SMPS Inductor 101
This paper will tell you all you need to know about inductors. All the math is there so i won't insist on it. However, what it does not tell you is how to actually measure them. Inductance and saturation flux density are the two important parameters.
For the first, the simplest method is a multimeter with inductance measurements. I have recently bought one too. A lot of people will tell you that inductance measurements with a multimeter are pretty much worthless in SMPS applications because the inductors operate with a degree of DC bias that the multimeter cannot apply. What they did not tell you is that there is a really simple circuit you can use with your multimeter, that applies bias to your inductor so you can measure its inductance in real-world conditions! And you can make it out of junk parts. If you have some dead PSUs lying around then you have the parts needed.
To measure maximum flux density (applicable mostly for ferrite, as iron cores have "soft saturation" and the limiting factor is core heating, not the inductance drop that occurs in saturation), you will need an oscilloscope. However, you should already have one if you're any serious about SMPS work. The beauty of it is that any old analog scope will do, while most circuits on the 'net say you need some math functions, or a scope with 2 channels - this couldn't be a bigger lie.
It goes like this - apply pulsed voltage of known peak voltage and period (this is what your scope will help you determine), to a number of turns on the inductor, via a 1 ohm resistor (or something close). If the resistor gets hot, then your core is saturating. Increase the number of turns till the resistor remains cool. The pulsed voltage source can simply be the secondary of a computer PSU transformer, just take 2 wires and solder them to the 12v winding of the transformer and there you have it. Then apply Faraday's Law and you get operating flux density. Saves a lot of time rewinding transformers and replacing blown switching transistors that would otherwise result from a saturating core. And yes i have a calculator for that... but you'll have to wait until tomorrow.
The circuits are soon to follow.
For the first, the simplest method is a multimeter with inductance measurements. I have recently bought one too. A lot of people will tell you that inductance measurements with a multimeter are pretty much worthless in SMPS applications because the inductors operate with a degree of DC bias that the multimeter cannot apply. What they did not tell you is that there is a really simple circuit you can use with your multimeter, that applies bias to your inductor so you can measure its inductance in real-world conditions! And you can make it out of junk parts. If you have some dead PSUs lying around then you have the parts needed.
To measure maximum flux density (applicable mostly for ferrite, as iron cores have "soft saturation" and the limiting factor is core heating, not the inductance drop that occurs in saturation), you will need an oscilloscope. However, you should already have one if you're any serious about SMPS work. The beauty of it is that any old analog scope will do, while most circuits on the 'net say you need some math functions, or a scope with 2 channels - this couldn't be a bigger lie.
It goes like this - apply pulsed voltage of known peak voltage and period (this is what your scope will help you determine), to a number of turns on the inductor, via a 1 ohm resistor (or something close). If the resistor gets hot, then your core is saturating. Increase the number of turns till the resistor remains cool. The pulsed voltage source can simply be the secondary of a computer PSU transformer, just take 2 wires and solder them to the 12v winding of the transformer and there you have it. Then apply Faraday's Law and you get operating flux density. Saves a lot of time rewinding transformers and replacing blown switching transistors that would otherwise result from a saturating core. And yes i have a calculator for that... but you'll have to wait until tomorrow.
The circuits are soon to follow.
