Brainstorm: Wireless Power
With wireless devices becoming smaller and smaller, what considerations must be made when designing power sources?
Barry Brents, TE Connectivity
As our smart phones, tablets, and other wireless devices get smaller and smaller, this creates more challenges for the electronic designer. And as we depend more on these devices, and give up our paper calendars, note pads, and other old fashioned ways of keeping track of our lives, the reliability of these wireless devices becomes much more critical. If the battery in our smart phone dies, we are suddenly unable to know when our critical meetings and lunch dates are, and our busy lives are plunged into chaos.
As these smaller devices become equipped with more processing power, more heat will be generated and it must be managed and conducted away from sensitive components. These sensitive components are often forced to operate at or near their maximum operating temperature. Components failures are likely to occur under these conditions, which can range from the dead smart phone that plunges our life into chaos, to the more catastrophic issues that can damage property or injure the user.
One common trend now for thin tablets and smart phones is the use of the lithium-polymer batteries. These batteries have a very high energy density and provide an amazingly long talk time in a very light weight package. However, they can have dangerous failure modes, so it is very important to manage the charging process and provide multiple layers of protection. Very often, the margin between the temperatures and currents for normal operation, and the temperatures and currents where damage or hazardous conditions can occur is very narrow.
All of these factors create many challenges for the electronic designer, such that power management, heat management, and circuit protection are more critical now than ever before.
Terry Chang, Field Applications Engineer, RECOM Power Inc.
Often times, power is the last part of an engineer’s design. The reason is because components may change during the design, which leads to different power requirements. With the wireless device shrinking in size, there is an increased demand for smaller and compact power supplies while keeping the same power output.
When designing power supplies, you must consider efficiency, reliability and additional features among other requirements. Discrete components may have a lower BOM cost, but it requires design and test time. RECOM’s low power AC/DC converters are potted with EN55022 Class B filter built-in, optimized efficiency and are EN and UL certified for European and US markets. With the EN55022 Class B emissions limit, it will help reduce interference for wireless devices and comply with FCC Part 15. This will reduce an engineer’s time spent on designing and testing the power supply. Since it is certified, there is no need to re-test the power supply when certifying the entire product.
For wireless devices that may be powered by a battery source, DC/DC converters help provide isolation and/or boost or drop voltages. RECOM has compact packages (SIP and DIP) that will provide the necessary voltage with high efficiency (up to 96%), which is important for battery sources.
Sol Jacobs, Tadiran Batteries
The smaller the wireless device, the more important it is to choose the right power management solution.
If the wireless device is intended for an inaccessible remote location, spends most time in an energy-saving “dormant” mode, and is off the AC power grid, then typically it will be self-powered by either a lithium battery or an energy harvesting device coupled with a rechargeable lithium battery.
Energy harvesting devices have seen limited commercial acceptance, primarily due to their expense and their complex electro-mechanical design, which makes them bulkier and less reliable. Most energy harvesting devices use rechargeable batteries to store harvested energy. While most rechargeable battery technologies are improving, they still suffer from limited operating life and cannot handle extreme temperatures. To address these problems, Tadiran recently introduced TLI Series rechargeable lithium batteries, which can operate up to 10 years, can be recharged up to 5,000 times, feature a low annual self-rate, can deliver high current pulses up to 5A, and offer a much wider temperature range (-40°C to 85°C) than other rechargeables.
The most common solution for self-powered remote wireless devices is lithium thionyl chloride (LiSOCl2) chemistry, which offers the highest energy density of any lithium chemistry commercially available. Higher energy density serves to minimize size and weight. This chemistry is also non-aqueous, enabling certain LISOCL2 batteries to operate in extreme temperatures(-85°C to 125°C). Certain brands of bobbin-type LiSOCL2 cells can deliver a self-discharge rate of less than 1% per year, enabling these batteries to operate maintenance-free for over 40 years. LiSOCL2 chemistry can also be modified using a patented hybrid layer capacitor (HLC) to deliver the high current pulses required for advanced two-way communications.
Ann Starks, Applications Engineer, Power MOSFET Division, Standard Products Group, ON Semiconductor
In industry today, the consumer is moving away from the home office setup and toward portable device solutions to meet on-the-go needs, looking for a balance between performance capability and device portability. Smart phones and media tablets, for example, are incorporating more functions and peripherals while striving to maximize battery life. As wireless devices become smaller and smaller, new demands are placed on the design of the internal power circuitry. These demands include component size, power density, system efficiency, and thermal management.
Form factor is a primary concern for designers, placing stringent requirements on the internal power electronics. A smaller form factor reduces the available board space and component spacing. Many designers are investigating components with smaller packages and lower profiles, as well as integrated chip options.
Wireless devices continually expand functionality, offering new features and enhanced performance capability. This may be anything from increased processing speed, to the capability of performing multiple tasks simultaneously. Circuit blocks must provide enhanced performance while meeting new space constraints as the wireless devices shrink in size. Device performance also increases demands on battery life, making circuit efficiency a key focus.
Designers seek to maximize circuit efficiency by selecting components optimized for the specific circuit application. MOSFETs, for example, continue to improve the QG*RDS(ON) for a given die area, enhancing device performance and providing the flexibility to fit into smaller packages. The shrinking size of wireless devices places new thermal demands on designers, and is an item that needs special consideration. Component size, power density, PCB copper content, and PCB layout all play a significant role in the internal thermal environment. Each of these items is critical to designing robust wireless devices to meet the demands of the market.