piclist 2002\10\26\081816a >
Thread: Son of simple step-up SMPS challenge
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face BY : Russell McMahon email (remove spam text)

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Challenge:

   Convert low voltage DC such as battery or 3v or 5v rail
   to "somewhat higher" dc voltage at modest power levels
   for e.g. processor operation, FET gate drive, LED drive,
   Op-Amp supply, Programmer Vpp supply, and more.

Somewhat over a year ago I proposed a design challenge for a simple circuit
to step up low voltages to somewhat higher ones for a range of applications.
I believe there was and is a demand for such capability and that such a
circuit may even provide functionality not readily achieved with available
ICs. While there was some initial response to this challenge it died an
early death, probably due to the events of September 11th.

I'd like to re-propose it and suggest a range of applications and target
parameters. Those interested are welcome top redefine the targets more
broadly or into several categories if felt desirable.

I have a specific prospective application in mind for my own use (4v to 12v
converter for FET gate drive) but this is only one of many possible
applications. I also would like a low cost, compact, efficient white LED
driver that provides essentially constant output and reliable starting over
a full range of single cell voltages (<0.9V - > 1.5V)

Ideally a single core design will allow a variety of input and output
voltages and power levels with suitable simple component changes. Low cost,
(non)complexity, efficiency, low parts count, gee-whiz factor, single
inductor, ...  all score brownie points.

Target power level is modest and will vary somewhat with application. e.g. a
white LED may require 10 to 20 mA at 3v, A FET gate supply a few mA at 12v,
an opamp supply 1 to 10 mA at 12v, a processor 5 to 20 mA at 3v to 5v etc.

Target efficiency is "as high as possible and as low as appropriate". e.g. a
design that rings an inductor and uses a clamp zener to dissipate any
surplus energy would have poor efficiency at less than full power output but
such an approach may well be acceptable in many applications.

A highly desirable aim is use of a single inductor with a single winding
although multi winding inductors or multiple inductors may well be
acceptable. (Single winding inductors at this power level are widely
available and low cost).

Lowest possible starting and operating voltage is a desirable aim although
the importance of this will vary with application (e.g. a 3v processor
supply
to 12v FET drive application only requires that the circuit start and run
properly on the 3v supply.) A "run the 1.5v cell utterly dry" design  will
ideally run on under 0.9v and will hopefully also start on less than a volt.

Number and type of active devices is "free" although obviously the less the
better commensurate with meeting other design aims. I anticipate that most
designs would use one to 3 transistors although more may be appropriate. Use
of an IC may be appropriate, although this may limit lower starting and
operating voltage. That said, a 4069 CMOS inverter IC has a minimum Vdd of 3
volt (but you will almost certainly need at least 1 transistor as well to
allow "flyback" voltage to be used).

Low cost is desirable although the real cost of a given design will vary
with use - e.g. a commercial product will consider PCB area and component
installation costs.

Too complex a design will be beaten in cost and simplicity by commercial
alternatives. Few ICs aim at the very low power / low voltage / low cost
market so there is a niche. The ability to start and run at voltages under 1
volt would place the design in a relatively exclusive club.


_______________


As my initial contribution to this challenge I submit my comments and
circuit
from a year ago for a white (or other) LED driver circuit (led1cel2.gif),
plus a modified version of this designed to produce a positive output
voltage and allow higher input voltages (smps512.gif) While I have named the
diagrams SMPS512 it in fact has NO means of regulation shown and no RC
component values shown. R2 is added to the LED flasher version to increase
Q2 turn on time during the inductor drive pulse and R3 is added to stop Q2
trying to apply full supply to Q1 base :-).

NB - neither of these circuits is optimal.
*** THE CIRCUIT DOES OSCILLATE ***
The circuit has been built and tested in practice.
This is not a paper only dream.
(Hopefully that makes it clear enough :-)
The above added because this circuit not surprisingly attracts detractors
due no doubt to its interesting mechanism of operation.

I imagine (but have not yet tested) that a zener from point g to point c
would provide regulation of sorts. When out exceeded approx Vin + Vzener -
0.6 Q2 would be held off. This has the disadvantage of reducing Vout with
decreasing Vin - less of a problem for Vout >> Vin. In most applications an
output zener shunt regulator would suffice.

Brief description of operation.

NB      R1 is sized large enough to NOT be able to support max drive
required by Q2 to keep Q1 turned on at peak inductor current. !!!!!!!!!!
It's function is to START on drive and possibly provide timing.

R2 small wrt R1

Letters a to h refer to equivalently labelled points on circuit diagram.
Startup. Q1, Q2 off.
c, f at Vin as C1 uncharged.
C1.c charges downwards via R1.
Q2 turned on via R1+R2 when b reaches approx Vin-0.6 (setting lower bound on
starting voltage).
Q2 turning on turns on Q1 via R3.
Q1 turning on pulls f and therefore c low via C1, turning Q2 harder on
causing regenerative turn on.
c now increases as C1 charges via base Q2,
Current increases in L (approx a ramp)
As drive to Q1 DECREASES as C1 charges, drive to Q2 (BetaQ1 x !bq1)
decreases until current in L cannot be supported by Q2. As current in Q2
prevents current ramp up in L field starts to collapse and inductor starts
to 'ring".
f rising increases c further adding to regenerative turn off.
f will ring either to
   - limit set by load
   - limit set by clamp output zener (not shown)
   - limit set by LC in tank cct (as no C across L c is only parasitic and
small)
   - reverse breakdown Q2 be junction (R2 reduces this prospect)
In practice the first two effects are most likely unless there is no load at
all.

With f held high c will now start downwards due to R1 (something like
bricklayer and barrel story :-) )
This will be hastened with increasing load as f will decay sooner.
Voltage at c will sooner or later reach starting point and cycle repeats.

Notes:
- R2 & R3 are refinements from LED driver version but don't alter basic mode
of operation.
- R1 size and Q2 beta are important factors in achieving turnoff.
- IF L saturates it should do so not too far below the R1-Q2beta limit or
there will be large unproductive current spikes in Q1 collector/inductor.
Saturation is not necessary but provides another rmeans of triggering
regenerative turnoff/ring phase.
Efficiency is not liable to be fantastic but Roman can probably tweak it to
get 80% plus :-)


++++++++++++++++++++++++++++++++++++++++++++++

LED flasher / driver

FROM LAST YEAR

           ********   Before we start:   ************

       - This is a REAL operating circuit.
       - This circuit DOES oscillate (& very well) in practice *.
       - It IS possible to describe a formal feedback mechanism and mode of
          operation.

                                   OK

WHAT IT DOES:

                                    1 cell to LED driver

                   or              Low voltage to higher voltage step up

                   or              Flasher

   With minimal parts count.

I suspect this wins the minimum component count with a single winding
inductor as per Roman's spec.

I put this under the above heading as in some ways it's a continuation of
that theme.
It also addresses the LED torch and LED from 1 cell applications.

I expect that Roman & Alice will have lots of fun further developing this
circuit, Jinx will use it in 3 unexpected applications in the next two weeks
and xxx & yyy will have lots of fun "commenting" on it :-).

This is based on a very time honoured flasher circuit which I have used for
other applications.in the past.
This is as stripped down as it seems to be possible to get it with an
inductor added to provide voltage step up.
Adding some components (typically 1 or 2 resistors) alters and may improve
performance.

Other applications might be a voltage step up for e.g. 5 --> 12v for
programming, solenoid drive as per Jinx's recent application, RS232 supply
etc.
I haven't optimised this or measured efficiencies but as shown it has a
remarkably square drive waveform and may even be somewhat efficient.

OUTPUT

As shown the inductor "rings" when Q1 is turned off delivering NEGATIVE
output below ground.
To reverse the circuit to supply positive output above Vin swap Q1 and Q2
types and swap ground and Vin connections.

LOAD

To use as a voltage supply replace the LED with a diode and filter
capacitor.
Usually only a single LED would be used.
LED1 is driven solely by the flyback voltage
LED2 sees" both flyback voltage and input voltage.

Arrangement 2 is more efficient as the input voltage is added to the flyback
and this part of the supplied voltage is essentially "100% efficient"

The LED1 arrangement has the advantage that if Vin exceeds LED normal
forward voltage somewhat (say up to 5 volts) the LED will still operate at
less than destruction current. Efficiency will suffer in this mode but we
now have a LED that will operate from Vin = 0.7 volts to Vin = ???

When operated with only an inductor as load (no LED) my example rings to
about 25 volts limited (probably) by L1 to C1 ratio and scope and stray
capacitive loading.

A LED which will not "glimmer" whatsoever when connected to a single cell
can be run across the whole cell operating range.

COMPONENT VALUES

I have not shown component values (but see example below) as performance is
immensely affected by component choice.
The circuit is reasonably "designable" but the operation is surprisingly
tricky considering the component count.

Rather than play with this further I thought I would release it to the eager
masses (well, Roman and Alice anyway :-) ) as they are much more likely to
extend and optimise it than I am at present.

EXAMPLE:

Single no-name brand le Clanche AA cell (standard penlight battery) half
flat. V = 1.2 volt
L1 = 330 uH miniature choke **
R2 = 1M
C1 = 100 pF
Fosc approx 10 kHz
HP high brightness Red LED
I_Battery approx 4 mA
Surprisingly bright.

** - Dick Smith Electronics R5234
A slightly physically larger 2.5 mH inductor gives somewhat brighter output
at somewhat increased current. I gain the impression that the higher
inductance is more efficient (as would be expected).

CAPACITOR

I have shown C1 as an electrolytic to denote polarity.
When used for a continuous supply (as here) the cap will be so small that a
non electrolytic will invariably be used.
When used as a flasher (see below) an electrolytic may be appropriate.

VARIATIONS.

This circuit can be extended and amended vastly.
A few guidelines:

Placing a resistor "R1" in series with C1 will have a significant effect on
discharge times (as it removes the Q2 Vbe clamp effect on the capacitor).

Placing a resistor between Q2 collector and Q1 base (try 1K to start) will
affect discharge and on cycle times.

This circuit delivers a negative voltage relative to ground.
A mirror image circuit may be built by swapping Q1 with Q2 and ground with
Vin to make a circuit providing voltage ABOVE Vin.

L1 may be altered significantly, varying energy storage for a given
frequency.

Changing frequency will affect energy delivered and therefore LED brightness
(or available output power)

Adding resistors to alter mark space (charge discharge) will affect power
delivered.
.
OPERATING VOLTAGE

Oscillation starts at about 0.65 volts but with components shown above
doesn't give notable LED brightness till about 0.8 to 0.9 volts in.
More is better.
Output waveform squares up nicely by Vin = 0.8 volt or so.

EFFICIENCY

Doesn't look marvellous.
Didn't do formal tests but some rough measurements suggest well under 50%
efficiency !!!!
If so, should be able to be improved substantially, at cost of extra
complexity.
.

REGULATION

No ! :-)

If used as a voltage supply I suspect that a simple zener and series R
connected from output to an appropriate transistor base will allow the
oscillator to be disabled when desired Vout is reached.

Given the power levels involved a simple shunt zener may be better and
easier. If low power maintenance of a voltage is desired then the zener
scheme would allow the oscillator to "burst" as required to maintain
voltage.

FLASHER

When C is made large (say 1 uF range) the frequency of operation will be so
low that individual output pulses will be individually distinguishable. In
this case, provided the energy in the inductor is adequate, the LED
'flashes". The design will need to be arranged to provide requisite energy.

WORTHWHILE?

Possibly not - but it's fun.
A simple 2 transistor cross coupled multivibrator would use  a few more
parts but be rather more designable.
Probably worth a look though.


SO

OK - over to Roman, Alice, Jinx, ... - any other improvers out there ???



regards

               Russell McMahon



_______________________


* For oscillation the current in R2 *Beta Q1 x Beta Q2 MUST be lower than
the load current.
This means that it will stop oscillating when the load resistance is
increased above this level.
The low resistance inductor load generally meets this requirement.







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