SMPS Inductor oversized vs properly sized


I am working on an SMPS for boosting an input voltage to drive a single string of LED's. The input voltage range is from 18 to 32 volts. The output voltage needs to be about 60V and the drive current is around 0.760 Amps. I am working on using the HV9912 as the driver chip. The frequency is set to approximately 200KHz.

The question I have is this:
I have calculated a required inductor rating of 90uH, what happens if I use something significantly larger(as in 2 or 3 times as large)? Will the duty cycle be the only thing affected? What effect does this have on the inductor current?

Best Answer

The inductance is merely a requirement of the switched frequency. Also a higher inductance usually means higher package/device size for a given current rating. What you should be worried about is not the Henries, rather the actual current carrying capacity and saturation point of the inductor used.

Your boost converter's inductor should have a current rating (and saturation point) easily 1.5x times more than your intended output current. Ripple currents for boost converters can be quite high, so make sure your output and input capacitors have suitable ripple current ratings and low-ish ESR (their ability to handle ripple current, they get hot due to ESR).

Google around for some application notes on SMPS boost converter design, they will cover the important aspects for selecting and sizing inductors appropriately. I know that Microshop and Texas Instruments both have good ones i've seen before. here is one from TI on basic calculations for a boost converter's power stage.

Also note that having inductances x3 more than required can very heavily damp the start-up current and settling time of the SMPS control system