RMS current rating

So what are the temperature multipliers really telling us All they really tell us is how the vendor has designed his capacitor from a thermal point of view, or what exactly is the capacitor s core temperature. As we will see, if we stick to the RMS current rating of the capacitor (without applying temperature multipliers), we don t really need to know the details of the core temperature either. Temperature multipliers were therefore just objects of abuse by some designers in the past. This is perhaps why nowadays most elko datasheets are no longer even carrying that information. 

Applications include power supplies, motors and drives, induction heating and military use. These designs are preferred to electrolytic types because of their longer life, high RMS current ratings and superior tolerance of surge voltages. 

I decided to also check the statement that the current rating of the capacitor needs to be 30% of I0. In Figure 12-14 I have carried out a hand derivation of the general equation for output RMS. We thus get... 

One method by which transformers may be rated for suitability to handle harmonic loads is by k factor ratings. The k factor is equal to the sum of the square of the harmonic frequency currents (expressed as a ratio of the total RMS current) multiplied by the square of the harmonic frequency numbers ... 

The transformer specified should be capable of handling 559 A of total RMS current with a k factor of not less than 4.378. Typically, transformers are marked with k ratings of 4, 9, 13, 20, 30, 40, and 50, so a transformer with a k rating of 9 should be chosen. Such a transformer would have the capability to carry the full RMS load current and handle winding eddy current losses equal to k times the normal rated eddy current losses. 

Also, remember that the general form expression for the individual harmonic distortions states that /n is equal to the RMS value of the nth harmonic current divided by the RMS value of the fundamental current, thus an expression for the current rating factor for cables can be formulated. The current rating factor (q) is the equivalent fundamental frequency current at which the cable should be rated for carrying nonlinear loads containing harmonic frequency components ... 

The rule-of-thumb is to pick a switch with a current rating at least equal to, but preferably at least twice the worst-case RMS switch current calculated above (for low losses, since the switch forward drop will decrease substantially if its current rating is increased)... 

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. 

The maximum value of the 20 minute RMS for the duty in the example used is 955 A, and the maximum value of the 5 min RMS is 1,130 A. For this application the drive inverter must be rated 22% higher than the calculated motor Constant RMS current. 

Rated apparent power - a measure of the alternating current (AC) power that is computed by multiplying the root-mean-square (rms) current by the root-mean-square voltage. It is determined by the following formula ... 

Note that ceramic capacitors have such low ESRs that it is not very practical to talk about the current passing through them. So manufacturers typically provide a derating based on the changing voltage waveform across them. Typically, if the rated DC voltage is VR, then the peak-to-peak voltage of the waveform is not allowed to exceed 80% of VV In addition, its RMS value is not allowed to exceed 28% of VR. 

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