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Designing a brushless motor controller

Howdy - I've been thinking alot about brushless motor control lately. I've been using some COTS brushless motor controllers and have been very unsatisfied with them - they've all been overly large, overly complicated, and overly costly. So I'm thinking about trying to build my own. I'm thinking about having it be something like this:

25-75VDC supply voltage 10A continuous output hall, quadrature encoder, and analog position inputs small (in physical size) CAN connectivity ~25KHz switching speed or faster

The first thing that I'd tackle, of course, would be the hardware design. I'm looking for very precise control - so I want a good deal of built in smarts. My gut instinct says to go the FPGA route. Somehow that just strikes me as a nice clean way of handling things. I'm more experienced with MCUs - but I'm always happy to learn a new technology.

Next up - and the topic that I'm having the most trouble with - is the design of the half bridges. I'd want to use N-FETs throughout to maximize efficiency. Driving the high side FET gates is troubling. The only method of driving FETs like this is with a floating power supply grounded on the source of the FET, and then optotransistors switching the FET with that. There has to be a simpler solution. First of all - is there a clever way to combine the floating power supplies such that you only have one high side FET power supply?

This is the problem that I'm having the most trouble with. I mean - to keep from blowing through the gate of the FETs (by going over the max VGS) - you could put a zener across VGS on each high side FET (with a series resistor of course) - but then you're going to be draining the floating supply when you turn that FET on until the source voltage goes high enough to turn off the zener. That is why it's very attractive to have a separate floating supply on each FET's source - but yeah... size is important here.

Lastly - driving the FETs - the standard solution I seem to see for problems like this is to have some sort of a low side high voltage (well, high enough to handle the bus voltage) N-FET with the source grounded and a resistor between the gate supply and the FET's drain, and the gate of the half bridge high side FET connected to the FET's drain. This has the clear problem of having a huge trade off between FET turn on speed and power drop across that resistor. I've also seen optoisolators used for this, which takes care of that problem but also adds large parts and would seem to me to place a limit on the switching speed (as I feel like optos are typically substantially slower than normal transistors).

So I guess - my big questions here are:

  1. FPGA/MCU?
  2. How to combine gate power supplies?
  3. How to drive high side FET gates?

Thanks so much!

-Michael

Reply to
Michael
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A small inverter supply for the floating supply. Torroids are not that big.

Bob

Reply to
sycochkn

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Reply to
GPG

Modern brushless drives are very complex, you should reconsider.

I'm thinking about having it be something like this:

Most new ones have FPGA, older ones had microcontrolles. If you've never done FPGA design before you have a very big mountian to climb. Another good reason to forget the whole idea.

This is the easy part, not only have there been one chip soloutions around for years, they are also well documented look on IR's website. If you can t work this bit out on your own you have yet another good reason to forget the whole idea.

Well if your still determined get one of internationals "power train" kits (I'm not sure if they still make them) for the power design side. Fujitsu have a new brushless design kit available from Farnell for all the control side, mcu, software theory ect. Of course you still have time to FORGET the whole idea.

Reply to
cbarn24050

I appreciate your concern, however, I am not worried.

I've done a decent bit of FPGA work - it's just been a while so I'm rusty. That's good to hear that I'm on the right track with the FPGA route. It just seems the more logical route for high performance stuff.

The less chips there are the happier I am. I prefer doing things discretely instead of connecting together black boxes. However, the IR2101/2 looks like it might take care of a lot of problems for me. =46rom the datasheet, it uses the way I was talking about with a high voltage FET + resistor driving the half bridge FET gate.

Software I'm not worrying about at this point - only the hardware. Somebody else is signed on for that side of things. I don't plan on forgetting the idea :) I don't seem to see anything about "power train" kits on IRF's website, unfortunatley. Thanks.

-Michael

Reply to
Michael

That was some time ago but search for this instead on their website, it's just what you need. IRMD22381Q

Reply to
cbarn24050

Well - if you implemented my suggestion in using a resistor in series with a zener then you could use a combined supply. I did exactly this fairly recently for some low speed high side switching. Worked fine - but you do waste some power in the zener and reisistor, while also slowing switching speeds. The IR2101 seems to be exactly what I was describing in my first post - except all wrapped up in a single chip.

I've never seen this done before - are there any half bridge driver chips that implement this? Also - why would this be necesarry? I mean once your VGS is below VTH there should be no conduction channel...

Three!

-Michael

Reply to
Michael

Sounds like you may be about to find out why.

You'll need a separate supply for each since each phase is moving relative to the others. Check the IR 2100 and friends for one approach to high side driving. Note that there is a limit on how long their scheme can hold the high side on.

For low threshold voltage FETs it can be usefule to have a negative gate drive to help turn off the FET and keep it off.

One item missing from your spec list is the number of phases.

Robert

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Reply to
Robert Adsett

To answer your questions, in order:

  1. No you didn't ask it, but you should have: consider your engineering time when you make your decisions. Unless your production volumes are huge, or the controllers that you buy don't meet your specifications for some reason, or you have to stand the rest of the system on it's head to accommodate the drives you're using, you're probably going to end up spending more money in the end by rolling your own, even if you drove the BOM cost to zero.

  1. I'd consider a DSP. TI, Analog Devices, and Microchip all make DSP chips that are designed for brushless service. I _think_ that all of them are available with CAN controllers built in, to boot.

2 & 3. Have you looked at high-side driver chips? These address many of the issues that you have, and IIRC there are ones that are good for up to 300V supply rails. I've seen these used with great success for class D synchronous amplifiers (by a company with low production volumes, but very stringent requirements, so they always ended up rolling their own and paying the $$ for engineering). 
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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

There is a parasitic capacitance from the drain to the gate. If the Drain-Source voltage changes quickly (say due to a low side turning on) you get voltage coupling from the drain to the gate, in the worst case turning or keeping the FET on. Even if it does not turn fully on you can get siginificant shoot through currents. Driving the gate slightly negative increases the margin. It's not a problem with high threshold devices but a lot of the modern high performance FETs have low thresholds and are more sensitive to this. For some the only difference between a logic level FET and the normal FET appears to be the label.

I think IR has an appnote on developing a negative gate drive using a

2100. It does use a number of extra components.

IGBTs often need a negative turn-off drive as well I understand.

I've seen three and four. I recall there being a 3 phase (3 1/2 H's in a package) chip. Maybe from IXYS? Capable of more current than you need as I recall. It might be worth a quick look, Ah here's one of them

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I don't know the cost of these but I'm curious.

I expect the needed capacitors are going to take more room than your power devices anyway.

Robert

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Reply to
Robert Adsett

MCU will be more straight forward. Look at the TI TMS320F2812 (or 2811). there is a soltware library already written for brushless DC operation. Freescale, Atmel and Microchip also have some DSPs and MCUs that have canned software as a base.

for the HW, you could go with something as simple as a IR2110 driving some low RDS fets. be careful about going with too low of an RDS fet because they have very high capacitinace which can increase switching loss. Besicaly select a device that can handle you current load plus a 2x margin of safety (say a 150V 20A device) and not much higher and you should be OK. If you get a big 60A wil be loaded with capacitance and the switching losses will more than offset the gains from the lower Rds.

Layout will be critial if you want high efficiency and low noise. That is a whole new subject.

Reply to
Mook Johnson

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