How it Works
A buck converter is a switching DC-DC power supply that steps down voltage with high efficiency. The MOSFET switch turns on and off at switching frequency f_sw. When on, the inductor stores energy; when off, it releases energy through the freewheeling diode. The average output voltage equals D·V_in.
The canvas shows three waveforms scrolling in real time: the switch node voltage (green, top), the inductor current (amber, middle), and the output capacitor voltage (teal, bottom). Adjust duty cycle, inductance, capacitance, and frequency to see the direct effect on ripple.
ΔI_L = (V_in − V_out) × D / (L × f_sw)
ΔV_C = ΔI_L / (8 × C × f_sw)
L_crit = (1−D) × R_load / (2 × f_sw)
Frequently Asked Questions
What is a buck converter?
A buck converter is a DC-DC switching power supply that steps down (reduces) voltage from input to output while ideally maintaining 100% efficiency. It uses a transistor switch, inductor, diode, and capacitor to regulate voltage.
How does PWM duty cycle control output voltage?
In a buck converter, the output voltage equals the input voltage multiplied by the duty cycle D (0 to 1): V_out = D × V_in. A 50% duty cycle halves the voltage. The PWM switch alternates at high frequency, and the inductor averages the switching waveform.
What causes inductor current ripple in a buck converter?
Inductor current ripple ΔI_L = (V_in − V_out) × D / (L × f_sw) arises because when the switch is on, voltage (V_in − V_out) is applied across the inductor, linearly increasing current. When off, the inductor freewheels through the diode, decreasing current.
What is the role of the output capacitor?
The output capacitor smooths the pulsating inductor current into a steady DC voltage. The capacitor voltage ripple ΔV_C = ΔI_L / (8 × C × f_sw) is minimized with larger capacitance or higher switching frequency.
Why operate at high switching frequency?
Higher switching frequency reduces ripple in both inductor current and output voltage, allowing smaller inductors and capacitors. However, switching losses in the transistor increase with frequency, so there is an efficiency tradeoff.
What is the difference between CCM and DCM?
Continuous Conduction Mode (CCM) means inductor current never reaches zero. Discontinuous Conduction Mode (DCM) occurs at light loads when inductor current hits zero each cycle. CCM provides lower ripple but DCM can be more efficient at light loads.
How is efficiency calculated for a buck converter?
Ideal efficiency is 100% since no energy is dissipated in an ideal buck converter — only stored and transferred. Real converters lose energy to switch resistance (conduction losses), switching transitions, inductor ESR, and diode forward voltage.
What is the voltage conversion ratio in continuous conduction mode?
In CCM, V_out/V_in = D (the duty cycle). This linear relationship makes control straightforward. In DCM, the conversion ratio depends on duty cycle, inductance, switching frequency, and load current.
How does inductor size affect buck converter performance?
Larger inductance L reduces current ripple ΔI_L, which means smoother output current and less capacitor stress. However, larger inductors are physically bigger, heavier, and have more resistance. The critical inductance separating CCM and DCM is L_crit = (1−D)·R/(2·f_sw).
What is a synchronous buck converter?
A synchronous buck converter replaces the freewheeling diode with a second MOSFET that turns on when the main switch is off. This reduces the forward voltage drop loss (from ~0.7V diode to ~millivolts across MOSFET), significantly improving efficiency at high currents.