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I'm looking for some input on a power supply problem...

I'm using a Mean Well LPV-60-36 power supply (output of 60 watts @ 36 volts, rated for 1.67A RMS) to power a DM542T stepper motor driver that's driving a NEMA23 stepper motor (rated up to 4.2A). The power supply is nominally marketed as a constant voltage "LED power supply", but my use case should be within its specs, and I need the IP67 rating to be used outside in Seattle's late Fall weather (it's for a Halloween prop).

As a baseline, this whole setup works fine when I'm running it off of my "cheap" switch-mode benchtop power supply @ 30 volts.

I'm facing a problem where the power supply seems to be triggering one of its overload protection mechanisms, and it shuts itself off until I power cycle with a ~60 second off time. The trigger is definitely instant velocity changes of around 20% or more (either positive or negative acceleration). The stepper motor driver has configurable current settings, ranging from 0.7A to 3.2A RMS. The problem seems to be more prominent when I've configured the driver to run at lower currents, and starts to go away as I run at higher currents near the power supply's rated output current limit.

I've put a 1000uF electrolytic capacitor (it's really 2 caps in series, since they're only rated for 35V each) on the motor driver's input / power supply output. This maybe helped just a bit, but the problem is still present.

I first suspected that a surge current was tripping the OCP, but I'm not able to measure it on any of my equipment. Using a multimeter in series in the circuit to measure amperage, the "max hold" mode never sees the current spike above what it was measuring just prior to the overload protection tripping - just under 0.7A, while the power supply is rated for 1.67A, so there should be plenty of headroom. Similarly, an AC current clamp meter on the power supply's input also doesn't measure a spike in input current. This doesn't seem to be over current protection.

I then measured the output voltage of the supply with my oscilloscope, and there's a fairly large ~1.6V peak-to-peak ripple when running the motor at higher speeds, but that doesn't seem to change when the overload protection is tripped. That's a much higher voltage ripple than what the power supply datasheet specifies, 150mV peak-to-peak, which is suspicious.

Is a "LED power supply" nominally different than a "standard" power supply? Maybe the power supply is tuned for the load characteristics of LEDs, but the load characteristics of the stepper motor driver is just confusing its overload protection?

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  • \$\begingroup\$ Note: I'm aware that some LED power supplies are constant current, not constant voltage. But the power supply that I'm using is indeed constant voltage. More on this at electronics.stackexchange.com/questions/283861/… \$\endgroup\$ Oct 20, 2023 at 23:22
  • \$\begingroup\$ For reference,.for discussion, From Meanwell Datasheet: #1. "Suitable for LED related fixture or appliance (such as LED Decoration or Advertisement devices) (Note. 11)" #2. "Note 11: This product is not intended for LED lighting luminaire applications in the EU. (In the EU the LPF/NPF/XLG series are recommended.)" #3. "Constant voltage design" #4. "Protections: Short circuit / Over load / Over voltage" #5. "Fully encapsulated with IP67 level (Note. 7)" #6. Note 7: "Suitable for indoor use or outdoor use without direct sunlight exposure. Please avoid immerse in the water over 30 minute." \$\endgroup\$ Oct 21, 2023 at 0:12
  • \$\begingroup\$ The datasheet does not call this an "LED power supply". To me, what they are saying is that people use this power supply to feed their LED drivers. You still need current limiting for your LED's, either a current-limiting resistor, or a constant current LED driver. \$\endgroup\$ Oct 21, 2023 at 0:20

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The act of posting this question made me think of a different testing method with the oscilloscope, which led me to the answer.

  • Blue (channel 3) is the voltage of the power supply output, measured with DC coupling.
  • Yellow (channel 1) is the same voltage of the power supply output, but measured with AC coupling.
  • Both probes are attached near the power load (the stepper motor driver), and have ~8 inches of 16AWG wire between them and the power supply.
  • I set a trigger on the Blue line (DC voltage) falling below 24 volts.

enter image description here

Since the voltage is rising to over 53 volts, 17 volts higher than the nominal voltage of the power supply, I would expect the power supply's over voltage protection to trip.

My theory of what's happening is the stepper motor simply can't accelerate as fast as the steps are telling it to, and the stepper motor driver probably goes into some kind of overload condition. This quickly and significantly decreases the amount of current being pulled by the stepper motor driver, which results in one of the following scenarios:

  • The switching mechanism within the power supply paired with the output inductor in the power supply don't react fast enough to the sudden drop in the load's current.
  • The inductance in the wires from the power supply result in a large spike in voltage, which reflects back to the power supply.

In either case, it's easy to see how the power supply's over voltage protection would be triggered.

Since driving the stepper motor with a higher current would increase the maximum amount of acceleration it could sustain, it makes sense that increasing the drive current makes this problem less prominent.

Now I just need to figure out how to avoid this from happening. Maybe a 36-40V Zener diode would prevent the power supply OVP from triggering. But I probably just need to avoid sudden accelerations of the stepper motor in the first place...

Update:

I confirmed that the power supply was not the problem after all. The voltage spike must be coming from the stepper motor controller.

I confirmed this by attaching the same power supply to an electronic load. When I switched the load from 1.6A (near the full rated current of the power supply) to 0 amps, the voltage only briefly rose above 400mV.

The stepper motor is now reaching speeds nearly 4x faster than before, without tripping overload protection of the power supply. Here's what I changed:

  • I implemented acceleration on my controller board. I increase/decrease velocity once every 1ms at a configured maximum acceleration until the target velocity is reached. This had the largest impact on performance.
  • I added a large capacitor bank and a voltage clipping circuit to the stepper motor. This had a small impact on operating the motor at high speeds and high acceleration, and mostly serves as a reliability buffer.

The capacitor bank and voltage clipping circuit that I made is shown below. I'm a software engineer, and EE is just a self-taught hobby, so there's likely a better way of doing this.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ The inductance of the wires does not cause that spike, the timescale is orders of magnitude too long for that. It’s probably your first hypothesis - the power supply is slow to react to a load dump, probably because it’s meant for LEDs (a load which remains constant over time, or varies slowly if an in-line dimmer is used) \$\endgroup\$
    – jms
    Oct 21, 2023 at 0:43
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    \$\begingroup\$ Using the scope to measure the load current would give you a better idea what is going on. \$\endgroup\$ Oct 21, 2023 at 0:50
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    \$\begingroup\$ Or the motor controller is braking and this increases supply voltage. You could check the direction of current: it should go out of the supply, not into it. For this you need to insert a current sense resistor in series and probe it with your scope. \$\endgroup\$
    – bobflux
    Oct 21, 2023 at 0:51
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    \$\begingroup\$ This transient suggests that your reasonable attempt at applying capacitance to the stepper driver was ineffective...either faulty capacitors with too-high ESR, or not enough capacitance. \$\endgroup\$
    – glen_geek
    Oct 21, 2023 at 1:51

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