Simple BLDC controller

[Jeremy Harris]

Jeremy,

I find it interesting that you actually can build a controller that may work better than a Chinese one. I admire your work, to bad I can't learn everything I want in detail. Do you think this controller can drive different mosfets? Since you want to use it at some very big mosfets? If so, this may be one really good controller for all the guys trying to get the rc motors going.

Ratking

The design as it stands will drive pretty much any FET you want. The key thing with driving FETs is to make sure that the gate drive current is adequate, and free from any ringing or noise. The drivers I'm now using are capable of driving 2A maximum into the FEt gates, far more than would be needed even for a multiple FET controller. By comparison, the XieChang/Infineon controllers typically drive the FET gates with only around 0.13A.

This means that this basic controller can be adapted to drive a fairly wide range of FETs, perhaps with just a change of gate drive resistor to ensure stability and set the gate drive current to the right sort of value for the total gate charge of the FETs being used. For example, the gate resistor on the XieChang controllers is 91 ohms (giving 130mA peak gate current from 12V) whereas that on my controller is 10 ohms, giving a peak gate current of 1bout 1.2A, probably a bit high for the FETs I'm using.

It doesn't make sense to use FETs in small packages to drive big RC motors, as they need a lot of current and that demands a low FET Rdson. It also demands good thermal performance, something that the small FET packages, like the TO220 size ones found in most controllers, can't provide.

Undoubtedly some of the big packages, like the SOT227 and the even bigger ones like the Ixys modules, are excellent at getting rid of heat from the FET junction, which mean they can handle a lot more current than their smaller brothers. The down side to big FETs is the need to provide more gate drive current, as gate total charge increases with the size of the FET. 2A should be enough to drive even big IGBTs, if anyone wanted to venture in the >100V realm for extra power.

Jeremy

 

[Ratking]

Jeremy,

I find it interesting that you actually can build a controller that may work better than a Chinese one. I admire your work, to bad I can't learn everything I want in detail. Do you think this controller can drive different mosfets? Since you want to use it at some very big mosfets? If so, this may be one really good controller for all the guys trying to get the rc motors going.

Ratking

The design as it stands will drive pretty much any FET you want. The key thing with driving FETs is to make sure that the gate drive current is adequate, and free from any ringing or noise. The drivers I'm now using are capable of driving 2A maximum into the FEt gates, far more than would be needed even for a multiple FET controller. By comparison, the XieChang/Infineon controllers typically drive the FET gates with only around 0.13A.

This means that this basic controller can be adapted to drive a fairly wide range of FETs, perhaps with just a change of gate drive resistor to ensure stability and set the gate drive current to the right sort of value for the total gate charge of the FETs being used. For example, the gate resistor on the XieChang controllers is 91 ohms (giving 130mA peak gate current from 12V) whereas that on my controller is 10 ohms, giving a peak gate current of 1bout 1.2A, probably a bit high for the FETs I'm using.

It doesn't make sense to use FETs in small packages to drive big RC motors, as they need a lot of current and that demands a low FET Rdson. It also demands good thermal performance, something that the small FET packages, like the TO220 size ones found in most controllers, can't provide.

Undoubtedly some of the big packages, like the SOT227 and the even bigger ones like the Ixys modules, are excellent at getting rid of heat from the FET junction, which mean they can handle a lot more current than their smaller brothers. The down side to big FETs is the need to provide more gate drive current, as gate total charge increases with the size of the FET. 2A should be enough to drive even big IGBTs, if anyone wanted to venture in the >100V realm for extra power.

Jeremy

Thank you for a long explanation

Is there any drawback using a igtb instead of a mosfet? Ebay is flooding over with 300-400A itgb's and they are cheap also. I need three of them and I feel that they can handle alot more abuse than a tiny little to 247 fet rated for 200A. I don't know why, but in this case I feel bigger is better.

Ratking

 

[liveforphysics]

If you're running over ~300v-400v, and using motors with lots of inductance and winding resistance so you can have slow PWM rates, then yes the big IGBT's would make sense.

 

[grindz145]

That's awesome. Seems like quite a mechanical challenge to make happen. I keep saying that I think we'll look back at this time as the dark ages when we weren't using transmissions on electric drivetrain.

What is the PN for the IXYS modules you're talking about? I would like to lay something out with a couple of different FET footprints for flexibility. Thinking about making it a batchPCB kind of thing... What do you think? That way anyone could order them really cheaply if they wanted it.

The Ixys FETs that LFP, Arlo, myself and a few others have are obsolete parts, I think. Ixys seem to have only ever put these into limited production - maybe demand wasn't great enough. The part number is VMM 650-01F. They are half bridge modules rated at 680A, 100V, with an Rdson of 1.8mohm. They have big bolt on tabs for the heavy current connections, so will bolt directly to big bus bars. I've attached a copy of the data sheet. They are as close as we are likely to get to FETs that we can't physically blow with the amount of energy we have available in an ebike battery; it's likely that even a fuse would be able to protect these FETs from damage.................

Jeremy

Thanks. I still would like to throw a couple different package througholes on there. It would be great for testing. Ill have to take a stroll over to Luke's FET resource page and have a look.

 

[grindz145]

Jeremy,

I find it interesting that you actually can build a controller that may work better than a Chinese one. I admire your work, to bad I can't learn everything I want in detail. Do you think this controller can drive different mosfets? Since you want to use it at some very big mosfets? If so, this may be one really good controller for all the guys trying to get the rc motors going.

Ratking

The design as it stands will drive pretty much any FET you want. The key thing with driving FETs is to make sure that the gate drive current is adequate, and free from any ringing or noise. The drivers I'm now using are capable of driving 2A maximum into the FEt gates, far more than would be needed even for a multiple FET controller. By comparison, the XieChang/Infineon controllers typically drive the FET gates with only around 0.13A.

This means that this basic controller can be adapted to drive a fairly wide range of FETs, perhaps with just a change of gate drive resistor to ensure stability and set the gate drive current to the right sort of value for the total gate charge of the FETs being used. For example, the gate resistor on the XieChang controllers is 91 ohms (giving 130mA peak gate current from 12V) whereas that on my controller is 10 ohms, giving a peak gate current of 1bout 1.2A, probably a bit high for the FETs I'm using.

It doesn't make sense to use FETs in small packages to drive big RC motors, as they need a lot of current and that demands a low FET Rdson. It also demands good thermal performance, something that the small FET packages, like the TO220 size ones found in most controllers, can't provide.

Undoubtedly some of the big packages, like the SOT227 and the even bigger ones like the Ixys modules, are excellent at getting rid of heat from the FET junction, which mean they can handle a lot more current than their smaller brothers. The down side to big FETs is the need to provide more gate drive current, as gate total charge increases with the size of the FET. 2A should be enough to drive even big IGBTs, if anyone wanted to venture in the >100V realm for extra power.

Jeremy

Thank you for a long explanation

Is there any drawback using a igtb instead of a mosfet? Ebay is flooding over with 300-400A itgb's and they are cheap also. I need three of them and I feel that they can handle alot more abuse than a tiny little to 247 fet rated for 200A. I don't know why, but in this case I feel bigger is better.

Ratking

Biggest thing is that IGBTs switch VERY slow. This controller probably wouldn't be the optimal place to test them, since there isn't any way to control PWM frequency etc.

 

[texaspyro]

They are as close as we are likely to get to FETs that we can't physically blow with the amount of energy we have available in an ebike battery

Awww, you're not trying hard enough...

 

[Jeremy Harris]

Biggest thing is that IGBTs switch VERY slow. This controller probably wouldn't be the optimal place to test them, since there isn't any way to control PWM frequency etc.

Not only do IGBTs switch relatively slowly, but they also have a high and fairly fixed voltage drop when conducting, so have higher losses than FETs at voltages where you can still get low Rdson FETs. As LFP says, IGBTs are better at much higher voltages than we typically work at, I reckon the break point, in terms of efficiency, is probably around 150V or so. Above this IGBTs are probably a better bet, below this FETs are probably better.

The PWM frequency of this controller can be changed easily, BTW. It will run happily from a kHz or so up to well over 100kHz. I have it set for around 22kHz, but a simple component change (literally just changing the value of a capacitor or resistor on the board) will set the frequency to anything you want. If you wanted to build a lower frequency IGBT driver with it then it would do that job OK, I'm sure.

Jeremy

 

[grindz145]

Biggest thing is that IGBTs switch VERY slow. This controller probably wouldn't be the optimal place to test them, since there isn't any way to control PWM frequency etc.

The PWM frequency of this controller can be changed easily, BTW.

Jeremy

Ahh I see now. Add a 400 - 12V switcher to power and it actually is quite feasible.

 

[ev_nred]

this is an epic controller! keep up the good work!

 

[Jeremy Harris]

Thanks, folks.

I've spent the day building and debugging the second controller, where I've added the 'kick start' comparator to the board, done away with the bodged daughter board and had a few frustrating moments getting this to work.

Apart from a couple of idiotic issues caused by my errors (including a blown driver chip caused by a solder splash I didn't see on the back of the board) things have gone well. It has been a learning experience though, particularly with regard to the way the high side driver bootstrap circuit responds dynamically. With the Mk1, I calculated the value of bootstrap capacitor needed as being just a couple of uF. This seemed far too low, as the Chinese controllers typically use around 47uF, so I fitted 47uF capacitors on the basis that 'more is good'. This happened to work for the first controller, once I'd fitted the 'kick start' comparator, but exactly the same circuit refused to start reliably on the second controller. Flicking the motor with a bit of throttle applied would start it up fine, but it wouldn't always reliable start from a standstill.

I thought I'd solved this puzzle with the first controller, so was more than mildly annoyed that it had apparently returned on the Mk2. It turns out that more isn't better as far as the bootstrap capacitors are concerned, Changing them for 10uf ones has fixed the problem completely. The give away was when I looked at the bootstrap capacitor voltage when the motor tried to start - it wasn't quite getting high enough from the brief pulse as the controller outputs were enabled to reliably turn on the high side FETs. The Mk1 probably worked because these capacitors have a big tolerance, +10%, -20%, so by luck I'm guessing that the first board had lower values capacitors than the second board.

This is probably something that might be useful to anyone else building a controller, so is worth noting. I think that a better solution for high side drive might be to use an isolated DC-DC converter to generate the required 12V or so above the +ve rail. That way the controller wouldn't need the 'kickstart' circuit and should reliably start from any condition.

The only other thing I need to look at more closely before releasing this design out into the wild is the high side FET turn off time. It's not as fast as I'd like, as it looks like the FET gate capacitance is slowing down the tail end of switch off. I may try adding a fast diode across the gate resistors to turn the FETs off more quickly, but need to do a bit more study of the driver datasheet first.

Jeremy

 

[bigmoose]

Jeremy I have been using Recom Power Inc RO-0515S Isolated DC DC converters for this application for a while, less than $5/ea. http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=ro-0515s
Datasheet http://www.recom-international.com/pdf/Econoline/RO.pdf

 

[Jeremy Harris]

Thanks Bigmoose. I've been looking at a very similar one that I can get over here easily (Digikey are mighty expensive here), but with a 12V input: http://uk.farnell.com/xp-power/ie1212s/converter-dc-dc-1w-12v/dp/8727775

I think I'm going to bite the bullet and order some, along with some more components for a third, and hopefully final, board. One things for sure, developing this simple controller is costing a few bob in parts! So far I think I've probably used up the price of a couple of cheap Chinese controllers just in capacitors that I can't easily retrieve from the prototype boards..............

The big advantage of the DC DC converter option is that it will remove the need for the 'kick start' circuit, and make the board layout a little simpler.

Onward, ever onward...........

Jeremy

 

[bigmoose]

The XP Power DC/DC should do nicely. I agree about DigiKey pricing. It has gone through the roof in the last year. I am currently putting my year end pieces parts order together and going to try Future for it.

 

[rhitee05]

Jeremy, does the PWM generator allow you to set a duty cycle ceiling, or does it go all the way to 100%? I'm wondering because I think limiting the duty cycle to 99% or 98% might help with the bootstrap circuit. That would ensure the bootstrap gets a chance to recharge each cycle. It wasn't clear it this is the whole problem or not, but 100% duty cycle with the motor stopped is where a bootstrap would run into trouble.

I'm following your project with interest, BTW. Nice work.

 

[liveforphysics]

The XP Power DC/DC should do nicely. I agree about DigiKey pricing. It has gone through the roof in the last year. I am currently putting my year end pieces parts order together and going to try Future for it.

Just had another order from Future arrive. Same parts worked out to be less than half digi-rips price, and future actually had them in-stock. Arrived 3days from the time I ordered, all well boxed.


I hate to say "told you so" on the bootstraping my friend, so I will just keep it to myself. Its a good method if you're trying to penny-pinch on a 1000 unit batch or something, but otherwise the $15 to do it right is by far the cheapest solution when you factor in the tuning/development time to make boot strapping un-buggy, and then you're still got the risk of blowing a $$ fet stage over a silly bootstrap failure to start-up, or if the cap gets depleated on a long continous WOT run.

DC-DC ftw.

 

[oldpiper]

I know what you mean about DigiKey, but sometimes when you search for parts you get a $700 toilet seat, and then if you do a bit broader search, you'll find what looks like the same thing for a reasonable price. (I know this happened when I was looking at current-limiting resistors just a week ago or so - search came up with one for $23, IIRC, and sifting down, there were listings of seemingly identically-spec'ed parts for $0.50 or so. It makes me suspect, although I haven't taken the time to check it out, that the high-dollar part that comes up first might be a mil-spec item (individually-tested, gold/iridium plated, etc.) and the others are for plain folks like you and me.

Cameron

 

[Arlo1]

Jeremy do you have a list of parts to build this? I would like to get an order out soon and I would like to incl all the parts to build a couple more of these.

 

[Jeremy Harris]

Jeremy, does the PWM generator allow you to set a duty cycle ceiling, or does it go all the way to 100%? I'm wondering because I think limiting the duty cycle to 99% or 98% might help with the bootstrap circuit. That would ensure the bootstrap gets a chance to recharge each cycle. It wasn't clear it this is the whole problem or not, but 100% duty cycle with the motor stopped is where a bootstrap would run into trouble.

I'm following your project with interest, BTW. Nice work.

Thanks, it does go to 100% PWM at full throttle, but it wouldn't start even with just a small amount of throttle applied. The problem may partly be down to the simple nature of this design; it's a simple logic array that just decodes the Hall states to the commutation output states. With the motor stationary, the controller tries to drive the FETs to the state determined by the Hall outputs. The selected high side FET can't turn on because the bootstrap capacitor isn't charged. I was getting around this by turning off all the outputs with the throttle closed, then enabling them once the throttle voltage exceeded a set threshold, just before motor start-up. This had the effect of turning on some outputs, so pulsing enough charge into the bootstrap capacitors to allow the thing to start.

Normally, it won't go to 100% at low rpm, because this implies a high torque condition that will invoke the cycle-by-cycle current limit, cutting back the duty cycle.

Jeremy

 

[Jeremy Harris]

Jeremy do you have a list of parts to build this? I would like to get an order out soon and I would like to incl all the parts to build a couple more of these.

Best hang on until I get the parts finalised, Arlo, I'd hate to give out a list of parts that was wrong. I've just ordered some more parts for the third (and hopefully last) prototype, and although I'm reasonably confident that it'll work it would be better if I can test it first.

Rest assured that as soon as I have the final circuit built and tested, I'll publish it here for anyone that wants to have a go at building one.

Jeremy

 

[rhitee05]

It sounds like your "hack" fix is actually doing more or less what you need to do for startup. To charge the bootstrap requires you to hold the phase low (low-side on, high-side off), the cap will then charge from the Vcc rail. It shouldn't take very long for this to happen, just a brief delay would be enough. Besides the circuit you added, another way would be to use an R-C circuit with an appropriate time constant. You might even be able to get away with something as simple as adding a capacitor to put this R-C right on the output enable pin, in addition to the pull-up you have now.

 

[liveforphysics]

It sounds like your "hack" fix is actually doing more or less what you need to do for startup. To charge the bootstrap requires you to hold the phase low (low-side on, high-side off), the cap will then charge from the Vcc rail. It shouldn't take very long for this to happen, just a brief delay would be enough. Besides the circuit you added, another way would be to use an R-C circuit with an appropriate time constant. You might even be able to get away with something as simple as adding a capacitor to put this R-C right on the output enable pin, in addition to the pull-up you have now.

Honestly. Does the $15 for 3x DC/DC converters really seem out of line to solve all these types of problems and enable the ability to run 100% PWM?

 

[texaspyro]

Well, 15 bucks is approaching the total component cost of the controller. Another fix might be to use a small 8 pin micro (ATTINY13?) to do the startup trick... sort of like an intelligent comparator.

But then, I utterly hate bootstraps and high-side driver chips because of the very issues that Jeremy is dealing with.

Or for considerably less than 15 bucks you could cobble together your own three output DC-DC

 

[Jeremy Harris]

High Side Drive

Eric is right, the trick with turning all the FETs off when the throttle is closed does work fine. I did try an RC on the enable pin, but although that works fine for initial power-on, if the thing is left powered up with the throttle closed it will get itself into a state where all the low side FETs are off and the bootstrap caps discharge, so it then won't start up again. The comparator sensing the throttle voltage works reliably enough, as there is around 1.4V of 'dead movement' at the lower end of the throttle range. I have the throttle set so its output sits at around 1V when closed, the comparator trips at about 1.2V and the motor starts to run at about 1.4V, with full throttle being around 5V (the throttle reference supply is 6.2V). This seems to work well, as closing the throttle disables the outputs and the first movement of the throttle enables them, ensuring that at least one low side FET turns on and allows its bootstrap to charge.

I've ordered some small DC DC converters and am going to use them on the Mk3, simply because it removes the need to do the throttle sensing thing. My guess is that the Chinese controllers are running a bit of code to pulse the outputs, or maybe just hold all the low side FETs on, when there's no throttle input. This might be a useful thing to note for those building uP based controllers with bootstrap high side drive.

The good news is that the NCP5181 drivers seem quite easy to use, with nice clean gate drive signals. There's no ringing I can see (with 10 ohm gate resistors) but the turn offs do have a small tail right at the bottom of the falling edge. I was going to try and do something about this, but having looked more closely the tail starts after the voltage has dropped below the gate threshold, so the FET is already off.

Texaspyro, I agree, high side drive IS a pain! Like LFP, I've decided that bootstrapping is a bit of a bodge - it works fine, but I just don't like it much.

Layout

I'm just doing the layout for the Mk3 and have decided to make it modular, for a couple of reasons. The first is that I want to build two versions of this controller for myself, a compact one that drives a set of six IRFP4368s, with a 70V 120A capability and a second one that drives three Ixys HiperFET modules, with a 100V, lots of amps, capability. The other reason is that the board size is big, I'm sure I can make the controller more compact by dividing it into two boards, stacked on top of each other. One will hold the drivers and FETs, the other the controller. All this needs is a ten way link between the two, which could be a couple of molex headers and connectors or just ten wire links. I'm going to aim to have the controller board facing down over the power board so that I can try and arrange the tall components to interlink between the two boards and reduce the overall height. This will make a big difference to the size of the Ixys HiperFET version, as that was getting pretty broad with the board hanging out the side of the big modules.

Commutation Capacitors

Having spent hours looking for small but capable capacitors, I've come up with what I think will do the job for a reasonable price, and without taking up too much space. It's hard to beat those small Rubycon ZL series caps for size and ESR, so I'm using those on both versions. To deal with the fast edges, I'm using a few 1uF metallised film caps that will take a couple of amps of ripple current and seem to have an acceptably low impedance at high'ish frequencies. The end result will be an array of capacitors with a total ESR of around 5 mohms over a frequency range that I think should be OK, with a total ripple current rating of around 20 amps or so. I'm hoping that this should be OK; it's massively better than the capacitors in the XieChang controller, so I think it should be.

The IRFP4368 version will have the capacitors on the power board, like the existing layout. The Ixys Hiperfet version will have a separate strip PCB, with two busses on it, that will bolt to the main copper buss bars. This strip PCB will hold all the capacitors in a row and, because it will be bolted copper side down to the copper bars, should make for as good a scheme as any.

If it turns out that I get too much ripple on the big version, then I'll just have to put up with making the thing bigger and bolting some bigger capacitors to the bus bars, which will be an easy mod. If it works with a big array of small capacitors then it does make it cheaper and more compact, though.

Jeremy

 

[ZapPat]

Honestly. Does the $15 for 3x DC/DC converters really seem out of line to solve all these types of problems and enable the ability to run 100% PWM?

You are aware that the bootstrap cap recharge issue would be totally eliminated by actively switching the low side FET along with the high side FET instead of using it as a passive diode (synchronous switching of a buck converter)? This also makes a *huge* difference in the controller's efficiency at lower duty cycles since that low side FET being used as a diode dissipates much more heat than if it were actively conducting. We can throw as many low resistance super-duper FETs in parallel as we want in a controller, if we are using them as diodes they'll still suck almost as bad as crappy FETs.

Of course there is a down-side to using synchronous switching, which is control complexity. Anyways, bootstraping may not be for everyone, but I personnaly like keeping my controller circuit simple (low part count) and have had no issues with this. I do not know of any other specific "problems" a bootstrap circuit has other than the need to be pulled low to recharge the cap once in a while? BTW, when chosen wisely a bootstrap cap is be able to keep the high side FET fully on during the whole commutation period, which means 100% PWM is no problem. The only situation where it could be a problem is at extremely low RPM (like when stalled), but actually this is almost never a problem since at such low RPMs you need to be doing PWM for current limiting or speed control anyways.


Jeremy - those rubicon ZL's look like a good choice. You can also add nipon chemi-con KZE's or panasonic FC's to your low ESR cap source list, they look all pretty much the same but the prices sometimes vary quite a bit even from one size to another so choice is good. As for bootstrap caps, 10uF still seems quite big to me - what kind of cap is it? I've always found those 47uF caps on the chinese controllers pretty weird! It's more important to have a good cap there than to have a big cap there, so the best would be to aim for something like a multi-layer ceramic of at least 2.2uF maybe. It only needs to be rated at your gate voltage. And remember that the bootstrap diode is also important - the bigger and lower ESR you make the bootstrap cap, the more peak stress you are putting on your diode. Another thing is that it is your driver chip's own decoupling cap that will be the one working the most to recharge the bootstrap cap, so making it also a ceramic low ESR cap is also necessary - and it's got to be quite a bit bigger than your bootstrap cap too, like 10X the size or so.

What kind of FET transition (switching) times are you getting BTW? I think you mentioned adding in a parallel gate resistor to make your turn-off times faster - I've had good results doing this. If it gets *too* fast for your liking after doing this, you could always add in another smaller series resistor that isn't in parallel with the diode and the other bigger turn-on resistor. Also, if you do choose to add a gate diode, make sure it has not too much voltage drop at the peak currents you will be driving the FETs at or it may not pull down the gate low enough.

Pat

 

[Jeremy Harris]

Jeremy - those rubicon ZL's look like a good choice. You can also add nipon chemi-con KZE's or panasonic FC's to your low ESR cap source list, they look all pretty much the same but the prices sometimes vary quite a bit even from one size to another so choice is good. As for bootstrap caps, 10uF still seems quite big to me - what kind of cap is it? I've always found those 47uF caps on the chinese controllers pretty weird! It's more important to have a good cap there than to have a big cap there, so the best would be to aim for something like a multi-layer ceramic of at least 2.2uF maybe. It only needs to be rated at your gate voltage. And remember that the bootstrap diode is also important - the bigger and lower ESR you make the bootstrap cap, the more peak stress you are putting on your diode. Another thing is that it is your driver chip's own decoupling cap that will be the one working the most to recharge the bootstrap cap, so making it also a ceramic low ESR cap is also necessary - and it's got to be quite a bit bigger than your bootstrap cap too, like 10X the size or so.

What kind of FET transition (switching) times are you getting BTW? I think you mentioned adding in a parallel gate resistor to make your turn-off times faster - I've had good results doing this. If it gets *too* fast for your liking after doing this, you could always add in another smaller series resistor that isn't in parallel with the diode and the other bigger turn-on resistor. Also, if you do choose to add a gate diode, make sure it has not too much voltage drop at the peak currents you will be driving the FETs at or it may not pull down the gate low enough.

Pat

I've had a good look around and, at least over here, the Rubycon ZLs seem to be the cheapest that I can get hold of before the holidays (odd buying criteria, I know..........), but it's nice to know we have a few more choices.

I've now moved away from the bootstrap drive circuit and have redesigned the board to take small DC DC converters. They make the board layout a bit simpler and although little more expensive than the bootstrap circuit I was using at least I can be assured that I have robust high side drive at all times. It also removes the need for the 'kick start' comparator driving the output enable pin that I was using to get around the inability of the controller to start from some motor positions without a flick, due to the lack of bootstrap charge.

I can't accurately measure the turn on/off times, as my old 'scope's too slow and the USB one I have has a far to a low sample rate to be useful for this job. I'm reasonably sure that they are fast enough to not be wasting much power, though, as the test motor will idle with a FET current (excluding the control circuitry) of about 70mA at 36V, which is about half the current the same motor draws when running on a XieChang controller at around the same speed. The actual FET switch off looks fast on my 'scope, it's just the gate drive that has a small tail at the base of the falling edge. I'm sure this is starting after the FET has, to all intents and purposes, turned right off, other wise I'd be seeing it on the FET output, too.

Jeremy