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Analog challenge exercise

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T3sl4co1l:

--- Quote from: Kleinstein on February 06, 2020, 04:35:02 pm ---With the wide input range the simple zener shunt regulation may not be adequate - unless the converter does at least some coarse stabilization.

--- End quote ---

It does, actually -- the RC timing is fairly frequency-stable (considering), for similar reasons why the 555 is stable; the switch charges the inductor to a fixed peak current, therefore as long as the output is DCM (which can be guaranteed, if a low enough duty cycle is chosen, which is also to say, a low enough nominal output current relative to the peak switch current), so the zener really doesn't have to do much with respect to line variations, only load variations. :)

Tim

Marco:
How much common mode current are those cores able to take before saturating?

T3sl4co1l:
Asking the real questions! ;D

I measured a Pulse brand, 1812 footprint (I don't have the exact part number on me at the moment..), 51uH part with saturation over 200mA.  A pleasant surprise!

Tim

Marco:
A constant power design is likely unbeatable on component count.

If you weigh efficiency a little heavier you could probably find some interesting circuits though. Usually the initial primary voltage spike before the current gets going on the secondary makes primary side regulation difficult with flybacks ... but what if you put a two transistor+zener overvoltage protection circuit on the secondary? When that blocks the current you'll get much larger primary voltage spikes, should be some way to use that for regulation with more consistent efficiency across loads.

T3sl4co1l:
Y'mean an SCR to shunt the secondary when it's at nominal voltage?  Has problems with reset flux and primary timing of course, but could be done.

Or a... the opposite of an SCR, it's on by default then snaps off at the threshold?  That would certainly be a cool way to reduce isolation component count -- even in regular circuits, where you just want to save an opto or whatever!  Not sure how you'd do that, maybe bias resistors to keep a transistor on then something like a foldback current limiter but sharper so it snaps off.

The newest and cheapest offline regulators do this sort of stuff; primary side regulation, not with secondary side shenanigans, of course; impressive simplicity, low parts count, low pin count even, mediocre efficiency but who cares when you're doing a stupid 5V 1W charger or whatever.


For those curious about something even simpler, here's a classic:
http://www.romanblack.com/smps/smps.htm
Downsides are high output ripple, no protection, narrow input range, etc.  You can't expect much from just two transistors, after all; it's only barely enough to even oscillate!

We can of course oscillate with one, if we allow transformers; my response to that would be: alright, where are you buying that transformer, off the shelf, for as cheap? :)

Heh... which, there's another thought: so what about transformers, we don't need steenkin' components, do a planar winding in the PCB!  And yes you can put core around it, but planar cores aren't terribly cheap in modest quantity either, nor are ferrite plates, etc.  But, y'know, you don't even need them if it's high enough frequency.  Now, you'd be right to guess RF transistors aren't going to be affordable outside of mass production quantities... but there's power GaN.  The EPC2036 is $0.47 in 100s from Digi-Key.  That's not terrible, and would make for one hell of a blocking oscillator.

Sheesh... I wonder if you can even pull off a GaN oscillator, without blowing up the gate in a single (or few) cycle(s)... not to mention minding the power dissipation rating of that dinky chip. :D

Tim

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