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Switching Converters


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An inductor makes a high voltage across itself when current flowing in it is stopped, because the collapsing magnetic field cuts across the windings.
So the transistor creates a high current in the inductor, then stops the current which makes a high voltage that is collected by the rectifier and filter capacitor.

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Your schematic is of a boost converter running open loop. The duty cycle of your generator connected to pin 3 of the LM311 will control the output voltage. When Q3 turns on, you have in essence placed a voltage step across and inductor. The current will start to linearly ramp up until the transistor turns off.  During this, on time, D10 is back biased and all the energy being delivered to the load is coming from C10. When the transistor turns off, the flux collapses, the voltage across the inductor reverses, and the inductor current now starts to ramp down as you draw energy out of it. The output voltage in a boost converter is ALWAYS larger than its input voltage, so the output will be the input voltage plus the voltage across the inductor.

There are of course limits to the way you run this converter… you have to consider the switching frequency, the duty cycle and the saturation current of the inductor. For example… if the switching frequency is really low and the duty cycle is high, the inductor could saturate, at which point it stops being an inductor, and you can say bye-bye to Q3 because there is nothing to limit the current.

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That is correct, R17 is acting as a load. R14, R16 and R18 aren't needed. The LM311 isn't a open collector comparator.

I'd use a logic level MOSFET for Q3 and a schotty for D10. Also, the values of L10 and C10 seem disproportionate. It looks as if the schematic is from a simulation program. Some of those components might be needed to make the thing run in the simulator.

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Thank's indulis!

Seems like you are the one to ask when it come to dc/dc converters  :D
As I see it there is no need to bias Q3 as it will degrade the efficiency, am I right?
What's the reason for choosing a logic level MOSFET?
Sorry kerem for sort of hiijacking your thread but I hope it's ok as long as we stick to the subject.

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Vcc for the LM 311 is 6V, so it's output and base drive voltage will be some level less than that. All that is needed is a series base resistor to limit the current and drop the voltage. The function of Q3 is that of a switch.... it want's to be either hard on, or hard off. A MOSFET, almost always, is a better "switch" than a bipolar. A logic level MOSFET would be the best choice because it is hard on with "logic level signals" i.e. 5V. The down side of logic level FET's is they have a higher "total gate charge" to get the nice low Rds on numbers.... sub .01 ohm FET's are VERY common nowadays. The higher gate charge means more losses due to the switching element (Q3 in this case). Anything that draws power from the 6V source that isn't needed in the circuit to make it function will degrade efficiency.

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