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'[EE:] simple step-up SMPS challenge'
2001\09\12@054124 by Roman Black

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Hi Vasile, how about a challenge, for a simple
step-up circuit to power a 4v to 5v PIC from
a 1.5v battery, or even a 1.2v NiCd cell.

I know chips are available to do this, but
with price and availability problems.
A cheap 2-transistor circuit that does this
(and regulates ok) would be very useful.

So, who can suggest a super cheap circuit that
does this:
* 2 or 3 cheap transistors
* ONE standard pre-wound cheap inductor
* ok for 1.1v input (discharged NiCd cell)
* 4v or 5v output, reasonable regulated
* cheap total parts cost and small size
* decent efficiency

Maybe a simple boost or buck-boost circuit?
Similar to the high-gain self oscillating things
that Russell and myself have been playing with?
:o)
-Roman


Vasile Surducan wrote:
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2001\09\12@062816 by steve

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> Hi Vasile, how about a challenge, for a simple
> step-up circuit to power a 4v to 5v PIC from
> a 1.5v battery, or even a 1.2v NiCd cell.

Pity you put the PIC restriction on it. If you were using a Cypress
PSoC microcontroller the SMPS would require 1 diode, 1 capacitor
and an off the shelf inductor. 0.9V in -> 5V out.

Steve.

======================================================
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TLA Microsystems Ltd         Microcontroller Specialists
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2001\09\12@063611 by Vasile Surducan

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Roman, a cheap step-up with ordinary transistors from 1.1 V is a real
challenge ! You need very small VCE if you think to bipolar transistors,
but I think bipolars can't be used here.
Even serious circuits like LT 1930 ( 1Amp ) running at 1...2MHz have a
minimum 2.6V input voltage.
About efficiency, I think 60% will be pretty good.
But I preffer to design a little expensive circuit than to have problems
with sorting transistors for high-est gain.
Vasile

On Wed, 12 Sep 2001, Roman Black wrote:

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2001\09\12@090854 by Russell McMahon

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This is going to be fun.
Definitely doable.

The biggest challenge is STARTING oscillation reliably at low voltages.
Once started the output voltage can be used for bias supplies etc.

The important parameter is saturation voltage. This can be well under 0.1
volts with a bipolar at low current.
Efficiencies are not going to be marvellous.


Silicon Chip magazine December 2000 did a single cell to white LED
converter. This uses 4 transistors and could use less. They use a basic
cross coupled astable multivibrator driving a buffer which drives the pulsed
coil. Output rings till the LED absorbs the available energy, This could
easily be adapted to a higher voltage design.  3 x BC548, 1 x BC337, 1
diode, 1 inductor, 3 caps, 6 resistors.  Runs on under 1 volt. No efficiency
stated. 220 uH inductor (custom wound). Runs at 11 KHz. Easily adapted.

I calculate power at about 50 mW input with values used. (0.5 L I^2 f) ipk =
220 mA. (They claim 64 mW output power which appears wrong).
ie 10 mA at 5 v at 100% efficiency. This sounds about right for a LED (maybe
a bit low). This could obviously be increased..

I could (probably) do this with a self oscillating design with 2 transistors
(regulated) or one transistor with questionable regulation and a
multiwinding inductor. Depends on the value of using an off the shelf 1
winding inductor. Note that the relatively high currents here make the
inductor more demanding than for the recent buck designs (saturation of core
and losses).

Roman ?

More anon.



     Russell McMahon
_____________________________



{Quote hidden}

with
> > > drain. Source to ground , gate to a simple oscillator ( could be a
> > > positive feedback self oscillator, but then instead of a simple
inductance
> > > will be a transformer ). At terminal "c" you'll have a magnified
> > > oscillation voltage. Rectifying with fast swiching diodes and
filtering
{Quote hidden}

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2001\09\12@102558 by Roman Black

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Russell McMahon wrote:
>
> This is going to be fun.
> Definitely doable.
>
> The biggest challenge is STARTING oscillation reliably at low voltages.
> Once started the output voltage can be used for bias supplies etc.

Good point. Multivibrator type design would
be a good place to start, using one side as
the power driver. If the other side could be
adapted to control the regulation, excellent,
but even a third transistor for regulator
control would be ok.

> The important parameter is saturation voltage. This can be well under 0.1
> volts with a bipolar at low current.
> Efficiencies are not going to be marvellous.

Sure, I got 19mV Vce (from a larger transistor),
there may be some issues finding the right small
package bipolar transistor. But once found, if it
is a common type of transistor the circuit would
fulfil the requirements of being super cheap...


{Quote hidden}

Ooh! You are on! Let's see the circuit. :o)

> Depends on the value of using an off the shelf 1
> winding inductor. Note that the relatively high currents here make the
> inductor more demanding than for the recent buck designs (saturation of core
> and losses).
>       Russell McMahon

I think the off the shelf inductor is fairly vital
in the challenge, simply because if the thing
is to be practical you need to be able to make
them cheap and in quantity. Anyone can build one if
time and cost are no problem, using specialty wound
toroids that take 1 hour to wind or $4 chips.
Being able to use it in a small or medium sized
run would give many options for a small PIC product.
Especially low current devices like 10mA or 20mA etc.
-Roman

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2001\09\12@102812 by Roman Black

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Hi Vasile, small Vce is possible with cheap
bipolar transistors, at low currents anyway.
The main use for this would be to use a D cell
or C cell (etc) battery to power your PIC
product, instead of a 9v battery. So the
5v output current needed would only be low,
for most of my projects 5mA to 20mA is plenty.
-Roman


Vasile Surducan wrote:
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2001\09\12@112130 by Mark Skeels

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> > Silicon Chip magazine December 2000 did a single cell to white LED
> > converter. This uses 4 transistors and could use less. They use a basic
> > cross coupled astable multivibrator driving a buffer which drives the
pulsed
> > coil.


I used a astable multivibrator to drive a voltage doubler circuit. I had to
add a JFET as a buffer to the output because it drew too much current and
messed up the duty cycle.

But, I'm thinking of going to some sort of inverting logic gate oscillator
because it costs us about $.06 to put down each smt component.

There were 10 components for the multivibrator and JFET buffer in the
original design. I think I could use a hex inverter and parallel the left
over inverters at the output, like, say, a 4049. I'd use this to drive the
doubler network. I need to generate roughly 5 or 6 mA at the output. It runs
at about 10KHz; the frequency and voltage outputs are not critical.

Of course, this is unregulated. I needed it to generate a higher dc voltage
to drive an op amp driving P-channel fets so that I could turn them off all
of the way.

Mark

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2001\09\12@164645 by Alice Campbell

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Hello Roman,

There was also a one-cell LED flashlight in Electronics Design about a month ago.  It uses 2 transistors and a simple inductor.  They mentioned in the text that another transistor could be added to get some regulation.  I would send the ckt but its at home just now.  I have tried the ckt but it appears to be touchy, I didnt have exact resistor values and had a hard time getting it to go.  But the leading indicators are that it is possible to torture this ckt into an SMPS.  I will send ckt later.

Alice


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2001\09\12@170926 by Octavio P Nogueira

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This circuit is at
www.e-insite.net/ednmag/index.asp?layout=article&articleId=CA90758&pu
bdate=07/05/2001


Friendly Regards

Octavio Nogueira
===================================================
nogueiraSTOPspamspamspam_OUTpropic2.com                  ICQ# 19841898
ProPic tools - low cost PIC programmer and emulator
http://www.propic2.com
===================================================

{Original Message removed}

2001\09\13@022620 by Vasile Surducan

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Yes, you have right, I had thought to large current.
Thanks also to Octavio which is thinking to the piclist people.
Vasile


On Thu, 13 Sep 2001, Roman Black wrote:

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2001\09\13@070628 by Roman Black

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Mark Skeels wrote:
{Quote hidden}

Hi Mark, this is interesting, using a hex inverter
or other logic chip, but surely this will be limited
to applications that step up from 5v to higher voltages?
Not for 1.1v to 5v application?
-Roman

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2001\09\13@073303 by Roman Black

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Cool! I'm really keen to hear whatever
you find out. :o)
-Roman


Alice Campbell wrote:
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2001\09\13@073528 by Roman Black

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The circuit looks simple enough, i'm not
convinced that it can be converted to
regulate and give top performance but it's
a good starting point. Thanks!
-Roman



Octavio P Nogueira wrote:
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2001\09\13@110617 by Mark Skeels

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>
> Hi Mark, this is interesting, using a hex inverter
> or other logic chip, but surely this will be limited
> to applications that step up from 5v to higher voltages?
> Not for 1.1v to 5v application?
> -Roman

Yes, Roman.

12 up to about 22v.

Mark

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2001\09\13@133230 by Alice Campbell

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Here is another I've looked into.
archives.e-insite.net/archives/ednmag/reg/1998/021698/04di.htm
(about halfway down page, look for '5 to 24V switcher')
It's 5V to 24V, and regulated.  It runs fine as speced, could it be tortured to go to 1.5V supply?  I don't know enough to recalculate the resistors to drop the input voltage down, but would truly appreciate someone explaining (slowly, in words of just a few syllables) how to do such a redesign.  I imagine the tricky part is getting the regulator transistor to work with the low voltage headroom, since it has to share the headroom with the second half of teh oscillator.  That's where my 2 neurons finally gave out.

alice


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2001\09\14@112750 by Russell McMahon

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part 1 2135 bytes content-type:text/plain; (decoded 7bit)

Here's a bare bones circuit for people to criticise unmercifully and
hopefully improve.
Any resemblance to other circuits living or dead is only partially
coincidental.

It's a truly horrible circuit.
But a simpler one would be hard to think of..
Versions of it work over limited voltage ranges.
It could be worked on to make it operate for a given load range and 1.0 to
1.5 volt operating range.
It may be fatally flawed.

Testing really wants a variable voltage supply as it is possible to get
versions which start only over a limited voltage range and operate over a
wider but still restricted range. Attempst to operate from a battery during
development may lead to the cct never starting. Whether it can be made
flexible enough I leave as an exercise for the student :-)
Try R2 = R3 = 1k, R1 = 3k3, R4 = 10k for starters.

This is kin to the 2 transistor LED driver of the other day and suffers
similar defects (and a few of its own).
Switching off of Q2 (main switch) is is controlled by R1 not providing
enough current to hold the transistor in saturation. Not nice.

D1, D2  provide enough isolation to hold Q1 off when Q2 is on and its base
is at Vbe above gnd. As Ic Q2 rises it comes out pf saturation when Icq2 >
betaQ2 x (Vin-0.6/R1) (approx ;-))
Collector of Q2 rising takes d2 d1 R2 highand turns Q1 on which clamps Q2
off. L rings to deliver power via D3 to load. R4.

Having load voltage high helps efficiency due to drop across D3.
A LED load can be connected across L1 as per previous circuit BUT better
efficiency is obtained with LED from Q2 collector to ground as then the
battery provides part of the LED voltage directly.

Using a small 100 uH inductor efficiency was poor.
At Vin = 2 voltsIin = 43.4 mA, Rload = 10K, Vout was 15.7V for an efficiency
of 35%. Not good.
This circuit (or a properly behaved variant) MAY be suitable for eg a 12v
programming supply from eg 5 volts.

I would prefer a driven switch using eg a multivibrator - one more
transistor but much more designable.
Fun though. Bed time here. (and then some)


regards


               Russell McMahon



part 2 2050 bytes content-type:image/gif; (decode)


part 3 154 bytes
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2001\09\14@141810 by Olin Lathrop

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> Here's a bare bones circuit for people to criticise unmercifully and
> hopefully improve.

I've only looked at the schematic and not your description.  But c'mon
Russel!  Just a few brain cells in gear before fingers in motion would save
the rest of us save a lot of time.  This circuit is stable and won't oscillate
because you've got two inverting stages tied in a ring with DC paths, which
makes a flip-flop.

As Vin goes up when power is turned on, Q2 will go on via R1 and keep its
collector low, thereby preventing Q1 from ever going on.  If R1 is low
enough Vin will continue to drain thru L1 and Q2 forever or until something
fries (can't tell since you didn't provide component values).  If R1 is too
high to keep Q2 saturated, then Q1 might (again, no component values) turn
on, turning Q2 off.  Once that happens it will stay that way until power is
removed.  At most one single brief pulse will be delivered to the output
each time power is applied.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spamBeGoneolin@spam@spamspam_OUTembedinc.com, http://www.embedinc.com

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2001\09\14@200902 by Dave Dilatush

picon face
Russell McMahon wrote...

>Here's a bare bones circuit for people to criticise unmercifully and
>hopefully improve.

>Any resemblance to other circuits living or dead is only partially
>coincidental.
>
>It's a truly horrible circuit.

[description snipped]

Yep, it is indeed horrible.  As Olin has pointed out, it can't
oscillate.

You'd be better off with a classic blocking oscillator.  
That reminds me of a story.  Years ago, I worked at a place where we
made propane igniter modules for portable heater/dryers used by the
telephone companies when servicing underground cable vaults.  These
modules used three windings on a saturable core, in an oscillator
circuit that contained a transistor, two or three resistors, a
capacitor and (IIRC from a quarter-century ago) a diode.  The primary
winding connected between VCC (a 1.5V cell) and the collector of the
switch transistor; a secondary winding, with somewhat fewer turns,
provided regenerative base drive to the transistor; and the third
winding generated the output to a HV rectifier diode and a 5nf, 10KV
capacitor.

The transformer for this particular unit looked like a TV flyback
transformer with a serious attitude problem, and it REALLY packed a
wallop.  With 1.5 VDC in, we got about 7000 volts out.

At one point in our blind quest to wring more profit out of this
little monster, we got a brilliant idea: why not use this same circuit
design to make a high-efficiency cattle prod (our plant was just
outside of Denver, Colorado, near the stockyards)?  
The idea of an "efficient" stock prod sounds odd, I suppose, but most
stock prods of the time used a high-voltage generator based on a
magnetic vibrator (sort of like the old Model T Ford spark coils) and
were horribly inefficient.  The batteries, so the stockyard folks told
us, only lasted but a few minutes and all the yardhands walked around
with their pockets bulging with flashlight batteries.  
Our little gadget could go on and on for hours on a single "D" cell,
giving off a fat, juicy quarter-inch spark and a loud "SNAP!" every
couple of seconds.  So we really thought we had something.

Anyhow, we whipped up a couple of prototypes and primed Earle, the
sales guy, to go out and do some field tests.  The next day, Earle
went off to the stockyards dressed in about $500 worth of hand-tooled
leather dudewear from L.L. Bean and a brand-new Stetson, cattle prods
tucked under his arm.

The poor guy came back a few hours later- dripping wet, covered with
big brown splotches and smelling like a cow that's just suffered a
HORRENDOUS case of diarrhea.

Seems old Earle had violated the cardinal rule of dealing with cattle:
never, never, NEVER stand directly behind a cow for an extended period
of time.  Feed cattle are bred for beef quality, disease resistance
and the ability to gain a lot of weight in all the right places in a
short period of time when fed an unlimited supply of grain.  
They are **NOT** bred for either intelligence or good anal sphincter
control.  Being mammals, they do occasionally sneeze- and when a cow
sneezes, stuff likely as not comes flying out both ends of the cow.  
So, one simply does not tarry behind a cow.  Ever.

The way Earle told it, he drove out to the feedlot and got permission
from the shift boss to go into one of the pens and run a few
experiments.  He took the stock prods, let himself into the pen, and
went looking for a victim.  He settled on one particular beast that
was busily munching its way through a feed-trough full of corn, and
quietly snuck up directly behind it.  Planting his shiny leather
L.L.Bean boots firmly in the dirt, he pushed the "ON" button and held
it in to let the prod build up a good, hefty charge.

Then, with a decisive lunge worthy of D'Artagnon, he thrust the
business end of our prototype straight into the cow's backside.  
Bad move.  Old Earle thrusted alright, but the cow didn't give him a
chance to parry; the result was spectacular.  
"That damned cow didn't just crap me," Earle said later.  "The ass end
of that thing f***ing DETONATED!  I jabbed and then my whole world
turned brown!  Covered with cowshit, and now they're mad at me down at
the yard because it took 'em an hour to catch the cow and get it
calmed down again."

Seems there aren't really any good engineering guidelines on how much
electrical energy is required to get a feed cow's undivided attention
without also sending the cow into an all-out, bellowing panic.  Some
things in engineering are simply not on the datasheet, and this was
one of them.

Anyhow, eventually the thing worked just fine after we cut the number
of turns on the output secondary by half and reduced the output
capacitance to only a couple hundred picofarads; Earle said the cow
just grunted, raised its head out of the feed trough, and started
slowly shambling away.  Perfection is sweet.

Anyhow, Russell, try a blocking oscillator design.  They can be made
to work nicely on a very low supply voltage.  If all you want out of
the thing is 5 volts or so, there's no need for a third output winding
like we had.  Just use the primary.

DD

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2001\09\14@212517 by Russell McMahon

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part 1 6627 bytes content-type:text/plain; (decoded 7bit)

> > Here's a bare bones circuit for people to criticise unmercifully and
> > hopefully improve.
>
> I've only looked at the schematic and not your description.  But c'mon
> Russel!  Just a few brain cells in gear before fingers in motion would
save
> the rest of us save a lot of time.  This circuit is stable and won't
oscillate
> because you've got two inverting stages tied in a ring with DC paths,
which
> makes a flip-flop.

Olin,

OK - that's merciless criticism as requested :-).
I'm sorry that your brain cells and my moving fingers don't reach the same
conclusion.

       This is a real working circuit.

I didn't get this circuit just from a "thought experiment" or a SPICE model.
I thought from what I'd written that it was clear that this was a real
working circuit, albeit a flawed one,  but obviously I should have made this
clearer, somehow.
As noted, it was offered as a starting point and a demonstration of the
basic elements of such a circuit .

You will note that I gave some efficiency figures at the bottom of the post
and I made some practical comments about it working over various voltage
ranges with various parameters. I also provided a specific set of resistor
values in the text under which I did some measurements. (I said : "Try R2 =
R3 = 1k, R1 = 3k3, R4 = 10k for starters".) I didn't put these on the
diagram because, as I noted, this is not a nice circuit as it stands,
different values will be required depending on load and Vin etc and, as I
noted, I was offering it as a basis for possible development for those who
may want to play with it.
.
Putting it more clearly, hopefully:

I drew this circuit up essentially as shown.
I then built it,
This circuit is working on my workbench.
The results given were measured with standard test equipment.
I monitored waveforms with an oscilloscope.
It oscillates very well when it is in oscillation.
The circuit works over only a limited input voltage range.
The starting voltage range is within but less than the operating voltage
range.
As noted, I do not suggest this as a final circuit.
Something may be able to be made from this circuit.
It MAY take a very simple change (made possibly with lots of development
effort) to make it much more useable.

As to mechanism of operation -

> As Vin goes up when power is turned on, Q2 will go on via R1 and keep its
> collector low, thereby preventing Q1 from ever going on.  If R1 is low
> enough Vin will continue to drain thru L1 and Q2 forever or until
something
> fries

The mechanism by which Q2 comes out of saturation and causes switching was
briefly covered in my original text. This is a fairly common and very time
honoured means of causing switching transition in simple low power, power
conversion oscillators. As I noted, it relies on the beta (current gain) of
Q2 and is therefore not a nice method.

Again:

- Q2 turned on by R1.
- Q1 turned off by R3 and not enough voltage from Vin to turn it on due to
D1,D2 and then R2/R3 divider.
- Q2 collector current via L1 increases (i ~- Vt/L)
- Q2 collector current driven by Iba2 x Beta q2 via R1.
- As current ramps up a point is reached where Icq2 exceeds beta x Ibq2 and
Q2 will start to come out of saturation.
- At this stage L1 attempts to maintain the peak current flow while Q2 is
reducing its ability to handle this. Vq2 will start to rise regeneratively.
At this stage it still has drive via R1.
- When (IF) Vcq2 rises high enough to start to turn off Q1 the process
becomes fully regenerative and Q2 is shut off completely.
This happens about when
           (Icq2 - 2 x 0.6) x R3/(r2+r3)m = 0.6 volts

ie when Q2 collector voltage - the 2 diode drops of D1,D2  divided by R2,
R3 reach 0.6 volts approx and turn on Q1.
Clearly this is not a very nice process and the range of input voltages
which it works over are affected strongly by component values and Q2 beta.

Nothing will "fry" as long as the circuit is "designed". ie Icq2 max does
not
saturate L1. R1 is dimensioned to cause beta starving of drive to Q2 at
designed peak current. Beta of Q2 is known (or its range).

I'll say again as I said in the last post (not in exactly these words).

- not a nice circuit
- not a final circuit
- has possibilities
- works in practice
- flaws make other circuits more attractive
- may be redeemable.
- interesting to note its existence.
- may have applications for simple higher voltage tasks eg 5 to 12 volts.

I also replaced D1,D2 with a 5v zener and got some interesting results but
that's another story.

Having such a simple WORKING circuit to look at allows people to examine and
understand the mechanism of oscillation. More complex circuits which have
been "optimised" for practical use hide the core processes and are hard for
beginners (or experts ;-) ) to understand.
This circuit is fundamentally the same as the EDN circuit forwarded by Alice
a few days ago but its operation is much easier to understand. For that
reason alone it is potentially valuable even if it is never used in this
form.

Roman has suggested (offlist) an emitter resistor for Q2 to stabilise
oscillation.

As shown there is NO explicit voltage regulation but this could easily be
added in rudimentary form with a zener from Vout to left hand end of R2 or
(probably better here) a zener across the output. Power output can be
adjusted by varying R1 to limit Icq2 peak appropriately.

When there is no load this does go into a self limiting mode as Q1 is held
off longer under the larger inductive decay time but this is not a
controlled method of voltage limiting (Vout about 25 volts for Vin = 2 volts
under that condition in one arrangement).



>(can't tell since you didn't provide component values).  If R1 is too
> high to keep Q2 saturated, then Q1 might (again, no component values) turn
> on, turning Q2 off.  Once that happens it will stay that way until power
is
> removed.

This is the key point.
The rise in Vcq2 due to inductive ringing of L1 causes Q1 to turn on and
regenerative turnoff commences.
Very nice and square waveform at Cq2 over a limited Vin range.

> At most one single brief pulse will be delivered to the output
> each time power is applied.

This is probably true for Vin outside the working range
The circuit is dependant on Vin being neither too high nor too low.
I can think of several possible ways of improving it but they will have to
wait a while.

Roman may turn it into a work of art in the meantime :-)


regards,



               Russell McMahon.





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2001\09\14@214904 by Dale Botkin

flavicon
face
On Sat, 15 Sep 2001, Dave Dilatush wrote:

<long cattle prod story snipped...>

Bloody hell, Dave, I laughed so hard I nearly duplicated the cow's
performance!!  GOT to send that one to my Dad.

Dale
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Hallo, this is Bill Gates and I pronounce 'crap' as 'Windows'.

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2001\09\15@091038 by Russell McMahon

picon face
>Here's a bare bones circuit for people to criticise unmercifully and
>hopefully improve.
...
>It's a truly horrible circuit.


//
Yep, it is indeed horrible.  As Olin has pointed out, it can't
oscillate.You'd be better off with a classic blocking oscillator.
//

But, it does oscillate ! (Maybe not in SPICE?).

That is an actual working circuit that I have shown.
See my other two posts for comments and mechanism of oscillation.
As I noted there - this is a bare bones idea starter that shows how simple
(and horrible ;-) ) such things can get.

I have no problems with using blocking oscillators or their variants.
Roman, who proposed this challenge, wanted, if possible, a single winding
design so that cheap off the shelf inductors could be used.
I mentioned recently that I have a very nice forward converter design but
that it uses 3 windings. Being a forward converter it does not have the
stored energy limitations of the typical small blocking oscillator designs.
This allows better power transfer per core volume. Fwd converters have their
own limitations but can be excellent in some applications. I'll wait until
I've trialled a working version at 0.9 - 1.5 volt in and 5v in before I give
details. Long ago we built many of these in a number of forms. Vin was
typically, 12, 24 and 50 volts. Various outputs. Some with regulation.
Interesting design. Stay tuned.



Russell McMahon

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2001\09\18@084329 by Roman Black

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face
Russell McMahon wrote:

{Quote hidden}

And a VERY functional way of designing something.
:o)


{Quote hidden}

This would increase stability and provide some
relief from changes in Vbe and Beta with each
device.

{Quote hidden}

Sorry I've been away for a few days riding my
motorcycle but I am keen to have a fiddle. :o)
Apart from the emitter resistor already mentioned,
I can see some benefits from adding a capacitor
B-E on Q1, this could help tune the circuit when
matched with the performance of the inductor at
set currents, and may help oscillation over a
slightly larger current/voltage range than you
are getting now.

I think the main issue will be regulaton, and how
cheaply and simply it can be added, since you have
already proven that the circuit works and has an
acceptable efficiency for a quick nasty prototype.
-Roman

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2001\09\27@023449 by Russell McMahon

picon face
part 1 6666 bytes content-type:text/plain; (decoded 7bit)


           ********   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 eg 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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2001\09\27@234209 by Russell McMahon

picon face
Extract from some offlist correspondence:

> Things were slow today so I soldered up your circuit - bingo! Very
> interesting combination of r-c relaxation oscillator and saturating
inductor.
>
> A junk box toroid started things off; then I found a Coilcraft smd 68uH
> part and tried it. It pulled about 6mA @ 1.0 volts.  Then I played around
> with the r-c values and finally tried removing the led and replacing it
> with a diode/cap to see what sort of negative bias supply could be made.
> That one wasn't as encouraging but it was all fun to do.

Note that when the inductor "rings" it is driving the timing capacitor and
there is going to be an LC energy storage transfer / resonance effect. This
is not the primary timing mode of the oscillator proper but will interact.
The peak voltage that it will ring to will be affected by, amongst other
things, the ratio of L to C - I mentioned this in passing in my original
post. By decoupling the inductor from the oscillator proper you would remove
this interaction. The easiest ay to do this would be to add another
transistor as the inductor driver. This adds minimal complexity but two more
components. By now we have 3 transistors and some other circuit may begin to
commend itself. Still fun though.

After a little playing I was getting about 25 volts peak when the inductor
was rung without a LED.

As resonant energy storage is proportional to L, C, i^2 and V^2 then
increasing the inductor by N willincrease the capacitive peak voltage by
N^0.5 IF the peak inductor current is the same. (If increasing L by a factor
of N caused the peak current to drop to 1/Nth (as it may) then the overall
energy storage may drop with increasing inductance due tot eh I^2 term).

As I noted, this circuit is "designable" but has many complex and not
initially evident interactions. Chances are that some basic experimenting
will reveal a lot more about what "really" happens with changes than any
amount of modelling.


   Russell

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2001\09\29@015428 by Tom Messenger

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face
I tried out Russell's circuit, mainly out of curiosity.  It works quite
well, but aside from the novelty, I did not need a method to light an LED
from a single cell.  But it *is* interesting...

The best LED drive seems to be to connect the LED from the output to the
supply, as Russell mentioned.  Also, since NPN's are typically faster than
PNP's, I reversed the topology and achieved faster switching times. This
improves efficiency slightly and I think allows for higher voltages to be
produced.

So if you have a handful of old alkaline batteries sitting around waiting
to be disposed of, you can squeeze every last electron out them and have a
bunch of blinking LEDs decorating your lab in the dark.

In practical terms, aside from LED's, there is a good use for this circuit.
There have been several requests on the piclist for power supply circuits
so if you want one to play with, this is a good one as it is very low power
and won't likely let out the magic smoke from inside the parts.  And it's a
lot safer than, say, offline switchers.

Finally, if one needs to power up a pic circuit from a single cell (key fob
transmitter, rtc, etc.), you could do a lot worse than to try this out.
With the led removed, it's not too hard to get 5 or 6 volts out of it. And
since pic's can be made to run down to around 3 volts, it should do nicely
to power them.

Tom M.

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'[EE:] simple step-up SMPS challenge'
2001\10\23@231357 by Jinx
face picon face
part 1 778 bytes content-type:text/plain; (decoded 7bit)

As I (think) was the reason for the step-up challenge, may I say thanks
to Russell for his low-parts circuit and others for their contributions.
Sorry it's a bit slow coming but I'd been without a PC for nearly 5
weeks. Gosh I missed you guys, ya big lugs ya. The circuit attached
is what I'll use to charge a cap that then gets dumped into a solenoid

The unloaded voltage is 40V, with a 2200u cap attached it tops out at
24V. It reaches this in just short of a minute and reaches 20V in less
than 30 seconds, which is OK for 30 and 60 second clock movements.
They'll pulse alright with just 1000u so there's some room to play with.
Russell suggested a R/Zd on the output to feed back to the PNP base
but in my case that won't be necessary



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part 3 105 bytes
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2001\10\24@071104 by Russell McMahon

picon face
> The unloaded voltage is 40V, with a 2200u cap attached it tops out at
> 24V.
> It reaches this in just short of a minute and reaches 20V in less
> than 30 seconds, which is OK for 30 and 60 second clock movements.
> They'll pulse alright with just 1000u so there's some room to play with.
> Russell suggested a R/Zd on the output to feed back to the PNP base
> but in my case that won't be necessary

Amount of energy transfer and therefore peak voltages and charging rates can
be altered by using different inductor size and/or oscillation frequency.
Due to fairly massive interaction of several factors its a bit of a "try it
and see" circuit.
For anyone trying this - the inductor can be one of the small 1/4w resistor
sized chokes sold mainly for RF purposes.
Should be an OK cct for eg 12v supply for programming.




       Russell.

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