Active PFC and a UPS

[[Spectre]]

Thanks to Ice Czar

http://powerelectronics.com/mag/power_power_supply_system/index.html

UPS Nonlinear Loads

Most end users are oblivious to the problems encountered when an online UPS interacts with nonlinear load, such as a switchmode power supply. Fig. 3, on page 42, illustrates the interface between an online UPS and a switchmode power supply. The switchmode supply within the host system consumes its current in high amplitude pulses, and as a result overloads the UPS, distorts its output and degrades in performance. This isn't the case if the SMPS has power factor correction (PFC); and in the United States most SMPS don't have PFC.

Repeated measurements under practical conditions show that in non-PFC SMPS, the current peak to rms ratio reaches a crest factor in the range of 2.5 to 3.5. This high crest factor imposes a difficult load for the UPS.

Manufacturers specify UPS capacity in volt-amperes, and set the UPS current limit with resistive loading. This affects the user in two ways: first, the actual power (in watts) that the online UPS can deliver is 75% to 80% of its VA rating. Second, the UPS cannot support a nonlinear load like a SMPS to full power capacity. In fact, it can support an SMPS only rated to 30% to 40% of its capacity. A 1kVA online UPS will only be suitable for backing up a system energized by a 300W to 400W (non-PFC) switchmode power supply. Attempts to draw higher power by the SMPS will severely distort the output of the UPS and may result in shutdown. Some UPSs support nonlinear loads to various degrees, but the above rule is safe to use as a guideline.

The whole idea of generating a high-purity sinewave within the UPS, and then rectifying it within the SMPS is wasteful. A UPS may be deployed to run non-electronic loads, such as lights or motors. However, this is a minor portion of the market for online UPS. Many applications back up computers and telecom devices, as well as instrumentation systems used for critical applications. In all of these applications, the immediate load for the UPS is the SMPS within the systems. Peak current and inrush current are associated with this load at startup. This current of 40A to 80A per SMPS will cause the UPS to shutdown due to overloading, or the UPS must transfer the load to utility to draw its inrush.
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The exception being ferroresonant based UPS units.