Technical Article

UPS in Critical Data Centers

June 17, 2019 by Sean Evanuik

Data center applications require reliable power sources. Learn more about uninterruptible power supplies in data centers.

Data center applications require reliable power sources. Learn more about uninterruptible power supplies in data centers.

Today’s world is seeing an increase in the use of cloud computing, and being connected to the internet at all times is the norm. People walk around with smartphones continuously connected to the internet. Binging entire seasons of shows from streaming services has become the preferred method of watching television. There are so many widespread uses of the cloud that I could not possibly list them all here. One thing is for certain. All of this use of the internet and the cloud requires extraordinary amounts of storage space and infrastructure. That is the job of the data center.

The data center is the heart of the internet. In order to make sure this information is available 100% of the time, data centers are designed for maximum reliability.


Image used courtesy of Manuel Geissinger


The uninterruptible power supply (UPS)  is one piece of equipment every data center uses to make sure the servers and all sensitive pieces of computing equipment are never susceptible to power line disturbances and power quality issues.  


DC Power Supplies in Data Center Systems

Computing equipment such as servers and routers all rely on an internal power supply to provide the regulated DC power required to run the processors and peripheral devices. These power supplies can only handle a certain variance in supply voltage before the computing equipment becomes susceptible to shutdown or overload.

The Information Technology Industry Council (ITI) has created a curve and application note that describes the input voltage envelope that typical information technology equipment can tolerate. It is called the ITI(CBEMA) curve.


The ITI (CBEMA) Curve. Image from Keysight


The application note defines seven power quality events that are described by the ITI curve. As an example, the curve shows that most equipment would be able to withstand a voltage dropout up to 20 ms. A UPS is designed to ensure the input voltage to computing equipment is within the “No Interruption in Function Region” of the voltage envelope.

Components of Uninterruptible Power Supplies

There are several types of uninterruptible power supplies, which will be defined below, but all UPS systems will make use of the following components.

  • Rectifier: The rectifier converts the input AC power into DC power. This DC power will be used to feed an energy storage system.
  • Energy Storage: Every UPSwill use some type of system for storing energy in case of input power failure. This energy may be stored in the form of batteries, flywheels, or supercapacitors and is what allows a UPS to supply uninterrupted power.
  • Inverter: The inverter converts the DC power from the rectifier or the energy storage system into the required AC power to be used by the load.


Basic system configuration of an example UPS, an Eaton E-series DX three-phase UPS. Image used courtesy of Eaton.



Types of Uninterruptible Power Supplies

  • Standby/Offline: The standby UPS has two modes. During normal operation, the input power is fed directly to the output load with no filtering. A solid state switch is used to transfer the load to the battery source when power loss is detected.
  • Line Interactive: Similar to a standby UPS but has the ability to adjust the output in response to over and under voltage scenarios without switching to a battery. A solid state switch is used to transfer the load to the battery source when power loss is detected.
  • Online/Double Conversion: An online UPS makes use of double conversion power electronics. In this topology, the battery system is always connected and does not require switching to the backup source. Normal operation power flow is through the rectifier, charging of the energy storage system, and through the inverter.

Online Double Conversion UPS Systems

Critical data centers will typically make use of online double conversion UPS systems.

We already know the online UPS will have a rectifier, energy storage system, and an inverter. To further increase the reliability of the system, an internal automatic static transfer switch will also be included to enable transfer to a bypass source. The transfer will automatically happen if there is a problem or fault detected within the UPS.

UPS systems in critical data centers also make use of an external maintenance bypass. This is a piece of switchgear that allows manual transferring of the critical load from the UPS to a bypass power source. Manually transferring to the external bypass source allows power to be completely removed to allow safe maintenance work inside the UPS.


UPS Redundancies for High Uptime Requirements

In order to meet the high uptime requirements for data centers, UPS systems are often deployed with redundancy.


N+1 Redundancy

Let’s define “N” as the full UPS capacity required to handle the total load. For simple redundancy, an extra module is installed.

In this multi-module system, each UPS is capable of providing the required “N” power. This is referred to as N+1 redundancy.


2N Redundancy

Enterprise level IT equipment often supports dual power supply operation. This equipment can be connected to multiple power sources. In a data center, these two sources would be independent UPS systems. An “A side” and a “B side” can feed the computer equipment. Each side would be able to handle 100% load capacity. This is referred to as 2N redundancy.


2(N+1) Redundancy

These two concepts can also be combined. Take for example having a 2N redundant power distribution system. Instead of a single module UPS on each side, there would be multiple modules. This provides N+1 redundancy on each side. This is referred to as 2(N+1) redundancy.


Transformerless Multi-Level UPS Topology

Today, the most advanced UPS systems deployed in data centers use a transformer-less multi-level topology. The topology ensures the highest reliability and efficiency.

Manufacturers are also starting to use wide bandgap transistors such as silicon carbide (SiC). This is increasing the efficiency of UPS systems up to 98% in double conversion mode.



As internet usage increases day after day, more and more data centers are being built. The UPS system is the heart of making sure every computer within a data center is always running.

Next time when you are streaming your favorite show, just remember all that information is being provided uninterrupted thanks to the UPS.