Mr RB
Well-Known Member
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.
That will give you an oscillator that turns off based on Q2 base >0.6v as it should, and after that you can increase the current in stages (with R2) to find the point the inductor saturation is causing an issue.
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), and then any Vout regulation you add simply increases the OFF time, as your previous mod seemed to be doing quite well.
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.
That will give you an oscillator that turns off based on Q2 base >0.6v as it should, and after that you can increase the current in stages (with R2) to find the point the inductor saturation is causing an issue.
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), and then any Vout regulation you add simply increases the OFF time, as your previous mod seemed to be doing quite well.
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