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Hot Topic: Prevent Thermal Runaway Damage in Power Electronic Systems

Thu, 08/04/2011 - 12:45pm
Matt Williams, TE Circuit Protection, www.te.com
The continued growth of power electronics in our homes, offices and vehicles is fueling a trend toward new materials and more efficient power components. High-power, high-temperature applications place greater demands on power electronic systems, resulting in the potential for serious thermal issues when components fail due to long-term exposure to harsh environments. As a result, most industrial and consumer equipment now incorporates thermal protection devices to improve reliability and safety.

When designing for thermal management, the heat generated by resistive and inductive loads, power capacitors, current drivers, switches, relays and MOSFETS present engineers with significant challenges. These heat-generating components are found in applications such as switch mode power supplies (SMPSs), high-voltage power supplies and switching applications for train traction motors and hybrid vehicles.

Improving power component performance, using design techniques that spread heat more evenly, and incorporating new heat sink materials are some of the solutions that have been utilized to enhance thermal management. Nevertheless, many designers currently rely on secondary protection to help prevent damage caused by thermal runaway events that may occur as a result of component failures or corrosion-induced heating.

There are a number of innovative technologies that have been designed to assist in protecting the application and end-user from catastrophic thermal events by interrupting electrical current flow when a power component is heated to its specific rated trip temperature. The most common approach is to use a thermal fuse/Thermal Cut Off (TCO) or a thermal switch. These devices offer the designer wide and specific temperature activation characteristics in both AC and DC applications. Available in bolt-in types, clip-on, pig-tail and lead-type styles these shapes can present complexities in both design and manufacturing processes, and require careful handling procedures in order to guarantee that the protection device is not damaged during assembly.

Because more and more printed circuit boards (PCBs) utilize only surface mount device (SMD) components, using a through-hole device can translate to special mounting procedures and higher cost. Additionally, standard devices may not provide the ruggedness and reliability needed for industrial applications; whereas components that are qualified for the automotive and industrial environments are fully tested to meet stringent shock and vibration specifications and to provide the proper DC ratings.

A new surface-mount component, the Reflowable Thermal Protection (RTP) device helps prevent thermal damage caused by failed power electronics. The device helps protect against damage caused by resistive shorts which may produce undesirably high temperatures through I2R heating, as well as hard-short overcurrent conditions. The device can be installed using standard lead (Pb)-free reflow processes and can be used to replace redundant powerFETs, relays and heavy heat sinks typically used in automotive and industrial designs.

Secondary Protection for powerFETS

Although powerFETs are increasingly robust, they are prone to failures which can occur very quickly if their ratings are exceeded. If the maximum operating voltage of a powerFET is exceeded, it goes into avalanche breakdown. If the energy contained in the transient overvoltage is above the rated avalanche energy level, the device will fail; causing a destructive thermal event that may result in smoking, flame or desoldering.

Automotive and industrial powerFETs have been shown to be more prone to fatigue and failure than devices that are installed in less demanding applications. When comparing powerFET failure rates over time, devices used in harsh environments exhibit greater parts-per-million failure rates. After five years in the field, the difference can be more than a factor of ten.

thermal runaway prevention Figure 1Although a powerFET may pass initial testing, it has been demonstrated that, given certain conditions, random weak points in the device can result in field failure. Even in situations where the powerFET is functioning within specified operating conditions, random and unpredictable resistive shorts at varying resistance values have been reported.

The resistive mode failure is of particular concern, not only for the powerFET but for the PCB. As little as 10 W may generate a localized hot spot of more than 180°C, well above the typical PCB’s glass transition temperature of 135°C, which can lead to damage of the board’s epoxy structure as well as a thermal event.

Figure 1 describes a scenario where a failed powerFET may not generate a hard short overcurrent condition but rather a resistive short, therefore producing unsafe temperatures through I2R heating. In this case the resulting current may not be high enough to blow a standard fuse and stop thermal runaway on the PCB.
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prevent thermal runaway Fig2If a power component failure or a board defect generates unsafe overtemperature conditions a secondary protection device can be used to interrupt the current and help prevent a thermal runaway condition. As shown in Figure 2, when the RTP device is placed in series on the power line in close proximity to the FET, it tracks the FET temperature and opens the circuit before a slow thermal runaway condition can generate an undesirable thermal condition on the board. 

 How It Works

To allow it to open at 200°C in the field, the RTP device utilizes a one-time electronic arming procedure to become thermally sensitive. Before arming, it can withstand three Pb-free solder reflow steps without opening. Timing of electronic arming is user-determined, and can be implemented to occur automatically at system power up or during system testing.

The RTP device’s 200°C open temperature helps prevent false activations and improves system reliability since 200°C is a value above the normal operating window of most normally functioning electronics — yet is below the melting point of typical Pb-free solders. As a result, the device will not open if surrounding components are operating in their target temperature range, but it will open before a component de-solders and creates the potential risk of additional short circuits.

prevent thermal runawaySummary

The RTP device can help designers reduce component count, provide a safe and reliable product, comply with regulatory agency requirements, and reduce warranty and repair costs. Its SMD packaging allows it to be quickly and easily installed using industry-standard pick-and-place and Pb-free reflow equipment. As with any circuit protection scheme, the effectiveness of a solution will depend on the individual layout, board type, specific components, and unique design considerations.


Author’s Biography

Matt Williams is Global Applications Engineering Manager for TE Circuit Protection, and is responsible for applications support for in-house and OEM customers. He has more than 25 years experience in applications engineering and semiconductor design within the digital, analog and RF communications disciplines. He earned his two B.S. degrees in electrical engineering and electronic communications from Phoenix Institute of Technology. 
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