An ongoing theme is "how can I supply eg 5 volts from a much higher voltage
supply without excessive power dissipation?"

I have a design I would like to share that will meet many such needs cheaply
and simply. Rather than reveal it immediately I propose a design challenge
along the lines below. Hopefully some useful ideas will come out of this.


Requirement

Step down voltage regulator suitable for powering small low voltage circuits
from a higher voltage supply with a reasonable power dissipation.

Overall figure of merit = finished cost while meeting design specification.
Costing rules provided below.

Switching regulator for reasonable efficiency.
    Any topology.
    Efficiency need not be superb but should be substantially
    better than an equivalent liner regulator over most of the
    input range.

Power out
    In this instance "'small" - say a few tens of milliamps
    Ideally scalable as required.

Output voltage
        Fixed but selectable over say 2 to 12 volts.
        Output would ideally be useable as is for operating typical
        processor based circuits but the use of a linear post regulator
        is acceptable if required. Cost of post regulators not included in
        costing.
.
Input voltage
        Always greater than output.
        Minimum: as little as 1  volt above design output
        Maximum: More the better -
            (preferably at least 50 volts )
     A design with a varying input voltage is desirable.
     Fixed Vin/Vout (eg 24 to 5) designs would be of
     interest but less useful generally.


Output regulation
    Smaller change the better
    Ideally +/- 0.1 volts across all Vin range and across
    load variation from 10% to 100%

Isolation
    Input to output isolation would be a bonus but is not required. This
allows almost any desired regulator topology (buck, flyback, forward, CUK,
etc) as desired.

Noise / EMC
    Output noise should be "reasonable" but post filtering for noise may be
necessary and is not included in the costing. A design that does not need
any post filtering is better. Extra filtering should NOT be required to meet
regulation specification. EMC / radiated or conducted noise should be
amenable to limiting by normal means.


BASIS OF COSTS

Components used are costed in Arbitrary economic units as follows. Any
relationships to $NZ in volume is entirely coincidental. All names have been
changed to protect the guilty. No animals used in testing this product. Each
of these units is very approximately 0.5 US cents. This costing is assumed
to be proportional to all up cost in an amateur environment. ((Real costs
can be scaled by an appropriate factor to match the currency of your
choice. ))

1    resistor 1/2w 200v max rated
5    transistor, TO92 pkg, general purpose (BC337 etc)
10    FET small signal TO92.
1    diode, small signal, 75v 150 mA (1N4148)
2    diode, low frequency, 1A, 400 volt (IN400x)
10    Diode, high frequency
10    Cap plastic up to 0.1 uF
20    Cap plastic 0.1 to 1 uF
5    Capacitor electrolytic 1 uF - 10 uF
10    Capacitor electrolytic 10 uF -100 uF

50    Inductor, single winding
10    extra isolated windings
5       extra winding connected to existing winding.

25    Comparator, 4 in pkg (LM339(
30    Opamp, 4 in pkg (LM324)
100   Switching regulator IC (384X, MAX, ... your choice)
50     TO92 Linear regulator (78L05 etc)

If you don't find what you need here describe it and suggest an appropriate
cost.
Also adjust any of the above if you feel it justified (but say why).

Provide brief estimated specifications (selectable output range, max and min
input range, dropout (minimum in to out voltage), regulation with changes in
load and input voltage,  noise, limitations and advantages of design.

Example:
=======

Simple switcher / MAXIM style buck regulator, with input filter cap, output
cap, inductor, all internal components

Cost    Item

15       caps
100    IC - super whizzbang all up one chip
50      1 winding inductor
2         resistors for Vout selection

167    Total

Comments: in reality usually more "glue" components would be needed.
Most  dedicated switching regulator ICs won't work at 50 volts input.
A very simple design with low PCB area.
Power limited by IC ratings and dissipation.
Power switch and flyback diode included in IC


           ---- INDUCTOR---
          |     -----------------    |
Vin--o---|        IC          |--o---.----> Vout
          |      -----------------   |
        ==  Cin                  ==  Cout
          |                              |
---------------------------------------------




        Russell McMahon

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