Ripple and noise are AC components superimposed on the DC output voltage. The measurement of ripple and noise is carried out under rated load and normal temperature. For a switched DC-DC exchanger, the output ripple voltage is a small pulse with high frequency component in the system, so the peak to peak value (mvp-p) is usually measured instead of the RMS. For example, when the ripple peak to peak value of a dc/dc exchanger is 50mV, its RMS value is very low, only 5mv, but whether it can be used in a certain system must be further considered.
Because the measured ripple contains high-frequency components, special measurement techniques must be used to obtain correct measurement results. In order to measure all high-frequency harmonics in the ripple spike, an oscilloscope with 20MHz bandwidth is generally used.
Secondly, during ripple measurement, great attention must be paid to prevent introducing wrong signals into the test equipment. The probe ground clamp must be removed during measurement, because in a high-frequency radiation field, the ground clamp will receive noise like an antenna and interfere with the measurement results. The measurement method shown in Figure 1 is used to eliminate interference. In order to further reduce noise, a 47uF capacitor can be connected in parallel.
The power supply is placed at a place 25mm away from the grounding plate, which is composed of aluminum or copper plate. The output common terminal and AC input ground terminal of the power supply are directly connected to the floor. The ground wire should be very thick and not longer than 50mm. Use 16amg copper wire to make 300mm twisted pair, connect power output at one end, connect a 47uF capacitor in parallel at the other end, and then connect it to the oscilloscope. The lead of the capacitor shall be as short as possible, and the polarity shall not be reversed. The "ground wire" of the oscilloscope probe shall be connected to the ground wire ring as far as possible. The bandwidth of the oscilloscope shall not be less than 50MHz. The AC of the oscilloscope itself shall be grounded.