Safety and reliability design of smart power socket in smart home

This article provides designers with recommendations for protection and low-power control components for power outlet designs that prevent overloads from damaging sensitive circuits while maximizing device efficiency.

Advances in wireless communications, the Internet, and electronic circuits have enabled the development of smart devices. Non-smart devices are evolving into smart devices through the use of Internet of Things (IoT) technology. Smart devices that provide power control, security, environmental control, and entertainment functions are becoming more common in homes. Examples of non-smart power control devices that now have smart versions include dimmers, power outlets, and ground fault circuit interrupters (GFCI)/arc fault circuit interrupters (AFCI).

While non-smart devices can only be controlled manually or are always running, smart devices have electronics and firmware that allow for automatic control and status feedback. Smart devices are a component of the IoT space that respond to controls from a personal computer, tablet, smartphone, or virtual assistant. These products use wireless communication protocols such as cellular, Wi-Fi, or Bluetooth to access smart devices.

Safety and reliability design

The challenge for designers is to ensure that these new smart devices are safe and reliable so that consumers can have high reliability without the risk of service interruption. Therefore, these devices require overvoltage protection and overcurrent protection to maintain operation, even when subjected to various environmental hazards such as lightning surges, inductive surges, electrostatic discharge (ESD), and electrical fast transients. This article provides designers with recommendations for protection and low-power control components that can prevent overload from damaging sensitive circuits while maximizing device efficiency.

Protect smart dimmers and power outlets

Dimmers and power outlets are connected to the AC power line and are subject to overcurrent and transient overvoltage conditions that may occur on the AC power line. Transients such as lightning strikes, power line voltage changes caused by load surges, inductive transients caused by motors turning on or off, and electrostatic discharge (ESD) can damage the electronic circuits that control smart dimmers and smart power outlets.

Figure 1 shows the protection and control components recommended for protecting electronic circuits and effectively controlling dimmers and smart power outlets.

Figure 1. Recommended protection and control components for smart dimmers and smart power outlets.

Protection and control components for smart dimmers

Dimmers are common devices used to control lighting in home lamps. Smart dimmers can provide precise remote control or timed control of a lamp or a group of lamps. Figure 2 shows the block diagram of an electronic dimmer switch and shows the specific circuit blocks using recommended protection and control components.

Figure 2. Block diagram of a smart dimmer. Recommended safety and control component options for the circuit module are shown in a list next to the block diagram.

AC input protection circuit

The AC input protection circuit module is directly connected to the AC power line and requires overcurrent and transient voltage protection. Designers should fuse the module to prevent overcurrent from causing damage to downstream circuit modules. We recommend using slow-blow fuses to avoid unnecessary shutdowns due to inrush currents (e.g. from switching power supplies). The fuse rating should exceed the rated AC line voltage.

A critical parameter of a fuse is the interrupting rating. Make sure the selected fuse will not melt or vaporize in the event of a large overload. Estimate the maximum current output of the power line and select a fuse with an interrupting rating that exceeds your estimated potential available current. Fuses can have interrupting ratings from 10 to 100 kiloamperes (kA).

To protect the AC input protection circuit from transients on the AC line, we recommend using a Metal Oxide Varistor (MOV). The MOV can withstand the maximum voltage generated by the transient and can absorb the current surge caused by the transient voltage. We recommend that you consider an MOV that can absorb a current pulse of 10,000 amps and 400 joules of transient energy. Good design practice is to place the MOV as close to the PCB input as possible to prevent transients from propagating into the circuit.

On the secondary side of the AC input protection circuit, transient voltage suppression diodes (TVS) are used to protect the downstream secondary circuits. You can choose unidirectional or bidirectional diodes, depending on the likelihood that the circuit will be subject to positive and negative transients. TVS diodes respond very quickly to transients, less than 1 ps. They can absorb 1500 W of peak pulse power and have a low clamping voltage to protect low-voltage electronic circuits.

Switching Circuit

The switching circuit controls the output to the lamp. Minimizing power dissipation maximizes circuit efficiency and minimizes heat buildup in the dimmer. We recommend using triacs (thyristors) with low holding current.

The holding current of the bidirectional thyristor is less than 10 mA. They can also operate safely at junction temperatures exceeding 100°C. To further improve efficiency, consider using a MOSFET to control the power of the bidirectional thyristor. Select a power MOSFET with a low RDS(on) resistance of less than 0.5Ω and a fast switching time to reduce power losses during device transitions and power dissipation when the MOSFET is in the on state.

You can simplify the management of driving the MOSFETs using a single-chip gate driver. The gate driver chip can contain two driver amplifiers to control the high-side and low-side power MOSFETs and maximize their switching speed. Select a gate driver with sufficient current carrying capacity to drive the MOSFETs. Finally, protect this circuit from line voltage surges in the switching circuit using an MOV that can withstand a similar voltage rating as the MOV recommended for the AC input protection circuit.

Wireless communication circuit

The wireless communication circuit uses the wireless LAN (Wi-Fi) protocol to communicate with a personal computer, tablet or smartphone for remote control of the dimmer. This circuit is connected to the external environment and is subject to ESD, which is mainly caused by the user of the smart dimmer.

We recommend using bidirectional TVS diode arrays (as shown in Figure 3) or polymer ESD protection devices to protect wireless communication circuits.

Figure 3. Bidirectional TVS diode array with two back-to-back diodes

With capacitance of less than 1 pF, both devices can protect I/O ports with minimal impact on circuit performance. Also, both components are available in surface-mount packages to save limited printed circuit board space. In addition, they draw less than 1 µA of leakage current, reducing the power load on the circuit. Best of all, either device will withstand ±12 kV ESD strikes per the IEC 61000-4-2 ESD standard.

Local switch

The local switch allows the user to manually control the output power of the dimmer. Like the wireless communication circuit, this circuit is connected to the external environment and is easily subject to ESD strikes. This circuit requires the same protection components as the wireless communication circuit. Again, select a diode array or polymer ESD protection device.

Protection and control components for smart sockets

Figure 4 illustrates the circuit modules in the smart socket and the recommended components for providing protection and effective control. Like the smart dimmer switch, the smart socket also has an AC input module, an AC-DC conversion power module, a wireless communication circuit, and a manual switch control circuit.

Figure 4. Smart outlet block diagram shows the location of required protection and control components. Recommended component selections are listed in the table.

AC input protection and rectification

The AC input and protection circuits connected to the AC power line, like the AC input protection module of the dimmer switch, are subject to large overcurrent surges and high overvoltage transients that are induced and propagated on the power line. Therefore, the AC input of the smart outlet circuit requires a fuse, an MOV, and a TVS diode with the same characteristics as those recommended for the dimmer input circuit.

power supply

Considering the space and efficiency of the smart socket, it is recommended to use a switching power supply to generate the DC voltage required for the control circuit. We recommend maximizing efficiency through high-frequency design. Consider using Schottky rectifier diodes in the circuit. These devices have a low forward voltage drop of typically less than 0.5V and can operate at a higher switching frequency, allowing small, space-saving designs to operate at high efficiency.

Wireless communication and local On/Off switch

Like the smart dimmer switch, the wireless communication and local on/off switch circuits are also exposed to the external environment and are susceptible to ESD strikes. Protect these circuits from ESD with TVS diode arrays or polymer ESD suppressors.

Protects GFCI, AFCI outlets and USB power outlets

GFCI outlets have been used to protect individuals from moisture since the 1970s. The National Electrical Code and the Canadian Electrical Code require the use of AFCIs in new residential installations and homes starting in 2014 and 2015, respectively. GFCIs sense when load current on the live line does not return to the neutral line.

If the current imbalance exceeds a predetermined trip level, the GFCI shuts off power to the outlet to prevent a shock hazard. The AFCI detects an arc condition and shuts off power to the outlet to prevent a fire. Figure 5 shows the recommended protection and control components for a GFCI, AFCI, and an electrical outlet with a USB charging port.