Capacitors can be used to smooth out voltage, a process also known as filter ripple. They can also be used as reservoirs for electrical energy storage and to block DC current. A capacitor consists of two metal plates that are separated by an insulator, the dielectric. One of the most notable features of capacitors is that they resist voltage changes, meaning that if suddenly the voltage applied to a capacitor is changed the capacitor cannot react immediately and the voltage across the capacitor changes more slowly compared to the applied voltage.
What is the "Primary" Side and "Secondary" Side?
In a PSU, capacitors are used in both the "primary" side and the "secondary" side. The primary side is the part of a PSU before the power transformer, where the AC comes in. The secondary side is after the power transformer and this is the part that actually generates the DC outputs. More on this in the SMPS section.
Capacitors allow DC to pass for a very short period of time before they block it. On the contrary, AC passes freely through them, but with a changed, rectified, shape. We calculate the charge a capacitor can store, called capacitance, in farads. However, a farad (F) is a very large unit so you will usually see micro-farad (µF or uF) or pico-farad (pF) units used instead. Besides their capacity, the two most significant features of a capacitor are its working voltage and the temperature rating (and for those that have polarity, the negative lead marking).
In PSUs, the best electrolytic capacitors are considered those rated at 105 degrees Celsius, since they have increased life span compared with ones rated at 85 °C. Of course, the capacitor manufacturer plays a key role, with Japanese-made capacitors always being the preferred choice.
There are various types of capacitors depending on their construction and the materials used. Some of the most common types are dielectric, film, ceramic, electrolytic, glass, tantalum and polymer. In PSUs, we mostly see electrolytic and polymer capacitors and, in the transient filtering/APFC (active power correction factor) stage, Y (ceramic) and X (metalized polyester) capacitors. In all cases, Y capacitors are placed between line and earth (or chassis) and always come in pairs, while X capacitors are placed across the line (connected between line and neutral). And since X capacitors tend to keep their charge for quite a long time, a bleeding resistor is often used to quickly de-charge them once the AC voltage is removed. If there is a short circuit in Y caps, there is a high risk of an electric shock to the user, and if an X cap shorts out, there is a fire risk.
If we place two or more capacitors in parallel, then their capacitances are added (equation 1 below). On the contrary, if we connect them in a series, then their total capacitance is reduced (equation 2).
The perfect capacitor should have zero resistance, which is defined as an object's opposition to electron flow. However, as this isn't a perfect world, all capacitors have some resistance, and the lower it is, the higher the quality of the capacitor. A cap's resistance is called equivalent series resistance (ESR), and it can hugely affect performance.
When we are trying to figure out the cause of a malfunction in a PSU, we shouldn't only measure the capacitance of its caps; we should also check ESR readings using the proper instrument. In many cases, capacitance might be within specifications but the ESR is way off, resulting in poor performance. Also, increased ESR greatly affects the operational temperature of a cap leading to its fast degradation and a much shorter life span. Even a 10 °C increase in the operating temperature of an electrolytic cap cuts its estimated lifetime, which shows the importance of keeping an electrolytic cap's temperature at the lowest possible levels.
In short, the most crucial specifications of a capacitor are the following:
- Working voltage (if exceeded for prolonged periods, the cap will most likely fail, making a loud bang).
- Working temperature.
- Tolerance (expressed in percentages, it shows how close a cap's capacitance is to its nominal level).
- Polarity (for electrolytic caps).
- ESR (equivalent series resistance).
- Ripple current.
- Leakage current (current "leaking" through the dielectric due to its poor isolation resistance).
- Size (since larger caps can dissipate heat easier and on top of that have more dielectric quantity).