What is an oscilloscope?

This measuring instrument can display voltage changes as a graph over time. It is indispensable for evaluating power supplies because it can visualize the operation of electronic circuits as "waveforms."

Why is waveform measurement important?

To be honest, at first I thought that it would be enough to measure the input and output voltages with a tester. However, there are cases where output is produced even if the switching is not working properly. By checking the waveforms with an oscilloscope, you can check even parts that cannot be seen with a tester, such as whether the IC is really switching correctly and whether there is any abnormal noise or oscillation.

Beginners in particular often encounter problems when designing. In such cases, it is very important to be able to properly "debug." An important element of debugging is measuring waveforms with an oscilloscope, which can help identify the cause of any problems.

Evaluation board and measurement environment

The board used this time was Analog Devices' LT8609A evaluation board, DC2958A.
This evaluation board features a manufacturer-recommended layout and component placement to maximize the performance of the LT8609A.

LT8609A Evaluation Board DC2958A Schematic

The measurement conditions are set as follows:

Input voltage: 12V

Output voltage: 5V

・Output current: ~2A

・Switching frequency: 2MHz

Introduction to the oscilloscope (Tektronix MSO58)

For the measurements, we used the Tektronix oscilloscope "MSO58".

The MSO58 offers high bandwidth and high resolution, making it ideal for evaluating switching power supplies.

Important points to note when measuring with an oscilloscope (switching, output ripple, etc.)

　　

- Probe connection: Minimize the effects of noise by keeping the GND lead as short as possible and selecting the measurement point accurately.

- Accurate waveform measurement: To accurately capture the waveform, it is important to measure close to the pin.

　

For reference, the probing environment is attached below.

To minimize the effects of noise such as switching and output ripple, it is important to perform measurements with the GND lead shortened as shown below.

Reference image for probing

1. Input voltage/output voltage

In the diagram, CH1 indicates the input voltage and CH2 indicates the output voltage.

The output maintained a stable 5V for an input of 12V. The input and output waveforms were measured at 2A.

Comment: You can check the input and output voltages with a tester, but by observing the waveforms with an oscilloscope, you can also see sudden noise and temporary drops.

2 Switching and output ripple

In the figure, CH1 shows the output voltage (AC coupling) and CH2 shows the switching waveform. The load is 2A.

3 Load transient response

In the diagram, CH1 indicates the output voltage (AC coupling) and CH2 indicates the load current.

We checked the responsiveness and stability when the load current was changed sharply from 0.5A to 1.5A to 0.5A. Specifically, we observed the magnitude of the undershoot when the load current was increased and the overshoot when it was decreased, the time it took to return to the set value, and the presence or absence of ringing. The undershoot / overshoot was around ± 150mV, the time it took to return to the set value was around 50us, and there was almost no ringing, making it a stable circuit.

Comment: If the load transient response is insufficient, improvement can be expected by reviewing the capacitance of the output capacitor and the feed-forward capacitor between the output and FB terminal.

Measurement results (homemade board)

Output ripple and switching waveform of homemade board

In the diagram, CH1 indicates the output voltage (AC coupling), and CH2 indicates the switching waveform.

Ripple voltage: On the evaluation board it was about 12 mVp-p, but on my board it reached several hundred mVp-p, causing the output to be unstable.

Switching waveform: A clean, periodic waveform was observed on the evaluation board, but on my board the waveform was distorted, indicating that the switching was unstable.

Comment: When I measured the output voltage with a tester, the target voltage was being output. However, looking at the diagram, I can see that the switching and output were unstable. I was able to observe problems that were not visible when measuring with a tester. I hope that my example has shown you how important it is to measure with an oscilloscope. The causes of these problems are likely to lie in design elements such as component placement and wiring. Comparing it to an evaluation board can give you an idea of where the problem lies.