The MDO3PWR Power Analysis Application Module installed on an MDO3000 Series oscilloscope, an embedded designer who rarely deals with power measurements can quickly get the same accurate, repeatable results as a power supply expert. A Power Analysis Application Module with an oscilloscope and differential voltage and current probes form a complete measurement system for power supply design and test.
Switching loss measurements
Although almost all components of a power supply contribute to energy losses, the majority of energy losses in a switch-mode power supply (SMPS) occur when the switching transistor transitions from an OFF to an ON state (turn-on loss) and vice versa (turn-off loss). By measuring the voltage drop across the switching device and the current flowing through the switching device, the Power Analysis application measures the switching losses as shown in the following figure.
Input analysis
Power quality measurements and current harmonics are two common sets of measurements made on the input section of a power supply to analyze the effects of the power supply on the power line.
Power quality
Power quality refers to a power supply's ability to function properly with the electric power that is supplied to it. These measurements help to understand the effects of distortions caused by nonlinear loads, including the power supply itself. The measurements include RMS voltage and current, true and apparent power, crest factor, line frequency, and power factor, as shown in the following figure.
Safe operating area
The Safe Operating Area (SOA) plot is a graphical technique for evaluating a switching device to ensure that it is not being stressed beyond its maximum specifications. SOA testing can be used to validate performance over a range of operating conditions, including load variations, temperature changes, and variations in input voltages. Limit testing can also be used with SOA plots to automate the validation. An example of an SOA plot is shown in the following figure.
Current harmonics
Because a switching power supply presents a nonlinear load to the power line, the input voltage and current waveforms are not identical. Current is drawn for some portion of the input cycle, causing the generation of harmonics on the input current waveform. Excessive harmonic energy can affect the operation of other equipment connected to the power line, as well as increase the cost of delivering the electric power. Therefore, power supply designers can use the current harmonics measurements to assure pre compliance of their designs to industry standards (such as IEC61000-3-2 Class A and MIL Standard 1399 Section 300A) before investing in the official compliance testing. An example of the current harmonics graph display is shown in the following figure.
Modulation analysis
Modulation is important in a feedback system to control the loop. However, too much modulation can cause the loop to become unstable. The Power Analysis application calculates and shows the trend in the on-time and offtime information of a modulated signal controlling the output control loop on a power supply, as shown in the following figure.
Output analysis
The ultimate goal of a DC-output power supply is to transform input power into one or more DC-output voltages. Especially for switching power supplies, the output measurements are essential. These measurements include line ripple, switching ripple, and modulation analysis.
Line and switching ripple
The quality of a power supply's DC output should be clean with minimal noise and ripple. Line ripple measures the amount of AC-output signal related to the input line frequency. Switching ripple measures the amount of AC signal related to the switching frequency.