Ripple current rating

To properly design the capacitance for the output stage, one should place enough capacitors in parallel so that each capacitor operates at about 70 to 80 percent of its maximum ripple current rating. The sum of the capacitors should equal the final calculated value, but each capacitor should have the value of Ctot/fi, where n is the number of capacitors in parallel. 

It is unusual to be able to And one capacitor to handle the entire ripple current of the supply. Typically one should consider paralleling two or more capacitors (n) of I/n the capacitance of the calculated capacitance. This will cut the ripple current into each capacitor by the number of paralleled capacitors. Each capacitor can then operate below its maximum ripple current rating. It is critical that the printed circuit board be laid out with symmetrical traced to each capacitor so that they truly share the current. A ceramic capacitor ( 0.I pF) should also be placed in parallel with the input capacitor(s) to accommodate the high frequency components of the ripple current. 

The rule-of-thumb is to pick an output capacitor with a ripple current rating equal to or greater than the worst-case RMS capacitor current calculated above. Its voltage rating is usually picked to be at least 20 to 50% higher than what it will see in the application (i.e. Vin.max for all topologies). The input voltage ripple of the converter is also usually a concern because a small part of it does get transmitted to the output. There can also be EMI considerations involved. In addition, every control IC has a certain (usually unspecified) amount of input noise and ripple rejection, and it may misbehave if the ripple is too much. Typically, the input ripple needs to be kept down to less than 5% to 10% of the input... 

The most important datasheet parameter is the ripple current rating. This is typically stated in Amperes RMS at 120 Hz and 105°C. It essentially means that if the ambient temperature is at the maximum rating of 105°C, we can pass a (low frequency) current waveform with the stated RMS, and in doing so we will get the stated life. The declared life figure is typically 2000 hours to 10,000 hours under these conditions. Yes there are lower grade 85°C capacitors available, but they are rarely used, as they can hardly meet typical life requirements at high ambients. 

Let us now understand what a frequency multiplier tells us. The ESR of an elko is also usually stated at 120 Hz. The vendor may have directly provided a ripple current rating at 100 kHz in addition to the 120 Hz number. If not, he would certainly have provided frequency multipliers. Atypical frequency multiplier is 1.43 at 100 kHz. That means that if we are allowed 1 A ripple current at 120 Hz, then at 100 kHz we are allowed 1.43 A. This, by design, will produce the same heating (core temperature rise over ambient) as 1 A causes at 120 Hz. Therefore this is also equivalent to saying that the ESR at 100 kHz is related to the ESR at 120 Hz by the following equation ... 

One caution Whenever the resonant capacitor is placed on a lower voltage output, the ripple current entering the capacitor increases. So, carefully check the ratings against the application. 

This means that if, for example, the rated ripple current is 1A (at a maximum rated ambient of 105°C), then we can pass 1.73A at an ambient of 85°C and 2.23A at an ambient of 65°C. But in doing so, the core temperature will remain the same (not necessarily the life, though, as we shall soon see). 

There is actually another complication. It has been determined that not only is the absolute value of the core temperature important, but the differential from can to core is critical too. So if we increase the differential beyond the designed-in 5°C, the life can deteriorate severely, even if the can itself is held at a much lower temperature. But the designed-in differential of 5°C occurs ONLY when we pass the maximum specified ripple current (no temperature multipliers applied), irrespective of the ambient. Which means that as a matter of fact we cannot use any temperature multipliers at all. So, if the capacitor is rated to pass 1A at 105°C, then even at an ambient of, say, 65 °C, we are allowed to pass only 1A, not 2.23A. 

Question If we pass the rated ripple current through a 2000 hour capacitor (no temperature multipliers applied) at an ambient of 55°C, what is the expected life (first pass estimate) ... 

Answer At the rated current we can expect that the core is at 55°C + A7 Core amb- Since we are passing only the rated ripple current, ArCORE AMB is the manufacturer s designed-in core-to-ambient differential. So the temperature advantage we have thus gained (measured from the maximum rated temperature) is (105°C + A7core amb) minus (55°C + ArCORE AMB), or 50°C. Since this capacitor provides 2000 hours at the maximum temperature, at the reduced ambient we may get a life of... 

Capacitor manufacturers recommend that in general we don t pass any more current than the maximum rated ripple current. This ripple current is the one specified at the worst case ambient (e.g., 105°C). Even at lower temperatures we should not exceed this current rating. No temperature multipliers should be used. Because only then is the case to core temperature differential within the design specifications of the part. And only then are we allowed to apply the simple 10°C doubling rule for life. 

If the measured ripple current is confirmed to be within the rating, we can then take the case temperature measurement as the basis for applying the normal 10°C doubling rule, even if the heat is coming from adjacent sources. Again, that is only because the case to core temperature differential is actually within the capacitor s design expectations. 

However, in direct customer communications, Chemicon has, at least in the past, allowed a higher ripple current than the rating. But the life calculation method given is then slightly different. This amounts to a special doubling rule every 5°C, which we will describe below using a practical example. 

Question We are using a 2200 xF/10V capacitor from Chemicon. Its catalog specifications are 8000 hours at maximum rated 1.69A, stated at 105°C and 100kHz. The measured case temperature in our application is 84°C and the measured ripple current is 2.2A. What is the expected life ... 

Answer Since we are passing more than the rated ripple current, we need to replace the usual doubling every 10°C formula with the more detailed formula made available to us by Chemicon. The calculation proceeds as follows ... 

The Access term in the previous equation should be omitted if the ripple current is equal to or less than its rated value. In other words, Access is not allowed to be negative. In that case we revert to the usual 10°C doubling rule (i.e., just omit the 2 /5 term in the equation above). 

Blog Entry 3 The capacitor s ripple current is rated at 950mA at 100kHz, which falls within our application spec, I believe. Its impedance at 100kHz is given as 0.0560hms. 

---