It’s a Small World And Getting Smaller

Sensors Insights by John Beigel

Doing more with less is the mantra of our era and nowhere is this more apparent than in the drive towards miniaturization in next-gen electronic components and systems. The push for smaller parts is coming from both the need for smaller assemblies in specific apps and the need to reduce material costs via smaller parts that work as well as larger ones. One key area where this trend is playing out is with reed sensors and planar transformers, where new manufacturing techniques are pushing the limits of smaller, faster, and cheaper.

Driving Toward Smaller, Lighter Components

Without question, aerospace was the initial driver towards miniaturization. Electronic components in rockets and controls had to be smaller and lighter as they left our atmosphere.

The advent of the semiconductor industry and the move towards integrated circuits was the next driver, and today thousands of transistors exist in a single micron of space. As we started placing semiconductors on printed circuit boards (PCBs), passive components were just too large to fit, looming up like skyscrapers. Obviously, these passive components—reed switches, reed sensors, reed relays, transformers, resistors, capacitors, and inductors—needed to trim down as well.

Next in line was the medical industry, which developed more devices for placement inside the body. These need to be ultra-reliable, very small components that use minimal power so they do not require removal for battery replacement.

Many medical applications may have been handicapped because the designs were large. For instance, the typical heart pacemaker/defibrillator design of yesteryear was 4 in. x 4 in. x 2 in., which did not easily fit in the chest, protruding out with a large and unsightly lump. The drive for smaller components, including reed switches, batteries, and microprocessors, gave us a pacemaker that fit reasonably in one's chest cavity. And this in turn spurred a much wider use of heart pacemakers worldwide.

Last, but definitely not least, is the consumer trend towards compact, portable, smarter devices that work faster and have loads of added features, for which we want to pay less and less. Clearly the ability to cut back in size has made a dramatic and positive contribution to our lives. Let's dive deeper into just how miniaturization affects two important passive components: reed sensors and planar transformers.

Reed Sensors

A simple device, the reed sensor consists of a hermetically sealed, two- or three-lead reed switch. Leads are ferromagnetic and react to each other by closing when a magnetic field is present. If a magnet comes close enough to a reed sensor, its contacts close. When the magnet withdraws, the contacts open. The reed sensor requires no external circuitry and switches loads directly. Importantly, reed sensors do not draw any current in the off state as do semiconductor sensors.

The heart of a reed sensor is the reed switch. A typical reed switch used to be about 25-mm long, but now they measure less than 4 mm. The availability of extremely small electromagnetic, hermetically-sealed switches is clearly driving more and more applications. At the same time, advancements in contact materials and construction have led to the ability of these smaller reed switches to maintain the contact ratings of their big brothers, at least in some cases.

Reed switches find employment in test, telecommunications, security, medical, automotive, appliances, industrial, and aerospace designs among others. Two of the most critical areas for miniaturization are the electronic/semiconductor equipment testing market and medical devices.

In the electronic/semiconductor equipment testing market, small size makes reed switches ideal for integration within reed relays that can pass extremely fast digital pulses and high frequencies. Semiconductors have to be able to process digital pulses in the order of billions of times per second and reed relays do this in an efficient manner with minimal signal loss.

All major semiconductor manufacturers need to reduce line width on microprocessors to allow them to manufacture the devices faster. To do this they need test equipment with the ability to test these digital pulses. And for that, they are going to continue to need ever-smaller reed switches.

Secondly, ultra-small reed switches are suitable for use in reed sensors for medical devices such as implants. They also outfit pill cameras, defibrillators, glucose monitoring devices, nerve stimulation devices, and more. The beauty of reed sensors in these applications is that they use no power, simply sitting in the body until called upon to act. Unlike semiconductor-based sensors that drain the battery by constantly drawing power, reed sensors can sit in the body for many years without the need for removal.

In addition, when activating reed sensors, doctors can change operating characteristics, i.e., reduce glucose amounts, or change heart pacing), as well as extract data, and perform device calibration. Reed sensors are also extremely small and take up very little board space. This is a key factor since most medical devices have several hundred components and each one has to be small or patients would see an obvious lump protruding out just below the skin level.

The latest reed switches are less than 4-mm long. However, engineers continue to work on reducing their overall length for emerging applications. To further meet the demands of miniaturization trends, research and development in Micro-Electro-Mechanical Systems (MEMS) show even greater promise.

Planar Transformers

Planar transformers maintain a very flat, low profile that minimizes component height. To accomplish this, designers eliminate the traditional copper wound coil approach and replace it with a laminar approach that uses multiple PCBs stacked together. These planar transformers can be through-hole or surface-mount and they satisfy requirements for low-profile applications.

Planar transformers are steadily replacing traditional wire-wound transformers in designs where more efficient and enhanced electrical capacity is necessary. A real advancement in transformer design, compact high power density planar transformers are typically 30% of the volume and weight of traditional wire-wound components. This reduction in size from a bulky part eliminates many design constraints.

Unlike a traditional transformer's turns of copper windings, planar transformers etch spiral patterns on a PCB. The planar design allows more efficient transformers because it uses flat rather than round conductors, so the conductors are always closer to the transformer core material. The magnetic circuit existing between the core and the conductors is more efficient with lower losses, which improves the transformer design. A planar transformer can handle more power than a wound transformer of the same size and weight, so planar transformers reduce the space the transformer requires in the end product.

Insulated planar transformers offer significant improvements over traditional copper wire wound transformers. In addition to being bigger and bulkier, manufacturing of traditional wire wound transformers in distant countries often leads to repeatability issues. Housings for planar transformer cores are machineable to meet design or application requirements. Another major advantage is that planar transformers are extremely energy efficient and have lower leakage inductance and AC loss, reduced electrical stress, and improved thermal performance.

Planar transformers provide more flexibility in how designers choose to process power. This is a dramatic selling point in aerospace applications where there are growing requirements for increased efficiency. One key to their use is proper design of the planar transformer. There are a great number of subtleties in design that need attention to ensure the planar transformer provides repeatable inductance from one component to another.

With a traditional transformer, layers vary, but a planar transformer uses precision-stamped or etched lead frames that easily, accurately, and automatically layer together. It is critical that the mechanical and thermal designs for planar transformers include precise electrical characteristics like capacitance, output and aspect ratio.

New Designs Must Push Size Limits

As miniaturization of devices proceeds, designs for passive components must follow suit. Manufacturers must embrace the challenge by focusing creative engineering talent to push the limits of size. Current R&D work is pushing the envelope to develop switches measuring well below 4 mm to meet both requirements in the medical field as well as the need for even smaller reed switches in the test-equipment market.

In some instances material costs may be the driver. For example, if you can redesign power transformers to do the same job as existing transformers and the new design uses less steel and copper wire, all parties benefit both by direct material cost savings and indirect cost savings that result from the smaller enclosure needed to hold the transformer.

In other instances smaller components may carry a cost premium. For example, when looking at medical applications like pill cameras and hearing aids, the driving requirement is usually size, not cost. Either way, it's simply this generation's version of "small is beautiful."

John Beigel is Technical Expert and Member of the Board of Directors, MEDER Electronics.