Looking for help, confused about terminology of Power Inductors.

[Zaprooder]

[SOLVED] Looking for help, confused about terminology of Power Inductors.

Hi All

This is my first post and a cry for help. I'm relatively new to the electronics scene and although I have made some quite complex circuits I have not yet touched inductors. The circuit I want to make will be a switching power supply for high current loads in the region of 0.2 to 4.0 ohms. The power supply will be variable (using PWM duty cycle) and will usually be in the region of about 3.5 to 4.5 volts although it is possible for the voltage to be as high as 10 (but not with a load of 0.2 ohms obviously). The PWM will be from a 12V(ish) source (typically a 3S LiPo pack). In order to smooth the outputted PWM (100KHz) generated by a micro controller (switching mosfets), It seems that using the diode/inductor/capacitor approach would probably work.
**broken link removed**
Size is a very important factor, the finished product has to be as small as possible, so I searched for SMD components. I found that inductors that can handle this kind of current and be a reasonable high inductance (100-300uH) as suggested by many circuit diagram examples I saw are non existent. The highest inductance I could find with a decent current capcity was this one.
http://uk.farnell.com/coilcraft/ser2915l-333kl/inductor-pwr-33uh-30a-0-1-7mhz/dp/2288911
And after doing some simulations in Proteus it seems it could work. I get ripple of about +-0.2 volts, but that's manageable.

So that's basically what I want to do, but, I came across something in the description of the Inductor.
RMS Current (Irms): 30A
Saturation Current (Isat): 3A
I really don't know what this means. Of course I did some googling but I simply cannot understand what anyone is going on about, goes right over my head. Is there an easy to understand description of what these Current limits mean and what I should be looking for when selecting the inductor? I think I understand what RMS is, Root Mean Square, which appears to be a way of calculating an average of wave keeping power constant ?? But the saturation current has lost me. Would anyone please point me to what I should be looking for in an inductor for this application? Thank you.

 

[mtwieg]

Isat refers to the saturation current. Power inductors are typically made out of ferrite or powdered iron cores, which increases their inductance. However this only works with currents up to a certain point. Basically above a certain current called Isat, the effective inductance of the inductor will start to decrease significantly (by some specified amount, like 10% or 20% in that datasheet), so you would see ripple current increase when the average current is above Isat.

The Irms current rating is roughly the RMS current that the inductor can tolerate without suffering damage. Often Irms is larger than Isat, sometimes it's smaller. Exceeding Isat without exceeding Irms won't hurt the inductor, but the inductor will behave nonlinearly. To simplify things you should aim to keep your current below both Isat and Irms.

If you have trouble meeting your size requirement, consider higher switching frequencies, and possibly interleaving multiple phases.

 

[Zaprooder]

Thank you. That basically boiled down a 3 page essay I was reading on another site

I think this one may do the trick
https://uk.farnell.com/vishay/ihlp8787mzer750m5a/inductor-shld-75uh-20-12a-smd/dp/2419058
It'll limit me to 12A but I can live with that. At least I know what I'm buying now.

Thanks again.