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A boost converter circuit such as TPS61022 is shown in Figure 1-1. With alternate turning on and off of the two integrated MOSFET, the inductor stores energy and then released to VOUT which is higher than the input voltage. The average value of VOUT is set by the FB pin and R1, R2 resistors.
The ideal operating waveform at heavy loading condition are shown in Figure 1-2. In the image:
In this equation:
However, one may observe much larger output voltage ripple than the calculation result in real circuits. This application details the root cause of the observation and proposes solution to solve the problem.
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 :