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3.6. Ripple and noise

In a switched-mode power supply, output voltage ripple, as shown in figure 19, is relatively complicated compared to the linear power supply. In ripple, three components with very different frequencies can be found. In the first place low frequencies of 100 Hz can be found due to the fact that the feedback loop rejection is not able to fully remove the ripple from the rectifier and the input filter. Secondly, there is the ripple at the switching frequency which usually has the highest level. And thirdly, in coincidence with the switching times, overlapped to the ripple signal, voltage peaks with components of much higher frequencies, also called noise, can be found.
 
Output noise measure-ments should be carried out very carefully in order to reduce the error introduced by measuring equipment. If measurements are taken with an oscilloscope, for example, the ground clamp must be removed, because it acts as an aerial introducing an irradiated noise into the measurement being carried out. A simple way to minimize this disturbance is by winding a conductor onto the sensor-end ground and making the length between this and the test point as short as possible, as shown in figure 20.

Figure-19

Figure-19

Figure-20

Figure-20