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Well, if you do 240vac to 12vdc off line and something goes wrong it could possibly send the peak of a 240vac rms sine out where 12v is supposed to be. Can you spell "Fry Daddy"
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BTW have you ever tried to run a regulated switching wall wart off of a 12v to 120vac power inverter without it being a pure sine (like the 'modified' sine type which is really just a plus and minus 145v pulse) ?
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I dont want to repeatedly stress the filter cap any more than it gets with the regular sine input. I know they do get pulsed even with a sine because of the conduction angles of the diodes, but i'd have to see how much worse it is with a super fast rising pulse instead of a sine.
You see where i am going with this? Maybe it would be a good idea to connect something between the pulsing 120vac output and the wall wart rather than connect it directly as with a regular ac sine line.
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Disappointment: with 1 amp output from the little switching wall wart (i did connect it finally for the test) the inverter draws 0.84 amps, which makes the whole setup only 50 percent efficient when powering the Tablet. I used a dummy load of 5 ohms instead of the tablet, but the output was a very clean 5v.
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Im not sure if you are aware of this or not, but the 34063 chip has more ripple than a conventional switching controller, for the same component values and operating load and such. That's because it uses a partly digital regulating technique rather than a pure analog technique that almost all other converter chips use. Does that matter that much? Well, maybe not as much if the inductor size is increased somewhat, like 2x times.
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About the peak currents into the wall wart (with 1 amp load on the wall wart)...
With the wall wart plugged directly into the 120vac mains line, the peak is up around 400ma, and it is a very sharp peak and there is no conduction for part of the cycle.
With the wall wart plugged into the inverter the main peak goes down to about 125ma, but there is a very sharp peak that lasts for a very very short time that could be 500ma or so. That pulse is so short though i have to wonder if it matters. The main current wave shape into the wall wart looks almost like the inverter output, very unlike the 120vac mains line current into the wall wart which is a sharp rise and then lowers down followed by a longer time where it ramps down more slowly, then goes to zero and stays there until the next cycle. So it could be the inverter is actually more gentle on the wall wart than the mains line is.
Oh you have a couple hundred in stock? No wonder you like them so much
What i did was i purchased a couple (i think 2 of them) before doing a simulation.
When i got them then i did a simulation and found the output ripple was so bad
i just couldnt believe it, and so i investigated and that's when i found that the
digitial technique is not such a great idea unless we really have to do it that
way.
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too large a difference to ignore because it's much easier to get high efficiency
with a 100uH inductor than with a 1mH inductor.
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I dont think i have any 1mH inductors around either except for low current ones
with somewhat higher than desired esr. I do have lower values like 50uH, 100uH,
150uH, etc., which can take up to 3 amps. ...
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Yes you're right if you can find an inductor that has low esr and also low Rac then it doesnt matter as much what the inductance is, but i automatically figure in the cost too when i talk about these. For example, the 100uH inductors i can get cost at most around 1.75 USD, usually less, and handle up to 3 amps or more (5 amps perhaps). Im shooting for a 2 amp output (max) so i want something that can handle say 3 amps before the inductance decreases significantly. ...
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If you can find me a ready made inductor from a reputable source (not eBay please ha ha) that is 1mH and has 10mOhm series resistance for a comparable cost i'll definitely consider using the 34063, but i havent had any luck finding them with that inductance and that low of an esr that dont cost a small fortune. Maybe im not looking in the right place Digikey? Winding my own means not only achieving the inductance but also keeping the core from saturating with DC current.
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BTW the problem with the said chip is NOT that it goes into discontinuous mode, that's entirely different, it's that it skips switch cycles because of the LC resonance. The LC "plant" reacts as it is driven, it is not driven to a predetermined output by a predetermined input because it has it's own idea of what the output voltage should be based on the energy that it took in. There's no way to stop it from reacting and get high efficiency at the same time without sync'ing the resonance to the LC natural frequency, which is not reliable.
If the above doesnt make any sense or seems strange, i'll post some wave forms which would show exactly what is happening and that clearly it is not entering a discontinuous mode of operation but is another 'mode' altogether not seen in more conventional (pure analog control PWM) switchers.
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LATER:
I found an interesting trick to help fix the situation above. It stems from the theory of boost converters where the output becomes unstable when the filter capacitance ESR becomes lower than a certain value. It's the one time when we dont want too low of an ESR for a DC power supply filter cap. Well the trick is to increase the ESR on this converter too, and that helps stabilize the output significantly.
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What i found in the past by calculation and by testing was that the smaller cores saturate too easy and the higher the mu the easier they saturate. Also, with a certain number of turns i might get the required inductance but then it saturates so i have to either use a gap and then increase the number of turns or else go to a lower mu core and they again add more turns. So either way it seems to increase the number of turns which raises the series resistance. Maybe a big enough core where it would not saturate as easily might work, so knowing the dimensions could help here.
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So i guess what i am asking is if you have an inductor you wound yourself that is 1mH and can handle say 3 amps i need to know what mu the material is and what the dimensions are.
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