Eric:
I thought the experiment was going to be to compare the two circuit waveforms side by side, ie with no battery (connected normally) and then with the battery instead of the emitter connected to the output?
Can you show the waveform with and without the battery? I think that would do it. They should be similar.
Very cool effect! Thanks MrAl and EricGibbs for testing that, it's interesting to see it will still oscillate based on collector current (collector saturation) of Q1 plus possibly some inductor saturation effect in there too.
However I think this will require higher peak inductor current (with the associated loss in efficiency) and of course the very real issue that there is no load regulation!
In one of my early tests before this design I had an extra resistor of 1 ohm or so between the inductor output and the load cap. Feedback was taken from the front of the resistor to Q2 base, so there was a much larger voltage ripple to activate Q2.
Once the RC delay was added there was no longer a need for the load voltage to have ripple to give a reliable low freq oscillation.
Eric:
Can you repeat that but take the time shot after say 10ms ? We're trying to see the operation after a decent start up period after all the active elements have reached their nominal values.
If that instability persists, we can get rid of it quite easily i think. The instability creates problems one being increased output ripple...but that's only if it persists after a decent start up time period.
Hi again Eric,
Can you try adding a 1 ohm resistor in series with the emitter of Q2 ?
Hi Eric,
Ok. There's got to be a reason it is so unstable. Perhaps it is because we are not using any series resistance for the coil?
Maybe 1 ohm in series with that too.
That instability does not occur in the actual circuit, apart from the situation where output current is too low to really get the inductor and output cap "working". If you check my 'scope photos you can see how stable it is once current is greater than about 15mA.
Also there should be some mV of voltage ripple on the output, I'm curious what you are getting there?
Ideally to tune the circuit the RC delay keeps the SMPS off long enough to get some reduction in Vout, I can't remember exact ripple voltage but 20mV might be ok. Then when the RC delays ends and the regulator is re-enabled the ON time will be longer, as it takes that bit longer to recover the lost Vout and oscillate nicely around the regulated Vout.
I think some of the operation you are seeing is due to reduced Q1 and Q2 gain, and it's oscillating a bit more based on Q1 characteristics and not as much on Vout characteristics as it should, if you get my meaning.
One thing you can try is to set the transistors to BC337-40 and BC327-40, these are the ones I stock and they are the higher beta versions (I think the -40 means beta of 400, so they are a better grade of BC series transistor).
To improve efficiency on most of these designs I was reducing the Q1 base current by increasing R1 and R2, in line with what worked well with the -40 transistors. You will probably get better stability and better Q1 saturation if you increase Q1 base current a bit, especially if you are not using good transistors. Many of my BC transistors measure >500 beta on my beta tester.
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Roman where are the scope pics you are talking about?
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8 or 9 ohms sounds a little high though. Is that the series resistance of the real life circuit too (inductor)?
Tvtech said:...
Even though I said this discussion was way above me....if circuits rely on specific components meeting A1 rating...as in BC 337-40 (best quality/highest gain)....then the circuit is flawed.
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EricGibbs said:...
I have tried various combinations of transistors, it works OK in simulation, only a small change in running frequency.
Even tried a 2N3055 and its PNP equivalent, works OK.
As I said in Post #28, with the 47uF having a low internal resistance the frequency is steady.
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I tried 2N4401 and 2N4403.
I found that with the 47uf cap having an ESR of 0.01 the circuit was unstable, but increasing that to 0.1 made it stable.
That's with the 50 ohm load.
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I also found that taking the 1nf cap out of the circuit it still workedI believe that is because there is a delay introduced with the inductor so the negative feedback is not felt by Q2 until after some small delay period anyway. Interesting. Also interesting is that all instability goes away regardless of the 47uf cap ESR.
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ericgibbs: Moderator extraordinaire said:hi,
With that version of the circuit I get this simulation.
I suspect the falstad sim is very limited.
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