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Q1. What is ripple current?
A1. Ripple current refers to the effect of internal self heating within the capacitor. Once the capacitor reaches the maximum temperature, the associated current level is referred to as ripple current. Ripple current rating does not directly apply to MLCCs.
Q2. Why are MLCCs not rated for ripple current?
A2. MLCCs are not stressed by current. Instead, it is the temperature that is the critical factor here. Ripple current, in this case, would vary depending on the MLCC temperature.
Q3. Why would the ripple current rating for a MLCC vary?
A3. Since MLCCs are dependant on temperature and not current, variation in this temperature will determine the max ripple current allowed.
This can be explained by looking at an example of a Class Ⅱ (X7R) capacitor. The X7R capacitor has a maximum operating temperature is 125℃. If the circuit was operating at a room temp of 25℃ then the MLCC could handle enough current until the part heated up to 125℃. If the same circuit was initially in a 100℃ environment, the ripple current would be less since there is a lower temperature differential.
If the MLCC could be kept below the maximum rated temperature, it could handle more current. Due to this dependency on temperature, ripple current measurement on MLCCs are performed at room temperature.
Q4. How is ripple current measured?
A4. Many MLCC suppliers measure ripple current differently. There really is no industry mandated method. Therefore when trying to compare ripple current data between suppliers or different test methods, it is important to understand the test conditions first before analyzing the results. There are direct and indirect methods to measure ripple current.
One direct method would involve gauging the amount of input current until the capacitor temperature rose to some delta T such as 10℃. Often a thermocouple would also be placed physically on the capacitor to measure the temperature rise.
Indirect methods include using ESR measurements to obtain ripple current. This principal is based on a common power formula P = i²R where in this case R is the equivalent series resistance (ESR). The capacitor is typically at room temperature.
Q5. What are some of the factors that affect ripple current measurement?
A5. There are many factors that affect ripple current. Factors such as frequency and temperature are more easily controlled.
Due to the ESR component, ripple current is directly related to ESR. Both are frequency dependant therefore since ESR is lowest at the self resonant frequency (SRF), the corresponding maximum ripple current is also at the SRF.
Q6. What happens if I exceed the maximum ripple current?
A6. As with any performance parameter, the manufacture does not recommend exceed any specification parameters. Some heating beyond the maximum rated temperature may not necessarily cause immediate failure. If the MLCC is heated beyond the maximum rated temperature, waveform of the ripple current would contribute to reducing the life of the capacitor. Some ripple current waveform factors include duration, intensity, and shape of the current.
Q7. Does polarity affect ripple current for MLCCs?
A7. For a polarized capacitor such as aluminum electrolytic, application of reverse voltage does accelerate the capacitor breakdown. MLCCs are not subject to this because they are not polarized. This is another advantage to use MLCCs for both design and manufacturing engineers.
Q8. Does case size affect ripple?
A8. From an electrical standpoint there are too many variations on design to generalize. Physically the larger case size MLCC does have greater volume and thermal mass. Due to this, the larger case size can absorb more energy (i.e. ripple current) before reaching the maximum rated temperature when compared to the same component in a smaller size.