Some general guidance comments...

 The slower the clock, the higher the indutance needed to get to where
you want to get,=20
 as you have already observed.  However, if you increase the operating
frequency, you can=20
 use lower inductances to get the output you want because you are
switching on and off more
 often.
=20
 Higher operating frequency has another benefit in that you can use
smaller capacitors for=20
 filtering.  For learning about SMPS's, this probably isn't an issue.=20
But for real world=20
 stuff, it is definitely something to consider.

 Another specification to look at regarding inductors is the "Series
Resistance".  If this=20
 value gets too high, you'll incur I2R losses in the coil, which will
lead to heating.
 While on the subject of series resistance, make sure the capacitors you
use has a relatively=20
 low "ESR" (Equivalent Series Resistance).  This is mostly for
efficiency benefit.

 There are many many apnotes and tutorials regarding SMPS's online.=20
Including some from Microchip.
 Seek these out and look them over.  There is much to learn, and these
resources will definitelty help.
 There are complete college courses on SMPS's.  Many of them online too.
 (Check MIT Opencourseware).

 I have some experience with SMPS's, so if you have sepcific questions,
I can try to answer them.
 However, note that my experience involves switching controllers from
various mfg's, and not PIC's=20
 specifically.  But the principles are the same.

 Regards,

 Jim

=20
=20

=20










Regards,

Jim

> -------- Original Message --------
> Subject: [EE] Boost SMPS turn-OFF best current value
> From: Electron <electron2k4@infinito.it>
> Date: Mon, January 06, 2014 9:04 am
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>=20
>=20
> Hello,
> I'm playing with a self-made boost converter, all the logic
> is made via a PIC and its ADC's.
>=20
> I'd like to ask some general questions:
>=20
> in my first experiments, I was turning ON the transistor switch,
> then monitoring the current in the inductor, then turn OFF the
> transistor till the current reached 0, and again in a loop.
>=20
> My aim is to get the most possible power out of the power supply.
>=20
> Now I think that if I turn OFF the transistor earlier, say 100mA
> below the "OFF" threshold (maximum current), instead of reaching
> 0, then the average current will be higher, and I will get more
> power from the circuit.
>=20
> Is my reasoning correct?
>=20
> What is the criteria to choose the inductor? As my PIC-based
> circuit is relatively slow, I noticed that only inductors around
> 10mH work, whatever has a smaller inductance probably charges
> and discharges too quickly to work.
>=20
> So the best inductor I should choose is the one that has (of
> course) the least possible resistance, but an high enough
> inductance to let my slow PIC work correctly?
>=20
> No other recommendations?
>=20
> For example, don't inductors have a maximum voltage rating?
>=20
> Thanks.
>=20
> PS: it's really funny to play with SMPS via PIC software!!
>=20
> Cheers,
> Mario
>=20
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