Buck Converter

In this article:

- What is a buck converter and why you'd like one.
- How it works in simple terms.
- Pros and cons and how to avoid common pitfalls.

What is a buck converter

Every circuit, every device need a specific voltage to work properly. However sources of electric power not always, if ever provide the correct voltage for a given device. To adderess this problem typically a voltage regulator is used, and the most commonly used one is the linear voltage regulator. This kind of regulator chokes the output voltage by making it to drop at a transistor that works as a variable resistor controlled by a feedback circuit (see conceptual diagram).

Linear voltage regulators such as the LM7812 provide good noise rejection and stable voltage. Though they are not much efficient, in particular when the difference between the input and output voltage is large and some high current must go through the regulator.
The following table gives an idea:

As you can see as the current or voltage drop raises the power losses increase quite significantly.

Switching is the key word

Because to make the output voltage stable linear regulators work by linearly dropping voltage across them, they unavoidably have low efficiency. The key here is the linear relationship between the output voltage and the voltage drop across the regulator, so what if we break this linear relationship?

Well, this can be achieved by switching on and off continuosly and making some kind of average at the output. This is in essence a buck converter: it chops the input power into a smaller output voltage, and related current.
This is an interesting feature because on linear regulators the input current is the same as the output current, while with the buck converter this fixed relationship is broken and we can get more current at the output than the one that flows at the input, provided the overall power is the same.
More precisely the output power must be less than the input power to account for the losses introduced by the process of conversion.

How a buck converter works

Referring to the picture above, we have two phases: On-state and Off-state. While in on-state the current is pumped into the inductor to charge the capacitor (see the red lines). In this phase the energy is stored into both the inductor and the capacitor, and also going through the load.
When enough energy is stored and the output voltage reaches the desired level, it switches into the off-state.
The switch opens (in a real circuit the switch will be replaced with a voltage controllable switch such as a MOSFET) so the sudden change in current across the inductor would make it to accumulate charges at its terminals creating a voltage.
That would keep the current to flow if a path for the charges is found to reach the other terminal of the inductor. This is achieved across the diode, so the current can flow from the inductor, through the load and back to the inductor up to the point the stored energy is dissipated. At this point the cycle starts over again.

My own Buck Converter

The above schematic shows the buck converter I've designed and built. You can spot the main switch as the IRFU5305, the main diode as the BYT30P400, the main inductor is a 78 μH, the output capacitor is the 2200 μF. The other inductor and capacitor are used to filter out the switching spikes. It is important to note that the first capacitor (the one connected with the main inductor) must be a low ESR cap to reduce noise and to make the circuit more efficient.
Also about the diode a better choice would have been to use a schottky diode, but I didn't have any suitable diode at the time of the build so I used a silicon fast recovery switching diode, and to help the transition between the conduction states a snubber has been added. It is composed by the 18 Ω 3W carbon resistor and the 1.5nF 100V polyester low ESR capacitor. to avoid the risk of spurious ringing the chosen resistor must have a very low parasite inductance.

The remainder of the circuit is to just drive these main components. The 741 Op Amp is used as an error amplifier to compare the output voltage in respect to the control voltage (CV). Its output goes to one of the two comparators that fit a LM393 dual comparator. This error voltage is compared against a sawtooth waveform generated by the other comparator and the BC327 transistor configured as a constant current source that charges linearly the 10nF capacitor generating the rising ramp. Since the comparator sports an open collector output this is also used to quickly discharge the capacitor, completing the sawtooth waveform.
This sawtooth is then fed into the first comparator to generate a PWM signal that controls how long the MOSFET should stay at on-state.

Notice that, to increase efficiency by speeding up the time the MOSFET takes to turn off, the driving circuit is configured so that the MOSFET is normally ON when no driving signal is issued to the first 2N3904 transistor (or ZTX452 if the circuit works at 60V), because the MOSFET is normally kept to GND through R 680 Ω.

Features

• Input voltage: 28V or 60V DC
• Output voltage: 6 to 28V or 10 to 60V DC
• Max current: 10A
• Efficiency: 93%
• Switching frequency: 32KHz

Pros and Cons

Now its time to have a look at what are the pros and cons of a buck converter in place of a linear regulator. First off we can notice that the buck converter works by swicthing the current through different paths. This means we may have a lot of noise emitted from these different paths. Thus one important point when designing a buck converter is the layout of the components and to pay attention at the path the current will follow. Reducing the distance between these two paths, and in particular to avoid the GND bouncing, it is of paramount importance to limit the noise both radiated and induced. So buck converters may be hard when dealing with EMI, or passing an electromagnetic compatibility test. Clearly linear regulators do not suffer from this kind of problem.
Of course swicthing circuits have bulkier and more expensive components, such as inductors and low ESR capacitors.

So we can summarise the pros and cons as follow:

More information and how to avoid common pitfalls

Watch the video to get more information, waveforms, a theoretica explanation and how to avoid common pitfalls.

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