Introduction to Buck Converter: Basic Configuration and Operation
Purpose of a Buck Converter
- A DC-DC converter is designed to step down a higher input voltage to a lower regulated output voltage.
Building Blocks
- A switch (MOSFET): Controls the input voltage flow.
- An inductor: Smooths out the current by storing and releasing energy.
- A capacitor: Filters and stabilizes the output voltage.
Switch (MOSFET)
- Alternates between ON and OFF states, controlling energy flow.
Key MOSFET Selection Criteria:
Package / Thermals
Breakdown Voltage (BVdss)
Current rating
Power loss
Conduction loss (Rdson)
Switching loss (turn-on/off loss)
Charge loss (Qg, Qoss)
Inductor
Stores energy during the ON state of the switch and releases it during the OFF state to maintain continuous current.
Key inductor selection criteria:
Current rating
Inductance vs. current
Saturation
Parasitics
DC Resistance (DCR)
Power loss
Conduction loss
AC loss (core loss predominantly for buck LOUT)
Capacitor
- Smoothens voltage ripples at the output, ensuring a steady DC output.
Key Output Cap (COUT) selection criteria:
Voltage rating
Temperature rating
Capacitance vs. voltage & temperature
Parasitics
Equivalent Series Resistance (ESR)
Equivalent Series Inductance (ESL)
Key Input Cap (CIN) selection criteria:
Voltage rating
Temperature rating
RMS current rating
High RMS current for buck input caps
Capacitance vs. voltage & temperature
Parasitics
Equivalent Series Resistance (ESR)
Equivalent Series Inductance (ESL)
Operation Principle
- When the switch is ON, the inductor charges while the capacitor supplies power to the load.
- When the switch is OFF, the inductor discharges, supplying energy to the load while the capacitor continues smoothing the output.
Output Voltage Regulation
- The output voltage is determined by the duty cycle of the switch: Vout = Vin × Duty Cycle.
Conclusion
The buck converter is an essential building block in power electronics. It combines simplicity with high efficiency for voltage step-down applications. Its fundamental components—switch, inductor, and capacitor—work together to provide regulated power for a wide range of devices.