Circuit Protection, Control, and Sensing Design Considerations for Smart Home Security Applications

Advancements in IoT technology are enabling the development of smart home security systems that offer consumers security, convenience, and energy efficiency. New sensing technology and wireless protocols such as wireless LAN (Wi-Fi) have created a wide range of devices that can monitor and control security equipment, access control equipment, appliances, energy management equipment, power outlets, lighting, and entertainment systems. Figure 1 illustrates the breadth of IoT technology that is enabling the smart home.

The smart home security market is a dynamic market. Unit shipments of residential security cameras are expected to grow from 54 million units in 2018 to 120 million units in 2023. That represents a substantial compound annual growth rate (CAGR) of 17%. There is a significant opportunity for innovation and new products in this market.

Smart home security products will need to be highly reliable in order to win market share. As more vendors enter the market and product differentiation narrows, product quality will become a greater deciding factor for consumers. Thus products will need to be robust to overloads and transients. The following paragraphs will give designers of home security systems recommendations for using and selecting components that protect and control their circuits. Components for protecting the following products will be presented:

Wired security camera systems are typically fixed mounted security systems for monitoring areas surrounding a residence. Figure 2 shows a wired security camera and recommended components for protecting and controlling the camera. These cameras are AC line powered and subject to overloads and transients propagated on the AC power line. In addition, all connections to the external environment are subject to electrostatic discharge (ESD).

Designers will need to protect their camera circuitry from damage due to these hazardous conditions. Figure 3 shows a block diagram of a wired security camera and the recommended protection and control components to ensure a robust product.

The power adapter interfaces with the AC power line and needs protection from power line current surges and voltage transients. We suggest fuses for overload protection, a metal oxide varistor (MOV) for protection from transient current surges, and a transient voltage suppressor (TVS) diode for protection from voltage transients. These three components, with parameters selected for the specifics of your design, will ensure the Power Adapter is robust to overloads and transients.

Select a slow blow fuse to avoid nuisance openings of the fuse due to switching supply inrush current. However, to avoid damage to the Power Adapter circuitry, choose a fuse that will respond quickly to a continuous overload such as within a few seconds to a 200 % overload. Do not overlook other key parameters when selecting a fuse. Verify that the fuse’s voltage rating exceeds the maximum line voltage that the power supply will see. Also, ensure that the interrupting rating is higher than the worst-case current overload that the circuit would be expected to encounter.

An appropriate interrupting rating will avoid physical damage to the fuse due to a large transient overload from the AC line. Since space is at a premium in the power supply, look for small fuses that meet the requirements. Finally, use a fuse with UL/IEC certifications to simplify and speed up product safety compliance qualification.

For large, high energy surges induced by lightning, for example, protect the supply with a metal oxide varistor (MOV) located as close to the input of the supply as possible. MOVs can absorb surge currents as high as 10 kA, and they can absorb over 500 J of peak pulse energy. Since the MOV is in contact with the power line, use a UL/IEC-certified component.

The third protection element you want to consider is a transient voltage suppressor (TVS) diode to protect downstream circuitry from voltage transients. TVS diodes respond to transients extremely quickly, in under 1 ps. When suppressing a transient, the TVS diode clamps its output at a low voltage to protect sensitive circuitry. TVS diodes are available in either uni-directional or bi-directional versions and have a wide range of clamping voltages which can be as low as 10 V.

The DC Input Stage supplies DC voltage to power the control board, the motor drive, and the Image Sensor Interface. We recommend both current overload protection and transient voltage protection for this circuit. Either a conventional fuse or a resettable polymeric positive temperature coefficient (PPTC) fuse are options. Surface mount versions of both these types of fuses are available so consume a minimum of PCB space. TVS diodes are also available in space-saving, surface-mount packages and can withstand transients with up to 5000 W of peak pulse power.

The control board contains the intelligence for the camera circuit. The control board includes the MCU, the video memory storage, and the wired interface circuit which transmits information over the AC power line using PoE protocol. Two of the control board’s circuits should have protection components. Those two circuits are the Wired Interface and the memory circuit.

The wired interface connects with the external environment and should have current overload and transient voltage protection. Like the DC input stage, select either a conventional fuse or a resettable fuse for current overload protection. For protection against voltage transients, we recommend a special component, a TVS diode array. The TVS diode array protects the PoE I/O lines from ESD and other high voltage transients.

Figure 4 shows an array of bi-directional diodes combined with a Zener diode to withstand a ±30 kV ESD strike. These diodes have a low 1pF/port capacitance which minimizes their impact on the POE protocol transmission and reception signals. Diode array configurations such as shown in Figure 4 are available in surface-mount packages, which take up the least amount of PCB space.

The SD card holds the data acquired by the camera. We recommend a TVS diode to protect this important circuit from up to ±30 kV of ESD. TVS diodes have low leakage currents under 1 µA to reduce power consumption.

The motor drive block has a pan and tilt motor circuit which positions the camera to track movement. For this circuit, we recommend a control component, an optically isolated solid-state switch. The optical isolation prevents motor noise and transients from coupling back into the pan and tilt control circuit. Solid-state switching provides bounce-free switching for smoother control of the motor. Look for solid-state switches with zero crossing switching to minimize motor start-up transients. Also, look for switches with current limiting to protect against a stalled motor condition.

For monitoring internal spaces, wireless security cameras are small and can be placed easily anywhere. See Figure 5 for an example wireless camera. Figure 6 shows a block diagram for the wireless camera and the recommended protection and control components for this device.

The USB adapter is the power supply for the wireless security camera. Since it connects to the AC mains, the USB adapter must be protected against current overloads and voltage transients. Similar to the power adapter that powers a wired security camera, the USB adapter should have a fuse for overcurrent protection, an MOV for lightning strikes, and a TVS diode for ESD protection.

If the design will use a USB Type-C protocol, then the USB Type-C, high-density connectors require thermal sensing to detect a short between connector pins. A thermal sensing element designed to be compliant with USB Type-C designs will avoid damage to cables and connectors if dust or dirt shorts adjacent connector pins. Refer to the USB Type-C Standard.2 The sensing element resets itself with a low resistance when the problem causing the temperature rise is corrected.

The battery pack should be monitored for an overcurrent condition if a battery cell fails in a shorted condition and overheats. Also, the battery management unit and the USB can be subject to ESD. We recommend a resettable fuse, a PPTC surface mount fuse, to protect against an overcurrent due to a battery cell failure. A PPTC fuse will trip in under one second to a 200 % overload to prevent one shorted battery from causing damage to multiple batteries.

The battery management unit and the USB circuit should have ESD protection. We recommend multi-layer, metal oxide varistors that have low clamping voltages to protect semiconductor components. When saving PCB space is an important consideration, versions of multi-layer MOVs in space-saving, 0402 surface-mount packages can be deployed.

If the USB circuit complies with the USB Type-C protocol, then a thermal sensing element identical to the one recommended for the USB Adapter should be used to protect the mating USB socket in the USB circuit. More details on designing a thermal sensing indicator into a USB Type-C circuit are in our design and installation guide.3

The control board contains digital electronics that control the camera and save the video data. The control board needs protection from transients and ESD. For this circuit, we recommend a Zener TVS diode that withstands up to a ± 30 kV strike. The TVS diode, with a leakage current of under 0.5 µA, draws a minimum of power from the circuit. The diode also takes up a small amount of PCB space and can be housed in 0201 surface-mount packages.

The wireless interface communicates with external wireless devices such as computers, smartphones, and mobile tablet computers. Since this circuit is exposed to the internal environment, the circuit is susceptible to ESD. In this case, consider a polymer ESD suppressor. A polymer ESD suppressor will protect a circuit from ESD strikes as high as ± 15 kV through the air and ± 8kV from human contact. In addition, its low capacitance of under 0.2 pF does not degrade signal transmission or reception.

The wired doorbell camera has become a very popular product for remote viewing of visitors. Figure 7 shows an example doorbell camera and Figure 8 shows a block diagram with recommended protection and control components.

The Power Supply Unit interfaces with a stepped down AC voltage from the AC power line and can be subjected to current overloads and voltage transients such as transients induced by lightning. As with the other supply circuits, a fuse will protect against current overload conditions. To protect against the voltage transients on the AC line, select a TVS diode which can absorb a transient with power as high as 4 kW. Consider using an optically isolated solid-state relay to control power to the other circuits and to keep noise and transients from the power line from propagating to the downstream circuits.

The Battery Pack provides back-up power to the Control Board and the User Interface. The Battery Pack and its sub-units, the Battery Management Unit and the USB circuit need protection from overcurrent conditions and transients using similar components recommended for the identical circuit in the wireless camera. Adding a TVS diode will ensure suppression of voltage transients and ESD strikes. As with the battery pack in the wireless camera, we recommend thermal sensing for the USB circuit if the USB Type-C protocol is a design requirement.

The user interface is subject to ESD from human contact. Use a TVS diode to protect the User Interface from ESD. If the doorbell camera will be marketed in Europe, you will need to include a switch between activating the door chime and the camera to comply with European privacy regulations. Consider using a solid-state relay to eliminate mechanical relay contact bounce and generation of electromagnetic interference (EMI).

The control board and the wireless interface circuits function for the doorbell camera as they do for the wireless camera. The protection components recommended for these circuits in the wireless camera would apply to the same circuits in the doorbell camera.

Some security camera manufacturers are adapting to the “new normal” of life resulting from the COVID-19 pandemic. They are adding infrared thermal sensing technology to the cameras so that the temperature of visitors to the home or business can be discretely monitored. Temperature measurement could become a standard feature in future smart home security systems. As security cameras become increasingly sophisticated to include this aspect of health safety, incorporating proper circuit protection helps develop more reliable and robust devices.

Two safety standards, shown in Table 1, define requirements for audio/video communication technology equipment and for batteries such as lithium batteries. IEC 62368-1 addresses safety requirements for surveillance equipment; and, the second standard, IEC 62311-2, dictates requirements for ensuring the safety of lithium cells and batteries. Improper assembly and operation of lithium batteries can lead to the potential of a fire; thus, monitoring of these types of batteries is essential. Failure to adhere to these standards will result in re-design work and a second submission for certification to a standards body. Project costs will increase and introduction of the product can be delayed.

Table 1. Safety Standards for products with surveillance cameras

By including protection and sensing components as a design objective and incorporating standards compliance early in the project, you can cost-effectively develop a robust and reliable smart home security system. Of course, your design is always a trade-off between performance and total cost of ownership. While some technologies may cost more than others, additional factors may show that the lowest total cost of ownership requires a higher cost component technology. Take advantage of component manufacturer’s engineering expertise when selecting protection, control, and sensing components. Using a trustworthy vendor saves your time and resources.

Reliability and efficiency enhance the reputation of your smart home security system. A reputation for high quality can result in market share growth and revenue growth. That leads to improved profitability.

For more information on circuit protection, sensing devices and component selection criteria, see the Circuit Protection Selection Guide and the Sensing Products Selection Guide courtesy of Littelfuse.

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