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Hi. I'm Mark Davis Marsh, and I'm going to be talking about inductor selection for your different applications, specifically we're going to be talking about inductive selection for a buck regulator. This is part of switching power supply component selection.

So what is the role of a inductor and a switch mode power supply? It's basically used as a filter for your switching currents. So if we look at a buck converter and we go into the on time of the high side MOSFET, you can see that basically the current is going to be traveling through the inductor and return through the output capacitor.

So what does this do? This supplies a voltage across the inductor, and that voltage is VG minus Vout, where VG is the input voltage and Vout is the output voltage. And if we apply that into our inductor equation, this basically implies that you'll get a ramp rate of your current VG minus Vout over your inductance of your current. So your current will be ramping up at this rate.

If you then turn off the high side FET and you turn on the low side FET, you will basically be taking away the input voltage of VG. And that terminal is now being shorted through the MOSFET. So the voltage applied across your inductor is now a negative output voltage. And so if you work through the equation again, you can see now that your inductor current is now ramping down at a rate of minus Vout over your inductor. And basically what you're doing is you are creating a triangular current that's proportional to the square wave that you applied from your input MOSFET.

So your goal for the inductor selection is to design the inductance such that ripple current that you are generating is low enough that your output voltage ripple is acceptable and also so that you don't exceed your current limits of your devices. So here we have a picture of that triangular current. This is at the bottom of your screen. And you can see how the current is ramping up during the on time of the high side MOSFET and ramping down during the on time of the low side MOSFET. And what you need to do is to basically derive an equation for the magnitude of this ripple, and that's the top equation that you see.

Basically it is calling delta IL half of your ripple current. So two times delta IL is your entire ripple current, and this is equal to the VG minus V over L times basically DTS, which is the on time of your high side MOSFET. And what you can do from this is you can calculate the inductor based on the amount of ripple that you would like to see during design.

So if we look at a typical buck converter design, you have several sets of specs that you need to meet. Let's say that you have an input voltage that's around 5 volts, you have an output voltage around 1.8 volt. Your switching frequency is nominally 2 megahertz but can range from 1.6 to 2.6 megahertz. And you have a current limit on that part that has a minimum of 830 milliamps.

There's a couple of things that you're going to need to look at. First, generally for most designs, you allow your ripple current to be in the 30% range. The 30% range has been determined by lots of people to give you a nice balance of the amount of ripple current versus losses in your design.

So if we use 30% as a base benchmark for our total ripple, or 40%, depending on how lose we want to spec, then you will see that the inductor current ripple will rise 20% above your output current. So you need to make sure that that 20% above your output current is still below the lower current limit of the part that you're using. And here you can basically work through the equations, where you keep your output current plus the ripple added from the inductor current. And you keep that below your current limit, and that will give you the max of current that you are allowed to use in this design.

So basically with that current limit of that part minus 1/2 the ripple current, that gives you the maximum allowed output current. So you can work through the equations. And you can basically determine your inductive value that you need for this to give you the required output current that you need for that design.

Then you'll go to one of the capacitor vendors, and you'll see their spreadsheet, which will look something like this. And on the left you have lots of different inductive values. And all across the top you basically have several different inductor sizes and types of inductors. And you'll look through for an inductor that has the inductance that you wanted.

You also look for an inductor that has a saturation current that is well above the current limit of the device that you're looking at. And you will also look for a device that has DCR that's low enough that you'll get the efficiency that you require for your design. So if you look at the curve on the bottom, we took several different inductors that we selected out of the previous slide, and we compared their results. And you can see that we have different efficiencies for each of these.

So when you're doing your design, realize that the size of the inductor, the amount of inductance, is also going to affect efficiency. And this will be part of your design requirements, your design criteria, for selecting the final inductor choice. So you've picked the inductance that you think will work for your design, what happens when one of the parameters of the inductor you chose is not really where you want it to be?

So you had a three or four microhenry inductor. But maybe it saturates early, or maybe you have limitations and you can only pick at 1.5 microhenry. What does this cause? Basically, if you pick a smaller inductor, you're going to get higher ripple current. This is going to increase your losses in the design.

The higher ripple current will translate to higher losses. It's also going to give you a lower average output current that you can meet. And so you're basically you'll be hitting that current limit earlier, and you'll have a higher ripple current. So the output current that you can provide is lower.

So what else can happen if the inductor's not quite up to snuff? If you have an inductor that has a nice saturation or an Irms that is lower than what you need, if the Isaturation for that inductor is much lower than the current limit of the device, what you can have happen is as the output low current increases and it passes the saturation current, the inductor is no longer an inductor. And you'll now get a very high ripple current, and this can cause lots of problems, including destroying the device.

If you're RMS current rating is too low, that usually means that you'll get a lot more DC resistive losses. So look for an RMS current that is higher than the load curve that you think that you'll be using and an I saturation current that's higher than the current limit of the device. This has been inductor selection. Thank you for watching.