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# Choice of inductor for buck converter

## Choice of inductor for buck converter

(OP)
I am implementing a MPPT solar battery charger (buck converter). The type of converter design have been discussed in my previous post http://www.eng-tips.com/viewthread.cfm?qid=350101

I have finally decided to implement a Interleaved buck converter with 4 leg. With separate driver and mosfet pairs.

My Design Specs:

12V - 500 Watt Charger (That gives a max current of 41.67A)

Current per converter - 41.67A/4 = 10.42A(per converter)

Vinmax- 18V
Vinmin - 15V
Vout - 11V
Iout - 15A/leg (4 leg in parallel therefore 60A in total, well above the max current produced from panel to be safe)
Freq - 500KHz
Iripple - 1A

Substitute the values here http://schmidt-walter.eit.h-da.de/smps_e/abw_smps_... for inductance calculation

Inductance - 8.759x10-6 H

I have chosen the inductor http://in.mouser.com/ProductDetail/Coilcraft/SER29...

Questions:

1. Is my choice of inductor good?
2. Do i only have to worry about the max current in the inductor (Isat) or do i have to worry about the voltage across an inductor too?

Thanks
Regards
Gokkul

### RE: Choice of inductor for buck converter

Looks safe to me.
You may meet saturation, but that seems unlikely with your 18 V max in and 11 V out specification. You are running at 500 kHz, so your maximum voltsecond surface will be 14 microvoltseconds (at near 100% conduction). I don't find any data for absorbed voltseconds in the data sheet. But, since your operation is in "the lower left corner" in most of the diagrams, I wouldn't worry. If you see saturation, you can easily increase the switching frequency.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

### RE: Choice of inductor for buck converter

Isat is a bit close to the wind for me. Should be fine in steady-state, but during ramp up the current can run a little higher as the filter cap is charged, and the legs may not share perfectly anyway. A good control scheme will prevent this, but how confident are you in your control scheme? With a bit of heat in the product, the inductance fails away very quickly as current increases. During development I'd be tempted to go with the SER2918H until you gain some confidence. Then if everything checks out, you could substitute the SER2915H to increase efficiency.

Frequency response and Isat are the critical factors for this inductor, rather than voltage drop (since the first two determine the latter). With the caveat about Isat tolerance, the inductor looks like a good choice for the application.

### RE: Choice of inductor for buck converter

(OP)
Thanks for the feedback guys. Appreciate it.

#### Quote (LiteYear)

I'd be tempted to go with the SER2918H until you gain some confidence. Then if everything checks out, you could substitute the SER2915H to increase efficiency.

Will keep that in mind :).

Thanks
Regards
Gokkul

### RE: Choice of inductor for buck converter

(OP)
I came across this pdf in which there are detailed tables for different inductors and each table table has "Power Capacity @ 100 KHz" mentioned.

Does this mean each inductor can operate only at a specified power level?
If so how do i know if the inductor i have chosen can operate at my required power level.

PDF attached.

Thanks
Regards
Gokkul

### RE: Choice of inductor for buck converter

That's a strange way to rate an inductor. I guess they're just trying to simplify things for the end user, provided the end user is building a bog standard switch mode power supply (with switching f = 100kHz). It doesn't mean it will only operate at that level. It just means it (probably) wont burn up or lose saturation if used in a switch mode power supply switching at 100kHz and with that output power (or less).

You know yours is okay because you've done the calculation yourself, and satisfied the Isat limits and frequency response.

### RE: Choice of inductor for buck converter

(OP)

#### Quote (LiteYear)

You know yours is okay because you've done the calculation yourself, and satisfied the Isat limits and frequency response.

That is reassuring.

Thanks for the response.

Thanks
Regards
Gokkul

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