Impact of line voltage sag on switch mode power supply operation

Switch-mode power supplies (SMPS) provide low-voltage dc power to almost every type of computer, control and instrumentation equipment. As such, the performance of this equipment operating in unprotected ac supply systems reflects the behavior of the SMPS to the voltage surges and sags that are endemic to power systems not protected by UPS. One purpose for this paper is to analyze the operation of SMPS during line voltage sags over and above merely observing the discharge of the dc link capacitor. A further purpose is to show that the tolerance of an SMPS to line-voltage sags can be improved by: (1) using a larger dc capacitor; (2) operating at less than the power rating; and (3) using power factor correction. Inrush current control circuits must be designed to operate effectively for line voltage sags of all predictable depths and time durations. Moreover, voltage disturbance plots of events do not indicate SMPS failures for each event outside the ITI limits. A percentage of SMPS will be operating at part load and not fail.