Electronic Circuit Protection for Wearable

wearable technologyAs consumers become increasingly dependent on wearable like smart glasses and fitness wristbands, circuit designers are tasked with incorporating advanced circuit protection technologies that safeguard the device as well as the user. Without proper protection, the device’s sensor circuits, battery charging interfaces, buttons, or data I/Os could be irreparably damaged by electrostatic discharge that is often caused by close interaction with the user. Circuit designers face many challenges in the design process stemming from the shrinking form factor of most wearable and the demand for improved device performance. This story has highlighted some of the aspects in ESD security of devices & innovations in the field.

Wearable devices are changing the trend of consumer electronics and will be met with rising demand in wide fields including health care, sports, fitness, fashion, entertainment, mobile communications, and the entire Internet of Things as people’s lifestyles change, thus powerfully driving the growth of this technology market. According to a research report of Research & Markets, the global wearable electronics market currently has a value of approximately USD 3.5 billion, which is expected to reach approximately USD 8.4 billion by 2018, representing a compound annual growth rate of up to 17.71%. In addition to chips, sensors, integrated design and other technologies which have received a lot of attention, printed circuit board (PCB), a basic electronics component of wearable devices, is also set to get great development opportunities. One of the challenges that wearable technology poses, is the very nature in which a device will be used, and primarily, the environments that it may be subjected to. For instance, a sensor in a wearable device has the added consideration of physical interactions; the device will be moved, worn, may see impact, may be flexed and potentially exposed to a number of additional elements, such as water or chemicals. It is therefore imperative that these devices are protected accordingly to ensure reliable performance when utilized in their end-use environments. Here comes the requirement of Electric Circuit Protection for Wearable Devices.

There are a bevy of challenges the circuit designer must consider when working on delicate sensors & components into the gadget itself; chief among them is protecting the gadget’s internal components, including sensor circuits, data I/Os, buttons, etc., from things like user-generated electrostatic discharge, or ESD for short. ESD can be generated by the user simply touching the device with his or her finger, and without proper protection in place, a small amount of ESD can cause irreparable damage to the device. This is where circuit needs proper protection to come up with smaller, safer and more reliable product. A simple human touch can be all it takes to initiate an electrostatic discharge (ESD) transient due to the fact that any of the sensor circuits, buttons, battery-charging interfaces, or data I/Os could provide a path for ESD to enter the device. Manufacturers of semiconductor-based ESD protection components are constantly trying to improve the capabilities of devices such as:

Lower clamping voltage to protect even the most sensitive circuits: During an ESD event, the main job of the ESD protector is to divert and dissipate as much of the ESD transient as possible. This characteristic is improved by reducing the on-state or dynamic resistance.

By reducing the dynamic resistance, the ESD protector carries significantly more of the surge current than the circuit being protected. In doing so, it reduces the electrical stress on the integrated circuit and ensures that it survives.

Lower capacitance to avoid interfering with high speed data transfer: Although circuit protection is vital to an ESD protection device’s purpose, it is important to remember that it must perform this role without interfering with the day-to-day functioning of the circuit it protects.

For example, on an RF interface (Bluetooth, ZigBee, etc.) or wired port like USB 2.0, the ESD protector must be prevented from causing distortion or loss of strength of the data signals. To ensure signal integrity, the capacitance of the ESD protector must be minimized without compromising protection levels.

Smaller form factors to fit the limited board space available in the wearable devices: Wearable medical devices will gradually get thinner and smaller (watches, wristbands, chest bands) or be incorporated directly into clothing, so that the circuit boards will have minimal space available for the ESD protection solutions.

Challenges in Designing a Circuit

It’s easy to select a cool gadget based on the features, but when we look inside the circuit it is not easy for the designer to make choice for the components inside. Alongwith performance some more aspects to be considered while designing a Smart gadget circuit are, Flexibility: Bodies are bendy and it just so happens that flex sensors sense a flex or a bend. They’re very good for areas of the body that bend in a broad, round arc. They work well on elbows, knees, fingers, and wrists. They are variable resistors and need to be used in combination with a voltage divider circuit in order to be read by a microcontroller. The biggest challenges in working with flex sensors are positioning and protection. In order to get an accurate reading of the flex of your elbow, the sensor needs to be positioned in the same place on your elbow every time. Force: Bodies often touch and get touched. One way to sense touch is through the use of force-sensing resistors (FSRs). FSRs have a makeup that’s similar to flex sensors but are configured to be sensitive to pressure rather than bending. They are also variable resistors and have delicate connections similar to flex sensors. Stretchy: From the bend of a knee to the expansion and contraction of a rib cage with each breath, properly positioned stretch sensors can capture the fluctuating nuances and curves of the human form. Movement & environment: People are active and mobile creatures. They reach for things they want, turn toward loud noises, and crouch down to coax the cat from under the bed. When creating wearable that react to events such as these, it is helpful to be able to sense movement. There are also far more sophisticated sensors that you can use. Accelerometers measure acceleration or changes in speed of movement. They can also provide a good measurement of tilt due to the changing relationship to gravity. Heart Rate & Pulse sensing: Your heart beats faster when you’re excited, and your skin gets clammy when you’re nervous. Besides sensing your environment and your movements, you can also use sensors to learn more about what is happening within someone’s body. A great place to start sensing these biometrics is pulse or heart rate. Optical heart rate-sensors, such as the Pulse Sensor Amped, are a small, lower-cost solution for measuring pulse. This type of sensor measures the mechanical flow of blood, usually in a finger or earlobe. It contains an LED that shines light into the capillary tissue and a light sensor that reads what is reflected back. It produces varying analog voltage that can be read by the analog input on any Arduino. Chest-strap heart monitors are a more expensive but more accurate solution for measuring heart rate. They measure the actual electrical frequency of the heart through two conductive electrodes (oftentimes made of conductive fabric) that must be pressed firmly against the skin. Polar produces heart rate monitors that wirelessly transmit a signal with every heartbeat. Such a sophisticated circuit needs a good protection mechanism further to ensure long life & cost optimization. Some of the considerations is selecting proper Circuit protection are:

  1. Impact Resistance: A wrist-mounted device is going to get smacked into hard surfaces on a regular basis. Just look at what happens to a watch face if it is not designed with a sapphire crystal. Take a look at an everyday watch with a standard glass crystal and you will notice how beat up the surface gets in as little as a year. While a headset or glasses are less likely to suffer direct impacts in use, such devices will see an increased probability of drops (most often at inopportune moments). The displays, electronics, and contacts need to be designed to survive such abuse.
  2. Sealing: It’s important, because normally, people perspire. Perspiration is salty, so you are not just dealing with moisture. When salty water intrudes into the device, it wreaks havoc with contact corrosion and possible circuit board damage. The problem is prevalent in any technology product coming into direct contact with skin. The issue is exacerbated by heat from the device and non-breathable soft goods. If the product is going to have any long-term reliability, it has to be tightly sealed.
  3. Ergonomics: A wearable that is comfortable for 10 minutes in a conference room may not necessarily be comfortable when worn for hours. Watch out for “hard” edges, non-breathable soft goods, or pressure points. In hiking boots, a minor skin contact at the start of a long hike can be noticeable. Two hours later, it is a blister. With a wearable product, a slight pressure point in a few minutes becomes a sore red mark after a few hours even if it does not produce a blister. The effect of device heating will not help the cause, either. This is crucial in wearable technology because poor power management translates into battery drain. Sure, one can always select a nice, compact battery, but how long will the device run under typical use conditions? Characterizing a usage profile is a non-trivial design activity. The Apple Watch is only expected to operate for two to five hours in actual use. What happens over an entire waking day of 16 hours? Ideally, one does not want to take off a watch during the day to charge it (unless a person works night shifts). Using Google Glass for video will kill the battery in an hour. It remains to be seen if such limitations restrict widespread, long-term adoption of consumer wearable technology. Battery life has a direct impact on a product’s real usefulness. The challenge is balancing battery capacity and size with efficient power management and a realistic usage profile. It can be quite tempting to make assumptions about usage profiles that would lead a designer to believe that a lower-capacity battery will do the job. These trade-offs should be made with a mind toward practical utility to the user.
  4. Heat: This has some relation, obviously, to power management. Realize that all electronic devices get warm (or hot). The harder you work them, the hotter they get. While the risk of an actual burn may be remote, heat can make a device very uncomfortable to wear. This is one of the observed issues with Google Glass. The more use you want to get out of a wearable device, the more heat dissipation is an issue. The designer needs to consider how heat will be managed, from both energy consumption and comfort viewpoints. Overall, wearable tech devices are here to stay.

Considerations While Designing a ESD Protected Circuit

The Most challenging aspect of protecting new applications

The biggest challenge is convincing circuit designers that circuit protection is important from the beginning of the design process. Because circuit protection devices do not add perceived value to the customer’s product or enhance the capabilities of the application, they are typically ignored until the end of the process when it’s time for electromagnetic compatibility (EMC) testing. It’s very important for designers to understand that board-level ESD protection ensures that these innovative devices are able to deliver safe, reliable performance throughout their intended operating life.

Iimportance of incorporate circuit protection in the design phase
Circuit designers should consider ESD protection early in the design phase so that they can choose the ideal protection device and select the optimal layout/ location before their options become limited by other components, including transformers, common mode chokes, and passives.

Will incorporating Circuit protection increase board area & Cost

In general, most designs do not require board-level TVS diodes at each of the IC’s pins. Instead, the designer should determine which pins have exposure to the outside of the application. Typical circuits include USB, audio, button/switch control, and other data buses. Since adding these discrete devices will take up board space, it’s important to concentrate on reducing their size. For some applications, there are some space-saving multi-channel arrays available.

Will it alter end device performance

ESD protection device manufacturers continue to work to develop products with ever-lower clamping voltages. They also work with board-level designers to ensure that proper circuit layout practices are followed to protect the reliability of the application. Coupling products with low dynamic resistance and proper board layout practices will ensure that applications with even the most sensitive IC will still perform reliably.

What are the key considerations for trace

Unlike lightning transients, ESD does not unleash a large amount of current for a long duration. Instead, it is important to move the ESD charge from the protected circuit to the ESD reference in a very short amount of time (<200 ns). The length of the trace (from the I/O line to the ESD component; from the ESD component to ground), not the width of the trace to the ground, is the overriding factor. This length should be kept as short as possible to limit parasitic inductance. Because of this, recent package developments include μDFN outlines that fit directly over the data lanes to eliminate the need for stub traces.

In terms of form factor, how ESD disruptive discharge can be done

The inductance will act as a choke for the ESD transient and degrade the protection performance. The parasitic capacitance will degrade high-speed signal performance. Designers should not only minimize the length/width of the stub trace, but also attempt to eliminate it. For the connection to the data or signal line, place the solder pad right on or adjacent to the trace, if possible. This can be done for discrete diodes as well as a number of arrays that are designed to be placed right over the data tracks. This underscores the need to consider the board-level ESD protection approach as early as possible in the design process.

Determine the ideal Human Body Model (HBM) or Charged Device Model (CDM) for a particular wearable technology

HBM and CDM are designations for the ESD robustness of the ICs that run the application, including the processor, memory, and ASIC. They are not designations for system-level ESD robustness or immunity. In general, applications like smart phones and PCs are characterized to the IEC 61000-4-2 standard. As a system-level specification, it allows the designer to determine whether the ESD device has the robustness and clamping capability needed to survive the system level transient and protect the system. For board-level ESD devices, the designer can determine their robustness by reviewing the IEC 61000-4-2 rating (8kV contact, 12kV contact, 15kV contact, etc.). In addition, review the dynamic resistance value (1Ω, 0.5Ω, etc.) to determine the level of protection provided by the devices. For wearable technology, a high IEC rating and low dynamic resistance are especially important.

Based on the connection among capacitance, signal integrity, and ESD clamping voltage, does low capacitance affect performance

Since significant advancements have been made regarding capacitance and ESD performance, there is no longer a tradeoff between these two characteristics. For example, new wafer fabrication processes allow the designer to create a protection device with 0.5pF of capacitance (or lower) and still have less than 1Ω of dynamic resistance. So, these low-capacitance diodes do not store any charge or impact high-speed signal integrity. The low dynamic resistance value ensures that they also have a low clamping voltage. Some application designers consider polymeric devices and zener diodes as viable alternatives to the semiconductor ESD solutions we have discussed. However, it is important to recognize that all these options have very different characteristics, including dynamic resistance. For chipsets with robust on-chip protection (per HBM) or that were made on old processes (e.g., 130 nm), polymers and zener diodes provide sufficient protection. In contrast, modern ICs made on very dense topologies (e.g., 45 nm and 22 nm) are very sensitive to ESD pulses and require the lower dynamic resistance associated with semiconductor TVS diodes. It is vital that the circuit protection solution matches the needs of the IC that requires protection.

Could ESD protection Can be incorporated at a later stage

Fortunately, there are multiple solutions available for protecting against ESD. To find the best solution for a specific application, it’s vital to consider the circuit’s characteristics, cost, and board implementation. Solutions like zener diodes will clamp an ESD event. However, they are optimized for voltage regulation, not transient voltage suppression. Because of this, their dynamic resistance and clamping voltage are higher than that of TVS diodes and diode arrays. Another shortcoming is that they won’t be able to survive a high number of ESD hits.

Conclusion

Wearable tech is see a rapid growth in consumer market and so a upward trend in stylish, long battery run and better performance gadgets, this increases the complexity of circuit design & protection. Many big companies like Littlefuse, provides best possible solutions for the designers to meet protection criteria maintain the performance lines.

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