[Design Application]
Online UPSs Save "Mission-Critical" Applications
But Designers Must Understand The Various Architectures and Options Available In Order To Make The Right Choice.
Uninterruptible power supplies (UPSs) fall into two distinct classes—standby or online. While the relatively low-cost, standby UPS has found favor in the consumer desktop market, the 5- to 10-ms delay these devices introduce when switching from utility to battery cannot be tolerated in critical applications. Here, the online UPS, which avoids this switching action, is preferred. But not all UPSs are created equal; each has its own balance of size, weight, cost, reliability, and performance. So system designers and integrators must understand the internal workings of these devices to ensure that choosing among the various topologies does not result in costly error.
The basic premise of all online UPSs is that the output is glitch free—a sinewave, well regulated, and well protected against overloads and shortcircuits. Also, it should be able to drive both resistive and certain reactive (nonlinear) loads. Because most systems running on UPSs are not purely resistive, their current and voltage consumption are not completely in phase, resulting in a reactive power component known as the "power factor." The rating of a UPS reflects its ability to deal with this reactive load. So, UPS capacity is measured in volts * amperes (VA), rather than watts.
Commonly available online UPSs start at 1 kVA (the size of a computer box), and go as high as several hundred kVA (large cabinets weighing hundreds or thousands of pounds). For backup, sealed, maintenance-free, lead-acid batteries are used for their ruggedness, durability, and ability to supply high inrush currents. They are easy to charge, and they maintain that charge over long periods when not in use. However, the batteries deteriorate at temperatures of 60°C and above, and at low temperatures they lose capacity—in terms of ampere hours (Ahr)—very quickly. The batteries are connected in series to arrive at the desired voltage (12 V * the number of batteries), and in parallel to arrive at the desired capacity.
If the inverter within the online UPS runs on battery at all times, the batteries must be charged continuously from the utility supply. The charger is rather large because it supplies the entire current needs of the inverter, as well as furnishing current to charge the battery. Because the charger is a dc-dc converter (from the rectified utility voltage to battery voltage), and the inverter follows it, this online topology, in effect, completes a double conversion. Hence, the name "double-conversion UPS."
The inverter does not recognize a utility failure because, during the absence of utility power, it will continue running on the battery and delivering its output. But the battery is now in a discharge mode, and will run the inverter for a period proportional to its capacity, temperature, and current drain. The smaller the load, the longer the backup time.
Several Architectures Online UPS manufacturers employ several major design architectures. All of them are in common use in commercial, industrial, and military UPSs. As noted, in online UPS, the battery powers the inverter stage at all times. The utility (ac) drives the high-power battery charger, which charges the battery, and supplies the inverter simultaneously (Fig. 1). The inverter stage—equipped with a stable oscillator, pulse-width-modulator (PWM), and feedback loop—resembles a switch-mode power supply, except that its output is ac rather than dc. The high-frequency PWM stage is modulated by the line frequency (50 or 60 Hz) in a class D fashion, resulting in a high-quality sinewave (after some minimal filtering).
Were it not for the fact that the inverter is limited in capacity and peak current capability, the load would not know the difference between the utility and the UPS output. The UPS proves advantageous however, as it is well regulated (±2%), while the utility voltage fluctuates widely (±15%). This regulation is provided by the feedback loop. A small transformer at the output generates a signal for the feedback loop for error amplification. A ±2% regulation at the output is feasible in this topology from no-load to full-load conditions. The battery-voltage fluctuation is ±15%.
The power transformer at the output of the inverter is a line-frequency transformer, and is, therefore, large and heavy (depending on the UPS capacity). After the battery bank, this transformer is the single most dominant weight and size element within what could already be quite a heavy UPS system.
Notice also, that this transformer, together with the capacitor across its output, acts as a filter inductor (shown by dotted lines)(Fig. 1, again). This bonus feature is accomplished by purposely allowing a high leakage inductance between the primary and secondary of the transformer. The result is a virtual filter inductor, which, together with the capacitor, forms a low-band filter for the high-frequency carrier.
The current-sense circuitry provides overload and short-circuit protection to the output. In addition, the input is bridged to the output of the UPS by a bypass switch. This switch can connect its output to the input utility in case of inverter failure. The transfer switch is a key feature in online UPSs, but it is not standard. Far-East manufacturers make another good use of this switch. At turn on, they connect the load to the utility for 10 s, and only then transfer it to inverter. The first 10 s permits loads with high inrush current—such as switch-mode power supplies (SMPSs), motors, and compressors—to start well on utility, and only then transfer to the inverter under more stable conditions.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
