What Is a Buck Converter?

A Buck Converter takes a high DC input and generates a lower output, stepping down the voltage. It can take a direct DC input from a battery, or it can be derived from AC at line level either directly via a rectifier circuit or indirectly through a step down transformer.

The operation of the buck converter is based on the principle that an inductor has a characteristic current/voltage relationship, where changing the current through it will result in a change in the voltage across its terminals. As the switch in a buck converter is constantly turned on and off, the inductor will store energy in its magnetic field in an attempt to maintain a constant current through it.

When the switch is in the open position (off-state), the current through the inductor is zero. When the switch is closed, the current will start to increase, resulting in the inductor producing an opposing voltage drop that reduces the net input voltage to the system. The inductor stores this energy for a short period of time before the rate of change decreases, causing the voltage to rise again until it reaches its maximum value.

In order to reduce the output voltage to a specific value, a capacitor is used on the output side of the converter. This helps to filter out the voltage spikes that would otherwise occur when the switch is cycling between the on and off states, making it possible to produce a smoother DC output voltage that can power electronic devices.

To enable a more efficient conversion process, a feedback mechanism is used to monitor the output voltage and adjust the switching duty cycle, or the proportion of time that the switch is on, to ensure that the output always matches the input. This is done by using a feedback network consisting of a voltage divider and a control IC. The feedback network can also be used to compensate for any fluctuations in the input voltage or load current.

Buck converters have a number of advantages over other types of switch-mode DC-voltage regulators. In particular, they are highly efficient, with efficiencies of 90% or higher common in well-designed circuits. This makes them very popular for use in a variety of applications, including computer power supplies, mobile phone chargers, and automotive or renewable energy systems.

In this article, we’ll be looking at some basic principles of the buck converter using LTspice. We’ll explore the underlying theory behind the buck converter, and then we’ll look at some example circuits and simulations to demonstrate how they work.

As you may have noticed from the diagrams above, the buck converter has a fairly complex structure, involving both a switching transistor and a body diode. The hysteresis of the threshold on the low side MOSFET results in a considerable loss of current to the drain, and this is why the diode is included. For a buck converter to be reliable, the hysteresis and losses need to be minimized. To achieve this, the input current can be controlled using a PWM control signal.