The traditional solution is the direct battery connection solution, like Figure 2-1. The communication model such as NB-IoT is connected with the LiSOCl₂ and HLC package directly. The voltage of the LiSOCl₂ and HLC package is about 3.6 V at room temperature. When the smart meter does the transmission, the HLC supports the high pulse current for the NB-IoT. During sleep mode, the LiSOCl₂ charges the HLC and supports the whole system consumption.

The disadvantage of the direct battery connection solution is customers must choose HLC1550 instead of HLC1520. Because the LiSOCl₂ and HLC package has the poor performance at cold temperature (-25 degC or -40 degC) , like in Figure 2-2 and Figure 2-3. Figure 2-2 is the performance of ER26500 and HLC1520 at high pulse current (250 mA / 250 ms). In the waveform, the voltage of the battery package is down to 3.4 V at high pulse current. It is too little margin to power the whole system. Figure 2-3 is the performance of ER26500 and HLC1550, because HLC1550 has a bigger size and higher current capability, the voltage is down to 3.6 V, and it can support the whole system and do the transmission. But HLC1550 has bigger size and higher cost.

Another disadvantage of this solution is the discharge current of LiSOCl₂ is uncontrolled. In the Figure 2-2 and Figure 2-3, the discharge current of LiSOCl₂ is up to 15 mA and 5 mA, respectively. The LiSOCl₂ cannot achieve the maximum capacity, referring Figure 1-2.

One of the cost competitive solutions is the pure boost (TPS61094 or TPS610995) solution (reference 4), similar to Figure 2-4. In this solution, customers could use HLC1520(vender: Tadiran), SPC1520(vendor: EVE) or UPC1520(vender: HCB) and they need to add a pure boost (TPS61094 or TPS610995) to regulate output voltage to about 3.6 V over the whole temperature range. This solution is not sensitive to HLC vendor and size, so the total cost is more competitive than the connecting battery directly.

The weakness of the pure boost solution is that the discharge current of LiSOCl₂ battery is uncontrolled. We cannot get the maximum LiSOCl₂ battery capacity, and the end-off voltage of the battery package is not controllable. If the terminal voltage is too low(< 2 V), it has unrecoverable effects on the battery lifetime.

The TPS61094 could offer a solution exchanging the HLC to a supercap, which can reduce the total solution cost while still controlling the LiSOCl₂ battery discharge current to achieve the maximum capacity and working lifetime, as shown in the Figure 3-1.

The TPS61094 is a synchronous bi-directional buck/boost converter with a bypass switch between input and output (reference 5). When the TPS61094 works in buck mode to charge the supercap, the charging current and the charging termination voltage are programmable with two external resistors (R₃ and R₂). When the TPS61094 works in boost mode, it can boost the supercap and regulate output voltage to the programmed voltage, set by R₁.

The TPS61094 has four operation modes: the auto buck or boost mode; the force buck mode; the force bypass mode and the true shutdown mode, set by the EN and MODE pins. Customers can choose the suitable mode based on their application.

The TPS61094 has 60-nA quiescent current in buck mode or boost mode and 4-nA quiescent current in force bypass mode, which could help the system achieve long lifetime.

In the TPS61094 with supercap solution, the MCU doesn’t need to control the TPS61094. TPS61094 can switch between buck charging mode and boost mode automatically. It can boost the supercap to power the high pulse load at data transmission and then charge the supercap during standby mode.

By setting EN = High and MODE = High, the TPS61094 is enabled to work in the auto buck or boost mode. TI suggests to set output target voltage (setting by R₁) is 3.3 V (> 3.6 V – 150 mV), which could help TPS61094 enter the buck charging mode automatically; set charging current to 5 mA, which could help get the maximum LiSOCl₂ capacity, according to Figure 1-2; set charging terminal voltage to about 2 V, which could help supercap get lower leakage current and longer working life time.

TI suggests to add a series resistor (R_in) of about 40 Ω between LiSOCl₂ battery and TPS61094 VIN pin. It could help limit the LiSOCl₂ battery discharge current during data transmission. The LiSOCl₂ battery discharge current is as

The TPS61094 operation is as shown in the Table 3-1. During stand-by operation in the smart meter, because input voltage is higher than output voltage + 100mV, the TPS61094 enters auto buck mode. The bypass MOS turns on and NB-IoT is powered by LiSOCl₂. TPS61094 charges the supercap until it is fully charged. When the NB-IoT does the Rx / Tx transmission, there is a high pulse current at the output of TPS61094, because LiSOCl₂ can’t support high pulse current, the input voltage will drop. When TPS61094 detects the input voltage is lower than output voltage + 100mV, the boost mode actives automatically. So the supercap mainly supports the high load current.

There is a summary of these three power solutions in Table 4-1. TPS61094 could provide a cost competitive and long lifetime solution in the smart meter. This solution can help smart meter customers exchange hybrid layer capacitor (HLC) to supercap, which could have lower total solution cost.

The TPS61094 with the supercap solution can support 18.4 years and increase operation time by 20% than the pure boost solution. And because the TPS61094 shares the same inductor and input/output capacitors in the supercap charging and discharging, the TPS61094 reduces component count by 50%.

Note:

The smart meter lifetime estimation is based on the following conditions:

• Calculation is based on Tadiran(LiSOCl2 TL-5920) and the capability de-rated according to the Figure 1-2.
• Assumption that ten months is 25 ℃ and two months' temperature is lower than 0 ℃.
• NB-IoT power consumption is about 134 mAh each year at supply voltage is 3.6 V
• LiSCL2 Battery self-discharge: 25 ℃: 1 % / year, 40 ℃: 2 % / year
• Hybrid layer capacitor self-discharge: 25 ℃: 3 μA, 40 ℃: 6 μA
• Super capacitor leakage current: for 3 F cap, working at 2.0 V can reduce the leakage current to 20 %, the leakage current: 25 ℃: 1 μA (5 μA * 20 %), 40 ℃: 2 μA
• Battery activation is about 30.4 mAh each year
• Valve power consumption is about 35.8 mAh each year
• Standby power consumption (including MCU, counting hall sensor, anti dismantling hall sensor, power supply, NB-IoT standby power consumption) is about 87.6 mAh.

The supercap lifetime is related to the operating temperature and operating voltage. The classical aging model for supercapacitors is Eyring's law that estimates the aging rate, as the Equation 2. This law stipulates that a 200-mV voltage surplus increases the aging by a factor of 2 and have the same effect as a temperature increase of 10 °C. (reference 6, 7, 8)

where

t_ref Reference lifetime (hours)

V_ref Reference applied bias voltage (V)

T_ref Reference aging temperature (K)

The smart meter customer can lower the operation voltage according to their life time and operation temperature requirement. There is the estimation lifetime from VINA Tech 3.0V series supercap, as shown in Table 5-1

Note: 30% capacitance degradation is considered as the end of life (reference 9).

The supercap leakage current is an important part of performance in the smart meter, which is related to the operating life time. The leakage current depends on temperature, working voltage, capacitance and other parameters (similar to charge duration and short-term history) (reference 10).

For the leakage current, when the supercap working voltage decreases, the leakage current could also decrease, similar to Figure 5-1. For example, the supercap works at 1.8 V, the leakage is about 8 uA (18 % of data sheet spec) at 25 ℃.

The supercap leakage current is related to working temperature, too. The supercap leakage current at 65 ℃ is about 3~4 times as the 25 ℃, in the Figure 5-1.

Note: the test data is based on WEC3R0156QG (3 V 15F) (reference 9).

Because the supercap lifetime and leakage current are strongly related to working voltage, TI suggests to set supercap charging terminal voltage to 2 V, which could achieve 20 years life time at 65 ℃ and the leakage current is about 18% of the datasheet spec.

The capacity of the supercap depends on Rx/Tx transmission loss. Let's take NB-IoT as the example. Assume the transmission internal is 24 h that is once data exchange every day, 3.3 V supply voltage, and a payload of 200 Bytes. The power consumption of one transmission is about 4 J. To leave 20 % margin, the target storage energy is set as 4.8 J (reference 11 and 12). The TPS61094 could support the supercap operation until supercap voltage is down to 0.7 V. So the supercap will discharge from 2 V to 0.7 V, the total discharge power is

The supercap discharge power should be higher than the total loss of NB-IoT transmission, 4.8 J, so smart meter could choose 3 F supercap.

The TPS61094 with supercap solution test waveform overview is as shown in the Figure 6-1. There are three phases in this solution. Phase 1 is the NB-IoT data transmission. The load current is about 250 mA for 250 ms. The TPS61094 could regulate output voltage to 3.3 V and control the battery current within 5 mA. In the phase 2, the NB-IoT stops doing the data transmission, so TPS61094 charges the supercap in setting current about 2.5 mA. As shown in the Figure 6-1, the supercap voltage increases and triggers charging terminal voltage (2 V). The TPS61094 stops charging and this is phases 3. The whole system enters the standby mode and waits for the next NB-IoT transmission.

Note:

The dark blue signal (Channel 1) is TPS61094 output voltage, The purple signal(Channel 1) is LiSOCl2 battery output current, The green signal(Channel 4) is the supercap voltage, The light blue signal(Channel 2) is load current.

The NB-IoT data transmission (phase 1) zoom in waveform is as shown in the Figure 6-2 and Figure 6-3. It can be seen that during high pulse current load emulating NB-IoT module, the output voltage of TPS61094 is regulated at 3.3 V to maintain normal work of system. At the same time, the output current of LiSOCl₂ battery is about 5 mA or 6 mA both at -25 degC and 25 degC, so that the LiSOCl₂ battery lifetime can be maximized referring Figure 1-2.