Re: 5V switching IC



Roger Hamlett wrote:
"Bob Monsen" <rcsurname@xxxxxxxxxxx> wrote in message news:pan.2005.11.29.01.04.32.296552@xxxxxxxxxxxxxx

On Mon, 28 Nov 2005 16:55:53 +0000, Mike Young wrote:


"Don Foreman" <dforeman@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:974ho1pnforfpol4vkpjs89b0fvc7c9e1r@xxxxxxxxxx

On Sat, 26 Nov 2005 11:24:18 GMT, "Roger Hamlett"
<rogerspamignored@xxxxxxxxxxxxxxxxxxx> wrote:



I'd suggest going discrete!.
If you look at:
http://www.romanblack.com/smps.htm

This circuit costs the least of any design that I know of, with reasonable
efficiency, and the parts cost will be less than the IC solution,
especially once you have added the discrete parts to the latter. :-)

That is a neat circuit!

Can you help me understand this better? Even with Roman's explanations, I
don't have enough of a basic grasp to figure out what's doing what when. I
would like to drop 48VDC unregulated to 12VDC and 5 or 3.3 VDC supplies. It
seems straightforward enough, but the waveforms in SPICE are not very
encouraging. The duty cycle on the main chopper can be quite short depending
on load, and this has strange effects on the oscillator. I can continue
changing the inductance and capacitance values randomly until it looks good,
but that would like to understand the relationships better.


There is a feedback loop, through Q2's emitter to the base of the PNP pass
transistor (Q1). That is the source of the instability that causes it to
oscillate. When Q1 is on, current is poured into the smoothing cap through
the inductor, causing the output voltage to increase. Once it increases
enough to shut off Q2, Q1 gets shut off too. The current through the
inductor stays on for a bit, and thus the voltage continues to increase as
its magnetic field collapses, but eventually stops, allowing the load to
run on the smoothing cap. Thus, the output voltage eventually falls. When
the output voltage finally goes Vbe below the fixed 5.6V threshold, Q2
starts up again, which again starts up Q1, restarting the cycle.

The thing that makes it oscillate is that the big inductor creates a
delay between turning off Q1, and when the output voltage actually
responds to the cutoff. The voltage continues to go up even after Q1 turns
off, and then droops once the inductor's magnetic field is gone. Same is
true of turn-on; turning it on results in current through the inductor,
which gradually builds up to the point where it overcomes the load and
starts charging the capacitor. So, the frequency will depend on the size
of the inductor (how far the charge lags the voltage changes), the size of
the smoothing cap (how long it takes for charge to change the voltage),
and the current draw of the load (how quickly the inductor will overcome
the load).

The cap C2 gives the turnon-turnoff a tiny bit of speed up, in that when
Q1 turns on, it'll yank Q2 on just a bit more with positive feedback, thus
making it return the favor to Q1. When Q1 turns off, it'll yank Q2 off
just a bit more, speeding up the Q1 turnoff. This sharpens up the edges a
bit, leading to a bit better efficiency.

Sadly, for certain loads, I've found that the circuit will simply fail to
oscillate, reverting to being an inaccurate linear regulator without
current protection or temperature compensation. If you are trying to drop
48V to 3.3V with any kind of current, that is clearly a bad thing.
Dissipation will go from Iload*3.3V/E to Iload*48...

Yes.
There are a couple of comments here.
The original poster, was asking about 10 to 20mA at 5v, from 24v. For this the little Black inverter is pretty ideal. However once the supply goes much higher than this, it becomes necessary to redesign the front end of the circuit. Also all switchers of this type, can potentially have problems with very large load variations. As load variations increase, chosing the time constant of the output components becomes increasingly critical. I too have seen some problems occasionally getting the circuit to start, usually where the circuit attached has a very 'soft' switch-on characteristic.
The second poster, seems to have different input and output requirements. Given the need for multiple 'rails' here, It might well become a balancing act, between a multi stage version of this, having a version of this as a 'pre-feed' to a couple of linear regulators, or using a transformer with two secondaries. The best choice will depend on the loads involved, the accuracies required etc. etc..

Best Wishes



The circuit is a hellacious p.o.s., and it doesn't work the way you describe, plus that comment about reducing the output filter capacitor to encourage oscillation is a nice try if your load does not mind 100% Vcc overshoot. So the circuit as shown is totally unreliable, it can blow out the load on turn-on, it can blow itself out by hanging in linear mode, it can blow itself out by destroying the NPN EB junction, nobody seems to have a handle on the critical time constants if any, the output voltage is largely an unknown with the zener ringing from an indeterminately large current spike, nobody seems to know what value of inductor is best, nobody seems to know how much filter capacitance to use, nobody has a handle on efficiency, and nobody seems to know what range of loading may be required, or not, for their blind random selection of components. That's quite the curiosity piece, Roger.

.

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