Author Topic: Selecting an equivalent Inductor  (Read 908 times)

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Offline solexious

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Selecting an equivalent Inductor
« on: March 14, 2016, 01:40:27 pm »
Hey all,

I'm currently setting up a boost led driver for the first time (part of a project that runs 3w rgb leds from li-ion batteries controlled by esp8266's). As its a boost driver it needs an inductor. The example I'm using (page 30, the 3 x 350ma 3.6v leds http://www.farnell.com/datasheets/1882103.pdf) uses this inductor: http://uk.farnell.com/coilcraft/do1608c-103mlb/inductor-pwr-10uh-1-5a-20-35mhz/dp/2287199

That inductor costs almost double the driver, so I went looking for a cheaper alternative. The 3 things that seem important are the inductance, dc resistance and rms. (I think?) So following that I found this inductor: http://uk.farnell.com/panasonic-electronic-components/ell6uh100m/choke-power-6x6x5mm-10uh/dp/1717483.

It seems to be a good replacement to me, but not having dabbled before I'm not sure.

Does that look like it will fit the bill? Am I missing something?

Cheers

Sol

p.s. alternative recommendations welcome, but will need to be available in the uk to be useful
 

Offline SebG

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Re: Selecting an equivalent Inductor
« Reply #1 on: March 14, 2016, 06:20:36 pm »
Hey.

Even if the data sheet recommends the DO1608C inductor you don't need to use that one.  What matters is that you match the specs like inductance, resistance and current rating. If one device has lower resistance then your circuit will be more efficient but that value shouldn't be too critical. Mainly it's the inductance and the current rating.  Other parameters can slightly improve the efficiency of the circuit but it could also drive the cost up.

However current rating comes in two flavors: RMS and Saturation and they are usually not the same. RMS current means there is some defined temperature rise at that current whereas Saturation current means that the inductance is reduced at those currents.  In the case of the ELL6US100m the Rated current of 1.8A means 45 degrees Celsius rise above ambient and 70% the rated inductance. So it would behave as a 7 uH inductor at 1.8A.  (I think i got that correctly since the translation in the datasheet is not accurate) The DO1608c part specifies ISat as 1.1A with a 10% drop in inductance (9 uH at 1.1A) and 20 degrees C rise in temperature and Irms as 1.5 A with a 40 degree C rise.  They are not easily comparable so ensure that the current in the inductor (i.e. the peek current) in your circuit doesn't reach saturation of any device you choose otherwise the efficiency will suffer.
Sebastian
 

Offline solexious

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Re: Selecting an equivalent Inductor
« Reply #2 on: March 15, 2016, 03:47:18 am »
Thanks for the reply.

Okie doke, that makes sense to me, thank you for pointing out the RMS and Saturation differences. On that note, I'm not sure how to workout how many amps the inductor will put put under with my leds. I'm assuming its not the 350ma that I will be drawing on the led side with my 3.6v 350ma string of 3 leds in series. How can I work that out so I can check where on the RMS/Saturation scale I will sit?

Cheers

Charles
 

Offline SebG

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Re: Selecting an equivalent Inductor
« Reply #3 on: March 17, 2016, 11:28:44 am »
The regulator device datasheet says that it's switch is rated for a switching current of 2.8A so that would be your maximum inductor current. Another way to look at this is how much power you are delivering to the LEDs. So if each led is 3.6V times 3 LEDs at 350mA is 3.78W. The regulator is say only 85% efficient so you need about 4.4W. Since you are powering it from a lithium battery at 3.4V then you'll need an RMS current of 4.4W/3.4V = 1.3A.  That RMS current can be a good ball park of the inductor's current rating.  So between 2.8A and 1.3A should be a good place. 
Sebastian
 


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