Uninterruptible power supplies (UPS)

The primary function of a UPS is to provide temporary backup power in the event of a loss of facility power. Industrial Uninterruptible power supplies (UPS) are for use in industrial/manufacturing situations, such as plant facilities and factories. Continuity of process control is critical across a wide range of Industrial segments: Water/Waste Water, Biotech/Pharmaceutical, Transportation, Chemical, Food & Beverage, Semiconductor, Automotive, and Renewable Energy. Equipment deployed in these segments is often subjected to harsh environments including wide temperature ranges or air characteristics with elevated moisture and salt content. A robust UPS power design is crucial here to ensure key data or work flow is not lost due to a compromise in a system’s operating voltage levels.

On-line UPS systems process all the power needed from the power line and output high-quality AC voltage for critical loads in the event of a main power failure. The rectifier in the UPS converts AC power into DC, then the battery stores the DC power. See Figure 1, at right.

Figure 1: On-line UPS block diagram (Image from etechnog.com)

High power density is important in these designs because of limited space in common applications such as data centers. High power density is achieved, in part, due to bulky transformer elimination. In this case, there must be a common neutral between the input and output AC ports because of safety importance in ground requirements. Higher switching frequencies will further reduce the size of these systems.

Enter wide bandgap (WBG) power transistors. The use of WBG devices allows for smaller, lighter weight and more efficient UPS systems due to faster switching capability with lower switching losses. Greater efficiency leads to extended backup time compared to less efficient Si devices.

Market Watch estimates the global UPS systems market will reach $16.6 Billion by 2027, predicting growth at a CAGR of 5.2% over the analysis period 2020-2027. The U.S. accounts for more than 27% of global market size in 2020. China is forecasted to grow at an 8.4% CAGR for 2020-2027.

Types of UPS topology

On-line
This type of UPS has the ultimate protection. The output is sinewave and it will have an automatic bypass switch to protect the UPS in the event of a fault or overload condition. The output is monitored and if a short-circuit is detected the bypass routes the load to the main power until the fault is removed. Voltage regulation is achieved via the AC-DC-AC process. See Figure 1, above.

Off-line
The Off-line UPS supplies AC power directly to the load by switching ON the transfer switch. In the event of a power failure, the offline UPS supplies the power from the battery backup. An important difference between On-line UPS and Off-line UPS is the requirement of a larger heat sink in an On-line UPS. Since the current drawn by the AC load is continuously flowing through its whole circuit, the temperature of the system increases. Therefore, it needs comparatively larger heat sinks and the components that can withstand high temperature and it can tolerate current flow for very long duration. Due to such a requirement, the cost of Online UPS increases significantly. See Figure 2, at right.

Standby
The Standby UPS is most commonly used for Personal Computers. The transfer switch is set to select the filtered AC input as the primary power source and also switches to the battery/inverter as the backup source in the event of a failure in the AC primary source. In the event of a power failure, the transfer switch will open to switch over to the battery/inverter backup power source. The inverter will only start when the power fails. The main benefits are high efficiency, small size, and low cost. See Figure 3, at right.

Line interactive
This type of UPS has intermediate protection. The output may be sinewave, step wave, or square wave and there is no automatic bypass. Voltage regulation is achieved via a built-in Automatic Voltage Regulator (AVR)/Automatic Voltage Stabilizer (AVS). See Figure 4, at right.

Figure 2

Figure 2: Off-line UPS block diagram (Image from etechnog.com)

Figure 3: Standby UPS block diagram (Image from elprocus.com)

Figure 4

Figure 4: Line-interactive UPS block diagram (Image from elprocus.com)

Traditional silicon design

Most of today’s online UPS systems are based on two-stage, transformer-less topologies with a common-neutral between the input and output AC ports. Two-stage, transformer-less online UPS systems are most often based on hard-switched designs that use four-quadrant switches. These switches introduce large high-frequency-loop inductances, limiting UPS volume reduction by operating these topologies at high switching frequencies. Various soft-switched UPS topologies amiable to high frequency operation can be used; however, these topologies either contain extra passive components to accomplish soft-switching or require a transformer to achieve a common-neutral between the input and output AC ports.

Silicon switching devices cannot achieve the high frequency provided by Gallium Nitride and Silicon Carbide devices which offer optimum power densities and performance.

WBG design

Today’s designers are forced to achieve higher power density and higher efficiency, but the standard Silicon topologies are limited in their high frequency operation capabilities. GaN and SiC are more efficient, more thermally stable, and certainly more capable for use in power devices that demand more load or higher frequencies at higher temperatures than Silicon.

Enter a GaN soft-switching transformer-less online UPS topology which enables significant size reduction by operating efficiently at high switching frequencies using GaN power devices. See Figure 5, at right.

Figure 5

Figure 5: A GaN UPS topology with a single DC bus using half-bridge switch elements (Image from Reference 1)

The proposed UPS employs standard GaN half-bridge structures with a common-neutral between the input and output and is able to achieve zero-voltage switching (ZVS) operation, in the boundary conduction mode, with no additional complex circuit design. This design employs a new control methodology for the UPS that has a dual-mode digital controller for the input PFC rectifier stage. The digital controller regulates the output voltage of the converter across both resistive and reactive loads.

The inverter (DC/AC) stage is also operated in dual-mode, and a digital controller regulates the output voltage of the converter across resistive and reactive loads. This converter architecture is capable of delivering 1-kVA of output power while maintaining unity power factor at its input. This GaN-based 1-kVA online UPS is operated using the proposed control technique in Reference 1, and is designed, built, and tested. The prototype UPS, operated up to 2MHz, achieved a power density of 26.4W/in3.

WBG semiconductor additional benefits

Reference 2, below, is a 2010 white paper regarding 1,200V SiC MOSFETs vs Silicon IGBTs in UPS applications. Although this is an older paper, and SiC has since greatly increased its lead in performance over Silicon, the results still show why WBG devices greatly improve efficiency and reduce conduction and switching losses because of their I-V gate characteristics and higher gate voltages. Gate drive current requirements of SiC devices also improve efficiency. SiC and GaN higher frequency capability helps to reduce losses better than Silicon. Passive component VA rating reduction also is achieved with SiC replacing IGBTs.

References
  1. Control of a GaN-Based High-Power-Density Single-Phase Online Uninterruptible Power Supply, Danish Shahzad, Saad Pervaiz, Nauman Zaffar, Khurram K. Afridi, IEEE 2019
  2. Performance Comparison of 1200V Silicon and SiC devices for UPS Application, James McBryde, Arun Kadavelugu, Bobby Compton, Subhashish Bhattacharya, Mrinal Das, Anant Agarwal, IEEE 2010