Use Supercapacitors to Make IoT Nodes Immune to Brownouts

When Internet of Things (IoT) or Industrial IoT (IIoT) networks are placed on the same main power grid used by residential electricity customers they are subject to power fluctuations, brownouts, or even a full loss of electric power for tens of seconds. Stateless nodes can resume operation on power-up; however, nodes that must maintain state over time will be reset on power-up, which can result in malfunctions, delays, or lost performance of the network.

Backup batteries are one way to avoid resets, but they have a limited lifespan and are potentially more expensive over the lifespan of the device. Instead, supercapacitors—polarized electrolytic capacitors with ratings of 1 farad (F) or higher—can be used.

This article discusses the importance of ensuring IoT and IIoT devices that are running critical processes remain powered. It then shows how to apply supercapacitors to provide protection against brownouts and power drops using two example devices, one each from AVX Corporation and Illinois Capacitor.

The problem with putting IoT nodes on residential power mains

Industrial facilities that run critical processes often have backup power generators in case of a temporary power failure from the electric company. Redundancies and multiple generators ensure that power is constantly maintained except in extreme cases of extended power outages. The factory may even have its own dedicated power line from the main power grid to ensure a continuous supply of electric power, which may also provide some immunity from power outages on the rest of the grid.

Many small or non-critical IoT nodes use power off of the same residential power grid used by homes in the area without any expensive battery backup systems. Depending on the design of the network, a brownout or momentary loss of power can reset systems, shut down machines, and result in loss of data and performance.

There are several options to prevent this loss of data. Backup generators that run on gasoline or natural gas can supply power for an extended period of time but are expensive in terms of materials and installation. Backup generators also require periodic maintenance and testing. This added cost and labor can quickly reach a point where the IoT network is no longer cost efficient to the point of being impractical for its initial purpose.

Another option is a battery backup unit. These use lead-acid batteries and are a reliable way of providing backup power for short periods of time. However, these require regular inspection and testing. Moreover, lead-acid batteries have a limited life cycle and so require regular replacement, adding to cost and labor. To further complicate maintenance, lead-acid batteries in battery backup units sometimes do not fail predictably and can fail shortly after being activated during a power outage.

Both backup generators and battery backup units are bulky and require additional space. For compact IoT networks it can be impractical or impossible to implement these solutions.

A common option is to put a small backup battery on the IoT node. This is an inexpensive option compared to generators and battery backup units. A lithium-Ion (Li-Ion) battery takes up little space and requires limited maintenance. However, Li-Ion batteries do have a limited cycle time—often as low as 500 charge/discharge cycles—resulting in periodic replacement. Li-Ion batteries also have a very limited operating temperature. Freezing temperatures reduce the capacity of a Li-Ion battery leading to permanent damage, while very hot temperatures can progressively damage the battery and lead to thermal runaway.

Instead, an easy and cost-effective way to provide instantaneous temporary power during brownouts or short-term power loss is to put a supercapacitor in the IoT node.

Supercapacitor characteristics and capabilities

Supercapacitors are polarized electrolytic capacitors rated at 1 F or above. As capacitors, they can charge and discharge in a matter of seconds, so they can act like a short-term rechargeable battery for IoT nodes. With a constant-current discharge; the voltage across the supercapacitor’s terminals will drop off linearly over time.

Supercapacitors have virtually unlimited cycle times of over one million cycles, allowing them to be constantly charged and discharged with no effect on capacity or lifespan. Unlike chemical batteries, cycling the charge on a supercapacitor has a minimal effect on the capacitor dielectric or electrodes. Supercapacitors are relatively unaffected by heat and cold and can safely operate at temperature extremes that would damage a Li-Ion battery.

Charging a supercapacitor is simple and does not require a sophisticated circuit to maintain the state of charge as capacitors cannot be over-charged. However, applying a reverse voltage to the polarized terminals or applying a voltage to a supercapacitor that is higher than the rated maximum can shorten its life.

Selecting a supercapacitor is a series of trade-offs. Of course, the larger its capacity, the longer it will be able to supply power, all things being equal. However, this increase in capacity not only comes with an increased cost, but also with a significant increase in size: supercapacitors are bulky components and the length and diameter are important to consider in the pc board layout, especially if room must be made for a larger supercapacitor later on.

The increase in the size of the pc board may be unacceptable for some applications, limiting the capacity of the supercapacitor. A larger supercapacitor may also interfere with airflow around the IoT node, which can hinder heat dissipation. These are all important considerations when designing in a supercapacitor for brownout or power dropout protection.

Supercapacitor discharge time

Equation 1 can be used to calculate the estimated discharge time of a supercapacitor, giving a good estimate of the length of time it can drive a circuit in the event of a power loss.

 Equation 1

Where:

t_seconds = Time in seconds the supercapacitor can supply sufficient power to the circuit

C_Farads = Capacitance in farads

V_max = The voltage across the capacitor at the initial time of discharge

V_min = The minimum voltage to which the capacitor can discharge before it is insufficient to power the circuit

I_max = The maximum (worst case) current draw of the circuit in amperes

Like all capacitors, supercapacitors have an equivalent series resistance (ESR). However, the ESR varies based on temperature, capacitor voltage, and current draw. For capacitor values over one farad, the ESR is less than 10 milliohms (mΩ), which makes the effect of the ESR on discharge times minimal.

For effective use in brownout protection, the engineer needs to select a supercapacitor that can satisfy Equation 1 for the given application. Developers should also test their systems under simulated brownout and power loss conditions to observe actual operation using selected board components. Since the capacitor may be initially charged to a voltage higher than the required operational voltage of the circuit, a low dropout regulator (LDO) is recommended to manage the capacitor voltage output.

Simple brownout and power drop protection

For simple brownout protection in case of a drop in electric line power that lasts only a few seconds, or for power drop protection that lasts less than a minute, smaller supercapacitors can keep small IoT nodes working. For example, the AVX Corporation SCMR22L105SRBB0 1.0 F supercapacitor is 8 millimeters (mm) thick and 22 mm wide (Figure 1). It is appropriate for harsh environments and has an operating temperature of -40°C to +65°C, temperatures unsuitable for any Li-Ion battery. Radial leads for vertical mounting help save space on the pc board.

Figure 1: The SCMR22L105SRBB0 is a radial-leaded supercapacitor measuring 8 mm x 22 mm. (Image source: AVX Corporation)

The ESR on the SCMR22L105SRBB0 is only 840 mΩ, ensuring very low power loss during discharge. Its maximum charge voltage is 9 volts.

Using Equation 1, the discharge time can be calculated for a simple IoT node drawing 80 milliamperes (mA). For a 3.3 volt system with a common AC adapter supplying 9 volts to charge the capacitor to maximum voltage, with an ideal low-dropout (LDO) regulator, this 1.0 F capacitor could supply power for 71 seconds under optimal conditions. The SCMR22L105SRBB0 has a capacitance tolerance of ±30% over rated temperature and voltage, so with a worst-case capacitance of 0.70 F, it can supply 80 mA for an estimated 50 seconds. This will vary based on the manufacturing tolerances of individual capacitors, so it is best to design for the worst case.

With a worst-case 50 seconds of current supply for this example, the SCMR22L105SRBB0 is more than adequate to handle power drops in brownout situations.

When laying out a supercapacitor, the leads should be routed as if they are power traces to minimize electromagnetic interference (EMI). In addition, the insulating sleeve of the supercapacitor should not come into contact with the pc board or any other component. If the sleeve is damaged by extreme solder temperatures or external forces, the metal can of the supercapacitor can short, resulting in circuit malfunction.

For higher capacity applications, Illinois Capacitor makes the DSF407Q3R0 400 F supercapacitor rated at 3.0 volts (Figure 2). With a diameter of 35 mm and a length of 60 mm, it is significantly larger than the previously mentioned device. Because applying reverse polarity on a 400 F supercapacitor can destroy the component, the DSF407Q3R0 has two no-connect keyed pins to prevent errors in assembly.

Figure 2: The Illinois Capacitor 400 F supercapacitor requires extra board space with a diameter of 35 mm and a length of 60 mm. It has two keyed pins to prevent reverse-polarity assembly. (Image source: Illinois Capacitor)

While the 3 volt rating may not seem impressive in the context of Equation 1, the 400 F rating provides for plenty of capacity. Capacitance tolerance is ±30%, resulting in a worst-case rating of 280 F. For a 2.7 volt system drawing 350 mA, as per Equation 1, charging the capacitor to its 3.0 volt rating results in 343 seconds of typical standby power at 400 F, and 240 seconds worst case at 280 F. This assumes an ideal LDO, so it’s important to perform in-circuit testing to see how the supercapacitor performs under simulated brownout and power loss conditions.

A 400 F capacitor can get hot, so it’s important to provide proper distancing from other components. This capacitor has a vent at the top, so there must be adequate spacing above it to allow heat to dissipate.

Conclusion

Supercapacitors can be used to provide backup power during brownout and short-term power loss situations in IoT and IIoT nodes. They have significant advantages over Li-Ion batteries, including virtually unlimited charge and discharge cycles, excellent high voltage operation, and high efficiency and reliability. Proper usage of supercapacitors in IoT and IIoT nodes powered by residential AC mains can reduce maintenance and system costs while improving the performance of the entire network.