Electronic – Buck converters duty cycle

Here is a hint: -

Both switching devices will be MOSFETs

This tells me that the buck converter is a synchronous type with a push-pull type switch arrangement - this makes estimating the duty cycle very easy; one switch connects the output to the incoming positive rail then the other switch grounds the output. The average voltage of the switched waveform for an input voltage of 3.7 volts and an output voltage of 1.2 volts is ......? Please fill-in the gap

If you don't quite understand where I'm coming from consider the "proper" output after the inductor and capacitor and think of what the inductor is - it passes DC i.e. if the input voltage has an average value of X then the output voltage also has an average value of X (yes there will be a small volt drop due to current thru the inductor because of the inductor's dc resistance but this can be largely ignored for a decent approximation).

I will try to answer your points assuming I have read them correctly.

1) The driver for the buck circuit will generally need some kind of regulation, but this can be a low power regulator. You then use the buck circuit to step down your supply voltage and regulate it, this will provide a lower stable voltage with more current for your load.

2) If your have either a dynamic supply voltage or load then the buck converter will need to be regulated to maintain a stable voltage. This is achieved by varying the duty cycle to the power MOSFET and feedback is required to the driver circuit to determine how the duty cycle is adjusted. You can use voltage feedback and then some closed loop control circuit to alter the duty cycle pending on the feedback signal.

To summarise a buck circuit is a good way to step down and regulate a large voltage to a smaller voltage, this can then provide power to a small, medium or large load. Additional regulation is generally required for control circuitry such as a microcontroller and driver circuit, but these circuits generally have very small power requirements.