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Posted on 02 July 2019

Circuit Protection Solutions Address Emerging Market Trends

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RTP device helps meet the reliability requirements of automotive power electronics systems

Electronic components manufacturers are perpetually challenged to keep pace with the evolution of the industry. In particular, the growth and density of electronic content in automotive, industrial, consumer electronics and telecommunications applications has been dizzying.

By Faraz Hasan, Global Strategic Marketing / Business Development Manager, Appliance / Industrial / Lighting, Tyco Electronics Circuit Protection Devices (CPD)

 

This article describes how the circuit protection business unit of Tyco Electronics (TE) is addressing some of the rapid changes in these markets. The company’s approach has allowed it to make customized products available for the development of emerging technologies, and deliver innovative, cost-effective solutions for existing technology.

Existing Materials and Expertise

Since its inception 30 years ago as Raychem Circuit Protection, TE has emphasized collaboration with electronics equipment designers coupled with forward-looking research and development of advanced materials. This approach has expanded the company’s initial breakthroughs in PPTC (polymeric positive temperature coefficient) technology into an ever-wider range of industries and applications. TE has also advanced the reach of circuit protection technology by integrating polymeric materials with other protection technologies, such as metal oxide varistors and Zener diodes to provide coordinated overcurrent/overvoltage protection. Figure 1 illustrates some significant product advancements that specifically address the evolving industry requirements.

TE technology breakthroughs from 1980 to 2010

New Approach to Li-Ion Battery Protection

The portable battery market is evolving from traditional low-power portable applications to much higher power applications. This trend is seen in lithium ion (Li-ion) battery applications for power portables, such as power tools, motive power (e.g., electric bikes) or standby power (e.g., solar panel back up systems). While Li-ion batteries can be more powerful, lighter and more environmentally friendly, they require more rigorous safety designs than nickel-cadmium (NiCd) batteries, and emerging safety standards that address Li-ion battery designs for high power applications will require new levels of protection.

Currently there are few protection solutions for high-rate-discharge battery applications at ratings above 30VDC/30A, and many of the conventional circuit protection techniques are large, complex and/or expensive. One approach can be to use a combination of ICs and MOSFETS. Another design technique may use a conventional bimetal protector in DC power applications requiring 30A+ hold current. However, the contacts must be large enough to handle the high current. Additionally, the number of switching cycles must be limited due to contact damage that may result from arcing between the contacts.

In November 2010, TE introduced the Metal Hybrid PPTC (MHP). The MHP device addresses the need for a cost-effective circuit protection device that can replace or help reduce the number of discharge FETs and accompanying heat sinks used in complex IC/FET battery protection designs. Essentially, the device offers spacereduction, cost-reduction and protectionenhancement benefits for emerging highrate- discharge Li-ion battery pack applications.

This new hybrid device connects a bimetal protector in parallel with a PPTC device, providing resettable overcurrent protection while utilizing the low resistance of the PPTC element to help prevent arcing in the bimetal protector at higher currents.

As shown in Figure 2, because contact resistance is very low during normal operation, most of the current goes through the bimetal. When an abnormal event occurs, such as a rotor lock, higher current is generated in the circuit, causing the bimetal contact to open and its contact resistance to increase.

MHP device principle of operation

If the contact resistance is higher than the PPTC device’s resistance most of the current goes to the PPTC device and no — or less — current remains on the contact, therefore preventing arcing between the contacts. When current shunts to the PPTC device, its resistance rapidly increases to a level much higher than the contact resistance and the PPTC heats up. After the contact opens, the PPTC device begins to heat the bimetal and latches it until the overcurrent event ends or the power is turned off.

Addressing New Automotive Power System Trends

The automotive power electronics market has grown quickly, with comfort and active safety features becoming more common. Some conventional mechanical functions such as power steering and electronic parking systems are migrating to electronic applications.

In parallel, the communications market is evolving. User demand for constant-connectivity is leading to the increasing density of IT server farms and telecom centers around the world, along with higher power machines and denser PCBs. These market trends place greater demands on power electronic systems, resulting in the potential for serious thermal issues when power components, such as powerFETs, capacitors, resistors or ICs fail due to harsh environmental conditions. This is also a concern in the industrial market, which is evolving and expanding globally.

In response to these trends, TE introduced the Reflowable Thermal Protection (RTP) device, also in November 2010. Developed in collaboration with automotive power electronics designers, this secondary protection device addresses the need for more robust thermal protection in automotive electronics systems. It also addresses applications such as telecom power systems and high-end test equipment.

This first-of-a-kind device helps prevent thermal runaway events that can be generated by multiple factors, including power component failures or corrosion-induced heating. The surface-mount device will open in the event that it achieves its critical temperature set at 200°C or below, remaining at higher than normal operating temperatures but lower than lead (Pb)-free solder melting levels.

Automotive powerFETs have been shown to be more prone to fatigue and failure than devices that are installed in less demanding applications. Although 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 Printed Circuit Board (PCB). As little as 10W may generate a localized hot spot of more than 180ºC, well above the typical PCB’s glass transition temperature of 135ºC, damaging the board’s epoxy structure and leading to a thermal event. Figure 3 describes a scenario where a failed power- FET may not generate a hard short overcurrent condition but instead a resistive short, producing potentially 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.

PowerFET failure in resistive mode can lead to overtemperature conditions

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 overheating condition, the RTP device activates and opens the power source line.

In a slow thermal runaway condition, the RTP200 device tracks the powerFET temperature until it opens the circuit at 200ºC

The RTP device helps meet the reliability requirements of automotive power electronics systems such as cooling fan applications, as well as ABS, power steering, PTC heaters, etc. The RTP200 device’s 200°C open temperature helps prevent false activations and improves system reliability since it is a value above the normal operating window of most normally functioning electronics, but below the melting point of typical Pb-free solders. As a result, the RTP200 device will not open if surrounding components are operating in their target temperature range, but it will open before a neighboring component de-solders and creates the potential risk of additional short circuits.

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 200°C and yet, in the field, will open when it detects temperatures above 200°C. (The 200°C value is the opening temperature of the RTP200 device, the first device introduced in a product family. Additional temperature options are scheduled for future releases.)

To allow it to open at 200°C 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.

Continuing Innovation

The new MHP and RTP technologies demonstrate the company’s commitment to providing leading-edge solutions to electronics equipment manufacturers. These industry-first technologies also affirm how long-term investment in material development and collaboration with customers have allowed TE to push existing performance levels into new, smaller, and more convenient packages.

These circuit protection devices are the initial offerings of two new product families. Product line extensions for the MHP and RTP devices are now being developed to target a broader spectrum of application requirements; such as an MHP device for 400V/60A applications, and various resistance or opening temperatures for RTP devices. 

 

 

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