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:

• I_L is the inductor current.
• I_OUT is the output current of the boost converter.
• I_D is the current through the synchronous rectification MOSFET.
• SW is voltage waveform in SW pin.
• ΔV_OUT is output voltage ripple.

The output voltage drops when the inductor is storing energy, and increases when the inductor energy is released. This behavior results in output voltage ripple ΔV_OUT defined by Equation 1. This formula is commonly found in boost converter data sheet. The ΔV_OUT is typically lower than 1% of the average output voltage if the boost converter is properly designed.

In this equation:

• C_OUT is the effective output capacitance.
• f_SW is the switching frequency of the boost.

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 :

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.