Monthly Archives: February 2018

Heat Pipes for Electronic Applications

Electronic applications such as mobile, embedded computing, and servers often use intermediate level heat pipes to cool systems dissipating 15-150 Watts. Usually, such heat pipes use copper tubes and sintered copper wicks with water as the working fluid.

System designers incorporate heat pipes when the thermal design has limitations of space and/or weight and other materials such as solid aluminum or copper heat sinks cannot achieve the desired cooling. However, heat pipes for electronic applications require several considerations.

Manufacturers often publish thermal conductivity of heat pipes as ranging from 10,000 to 100,000 W/mK, which is nearly 500 times that of solid aluminum, and 250 times that of solid copper. However, unlike solid metal, the effective thermal conductivity of copper heat pipes varies extensively with their length and other factors affect this figure as well, although to a lesser extent.

For instance, a sample heat pipe could reach its published thermal conductivity of 100,000 W/mK when its length was 100 mm, when transporting heat from a 75 W power source. However, on increasing the length of the same heat pipe to 200 mm, its thermal conductivity fell to less than one-third the published figure.

Moreover, customization for a specific electronic application can severely limit the operational and performance characteristics of a heat pipe. This is much like the type of transformation a vehicle requires when fitting it out for a specific activity such as track racing or off-roading.

An engineer often has to customize the heat pipe when navigating the crowded route from the heat source to the evaporator or condenser. They do this by flattening or bending the heat pipe. For a lower thermal resistance, the designer may have to machine a heat pipe assembly to create a flat surface capable of direct contact with the heat source. This may require elimination of the solid metal base plate and the extra TIM layer. To keep the entire fitment small enough, the heat pipe may have to use a diameter that is different at one end from the other.

Drastic changes to customize the heat pipe to the electronic application often results in the heat pipe unable to meet the power handling requirements. To make it suit the application, the engineer may have to make changes to the internal structure of the heat pipe design.

This often requires changing the porosity and thickness of the wick, allowing the engineer to tune the heat pipe to meet specific operating parameters and performance characteristics. For instance, the heat pipe may have to operate at higher power loads or against gravity, requiring a larger pore radius or an increase in the capillary pressure of the wick.

However, a larger pore radius works against gravity, and therefore, in place of increasing the pore radius, increasing the wick thickness may make it more effective. Alternately, the engineer may increase both the wick thickness and porosity along the length of the tube. Some suppliers specialize in custom heat pipes and use unique mandrels and/or custom formulated copper powder to fabricate them. The outward physical characteristics also affect the performance of a heat pipe.

Using the Raspberry Pi to Secure IoT

The popular single board computer, the Raspberry Pi (RBPi), can effectively secure systems that traditional protection mechanisms often cannot. Industrial control system networks and Internet of Things fall under this category. You can use the RBPi2B and later models as an adequate medium for running the various security tools.

For this project, you need a Micro SD card of at least 8 GB size, and the bigger it is the better, as you can use the extra space to store a longer log data history, for instance, for logging data from Bro IDS. A case for the RBPi is preferable, and you can use one suitable to your individual taste and style. Although optional, a small form factor wireless keyboard is more helpful to configure the device on the fly, rather than using a full size keyboard.

Once you have configured the RBPi for networking, enable SSH and allow configurations from an SSH client. The hardware you will need includes an RBPi2B or later, an 8+ GB Micro SD card, a case for the RBPi, a Micro USB power cord, and an optional mini wireless keyboard.

Use the RBPi website to follow their getting started guide and install the Raspbian operating system using the New Out of the Box Software (NOOBS). Those familiar with the installation system can also use the traditional method of installing the Raspbian OS directly without the NOOBS, and it should work fine. Other OS distributions for the RBPi may also work, but you will need to try them out.

As the RBPi security solution places great reliance on lightweight open-source software, and the device monitors all traffic, you need to install software that inspects the traffic to learn what is going on. This requires installation on an Intrusion Detection System or IDS. Among the several free products available in the market, the one most suitable for the RBPi is the Bro IDS. The Bro inspects traffic at all OSI layers, and adds additional scripting that increases attack detection.

Bro IDS has some prerequisites before it can install on the RBPi. Install the prerequisites via apt-get, and after completing, download the latest source code for the Bro. Now, setup the environment to build, and to install the build—use configure, make, and make install. This allows you to manually control Bro, or use Broccoli to control it automatically.

Although the Bro IDS comes with an extensive signature base that can detect a number of common attacks, you can enhance its signature with Threat Intelligence. Another advantage in using the Bro IDS is the availability of Critical Stack, and you can integrate the threat intelligence with the Bro.

You can use Critical Stack, a threat intelligence feed, as a free aggregator. It functions as a simple point-n-click integration as it pulls data, such as addresses for Tor Exit Mode IP, known phishing domains and/or other malicious IPs. After pulling the data for threat intelligence, the Critical Stack agent formats it into a scripting language that Bro understands. The Bro IDS can pick up the new script automatically.

Tuning an IoT MEMS Switch

Menlo Microsystems, a startup from GE, is making a MEMS-based switch fit into a broad array of systems related to Internet of Things (IoT). Already incorporated into medical systems of GE, they can tune the chip to act as a relay and power actuator for several types of industrial IoT uses, including using it as an RF switch suitable for mobile systems.

Menlo first described their electrostatic switch in 2014. They have designed it with unique metal alloys deposited on a substrate of glass. The arrangement creates a beam that a gate can pull down, making it complete a contact and allow current to flow. Compared to a solid-state switch, this electrostatic switch requires significantly less power to activate and to keep it on. This single proprietary process creates products for several vertical markets.

The low power consumption of the device allows it to handle high currents and power switching. Unlike traditional switches, the MEMS switch does not generate heat, and therefore does not require large, expensive heat sinks to keep cool.

Currently, a tiny research fab run by GE is making the switch. Menlo expects to produce it in larger quantities in mid-2018, through Silex Microsystems, a commercial fab in Sweden. According to Russ Garcia, CEO of Menlo, their biggest challenge is to get the technology qualified in a fab producing commercial items.

The device has huge opportunities as it can replace a wide variety of electromechanical and electromagnetic power switches and solid-state relays. Menlo is planning to roll out several varieties of reference boards incorporating its MEMS chips, which will be helpful in home and building automation, robotics, and industrial automation.

For instance, IoT devices such as the smart thermostat from Nest face an issue of efficiently turning on or off high power systems such as HVACs. According to Garcia, the Menlo switch can do this while drawing almost zero current. Additionally, the Menlo switch offers a two-order reduction in the size of power switches and their power consumption.

It took a 12-year research effort by GE to incubate the design of the MEMS switches. They discovered that reliability issues were related to materials MEMS used, and overcame the issues with alternate unique metal alloys for the beams and contacts of the switch including generating a novel glass substrate. This combination allows billions of on/off switches to handle kilowatts of power reliably.

The medical division of GE will be among the first users of the chip. They will use the chip to replace a complex array of pin diodes in their MRI systems. This replacement by MEMS switches can knock off $10,000 from the cost of each MRI system. This includes the payment to five PhDs who presently tune each of the machines with pin diodes. The new MEMS switch will allow an automatic programming of the system.

Although GE will be an exclusive user for the chips in their MRI systems, Menlo is discussing future uses of the chip with other MRI makers as well. According to Garcia, GE wants to create a new strategic component supplier for the chips. Menlo is also planning to use the chips for RF switches.

Thermal Protection Prevents SSR Failure

Solid State Relays (SSR) are replacing conventional electromagnetic relays for load control applications in the industry, as they hold several advantages over the latter. However, SSRs often face overheating causing them to fail. Newer designs now come with integrated thermal protection that improves longevity, efficiency, and system safety by preventing overheating and failure of SSRs.

Machinery driven by large motors requires a system to switch off the power supply to the motor on sensing higher than normal heat, thereby preventing expensive damage. Usually, this is accomplished by an electrical relay accomplishes this by interrupting the power supply to the motor. Presently, the industry uses two main types of electrical relays for the purpose—an electromagnetic relay (EMR) or a solid-state relay (SSR). Although EMRs are the tried and trusted solution for load circuit management, SSRs are now making successful inroads into their market share.

One of the major drawbacks of EMRs is their limited life span, and their susceptibility to external influences such as shock, vibration, and magnetic noise, among others. This causes wear and reduces the life cycle. On the other hand, the all-solid-state construction of the SSR, without any moving parts, makes them highly tolerant of external disturbances. As there is no wear to reduce accuracy, SSRs enjoy longer life cycles and offer predictable operation. For instance, while an EMR may work reliably for hundreds of thousands of cycles, an SSR continues to perform satisfactorily even after five million cycles of operation.

SSRs carry a several-fold entry price hike over their similarly rated electromechanical counterparts, which are priced considerably lower. Therefore, unless the application demands exclusive seclusion from positioning, vibration, shock, and/or magnetic interference, using an EMR is often more economical. SSRs are more suited to harsh operating environments, and their longer lifespan soon provides their return on investment.

Unlike EMRs, SSRs generate heat when conducting current. Unless managed by a thermal component, overheating can damage an SSR, resulting in an outage of the manufacturing system or assembly line, leading to expensive repair expenses.

To address the challenge of overheating, designers now integrate a thermostat within the SSR. This prevents the device from overheating and ensures the relay always operates within its safe operating area (SOA). Furthermore, it protects the operation of the system and components from potential outages and/or damage.

The user can set the maximum operating temperature depending on the application. If the internal temperature of the SSR crosses the set threshold, the integrated thermostat embedded within cuts off power to the input circuit. The internal power-switching device mounts a metal plate, whose temperature the thermostat constantly monitors. If the temperature of the metal plate exceeds the normal range, the power-switching device signals the SSR to turn off the power.

By providing a trip during overheating conditions, the built-in thermal protection ensures   near-absolute equipment damage. This translates into reduced maintenance expenses and production downtimes. The user can choose to turn on power automatically when the temperature has returned to normal, or opt for an inspection before switching on the power manually. The second option helps to troubleshoot design issues in the system.