2 Observation in Bench Test

The Equation 1 assumes that the output capacitor is ideal and capacitor ripple is perfectly measured by the voltage probe. However, the actual ripple waveform would be related to the setting of oscilloscope bandwidth and voltage probe grounding method.

The bandwidth of a Tektronix oscilloscope can be set to 20 MHz or full bandwidth, which could be 500 MHz or higher depending on the oscilloscope model. The voltage probe grounding loop could be as in Figure 2-1. The left probe has long grounding wire and the test point is not closed to the capacitor. The right probe has very short grounding wire and the test point is on the terminals of the capacitor.

Taking TPS61022 as an example, the output ripple waveform with different setting are shown from Figure 2-2 to Figure 2-4. From the waveform, three conclusions can be derived :

1. The loop between the probe tip and its ground must be as small as possible, to avoid any noise coupling.
2. In additional to the switching frequency ripple, there is large and high frequency voltage spike across the output capacitor.
3. The voltage spike is much small at 20-MHz bandwidth setting as the oscilloscope acts at low pass filter.