Beginners: please read this post and this post before posting to the forum.
0 Members and 1 Guest are viewing this topic.
Beacause you use MOSFETs, snubbing diodes are not needed. The body diodes of the MOSFETs are sufficient.
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 [...]
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.
Quote from: jwatte on December 04, 2012, 12:48:06 PMBeacause 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!
Quote from: jwatte on December 04, 2012, 12:48:06 PMRunning 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!
Quote from: jwatte on December 04, 2012, 12:48:06 PMI 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?
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...
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.
Quote from: jwatte on December 04, 2012, 09:22:07 PMIf 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.
the 1N4148 is blindingly fast compared to Schottkys.
But what do you consider high voltage in that respect?
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...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.
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 PMSpeaking 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.
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!
-- I meant high current. Schottkys rated for > 10 A seem to be hard to find and expensive to purchase when you do.
That depends on driving current, doesn't it?
Most unipolars tend to be small, because if you want real power, you will want bipolar. I think we agree on that?
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."
I should probably have been more clear that I was looking specifically for high-side drivers with 100% duty cycle.
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?
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).
Quote from: jwatte on December 05, 2012, 02:41:31 PMThat depends on driving current, doesn't it?Yes, you referred to a "250mA/12V unipolar stepper motor".
Quote from: jwatte on December 05, 2012, 02:41:31 PMMost 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 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
QuoteBS170 as an example of a small use caseFor driving a stepper at anything beyond super slow, it's poor design, plain and simple.
BS170 as an example of a small use case
Quote from: jwatte on December 05, 2012, 02:41:31 PMI 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.
But here we were talking about the current driving the gate of the MOSFET that in turn drives the motor.
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!)
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.
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.
Naps are the best!
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.
QuoteQuote from: jwatte on December 07, 2012, 01:23:51 AMHence, 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++.
Quote from: jwatte on December 07, 2012, 01:23:51 AMHence, 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.
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
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.
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, [...]It sounds like you also haven't found a 100% duty-cycle high-side N channel driver, so I'll keep looking.
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 ;-)
just get them from where you found them (power management)