Powering the Internet of Things

In the Internet of Things (IoT), everything is linked. Cisco expects that around 50 billion devices will be networked by 2020. These devices are equipped with many different sensors exchanging data in real time, enabling efficient communication in a smart infrastructure. To guarantee a maintenance-free operation, these sensors need to be powered reliably. This can be done from a mains connection, through energy harvesting or from a battery.

The Internet of Things connects many different devices, which have to date been working independently of each other, into one big network. These devices act as intelligent systems, exchanging and recombining data and thus forming interconnected communication networks that are able to make smart decisions and respond in real time to signals from other systems as well as to human intervention. All these functions are of course driven by data, which is provided by sensors that act as the sensory organs of the Internet of Things. To do this, sensors need to be powered from reliable sources. Depending on the actual application, there are various options available, including batteries, energy harvesting devices or direct connection to mains power.

Sensors powered by coin cells

Many sensors are equipped with 3 V coin cells, as they are not only cheap, but also provide power locally so there is no need for running a wire to an existing mains power source. This approach has however a number of disadvantages. A fully charged CR2032 coin battery supplies approximately 3.2 V. After only a few operating hours, the voltage drops to below 3 V, which might be insufficient for certain wireless modules (WLAN, Bluetooth, LoRaWAN, etc). As a result, transmission can become unreliable, or the signalling range might suddenly be limited.

Also, batteries have a limited service life time and need to be replaced from time to time. Where devices are installed in locations that are difficult to access, a battery change can lead to significant costs. It is therefore advisable to design such systems in a way that makes best use of the available battery power.

That is why RECOM has developed the R-78S switching regulator series designed specifically for battery-powered applications. These modules are able to generate a stable 3.3 V supply output with a regulation accuracy of 1% from input voltages as low as 0.65 to 3.15 VDC. This means that the IoT device is reliably powered with a stable voltage until the battery is fully exhausted. The R-78S thus enables users to run microprocessors, WLAN/Bluetooth modules and IoT systems with a single 1.5 V battery or coin cell, which has a much longer service life time than a sole coin cell.

Figure 1: The R-78S boost switching regulator prolongs not only the service life of batteries in IoT applications, it also squeezes the last bit of power from the cells.

Energy-saving sleep mode

In order to use the available energy in the most efficient manner, modern IoT applications resort to a well-known trick. While they appear to be in switch-on mode, they are actually in sleep mode most of the time from which they are only woken the moment the application requires it. This reduces energy consumption to a minimum.

Figure 2 shows a typical application of the R-78S in a wireless module powered by a low-cost 1.5 V battery. The switching regulator boosts the voltage to the 3.3 V required by the microcontroller and the wireless module. By integrating a buffer capacitor that keeps supplying the circuit when it is off, the R-78S can be put into energy-saving sleep mode in which it remains for most of the time, until it is briefly woken up to so that the connected module can transmit data. For the short time the circuit is closed, it consumes around 600 µW. Subsequently, it returns to sleep mode where power consumption is only around 20 µW. In this mode the application requires as little as 7 µA. At some stage, the capacitor charge drops however below a certain threshold. At this point, the microprocessor activates the R-78S so that the full battery charge becomes instantly available. This prevents the circuit from remaining in sleep mode for too long, which would discharge the capacitor to a level where waking up would be impossible. In certain applications, the microcontroller is woken up by an external alarm signal. The smart solution available with the R-78S ensures that the power available from a coin battery is utilised down to its last gasp.

Figure 2: By integrating a buffer capacitor into the circuit, the R-78S can be set to sleep mode, saving valuable battery power.

Battery-powered IoT applications need to be optimised for efficiency. One way of achieving this is operating the applications in the lowest possible load range. The R-78S switching regulator module has been optimised for an impressive efficiency of 93% under full load. To meet the specific requirements of IoT applications, the efficiency is still above 80% in the crucial light load range of around 10%.

Power supplies for IoT applications

A viable alternative to batteries is the supply of the sensors directly from the power mains. There are already various highly affordable, compact and reliable power supplies on the market that make time-consuming battery changes unnecessary.

Power supplies for IoT applications must not only be extremely compact, but also be able to withstand the special operating conditions of sensor systems. The power supply for instance must be able to handle sudden load changes without causing extreme voltage peaks at the output. Another important aspect is transient behaviour. Most AC/DC data sheets only specify the control mode at load changes from 50 to 75%, which is obviously of little relevance to IoT applications where changes in the 0 to 25% range are the ones that count. Ideally, the settling time should be 500 µs or less.

Mains-powered IoT applications require compact power supplies that take up as little space as possible. Figure 3 shows the difference in size between a power supply launched a few years ago and the latest generation of supplies available from RECOM. Both modules provide 5 W.

Typical sensor applications are run continuously and are in energy saving or sleep mode most of the time. For PCB-mounted and integrated power supplies, the European ErP (Energy Related Products) Directives prescribes a standby consumption of less than 500 mW. Considering the ever-increasing popularity of IoT applications and the huge number of power supplies that will soon be required to run these, this standby consumption is still far too high. Preferably, power supplies should come with a standby consumption of less than half the 500 mW limit. Low standby power consumption however is not the only issue. Many IoT applications are run within an extremely low load range, or frequently switch from zero to a low load. Even if the data sheet specifies an excellent zero load consumption, the picture might be very different when there is a low load. This is due to the fact that the controller automatically switches to pulse mode the moment there is a zero load in order to save energy. When even the tiniest load is applied, it switches back to normal operating mode.

Innovative solutions for IoT applications

As the Internet of Things is really an internet of sensors, its future depends on innovative power supply solutions. RECOM offers intelligent solutions such as the R78-S boost switching regulator that help to save battery power. These regulators squeeze the last bit of energy from the battery so that applications can be run for very long periods without problems and no need for maintenance.

For applications that are to be powered from a mains socket, RECOM offers a comprehensive portfolio of 1–10 W AC/DC power supplies specially designed to meet the needs of advanced IoT applications.