Often overlooked in power supply, a stress is input capacitance (** KEMET tantalum capacitor** RMS current). If you do not understand it correctly, a current will cause the capacitance (** KEMET tantalum capacitor** Overheating and premature failure). In a buck converter, using the following approximation, the output current of the (Io) and duty cycle (D) can be calculated easily RMS:

In Figure 1 1/2 output current at the input capacitor RMS buck current peak

In the past few years, the two ceramic capacitor volume efficiency and cost has made great progress. The ceramic capacitor has now become the preferred level of power supply bypass.However, their low ESR will cause a lot of problems, such as: EMI filter in power oscillation and voltage surge accident. Parallel (electrolytic capacitor** KEMET tantalum capacitor** ) are often used to suppress these high Q. In these cases, you should pay attention to the ripple current in the electrolyte, because the power supply ripple current will eventually enter a large number of electrolytic capacitor. Figure 2 shows a 100 kHz input capacitor switch example, the input capacitance by an electrolytic capacitor in parallel with the 10 uF ceramic capacitor, series resistance and the electrolytic capacitor containing 0.15 ohmic equivalent (ESR). Assuming the electrolytic capacitor capacitance than the ceramic capacitor, in this case, RMS about 70% of the current in the electrolyte. To reduce the RMS current, you can increase the frequency or the ceramic capacitor equivalent series resistance (ESR). By using Fourier series capacitive current can draw the curve to calculate each harmonic (up to 10) of the electrolytic capacitor current, total current RMS and re combination of harmonic calculation of electrolytic capacitor. Please note that the current ESR and the difference of the ceramic capacitor 1/4 cycle in phase, therefore they should be regarded as vector. If you don't want to spend time in these calculations, you can do so by a current source and three passive components easily the circuit simulation.

Figure 2 using different types of capacitor electrolytic capacitors (please note that** KEMET tantalum capacitor** Current)

In short, attention to the RMS current input capacitor, because the current stress will reduce the reliability of capacitor. Combined capacitor type more in need of special attention, because ceramic capacitors usually allow ripple voltage high enough to form an overcurrent condition in parallel electrolytic capacitor. The question problem solving method is to add one or more of the following: working frequency, quantity, ceramic capacitor electrolytic capacitor ESR or RMS rated current.

The following is the derivation of the RMS input current in the capacitor, the assumption of infinite inductance. The rectangular pulse (D0.5*Ipk) of the RMS current as a start, and remove the DC component (D*Ipk).