8
\$\begingroup\$

The inductor in question is part L3, a power inductor with both coils in parallel. Datasheet.

The complete circuit is the power supply for an RFID reader, where +5V_A is the output for the antenna drivers and +5V for more general purpose stuff.

The question: Why use a "power" inductor in parallel instead of just a "normal" inductor with 22uH @ 100kHz as the datasheet says for this one in parallel configuration?

enter image description here

\$\endgroup\$
  • 4
    \$\begingroup\$ Could be a part used in other things the company makes, rather than adding a new line to the BOM \$\endgroup\$ – Colin Aug 16 at 12:56
13
\$\begingroup\$

It's a "power" inductor because it needs to carry a significant amount of DC current. Connecting the windings in parallel gives you the same inductance as either winding alone, but with twice the current capacity and half the DC resistance. It's exactly equivalent to a simple coil with the same number of turns, but twice the cross-sectional area of the wire.

\$\endgroup\$
  • \$\begingroup\$ Would it really need to carry significant current? If it's just an RFID reader? This is definitely not my area so am genuinely curious. I thought most would be below 100-200mA? \$\endgroup\$ – MCG Aug 16 at 13:43
  • 2
    \$\begingroup\$ 100-200 mA IS significant current for an inductor. Most "small signal" inductors of that value will have saturation currents much lower than that. \$\endgroup\$ – Dave Tweed Aug 16 at 13:45
  • 3
    \$\begingroup\$ You only get twice the current capability if the core doesn’t saturate and dramatically reduce its inductance. \$\endgroup\$ – Andy aka Aug 16 at 17:00
  • 1
    \$\begingroup\$ Doesn't two inductors in parallel give half the inductance, not same inductance? AFAIK paralleling inductors works similarly to paralleling resistors. \$\endgroup\$ – jpa Aug 17 at 5:50
  • 9
    \$\begingroup\$ @jpa: What you say is true ONLY if the two inductors do NOT share any magnetic field. When you have two windings on the same core, it's a completely different story. \$\endgroup\$ – Dave Tweed Aug 17 at 10:13
10
\$\begingroup\$

Unless the designer is here, there is no way to really know for certain. Perhaps it was a cheaper option. Maybe it had the best performance under tests conditions compared with other inductors?

If the company that designed this already use that component in another design, it makes sense to re-use it on another product. It saves having to buy a new component, and could reduce the price of the current one if they then have to purchase a higher quantity.

In some products I have designed I have used inductors for many different reasons. Some of them the reason I mentioned above. We already stocked that part and it happened to be the same value as I needed, so I used that.

Sometimes I just follow the datasheets of certain ICs and use recommended parts. This could have been any one of my suggestions above, or possibly something else.

Other time, I have just used a cheap one where things aren't performance critical. Other times I have needed performance so have tested the device with different parts and chose the one that worked best. I don;t think there will be a definitive answer unless the designer happens to come by. But the reasons I suggested are quite common.

\$\endgroup\$
  • \$\begingroup\$ Thanks for illuminating the engineering standpoint of it! \$\endgroup\$ – Horst Aug 20 at 8:57
8
\$\begingroup\$

Look into the Bourns SRF1260 datasheet: Multiple applications: parallel, series, dual-inductor and transformer.

Clearly the designer opted for parallel connection, thus doubling the current rating.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.