Choosing the Right Type of Output Capacitor for a Switching Regulator
Capacitors are a crucial component of step-down switching regulators. Learn about the different capacitor types and how each one affects regulator performance.
Previous articles in this series examined the electrical behavior of step-down switching regulators, provided guidance on initial inductor sizing, and discussed inductor current and inductance fine-tuning. Now, with help from LTspice simulations and the schematic below (Figure 1), we’ll explore the relationship between capacitor characteristics and the performance of switch-mode buck converters.
Figure 1. A schematic for a step-down converter implemented in LTspice.
Purpose of the Output Capacitor
The inductor in a switch-mode regulator allows on/off switching action to produce a ramp-up/ramp-down current waveform. However, we need output capacitance to store and release charge in such a way that the current flowing into the load and the voltage across the load remain stable despite the (potentially quite large) variations in inductor current. The plot below (Figure 2) shows what happens when I virtually eliminate the output capacitor by using a very small value for C1.
Figure 2. Voltage output for a simulated buck converter with an extremely low capacitance value.
We see, then, that the output capacitor in a switch-mode regulator fulfills a critical filtering function. This component should therefore be selected carefully, with attention to both the type of capacitor and its capacitance value. In this article, we’ll focus on capacitor type; in the next, we’ll discuss value.
The three capacitor technologies most commonly used in low-voltage electronic devices are ceramic (also known as MLCC, meaning multi-layer ceramic capacitor), aluminum electrolytic, and tantalum. I have summarized the pros and cons of each below, with an emphasis on qualities relevant to switching power supplies; please keep in mind that these are generalizations, and that generalizations by nature sacrifice a degree of accuracy for brevity and simplicity.
- Ceramic Capacitors
- Advantages: low cost; low ESR (see next section for an explanation of this); not polarized; compact.
- Disadvantages: subject to piezoelectric effect; capacitance unstable relative to DC bias, temperature, and time; not as readily available with high capacitance.
- Aluminum Electrolytic Capacitors
- Advantages: low cost; readily available with high capacitance and high voltage rating; high capacitance-to-volume ratio.
- Disadvantages: polarized; bulky; performance degrades over time; high ESR; ESR is unstable relative to operating temperature; less effective at high frequencies.
- Tantalum Capacitors
- Advantages: high capacitance-to-volume ratio; compact; long-term reliability.
- Disadvantages: high cost; polarized; less effective at high frequencies.
The trend in switching-regulator design is toward higher switching frequencies, which allow for lower output capacitance. This makes ceramics a more feasible choice for output capacitors; those who want more information on this topic may find my guide to ceramic capacitor types of interest.
Overall, I find ceramic capacitors the most useful. I only consider aluminum electrolytic or tantalum if there is a compelling reason to avoid ceramic.
Equivalent Series Resistance
Which characteristic most impacts your choice of output capacitor type depends, at least in part, on your priorities. If you’re focusing on electrical performance, though, equivalent series resistance (ESR) is probably the most important factor to consider.
In switch-mode converter circuits, lower ESR is usually better. Higher ESR leads to higher output-voltage ripple and lower efficiency; it may also negatively affect the stability of the control loop that the regulator uses to maintain a specified output voltage. In theory, however, a switcher’s control loop could be designed with higher-ESR capacitors in mind, so we can’t say that lower ESR is always better for stability. One crucial fact to keep in mind is that capacitor ESR is not constant over frequency. You need to use an ESR value that corresponds to the operating frequency of your circuit.
If you’re using an off-the-shelf switcher, the datasheet will hopefully include example capacitor part numbers or a recommended ESR range. You may also find simulations helpful in identifying successful capacitor parameters, especially if datasheet guidance is limited or unavailable, and you can add ESR to an ideal capacitor to make simulations more consistent with real-life electrical behavior.
A more complete model for a real-life capacitor includes both ESR and equivalent series inductance (ESL). You can read more about ESR and ESL in Part 2 of my series on bypass capacitors.
The following plot (Figure 3) conveys the output ripple for the switcher circuit shown in Figure 1 with an idealized output capacitor (ESR = 0 Ω, ESL = 0 H).
Figure 3. Output ripple for Figure 1. ESR = 0 Ω.
Now let’s modify the output capacitor so that its behavior is more consistent with that of an 0805 ceramic capacitor. We’ll set equivalent series resistance to 10 mΩ, which is a reasonable value for a ceramic capacitor at the 1.5 MHz operating frequency of our switcher. The new VOUT trace is shown below in Figure 4, with identical settings for the vertical and horizontal axes to make a direct visual comparison easier.
Figure 4. Output ripple for Figure 1. ESR = 10 mΩ.
There’s no significant difference here, which is good news. These results suggest that a good-quality ceramic capacitor will not seriously degrade our output voltage.
Now let’s increase the ESR to 400 mΩ, which is the sort of resistance that we might have with an electrolytic or tantalum capacitor operating at 1.5 MHz. Figure 5 illustrates the results:
Figure 5. Output ripple for Figure 1. ESR = 400 mΩ.
There’s a notable increase in output ripple, but nothing catastrophic.
If you’re looking for catastrophic degradation, you can achieve it by using an electrolytic capacitor in applications that must operate at very low temperatures. Figure 6 gives an example of how much an electrolytic cap’s ESR increases as temperature decreases below room temperature.
Figure 6. Electrolytic capacitor ESR as a function of temperature.
Thus, an electrolytic output capacitor that provides acceptable ripple or stability performance at 20 °C could be completely unacceptable at –30 °C. If you have an application where this low-temperature behavior is problematic, but not enough to completely abandon the electrolytic capacitor, you can improve the situation by adding a ceramic capacitor in parallel with the electrolytic.
Looking Ahead: Capacitance Calculations
We’ve discussed the characteristics of common capacitor types and how they affect the performance of switching regulators. In the next article, we’ll focus on output-capacitance calculations and how to choose the right capacitance value for your needs.
All images used courtesy of Robert Keim