. Schottkys are generally recommended as fast barriers, in data sheets for anything from switching power supplies, to gate drivers, charge pumps, etc.

Looking at the data sheets:
1N4148 has a switching speed of 4 nanoseconds, and a max current of 450 mA. It has an 8 pF capacitance.
Meanwhile, a Schottky like CSD01060 is rated at "essentially no switching losses" and 1.7A. It has an 11 pF capacitance, which is slightly higher. Maybe that's the "slowness"? The capacitance per rated ampere is lower, though.

Also: I take "essentially no switching losses" to mean "zero recovery" but maybe that's not what it means. If you could qualify your suggestion to look away from Schottkys with a more detailed reference, that would be great!

Quote
But what do you consider high voltage in that respect?

I really should proofread my posts, as I meant "current!" The above diode is rated for 600V -- I meant high current. Schottkys rated for > 10 A seem to be hard to find and expensive to purchase when you do.
A small-signal switcher like 1N4148 is cheap, but it's also small :-)


Quote
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...
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.

That depends on driving current, doesn't it? Although the reason I mentioned BS170 was that it's not a high-performance part, yet worked fine for a small, 12V, unipolar stepper motor. Most unipolars tend to be small, because if you want real power, you will want bipolar. I think we agree on that?

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.

You need to build up the gate charge quickly enough that you don't develop heat. If you can switch "slowly" (we're still talking less than a few microseconds) -- either PWM at perhaps a few hundred Hz, or even just on/off switching -- then low currents like 20-40 mA may be perfectly fine, and that can be delivered by many microcontrollers.
After all, the microcontrollers are built entirely out of MOSFETs themselves, and seem to switch their own internal MOSFETs just fine :-) Yeah, those are smaller-current integrated devices, with smaller gate charges, but I point this out to show that it's a numbers game, not a black-or-white.
Calculate time in transition zone times power lost while in transition zone, multiply by switching rate, subtract cooling, and you end up with "it works" or "it doesn't."

Quote
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.
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.

Yes, that's the whole benefit of MOSFETs! But a driver that uses a bootstrap capacitor will eventually see the gate voltage drive if you keep holding it high, and eventually the switched MOSFET will dip into the transition zone and then quickly overheat. This is why drivers have a rated duty cycle -- the time needed to re-charge the bootstrap capacitor. 100% duty cycle drivers can re-charge the capacitor without driving the gate low, which is clearly preferrable, but I only found those in surface mount packaging back when I was looking.

[quoite]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.
[/quote]

I looked at those, and they are not bootstrapping (high-side) drivers, only low-side drivers. Which is fine for a unipolar driver, but not for bipolars that need H-bridges. I should probably have been more clear that I was looking specifically for high-side drivers with 100% duty cycle.


Offline Soeren

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Re: Motor driver circuit - suggestions for improvements
« Reply #7 on: December 06, 2012, 11:47:48 PM »
Hi,

That's counter to all the design advice I've read until now. Schottkys are generally recommended as fast barriers, in data sheets for anything from switching power supplies, to gate drivers, charge pumps, etc.

Yes, well it's a relative world... Compared to regular PN diodes they're fast (with a few exceptions), but compared to fast/super fast/ultra fast diodes, they're slow.


Looking at the data sheets:
1N4148 has a switching speed of 4 nanoseconds, and a max current of 450 mA. It has an 8 pF capacitance.
Meanwhile, a Schottky like CSD01060 is rated at "essentially no switching losses" and 1.7A. It has an 11 pF capacitance, which is slightly higher. Maybe that's the "slowness"? The capacitance per rated ampere is lower, though.

When you read datasheets, you need to keep your eye on what you need the component for - in this case an inductive recoil - and hold that up against the (right) parameters the datasheet reveals. In this case, look at the pulse handling.
The 1N4148 has a max rating of around 1A+ for short pulses and up to 2A for a 1µs (non repetitive pulse.

The CSD01050, one of the "new breed" silicium carbide Schottkys, id not what I consider a normal Schottky, in the sense where amateurs can lay their paws on them, visiting a high street parts pusher, so in this context, you shouldrefer to regular (Old Skool) Schottkys, which is way slower than the SiC's.
If you have access to SiC's, I don't understand your problem of finding the appropriate current handling clamping diodes.
I'm not sure where you got the 1.7A rating, as the datasheet from Cree gives different numbers depending on temperature, but no 1.7A?


Also: I take "essentially no switching losses" to mean "zero recovery" but maybe that's not what it means.

Terms like "essentially" and "virtual" means (in this context) mens "little" compared to the older generations of ("Heavy Metal") Schottkys.
Saying a diode have zero recovery time (they talk about ZR current), would mean you could pour daylight through it ;)


If you could qualify your suggestion to look away from Schottkys with a more detailed reference, that would be great!

I'm not saying that you shouldn't use Schottkys! I was just referring you to fast (and faster) recovery diodes, which is a much older device than SiC Schottkys, as you wanted high voltage/current/whatever, but perhaps I should rather have asked you how you arrive at a need for >10A diodes for clamping a motor driver, as there's a widespread misconception around diode selection among amateurs (and  way too many pros I hate to say), which is usually realized in people installing 10A diodes if the motor is rated at 10A (whether unloaded or stalled).

A clamping diode needs to be able to safely handle the flyback pulse, which is a short duration, it doesn't need to handle the peak as if it was DC.

If you want the lowdown on the fast and faster diodes, I'll suggest reading a few of the papers about them (not datasheets and app notes, but what some of them call "linear briefs" - they're usually a lot of info in a light package.


-- I meant high current. Schottkys rated for > 10 A seem to be hard to find and expensive to purchase when you do.

The beefiest Schottky in my drawers are 60A (drawn from some long forgotten piece of electronics), but Fairchild (among others) have several 40A devices - one example here which is under  a buck at 1k units.
So, easy to find and fairly cheap I think ;D


That depends on driving current, doesn't it?

Yes, you referred to a "250mA/12V unipolar stepper motor".


Most unipolars tend to be small, because if you want real power, you will want bipolar. I think we agree on that?

I don't think that's why they're small :P
If I want "real power" I don't use steppers at all.
(It would probably be good to write eg. "bipolar motor", as diodes are unipolar (Schottkys) and bipolar (PNs) as well


You need to build up the gate charge quickly enough that you don't develop heat. If you can switch "slowly" (we're still talking less than a few microseconds) -- either PWM at perhaps a few hundred Hz, or even just on/off switching -- then low currents like 20-40 mA may be perfectly fine, and that can be delivered by many microcontrollers.

For on/off with a period of say a second or more between shifts it's OK.
For driving a stepper at anything beyond super slow, it's poor design, plain and simple.


After all, the microcontrollers are built entirely out of MOSFETs themselves, and seem to switch their own internal MOSFETs just fine :-) Yeah, those are smaller-current integrated devices, with smaller gate charges, but I point this out to show that it's a numbers game, not a black-or-white.

The internals of I/O is completely unrelated here and in a sense, it is black and white, as it boils down to knowing the components you work with and the environment they'll live their life - or not... Leave bad design to the Chinese, they're the experts in that discipline.


Calculate time in transition zone times power lost while in transition zone, multiply by switching rate, subtract cooling, and you end up with "it works" or "it doesn't."

No, with any design, you end up with something you'll stake your rep on, or add to the already too large pile of crap design that isn't totally in the "doesn't work" category, but is still a far cry from anything I'd put my name on.


I should probably have been more clear that I was looking specifically for high-side drivers with 100% duty cycle.

What? You're using N-ch. devices for high side?  ::)
That would explain it.

Oh well time for a short nap.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jwatte

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Re: Motor driver circuit - suggestions for improvements
« Reply #8 on: December 07, 2012, 01:23:51 AM »
If you have access to SiC's, I don't understand your problem of finding the appropriate current handling clamping diodes.
I'm not sure where you got the 1.7A rating, as the datasheet from Cree gives different numbers depending on temperature, but no 1.7A?

"access" in the sense that they're within my hobby budget in quantity "a handful." The 1.7A came from me searching Digi-Key, then finding a data sheet, and copying the part number for the cheapest I found.

Quote
perhaps I should rather have asked you how you arrive at a need for >10A diodes for clamping a motor driver, as there's a widespread misconception around diode selection among amateurs (and  way too many pros I hate to say), which is usually realized in people installing 10A diodes if the motor is rated at 10A (whether unloaded or stalled).

Right! And that explains where I'm coming from: I've used these guys for switching power supplies and high-side N-channel drivers. Motors are pretty new to me, and the advise I've heard is "put a fast diode across it if you use bipolar transistors; MOSFET body diodes are good enough otherwise."

Quote
That depends on driving current, doesn't it?
Yes, you referred to a "250mA/12V unipolar stepper motor".

But here we were talking about the current driving the gate of the MOSFET that in turn drives the motor.


Quote
Most unipolars tend to be small, because if you want real power, you will want bipolar. I think we agree on that?
I don't think that's why they're small :P
If I want "real power" I don't use steppers at all.
(It would probably be good to write eg. "bipolar motor", as diodes are unipolar (Schottkys) and bipolar (PNs) as well

Clearly, assuming context leads to misunderstandings. Transistors, too, can be bipolar :-)

Yes -- if you need a stepper motor, and want something more powerful, I would suggest a bipolar stepper motor over a unipolar stepper motor, because unipolar stepper motors are pretty slow even when driven as hard as you can. Chopping drivers and high voltages for bipolar steppers is what the CNC and servo guys seem to use these days when specifying steppers.

This is the background to the question to the original poster: I think he's driving a unipolar stepper, and I'm wondering whether it performs well enough.

Quote
Quote
BS170 as an example of a small use case

For driving a stepper at anything beyond super slow, it's poor design, plain and simple.

It worked fine for my egg plotting 'bot, driving small 12V unipolars. Engineering, in the end, is the art of solving the actual problem as effectively and cheaply as possible. The problem statement does not always include "must last 100 years and stand up to abuse outside normal operating parameters." (Even though the world would be a less frustrating, and much more expensive, place if it did!)


Quote
I should probably have been more clear that I was looking specifically for high-side drivers with 100% duty cycle.
What? You're using N-ch. devices for high side?  ::)
That would explain it.

Yes, I do, if I drive something like a bipolar (so I need an H-bridge or similar.)
I've found that P-channel devices have a similar problem, because you have to somehow switch their gate up to the source voltage to turn them off. Pulling down to turn on is easy with a small signal N-channel switcher, but pulling up to 24V or whatever when your control signal is 5V is harder. For example: A pull-up resistor that is small enough to switch the P-channel fast, means that you waste a lot of current through the signal N-channel when pulling the P-channel gate down.
Hence, if you're going with drivers anyway, you might as well go with high-side N-channel drivers, which lets you use the faster, cheaper, lower on-resistance N-channel power MOSFETS.

But, I've only found high-side drivers that require duty cycles less than 100%. When stepping motors for locomotion, as the OP does, this is probably fine, until you want to stand still...
So, to be able to recommend bipolar stepper motors as an alternative to the current unipolar design, the question of how to actually drive those bipolars probably should be solved. Integrated motor controllers can do it, of course, but that's significantly more complex than just a few MOSFETs that he's using now.

Quote
Oh well time for a short nap.

Naps are the best!

Offline MrWizard

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Re: Motor driver circuit - suggestions for improvements
« Reply #9 on: December 08, 2012, 11:58:12 AM »
Now it's your turn Soeren  ;D

I am learning lots from this....   ;D

Offline Soeren

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Re: Motor driver circuit - suggestions for improvements
« Reply #10 on: December 09, 2012, 06:55:22 PM »
Hi,

But here we were talking about the current driving the gate of the MOSFET that in turn drives the motor.

No, I was talking about the voltage drop over a BS170, which, at 250mA is larger than it would be with eg. a BC337.


Engineering, in the end, is the art of solving the actual problem as effectively and cheaply as possible.

Engineering has nothing to do with art, it applied science.
In art, the performer calls the shots, in engineering, the laws of physic does.

Engineering is not about solving a problem as cheaply as possible, that's a demand pulled down over our heads from the money men and when designing something for yourself, there's no need to save a few percent (which compounds to a huge amount when in mass production).


The problem statement does not always include "must last 100 years and stand up to abuse outside normal operating parameters." (Even though the world would be a less frustrating, and much more expensive, place if it did!)

Leave out the clamp and it certainly won't last for 100 years and we're not talkingabout abuse outside the normal operation. I know you ain't an engineer, but it may be a good idea to study the terms MTBF and MTTF, which is a bit more precise than "last nn years" :)


Yes, I do, if I drive something like a bipolar (so I need an H-bridge or similar.)

Err?
Weren't we talking MOSFETs?


I've found that P-channel devices have a similar problem, because you have to somehow switch their gate up to the source voltage to turn them off. Pulling down to turn on is easy with a small signal N-channel switcher, but pulling up to 24V or whatever when your control signal is 5V is harder. For example: A pull-up resistor that is small enough to switch the P-channel fast, means that you waste a lot of current through the signal N-channel when pulling the P-channel gate down.

When running a 24V motor om a 'bot, we're usually talking about 250W or more, so the loss, which is in the resistor (the MOSFETs RDS_ON means the it doesn't change whether you use 12V, 24V or 100V (if it can sustain that voltage), is a very minor thing and if you find this small loss problematic, just go active.


Hence, if you're going with drivers anyway, you might as well go with high-side N-channel drivers, which lets you use the faster, cheaper, lower on-resistance N-channel power MOSFETS.

A major problem with high side N-ch. devices is that you have to generate a voltage above V++.
Funny you mention faster, after advocating switching from an I/O port ;)
What numerical parameters do you need, since you think that a P-ch. device cannot handle it??

In case you're not just trying to substantiate your beliefs with guesses, here's a p-ch. device that should cover a lot. They come with better specs, this is just one I know and I think it'll cover most of your high power needs.


So, to be able to recommend bipolar stepper motors as an alternative to the current unipolar design, the question of how to actually drive those bipolars probably should be solved.

It is... With P-ch. high side ;D


Naps are the best!

I'd rather not have to sleep actually - it's time going from doing stuff and it hurts my back and neck, but when the books on the shelves seems to be waving at you, it's a sure sign that your brain are on leave ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jwatte

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Re: Motor driver circuit - suggestions for improvements
« Reply #11 on: December 09, 2012, 07:38:15 PM »
Quote
When running a 24V motor om a 'bot, we're usually talking about 250W or more, so the loss, which is in the resistor (the MOSFETs RDS_ON means the it doesn't change whether you use 12V, 24V or 100V (if it can sustain that voltage), is a very minor thing and if you find this small loss problematic, just go active.

With a 70 Ohm resistor, and 12V drive voltage (more common for robots) you'll see over 2 watts of loss through the pull-up resistor while pulling it down. For a 12V/5A motor, that's about 3%. Not to mention the resistor will need some kind of thermal management, and it's a pretty big and expensive resistor.

It certainly works, though -- I've built a H-bridge with P-channel top switchers that worked great for a 8v/10A pair of motors. With 220 Ohm pull-up, the PWM eventually made the P-channel devices overheat, but with 100 Ohm pull-up, it worked fine for the hour-long tests I ran. This H-bridge came to a death when I put it down on an exposed aluminum profile that shorted it out. From that, in turn, I learned to always conformal coat the back of all my circuits :-)


Quote
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Quote from: jwatte on December 07, 2012, 01:23:51 AM
Hence, if you're going with drivers anyway, you might as well go with high-side N-channel drivers, which lets you use the faster, cheaper, lower on-resistance N-channel power MOSFETS.

A major problem with high side N-ch. devices is that you have to generate a voltage above V++.

That was the whole point. I'm looking for high-side N-channel drivers that can support a 100% duty cycle. Maybe they don't exist outside specialized use cases like power management, which is the only place I've seen it mentioned. I imagine you could build one by letting the already-charged gate float for a microsecond or two while you re-charge the boost capacitor, and then keep it on for another long while; repeat. Or you could just use two boost capacitors and flip/flop between them. Seems like a simple enough circuit to build, I'm just surprised I can't find one already made and integrated.
For example, the LTC1163 only drives loads up to 5V (with a 9V gate boost.) IR2125 only works with PWM / non-100% duty cycle. TPS2811, billed as a "high side" driver, actually needs the boost to come externally. It sounds like you also haven't found a 100% duty-cycle high-side N channel driver, so I'll keep looking.

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I'd rather not have to sleep actually - it's time going from doing stuff and it hurts my back and neck, but when the books on the shelves seems to be waving at you, it's a sure sign that your brain are on leave

There's lots of good science suggesting that sleep is crucial to mood, intelligence, creativity, and learning. I'd rather not deplete whatever I've got of those qualities ;-)

Offline Soeren

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Re: Motor driver circuit - suggestions for improvements
« Reply #12 on: December 10, 2012, 07:48:00 AM »
Hi,

With a 70 Ohm resistor, and 12V drive voltage (more common for robots) you'll see over 2 watts of loss through the pull-up resistor while pulling it down. For a 12V/5A motor, that's about 3%. Not to mention the resistor will need some kind of thermal management, and it's a pretty big and expensive resistor.
Let's not argue over which is the most used voltage, as this depends on ones perspective :)

3.3% loss is not a big deal!
Thermal management? Just get a 5W resistor and be done with it - At $0.42 in one-off, I don't see how you can call that expensive?
And the size of 22x10x9mm - while larger than a 0.25W resistor is pretty manageable (they used to be twice that long for the same power)


That was the whole point. I'm looking for high-side N-channel drivers that can support a 100% duty cycle.
My point is that you don't need them with P-ch. devices.
I just banged together a crude 24V P-ch. driver (1.2 Ohm load), with 4 transistors and a handful of resistors.
Switching (in driven PMOS/load): Rise time 81.2ns, fall time 211ns. (And I only used one hand :P)
It's capable of holding a step (for hundred years ;)) of course.


Maybe they don't exist outside specialized use cases like power management, [...]
It sounds like you also haven't found a 100% duty-cycle high-side N channel driver, so I'll keep looking.
I haven't been searching for them, as I don't need them, but if you really want them, just get them from where you found them (power management)


There's lots of good science suggesting that sleep is crucial to mood, intelligence, creativity, and learning. I'd rather not deplete whatever I've got of those qualities ;-)
Yeah, I know, but still it's annoying that you have to break off a good session, just because you need a couple of hours in horizontal mode ;)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jwatte

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Re: Motor driver circuit - suggestions for improvements
« Reply #13 on: December 12, 2012, 07:03:47 PM »
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just get them from where you found them (power management)

Ah, but I only ever see them as parts of rather expensive full-solution switching controller ICs. I'd love something as simple as an IR2125 or MIC5021, but for 100% duty cycle.

 


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