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Circuit protection units ease transition to new automotive technologies

Posted: 08 Aug 2011 ?? ?Print Version ?Bookmark and Share

Keywords:fuel economy? circuit protection devices? polymeric positive temperature coefficient? overcurrent protection?

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 proposed to enhance thermal management. Nevertheless, designers still rely heavily on secondary protection to help stop thermal runaway events that can be generated by power component failures or corrosion-induced heating.

The most common approach is to use a thermal fuse/Thermal Cut Off (TCO) or a thermal switch. These devices both offer the designer wide and specific temperature activation characteristics in both AC and DC applications, but they present challenges in the board assembly process. Because more and more PCBs utilize only surface mount devices (SMDs), using a through-hole device can translate to special mounting procedures and additional cost and complexity. Additionally, standard devices may not provide the ruggedness and reliability needed for automotive applications; whereas components that are qualified for the automotive environment are fully tested to meet stringent shock and vibration specifications and provide the proper DC ratings.

TE Connectivity recently introduced an automotive-qualified Reflowable Thermal Protection (RTP) device, a secondary protection device that interrupts current and helps protect against damage resulting from failed power components and PCB trace overheating. This solution is touted to generally eliminates the need for heavy heat sinks, redundant FETs and relays in electronic control units, HVAC, ABS, and power steering systems, DC/DC converters and PTC heaters.

As shown in figure 4, when the RTP device is mounted in close proximity to a powerFET it tracks the FET temperature. If the FET exceeds its normal operating temperature limit and generates an overtemperature condition, the RTP device activates and opens the power source line.

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Figure 4: In a slow thermal runaway condition, the RTP200 device tracks the powerFET temperature until it opens the circuit at 200�C.

The surface mount device can be quickly and easily installed using industry-standard pick-and-place and Pb-free reflow equipment, and can withstand multiple reflow passes with peak temperatures exceeding well over 200C and yet, in the field, will open when it detects a temperature above 200C.

To allow it to open at 200C in the field after going through standard reflow installation, the RTP device utilizes a one-time electronic arming process to become thermally sensitive. Before the arming procedure, it can withstand at least three Pb-free solder reflow steps without opening. The arming procedure can be implemented to occur automatically at system power up or during manufacturing end of line system testing.

The connected car
Heading down the "infotainment highway," two clear trends are rapidly changing the face of automotive design: in-car entertainment and wireless connectivity. Vehicle manufacturers, as well as mobile device manufacturers, are keenly aware of consumers' growing demand to connect with their digital devices inside their cars in the same way they do in the home or office.

Today's vehicles have become mobile networks, integrating a wide variety of features and functions, including embedded controls, mobile media, and wireless options. Applications such as infotainment, telematics, safety, and electronic controls require the use of several established networking standards such as LIN, CAN, MOST, IDB-1394, FlexRay, byteflight, Bluetooth for embedded control, and others.

The Bluetooth standard has created new market opportunities for mobile electronic devices that can interact with the vehicle and its systems, such as cellphones, MP3 players and navigation equipment. Protecting exposed communication interfaces from electrostatic discharge (ESD) and other voltage surges induced through the antenna and the instrument's wireless (RF) interface is a critical design issue.

Figure 5 shows how a polymer-enhanced Zener diode (PolyZen device) helps protect the power input to the Bluetooth module. The device provides integrated overcurrent and overvoltage circuit protection. Low-capacitance polymer ESD (PESD) devices are also used here to help protect the antenna against damage caused by ESD surges.

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Figure 5: Typical circuit protection solution for Bluetooth modules.

Down the road
The increased number of power components and high-data-rate connections in automotive designs has created a greater need for reliable, cost-effective circuit protection solutions. Companies, such as TE Connectivity, now have research, design and application engineering teams collaborating with automakers, integrators and OEMs to develop the components required to advance a variety of new technologies, support emerging applications, and help automakers meet stringent safety standards.

- Tony Cilluffo
??Business Development Manager
??TE ConnectivityCircuit Protection Business Unit

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