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MBR0530 Datasheet(PDF) 10 Page - ON Semiconductor |
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MBR0530 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 13 page NCP1546 http://onsemi.com 10 COMPONENT SELECTION Input Capacitor In a buck converter, the input capacitor witnesses pulsed current with an amplitude equal to the load current. This pulsed current and the ESR of the input capacitors determine the VIN ripple voltage, which is shown in Figure 10. For VIN ripple, low ESR is a critical requirement for the input capacitor selection. The pulsed input current possesses a significant AC component, which is absorbed by the input capacitors. The RMS current of the input capacitor can be calculated using: IRMS + IO D(1 * D) where: D = switching duty cycle which is equal to VO/VIN. IO = load current. Figure 10. Input Voltage Ripple in a Buck Converter To calculate the RMS current, multiply the load current with the constant given by Figure 11 at each duty cycle. It is a common practice to select the input capacitor with an RMS current rating more than half the maximum load current. If multiple capacitors are paralleled, the RMS current for each capacitor should be the total current divided by the number of capacitors. Figure 11. Input Capacitor RMS Current can be Calculated by Multiplying Y Value with Maximum Load Current at any Duty Cycle 0 0.2 0.4 1.0 DUTY CYCLE 0 0.1 0.3 0.4 0.5 0.6 0.2 0.6 0.8 Selecting the capacitor type is determined by each design’s constraint and emphasis. The aluminum electrolytic capacitors are widely available at lowest cost. Their ESR and ESL (equivalent series inductor) are relatively high. Multiple capacitors are usually paralleled to achieve lower ESR. In addition, electrolytic capacitors usually need to be paralleled with a ceramic capacitor for filtering high frequency noises. The OS−CON are solid aluminum electrolytic capacitors, and therefore has a much lower ESR. Recently, the price of the OS−CON capacitors has dropped significantly so that it is now feasible to use them for some low cost designs. Electrolytic capacitors are physically large, and not used in applications where the size, and especially height is the major concern. Ceramic capacitors are now available in values over 10 mF. Since the ceramic capacitor has low ESR and ESL, a single ceramic capacitor can be adequate for both low frequency and high frequency noises. The disadvantage of ceramic capacitors are their high cost. Solid tantalum capacitors can have low ESR and small size. However, the reliability of the tantalum capacitor is always a concern in the application where the capacitor may experience surge current. Output Capacitor In a buck converter, the requirements on the output capacitor are not as critical as those on the input capacitor. The current to the output capacitor comes from the inductor and thus is triangular. In most applications, this makes the RMS ripple current not an issue in selecting output capacitors. The output ripple voltage is the sum of a triangular wave caused by ripple current flowing through ESR, and a square wave due to ESL. Capacitive reactance is assumed to be small compared to ESR and ESL. The peak to peak ripple current of the inductor is: IP * P + VO(VIN * VO) (VIN)(L)(fS) VRIPPLE(ESR), the output ripple due to the ESR, is equal to the product of IP−P and ESR. The voltage developed across the ESL is proportional to the di/dt of the output capacitor. It is realized that the di/dt of the output capacitor is the same as the di/dt of the inductor current. Therefore, when the switch turns on, the di/dt is equal to (VIN − VO)/L, and it becomes VO/L when the switch turns off. The total ripple voltage induced by ESL can then be derived from VRIPPLE(ESL) + ESL( VIN L ) ) ESL( VIN * VO L ) + ESL( VIN L ) The total output ripple is the sum of the VRIPPLE(ESR) and VRIPPLE(ESR). |
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