Buck Converters

Buck Converters Lower DC Voltage and Ensure a Given Voltage. Some Can Limit Current, Too.

There are many ways of lowering the voltage, for example a voltage divider (using resistors). In fact, any load will reduce the voltage by consuming part of the energy available.

If you want to lower the voltage without consuming energy though (i.e. wasting energy by dissipating some of it as heat), you need a Buck converter.

Turning Input Voltage On And Off

A Buck converter consists of a high-frequency switch that turns the input voltage on and off in rapid succession. This is the switching frequency (and explains why Buck converters are called switching power supply). Typically, Buck converters use a switching frequency of 50kHz up to a few MHz, so they switch the input voltage on and off between 50 thousand to a few million times per second.

If we’d stop here, we would have PWM: the output device would receive the full input voltage alternating with no input voltage at all. In the average, the output voltage would now be lower than the input voltage, and in fact loads such as LED and motors can be controlled this way because they don’t really care much about voltage spikes.

Hoewever, if you plan to supply more sensitive devices such as microcontrollers, you cannot use PWM output directly. You need to supply a constant voltage and cannot have a mixture of voltage spikes and no voltage.

This is why Buck converters add a coil, a diode, and a capacitor. Here is a circuit diagram showing the fundamental components of a Buck converter:

Phase 1: “Borrowing Energy”

Let’s first look at the Buck converter when its high-frequency switch is turned on. Here the schematics are very simple:

The only new thing is a coil that needs to be passed on the way to the load.

A coil is just like another load. and it consumes energy, too, just like any other load or resistor (to create a magnetic field). So as long as the magnetic field is still building up, the coil acts like a resistor in a voltage divider and reduces the voltage. Mission accomplished.

Unlike a resistor though, the coil does not dissipate the energy. Instead, it “invests” the extracted energy into a magnetic field. Magnetic fields store energy, similar to a battery, and can give it back. Dissipated heat from a resistor is lost.

Which brings us to the second half of operation: the off phase.

Phase 2: Running From Borrowed Energy Only

In this phase, the input voltage source vanished because the Buck converter turned it off. It does not supply energy to the output at this point:

Instead, the magnetic field starts to collapse, and the coil starts to return the energy it previously extracted. The energy that was needed to create the magnetic field is now used, and the coil acts like a battery.

Of course, current can only flow when there is a closed circuit. Since at this point the input source is separated, a diode now guarantees that the circuit remains closed.

Borrow, Return, And Repeat…

When the Buck converter switches back to on mode in time before the magnetic field has fully collapsed, the process repeats. In the on phase, the output voltage comes again from the input power supply and is again reduced by the energy that the coil needs to rebuild its magnetic field. The diode is now in blocking mode.

An additional capacitor can help even out the voltage which otherwise would be “rippled” because building and collapsing a magnetic field isn’t linear.

In short, a Buck converter is reducing the voltage much similar to a resistor. But instead of wasting the unwated voltage uncontrolled by dissipating heat, it temporarily stores this energy in a magnetic field, and returns it while at the same time, the input power supply is temporarily cut off.

A Buck converter works almost as if you’d charge a battery in series with your load, extracting and transferring energy to that battery. Then, once the battery is full, you run your circuit for a while from the battery until it is empty, and repeat. So in a Buck converter, the coil and its magnetic field act like a wear-free fast-loading and fast-unloading battery.

The switching frequency of a Buck converter determines the size of the coil and capacitor. The higher the switching frequency, the shorter the two on and off phases are, so less energy needs to be temporarily stored, and components can be smaller.

At the same time, with higher frequency the energy must be stored and retured more often. Any loss associated with this process increases. Higher switching frequencies can therefore decrease efficiency.

Here is a list to popular Buck regulator chips along with commonly available and ready-to-use breakout boards that use these:


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(content created Mar 08, 2024)