DC-DC buck converter noise on Replicape

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The Replicape has two DC-DC buck step down regulators, one for supplying the 5V to the BeagleBone (Black), a MCP16321 and one 12V so you can use 12V fans or LEDs for keeping your stuff cool. (Pun intended)

When both the Replicape and the Manga Screen were finished and I was finally able to test them together, an issue arose. The screen would suddenly cut out during boot because the power to the USB was cut by the BBB. Only by lowering the brightness significantly and adding an extra electrolytic capacitor across the 5V input was I able to make them work together. Why was this happening?

It turns out that Rev A3A has a transient load issue when powered through the Replicape board. This was traced to the 5V DC-DC step down regulator. But even with slow changes in load, for instance ramping up the brightness on the display, the display would suddenly be cut when the load was high.

The Beaglebone has a power management circuit for the USB, a TPS2051 which has an under voltage lockout of 2V. This is probably what kicked in when a combination of switching noise and a sharp transient occurred.

Switching noise issues are not the easiest to debug. It doesn’t help (much) to add lots of capacitance to the output of the converter, it simply will not help reduce noise in the 200MHz range.

One solution to this problem was to limit the slew rate of the N-MOS on the step down converter by adding a resistor in series with the boost capacitor. This does decrease the efficiency of the converter by something in the region of 1%, but that is a small price to pay for a working USB power.

Below is a picture of a Replicape with a slew rate limiting resistor and one without. It is clear that the switching noise has been dramatically reduced with the slew rate limiting resistor.
Heat management is an important on such a small board, but a 1% reduction in efficiency should be acceptable.

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A different solution would be to filter the noise by using an rc/lc filter or similar on the output, an RC snubber as they call it.

Sorry for the few updates lately, I’ve been in limbo trying to get ready for Maker Faire Rome. And the in the end I never did get the printer running in time for the event, haha! Still, it was a good show.

Ok, so this is the only hardware issue I am aware of, so hopefully CircuitCo will start production now.

Also, HipsterBot is dead! It’s now called Thing! You know, ‘cos it prints things? More updates in that later, I’m working on calibrating the printer now, so a few videos in a few days I’m sure!

7 thoughts on “DC-DC buck converter noise on Replicape

  1. Switching power supply design can be a bit of black-magic, particularly if you are unfamiliar with high-frequency RF design. Component selection and layout have a lot to do with getting a good working design. Look for parts without a lot of strays and try to keep the current loops small. Given your ‘scope traces, I’d say you either have a lot of stray capacitance in your inductor (or it’s surrounding PCB layout), or the output filter caps are not grounded well to where your FETs are sourcing/dumping current. Assuming you’ve got fairly decent components, you should review your layout.

    • Thanks a lot, Charles! I’ve got some experience with RF design, but not with switching PS designs. In the new design, I’ve increased the trace to and from the converter, added a few more ground vias, in addition to the C_boost resistor. Measuring across the output capacitor gives a whole lot less noise, so it might definitely be a ground loop issue. The main point is that it works good enough. Perhaps we will add some tweaks to later designs if it becomes an issue or if someone wants it CE-marked or something.

      • Great point about the probing! You absolutely have to have a good local ground to be able to determine anything about what you’re viewing on the ‘scope (I’ve been doing this so long, I sometimes forget to mention the basics). I typically measure directly across the capacitors, using the ‘stubby’ ground lead that comes with (good) probes.

        Also, for power-supply debugging (or anywhere you have good low-impedance signals that don’t mind a bit of extra load), you can use a 950 or 1K ohm resistor, some 50-ohm coax, and the 50-ohm termination in the ‘scope to make a “poor-man’s” 10:1 probe. It might look ugly, but you can easily get a probe with GHZ+ bandwidth for a few bucks! Here’s a couple examples of what I mean:
        http://www.sigcon.com/Pubs/straight/probes.htm
        http://koti.mbnet.fi/jahonen/Electronics/DIY%201k%20probe/

  2. Something else to try would be to put some low value (.1uF to .01uF) SMT ceramic caps across the supply ouput and close to sources of high frequency switching noise. The parasitic resistance and inductance in large caps like electrolytics makes them not very useful above a few hundred KHz. The ceramic caps, particularly surface mount have very low parasitic resistance and inductance, so they work well. Keeping them close to the source of the noise keeps the board inductance down.

    Good Luck,
    BobH

    • Yeah putting a couple of 22nf Caps on the output did the trick for me. On my one I have a built in 4port USB hub and it too was powering down when I connected the screen.

      Waiting for my scope to arrive, then I will check out how noisy this line really is.

  3. Pingback: KKT DC-DC 12V 24V 36V HRD Step Down DC 5V 3A Car Power Converter Regulator | INFO-BARRELS.COM

  4. Hi, had a quick lock at the layout. For the grounding of input and output capacitor and also the diode should have a close high current connection. Preferably the 3 connections should be the center point (as close to eachother as possible) of a star grounding.

    Best Regards
    Andreas

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