Overview of application
Switch mode power supplies are commonly used in many electronic products. These power supplies typically have input capacitors that cause high levels of peak inrush current to be drawn as they charge from the rectified line at turn-on. The peak amplitude of the inrush current varies with the turn-on phase of the ac voltage cycle. Usually, the highest peak inrush currents occur near the peak (90°) of the voltage cycle. Characterization of inrush current versus turn-on phase allows for determination of worst case inrush current condi- tions, which must be determined to properly select fuses and circuit breakers, to uncover component stresses, and to determine if a product will pro- duce ac line disturbances that interact with other equipment connected to the branch circuit.
For this example, the equipment under test requires an ac line voltage of 120 Vrms at 60 Hz.
Agilent 6800 series features used
•RMS voltage and frequency control
•Peak current measurement
•Pre-event current data capture
•Trigger synchronization to the output voltage phase
•Measurement and waveform generation synchronization
•High crest factor
Advantages/benefits of the Agilent 6800 series solution
The 6800 series provides a “One-Box” Solution for measurement and waveform generation and elimi- nates the worry of synchronizing separate instru- ments. The ability to turn-on relative to the output voltage phase allows worst case inrush characteri- zation, which results in a more reliable product.
Implementation details
How the 6800 series implements peak inrush current measurements
The RMS voltage is programmed to Step mode to generate a turn-on condition from 0 Vrms to 120 Vrms. The turn-on is synchronized to the phase of the output voltage. The current measure- ment is programmed to occur at turn-on with
10 milliseconds of pre-event data to ensure that the full inrush event is captured. To characterize the inrush current of the unit under test, the turn- on phase is initially set to 40° for the first peak inrush current measurement and is then increased at 10° increments up to 90° for succeeding peak inrush current measurements. Between tests, the input capacitors of the unit under test are allowed to fully discharge for proper characterization.