Design Talk - Industrial and Harsh Environments
Editor's note: No matter how good the tech, if it must address an industrial or other demanding and harsh application, it better be able to deal with its environmental surroundings without operational compromise.
Increasing Ruggedization and Viewability in Mobile Displays
When designing displays for mobile applications it’s often a balancing act between increasing ruggedization and keeping the product’s weight down. These two seemingly opposite ideas can be solved by optically bonding substrates to the LCD front surface thereby eliminating the air gap which increases ruggedization without adding weight. Additional benefits resulting from the removal of the air gap include: the elimination of condensation and fogging, better viewing experience, thinner display designs and the reduction of parallax issues especially in tablet PC applications
Another concern for mobile displays is outdoor readability. In displays using a non-bonded cover glass, the ambient light reflects off three interfaces resulting in as much as 13.5% reflectance. One approach to increasing viewability is to use coverplates treated with anti-reflective (AR) coatings. However this solution only minimizes light reflectivity and doesn’t address the need for greater display contrast. Instead, you can optically bond the AR-coated substrate directly to the LCD eliminating the air gap between the two reflective surfaces of the cover glass and the LCD allowing great reductions in reflectance and reducing the number of anti-reflective treatments needed. With optical bonding, the contrast ratio can increase by as much as 400% verses a non-bonded display.
For these reasons, optical bonding should be considered when designing displays that are going into high-performance consumer or industrial mobile applications. Since DuPont first developed DuPont™ Vertak™ direct bonding technology for optical bonding, it has been used extensively in marine electronics, medical applications, commercial avionics, notebook and tablet PCs, and touch screen devices. Vertak™ technology has continued to withstand the highly-demanding environmental challenges faced by these displays supporting stable performance under extreme temperatures and altitudes by increasing outdoor readability up to 400%, enhancing impact and scratch resistance by 300%, improving durability to withstand shock and vibration, providing a barrier to stains, dirt, moisture and scratches, and even enabling thinner and lighter display designs.
Spaced out on HiRel
The heavens have fascinated people since the beginning of mankind. Space travel and exploration remain a popular topic to all ages and are the subjects of international debate, pride, strength and domestic security. The US certainly prides itself in the amazing feat of manned lunar landings in the late 60s and early 70s. A feat not repeated again in 40 years since those first steps were taken by Neil Armstrong.
Science fiction involving space travel continues to fill our entertainment media, and most of us have heard those famous words of Star Trek’s Captain James T. Kirk, “Space – the final frontier!” For those of us who work in the industry, however, we quickly must separate fiction from fact. And we quickly find out that “space is actually a very deadly frontier.” Space is an extremely harsh environment, not only for humans but for electronics as well. Our atmosphere assists in additional shielding of incoming radiation, but produces terrestrial neutrons in this shielding process. These neutrons can produce logic upsets in electronic circuits by impacting other atoms releasing small amounts of energy, changing the material properties and producing energy pulses, causing “glitches” in integrated circuits. As semiconductor feature sizes shrink, the task of making circuits more immune to these effects, or hardening them against these effects, becomes more difficult.
Space, unlike the shielded environment in which we live, can be very dangerous. Low earth orbiting (LEO) spacecraft and satellites are exposed to these protons, electrons and heavy ions from space radiation. Geostationary orbiting (GEO) satellites during a 10–15 year mission will encounter large doses of radiation. Even with shielding they receive enough radiation to kill humans in just a few months. Shielding can help, but only to a limited degree. There is a point of diminishing return where shielding is no longer beneficial. Systems must be designed to operate in this difficult environment, and at the heart of these systems are the electronic components. Special semiconductor processes can be used along with special design techniques to ensure that circuits can operate reliably over long periods of time in space.
Our daily lives are more dependent on data sent or received over satellite systems than ever before. Whether it is international communication, music, DTV, GPS, weather gathering, surveillance, national security, etc., we rely on these systems to function properly every day.
Building circuits that can tolerate radiation effects are necessary for space and aircraft as well as today’s medical applications. Dental and medical X-ray diagnostic equipment now use electronic imaging and data conversion chips to capture images, when photographic film was previously used. These circuits must be able to withstand daily low doses of radiation in these medical and diagnostic applications.
Texas Instruments’ High Reliability group (HiRel) is a major supplier of radiation tolerant semiconductors. Circuits are developed and tested against the various radiation effects that can be encountered in medical, avionics and space applications. It just makes sense to use HiRel products in High Reliability systems. It is very important to not only understand the environment that products are going to be used in, but also that the circuits are reliable and tolerant in the specific application.
DC Conversion for Harsh Environments
Electronic instruments for subsurface applications, specifically for the oil industry, are frequently exposed to extremely high temperatures depending on friction generated while drilling, depth and the specific geology of a location being explored. The temperatures are usually well over 150ºC and in many instances reaching 200ºC or more. Very few commercially available electronic components are designed or characterized for such high temperatures. Certain technologies are inherently capable to provide reliable operation beyond what is specified in the component data sheets. As a result there are very limited suppliers offering DC-DC converters for operations of greater than 125ºC and none above 175ºC to date.
One such device, are capable of providing up to 20W of output power over the extreme case temperatures of -35ºC to +185ºC. They are available in single and dual output configurations. Input and output are galvanically isolated to protect the output loads from catastrophic system failures on the primary (input) side and to provide the flexibility of stacking several converters to obtain a higher output voltage. The converters are hermetically sealed thick film based microelectronic hybrids. Target applications include down-hole oil drilling, seismic survey, aircraft engine controls, other natural resources explorations, and any design applications requiring up to 185ºC of operations.
The HTA series utilizes a single-ended forward topology with resonant reset. Two high voltage power MOSFETs are used in series to accommodate the high operating input voltage and to minimize the voltage stress. The nominal switching frequency is 500KHz. PWM controller incorporates an IR’s proprietary custom ASIC to minimize size and components count. Input/output isolation and excellent output voltage regulation are achieved through the use of magnetically coupled feedback. Voltage feed-forward with duty factor limiting provides high line rejection and protection against excessive output over voltage in the event of an internal control loop failure. The design includes an LC input filter to control the conducted emission propagating back on to the input lines. A typical input ripple current is limited to less than 15mA peak-to-peak.
The output section uses two isolated windings with the traditional rectification arrangement followed by the individual low pass output filters to attenuate the higher frequency ripple and noise. The output overload and short circuit protection makes use of the resistance of the inductor wire to minimize power losses. Output voltage is sensed and the control loop is closed across the positive output. The negative output is expected to provide regulation when the loads of both the positive and negative outputs are balanced. For single output models, only one single secondary winding, associated rectification and filter circuit is used.
Assembly begins with attachment of electrical components by way of solder reflow or adhesive epoxy to bare beryllium oxide (BeO) thick film substrates with screen printed resistors. Transformers and other magnetic parts are attached to substrates and base of the assembly with thermally conductive epoxy. The assembled substrates are then solder reflowed to the base. Lead frames and wire bonds are attached from the substrate to substrate and substrates to the I/O pins. The assembly is then inspected for acceptable workmanship and tested to insure proper functionality. High profile components are secured with silicone based gap filler to enhance mechanical stability for the intended shock and vibration environments. The assembly is completed with laser sealing the lid. A completed assembly is retested to insure proper functionality and is subjected to additional reliability screening as required.
Steady increase in oil price is expected to enable the oil companies to re-open their existing wells for more oil at greater depths. As depth of wells increases, so are the temperatures and length of drill-string. New converters such as the HTA series lays the engineering ground work for future development of power converters to meet this expectation. Materials and manufacturing processes are just as critical as electronic components in new development of electronic instruments and power electronics. It remains to be seen whether or not the increase in demand will be sufficient to lure companies of the supporting industries to develop new technologies to meet the expected increase in demand.
Cooling High-Power Electronic Components in Small Packages
Using vapor chambers can be an efficient way to manage heat in today’s small, yet high-power electronic devices where effective cooling helps ensure long component life and reliability.
Imagine a high-power radar system used by our armed forces in the Iraqi desert. The components are engineered for top performance, but designed to be extremely compact to save space and weight. Failure is not an option for mission-critical devices like this, where hundreds of soldiers are counting on that electronic device to be the eyes and ears of the unit. These electronics must be cooled with reliable and compact systems to keep them operational even in the harshest environments.
As electronic devices become smaller and more powerful, effective cooling becomes very challenging and critical for component life and reliability. The air-cooled heat sinks attached to heat-generating components must have the appropriate surface area and airflow to dissipate the heat. However, surface area alone is not enough with highly concentrated heat sources such as high-power electronic components. As the electronic components get smaller and the heat sink’s base area increases, a large thermal heat spreading penalty is typically found in the base of the heat sink. As the heat travels to the extremities of the heat sink, the cooling fins furthest away from the heat source become ineffective, so no matter how big the heat sink gets, its thermal performance becomes constant and ineffective.
In order to show this spreading phenomenon, a CFD (computation fluid dynamics) simulation was created for a typical high-flux electrical component and its cooling sink (Figure 1). For this example, a 6” X 6” X 1” low-profile aluminum heat sink with a standard extruded pitch was used to cool an electrical component (0.5” X 0.5” source at 100 Watts, 62 Watts/cm2). As shown in Figure 1, the high temperature gradients are evident in the cross-section view. The fins toward the outside edges are quite cool compared to the hot spot directly over the electrical component. The performance of this heat sink with 300 LFM (linear feet per minute) of airflow is 0.46 °C/Watt, which is the temperature rise from ambient to the hottest spot on the component over the total power dissipated.
The challenge now becomes the ability to spread that heat efficiently through the base of the heat sink without changing its existing geometry. Meeting this challenge allows the designer to stay within the same form factor or original package size without a long and costly redesign of the component enclosure. In order to reduce this high thermal resistance, the heat sink metallic base needs to be replaced with a “super” conducting material. In this example, a vapor chamber can be used as the medium to spread heat in the base more efficiently.
Benefits of Vapor Chambers
Vapor chambers are essentially flat or planar heat pipes that use the principles of evaporation and condensation to produce a very high conductivity thermal plane. Vapor chambers are basically evacuated vessels with an internal wick and a working fluid. The wick helps transport the working fluid back to the evaporator surface without the use of any moving parts. Once the fluid evaporates, it travels to the cooler section of the chamber, condenses in the wick and the cycle continues.
Vapor chambers can have a number of different shell materials and working fluid combinations. The selection of these materials depends mainly on the operating temperature of the cooling system. The most common combination in the electronics cooling field is copper and water due to its operating temperature of about 10°C to 250°C, but other liquids and materials can be used for extreme temperature ranges.
Bulk conductivities for vapor chambers have been measured at over 30 times the conductivity of copper, and over 10 times the conductivity of pyrolytic graphite and diamond in the same flat plane configuration. In addition, vapor chambers can be bonded into an existing extrusion or used as the base itself, in which case fins can be soldered directly to it. Vapor chamber sizes can range from as small as 1” X 1” to as large as 13” X 20”. Standard thicknesses range from 3 to 9 mm so they can be easily inserted into existing bases.
In today’s electronic cooling market, vapor chambers are used in various applications. The military uses these high-conductivity heat sinks in cooling radar TWTs (traveling wave tubes), IGBTs (Insulated-gate bipolar transistors), and other high-flux
electronics. The medical industry uses them to warm blood uniformly. A multitude of heat sinks in mid- to high-range computer servers use vapor chamber technology to manage the heat from high-flux, high-computing-power CPUs that define the speed and performance of the system.
To illustrate the thermal performance improvement that a vapor chamber can provide to an all-metallic heat sink, let’s examine the same heat sink described earlier, but with a vapor chamber integrated into the heat sink base. As shown in Figure 2, the heat is spread much more evenly across the entire heat sink, causing a drop in overall thermal resistance of 37 percent. The heat sink shown here with an embedded vapor chamber and all of the parameters held constant exhibits a resistance of only 0.29 °C/watt.
As illustrated, the enhanced performance of the vapor chamber improves the thermal performance of an all-metallic heat sink significantly. The improved thermal performance allows the electronic component designer to easily manage component frequency speed and power increases within the existing architecture, and at the same time, allow for much more computing/transmitting power for new designs in a more compact space. For example, if an electrical component in a given system is reaching its maximum junction temperature at 100 Watts, the vapor chamber can potentially increase the dissipated power to 130 Watts without changing junction temperature. This is a great advantage for devices such as high power mosfets, RF transmitters, and densely packed CPUs.