All the electronics world find and need to reduce one DC voltage to a lower one. we may need to power a 3.3V micro-controller from a 12V supply. The solution is simple, we just add a 3.3V linear regulator IC like LD1117 with the 12V and it regulates the voltage down to 3.3V.
The Voltage regulators is called a buck converter or step down voltage regulators. It’s a type of DC-DC converter, so it accomplishes the task using a few transistor switches and an inductor.
It’s quite similar to a boost converter, but the placement of the inductor and transistor are switched. The switch shown in the above circuit will normally be a power electronics switch like MOSFET, IGBT or BJT. The switch will be switched (turned on and off) by using a PWM signal.
The working of Buck converter is slightly similar to that of PWM ‘dimming’. We’ve all heard of lights being dimmed by a PWM signal. A small duty cycle means that the average voltage seen by the load is small and when the duty cycle is high the average voltage is high too.
But average voltage is not what we need – a raw PWN signal oscillates between high voltage level and ground, something no delicate load MCU would like. Of course, connecting an RC filter to a square wave source renders the output clean. The voltage level of the filter depends on the duty cycle of the PWM signal – the higher the duty cycle the higher the output voltage.
So now we have a clean output voltage. The below graph shows the raw PWM signal in blue color and the filtered outputs in red and violet color.
We could now simply use this as a buck converter, but there’s one major drawback –the resistor in the RC filter limits the current and wastes energy in the form of heat, which is no better than the linear voltage regulator example.
To fix this problem, we turn to another type of voltage filter, the LC filter, which does the same job as the RC filter but replaces the R with an L, in other words the resistor with an inductor. The inductor resists changes in current and the capacitor resists changes in voltage, which results in the output being smooth DC.
And now we have a converter that is capable of stepping down DC voltages and doing it efficiently.
Since the switch is on the high side, using an N channel MOSFET or an NPN bipolar wouldn’t work, unless we have a bootstrapped gate driver. Though this is possible, it is quite complicated.
Using a P channel device in these circumstances would be recommended, they greatly simplify driving requirements, but remember that they turn on when the gate is low, so an inverted signal would be necessary. One can use the IRF5210, it has a decent on resistance of 60mΩ and a VDS of -100V,which should be plenty for most applications. However, there are many better devices available, the choice is entirely up to the designer based on the specific application.
Since this diode does not have to handle very high voltages, rather high currents, it would be a good design choice to use a Schottky diode with a low forward voltage drop to keep things efficient.
The capacitor value depends on the output voltage ripple and can be calculated using the capacitor equation, but generally a value between 100uF to 680uF for low current applications should suffice.