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Thread: 2-tran SMPS 5v reg now 90% efficicient!
www.piclist.com/techref/index.htm?key=tran+smps+reg+efficicient
picon face BY : Morgan Olsson email (remove spam text)



Thank you for your work Roman, this Black regulator really look nice :)

>For 1A maximum
>I think I would use 2x BC327 in parallel and 2x 1N5819
>in parallel.

Warning: Parallelling bipolar transisotrs is usually avoided, since their temperature characterstic will make only one of them operate:  Th eone whichis slightly warmer wil have lower Ube, so, it do more labour than the other, and get still warmer etc.  Finaly you have one very hot tranny and the other just sitting there.  Solution: use separate base resistor(s).

Parallelling bipolar diodes i have never seen, may be same problem, but cheaper to use one bigger eanyway.  But for shottkys like 1N5819 i have done and no problem.

>This is still smaller and cheaper than
>just about any alternative.

Maybe Bc640-16.  (that -16 defines hFE=100..250, plain Bc640 is only 40..250 which would require designing for higher base drive margin)

Btw for even cheaper design use i.e Bc546B for Q2.

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Possible problem: verify peak negative Ube of Q2.  I fear that for higher input voltages it will be over spec (which usually is 5V for transistors like theese)  Problem is when it switches off, and C2 pulls it far down.
Securely limited by the zener to less than minus 1V (w regard to neg supply) peak.
But for 5V out we may only go down to neg supply.
If output voltage is 4V or lower then we can rely on the zener to protect Q2 from too high reverse Ube.  But for higher out voltage C1/C2 ratio is to be checked:

Simply, C3/C2 divides the amplitude over D1 to make the amplitude at Q2 base.
Example: at 30V in the amplitude at D1 is almost 31V.
Say we want to design for an amplitude at Q2 base of 5V (Ube then swings between about +1 and -4 V)

Then C2=(C1 * 5/31) ;  If C1=6n8 then C2=1n1

(actually the amplitude will be slightly less due to positive clamping by the zener)

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Side note: all paragraphs on i.e http://www.romanblack.com/a02.htm gets line wrapped in browser Opera, making a mess of ASCII art and test data. (NO paragraphs are wrapped using Mozilla.)

/Morgan

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