Electronic water and gas meters represent some of the most vexing low-power design challenges for embedded control systems requiring RF connectivity. The nature of these applications requires them to be battery powered as electricity is rarely provided at the point of service for gas or water utilities.
The expected battery life for these systems is often greater than 20 years. This requirement is dictated by the utility provider since a single service call from a technician can often exceed the entire cost of the smart meter. Because of this long-life design requirement, nearly all water and gas meters use a battery chemistry of lithium thionyl chloride (LiSOCl2). This battery chemistry is chosen because of its very low self-discharge behavior and resulting ability to last for up to 20 years in these applications. However, these batteries are very expensive (as much as $1.5/A-hr) resulting in battery bill of material (BOM) costs of up to $10 to $15 per water or gas meter.
Many smart meter providers have determined that they can further differentiate their products by extending their communication range. In their system network topology, a fixed number of meters would communicate usage and billing information to a single repeater mounted on a utility pole through a sub-GHz proprietary network. The repeater would aggregate and transmit the collected information back to the utility provider over a cellular network modem or other backhaul channel.
A single repeater could support approximately 1000 meter nodes. However, the cost of the repeater can be anywhere from 10 to 100 times greater than a single meter node. Metering suppliers often face pressure from their customers to reduce the number of repeaters in a given network. This can be most readily achieved by improving the robustness of the transmitter (TX) link.
There are a number of ways to improve the TX link budget. The most obvious solution is to increase the output power of the transmitter using a power amplifier (PA). This is also the most costly approach in terms of battery life. Another strategy is to enhance the protocol to minimize the number of dropped messages and subsequent retransmissions. Although a much lower power approach than simply adding a larger PA, this technique can still increase the new TX power budget by as much as 40 percent over the current power budgets.
Let’s consider three design requirements for one particular smart meter redesign:
- Allocate 40 percent more power budget to TX functions to increase range
- Maintain existing LiSOCl2 battery size (A) and capacity (3650 mA-hr)
- Maintain existing battery service life of 20 years
The strategy is clear: Increase the power within the TX budget while not increasing its total power budget. The reductions would have to be found in other functional areas, namely the RX, active and sleep mode budgets. Figure 1 shows the original power budget and the target budget after redesign.
Fig 1: Comparison of power budgets for smart meter applications.Higher efficiency voltage conversion
To increase performance and reduce the power requirements of CMOS circuits, chip designers use the smallest practical device geometry to build their integrated circuits. It is common to find embedded processors and RF transceivers designed in 0.18 µm, 0.13 µm and even 90 nm geometries. One of the keys to reducing the power consumed by the device is reducing the internal operating voltage, thus reducing the CVf switching losses.
iswitching loss = Cgate × Vgate × frequency
Even though the battery supplying the device may have a terminal voltage of 3.6 volts, the device will operate at a much lower voltage internally.
Nearly every device on the market integrates internal low drop-out regulators (LDO) on-chip. This is the structure that takes a 3.6 V input and regulates the internal voltage of the chip to a lower value, typically 1.8 V or less. In other words, taking a 3.6 V input using a linear regulator with a 1.8 V output has a 50 percent conversion efficiency. Obviously, this gets worse as the output voltage decreases.
More advanced embedded controllers, such as the C8051F960 MCU shown in figure 2, have integrated switching regulators with much higher efficiency than their LDO counterparts. In many cases, these devices can have switching efficiencies as high as 85 percent. This high efficiency has the effect of reducing the total current sourced from the battery and extends battery life.
Click on image to enlarge.
Fig 2: Comparison of switching efficiencies between traditional and advanced MCUs.
Using this approach, we can greatly reduce the existing RX power budget.
In other words, the current that is sourced by the battery for use by the radio receiver is approximately 62.5 percent of what it would be using the DC-DC buck converter as opposed to just an LDO. This approach has the net effect of reducing the RX current power budget by that amount.
Fig 3a: Enhanced RX power budget enabled by DC-DC switching improvements.
With this change, we have nearly met the new RX power budget (i.e., from 30 percent to 19 percent as shown in figure 3a, although the target is 18 percent). We must continue to optimize the system in other operating modes.Lower sleep mode power
Battery-powered meters often reside in a low-power sleep mode 99.9 percent of the time. For this reason, it is critical to keep the power consumption of sleep mode circuits as low as possible. Best-in-class devices of just a few years ago could achieve currents on the order of 1 µA using a 32.768 kHz crystal driving a low-power wake-up timer at 3.6 V. Today, further optimizations have yielded devices that can perform the same function at approximately 700 nA at the same voltage. Although the net savings is only 300 nA, this is essentially at 100 percent duty cycle, so this value can be subtracted directly from the power budget.
Fig 3b: Impact of sleep mode enhancements on sleep mode budget
Using devices with lower power sleep modes, we are able to reduce the sleep mode budget from 8 percent to 5 percent as shown in figure 3b, thus meeting the design target. However, since we only met and did not exceed our goal, additional improvement is still needed to achieve our overall design goal. The last area of focus is reducing active mode power.