The poor man SMPS

[mag]

Hi All,
I have found on an old computer board a very strange and very simple SMPS
based on 78L08.
You can find the schematics that I reconstructed from the PCB layout.
After a spice simulation, it really works!! But why?

The switching frequency is dependent from the L1 and C2 value and also from the input voltage and output current.

I don't understand its opearating principle, I think it will use the 7805 as a kind of error amplifier and the circuit oscillated when the feedback becomes
positive.

Did someone of you already seen this kind of circuit? If yes please give me
an explanation on how it works because I am really interested in it (just for curiosity).

Thank you

ciao

-marco

[Sch3mat1c]

R3/R2 sets hysteresis, so it's using Q1 as an inverter and switch, and U1 as an inverter and error amplifier. The values of L1, C2 (and its ESR) should be significant, as this is a hysteresis buck converter.

U1 has poor switching speed, so it's not going to be very fast or efficient. I suppose one would hardly care about efficiency if you're switching with a BJT anyway.

Tim

[Electrone]

Originally Posted by mag

Hi All,
I have found on an old computer board a very strange and very simple SMPS
based on 78L08.
You can find the schematics that I reconstructed from the PCB layout.
After a spice simulation, it really works!! But why?

The switching frequency is dependent from the L1 and C2 value and also from the input voltage and output current.

I don't understand its opearating principle, I think it will use the 7805 as a kind of error amplifier and the circuit oscillated when the feedback becomes
positive.

Did someone of you already seen this kind of circuit? If yes please give me
an explanation on how it works because I am really interested in it (just for curiosity).

Thank you

ciao

-marco

Hi. I'll describe somewhat its operating process. It self-oscillates because the delay involving the output inductor charging the output capacitor causes the voltage at the feedback pin to alternately rise and fall. Those things happen as the signal overshoots and falls below the reference voltage inside the 78L05.
IC.

[Elvee]

All 3-pin regulators have switching regulators examples in their applications examples: LM109, LM117, etc.
Some examples p19:
http://www.national.com/ds/LM/LM117.pdf
It is an inefficient and expensive way of building a regulator, compared to a dedicated IC: it requires more components, a much larger coil, has a poor efficiency and poor dynamic regulation.
A real winner.

[star882]

For applications where regulation is not very important and extreme efficiency is not needed, a discrete design can often be a better choice than an integrated design. Such as DC power tools, LED lights, battery chargers, and small motor speed controls.

[Electrone]

Here is something pretty much related. I might try an IC like it for a regulated battery charger.

http://www.national.com/images/pf/LM2575/1147501.pdf