Question

Explain the operation of a synchronous buck converter.

Answer 1

A synchronous buck converter is a type of DC-DC converter used to step down a DC voltage level efficiently. It is widely used in various electronic devices to provide a stable and regulated voltage to power sensitive components. The main advantage of a synchronous buck converter over a non-synchronous (traditional) buck converter is increased efficiency.

The operation of a synchronous buck converter involves the following key components:

Switching Devices: The main components in the buck converter are two semiconductor devices: a high-side MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and a low-side MOSFET. These devices act as switches to control the flow of current through the converter.

Inductor: The inductor in the buck converter stores energy when the switches are closed and releases it when they are open. It smooths out the output current and acts as a filtering element.

Diode: In a synchronous buck converter, a diode is replaced with another MOSFET. This additional MOSFET is called the "synchronous rectifier," and it works in conjunction with the main high-side and low-side MOSFETs to improve the converter's efficiency.

Control Circuitry: The control circuitry consists of a feedback loop that monitors the output voltage and compares it to the desired voltage level. It then generates control signals to drive the high-side and low-side MOSFETs accordingly.

The operation of a synchronous buck converter follows these basic steps:

Initialization: Initially, both the high-side and low-side MOSFETs are turned off, and the inductor current is zero.

Switch ON Phase (High-Side MOSFET ON): The control circuitry sends a signal to turn ON the high-side MOSFET, connecting the input voltage (Vin) to the inductor. The inductor starts to accumulate energy and stores it as a magnetic field.

Inductor Current Ramp Up: As the high-side MOSFET is ON, the inductor current starts to increase linearly, and energy is stored in the inductor.

Switch OFF Phase (High-Side MOSFET OFF): When the inductor current reaches a predetermined value or time limit, the control circuitry turns OFF the high-side MOSFET. Now, the inductor current wants to keep flowing, but the high-side MOSFET is blocking the path.

Switch ON Phase (Low-Side MOSFET ON): The control circuitry then turns ON the low-side MOSFET. Since the high-side MOSFET is OFF, the current path for the inductor becomes Vin → Inductor → Low-side MOSFET (synchronous rectifier) → Ground. The inductor current now starts to decrease.

Inductor Current Ramp Down: As the inductor current decreases, the stored energy is released, and the inductor voltage reverses polarity, providing energy to the output capacitor and load.

Switch OFF Phase (Low-Side MOSFET OFF): When the inductor current reaches a predetermined value or time limit, the control circuitry turns OFF the low-side MOSFET. Now, both MOSFETs are OFF, and the inductor current falls to zero.

Output Voltage Regulation: The output capacitor smooths out the voltage ripple and supplies power to the load. The control circuitry continuously monitors the output voltage and repeats the ON-OFF switching cycle of the MOSFETs to maintain the desired output voltage level.

The synchronous buck converter offers higher efficiency compared to non-synchronous buck converters because the synchronous rectifier (low-side MOSFET) replaces the diode, which has a lower voltage drop during conduction. This reduction in voltage drop leads to reduced power losses and improved overall efficiency.