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What is the difference between a coupled inductor and an actual transformer? The LT3757 datasheet mentions that I need a coupled inductor for the SEPIC to work

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You don't have to use coupled inductors in a SEPIC configuration. The main advantage is that you can get by with lower inductance windings. If the two inductors are closely coupled, the ripple current is divided between them and the required inductance is halved. So if you want to replace a coupled inductor which has two 3.3 uH windings, you would need two individual inductors of 6.6 uH each at the same current rating.

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  • \$\begingroup\$ +1 because this explains well what is said in the data sheet. Thanks. \$\endgroup\$ – zebonaut Nov 6 '10 at 23:19
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I am no absolute expert on SEPIC converters, but as far as I can tell from the data sheet, individual inductors should work: "As shown in Figure 1, the SEPIC converter contains two inductors: L1 and L2. L1 and L2 can be independent, but can also be wound on the same core, since identical voltages are applied to L1 and L2 throughout the switching cycle." A coupled inductor may have been suggested because it uses less board space: "By making L1 = L2, and winding them on the same core, the value of inductance in the preceding equation is replaced by 2L..."

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I think coupled inductor is marketing techno-term. I fail to see difference between 100% coupled inductors and perfect transformer. and.. Look, you can buy whole 2 inductors for price of one transformer.

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  • \$\begingroup\$ So it's a massive scam by Big Transformer Co and Big Science?? what? \$\endgroup\$ – Thomas O Nov 7 '10 at 10:29
  • \$\begingroup\$ I mean the answer is "no difference". \$\endgroup\$ – user924 Nov 7 '10 at 14:07
  • \$\begingroup\$ Isn't the difference that the transformer can have a different number of turns on each side and is thus capable of stepping down voltages? I'm not sure coupled inductors work this way. If you're only interested in 1:1 then I think I agree they're the same. \$\endgroup\$ – AngryEE Nov 8 '10 at 14:08
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    \$\begingroup\$ No. Two inductances with different values and loose coupling will still be a transformer with some N:M ratio. The model of non-ideal transformer (goodle for "wiki leakage inductance") covers all cases between decoupled inductances to ideal transformer with everything in between. \$\endgroup\$ – user924 Nov 10 '10 at 2:05
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The inductor stores energy as magnetic flux, the transformer just transfers energy magnetically.

.. as far as I know anyway:)

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  • \$\begingroup\$ They still store it. \$\endgroup\$ – Kortuk Nov 6 '10 at 13:29
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    \$\begingroup\$ Much like a flyback 'transformer' is actually a coupled inductor, it's more a reference to how the device is used (energy storage and transfer during different cycles, vs. energy transfer during on-time) AFAIK \$\endgroup\$ – Adam Lawrence Nov 8 '10 at 2:32

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