Buck Converter

A buck converter is a step-down DC-DC converter that produces an output voltage lower than its input voltage. You will find them inside laptops dropping the 19V battery rail down to 1.2V for the CPU.

Core Concept

When the MOSFET turns on, current flows through the inductor to the load, storing magnetic energy. The inductor current ramps up linearly.

When the switch turns off, the inductor opposes the change in current and forces the freewheeling diode to conduct. The stored energy then supplies the load, and the inductor current ramps down.

The output filter capacitor maintains a steady DC voltage. The continuous swapping between source-fed and inductor-fed states regulates the voltage.

Key Formulas

Voltage ratio Vo / Vin = D. Inductor ripple current dI = (Vin - Vo) * D / (L * fs). Critical inductance Lc = ((1 - D) * R) / (2 * fs).

Given:\nVin = 24 V, Duty cycle D = 0.4, L = 100 uH, fs = 50 kHz.\n\nFormula:\nVo = D * Vin\ndI = (Vin - Vo) * D / (L * fs)\n\nSteps:\nVo = 0.4 * 24 = 9.6 V\ndI = (24 - 9.6) * 0.4 / (100e-6 * 50000)\ndI = 14.4 * 0.4 / 5 = 1.152 A\n\nFinal Answer:\nOutput voltage is 9.6 V and ripple current is 1.152 A.

Exam Tip: GATE trap: The output voltage of an ideal buck converter depends only on the input voltage and duty cycle. It is completely independent of the load resistance in continuous conduction mode.

Performance Parameters

• Voltage Gain: Strictly between 0 and 1.
• Inductor Current: Always continuous in high-power applications.
• Input Current: Pulsating, requiring a strong input filter capacitor.
• Switch Voltage Stress: Equals the maximum input voltage Vin.
• Diode Voltage Stress: Also equals the maximum input voltage Vin.

Quick Revision

• Steps down DC voltage.
• Inductor is in series with the load.
• Output current is continuous.
• Input current is discontinuous.
• Requires a high-side gate driver.
• Exam trap: Using the continuous conduction mode formula when the problem states the inductor current reaches zero. Always check if L is greater than Lc.