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What’s the best technique for temperature management of sensitive components in extreme environments?

Fri, 11/22/2013 - 4:33pm

What’s the best technique for temperature management of sensitive components in extreme environments?

Matt Cuhadar, application and sales engineer, Ametherm

Thermistors are very useful in a variety of temperature management applications, including sensing, controlling, compensating, and protection. Temperature sensing is commonly achieved with probes utilizing a negative temperature coefficient (NTC) chip or a disc thermistor. A voltage divider or Wheatstone bridge is often used to measure varying VOUT due to temperature, and with the aid of an analog-to-digital converter (ADC), the continuous V value is converted to a corresponding digital number that represents temperature. To control the temperature of a circuit board, fans can be utilized that rely on measured temperature from thermistors for their speed control system.

Temperature compensating is essential for solenoids, as the metals used to manufacture coils — such as copper or aluminum — exhibit a positive temperature coefficient (PTC). NTC thermistors are very effective in compensating for this increase in resistance, as the NTC range of -0.29 %/°C to -0.51 %/°C is a great match to copper's temperature coefficient of 0.50 %/°C or aluminum's 0.26 %/°C.

Temperature sensing, controlling, and compensating with thermistors all serve to protect sensitive components. In addition, PTCs (SMD or leaded form) are used to provide protection from overcurrent.

Advantages of Thermistors in Temperature Management:
• High temperature coefficients (compared to other options for temperature sensing) allow precise measurement
• Ease of use
• Cost-effective
• Customizable

The non-linearity of R/T (resistance vs. temperature) curves could cause difficulty when measuring a wide temperature range. Thermistor R/T curves are most linear between 0 °C and 70 °, their most effective range, and Beta values are often presented at these temperatures. A voltage divider is used to linearize the thermistor output with the help of a fixed resistor. In addition,  a more accurate ADC can be used.

Thermal time constant and dissipation factor are key values for thermistors in determining potential issues, such as "self-heating” or predicting cycle times.

Wilson Lee, Director of product marketing at Newark element14

Engineers may underestimate the importance of thermal management, but it is critical to utilize techniques at the production level that reflect the real environment in which their build or components may exist. In my experience, I have found engineers make two common mistakes during temperature management.

The first mistake is that engineers rely strictly on the product specification sheet, which tests components in a lab set up, without a true frame of reference for how the part operates or its functionality within a larger design  environment. There are nuances to the design process that the spec sheet won’t consider: forced air cooling for an out-of-bound temperature range, power source reliability and quality/cleanliness of the operating environment, to name a few. Instances where a design engineer is designing components for use in another part of the world can pose challenges too, as they are sometimes not able to sufficiently simulate the in-use environment. Disregarding the real operating environment in which components are used can result in thermal failure.

A second common thermal management mistake, particularly in extended temperature designs, is that engineers expect full performance characteristics across wide temperature ranges. This leads to over compensation or over driving of the devices, which often leads to component failure. In those situations, it is important to take into consideration performance drop offs and to build backup and redundancy measures. Also, it is important integrate other mechanisms into the cooling process, rather than relying solely on forced air cooling. There are several reliable options available: temperature sensors integrated into the board itself; and thermal padding to isolate “hot spots” on the board.

As board space continues to shrink, engineers are further challenged in how to incorporate design elements that manage temperature. However, in any extreme environment where thermal management is not well thought-out, new designs are more likely to fail and experience catastrophic (immediate) rather than soft failures.

John Gammel, Senior Staff Applications Engineer, Sensor Products, Silicon Labs

When it comes to environmental monitoring, most designers focus on temperature sensing for which there are a multitude of options. Few designers consider the benefits of including relative humidity (RH) sensing in their systems, which can be added to temperature sensing solutions without compromising board space and at very little additional cost.

There are numerous cases in which sensing RH can be beneficial:
• A telecommunications equipment cabinet requiring climate control for thermal management. Increasing RH due to higher cooling coil temperature can indicate air conditioning problems ahead of rising temperature, giving an early indication of potential system failure before electronics are damaged or an outage occurs.
• Outdoor electronics that have a weatherproof enclosure that vents to the outside. Generally, in this kind of cabinet, RH levels will be low even if outside humidity is high due to heat generated in the cabinet. However, if water accumulates in the cabinet, this will not be the case. High RH in this type of cabinet is a good indicator of liquid water, which could lead to corrosion.
• An electronic system used in a rugged application with a watertight enclosure. A small amount of desiccant in the enclosure should keep RH low. RH measurement provides a measure of the integrity of the seal.

Sensing humidity is no longer a complex or board space-consuming task. In fact, highly integrated CMOS-based, single-chip RH and temperature sensors are now available in small surface mount packages that operate on a single I2C interface.  Additionally, many RH sensing devices are factory calibrated, which means there is no added system test cost. RH sensors are also available with optional factory-installed covers to protect the devices from dust and liquids and to ensure reliable RH and temperature measurement needed to safeguard sensitive electronics in extreme environments.

Gary Steiger, Operations Manager  –  STEGO, Inc.

The first consideration should be the type of enclosure used to house the components. The minimum rating should be NEMA 3, though NEMA 4X is often considered for greater protection. While enclosures provide immediate physical protection for electronics, unless insulated, they provide very little in regards to extreme temperatures. Therefore, thermal management systems are recommended and must be engineered for each application, based on numerous factors.

When designing a system for extremely low temperatures, the starting point is determining the size and type of heater to be used. The required heating power can be calculated by taking into account the lowest possible ambient temperature, the desired interior temperature, the size and material of the enclosure, and how it is mounted. Once the properly sized heater is chosen, whether it be a convection heater or a fan heater, it must be controlled with an appropriate regulator. From simple bi-metal switches to sophisticated electronic thermostats, these devices are fundamental in maintaining the desired temperature within the enclosure.

Extreme heat is also a concern for sensitive components. There are a variety of cooling devices that offer the appropriate air flow protection, including filter fans and fan trays, typically regulated by thermostats or other control devices. Newer innovations such as multi-directional air flow nozzles provide precise cooling of heat sources and the prevention of heat pocket formation. Each of these solutions relies on existing ambient temperatures, either inside or outside the enclosure. In the case of extremely high temperatures, the use of air conditioners or heat exchangers should be considered.

Options like these allow the appropriate, responsive solution to be selected for each unique environmental challenge. And with constant innovation, today’s heating and cooling devices are becoming extremely energy efficient, even in harsher environments.

Ramesh Khanna, Texas Instruments

In harsh environments where temperatures can be as high as 210 degrees C, it is critical to have proper thermal management and thermal protection.  You cannot overestimate the importance of sensing system temperature as it’s a critical element that protects the system. There are various approaches commonly used to protect the system including:

1) Resistance temperature detector (RTD): Semiconductor IC sensors available in digital as well as analog output have a linear increase in resistance as temperature rises and are is commonly constructed of platinum;
2) Negative Temperature Coefficient Thermistor (NTC): Best suited for precision temperature measurements. The resistance of the material is linearly proportional to the temperature;
3) Positive Temperature Coefficient Thermistor (PTC): Used in conjunction with an op amp PTCs work as temperature monitor. PTCs are best suited for switching applications, where the resistance rises suddenly at a critical temperature; or
4) Platinum resistance temperature detectors (PRTD): Very stable temperature sensors that are not affected by corrosion or oxidation. PRTDs are a resistive device and require an excitation current, or a constant current drive. Voltage is then read across its terminals. Using the Kelvin connection force leads provide constant current drive to PRTD.  Constant current introduces voltage drop in the forced leads.  In order to avoid the drop in forced leads as part of measurements, sense leads connected across the PRTD are connected to the instrumentation amplifier configuration using OPA211-HT. This configuration ensures higher accuracy by removing the drop-in force leads, as very little current flows in the sense leads.

Each approach has its pros and cons depending upon the accuracy required for the application. And, mounting the thermal sensor at the appropriate location in the system is critical no matter what type of sensor is selected for the application.

Jacqueline Leff, Global Product Marketing Manager for Honeywell Sensing and Control

In medical applications such as CPAP machines, ventilators and other respiratory applications such as sleep apnea and anesthesia machines, it is important to precisely manage temperature while at the same time monitor and manage humidity levels. In the past, this was accomplished by mounting two different sensors on a single board as well as all the required electronics for them, and then developing packaging around the unit. However, by combining the temperature sensor and the humidity sensor into a single package, it can enable design engineers to specify a single, pre-certified component that includes all the required intelligence, processing and I/O’s (inputs/outputs).

Combining the two sensors, as well as all the associated connections, processing and intelligence, makes it simpler to design end-use products. The ultra-compact packaging allows for flexibility, occupies less space on the PCB (printed circuit board) and typically simplifies placement on crowded PCBs and in small devices.

Also, combining the components into a single module make it easier for OEMs to develop next generation products using the same sensor elements without having to redesign or re-certify the sensing section.  In addition, the sensor’s multilayer construction provides resistance to most application hazards such as dust, dirt, oils and common environmental chemicals. Hydrophobic filter and condensation-resistance features can allow for use in a wide range of challenging condensing environments.

Stable performance is very important as well – maybe the most important factor of all. Long-term stability minimizes system performance issues and helps support system uptime by eliminating the need to service or replace the sensor during its application life, and removing the requirement to regularly recalibrate the sensor in the application, which can be inconvenient and costly.

Steve Jackson, Business Development Manager, Thermal Management at Sapa Extrusions North America

Thermal management solutions of sensitive components in extreme environments are a multi-layered challenge.  It is necessary to solve two separate problems – the need to remove the heat from the component, as well as the need to protect the component from the environment.  Failure at either challenge is not an option.

First, consider the environment.  For example, desert or moist conditions require protection of the components from sand, dirt or water. The best option is to design the enclosure as both the heat sink, and as the “safe environment.”
Sealed aluminum extrusions are often the best option.  Ideally, a hollow enclosure is extruded with thermal management fins on the outside of the enclosure.  It is possible to provide a 12” x 12” enclosure in this manner.  If a larger enclosure is necessary, the extrusion engineers can design various methods of joining the extrusions to make larger enclosures.  If moisture is not a concern, then a “snap-fit” design is the most cost-effective solution.  If moisture is a concern, then Friction Stir Welding (FSW) is Sapa’s method of choice.  This allows for an enclosure up to six feet wide and 30 feet long.

Once the sensitive components are installed in the enclosure, there are various types of end caps that can be utilized. Typically, screw-bosses are part of the enclosure design and are in place via the extrusion process at no additional cost. The end caps can be attached to the end of the enclosure by self-tapping screws.

However, if moisture is a concern and/or the enclosure will need to be opened, it is possible to machine a gasket space into the end of the enclosure, as well as “drilling and tapping” holes into the end of the enclosure profile.  In this method the enclosure is both water-tight and can be opened repeatedly.

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