An efficient way to achieve backup power using supercapacitors

“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.

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Many modern line-powered smart Internet of Things (IoT) devices require backup power to safely power down or maintain communication 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 gained popularity 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.
• Significantly lower power and peak current compared to General Packet Radio Service (GPRS).

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 entire process is 30mA for a duration of 80s C loads during this period require backup power with sufficient capacity and seamless power switching in the event of a sudden mains power outage. TPS61094 60nA Quiescent Current (I_Q) bidirectional buck/boost converter enables 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

model	sleep mode	transfer mode							transfer mode
		wake up and scan	data transmission		RRC activity	RRC version		PSM
Current (mA)	0.003	28	310	40	20	310	20	8	30 (mean)
time (s)	*	2	1.32	12.68	30	1.25	1	30	80 (total time)

* For clients and end devices. May be minutes to days.

To implement an efficient backup power circuit using a supercapacitor and the TPS61094, 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. V_SYS directly to V_OUT powered by. When a power outage causes V_SYS When falling, the TPS61094 automatically enters Boost_on mode: the bypass FET is turned off and the charge stored in the supercapacitor is V_OUT powered by.

Figure 2 shows the results of a full cycle of backup power measured with an oscilloscope. V_IN Indicates the system voltage of the grid. V_OUT is the output voltage of the TPS61094, V_SUP is the supercapacitor voltage. I_OUT 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 V_OUT. 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 the system V_IN recover. 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 power supply ORing, charge current limit, and supercapacitor terminal voltage setting.

Table 2 lists the important characteristics of each backup power method.

Table 2: Overview of Backup Power Solutions

solution	TPS61094	1S boost design	2S Buck/Boost Design
core device	TPS61094	TPS61022	TPS63802
Device IQ (uA)	0.06	27	11
integrity	high	Low	middle
charging circuit	integrated	discrete	Partially integrated
Supercapacitor Configuration	1S	1S	2S
3.3VVOUT Average Maximum Output Current (mA)*	300	650	1,300
ORing circuit	integrated	discrete	discrete
Programmable supercapacitor terminal voltage	integrated	discrete	discrete
Programmable Charge Current	integrated	discrete	discrete
Programmable output voltage	integrated	integrated	integrated
VIN Range (V)	0.7-5.5	10-12	3.3-5
VOUT Range (V)	2.7-5.4	2.2-5.5	1.8 to 4.9 V
Key Elements of Backup Power	TPS61094	Discrete charger, TPS61022	TPS63802, LM6100, INA181A, operational amplifier

* V for TPS61094 and TPS61022_IN The minimum value is 0.7V. V of the TPS63802_IN is 1.3V.

Epilogue

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 are supported.

The Links:   FLC48SXC8V-11A   LM151X2T