The role of reference voltage in buck converters?

Question

I asked a question here which made it quite clear that it's probably not a good start to make something that I plug directly into the mains.

Instead, I've decided to build a low-voltage buck converter for starters. But I've run into something I don't understand.

1) You need a stable voltage to power your switching IC, or external circuit, or what have you. Usually lower voltage than the current you're trying to lower. 2) You need a stable reference voltage for your switching mechanism so it knows when it's duty-cycle is right on.

With these two things required in a buck converter, it seems you've already accomplished your end just to build the buck converter. I.E. you have to have pre-established stable, lower voltages to run a buck converter who's purpose is to produce a stable, lower voltage.

Why would you want a buck converter, and how does one establish a reference voltage and a voltage to power the switching mechanism?

EDIT

Reading both the answers I think I understand. For the switching mechanism, we KNOW the load it's going to draw and can plan accordingly. We can produce a very stable linear regulator if we know how much the load will be. We can even account for little fluctuations with a zener diode.

However, using the same process for the overall power supply would be inefficient, and if we drew too much current, the heat emanating from the components would be uneconomical. this is without mentioning that we would be unable to switch the load without getting a different voltage.

Correct?

Answer 1

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.

Answer 2

The reference voltage is required in the feedback loop of the switch mode supply so that the switching chip has something to compare the output voltage to - it needs to know whether it has to raise or lower the output voltage.

The reference voltage is usually a high-impedance, low output current reference, so it does NOT replace the entire switch-mode supply (which by definition has a low output impedance and high current capability). A simple (but not ideal) way to produce the reference is with a reverse-biased zener diode.

Answer 3

A voltage reference is at its heart a very precise linear regulator composed of a diode and a resistor. The higher the load on the reference, the more power wasted as heat. This of course defeats the purpose of using a switching regulator in the first place so it is recommended to use as little current from it as possible.