Re: 5V switching IC


"Fred Bloggs" <nospam@xxxxxxxxxx> wrote in message
news:438C6BB1.2010108@xxxxxxxxxxxxx
>
>
> 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.
At no time have I 'described' anything about the way the circuit works.
_perhaps you should learn how to read threads_, before posting.
However the oscillator can work quite well, if used with care. Though it
does take some thought about the whole circuit round it, rather than being
treated as the typical 'black box' (though a different 'Black' here), that
most regulators are treated as by many people.
The time constants for the output stage, will depend massively on the
circuit attached, and this is the biggest problem in the design, in that
it cannot really be treated as a lump on it's own.
It is a useable circuit, that can form the basis of thinking, for budget
designs, which is what the thread started about. However the assumption is
present that if it is to be used as the basis of something outside it's
original work enviroment, it'll need to be redesigned by someboy who knows
what they are doing....

Best Wishes


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