DIN-Rail Mounted DC Uninterruptible Power Supplies Protect DC-Powered Equipment

Uninterruptible power supplies (UPSs) are integral parts of many industrial applications. Wherever an interruption in primary power could lead to data loss, damage sensitive equipment, compromise safety, or threaten lives, a UPS is part of the system. These critical systems automatically connect the load to battery power when an external power loss occurs, buying time to connect another power source or allowing equipment to be shut down safely.

UPS systems are used in a wide range of applications, including telecommunications, hospitals, and data centers. UPSs designed for direct current (DC) loads protect equipment in transportation, power generation, factory and building automation, semiconductor fabrication, security, and more.

More than a simple switch between an external power supply and a backup battery, a UPS must also manage battery health and input/output voltages. An engineer adding a UPS to existing or planned equipment needs to choose a product that matches the needs of the load and battery, as well as considering the UPS’s interface and safe operating requirements.

Understanding UPS operating principles

Equipment designed for alternating current (AC) power input uses an AC UPS that converts a backup battery’s DC power into an AC output appropriate for the load with inverter circuits (Figure 1). Under normal operation, externally supplied AC power bypasses the battery and inverter to power the load directly. In the event of a power outage, energy from the battery passes through the inverter to power the load at the required voltage until the battery runs out, power is restored, or another power source is connected.

An AC UPS also contains a rectifier, which comes into play while AC power is available. The rectifier turns a portion of the supplied AC into DC at a voltage regulated to charge the battery.

Figure 1: An AC UPS bypasses the battery until it’s needed, then employs an inverter to convert DC from the battery into the AC required by the load. A rectifier charges the battery while AC power is available. (Image source: TDK-Lambda)

A DC load, on the other hand, can connect directly to a backup battery via a DC-UPS. A DC-UPS has two main functions. When external power is available, it works as an internal bi-directional DC/DC converter. This allows it to both charge the battery from rectified DC voltage and convert it to the appropriate voltage for the DC load (Figure 2). During an external power interruption, it acts as a DC/DC converter, supplying a regulated voltage from the battery to the load.

Figure 2: In a DC-UPS, the battery powers the load directly. When power is restored, rectified external AC voltage powers the load and charges the battery. (Image source: TDK-Lambda)

TDK-Lambda’s DUSH960 DC-UPS (Figure 3) work with DC input voltages between 12 V and 60 V. They output up to 20 A and 960 W at user-programmed voltages between 12 and 48 VDC. As programmable DC/DC converters, DUSH960 units can be used without a battery over an input and output range of 10 V to 60 V (12 V to 48 V nominal).

Because the backup battery can supply the load directly without the need for an inverter, DC-UPS systems are extremely efficient. They can reliably deliver 96% to 98% of the energy drawn from the battery.

Figure 3: DUSH960 DC-UPS models convert DC voltage from backup batteries to the voltage required by a DC load. They also manage the recharging of batteries when power is restored. (Image source: TDK-Lambda)

DC-UPS battery options

Because DC-UPSs, like those in the DUSH960 series, are DC-to-DC converters, they work with a variety of energy storage technologies. Lead-acid batteries, lithium-ion (Li⁺) batteries, nickel-metal hydride (NiMH) batteries, and supercapacitors all work with these DC-UPSs for storage capacities up to 20 A and 1,000 Ah.

Each type of battery or supercapacitor is available in a range of supply voltages that depend on power supply size and construction. In addition, battery voltage changes as the battery discharges, as the temperature fluctuates, and with the number of charge-discharge cycles.

The DUSH960’s DC/DC converter function eliminates worry about these fluctuations and ensures that the programmed DC voltage is delivered to the load, whether the backup power supply delivers 10 V, 58 V, or something in between.

Safeguards and safety ratings

Along with the DC/DC converter capabilities of the DUSH960 series, the units carry sensing capabilities and circuitry to protect both the battery and the load. The DC-UPSs defend against reverse battery connection, which could damage the battery and other sensitive electronics. Their sensors also monitor battery health, detecting and preventing deep discharge and overcurrent.

When power is restored and the system is recharging the battery, firmware provides the appropriate charging profile for the type of battery attached. The units also monitor the ambient temperature and optimize the charging voltage accordingly, lowering it for warmer ambient temperatures and raising it when the environment is cold.

DUSH960 units are rated for nominal operation throughout a wide temperature range: -40ׄ°C to +40°C. Between -50°C and +70°C, operation is permitted, but allowable amperage must be derated linearly by 12 W/°C from 15 A as temperature increases.

The units are certified to multiple safety standards, including IEC/EN/UL/CSA 61010-1, IEC/EN/UL/CSA 61010-2-201, and IEC/EN/UL/CSA 62368-1 (Ed. 3). They also carry CE and UKCA marks for the Low Voltage and RoHS directives.

DC-UPS installation and interfaces

In addition to meeting international safety standards, DUSH960 units are designed for standardized installation. Mounting the DUSH960 units is quick and easy since these are DIN-rail modules. At 55 mm wide by 115 mm tall by 110 mm deep (2.13 in. x 4.53 in. x 5.17 in.), the compact DUSH960 units weigh in at 470 g for the DUSH960-1248-1M and 500 g for the DUSH960-1248-0M. Both units are compatible with TS35/7.5 and TS35/15 DIN rails.

To allow users to make the most of onboard sensors and programmability, the DUSH960s have several interface options. On the front panel of the DUSH960-1248-0M model (Figure 4), a 1.5 in. color LCD screen and four control keys permit manual monitoring and programming.

Figure 4: The DUSH960-1248-0M has a color LCD screen with four control keys on the front panel. The panel also contains connection points for Modbus/RTU, battery temperature sensors, and more. (Image source: TDK-Lambda)

The DUSH960-1284-1M’s front panel features indicator LEDs in a cost-optimized model for less-accessible applications. These LEDs signal errors, warnings, and alarms, as well as indicating the DC-UPS’s operating mode: normal mains operation with battery float, normal mains operation with battery charging, or backup mode.

Other communication and sensing connections include an isolated remote on/off signal and two normally open (NO) dry contact relay contacts that provide signals when needed, but do not draw any power when open. Inputs for optional battery-temperature sensors are included on the unit; the sensors are available separately.

The DUSH960-1248-0M variant also features a 5 A auxiliary output from the battery. Although the voltage from this output is unregulated and equal to the battery discharge voltage, the port does provide overcurrent protection.

All models of the DUSH960 also connect to industrial automation systems. A mini-USB-B port facilitates connection via RS485 to Modbus/RTU control software. This connection, along with the provided PowerCMC software, enables users to monitor the unit’s performance, set templates for input and output voltage and other parameters, standardize across multiple units, and maintain alarm logs.

Conclusion

UPSs provide critical backup power to allow for a safe shutdown, save important data, ensure user and public safety, and avoid economic loss. With standardized DIN-rail mounting, safeguarded DC/DC conversion, and multiple interface options, TDK-Lambda’s DUSH960 DC-UPS products provide high-efficiency, customizable backup solutions over a wide range of input and output voltages, and are compatible with the most common electrolytic and electrostatic energy storage technologies.