How to Make the Best Switch Mode Power Supply Measurements

Application Notes

Characterizing the operation of switch mode power supplies requires a broad range of measurements. Most of the required measurements can be performed using an oscilloscope. However, many engineers today still perform these measurements manually on their scopes, which is very time consuming. Many of today’s mid- and high-end oscilloscopes offer power supply measurement options that can help automate the process of the characterizing a power supply. This application note will look at how to make the following switch mode power supply measurements with an oscilloscope’s automated power measurement application:

 

Input analysis

– Power quality

– Current harmonics

– Inrush current

 

Switching analysis

– Rds(on) and Vce(sat)

– Switching loss

– Slew rate

– Modulation

 

Output analysis

– Output ripple

– Turn-on/off time

– Transient response

– Efficiency

 

Frequency response analysis

– Power supply rejection ratio (PSRR)

– Control loop response (Bode gain and phase)

 

Quite unique in this set of measurements, for an oscilloscope that is, are the frequency response measurements. These types of measurements typically require a vector network analyzer (VNA), which is also called a frequency response analyzer (FRA). This application note begins with a brief discussion on probing tips, and then provides step-bystep instructions on how to perform the various power supply characterization measurements using an InfiniiVision 3000T or 4000 X-Series oscilloscope with the Power Measurement option (DSOX3PWR/DSOX4PWR).

 

This application note also provides explanations of each measurement and why it is important for complete power supply characterization.

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