All capacitors have losses because of internal equivalent series resistance (ESR), and to a lesser degree, equivalent series inductance (ESL). Values for ESL are not always easy to obtain. However, some manufacturers provide graphs of frequency versus capacitor impedance. These graphs typically show the capacitor impedance falling as frequency is increased (as shown in Figure 9-2). In Figure 9-2, X_C is the reactance due to the capacitance, X_L is the reactance due to the ESL, and f₀ is the resonant frequency. As the frequency increases, the impedance stops decreasing and begins to rise. The point of minimum impedance indicates the resonant frequency of the capacitor. This frequency is where the components of capacitance and inductance reactance are of equal magnitude. Beyond this point, the capacitor is not effective as a capacitor.

Figure 9-2 Capacitor Impedance versus Frequency

At f₀, X_C = X_L; however, there is a 180° phase difference resulting in cancellation of the imaginary component. The resulting effect is that the impedance at the resonant point is the real part of the complex impedance; namely, the value of the ESR. The resonant frequency must be well above the 800-kHz switching frequency of the device.

The effect of the ESR is to cause a voltage drop within the capacitor. The value of this voltage drop is simply the product of the ESR and the transient load current, as shown in Equation 1.

where

• V_IN is the voltage at the device input
• V_PK is the maximum value of the voltage on the capacitor during charge
• I_TR is the transient load current

The other factor that affects the performance is the value of the capacitance. However, for the input and the full wave outputs (single-output voltage devices), ESR is the dominant factor.