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Power tip: Understand capacitor parasitics

Posted: 03 Sep 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Power supply ripple? transient? equivalent series resistance?

Power supply ripple and transient specifications establish the requirements for the amount of capacitance you will need. They also set limits on the capacitors' parasitic components.

Figure 1 shows a capacitor's basic parasitic components, which consist of the equivalent series resistance (ESR) and equivalent series inductance (ESL). It also graphs the impedance of three capacitor styles (ceramic, aluminum electrolytic and aluminum polymer), versus frequency.

The table shows the values used to generate the curves. These are typical values you might find in a low-voltage (1V C 2.5V), medium current (5A) sync-buck power supply.

Table: Comparing three capacitor styles, each has its strength.

At low frequencies, all three capacitors show no signs of parasitic components as the impedance is clearly a function of the capacitance alone. However, the aluminum electrolytic capacitor impedance stops diminishing and begins to look resistive at a relatively low frequency. This resistive characteristic continues to a relatively high frequency where the capacitor turns inductive. The aluminum polymer capacitor is the next capacitor to deviate from ideal. Interestingly, it has a low ESR and the ESL becomes apparent. The ceramic capacitor also has a low ESR, but since it has a smaller case size, its ESL is less than that of the aluminum polymer and aluminum electrolytic capacitors.

Figure 1: Parasitics alter the impedance of ceramic, aluminum, and aluminum polymer capacitors differently.

Figure 2: The capacitor and its parasitic elements create different ripple voltages in a continuous sync-buck.

Figure 2 presents the power supply output capacitor waveforms from a continuous sync-buck regulator simulation operating at 500kHz. It uses the dominant impedances of the three capacitors in figure 1: capacitance for the ceramic; ESR for the aluminum; and ESL for the aluminum polymer.

The red trace is the aluminum electrolytic capacitor, which is dominated by the ESR. Consequently, the ripple voltage is directly related to the inductor ripple current. The blue trace represents the ripple voltage across the ceramic capacitor, which has small ESL and ESR. The ripple voltage in this case is the integral of the ripple current in the output inductor. Since the ripple current is linear, this results in a series of time-squared sections and appears sinusoidal in shape.

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