Ripple current

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The charging and discharging current (ripple current) in the smoothing capacitor C1 has significantly higher amplitudes than the current through the load R1. Because P = I 2 R, they cause a relatively high power loss in the internal resistance ( ESR ) of C1.

As ripple current (Engl. Ripple current ) of ripple in the German standards "superimposed AC" is called in the electrical one AC arbitrary frequency and waveform , of a direct current is superimposed. This can also lead to a change in polarity. The pure alternating current component of the ripple current occurs primarily in capacitors , which serve to reduce the residual ripple .

In particular, pulsating direct current causes alternating currents with high effective values in capacitors connected in parallel :

  1. after the rectification of a mains-frequency alternating voltage on the downstream filter capacitor : frequency of the ripple current 100/120 Hz for full-wave rectification or 50/60 Hz for one-way rectification. Also known as hum current .
  2. in the case of switched-mode power supplies and switching regulators in their input and output capacitors at their operating frequency (approx. 20 kHz to 1 MHz). The relative load can be very high, since the required capacitance value would actually be low due to the high frequency.
  3. in supply voltage backup capacitors for audio power amplifier and transmitter - power amplifiers

Effects

An alternating voltage superimposed on the direct voltage causes charging and discharging processes in a capacitor . These cause an effective current, which generates a power loss via the equivalent series resistance (ESR) of the capacitor , which is converted into heat . The condenser heats up. The service life and reliability are affected.

In the case of ceramic and plastic film capacitors , the heating has little negative impact on the service life if defined upper temperature limits are not exceeded. In the case of aluminum electrolytic capacitors with liquid electrolytes, however, the internal heat generated by the ripple current shortens the expected service life of the capacitors. Ripple currents that are outside the specified limits of tantalum electrolytic capacitors can lead to their immediate destruction.

Another side effect is the generation of electromagnetic alternating fields, which have a negative impact on the EMC behavior of a circuit.

Ripple currents are also created by rapid switching operations, e.g. B. in digital technology or in the commutation of DC machines . If these ripple currents cannot be prevented, an attempt is made to dissipate the majority of the alternating component directly at the point of origin using block capacitors .

Almost all capacitor types are specified for certain ripple currents. Often a value is given for 100 Hz and one for 100 kHz, which is close to typical applications in conventional and switched-mode power supplies.

In the case of aluminum electrolytic capacitors in particular, there are very large type-related differences in the ripple current load capacity. Failure to observe this property or the careless substitution of components (types with a higher load capacity are more expensive than conventional ones) leads to typical early failures and is often responsible for the unreliability of many cheap electronics products in the entertainment, lighting and IT sectors.

See also

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