Lighter, cordless equipment is great for mobility, but wireless charging factors must be addressed.
When it comes to surgical equipment, there are several reasons, and hurdles, associated with going portable.
The reasons: Lighter, cordless equipment reduces surgeon fatigue and increases mobility during operations. Furthermore, the reduced power consumption results in less heat and smaller form factors. As for roadblocks, one important one has to do with the sterilization process that must occur before any equipment can be used in an operation. Battery packs and matching equipment with exposed leads have long term issues when exposed for autoclaving (sterilization method that uses dry or steam heat, generally with constant pressure) unless extreme measures are taken to seal and protect the connection points.
This article discusses wireless charging and battery management technologies that can make surgical equipment more portable, reliable, and easier to use.
Portable sterilizing
With autoclaving, steam is the most widely used method, with a typical heat profile of 246°C for 30 minutes to ensure sterilization. The device’s various components must be able to maintain a seal, regardless of the differing expansion rates of the outer components. The ideal design incorporates a uniform exterior shell with no access points or entryways. Implementing power and communications needs – with no external connection points – are among the critical challenges for medical electronics.
While Wireless standards have emerged over the years to address the communication problem. Bluetooth Low Energy (BLE), personal area network (PAN), ZigBe, near field communication (NFC), and many others offer a low-power wireless communication path suitable for low bandwidth applications, hurdles having to do with power are another matter. Portable applications require batteries and methods of recharging them. Having access to batteries makes replacing them easier, but exposure to the autoclave’s high temperatures and humid conditions can shorten battery life. This issue is greatly mitigated by recharging internal rechargeable batteries through the equipment’s outer skin. Keep in mind that both wireless communication and power transfer benefit from a device with a no-metal case (at least where the coil and antennae are located). Particularly important to power transfer is the thickness of the case as this determines the distance between receiver and transmitter coils and directly correlates to power transfer efficiency.
Understanding batteries and wireless power systems
The battery itself is the most critical element of portability. Different battery types such as lithium-ion (Li-Ion), NiCd, NiMH, and others have different operating characteristics and performance. For example, NiMH has relatively low capacity and a high self-discharge rate as compared to NiCd, whereas Li-Ion batteries require a more precise charging circuitry. Designing battery management electronics to match the specific battery type maximizes the battery pack’s overall lifespan.
For example, understanding the battery’s voltage profile at varying load conditions, and having the ability to measure battery impedance is critical for accurately predicting the end of the battery’s utility. Semiconductor manufacturers such as Texas Instruments have battery management solutions that can charge, monitor, and protect the various battery types and maximize their usefulness, as well as determine battery life. Once a battery is chosen and matched with the correct charging and protection ICs, the power required from the wireless power source to charge the battery in a given time can be determined.
The two main components of a wireless power system are the transmitter and receiver. The transmitter radiates power by sending alternating current into a coil of wire, which generates a magnetic field. The receiver (when placed on the transmitter pad) captures that energy and provides a current output to the load. The alternating currents are then rectified and converted to a DC voltage. The magnetic field is contained by shielding material placed behind the transmitter and receiver coils. An easier way for power supply designers to look at this is to imagine a forward or transformer-coupled power supply, then physically separating the transformer’s primary and secondary windings. Varying output is done by varying the transmitter’s switching frequency. The transmit coil assembly has a resonant frequency, a point at which maximum efficiency occurs. This is achieved by moving the frequency closer to, or farther from, the resonant frequency. This results in a varying power output.
A simple approach to transferring power is to direct a pulse of current alternately through the transmitter while regulating the current through the coil. This sends a constant power to the receiver. The power transmitted is roughly calculated by the voltage applied to the coil times the regulated current. This assumes a 50% duty cycle pulse and minimum delay between alternating pulses. The receiver takes in the power and uses it to recharge the battery. The disadvantage of this scheme is that there is no feedback or communication between the receiver and transmitter. This makes the solution inefficient because the transmitter must always transmit – whether or not a receiver is present, which causes the receiver to burn power coming in even after the battery is charged. Therefore, a feedback path is used in the vast majority of solutions.
Feedback can be provided through either analog or digital methodologies. An extra winding on the transmit side couples to a reflected wave from the receive side, thus indicating that a receiver is in place and that power can be applied. Once the reflected voltage level is determined, the receiver can change the voltage reflected by grounding the pulse. This notifies the transmitter to shut off the power. More commonly used is the digital method of communication.
A string of ones and zeros is sent via the transmit and receive coils. Using a basic data structure consisting of a start bit, stop bit, and parity, reliable communication can pass between the two systems. This communication can be basic information, such as whether or not a device is in charging range and when to turn on the transmitter, all the way to encrypted communication that verifies the device’s identity, battery life information, device health, usage times, field strength, and others. This information-transfer greatly improves efficiency and reliability of the device being charged.
The amount of energy transferred from transmitter to receiver is dependent on many factors. The coil configuration and diameters, orientation and distance between the two coils, and frequency are the main components in determining the energy transfer efficiency. Emerging standards such as Qi from the Wireless Power Consortium (WPC) specify coil sizes, magnetics, wire gauge, output power, and communication protocol, in addition to specifications for the hardware and distance between coils.
The advantages of making standards-compliant solutions are guaranteed operation with any transmitter or receiver that meets the same standard, dependable, and consistent power transfer, and the ability to buy compliance-certified solutions for quicker time-to-market. This is especially important if you want to enable the market using chargers other than your own.
Disadvantages include a capped maximum power transfer with a form factor that accepts multiple sources (including competitors). However, this form factor could be overkill for the application, and may add significant cost. Figure 3 shows a reference design platform for a Qi transmitter and receiver. This reference design uses the magnetics in such a way as to “self-align” the receiver to the transmitter. The receiver is pulled into place by the permanent magnet pole alignment of the two units. This gives the user a tactile feel ensuring proper placement so that maximum power transfer occurs.
Conclusion
Wireless charging is still an emerging market with improvements being made daily. Coupling efficiency and ease-of-use are two top areas of development. Holding back the adoption rate are the added cost of adding the receiver to the product that may or may not be used by the end use, complexity, and multiple standards competing for customers and applications.
However, for the medical market, wireless charging can enable lighter, more efficient portable products that are not constrained by the multiple vendor consumer markets. The ability to develop custom hardware and data transfers specific to the medical application maximizes performance while providing secure, reliable, and consistent operation. The entire ecosystem of a portable device needs to be considered. This includes the battery type, capacity, battery protection, gas gauge and impedance measurement, power consumption, charge time, case thickness, coil size, communication needs, and physical placement of the charger and device. These all contribute to the performance, utility, and lifetime of the end product.
References
1. Wireless Power Consortium
2. Download a datasheet and application note for the bqTESLA: www.ti.com/bqtesla-ca.
3. “Building a wireless power transmitter,” Application Note (slua635), Texas Instruments, March 2012.
4 “How to implement a 5-W wireless power system,” by Upal Sengupta and Bill Johns, How2Power, July 2010.
5. “Universally compatible wireless power using the Qi protocol,” by Upal Sengupta and Bill Johns, Low-Power Design, October 2011.
