dorotaCMay 27

More #electronics questions.

The MCP1623 boost converter.

They recommend a 4.7µH inductor, and they support boosting from 3.4V to 5V. At low currents, the efficiency can drop to 0.6.

TI Application Note SLVA372D gives inductor ripple current as T*Vin*D/L. That works out to

> 3.4V * 0.6 / (270kHz * 4.7µH)
680/423, approx. 1.607565 ampere (current)

which is WAY above the converter's switching current.

And yet, it works.

What is wrong with my calculations?

#boost #smps

Show threadMalin DixonMay 27
@dcz The switching frequency of the MCP1623 is typically 500 kHz.
When boosting from 3.4V to 5V with a 4.7µH inductor, the current will increase at 3.4/4.7 = 0.72 A/µs and will fall at (5-3.4)/4.7 = 0.34 A/µs, so it will increase about twice as fast as it decreases, so it will be turned on for about 1/3 of the time, or around 0.7µs in a 2µs cycle of 500 kHz
In 0.7µs the current increases by around 0.5 A, so I would estimate the ripple at about half of that.
Show threaddorotaC

@diver300 What makes the equation I used be so off?

I realize I could pore down the numbers and figure it out but this is a distraction project and I'm after quick pointers. Will come back to it later :P

May 27 at 8:43amWeb
Show threadMalin DixonMay 27
@dcz I probably won't go into it in detail but problem appears to be the frequency. The period at the 270 kHz you used is nearly 4 µs which is very much more than the 0.7 µs which I estimated, so the current in the inductor would have a lot more time to increase if the frequency were 270 kHz