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Old 10th June 2010, 10:12 PM   #1
mag is offline mag  Switzerland
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Default The poor man SMPS

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
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File Type: pdf poor_man_smps.pdf (46.3 KB, 242 views)
File Type: pdf poor_man_smps_sim.pdf (48.7 KB, 114 views)
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Old 10th June 2010, 10:32 PM   #2
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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
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Old 10th June 2010, 10:46 PM   #3
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Quote:
Originally Posted by mag View Post
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.
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Old 11th June 2010, 07:53 AM   #4
Elvee is offline Elvee  Belgium
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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.
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Old 12th June 2010, 04:24 AM   #5
star882 is offline star882  United States
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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.
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Old 12th June 2010, 10:44 PM   #6
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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

Last edited by Electrone; 12th June 2010 at 10:55 PM.
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