Power electronics lab gear for current and motor controllers - Page 1
Forum section: electronics / beginners

author: NickEdison
author_rank: Contributor
timestamp: December 11, 2021, 02:29:36 am
content: Dear forum users,I am a student looking into setting up a small hobby lab for current and motor controllers (P > 200 ). I have some experience with electronic circuits (mainly digital) and would like to broaden it.A bit more details on my use cases:For now I am mainly interested into the production of motor drivers (BLDC and DC) including control boards using microcontrollers, but also battery chargers and protections circuits. I also like working with FPGA's but feel like I am not jet experienced enough to design my own boards for them. Autonomously driving robots are on the list as well.In the future I might look into designing a battery storage solution for POE devices at home, which will be charged using solar panels (omitting the power conversion of dc to ac and then back). Of course there will also be small projects along the way.My current setup:4 multimeters (I feel like these suit my needs for now)Soldering stationHomemade psu 0-26 at 1PC PSU for projects that require a significant current at 5 or 12  (It is not current limited, which results in some spectacular fireworks sometimes)I think that I have most of the handtools required and a reasonable stock of componentsThere is a pc in my lab as wellWhat I am considering to add:Siglent SDS1104X-E (I think my use case requires a 4 channel scope because I would like to be able to measure all 3 phases of the BLDC motor for example and estimated that I need a 100  scope. Given that I have a 30 PWM signal of 8 bits then the smallest pulse would be approximately 13  long, which I think can be approximated as a 7.68  square wave hence, a 50 could be sufficient, but I think the additional bandwidth would be nice.)Korad KD3305D or KA3305P (These were the best I could find in terms of bang for the buck and with decent power delivery. I initially considered buying 2 single channel PSU's, but like that the double supply allows me to put them in parallel for higher currents)A homemade loadMy questions:I am currently doubting between KD3305D or KA3305P because I am not sure if the ability to turn on/off the channels, the memory buttons and programable interface is worth the extra €50,-https://www.eleshop.nl/korad-kd3305d-5195.html,https://www.eleshop.nl/korad-ka3305p-2x-0-30v-2x-0-5a-programmeerbare-labvoeding.html(sorry I don't see an option to translate the website into English)Would I be able to use the Korad PSU for powering small circuits directly or will it be to noisy? (To test current measuring circuits for example)Given my use case; would these be a good purchase or would I be better of with something else?Am I missing important equipment or making incorrect assumptions?Best regards

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 11, 2021, 02:52:56 am
content: Yes, you are missing the probes. At that speed the ringing of differential probes make them useless.Single ended probes will be fine but only for grounded signals. For anything that is floating (high side voltages, current shunts) look for an isolated probe

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2021, 11:52:21 am
content: You really want the scope BW to suffice for seeing layout parasitic ringing. 50MHz is maybe barely acceptable, 100MHz is fine unless you do very tight small DC/DC converters. You are right about 4 channels. Although very rarely I actually scope all three of the actual motor phases! You can live with 2 channels, 4 makes everything just so much easier.I have had good success with just Rigol DS1054Z hacked to 100MHz designing and measuring DC/DC converters and motor controllers.The really big question is, are you going to build high voltage motor controls? Like mains voltage, over a few hundred volts? If yes, investing in differential or isolated probes with high CMRR (and also good CMRR as function of frequency!) would be very desirable. But as always, using your brain can compensate lack of equipment, I have successfully developed some AC induction motor inverters without such probes.For safety extra low voltage BLDC stuff, really, the only lab gear you really need is scope similar to Rigol DS1054Z (4 channels, 50-100MHz), soldering iron with both smaller and very large tips, adjustable temperature hot air gun, and normal lab power supplies.As always, it Would Be Nice to have large enough lab supplies to actually run the motor in question but I haven't always had that luxury either. You can start with small enough motors. You can verify your current sense and overcurrent protection works. You can verify amount of voltage ringing. Then you can connect the larger motor and run it with a battery. Remember fuses, they protect from fire damage.I have managed to limit number of burn semiconductors quite a bit by using step-by-step process. The price of the MOSFETs isn't the problem, the time wasted in desoldering blown parts (best to replace gate drivers as well even if they seem working) kills motivation.

author: NickEdison
author_rank: Contributor
timestamp: December 11, 2021, 04:54:11 pm
content: Quote from: Siwastaja on December 11, 2021, 11:52:21 amFor now I am mostly thinking about circuits of up to 75 DC.I have some batteries I could use for testing larger motors but hope that the lab PSU can suite most of my needs because, adjustability and protection when using batteries is fairly limited.Quote from: MasterTech on December 11, 2021, 02:52:56 amI might miss something but if I connect the driver to the PSU, I am able to connect the probes anywhere on the circuit (given that it is not grounded), right? Wouldn't I be able to connect the grounding lead to one side of the shunt and measure on the other side with the bandwidth specified by the probes?If not, do you have a suggestion for a budget isolated probe? I have no clue what I would need to look for in that case.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2021, 05:06:34 pm
content: Still, you may overestimate the "protection" lab supplies give.In case you drive the FETs wrong, or your current feedback circuit doesn't turn them off in time, the DC link capacitance in your converter (and even the output capacitance of the lab supply) is enough to blow up all the semiconductors. The lab supply current limit then only protects from fire - properly rated fuse on a battery does the same. Yeah, lab supply might be a bit quicker than the fuse, meaning it looks a bit less dramatic, but the end result is pretty much the same. Even the fuse is enough to protect the traces from lifting off the PCB.Hence, my suggestion of step-by-step verification. Because motor controllers require active current control anyway, start building and verifying that piece of circuit first. Overcurrent is easy to test by charging a capacitor to say 1V and connecting it directly over the current shunt resistor. Then verify on scope screen MOSFET gate drive going low in short enough time (maybe tens of microseconds; depending on what minimum output inductance you design for).Obvious solution to the grounding problem: make the shunts ground-side. The optimal and typical place in well designed controller is between ground and source leg of the lower MOSFET. High side shunt amplifiers are more expensive, or have worse CMRR in that place in circuit. In SELV BLDC controller you can pretty much probe all interesting voltages relative to ground. Using the math function of the scope to find Vgs of the top MOSFET is not optimal, but given budget limitations, gets the job done just fine. You are interested in the switch node voltage anyway.

author: NickEdison
author_rank: Contributor
timestamp: December 11, 2021, 05:40:07 pm
content: A step by step approach is indeed good practice, and I have a working current controller < 1, as this is indeed the first step. However, I have no way to properly test and verify it's behavior currently; hence the investment.About making the shunt ground-side; there could still be a small voltage drop over the ground plane. Wouldn't this be a problem if all grounds are not connected at the same place on the ground plane and how does connecting the ground to earth impact this?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2021, 07:21:09 pm
content: Just use solid ground plane. When measuring, connect the scope ground clip to the ground plane as close to the measured signal you can get; connect all in-use probe ground clips even when they are seemingly internally "connected". Slow (near-DC) ground plane currents cause small errors but these are irrelevant in motor controllers, you don't have such precision signals (trying to directly measure over the shunt using scope is futile anyway; you should just scope the output of sense amplifier). Fast (AC) ground plane currents follow the path below the signal.The ground clip wire is crappy anyway, acting as an antenna. But even cheap scope probes come with that tiny steel string, and if you accidentally lose it like I always do, you can make new (temporary, crappier) ones from cut off through hole resistor leads. Scrape off soldermask from the ground plane close to the measurement site. This is relevant when measuring small voltage ripple over capacitors.There are low cost tricks like that "lo-Z probe", google it.

author: NickEdison
author_rank: Contributor
timestamp: December 11, 2021, 07:40:31 pm
content: Clear, thanks a lot!I think to have seen a video on the "lo-Z probe" on the EEVBLOG. I will take a look when I need it.Anny thoughts on questions A and B?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 12, 2021, 07:42:12 am
content: "Noise" of lab PSUs is non-issue.Cheap no-name lab supplies are hit-or-miss, some are broken by design and fail in basics, some are just fine. I would just buy it and return if it breaks down or doesn't have a normally working CC mode.Output on/off feature is really good, I like it, but obviously you can live without by disconnecting the banana plug.

author: NickEdison
author_rank: Contributor
timestamp: December 12, 2021, 10:07:55 am
content: Disconnecting isn't the problem indeed, but I am mainly worried about reconnecting. If I would connect a component that should be current regulated, the PSU will start to drain the output capacitor before it begins regulating current.But I guess it can be omitted by setting the voltage to 0 and then change it to what I need.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 12, 2021, 10:37:44 am
content: This is a complex problem field where you just need to understand what you are doing and work around limitations.For example, even if you buy expensive lab supply, some are two-quadrant (able to sink current) some are not and it's impossible to say which is "better", it depends on what you are doing.Soft ON/OFF feature allows you to generate at least somewhat predictable voltage ramp, at very least without on/off bouncing. Mechanical switch / connecting the banana plugs sometimes adds contact chatter, sometimes it doesn't if you do it really quick.But really, I have been, in my own opinion, quite successful in tackling large enough part at a time (I realize I'm seemingly contradicting my above "take small steps" advice here). This means, design your circuit to accept normal off-the-shelf power supply or battery, and not rely on the characteristics of a specific type of lab supply. In practice, this means damping large low-ESR MLCC banks with elcaps, maybe even adding TVS in early phase of design, definitely use fuses, and so on. It saves time to do this from day one, and the question about lab supply becomes secondary; supplying enough current at adjustable voltage is enough. (I find surprisingly little use for current limiting (CC) when developing a BLDC controller! It has so much input capacitance anyway. Standard hickup protection mode of a laptop supply or similar pretty much offers the same protection.)

author: NickEdison
author_rank: Contributor
timestamp: December 12, 2021, 11:10:38 am
content: Makes sense, thanks again.ps, if someone else has another opinion, feel free to express it.