Online UPS Designer Reference Manual, Rev. 01-2 Freescale SemiconductorPreliminaryThese effects are aggravated by the long distances the electric distribution networks usually span.From the electrical utility’s point of view, the best possible load is the pure resistive: The currentwaveform should be a pure sinusoidal waveform identical to the voltage waveform and of thesame frequency and phase.In order to show a resistive load to the utility lines, the input current to the UPS is controlled (i.e.,modulated), to make it match a set point. This set point depends on the input voltage waveform,and its amplitude is dependent on the equipment’s power consumption.1.1.3 DC-to-DC ConvertersIf a rectifier is connected to the AC line supply, then the DC voltage will be equal to the peakvoltage of the line. (i.e., in a 120 VRMS line, the peak will be 120√2, 170V). If the battery bank isconfigured for 12 or 24 V DC , the UPS works by using DC-to-DC converters.For an online UPS, two power DC-to-DC converters are required. One converter operates as thebattery charger, and the other boosts the battery voltage in the absence of line input and generatesthe appropriate DC required by the inverter.1.1.4 Phase Locked Loop (PLL)This UPS can operate in the Free Running mode or in the Locked-to-Line mode. If the AC mainline frequency is at the nominal value of 50Hz or 60Hz ± 5%, then the PLL locks the inverteroutput to the line. If the AC main line frequency runs out of limits for any reason, the UPS willautomatically switch to run locked to the internal frequency reference.The UPS will also work in the Free Running mode if commanded to operate as a frequencyconverter. For example, it can connect to a 60Hz AC main line frequency and output a signal of50Hz frequency, and vice versa.The purpose of Phase-Locking the inverter to the line input is to enable the automatic bypassfeature, and to avoid signal “mixing” at the rails. These two features are detailed in the followingsections.1.1.5 Bypass OperationIn order to allow a UPS bypass without loss of power at the load, two conditions must be met:• The inverter output must be locked to the frequency and phase of the AC main line• The inverter output and the AC main line’s RMS voltages must be within 10% of oneanother
