8.2.2.3 Output Capacitor Selection

There are three primary considerations for selecting the value of the output capacitor. The output capacitor determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. The output capacitance needs to be selected based on the more stringent of these three criteria.

The desired response to a large change in the load current is the first criteria. The output capacitor needs to supply the load with current when the regulator can not. This situation would occur if there are desired hold-up times for the regulator where the output capacitor must hold the output voltage above a certain level for a specified amount of time after the input power is removed. The regulator is also temporarily not able to supply sufficient output current if there is a large, fast increase in the current needs of the load such as a transition from no load to full load. The output capacitor must be sized to supply the extra current to the load until the control loop responds to the load change. Equation 14 shows the minimum output capacitance, from the electrical point of view, necessary to accomplish this.

Equation 14.

Where ΔI_O is the change in output current, f_SW is the regulator switching frequency and ΔVOUT is the allowable change in the output voltage. For this example, the transient load response is specified as a 5% change in VOUT for a load step of 1 A. For this example, ΔI_O = 1 A and ΔVOUT = 0.05 × 2.5 = 0.125 V. Using these numbers gives a minimum capacitance of 160 μF. This value does not take the ESR of the output capacitor into account in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation. However, for space applications and large capacitance values, tantalum capacitors are typically used which have a certain ESR value to take into consideration.

Equation 15 calculates the minimum output capacitance needed to meet the output voltage ripple specification. Where f_SW is the switching frequency, VOUT_ripple is the maximum allowable output voltage ripple, and I_ripple is the inductor ripple current. In this case, the maximum output voltage ripple is 20 mV. Under this requirement, Equation 15 yields 168.75 µF.

Equation 15.

Equation 16 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple specification. Equation 16 indicates the ESR should be less than 7.41 mΩ.

Equation 16.

For this specific design, taking into consideration the stringent requirements for space applications, an output capacitor of 330 µF with ESR = 6 mΩ has been selected.