Brushless DC ESC wiggin out - Page 1
Forum section: electronics / beginners

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 06, 2016, 12:00:13 am
content: So i have a 4-in-1 hobby ESC (http://tinyurl.com/jv9ckyf) for mini-micro quads that is giving me a bit a trouble. three of the ESC's on the board are working perfectly and one is giving me hell. The motor stutters and occasionally it'll catch and run. To get a visual i hooked the board up to my O-Scope and this is what i've found so far:Setup:-Servo tester for PWM input-3 cell LiPo-Small RotorX motorImages from the Good ESCs:Low Output:Medium Output:High Output:And now the mysterious bum ESC of the board:Signal during stuttering:Low Output:Medium Output:High Output:So what i've gathered from this is the bad ESC has lost it's phase shift and i wantto say Ch 2 & 3 are the problems as only Ch 1 seems to keep a minor semblance of a good wave with a flat baseline periodically chopped. I'm aslo totally in this realm so i'm just making assumptions lol.What would really help is understanding how these DC ESCs create their 120 shift between their PWM drive signals. learning from doing is how i do and having a start point can really help my research on this project. My researching skills seem to be terrible as i only seem to find fluff and summary explanations on their operation. So a lazy finger in the right direction will be great.I've replaced it with a new board but i'd really like to give a shot at fixing or finding the issue asan exercise in troubleshooting.

author: retrolefty
author_rank: Super Contributor
timestamp: September 06, 2016, 01:33:37 am
content: QuoteMost hobby ESCs are of the 'sensorless' types where their timing reference is made by sampling the output phases line internally and sensing a small 'back emf' pulse which is used to trigger the next phase drive.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 07, 2016, 02:31:02 am
content: Thanks for the input! it's givin me a pretty awesome rabbit hole to go chasing down. I believe I may have found at least one of the issues. I checked the board out with an eye loupe to try and identify some of the type of components make up the circuit and found something interesting.One side's edges are lined with IC's marked 7426 BA332U and on the reverse 7426 BA3357. Haven't been able to find anything conclusive but i did find that on the side that's populated with the BA3357s there's a BA332U in place of one on the bad ESC. I haven't done anything to confirm this is the issue yet but i think it's a start.I think a wrong chip could mess with the starting sequence which then prevents back EMF from building up for the controller to read. When the motor does magically work, the chip being part of the output, would mess with the output signal and/or i think distort the read of the back EMF depending on how this circuit takes its measurements.Does this sound like it's around the ballpark?Any useful information on how to identify what ICs these likely Chinese chips are would be awesome. I've tried looking up the last 4-5 letters/digits of the identifier but I found too many options to start with but i'm thinking the BA332U is some frequency synthesizer and the 3357 is some type of transistor/switching IC.

author: CJay
author_rank: Super Contributor
timestamp: September 07, 2016, 08:46:57 am
content: Post pics of the chips with markings in view, neither number you posted gives any definitive hits

author: tatus1969
author_rank: Super Contributor
timestamp: September 07, 2016, 09:36:15 am
content: it with starting examining the MOSFETs in single. Check the gate waveform, must be rectangular with stable ON amplitude. Also show the corresponding output voltage swing of that channel. Without motor connected it should be rectangular either (ESC should attempt motor start then). Gate voltage depends on ESC, you can compare with a working one. Easy for the low side transistor, but hard for the high side. If you don't have a differential probe, use two channels, one at output, second at high side gate, and subtract them on the scope.This way you should be able to find the broken MOSFET or driver stage quickly.From the pictures I suspect that one of the low side MOSFETs is broken. The output signals don't go to GND close enough.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 07, 2016, 10:31:42 pm
content: So i have pictures here of the chips. May need to zoomin a bit but the resolution is descent enough to see the markings. I was also able to fit all the chip types in but i really think only the ICs at the edges will be relevant.I'll give the measurements a go when i have some free time from homework and studying today.So pics focused on the side in question on both sides of the board:Bottom Side:Left three are part of the suspect ESCTop Side:Right three = suspectThe far right chip has a little solder bridge but from what i can tell the left three terminals share a connection anyway. But going to remove it once i have time.you will also see that this where the middle chip is not the same as the other two as it is on the good ESC (the left three)

author: tatus1969
author_rank: Super Contributor
timestamp: September 08, 2016, 06:58:58 am
content: the big ones at the borders are all MOSFETs. My guess is that they used complementary types, one N and one P channel in pair per channel. The tiny 6-leg parts in the top picture are probably driving the gates, could be them as well.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 08, 2016, 07:58:03 am
content: First thing I'd do is to remove those solder blobs and make sure there aren't any pin-to-pin shorts. Then trace the pinout to see if it corresponds to this mosfet:http://www.aosmd.com/pdfs/datasheet/AON7426.pdfhttp://www.analogpowerinc.com/datasheet.php?data1=DS_AM7426N_1A_1

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 08, 2016, 01:03:21 pm
content: So if those first four are the IC's designation what are the last set? a lot or production code?Thanks for the data sheets though! holy hell that helps a ton. Both sides are populated with 7426 though so I'm assuming they're all N-Channel. I can't find any other info on other markings that may indicate a spec change through out the 7426 line. Going to check the chips on the new board to see how they read. Hopefully I get some time tonight to get the Iron and O-scope out.

author: tatus1969
author_rank: Super Contributor
timestamp: September 08, 2016, 01:13:11 pm
content: Quote from: CoffinDodger on September 08, 2016, 01:03:21 pmThe marking logic differs from manufacturer to manufacturer, and from part to part, especially if space is limited. The second line most probably contains information about production date and facility. But not necessarily, sometimes the first letters in second line belong to the part number. The datasheet does not have information about the marking format, so i would assume the parts are identical, but from different production lots.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 11, 2016, 05:40:03 am
content: So i finally got around to stickin a scope to my ESC again as Tatus suggested and most of the signals matched the others except for this guy.I don't know much about these circuits yet, I don't start studying MOSFETs for another week or so but i'm assuming the output comes off the drain (depending on the circuit design i assume, lot's of assumptions right now) and the gate is for the input signal. I'm also assuming (more assumptions!!) the low side is side receiving the original signal and the high side is the final stage. SO with that i bring you these images.Everything else was pretty nice and square except for this low side guy.It's Gate signal was just like the rest of the FETs:While it's Drain looked like a capacitor charging:And the High Side Gate had the same form coming through (the difference being the time scale was different):I'm thinking this little guy is my culprit. Thoughts?

author: tatus1969
author_rank: Super Contributor
timestamp: September 11, 2016, 08:39:17 am
content: For switching applications like this, you can compare a MOSFET with a relay. The gate-source voltage (Vgs) is the coil, and the drain-source path is the switch. When Vgs is greater than a certain voltage (in your circuit this is 5V), the transistor shorts out the drain-source path. For N-channel MOSFETs like here, the drain-source voltage (Vds) cannot become more negative than ~1V, since there is an antiparallel diode "built in". The power stage for one motor looks like this:For the low side it is simple. As the source is connected to ground, you can directly drive the gate with a square wave voltage. In many cases they directly use the output pin from their microcontroller.The high side is trickier, because when that transistor conducts the source will be pulled to to the positive supply rail. But the transistor always needs a positive voltage between gate and source in order to conduct. What does that mean? If +Vb is the supply voltage, then the high side gate must be +Vb+5V(!) The circuit that accomplishes this tricky part is called "high side driver" or "floating gate driver". There are chips that do this, but in many ESCs they use a discrete transistor circuit.This is also the reason why I said that, in order to check if the high side transistor is controlled properly, you need to use two probes, one at the gate, and another one at the source, and subtract them on the scope. Then you want to see a nice 5V square wave. If not, then the driver could be damaged, or the transistor's gate could be damaged. In fact, you measured both voltages above. In your last post, you would have to subtract the second from the third trace (low side drain is connected to high side source).But anyway, your pictures look strange. Where was your scope's ground clip connected to? It should always be the negative battery terminal (ground).I cannot clearly identify what might be broken. You have negative voltage at the drain, which shouldn't be possible because of that diode I mentioned above. The transitions are much too slow as well. I would next check both transistors of this phase for good soldering, maybe you also have a cracked copper track on the PCB. And I would put the high side Vgs on the scope as explained above.I rather suspect the drive circuit of the high side transistor here, because the falling slope of your last two pictures is fast, which indicates that the low side transistor is switching well. If that proves to be the case, the good thing is that you don't have to replace the MOSFETs, but the bad thing: you need to dig deeper into that tricky floating gate driver.

author: retrolefty
author_rank: Super Contributor
timestamp: September 11, 2016, 01:06:34 pm
content: Quote from: CoffinDodger on September 11, 2016, 05:40:03 amLooks like possibly there is an open path to the load winding in that first pic. Check for continuity.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 11, 2016, 05:17:32 pm
content: Alright so used the math function like you originally suggested. Confirmed my ground clip referenced the DC source ground. Also switched my probe coupling from AC to DC, i think that was resulting in the negative going part of those forms. I first checked a known good ESC on the same board to make sure i was measuring correctly and i got the 5v square wave just as you said. Then did a few more times just for that warm fuzzy feeling. This stuff is getting pretty cool so got a few giggles out of it.Anyway, I then did it with the questionable high side FET and the Gate matched the Source and so math was null.So this means the driver isn't overcoming (at all) the 5v requirement between the Gate and Source right? Maybe some other hooplah, either way looks like I've got some fun ahead of me.

author: tatus1969
author_rank: Super Contributor
timestamp: September 11, 2016, 06:23:53 pm
content: Sounds like good progress, congratulationsNow to check if the broken part is the MOSFET, or the driver, you can measure the gate-source path with a ohmmeter. If the transistor is good, then you should at least read a few kiloohms (if that transistor were soldered out lying on your desk, you should in fact read >> megohms, but the circuit driving it will probably limit that). Just make sure the ESC's power rail is at zero before measuring, maybe short it once to discharge the large caps.If it's the driver that is broken, you'll probably need to do some reverse engineering in order to understand how that is built. Do you know if the schematic of this ESC is around?

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 11, 2016, 07:02:01 pm
content: RgsCame up to 51k and i got the same measurement on the other FETs so looks like i'm diving into the Driver doesn't it? I haven't been able to find a schematic sadly as of yet. going to start digging a little more earnestly. Or take a few pics and try to make out the traces for a bit. Sadly this will be the last of the progress i make on this little foray until possibly the end of the week. Got some BJT amplifier home work to jump on and the week is usually full of information over flow.Retro: i really hope there's an open to the load, lol. i'm testing with no load connected at the moment

author: tatus1969
author_rank: Super Contributor
timestamp: September 11, 2016, 07:34:26 pm
content: looks like it is the driver. To rule out a firmware problem: was the ESC working before? From your pictures, I would say the driver is all that stuff on the top side - the one opposite to the microcontrollers. The small 6pin chips are either integrated high side drivers, or more likely dual transistors. Their marking would be interesting. What puzzles me is their number. For 3 phases I would expect 3 chips, not 2.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 11, 2016, 08:07:06 pm
content: Really wishing i had some needle tip DMM probes though. These 2mm are killing me on SMD components. Anyway, This is what i'm looking at for what i'm assuming is that fancy pants driver.looks like one cap, 2 resistors, and 1 undetermined semiconductor labeled T4 (has a nice solid line on one end some i'm guessing a diode) per channel and 2 little 6 legged dudes marked K6N89 from what i can make out. There's no middle 6-leg dude forthe middle channel. May be related to tracking the motor's Back EMF as Retro mentioned earlier. On all the ESCs on this board the middle channel gave me nothing with no motor connected. I'll confirm that in a bit.Comparing to the bottom set, looks like 5 resistors and 1 capacitor to each 6-leg midget pair. I'm suspecting those connect with what looks like a voltage regulator or power transistor at the top with a couple of resistors coming off of it. The real fun will come trying to trace these vias.Once i'm done studying, homework, and this bottle of Black Butte XXVIII, I'll start poking things again.

author: tatus1969
author_rank: Super Contributor
timestamp: September 12, 2016, 05:39:09 am
content: all three phases need high side drivers in a brushless ESC. The 2-leg part is a diode. From the picture it looks like the lower 6-leg part is controlling both the middle and the lower transistor.These ESCs use 6-step commutation to turn the motor, you may put that into Google. It is characteristic for this to have one phase to do a PWM, a second one is shorted to GND (low side conducting), and the third one is floating (both low and high side FETS switched off). The role of each leg changes over time, which creates the three phase AC that the motor needs to turn. The floating leg is the one that is sensed for back EMF.When there is no motor connected, the ESC needs to receive a command to start its motor. Then it should start this scheme on its own, without needing back EMF. That's the sensorless starting scheme, the stutter that you will feel when you hold the propeller.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 12, 2016, 05:08:54 pm
content: I agree about the 6-legged dude. was staring at it last night and sketched up the circuit i attached. I'm having a little trouble with the vias though. They don't seem to match up well to things on the other side. can there be more traces on the board than are readily visible? I haven't done much studying up on different board types and layers and such.I'm going to research the 6-step commutation after work today but the rest of your description makes sense to me.Edit to the Sketch:That PMF is supposed to be PFM (Pure Freakin Magic)

author: tatus1969
author_rank: Super Contributor
timestamp: September 12, 2016, 06:35:54 pm
content: close but probably not correct. Maybe comparing with the most likely circuit helps sorting it out.In this drawing, "load" is where the low side FET sits. "pad1" is the PWM from the microcontroller.The BJT would be 1/2 of these 6pinners.edit: 6pinner pinout is likely

author: tatus1969
author_rank: Super Contributor
timestamp: September 13, 2016, 10:29:03 am
content: another hint regarding my explanation of six step commutation. This software is doing it differently from what I explained above. Correct is: one phase is doing PWM with a certain duty cycle, lets call this 'd'. A second phase is doing PWM as well, but with opposite duty cycle (100% - d). The third phase is floating, allowing back EMF measurement. When you do it this way, then you can just sum up the voltages of all three phases with three resistors, all tied together at the other end. Because the two PWMs are exactly opposite, they cancel out, and after some RFI low pass filtering you get a nice back EMF waveform that the microcontroller can digitize with an ADC.

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 18, 2016, 09:16:38 pm
content: So this project is going to be taking a back seat for a while. Can't really do much now with out a fine tip for my soldering iron and some more appropriate porbes for my meter. i.e. I knocked off one of the diodes while trying to compare resistances between the DUT and the working circuits. SO now i'm working on an Octopus for giggles until the new tools come in. and of course the octopus is giving me trouble now too though I think it's more of an O-scope set-up issue than an octopus circuit issue. I was working at one point then not working the next. Anyway i'm rambling off topic now. Once I have the appropriate tools i'll be back on this project again. Thanks for all the help guys!!

author: tatus1969
author_rank: Super Contributor
timestamp: September 19, 2016, 08:08:56 am
content: Quote from: CoffinDodger on September 18, 2016, 09:16:38 pm