SMPS Capacitor Calculator

Understanding SMPS Capacitors & Topologies

Capacitors are essential in Switched-Mode Power Supplies (SMPS) for smoothing voltage ripples, storing energy, and handling pulsating currents. They are primarily categorized as input or output capacitors, each serving distinct roles.

Input Capacitor (Cin)

The **Input Capacitor (Cin)** filters the input voltage, suppressing noise generated by the switching action and providing a stable voltage source to the converter. Its primary role is to handle the pulsating input current drawn by the switching element, particularly in Buck and Buck-Boost topologies. For Boost converters, the input current is continuous, so the input capacitor mainly deals with high-frequency switching noise.
**Simplified Formulas for Minimum Cin (Ideal, based on voltage ripple):**

• **Buck:** $C_{in} = \frac{I_{out} \cdot D \cdot (1-D)}{f \cdot \Delta V_{in}}$
• **Boost:** $C_{in} = \frac{I_{out}}{f \cdot \Delta V_{in}}$ (Simplified for basic voltage ripple; actual calculation often considers RMS current and ESR for high-frequency noise.)
• **Buck-Boost:** $C_{in} = \frac{I_{out} \cdot D}{(1-D) \cdot f \cdot \Delta V_{in}}$

Output Capacitor (Cout)

The **Output Capacitor (Cout)** filters the output voltage, smoothing out the ripple caused by the inductor’s switching action and supplying instantaneous load current demands. It must handle the ripple current from the inductor and maintain the output voltage within a specified ripple range.
**Simplified Formulas for Minimum Cout (Ideal, based on voltage ripple):**

• **Buck:** $C_{out} = \frac{I_{out} \cdot (1-D)}{f \cdot \Delta V_{out}}$ (More precise calculations involve inductor ripple current and ESR.)
• **Boost:** $C_{out} = \frac{I_{out} \cdot D}{f \cdot \Delta V_{out}}$
• **Buck-Boost:** $C_{out} = \frac{I_{out} \cdot D}{f \cdot \Delta V_{out}}$

Note: $D$ is the Duty Cycle, $f$ is the Switching Frequency, $\Delta V$ is the Target Voltage Ripple, $I_{out}$ is the Output Current. These formulas are simplified for calculating minimum capacitance based on voltage ripple. In practical SMPS designs, the Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL) of the capacitors often play a more significant role in determining the actual voltage ripple. For reliable operation, capacitors must also be rated for the RMS ripple current they will handle.