Power supplies convert electrical energy as efficiently as possible with constraints of practical components and compliance to international standards such as FCC part 15 for EMC, UL and CE for safety and IEC61000-3-2 (harmonics) and IEC61000-3-3 (flicker). When designing power supplies it is essential to make accurate measurements of the input and output quantities to ensure the power supply is functioning correctly and / or to verify its design. Below is a description of some of the more critical power supply measurements.

Phase and amplitude samples of current and voltage are the fundamental building block for algorithms to derive the many parameters needed to characterize power supplies.

Measuring input and output power provides an efficiency specification for the power supply. High efficiency measurements demand high accuracy of input and output power measurements.

Power supplies are generally characterized for normal operating power and standby power (equipment with power continually applied but turned off by electronics). Current drawn will be non-linear making it essential to measure True RMS current and voltage.

The RMS value is the most commonly used and useful means of specifying the value of both AC voltage and current.

The RMS value of an AC waveform indicates the level of power that is available from that waveform and is equivalent DC at the same voltage. This is one of the most important attributes of any AC source. The calculation of an RMS value can best be described by considering an AC current waveform and its associated heating effect such as that shown in Figure 1 a below.

It is clear that, in comparison with DC systems, the transferred AC power is not simply the product of the voltage and current values. A further element, known as the power factor must also be taken into consideration.

Inrush is important for:

• Correctly specify I2t rating of fuses and circuit breakers.
• Determine peak current rating required for rectifier diodes etc.
• Determine effect of the inrush current on the a.c. supply

Inrush may also be significant after switch on during a power interruption.

Harmonic and Flicker measurements are required for compliance testing to IEC61000-3-2 and IEC61000-3-3