Electronic – Switching regulator efficiency TI LM2678 – measured vs. datasheet

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I'm working with a TI LM2678-ADJ regulator (granted, on a breadboard) to power a 40" led strip. Trying to figure out why the measured efficiency is about ~15% lower than what the charts in the datasheet claim.

Input: Vin = 20V, Iin = 0.925A (18.5W*)

Output: Vout = 12V, Iout = 1.255A (15.06W*)

*Didn't measure power directly, but using V x I to calculate power based on measured V and I.

So given those values it would seem the efficiency (Pout/Pin) is about 81.4%.

But the chart in the datasheet claims it should be about 95% efficient:
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For refecence here the schematic from the datasheet:
enter image description here

Could the ~15% discrepancy be attributed to the other components in the circuit? I'm not using exactly the recommended components from the datasheet but at least similar spec'd ones (from what I can tell).

This is the inductor I'm using: http://www.digikey.com/product-detail/en/2107-V-RC/M8852-ND/775391

Best Answer

Things that can likely go wrong: -

  • Building it on breadboard - contact resistance versus solder
  • Building it on breadbaord - contacts/track volt drops
  • The inductor gives no clue to how it will perform at 260 kHz - choose one that does
  • 33uH is probably not the best choice - 47uH is a better choice for 20V in and up to 2A out (assumed LM2678-12 and adjustable version)
  • Bad positioning of input and output capacitors
  • Bad choice of input and output capacitors (electrolytics are quite possibly a bad choice so follow the data sheet as to its recommendations - it has many)
  • Inductor leads too long
  • Measurement errors in meters
  • Measurement errors in probing
  • Schottky diode of correct type not used (DS says: From Table 10 a 5A or more Schottky diode must be selected)

Breadboard build is the worst culprit. Trust that it will work, lay out a PCB, come back here and get someone to review the layout. Don't build switchers on BB.

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