Many modern, line-powered smart Internet of Things (IoT) devices require backup power to safely power down or maintain communications in the event of an unexpected power outage. For example, electricity meters can provide detailed information on when, where and for how long the outages were made through the radio frequency interface. Narrowband Internet of Things (NB-IoT) has recently become popular for the above-mentioned uses due to the following advantages:
Use existing 2G, 3G and 4G frequency bands.
Supported by one or more operators in the Americas, Europe and Asia.
Compared to General Packet Radio Service (GPRS), the power and peak current are significantly reduced.
A well-designed backup power scheme helps to provide backup power of the right capacity, switch seamlessly between normal and backup power, and support multiple outages without maintenance. In this article, we’ll describe a simple way to implement a backup power scheme that uses TI’s TPS61094 buck-boost converter and a supercapacitor to meet NB-IoT and RF standards. We will also compare the TPS61094-based solution with an existing TI reference design.
NB-IoT Backup Power
Table 1 shows the current consumption over time for different NB-IoT operating modes. The peak value is 310mA in data transfer mode for 1.32s, and the load also varies significantly in different operation modes. The average current consumption of the whole process is 30mA for 80s. When the main grid suddenly loses power, backup power with sufficient capacity and load duration for seamless power switching are required. The TPS61094 60nA quiescent current (IQ) bidirectional buck-boost converter enables a reliable and simple backup power design while being a single-chip solution for supercapacitor charging and discharging without additional circuitry.
Table 1: Example of NB-IoT Load Curve for Saft Batteries
Using a supercapacitor and the TPS61094 to implement an efficient backup power circuit, Figure 1 shows how we configure the TPS61094 evaluation module (EVM) to provide sufficient backup power support for the NB-IoT load curves in Table 1.
Figure 1: TPS61094 EVM Backup Power Configuration
When the system power is turned on, the TPS61094 enters Buck_on mode: the bypass field effect transistor (FET) is turned on, providing a constant current of 500mA to the supercapacitor, and stops charging when the voltage across the supercapacitor is 2.5V. VSYS directly powers VOUT. When a power loss causes VSYS to drop, the TPS61094 automatically enters Boost_on mode: the bypass FET is turned off and VOUT is powered from the charge stored in the supercapacitor.
Figure 2 shows the results of a full cycle of backup power measured with an oscilloscope. VIN represents the system voltage of the grid. VOUT is the output voltage of the TPS61094 and VSUP is the supercapacitor voltage. IOUT is the current drawn by the load. In our example, the load draws 100mA, which is 3.33 times the average current draw of the load curve. We increased the load to determine how the TPS61094 switches the input power during grid outages under more extreme load conditions.
When the system power suddenly drops, the TPS61094 immediately enters Boost_on mode and uses the power of the supercapacitor to regulate VOUT. The buck/boost converter provides the required output current in 254.5s and can process 11.5 NB-IoT transactions. The TPS61094 discharges the supercapacitor until its voltage drops to 0.7V; at this point, the device enters shutdown mode until system VIN is restored. In Buck_on mode, the TPS61094 seamlessly charges the supercapacitor with constant current. As shown in Figure 2, the switching between discharge and charge of the supercapacitor is very smooth.
Figure 2: TPS61094 power-off and power-on measurement results
Other backup power implementations
You can also use other solutions, each with pros and cons. One is the Supercapacitor Backup Power Reference Design for Electric Meters, which uses a discrete circuit to charge the supercapacitor and a TPS61022 boost converter to boost the supercapacitor voltage to a higher system voltage during grid outages. The TPS61022 output current capability is higher than the TPS61094 solution, but requires more external components.
The other is a supercapacitor backup power reference design with current limiting and active cell balancing, which uses the TPS63802 buck/boost converter as a supercapacitor charger and regulator and eliminates the need for additional discrete charging circuit, but still requires additional external components for ORing power supply controller, charge current limit and supercapacitor terminal voltage setting.
Table 2 lists the important characteristics of each backup power method.
* The VIN minimum value for TPS61094 and TPS61022 is 0.7V. The VIN of the TPS63802 is 1.8V.
Table 2: Overview of Backup Power Solutions
Low-power wireless standards are becoming more widely used. With high integration, simple design and excellent light load efficiency, the TPS61094 is suitable for backup power applications using LTE-M, Lora, Bluetooth and other emerging wireless interfaces.
For higher output currents, a meter or current limit reference design is a very effective solution. Although this design requires more discrete components, higher power RF transmissions such as GPRS can be supported.