How to Quickly Differentiate Between Buck, Boost, and Buck-Boost Basic Topologies?

When it comes to DC-DC, we are familiar with these three circuits.

The Buck circuit, also known as a step-down circuit, is a common non-isolated DC-DC converter circuit, but it can only step down.

It mainly consists of a switch transistor Q1, a freewheeling diode D1, an energy storage inductor L1, an output filter capacitor C1, and a load resistor R1.

When Q1 is at a high level, Q1 conducts, D1 is reverse biased, and current flows through the inductor to supply power to the load. At this time, the current gradually increases, but due to the self-induced electromotive force of the inductor, the current rise is impeded. The inductor converts electrical energy into magnetic energy for storage.

When the control circuit pulse is at a low level, Q1 turns off, and since the inductor's current cannot change instantaneously, the inductor's ends become positive and negative. Then the current decreases, and the energy stored in L1 is converted back to electrical energy and released to the load R1.

Note that the capacitor here is a filtering capacitor (C1). Do you know its role?

Next, let's look at the Boost circuit, also known as a step-up circuit.

The circuit components are the same as the Buck circuit, but there are differences. The inductor L1 is on the input side and is called the boost inductor.

When Q1 conducts, the input voltage charges the inductor, with the circuit being: Vin --- inductor L --- switch transistor Q1.

When Q1 is turned off, energy from the input and inductor simultaneously supplies energy to the output. The circuit is: input Vi → inductor L → diode D → capacitor C → load RL. At this time, the power supply voltage of the load is equivalent to the input voltage plus the induced electromotive force of the inductor, thus achieving a boost.

The Buck-Boost circuit, also known as a step-up/down circuit, can be seen as a series connection of Buck and Boost converters, but with the switch transistor combined. Its output voltage can be lower or higher than the input voltage and is a type of single-transistor DC converter.

When Q1 conducts, the input current flows directly through the inductor to ground, and the output on the right is maintained by the capacitor discharging.

When Q1 is turned off, the inductor current flows from ground to the load and capacitor, then after passing through the diode, returns to the inductor. This is the process of releasing energy from the inductor and charging the capacitor.

So, is the output voltage stepping down or stepping up?

This depends largely on the duty cycle of the PWM wave controlling the switch transistor. If the duty cycle is greater than 1/2, it's stepping up; otherwise, it's stepping down.