What Size Capacitor Should I Use?
Take into account the capacitance, voltage rating, ripple current rating, and temperature when selecting a capacitor.
The physical size of a capacitor depends on the capacitance value. As the capacitance increases, the size becomes larger.
The capacitance variation is temperature-dependent. In case you need control over capacitance for a broad temperature range, select the capacitor with the smallest temperature coefficient.
Capacitors are key passive components used in the electronics industry
Capacitors are one of the main passive components in the electronics industry. They are used for coupling, decoupling, power supply filtering, signal filtering, impedance matching, energy storage, and snubber action in electronics circuits. Depending on the application, the size of the capacitor varies, either in its capacitance or physical volume.
When considering the capacitor size for a given application, parameters such as voltage, current ripple, temperature, and leakage current must be considered. Capacitor size selection is important, considering the physical size and capacitance aspects, as they affect circuit assembly and the performance variation of the circuit.
Let's discuss capacitor size and the parameters that influence it in this article.
What Size Capacitor Should You Use?
Selecting an appropriately-sized capacitor can be challenging. The selection of the capacitor should take into account the capacitance, voltage rating, ripple current rating, and temperature. The physical size of the capacitance is influenced by the variation in each of these parameters, and the variation in size is different for each capacitor type, including paper capacitors, mica capacitors, ceramic capacitors, and electrolytic capacitors.
The major parameters guiding capacitor size selection are:
The primary consideration for capacitor selection should be the nominal capacitance value. Knowing the application is important for determining the capacitance value. Either the designer calculates the capacitance or, in an integrated circuit application, the capacitance is recommended in the IC datasheet. Depending on the circuit requirements and the expected capacitor characteristics, the capacitor type is selected.
Some common capacitor types include:
- Pico-farad range ceramic capacitors
- Nano-farad multilayer ceramic capacitors (MLCC)
- Micro-farad aluminum electrolytic capacitors
- High-temperature range mica capacitors
The tolerance of the capacitor is worth considering, as it gives information about the actual variation of capacitance allowed. A higher tolerance capacitor is not suitable for precision applications, and in such cases, the lowest tolerance capacitor should be selected. Capacitors with the same capacitance but different tolerance levels are available. The physical size of the capacitor is dependent on the capacitance value; as the capacitance increases, the size becomes larger.
Working Voltage and Ripple Current
The voltage rating is the maximum continuous DC or AC voltage that a capacitor can withstand without failing. Exceeding the voltage rating may damage the capacitor, and this rating greatly influences a capacitor’s life expectancy.
Usually, capacitors are derated by the following rule of thumb: a capacitor is selected such that its voltage rating is two to three times greater than the expected operating voltage. Derating increases the footprint requirements of the capacitor because, with an increase in working voltage, the physical size of the capacitor also increases. For example, the diameter of an electrolytic capacitor for the same capacitance with different working voltage is different with a larger size for the higher voltage rated capacitor.
No capacitor is ideal, and in practical applications, ripple current or leakage current flows through the dielectric of the capacitor. The ripple current rating must also be considered, especially when the application involves a high load. For electrolytic capacitors that offer less ripple current rating and for high ripple content withstanding capabilities, film capacitors are recommended, provided the application supports this capacitor type. As the ripple current capability of the capacitor increases, so does its physical size.
Operating Temperature and the Temperature Coefficient
The operating temperature is an important environmental factor in the selection of a capacitor. You can find the temperature rating of a capacitor by looking at its datasheet, and can make an appropriate selection by choosing a capacitor with a higher temperature rating than the actual application temperature. However, it is important to provide some margin for accommodating the thermal rise over the operating temperature due to internal heating. If not given enough of a temperature margin, heating can cause the capacitor to explode.
If the circuit or application you are dealing with is temperature-sensitive, then it is important to consider the capacitor variation versus temperature. The capacitance variation is temperature-dependent. In case you need control over capacitance for a broad temperature range, select the capacitor with the smallest temperature coefficient. The physical size of the capacitor is directly dependent on the temperature range. As we select a capacitor for an extremely high and wide range of temperature applications, ensure enough space on the board to incorporate this.
Apart from capacitance, voltage rating, leakage current rating, and working temperature, other things to consider when figuring out what size capacitor to use in an electronics circuit are factors such as equivalent series resistance, reverse voltage rating, frequency, operating losses, operating lifespan, and the mean time before failure (MTBF). By considering each of these parameters in the capacitor selection, there will be dimensional variations in the component.
Cadence’s design and analysis tools aid designers in building circuit boards with various types of capacitors. Cadence software offers simulation tools that help you to find the difference in the performance of a circuit with different types of capacitors.
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