Choosing Capacitors For Power Supplies

The most important applications for capacitors in power supplies are in energy storage, snubbering, EMI suppression, and control circuits. As we look at each area, use the accompanying chart to see how each dielectric technology competes or complements each other with respect to the application area.

Snubbering

Modern power semiconductors that switch at high frequencies are subject to potentially damaging voltage spikes. Snubber capacitors, when connected across the power semiconductor, limit peak voltages by absorbing voltage pulses and protecting the semiconductor, making the snubber capacitor a critical component in the power stack.The current and voltage ratings of the semiconductor along with its switching frequency drive the selection of the snubber capacitor. Since these capacitors experience very steep dv/dt values, a film capacitor is the right choice for this application.Typical capacitor ratings range from 470 pF to 47 nF at voltage ratings as high as 2,000 Vdc. For high power semiconductors such as IGBTs, the values can be as high as 2.2 µF with voltages in the range of 1,200 Vdc.Do not choose a capacitor based just on the value/voltage combination. Consider the required dv/dt rating when selecting snubber capacitors. The dissipation factor determines the power dissipation within the capacitor. Therefore, select an alternative that offers a lower loss factor.

EMI/RFI suppression

These capacitors are connected to the input side of a power supply to reduce electromagnetic and/or radio frequency   interference   produced by the semiconductor. Being connected directly to the main input line exposes them to damaging overvoltages and transients. For that reason, there are different safety standards introduced by different regions in the world, including EN 132 400 for Europe, UL 1414 and 1283 for the US, and CSA C22.2 No. 0; 1, and 8 for Canada.X- and/or Y-class capacitors,   with plastic-film technology provide one of the least expensive suppression methods. The impedance of the suppression capacitor decreases with   frequency and the high frequency current passes   through   the capacitor.   An X   capacitor   provides   a   "short circuit"   for   this current between lines and a Y capacitor between the line and body of the equipment that is grounded.There are subclasses for X and Y capacitors, which specify the peak value of the surge voltages that it can see. For example, an X2 capacitor of capacitance up to 1 µF is rated for a peak surge voltage of 2.5 kV, whereas the rating for an X1 capacitor of similar value is 4 kV. Choose an appropriate class of interference-suppression capacitor based on the peak voltages occurring during load dump.

Control and logic circuitry

A wide variety of capacitors including tantalum, ceramic, film, and aluminum are used in the power supply's control circuitry. Unless used in a harsh environment, these devices are general-purpose   components   with   low   voltage   and   loss   values.   For   power   supplies   used   in harsh environments, high-temperature components are typically selected. For industrial or professional-   application   power   supplies,   selecting     low-ESR components, for higher overall reliability is a good   practice.To take advantage of automatic assembly,   reduced   size,   lower   assembly cost and,   consequently, higher productivity, most designers try to stay with SMD technology   for the capacitors used in the control circuitry. However, it is not uncommon for some engineers to choose mixed technology to take advantage of the substantially lower cost of some leaded components, such as film capacitors, that also exhibit better reliability.