piclist 2002\11\02\072734a >
Thread: Son of simple step-up SMPS challenge
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face BY : Russell McMahon email (remove spam text)

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*** Yes! This circuit DOES oscillate as shown ***

Here's a simple RC oscillator that MAY show promise for driving very low
voltage boost inverters.

The basic idea came from the 4 transistor boost convertern cicruit I posted
a few days ago but this has been changed so that the oscillator uses two
transistors only. This is essentially a 2 transistor Schmitt trigger with
feedback. Values shown work but are typical only and will need to be adapted
to a specific design.

The oscillator starts well at Vin down to less than 0.9 volt

Adding R5 & Q3 in place of R4 with an inductor to ground from collector of
Q3 gave enthusisatic negative going pulses (as would bee xpected). I played
with, rectification and a LED load. Efficiency was not marvellous but can
probably be made quite respectable. The oscillator proper can be designed to
have low current drain.

While its function may not be obvious to the uninitiated, Q2 is absolutely
vital for oscillation. It serves to provide Schmitt hysteresis by providing
a lower voltage across R1 with Q1 off, Q2 on and a higher voltage with Q1
on, Q2 off.

The only critical design demand for oscillation is that !q1_on is lower than
!q2_on. Fir fully saturated transistors tghis is satisfied by R3 rather
larger than R4 as shown.

The advantage and potential disadvantage of this circuit is that operation
is RC time constant controlled, independent of inductor flyback timing. This
can allow confidence in coil driving and on time etc but care must be taken
with duty cycle allowed and not driving the inductor long enough to allow it
to be severely saturated.

The circuit may be redrawn as a lontailed pair cross coupled oscillator (of
sorts) although the connection of both bases to one collector and no bases
to the other may bend the brain somewhat until it is appreciated that
feedback is via the common emitter coupling.

I tried an inductor in place of R4 and while this did produce flyback action
it was not very satisfactory. More experimentation MAY produce a useable
design. I wonder what Roman can do with this? :-)


       Russell McMahon


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