Buck Converter
A buck converter is a special type of SMPS that steps down the input voltage to a lower output voltage. It is widely used for power management and low-power microprocessors. A buck converter works by turning a switch (usually a transistor) on and off, storing energy in an inductor and capacitor, and then delivering it to the output in a regulated way.
Figure 1.0 Buck Converter
The above figure shows the current path when the switch is turned on and off (shown by the thick line).
- When the switch is on, the input voltage is connected to the inductor. The difference between the input and output voltage is then forced through the inductor, causing the current through the inductor to increase.
- During the on-time period, the inductor current flows into the load and the output capacitor (the capacitor is charged during this period).
- When the switch turns off, the input voltage applied to the inductor is removed. However, since the current in the inductor cannot change immediately, the voltage across the inductor will adjust to maintain a constant current.
- As the current decreases, the input of the inductor is forced to develop a negative voltage, eventually reaching the point where the diode conducts. The inductor current then flows through the load and back through the diode.
- The capacitor discharges to the load during the off period, thus making up the total current delivered to the load (the total load current during the switch off period is the sum of the inductor and capacitor currents).
Continuous vs Discontinuous Operation in Buck Converter
Buck converter can operate in two states:
- Continuous, where the inductor current never falls to zero
- Discontinuous, where the inductor current falls to zero at the end of each cycle.
In most buck regulator applications, the inductor current never drops to zero during full load operation (this is defined as continuous mode operation). Overall performance is usually better using continuous mode, and it allows maximum output power to be obtained from a given input voltage and switch current rating.
In applications where the maximum load current is quite low, it may be advantageous to design for discontinuous mode operation. In these cases, operating in discontinuous mode may result in a smaller overall converter size (because a smaller inductor can be used).
Discontinuous mode operation at lower load current values is usually harmless, and even converters designed for continuous mode operation at full load will become discontinuous as the load current decreases (usually without causing a problem).
Typical application example
The following figure shows a typical application example of a buck converter, LT8631.The LT8631 is a current mode PWM step-down DC/DC converter with internal synchronous switches that provide current for output loads up to 1A. It can be used in a system level solution that effectively converts the high-voltage rail to 5V.
The main advantage of the buck converter is its simplicity and flexibility. The following table summarizes the key considerations for buck converters.
| Advantages | Disadvantages |
|---|---|
| Buck converters are known for their high efficiency in converting higher input voltages to lower output voltages. They waste less energy in the form of heat than linear regulators. | Buck converters require complex control circuitry to operate properly, which increases design complexity and potential reliability issues. |
| Due to their high efficiency, buck converters generate less heat, which is critical for applications where thermal management is a concern. | Buck converters generate EMI due to switching, which may require additional filtering and shielding. |
| Buck converters are generally smaller and lighter than linear regulators, making them suitable for applications where size and weight are limited. | Some buck converter designs may have higher output voltage ripple compared to linear regulators. This can be a problem for applications that require extremely low noise levels. |
| Buck converters can support a wide range of input voltages, allowing them to operate properly from variable or unstable power supplies. | Buck converters can only reduce the input voltage and are not suitable for applications requiring higher output voltages than input voltages. |
| Buck converters have fast transient response, making them suitable for applications that require rapid adjustment to load changes. | Some buck converter designs have limitations in the maximum current they can handle. High power applications may require more complex buck converter configurations. |
| The buck converter provides a stable and well-regulated output voltage even if the input voltage fluctuates. | The design and selection of buck converter components can be challenging and may require careful consideration of factors such as inductor selection, switching frequency, and control loop design. |
| Buck converters are often used in battery-powered devices where energy efficiency is critical. They help extend battery life by minimizing power loss. |