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HOW TO MEASURE BUCK CONVERTER LOOP GAIN AND PHASE
Newsletter Issue 26 | Mar. 29, 2018
Understanding the basics of the control loop
In power converters, the control loop plays an important role in the regulation characteristics of the converter. During the design stage it is important to check the stability of the control loop. Stability measurements are often done by measuring the control loop transfer function over a wide frequency range, and plotting the loop gain and phase versus frequency in a Bode Plot.
Challenges of gain & phase measurements
Loop gain and phase measurements can be tricky, because measured signals are small, and the injection signal can easily influence the normal behavior of the power converter. For measuring loop gain and phase, a network analyzer or frequency response analyzer is normally used, but these are complex and expensive items of equipment, and only show results in the frequency domain.
A better way to measure loop gain & phase
For accurate gain-phase measurement results, it is important to ensure the converter is working correctly during the gain-phase measurement. This can be accomplished by checking the converter signals in the time domain as well, under the same conditions as during the gain-phase measurement. A PicoScope® in combination with a Frequency Response Analyzer (FRA) utility can be used successfully for converter loop gain measurements, and it easily lets you check the signals in both time domain and frequency domain.
An example of Buck Converter stability testing
We have used a PicoScope 5444B with the FRA utility to measure the loop gain and phase of a popular Richtek current mode buck converter RT7257GHZSP in the 12V to 5V/2A application example which we explained in the previous EVB DIY video. We will use the PicoScope in Scope mode to check the loop input and output signals and converter switching, and then switch the PicoScope to FRA mode to measure the loop gain and phase at different operating conditions.
FIND MORE PRACTICAL MEASUREMENT TIPS
IN THE VIDEO
The tutorial video “How to measure Buck converter loop gain and phase” will explain the basics of control loop measurements and will show you how to use the PicoScope® with the FRA utility. We’ll also explain how to make the signal injection transformer that you need for this measurement. The video gives many practical tips and examples on converter stability measurement that can also be applied to many other converter types.

PicoScope is a registered trademark of Pico Technology.
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