Side-by-side comparison
| Parameter | Buck | Boost Converter |
|---|---|---|
| Function | Steps voltage down (V_out < V_in) | Steps voltage up (V_out > V_in) |
| Voltage Conversion Ratio (CCM) | M = D (duty cycle 0–1) | M = 1/(1−D) |
| Switch Position | High-side (series with input) | Low-side (shunt to ground) |
| Inductor Location | Between switch and output | Between input and switch junction |
| Input Current | Pulsed (switch is series) | Continuous (inductor on input side) |
| Output Current | Continuous (inductor on output side) | Pulsed (diode conducts only during off-time) |
| Typical Efficiency | 90–97% (low V_in to V_out ratio) | 85–94% (lower at high step-up ratio) |
| Switching Frequency | 100 kHz–2 MHz typical | 100 kHz–1 MHz typical |
| Example ICs | LM2596 (150 kHz, 3 A), TPS54360 (600 kHz) | LM2577 (52 kHz), TPS61030 (1 MHz, Li-ion boost) |
| EMI Characteristic | Input pulsed current — needs input capacitor | Output pulsed current — needs output capacitor |
Key differences
In a buck converter, duty cycle D directly sets V_out = D·V_in — linear, predictable, easy to control. A boost converter's M = 1/(1-D) approaches infinity as D → 1, but practical limits (inductor DCR, switch R_DS(on)) cap boost ratio at around 5–10× before efficiency collapses. The inductor in a buck sits between switch and output, smoothing output current; in a boost it sits between input and switch, smoothing input current — this means boost converters have a naturally smooth input current (good for batteries) but pulsed output. Right-half-plane zero in the boost converter transfer function complicates compensation, requiring a much slower control bandwidth than a buck at the same switching frequency.
When to use Buck
Use a buck converter when the input voltage is always higher than the required output and a simple, high-efficiency step-down is needed. Example: a TPS54360 buck IC converts 12 V from a laptop adapter to 3.3 V at 3 A for an FPGA core supply, switching at 600 kHz with 93% efficiency.
When to use Boost Converter
Use a boost converter when the supply voltage is lower than the required output, such as battery-powered systems. Example: a TPS61030 boost IC converts 3.6 V from a single Li-ion cell to 5 V at 1.2 A for USB OTG output in a smartphone, switching at 1 MHz.
Recommendation
Choose the buck converter whenever your input is higher than your output — it is more efficient and easier to stabilise. Choose boost when stepping up is mandatory, but keep duty cycle below 0.85 to avoid efficiency collapse. If you need both up and down from the same supply, use a buck-boost or SEPIC, not two separate converters.
Exam tip: Examiners ask students to derive the inductor current ripple ΔI_L = (V_in − V_out)·D/(f·L) for a buck converter in CCM and then state the boundary condition between CCM and DCM — know that at the CCM/DCM boundary, ΔI_L/2 equals the average output current.
Interview tip: A placement interviewer at a power IC company (Texas Instruments, Microchip) will ask you to explain the right-half-plane zero in a boost converter and its effect on feedback loop bandwidth — state that RHPZ appears at z = (1-D)²R/(2πL) and forces a lower crossover frequency, making boost harder to stabilise than buck.