A capacitor is everywhere. In power supply, LED lighting, in commercial electronics, in signal processing, etc., you need a capacitor. What is its specific role basically? A capacitor has several roles. It will eliminate noise issues on the circuit, working as a filter. It is the major part in low pass, high pass, band pass, band stop filters and so on. It is also very vital in rectification to attain a DC straight voltage. In power supplies, capacitor acts as an energy storage device. Lot of applications for this simple electronic part. I will no longer discuss here what a capacitor made up and just focus on how to select capacitors instead.
How to Select Capacitor – Important Factors
There are important parameters to consider in capacitor selection for your circuit. Either you want to go on a chip or to a through hole one. Either a film or an electrolytic one and so on. Let’s discuss all the considerations here.
1. How to Select Capacitor Capacitance
Capacitance is the electrical property of a capacitor. So, it is the number one consideration in capacitor selection. How much capacitance you need? Well, it depends to your application. If you are going to filter output a rectified voltage, then you need a larger capacitance for sure. However, if the capacitor is only intended to filter signal noise in a small signal circuit, then a small capacitance in pico to nano farads will do. So, know your application.
Supposing the application is indeed to filter a rectified voltage, then you need a big capacitance in hundred micro farads. You can do trial and error until ripple voltage is within the requirements. Or you can do calculations to start with.
For a bridge and full wave rectifier, the capacitance required is can be computed as below.
Cmin = Load Current / (Ripple Voltage X Frequency)
Cmin is the minimum required capacitance
Load current – is simply the rectifier load
Ripple voltage – is the peak to peak voltage fluctuations when measured in the rectifier output
Frequency – for bridge and full wave rectifier this is twice the line frequency.
Below circuit is a bridge rectifier with input of 120Vrms at 60Hz, load current of 2A and a ripple voltage requirement of 43V peak to peak. We will estimate what should be the minimum capacitance needed for C1.
Cmin = Load Current / (Ripple Voltage X Frequency)
Cmin = 2A / (43V X 2 X 60Hz) = 387uF
Based on below simulation, the peak to peak ripple voltage using a 387uF is 35.5V. It is close to the 43V. Since the computation result is a minimum capacitance, by selecting a higher value capacitance, the ripple voltage will further decrease.
2. Tolerance – Also a Factor in Capacitor Selection
Aside from the capacitance, another thing to consider on how to select capacitors is the tolerance. If your application is very critical, then consider a very small tolerance. Capacitors come with several tolerance options like 5%, 10% and 20%. It is your call which is which. A higher tolerance is cheaper than a lower tolerance part most of the times. You can always use a 20% tolerance part and just put more margin to your design.
3. How to Select Capacitor Voltage Rating
Capacitor will get damage by a voltage stress. So, it is a must to consider the voltage in capacitor selection. You need to know the voltage level where the capacitor to be installed. A capacitor is most of the times is installed in parallel to a circuit or device or a sub circuit. Though there are few cases to install a capacitor in series. In my designs, I am not allowing to a voltage stress of more than 75%. This means, if the actual circuit voltage is 10V, the minimum capacitor voltage I will select is 13.33V (10V/0.75). However, there is no such voltage. So, I will go to the next higher level that is 16V. Can you use 20V, 25V or even higher? The answer is yes. It depends to your budget because the higher the voltage, the expensive the capacitor is. It will also depend on the physical size requirement. The capacitor physical size is directly proportional to the voltage rating in most cases.
For instance, in the sample circuit above, the maximum level of the voltage across the capacitor is the peak level of the 120Vrms that is around 170V (1.41 X 120V). So, the capacitor voltage rating should be 226.67V (170/0.75). And I will choose a standard value near to this.
4. Selecting Capacitor Current Rating – Know the Ripple Current
If you are not an electronics hobbyist or working on the field for some time, you may not familiar with the term ripple current. This is the term given to the current that will pass through the capacitor. In ideal case, there is no current that will flow to the capacitor when it is installed across a DC voltage line. However, if the actual voltage across the capacitor is not pure DC, like there is a small fluctuation on the voltage, this will result to a ripple current. For low power circuit and the voltage variation is very negligible, you should not worry on this ripple current rating.
However, for capacitors put in place to filter the pulsating DC from a rectifier, the ripple current is critical. The higher the load, the higher the ripple current. So, how to select capacitors for this application? For rectification, it requires most of the times a larger capacitance to get a near straight line voltage. Thus, the first option is to consider an electrolytic capacitor. In some applications that the ripple current is very high, electrolytic capacitor will not work anymore as its ripple current is smaller. In this case, film capacitors are chosen as they are having very high ripple current rating. The drawback however is the capacitance is limited to few microfarads only thus need more of them in parallel. Considering below rectifier circuit, a filter capacitor of 330uF and a load of 2A from an AC source of 120Vrms at 60Hz. This is the same as the above circuit but redrawn and simulated in LTspice. LTspice is a freeware circuit simulation tool from Linear Technology. If you want to learn on how to do simulation on LTspice, read the article LTSpice Circuit Simulation Tutorials for Beginners.
The simulated ripple current is 3.4592A.
If you don’t know simulation, you can estimate the actual ripple current using below equation.
Iripple = C X dV X Frequency
Iripple – is the actual ripple current flowing to the capacitor
C – the capacitance in the circuit
dV – this is the change of input voltage from zero to the peak
Frequency – this is the frequency of the AC voltage (not the rectified waveform frequency)
Let us do calculation of the above data:
Iripple = C X dV X Frequency
Iripple = 330uF X (170V-0V) X 60Hz = 3.366A
The computed value is very close to the simulation result. Then, I will consider here a maximum current stress of 75%. So, the chosen capacitor must have a ripple current rating of not less than 4.5A (3.366A/0.75).
5. Consider Operating Temperature in Selecting Capacitors
Environment factors are also needed to consider on how to select capacitors. If your product will be exposed to an environment temperature of 100’C, then do not use a capacitor that is only rated at 85’C. Likewise, if the minimum environment temperature is -30’C, then do not use a capacitor that can only withstand -20’C temperature.
This specification seems to be very straight forward. However, if the capacitor is exposed to a very high ripple current, there will be an internal heating, and this will result to a thermal rise above the environment temperature. So, you need more margin for the operating temperature. For example, the maximum ambient temperature where the product is going to install is 60’C. Do not just select a capacitor that can handle 60’C. Select perhaps a temperature rating of 105’C. This will give enough margin due to the internal heating.
6. Selecting Capacitor Dielectric Material
In chip resistor, you will encounter this option when you browse on online distributors like Mouser and Digikey. What does this parameter mean? This is the dielectric material used in fabricating the capacitor. I cannot elaborate further on the physics of the capacitor construction but in my designs I always consider a dielectric of X7R, NP0 or C0G. They are usually having higher temperature range. See below few samples of X7R, NP0 or C0G compared to just X5R.
7. How to Select Capacitor – Life Expectancy
Capacitor life or lifetime expectancy is the length of time the capacitor will stay healthy as designed. This is critical for electrolytic capacitors. For ceramic capacitors, this is not an issue and probably not worth to look in to when selecting capacitors for small signal circuits. There is still a life limit for it but more than enough to sustain through the entire life cycle of the product. Unlike the electrolytic capacitors, if not properly evaluated, it will fail before the product life cycle end and this should not happen. The ripple current will shorten the capacitor’s life. So better manage it. There are reference calculations on the datasheets or from suppliers on the capacitor life. These are straight forward equations that you can use in capacitor selection with regards to expected life. Some are also giving a graph for easier understanding. Below sample calculation and graph is taken from KEMET datasheet. KEMET is one of the leading capacitor manufacturers.
8. Physical Dimension and Mounting Style are Factors in Capacitor Selection
The last but not the least to think about is the physical dimension as well as the mounting style. Sometimes capacitor selection is dictated by the space available. Chip capacitors has small footprints but with limited capacitance value. On the other hand, electrolytic capacitors have bigger capacitance, but they are bulky. Are you going to use a surface mount or a through hole part? Well, this is up to you. Evaluate your space requirements before you go far in dealing with other parameters.
Sample Capacitor Specifications
Below is a capacitor specification rating I grab from Mouser electronics page. It has the capacitance, voltage, tolerance, ripple current, operating temperature, physical dimensions and mounting orientation and life. But take note, the life specified is just the base life or this is the load life at a maximum permissible operating temperature.