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I used TI's workbench to design a simple 24 to 12V @ 6A. I have it built in a PCB board, but I started doing some power measurements and notice the output voltage ripple is quite high at loads bigger than 1A. According to the WB design, ripple should be around 100mV p-p, but I'm seeing ripples as high as 700mV p-p. Can anyone help?

This is my schematic that I got from TI"s WB:

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This is my PCB Layout

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And this is what I am seeing on the scope:

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  • \$\begingroup\$ How are you doing the measurement ? What is your probe ? Is your input power supply the same as the original one or do you use a random power supply ? \$\endgroup\$ – zeqL Dec 5 '13 at 23:00
  • \$\begingroup\$ Could be a problem with the output RLC. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 5 '13 at 23:47
  • \$\begingroup\$ zeqL: My input power supply is a Bench Power Supply made by B&K Precision, I am also using an Electronic Load by the same company. \$\endgroup\$ – Gustavo Corona Dec 6 '13 at 0:00
  • \$\begingroup\$ Those high peaks are not ripple voltage - they are likely to be pick-up as said in AF's answer \$\endgroup\$ – Andy aka Dec 6 '13 at 0:10
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Your output ripple will be a triangular voltage waveform at the converter switching frequency; the triangular current waveform given to you by the output inductor superimposed onto the output capacitor ESR. What you have on your scope is huge CM (common-mode) pickup which is swamping the actual ripple.

  1. Turn your scope bandwidth limiter on. Your true ripple will be orders of magnitude lower than 20 MHz.

  2. Measure directly across your output capacitors with a short probe to minimize pickup.

  3. Use your scope cursors to ignore the junk at the switching transitions and just measure the triangular part of the waveform.

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  • \$\begingroup\$ Thanks for the feedback. I will try this tomorrow at the lab. \$\endgroup\$ – Gustavo Corona Dec 6 '13 at 1:54
  • \$\begingroup\$ +1 because it looks like you used the special scope probe grounding wire instead of an ordinary paperclip. \$\endgroup\$ – Phil Frost Dec 6 '13 at 3:46
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    \$\begingroup\$ This was it. I redid my measurement and ripply is now what I was expecting. \$\endgroup\$ – Gustavo Corona Dec 6 '13 at 13:59
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It's possible that the problem is in your measurement setup and not the converter. Switching converters can generate magnetic fields that the scope probes can pick up. Try removing the standard scope probe ground lead and "witch's cap" clip and instead using a small piece of bare wire wire wrapped around the ground sleeve. Also use an X1 probe instead of an X10 probe for a lower impedance (1 M instead of 10 M). If you are still seeing transients like this, try using a piece of coax cable with the scope in 50 ohm mode. It's much harder for magnetic fields to influence a 50 ohm load than a 1 M ohm load.

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