Design Considerations for Implementing Circuit Protection Devices in PC Designs

PublishTime：2020-11-16

Despite the recent slowing of personal computer sales in North America and Europe, Worldwide sales have remained relatively consistent due to increased sales in Asia and India. According to a Dataquest projection of PC usage, the number of systems worldwide will exceed 150 million units in 2002 and shipment of desktop and mobile systems will grow at 16.1% compound annual growth rate (CAGR). The constant demand for personal computers requires that developers and OEMs continue to improve on designs in order to remain competitive. One strategy that has proven to be effective in securing market share is reducing the size of the PC while improving the systems capabilities. As a result, the average system is capable of performing numerous tasks simultaneously without effecting the performance of the system. Designers have increased the number of external interfaces so users can have the luxury of utilizing more peripheral devices without having to disconnect one device to use another. “Plug & Play” interfaces such as USB, Fire Wire (IEEE 1394) and SCSI formats have become more and more common on PCs. One only has to look at the changes that have been implemented on systems over the past year to see that this is the case. The Intel PC 1999 and PC 2000 Design Guide mandates that two USB ports be available on every PC’s system board with most newer designs including up to eight. So what does this mean for component manufacturers of circuit protection devices? Simply enough, more opportunities for circuit protection manufacturers who are seeing an increase in demand for their products. In addition to an increase in demand for devices to protect interface ports, component manufacturers are faced with the responsibility of developing smaller components to meet the space requirement of the newest computer designs. For example, let us look at circuit protection of a standard USB port from a design engineer’s point of view. Using the Microsoft Windows Hardware Design Guide as an example, it promotes that a USB lines provide suitable voltage and current protection without the need for replacement of suppression devices every time that an overcurrent or overvoltage condition occurs. Therefore, when selecting suppression devices, a component that is self-resetting is ideal in these conditions. The Polymer Positive Temperature Coefficient device (PPTC) has gained much popularity as an overcurrent-limiting device in USB applications because of these properties. The PPTC device typically consists of a conducting polymer layer that separates two or more electrodes. PPTCs are rated similar to standard fuses that are rated in relation to a circuit’s typical operating current. When this rated current is exceeded, the polymer layer will begin to heat. The polymer material will then begin to transition from a solid to a liquid state. As the polymer material expands, conductive layers within the polymer begin to break causing the device to shift from a low resistance state to a high resistance state (Figure 1). The resulting shift in the resistance of the device results in reducing nearly all of the current through the device. After a fault condition is removed, the polymer begins to contract and cool. During the cooling process the conducting chains come back into contact with each other restoring normal current flow through the device.
It is obvious from a description of the PPTCs functional characteristics,why ithas become the device ofchoice for overcurrent protection onUSB ports. Now we should look a little further into the electrical response of a PPTC based on the electrical characteristics of a USB port.

Brand of Product：	Littelfuse
Part#：
Product Category：	Circuit Protection Devices More
Application：	More
Data Type：	Application note & Design Guide More
Safety & Environment Standards：	More
Operating Temperature：	Please see the document for details
Qualification Grade：	More
Qualification Approval：	More
Package：
Language：	English Chinese Chinese and English Japanese
Date of Creation：	2018/07/11
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Data Size：	535 KB

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