ADUM 4221

Question

My question is related to the capacitance C9,C10,C11 as shown in figure.What is the use of these capacitances .Also the value of these capacitances is recommended as 100 &micro;F electrolytic capacitor for C9 and 2.2 &micro;F ceramic capacitors for C10 and C11.But nothing is said about the voltage rating of these capacitances .Can any one tell me the voltage rating of these capacitances .Waiting for the replay

RSchnell · Accepted Answer

Hello, 
 Capacitor voltage rating is a tricky topic. At the first pass, you MUST choose a voltage rating that is higher than any voltage the capacitor will encounter. Choosing a value below your intended working voltage can lead to device failure.For C9, C10, and C11, these are the bus voltage capacitors, and MUST be rated above whatever bus voltage your system will see. It is a DC value in this circuit. Capacitor voltage ratings also come with a derating curve, and the closer the capacitors get to their rated voltage, the lower the actual capacitance is. It often surprises me when looking at the voltage to normalized capacitance curves, and at the rated voltage, many capacitors only have 10-30% of their labelled capacitance. This means that even if you choose a 100 &micro;F capacitor rated too exactly the bus voltage, at that voltage, the capacitance will be significantly lower, and must be accounted for. 
 The choice of the equivalent capacitance at the operating voltage is up to the designer of the circuit. More capacitance will produce less ripple when there is current flowing out of the capacitor bank. Remember the capacitor equation: 
 
 I = C * dv/dt 
 Where I = current in/out of the capacitor 
 C = capacitance 
 dv/dt = rate of change of the capacitor voltage 
 
 A larger C can accommodate a larger I for the same slope in voltage. The reason for multiple capacitors with different values is that different capacitors have different frequency response due to their impedances. Smaller capacitors generally have a better high frequency response, and also a lower ESR. This comes from the physical nature of the construction of the devices. Electrolytic capacitors are a good choice for getting large values (bulk capacitance) but usually require the application to be DC, as reversing the voltage on an electrolytic capacitor can lead to device failure. Bus capacitor banks are often constructed with a mix of electrolytic and non-electrolytic capacitors to cover many frequency ranges. 
 
 RSchnell)

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