Jacob, some details of the hardware design might help folk debug it for you "offline". Can you share the schematic and layout?
There are many traps for newcomers to design of switching hardware...
Clifford Heath
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Here are the kicad files, sorry the schematic is a mess, some of the components are redundant as I made it so I could test multiple configurations with one board design. It's relatively straight forward it plugs into either a TI MCU using instaspin or a STM32F4 board with VESC software. Voltage dividers and current sensors appear to be working without any issue. Mosfet drivers are 3x analogue devices adum7223 isolated 4A half bridge drivers. High side is bootstrapped with a schottky diode + 0.1uf and 10uf ceramic capacitors. low side just has the capacitors. Between the driver and the mosfet gate I have tried a few configurations, one with a diode + 30ohm resistors, sinking was done via a PNP bjt. This one did function once but was damaged afterwards. I then tried different variations with just resistors and no BJT since the driver is supposed to be able to sink quite a bit.
Everything I tried just killed gate drivers and mosfets due to shorts from not switching. I managed to prevent it damaging itself by adding capacitors between each motor phase and ground but I still haven't got it switching properly again. Generally when it was breaking everything on the 15V vdd section was being destroyed. The PCB is 2oz copper thickness.
This is how I have it configured for recent tests, each phase has an ADUM7223 driving the mosfets with an 82R resistor at the gate with a diode to bypass it when sinking. The ADUM7223 is configured wit the high side power provided by a bootstrap circuit, after the diode there are 10uf and 0.1uf ceramic caps. Low side is the same minus the diode.
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You definitely need hardware protection. I'd wager $20 you're turning both top and bottom FETs on at the same time. Use an inverter between INA and INB (one wire from MCU) and use DISABLE for off (one more).
The trace length thing could be glitching the MCU but that's a secondary concern.
Clifford Heath
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Just a note:
If you're using a 20V supply gor the motors, then you're 2V over the max listed for the output drive for thr ADuM7223
For now the power supply is just an isolated 20v 2a DC power supply, I saw a few articles that suggested low ESR high ripple current caps for low inductance BLDC when testing. Motor controller software should always keep one low side mosfet open. I did actually have it running once, but the working one was broken when I connected a different MCU dev board which turned out to have a fault and destroyed it. After that I was ill for a few weeks but my attempts to build another one didn't work this time. I was able to construct an oscilloscope using a cyrpress PSOC today, I think I found the problem, some flux residue may have been causing gate drivers not to be soldered correctly leading to signal issues that weren't visible without the oscilloscope. I suspect some of the highside gates were turning on while none of the low side operated properly and as you were saying the motor acts as an inductor it would then dump the energy into the gate drivers and mosfets. The capacitors must have been able to stop the mosfets being killed as they were very high ripple current low esr 260V caps for power supplies, so they likely can absorb anything the small motor can produce with 20V and a few amps.I have been looking at this reference design today https://wiki.analog.com/resources/eval/user-guides/ad-fmcmotcon2-ebz/hardware/lv_board it's relatively close to what I want run with mine, I will try similar values to what they used, 22ohm driver to gate and diode + 3ohm for sinking. They didn't seem to have any sort of hardware based shoot through protection, though it is driven by a FPGA, maybe it takes care of that. I have the PSOC working as a PWM + scope, so I will try testing each phase individually doing something simple like varying the intensity of an LED lamp before trying motor control with all 3 phases.
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On 10 Aug 2016, at 12:34 AM, ja...@varanidcs.com.au wrote:
> For now the power supply is just an isolated 20v 2a DC power supply
So when the motor is at full current, and the FETs turn off,
where does the current go? How long does it take for that
current path to "open up" - longer than 9ns? What is the
cross-section area of the loop of wire that leads from one
motor terminal round to the other one(s), and will radiation
from that loop cause glitches? How many joules of energy
exist in the motor's magnetic field (I^2*L/2) that must be
passed or absorbed?
If you can't answer these questions, you don't yet have a
"design" for a motor controller. Because that current *will*
not stop. It will find a path, even if it has to make smoke.
Clifford Heath.
> , I saw a few articles that suggested low ESR high ripple current caps for low inductance BLDC when testing. Motor controller software should always keep one low side mosfet open. I did actually have it running once, but the working one was broken when I connected a different MCU dev board which turned out to have a fault and destroyed it. After that I was ill for a few weeks but my attempts to build another one didn't work this time. I was able to construct an oscilloscope using a cyrpress PSOC today, I think I found the problem, some flux residue may have been causing gate drivers not to be soldered correctly leading to signal issues that weren't visible without the oscilloscope. I suspect some of the highside gates were turning on while none of the low side operated properly and as you were saying the motor acts as an inductor it would then dump the energy into the gate drivers and mosfets. The capacitors must have been able to stop the mosfets being killed as they were very high ripple current low esr 260V caps for power supplies, so they likely can absorb anything the small motor can produce with 20V and a few amps.
>
> I have been looking at this reference design today https://wiki.analog.com/resources/eval/user-guides/ad-fmcmotcon2-ebz/hardware/lv_board it's relatively close to what I want run with mine, I will try similar values to what they used, 22ohm driver to gate and diode + 3ohm for sinking. They didn't seem to have any sort of hardware based shoot through protection, though it is driven by a FPGA, maybe it takes care of that. I have the PSOC working as a PWM + scope, so I will try testing each phase individually doing something simple like varying the intensity of an LED lamp before trying motor control with all 3 phases.
>
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Clifford,
While I know some of the theory, it's definitely been good to read through it again from your explanation.
FWIW: I had a crap time (many many moons ago) figuring out pointers in C till I read through my 3rd book (K&R no less, which I was told "not" to bother with by everyone at the time) which explained it in a different way. It "clicked" and suddenly all the previous explanations I read made complete sense.
Sometimes, simply reading the same explanation done from someone elses perspective is all the enlightening you need. ;)
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I'll de-lurk to ditto what Bharanee said. Thankyou, Clifford, for the interesting reading.
Cheers,
Damien
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Hi guys
Brett from Perth here,
I posted interest in up-specing vedder's ESC on his forum a while ago and got a few bites but had to focus on other things.
Jacob, have you posted your design efforts anywhere? There are no less than three developers hacking on the same problem right now.
The flyback from fet turn off is normally either handled by conservative turn off speed, or by the body diode (or external flyback diode) of the complimentary fet.
In IGBTs, the transistor doesn't reverse conduct so flyback diodes are critical.
Clifford, I suspect that that particular gate driver is for inverter controllers that twiddle the dead-time cycle to cycle.
There are some cases where switch-on takes so long that you need to dial the dead-time all the way into negative at the logic side to get a clean handover fet-to-fet. It's important when you're using MOSFETs and seriously need to limit flyback noise.
There are also the cheapo atmega8 ESCs that have no hardware protecting against shoot through. (I've cooked plenty of those because of it)
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The DRV8302 is always a headache for some reason, people running the VESC controller report it exceeding the 30ma limit and destroying itself when FOC commutation is used. It also often fails around 50V leaving fairly limited voltage ranges. The main reason I replaced it in the first place was myself and many others want to use it for electric unicycles. At the moment the only controllers you can get are poor quality chinese designs. We even tried asking the manufacturers if we could pay them to design a better one but they aren't interested. Due to the nature of their large direct drive hub motors they require 60-90V to reach desired RPM. A few chinese manufacturers decided to try and make higher top speed models by reducing the number of turns in the motor windings which then either decreases torque to the point it falls over or their control board burns out due to the increased current required to keep it upright.This is an example of a "good" electric unicycle controller used in products sold for more than $1000 USD. They all pretty much have the same issues, cheap 2 layer PCB with TO-220 package mosfets, long traces with cheap drivers and no real heatsink. The design appears to be partially based on an old 250w ebike controller with various additions. Usually they overheat so they add more cheap high RDSon mosfets only to find switching losses are making too much heat so they then drop the switching speed ending up with a noisy and unreliable drive. I think one brand has resorted to 7khz or 9khz switching. On top of that they generally cannot be reconfigured, so each controller must match certain batches of motors if you assemble or repair it yourself.
Clifford, the major limitation people are redesigning for is the 60v limit of the DRV ic.
I need a 120v supply rail with 200A sustain; that looks like 300v survivability, and I need to put it in the hands of students, so that goes up to 500v.
I'll almost certainly use bootstrap diodes, and if I want to support Vedder's complete feature set, I'll need a secondary supply like the charge pump.
Designs like Jacob's asymmetrical gate drive can sometimes force a large enough dead time to hurt. (Mostly important in low noise applications; radio astronomy facilities for example)
I'm actually planning on using entirely separate high and low side gate drivers, they're available with much higher isolation voltages. So while I may not have hardware guarantees on non-overlap, it's something that merits thinking long and hard about.
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