# How do inductor specifications impact their effectiveness in a boost converter circuit?

I've been trying to understand how to select an adequate inductor for a boost converter circuit. I'm a hobbyist so I'm not always sure what to look for, so I haven't been able to find something that helps me understand holistically wether or not an inductor is adequate for a given step-up DC/DC converter circuit.

My question is what are the relevant specifications for an inductor, how does changing each one impact it's capability in the converter circuit, and how do you figure out what makes the inductor inadequate?

## Examples

### Inductance

The inductance seems to "set" the ripple current. The higher the inductance, the lower the ripple current. This appears to be desirable. If the datasheet recommends a 1uH inductor, can I substitute a 2uH inductor? What about 470uH? How do I know how much ripple current is acceptable?

### Maximum DC Current

The DC current rating seems like it's related to how much current I can draw on the supply side. If I think I'm going to pull 3A from the power supply, can I use any inductor, ceteris paribus, with a maximum DC current greater than 3A (plus some amount of derating)?

### Maximum DC Resistance

The maximum DC resistance seems like smaller is better, but how do I know how high the DCR can be before it negatively impacts the converter circuit or battery life?

### Self Resonant Frequency

The self resonant frequency has to be greater than the frequency of my switching circuit right? But, can I pick just about SRF that's greater than the switching frequency (plus some margin)?

Are there other factors that impact the inductor selection that I didn't consider (other than price and footprint . . . which is why I'm asking the question in the first place)?

# This turned out to be a pretty incredible resource

http://www.ti.com/lit/an/slva372c/slva372c.pdf

• This is a very broad question. A short answer would be: it depends. All these variables are of influence on the total design and it performance. It is the designer of the converter who makes the choices which are a result of balancing certain properties against each other. Feb 10, 2019 at 16:36
• You also have to make sure that the inductor current does not exceed the saturation current limit even in the worst case. Max DC current may be contolled by power dissipation limit. There may be a saturation current limit which could be higher (or even lower, but that is unlikely) than the max DC current. Feb 10, 2019 at 16:49
• For a lot of small pre-compensated DC-DC converters, the datasheet says to use a specific inductor and capacitor value for the output filter. If that is the case, just do what they say. It is compensated with those values in mind, and changing it will possibly create stability problems for you. If it is not compensated internally, you can possibly use a wide range of inductors depending on your load particulars. But you have to compensate it correctly. Usually max ripple current will be 50% or 40% of the full-load output current. Less is OK, too if you don't mind a physically large inductor. Feb 10, 2019 at 17:04
• Remember that the saturation current limit must be less than the peak current that goes through the inductor. If you have ripple current larger than 50%, you will be forced to choose an inductor with a higher and higher saturation current limit. It just gets ridiculous at some point. So you don't want to let the ripple current increase too much. Feb 10, 2019 at 17:17
• The DC limit is a thermal limit. RMS current will need to be less than the DC limit to avoid over-heating. Saturation limit applies to the peak current during switching. Basically, DC limit is the max output load limit. Max load + ripple must be less than the saturation limit of the inductor. Feb 10, 2019 at 17:31

If the datasheet recommends a 1uH inductor, can I substitute a 2uH inductor?

You can but you'd get a lower ripple current or the converter might not work as well as it would with a 1 uH inductor.

How do I know how much ripple current is acceptable?

That depends on your load and output capacitor. The ripple current causes a voltage ripple (through the output capacitor's series resistance, ESR) on the output voltage. That will be the supply ripple for the circuit operating on that voltage. Some circuits are very sensitive to supply ripple, others ignore it completely.

.. pull 3A from the power supply, can I use any inductor,.. maximum DC current greater than 3A

No, you need an inductor which can handle more than the expected peak current through the inductor. The 3 A delivered by the converter is the average current. The current through the inductor could be a lot higher. Especially when the a small input voltage needs to be converterted into a high output voltage. Then the inductor's charging cycle is long but the discharge cycle is very short.

but how do I know how high the DC Resistance can be before it negatively impacts the converter circuit or battery life?

You do system calculations and then you know.

Self Resonant Frequency

You don't "pick" a SRF, you just "don't pick" inductors that have a SRF which are too close to your switching frequency.

As commented, this is quite broad. Typically as a hobbyist you don't need to worry too much about all this. In general you would just buy a boost converter module that fits your needs or use an IC that can do what you need. Then just follow the chip manufacturer's datasheet. If you do what is suggested in the datasheet you will also get the performance as listed in the datasheet.

• Wouldn't a higher inductance value actually produce less ripple? After all, di/dt=V/L. As mentioned in one of the comments, it does affect the compensation though.
– Big6
Feb 10, 2019 at 17:16
• Yep, typo, fixed it Feb 10, 2019 at 20:05