I suspected it was inductor saturation dragging Vce up.
Try increasing R2 (current sense resistor) to a high value so the circuit turns off at about 10mA. So R2 = 0.6v / 0.010A = 600 ohms. You will have to reduce load current a lot too, or just remove the load resistor and use something like a 12v zener as per my original circuit.
In real life the BC337 is good for 500mA (some for 1A) continuous, and even at currents far above that the BC337 may still have quite a low Vce saturation voltage, as it can be used to pulse 2A or so at least if I remeber right.
I think your simulator might be simulating unlimited PSU amps or something, a situation that just doesn't happen in real life from a 3.3v battery!
It would be worthwhile using a 47uF - 220uF input cap on Vin, and setting a current limit for Vin of a few hundred mA, similar to a small battery.
As I said previously a good way to tune the circuit is to set the current switching point by R2 first, at a current that will just supply the max desired output current. That gives you a safe "max power" the circuit is capable of (when boosting)
any Vout regulation you add simply increases the OFF time, as your previous mod seemed to be doing quite well.
Perhaps I am missing something with regard to Vce of Q1, but it appears to me that the base resistor limits the base current of Q1 to .7 ma with the result that Vce becomes larger than is being discussed. Looking at the PN2222A datasheet
http://www.datasheetcatalog.org/datasheet2/9/0o9g9t5j0kwhicxjckjr12szz0fy.pdf
According to figure 4, for a base current of .7 ma Vce is about .02V when Ic is 10ma but before the Ic rises to 150 ma, Vce is offscale at greater than 1V. If the simulator is correctly modeling the PN2222a this should add to the voltage rise at the base of Q2.
skyhawk said:Yes, I agree with the cycle you describe, but I am confused by the use of the term "inductor saturation". By inductor saturation, I understand that the magnetic material in the inductor saturates with the result that the inductance decreases and the back emf from the inductor drops resulting in a further increase in the inductor current.
skyhawk said:What I am suggesting is that as the current through Q1 increases, Q1 moves out of its saturated condition as determined by its base bias resulting in a rapid increase in Vce part of which is fed to the base of Q2 by way of the voltage divider formed by R3 and R4.
skyhawk said:The average value for a triangular waveform is 1/2 the peak. I calculate the input power as 453 mw based on an average current of 122.4 at 3.7V.
ronv said:I think # 65 may be a wrong schematic.
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I'm using KN2222A's FYI. they are similar and are good for 600mA continuous.
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... I improved the battery model by adding a 3.9 Ohms resistor in series with it, to simulate 1 Amp current draw. I also added a capacitor in parallel with it.
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I'm not disputing this. I simply want to marginalize the power wasted in R2 significantly. I would remove it if at all possible. I was thinking of a high side current sense instead, and set up Q2 to trigger entirely off of inductor saturation. What say you to this?
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Thanks for that. Sadly, I'm afraid it will likely prove to be the only real contribution I make to the blasted infernal contraption. At least that's what it's looking like so far....I may want to go back to the buck design, this damn thing is giving me (MORE) psychological issues. Plus I recently realized this boost circuit discussion could be seen as thread jacking the OP /)
skyhawk said:...
Perhaps I am missing something with regard to Vce of Q1, but it appears to me that the base resistor limits the base current of Q1 to .7 ma with the result that Vce becomes larger than is being discussed. Looking at the PN2222A datasheet...
I can't get it to oscillate. Did I get a value wrong?
Mr RB said:Sorry if I sounded grumpy there...
Mr RB said:OK, I think it is a bad move forcing the circuit to oscillate based on the saturation. In that case, the inductoir becomes a lot less efficient and even worse; Q1 is forced into a linear mode where it has a higher Vce and this is extremely energy inefficient.
Mr RB said:What you could do is add a voltage divider between Vin and the top of R2. If the voltage divider gives say 0.45v, then R2 only needs to produce 0.15v for the base of Q2 to get the 0.6v it needs for good fast oscillation.
The zener is a nice touch.
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If you want we can start over with the original circuit + zener mod and rework the whole thing from the top? I'm aiming for good regulation, with good efficiency, while maintaining the low parts count. I want to "collect the whole set" so to speak. For efficiency, I want to aim for no less than 80%. For Regulation, we can't really ask for more than 0.1v p-p ripple I don't think, but then again maybe I'm just being pessimistic. For parts count, below 15 parts would be optimal, 20 would be survivable.
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From what I see, as soon as Q1 is pulled out of saturation, it's collector spikes rapidly, raising R4 as well. And this positive feed back does/could powers it through the whole liner region in a hurry. The idea is we use Q1 as our current sense, and tune it to jump right at saturation...
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Are you sure you really mean the top of R2? And not actually somewhere on Q2 base, or something?
With respect, I don't see the first thought as a working option since R2 would effect the voltage divider such that the top half would need to be very low resistance, this would draw excessive quiescent current, and do so constantly.
Re your last post with LTSpice, can you please also post a GIF etc as some people (like myself) don't use simulators and can't see an ASC file.
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