PolySwitch Device Enhances Overcurrent Protection

Apr 1, 2006 12:00 PM
By Khagendra Thapa, Systems Engineer, Zetex Semiconductors, U.K.; and Chris White, UK and Ireland Sa

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Where resettable overcurrent protection is a requirement, PolySwitch polymeric positive temperature coefficient (PPTC) devices offer a highly effective solution. A thermally activated device, it resets automatically and resists nuisance tripping.

Once a fault condition is removed and the PolySwitch device temperature drops, the device returns to a low-resistance value, restoring normal circuit operation. If a short duration event occurs — for example, inrush current into an inductive load — the PolySwitch device will not trip.

However, in certain systems an unexpected shutdown can result in lost or corrupted data. Therefore, the ability to provide early warning of an impending overload, in addition to monitoring the level of current flowing into the load, would be a valuable feature. While the use of only a PolySwitch device in its non-trip state can provide a rough estimation of current, both the monitoring and overload warning functions for the load current can be fully achieved by combining the PolySwitch device with a simple current monitor IC.

As shown in Fig. 1, the resistance of the PolySwitch device is “flat” in its hold-current range. Hold current (I_H) is the maximum current the device will pass without interruption at 20°C in still-air conditions. As the device reaches its definite trip current (I_T), it will change from a low-resistance state to a high-resistance state. The value of I_T is generally twice the value of I_H.

Because the resistance of the PolySwitch device is relatively constant up to I_H, the voltage developed across the device rises “linearly” as the load current increases. This is assuming the device ambient temperature is constant. These properties of the PolySwitch make it possible to track the load current throughout the PolySwitch device's hold-current range prior to tripping. If the PolySwitch device trips and changes to a high-impedance state, the full supply voltage driving the load will then develop across the device.

Fig. 2 shows how a PolySwitch device can be used in conjunction with a ZXCT1030 current monitor to provide an early warning that an overload current or a current surge is occurring. The ZXCT1030 has an internal comparator and voltage reference. The PolySwitch device is placed in series with the supply rail and load. The device is selected on the basis of its I_H so that it will remain in a low-impedance state for a required amount of load current flow.

From the data sheet, the maximum operating (non-trip state) resistance value of a PolySwitch device is defined by R_1MAX. At 20°C, the value will be less than R_1MAX unless the device has tripped or the load current is beyond the value of I_H.

Using the R_1MAX and I_H of a PolySwitch device, the maximumcan then simply be calculated:

And for an overcurrent tripped condition, I_T and R_1MAX can be used to approximate:

If the maximum V_SENSE voltage is to be exceeded, a 10-kΩ resistor should be placed in series with the V_SENSE- input of the ZXCT1030. This is because when the PolySwitch trips, the differential voltage across the V_SENSE pins of the current monitor could exceed the device ratings and damage the ZXCT device. The choice of sense voltage indication will depend on system requirements. The output voltage (V_OUT) of the ZXCT1030 or ZXCT1021 is, where 10 is the typical signal gain offered by the current monitors.

The ZXCT1030 has an onboard comparator and voltage reference. A reference level can be set on the noninverting input of the comparator to establish a trigger level to switch the comparator output. V_OUT is internally connected to the inverting input of the comparator. Once V_OUT exceeds the desired threshold set via Comp_IN, the comparator output is pulled low. The comparator output stage is an open-collector output. This can be connected to the supply rail via a pull-up resistor, typically 10 kΩ.

The output signal from the comparator can be connected to a microcontroller to inform the system that excessive current is flowing into the load. This allows the system to make an intelligent decision, such as saving data in nonvolatile memory. A typical application area for this function is USB port protection, where a PolySwitch nanoSMDC075F or nanoSMDC150F device will help provide single or dual port protection, while the Zetex ZXCT1030 will provide an error flag to denote that an overcurrent event is imminent, and/or has occurred depending on the trigger level set at the ZXCT1030 comparator input.