Accurately measuring power supply ripple is an art in itself. In the example shown in Figure 1, a junior engineer used an oscilloscope by mistake. His first mistake was to use an oscilloscope probe with a long ground lead; his second mistake was to place the loop formed by the probe and the ground lead near the power transformer and switching element; his last One mistake is to allow excess inductance between the oscilloscope probe and the output capacitor. This problem manifests as high-frequency pickup in the ripple waveform. In the power supply, there are a large number of high-speed, large-signal voltage and current waveforms that can be easily coupled with the probe, including the magnetic field coupled from the power transformer, the electric field coupled from the switch node, and the common mode generated by the transformer mutual winding capacitance Current.
Figure 1: Poor measurement results from incorrect ripple measurement.
Using the correct measurement method can greatly improve the measured ripple results. First, bandwidth limits are usually used to specify ripple to prevent picking up high-frequency noise that does not really exist. We should set the correct bandwidth limit for the oscilloscope used for measurement. Secondly, by removing the probe "cap" and forming a pickup (as shown in Figure 2), we can eliminate the antenna formed by the long ground lead. Wrap a short piece of wire around the probe ground connection point and connect the ground to the power source. Doing so can shorten the length of the tip exposed to high electromagnetic radiation near the power source, thereby further reducing pickup.
Finally, in an isolated power supply, a large amount of common-mode current flows through the ground connection point of the probe. This creates a voltage drop between the power ground connection point and the oscilloscope ground connection point, which appears as a ripple. To prevent this problem, we need to pay special attention to the common-mode filtering of the power supply design. In addition, wrapping the oscilloscope leads around the ferrite core also helps to minimize this current. In this way, a common-mode inductor is formed, which reduces the measurement error caused by the common-mode current while not affecting the differential voltage measurement. Figure 2 shows the ripple voltage of this identical circuit, which uses an improved measurement method. In this way, high-frequency peaks are truly eliminated.
Figure 2: Four slight changes have greatly improved the measurement results.
In fact, after being integrated into the system, the power ripple performance will be even better. There will almost always be some inductance between the power supply and the other components of the system. This inductance may exist in the wiring, or only the etching exists on the PWB. In addition, there will always be additional bypass capacitors around the chip, which are the load of the power supply. The two together form a low-pass filter, which further reduces power supply ripple and/or high-frequency noise. In extreme cases, when the current flows through a one-inch conductor of 15 nH inductance and 10 μF bypass capacitor for a short time, the cut-off frequency of this filter is 400 kHz. In this case, it means that high-frequency noise will be greatly reduced. In many cases, the cutoff frequency of the filter will be below the power supply ripple frequency, which may greatly reduce the ripple. Experienced engineers should be able to find ways to use this method in their testing process.