Power Design Tip 21: Please pay attention to the capacitor RMS ripple current rating! (For full text, please see PDF!)

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Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

SMD aluminum electrolytic capacitor

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One type of stress that is often overlooked in power supplies is the input capacitor RMS current. If it is not properly understood, overcurrent will cause the capacitor to overheat and prematurely fail. In a buck converter, the RMS current can be easily calculated from the output current (Io) and duty cycle (D) using the approximation provided in this article. Its trajectory is a circle with a maximum of 0.5 at 50% duty cycle and 2 zero crossings at 0% and 100% duty cycle. The curve is symmetric around the 50% duty cycle. Between 20% and 80%, the ratio between RMS current and output current is greater than 80%. Using this range of duty cycles, you can roughly estimate the RMS current as 1/2 of the maximum output current. Outside of this range, you need to do the calculations accordingly.

In the past few years, ceramic capacitors have made great progress in both volumetric efficiency and cost. Ceramic capacitors are now the first choice to bypass the power stage. However, their low ESR can cause a lot of problems in the power supply, such as EMI filter oscillations and unexpected voltage surges (see Power Design Tip 20). Parallel electrolytic capacitors are often used to suppress these high Q circuits. In these cases, you should be aware of the ripple current in the electrolyte, as a large amount of power supply ripple current will eventually enter the electrolytic capacitor. Figure 2 shows an example of a 100 kHz transfer switch with input capacitors. The input capacitor consists of a 10 uF ceramic capacitor in parallel with the electrolytic capacitor, which contains an equivalent series resistance (ESR) of 0.15 ohms. It is assumed that the electrolytic capacitor has a larger capacitance than the ceramic capacitor, and in this case, about 70% of the RMS current appears in the electrolyte. To reduce this RMS current, you can increase the ceramic capacitance, operating frequency, or equivalent series resistance (ESR). This curve can be plotted by the Fourier series of the capacitive current to calculate the electrolytic capacitor current for each harmonic (up to 10) and recombine the harmonics to calculate the total RMS current of the electrolytic capacitor. Note that the current in the ceramic capacitor is 1/4 cycle out of phase with the current in the ESR, so they must be treated as vectors. If you don't want to spend time on these calculations, you can easily simulate the circuit with a current source and three passive components.