Without an inductor your regulator will be linear, it doesn't matter
whether it is frequency switching the transistor or not.
If you do the math, you will see that the extra energy will be
dissipated in the resistor and transistor and will be the same as in the
standard linear regulator.
All that you have is a linear regulator with a lot of ripple voltage and
current.

Now, if you use an inductor then it may become a completely different
beast. The inductance acts as an "inertial flywheel" for the current,
limiting it without converting it in heat as in the resistor and
transistor (of course it is not an ideal inductor,  it has a tiny
resistance and thus will waste some power).
The inductor limits the raise and fall of the current, storing energy as
B field while the current is increasing and supplying current while the
transistor is off.


Isaac


Em 5/6/2013 17:30, Jesse Lackey escreveu:
> Hi all, I'm looking at dc/dcs for running 400mA - 1Amp highpower LEDs=20
> with some special requirements, and in thinking about various schemes, I=
=20
> wonder how (or if) something this simple would work.
>
> Setting aside the minor additional complexity of making a=20
> constant-current vs. constant-voltage dc/dc, what if there were a=20
> circuit like:
>
> +12V-->PFET-->Cap-->Output(say 5V target).
> The output would have a voltage divider (i.e. feedback) going to a=20
> comparator, which has a reference voltage for its other input.  The=20
> comparator would turn on the PFET if the feedback voltage were below the=
=20
> reference voltage, and turn off the PFET if not.
>
> Schematic...
> <http://www.celestialaudio.com/pfet-comparator-cap-dcdc.png>
>
> Now I haven't even tried a simulation (which I rarely do, I don't do=20
> much analog that isn't largely 'cookbook' with tweaks) much less built=20
> it, so I have no 'real data' on how well or if it works.
>
> But it sure seems like it would operate more or less correctly.  As C1's=
=20
> value gets larger, it takes more time to charge (thru PFET) and=20
> discharge (thru load), and as the comparator hysteresis gets smaller,=20
> the PFET switching on-time gets shorter, so it switches more often (less=
=20
> efficient) but you have less output ripple.
>
> If the comparator were an opamp, driving the PFET gate directly, you'd=20
> have a DIY linear regulator.  By using a comparator with hysteresis, you=
=20
> get a dc/dc.  I think.
>
> For my application, I will know the load (a string of LEDs being=20
> high-speed PWM dimmed), the approximate needed output voltage to light=20
> them at the designed current, and will be doing a little trickery to=20
> handle all that.  I can tolerate a moderate (humm... maybe 10%? TBD)=20
> output voltage ripple.
>
> This seems somehow too simple to work reasonably, but I'm not sure why.=20
>   C1 would have to handle the ripple current and the inrush current (as=20
> would the PFET - something that would need a bit of additional design=20
> work, can be handled a few ways), R3 has to be low enough value to turn=20
> off the PFET quickly when the NFET turns off, but high enough value to=20
> not be too wasteful when the PFET is on.
>
> I like that I can set the output ripple by choosing C1's value and the=20
> comparator hysteresis.
>
> Thoughts?
> What's going to fail first if run 24/7 for years?
> Similar, polished, known-working designs anywhere?
>
> Thanks all!
> J

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.