Output Filter(L,C) Calculator Switching Power Supply

In power supply electronics, designing determining the inductor ($L$) and capacitor ($C$) values is crucial for achieving stable operation with minimal ripple. This post explains how to calculate the $L$ and $C$ values for a such as buck converter, flyback converter etc that employs low pass filter circuit step by step, assuming the ripple current ($\mathrm{\Delta }{I}_L$) is percentage of the load current(IL), ripple output voltage, maximum output voltage tolerance to get minimum output capacitor value, duty cycle as a percentage of the total period of the desired switching frequency.

Key Design Assumptions

1. Output Voltage ($V_o$): $5V$.
2. Maximum Output Voltage ($V_p$):6V
3. Ripple Voltage ($\mathrm{\Delta }{V}_o$): $50mV$.
   
4. Load Current ($I_L$): $2A$.
5. Ripple Current ($\mathrm{\Delta }{I}_L$): $30\mathrm{\%}$ of the load current ($I_L$).
6. Switching Frequency ($f_{sw}$): $20kHz$.
7. Duty Cycle (D): 40% of switching period(tp).

Step 1: Inductor Design

The voltage across the inductor ($V_L$) during the "on" period is:

Here:

• $V_s$ is the secondary voltage of the transformer, that is the input voltage to the output filter.
• $V_s=\frac{V_{out}}{D}=\frac{5}{0.3}=16.66V$, where D =30% / 100%(duty cycle).

Thus:

The inductance ($L$) is calculated using:

Where:

• $t_{on}=D\cdot T=0.3\cdot \frac{1}{20,000}=15\mu s$,
• ΔIL​=0.3×IL​=0.3×2A=600mA(30% of Load Current IL​)

Substitute the values:

Step 2: Capacitor Design

The capacitor is designed to handle ripple current and voltage. For ripple voltage, we use:

Substitute the values:

However, a small capacitance may lead to unacceptable overshoot during sudden load changes. To address this, consider energy transfer from the inductor to the capacitor during load transients.

Step 3: Adjusting Capacitance for Overshoot

The energy stored in the inductor is:

The capacitor must absorb this energy without exceeding $V_p=6V$. Using:

Rearrange for $C$:

Substitute values:

Conclusion

For the given converter design:

Circuit Diagram

Below is a basic schematic of the buck converter showing $L$ and $C$ placement.

By carefully selecting these components, you can ensure efficient operation with minimal ripple and stable voltage.

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