Design Considerations for Implementing Circuit Protection Devices in PC Designs
By John Halpin, Marketing Engineer, PTC Products
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.
Figure 1. Graph of PTC performance with increasing heat caused by overcurrent.
It is obvious from a description of the PPTCs functional characteristics, why it has become the device of
choice for overcurrent protection on USB ports. Now we should look a little further into the electrical
response of a PPTC based on the electrical characteristics of a USB port.
USB Circuit Designs with PTC Overcurrent Protection
USB ports are configured as either self-powered or bus-powered. A self-powered USB hub must supply
current up to 500 mA on all of its ports. The self-powered hub does not draw power from the USB stream
but may utilize up to 100 mA from upstream devices or hubs to make functionality possible when it is
powered down. Bus-powered hubs can draw up to 500 mA from an upstream self-powered connection.
Typically, current of 100 mA is available for functions and processors internal to the hub. External ports in
a bus-powered hub can supply up to 100 mA per port, with a maximum of four ports per hub.
Voltage-drop calculations for several applications of port protection in USB circuit designs are presented in
Table I (Note: Device resistances reflect maximum on-board resistance of Littelfuse 1812L150PRT and
1812L260PRT PPTC devices). The calculations demonstrate the effect of several PPTC devices available
to provide overcurrent protection. In the design of these ports, consideration must be given to ensuring that
the voltage drop does not fall below the minimum of 4.75 V for a self-powered hub port, or 4.40 V for a
bus-powered hub port. The upstream voltage supplied to a bus-powered hub is 4.75 V.
Figure 2: Self-powered Hub Individual Figure 3: Self Powered Hub Multiple Port
Port Protection Protection
Self-powered hub (individual port, see Figure 2):
Power supply 5.000 V
Trace
20 m! x 0.5 A
= 0.010 V
Ferrite bead
5 m! x 0.5 A
= 0.003 V