Ripple current

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 major concern of both output and input filter capacitors is the ripple current entering the capacitor. In this application, the ripple current is identical to the inductor ac current. The maximum limits of the inductor current is 2.8 A for I peak and about one-half the maximum output current or 1.0 A. So the ripple current is 1.8 A p-p or an estimated RMS value of 0.6 A (about one-third of the p-p value). 

The best candidate capacitors are from AVX, which have very low ESR and thus can handle very high levels of ripple current. These capacitors are exceptional and not typical. One piece of these could handle the output demands. 

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. 

The datasheet usually provides certain temperature multipliers for the allowable ripple current. For example, for the old but still well-known LXF series from Chemicon, the numbers provided are... 

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). 

So the multiplier must be 5°5 = 2.236, which agrees with the published datasheet value. Therefore we see that from the vendor s published ripple current temperature multipliers, we can easily deduce his designed-in maximum core temperature. 

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. 

We see that we cannot have our cake and eat it too. We can increase the ripple current (but degrading its life) by applying the temperature multipliers. Or we can increase the life (but not the ripple current) by not applying these multipliers. We just can t have it both ways ... 

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... 

Therefore a case temperature measurement may not suffice. We should also measure the ripple current passing through the capacitor. 

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). 

Rather than take the case temperature as the local ambient temperature of the capacitor, which is more of a worst-case calculation, we could try to actually measure the local ambient. Assume that the general ambient is Lamb ext- The local ambient near the capacitor is Lamb. The procedure to factor out the heat from nearby components (i.e., heat which is not due to ripple current) is as follows ... 

There is also a major issue concerning secondary-side trace inductances, one that we will discuss a little later. Other than that, there are no issues, except of course the fact that because there is only one capacitor, the effective ESR won t be very good (nor the RMS ripple current-handing capability). 

The value of the inductor may be increased above the minimum recommended value to reduce input and output ripple. However, once the ripple current is less than 20% of the average current in the inductor, the benefit to output ripple becomes minimal. 

Input Capacitor RMS ripple current ICin IRMS 0.57 A ... 

Output Capacitor RMS ripple current cXout IRMS j0.21 ... 

Inductor ripple current, peak-to-peak value, Steady State PWM Duty Cycle, range limits from 0 to 100 L Ipp 12.0 A Duty Cycld 50.4... 

Average input current 5 [inductor ripple current, peak-to-peak value Jin Avg [i-IPP 0.93 A (0.65 A... 

Inductor ripple current, peak-to-peak value... 

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