> My solution is given below - its a classic buck regulator but without most
> of the components usually used.
>
> This design is simple to build, forgiving, well behaved, easily scaled for
> power and voltage and reasonably efficient.
> It SHOULD have use in many amateur applications.
>
> This circuit is special - it was provided by God (no kidding).

And it is a Godsend!

Switching regulators has been on my list the last few months due to my
perpetual desire to power PC's in my vehicles. Starting out with MP3
applications but considering moving into DVD and other digital video apps.

There is a bunch of stuff out there about buck and boost theory. But two
things became apparent: No details are discussed on the error/control/feedback
channel and that everyone expects a fixed frequency timing, with just a
variation on the duty cycle to control the voltage.

You've blown both of those out of the water! The discussion of the feedback
through ZBUK1 and QBUK1 is clear and inspired. And since ZBUK1 is the sole
voltage control element, one a trivially change the voltage by replacing it.

Quite impressive.

Now on to my questions:

1) You talked about replacing BUKFET with a PNP bipolar. What about an NPN
bipolar? Remove QBUK2, RBUK3, ZBUK2, and RBUK4 and tie the bottom of RBUK2
and the collector of QBUK1 to the base of the NPN. When QBUK1 is off then
RBUK2 will turn the power NPN on. When QBUK1 conducts, the NPN will turn off.
I'm sure that the base current from RBUK2 will need to be higher so that
the power NPN will saturate.

2) Is RBUK3 the current limiting resistor for ZBUK2? Just making sure.

3) What element requires the 1V headroom?

4) Is the BUKFET and the inductor the only power elements? My goal is to build
a 5V 10A supply.

And my new design challenge:

How could this circuit be augmented so that it could output 12V from an
input between 6V and 15V? 15V: Car running normally. 6V: Car starting.
In my previous design I used a gel cell battery to steady the input current.
However the battery doesn't have enough headroom to satisfy the design
requirement.

Some ideas:

1) Create a boost/buck design where the boost stage always pumps to 16V or
so giving the required headroom. A boost design can be done from this same
circuit with a small rearrangement of the power elements. Instead of switch
(BUKFET), diode, inductor use inductor, switch, diode:

Vin ----IIIIIIIII------+-------DDDDDDDDDDD--------+------ Vout
                       |                          |
                       S                         Cap
                       |                          |
                      GND                        GND

Use exactly the same control system and Vout will rise to the ZBUK1 control
voltage.

2) Since I'm going to have a 5V circuit anyway do it in the opposite direction
and boost the 5V output to 12V.

3) Add a couple of more control elements so that the supply can switch from
buck to boost. consider:

Vin ---S1-+--IIII------+---+--------D2-------+----+------ Vout
          |            |   |                 |    |
          S2           S3  +--------S4-------+   Cap
          |            |                          |
          D1          GND                        GND
          |
          GND

In buck mode:

S1: osciallates. It's BUKFET in the original design.
S2: on. catch diode for buck
S3: off. Boost power element
S4: on. Boost catch diode D2 bypass

In Boost mode:
S1: on. Passes Vin to boost
S2: off. buck catch diode bypass
S3: oscillates: The boost BUKFET.
S4: off. Use the D2 boost catch diode

At first glance it seems like it's more work than it's worth.

Any ideas?

I plan to build my 5V version in a short time frame. I'll probably implement
suggestion #2 above unless someone has another idea.

BAJ

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