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Tensop 20th December 2003 08:20 PM

A simple question regarding inductors

I've been looking at a few designs involving inductors and have wondered.. what is the effect of having 2 inductors in parallel?

Does the total inductance double, or divide?

Does the current handling ability double or remain the same?



sreten 20th December 2003 08:48 PM

The current handling is obviously double.
The effective inductance halves.

:) sreten.

Tensop 20th December 2003 10:29 PM

danke :)

Tensop 21st December 2003 08:54 AM

Also, while im at it.. another Question..

as i mentioned im working on something involving inductors.. i might aswell shed a little detail on the subject..

i'm currently designing an SMPS. With inductors on the output as they simply dont work well without them(spikes, stress on the mosfets etc) the problem is although i can source inductors.. they are out of computer power supplies(generic chinese junk electronics) that i plan to use.

So i may of found a solution to the problem i face. Am i able to use 3 inductors(keeping in mind that i have compensated for the the inductance value change) in parallel to increase the current handling ability?

In the picture i have attached i've labled the direction of current. from 1(a diode) through to 2(inductors) and into 3(capacitors, and screw terminals.. eventually ending up at a set of amplifiers)

sreten 21st December 2003 01:30 PM

Yes, you can use 3 inductors, each one three times the nominal value, in parrallell.
The only reason to do this would be the inductor cores are too small for one to suffice.

:) sreten.

paulb 21st December 2003 02:49 PM


Originally posted by sreten
The current handling is obviously double.
The effective inductance halves.

:) sreten.

Assuming that the inductors are identical. Obviously.

Tensop 21st December 2003 08:06 PM


The original specification recommended 100uH here are the possible combinations.

1x 100uH for 5 amps = 100uH
2x 220uH for 10 amps = 110uH
3x 220uH for 15 amps = 73.3#uH
3x 470uH for 15 amps = 156.6#uH

It will be used on the output stage as a filter in an smps circuit, with a switching speed that is varied, and may be anywhere between 20khz and 400khz(depending on chosen speed)

Tony Salsich 21st December 2003 10:09 PM

Since neither of your two options generates a perfect solution, you may wish to trim the value of each inductor to yield a value that is closer to the ideal. If the coils are not impossibly glued to the cores (or carriers as well), then you can reduce the number of turns on each inductor by the square-root of the ratio of 300/470. In other words, reduce the number of turns to 80% of the original count. There are implications to this - the cores are big enough to accept 80% of the turns with ~25% heavier wire.

I assume that these are not gapped ferrite cores, but are powdered iron (distributed gap). They should be fine as post filter elements for a switcher. Can you give more detail on the switcher architecture?


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