PolySwitch Device Enhances Overcurrent

POWER DESIGN
PolySwitch Device Enhances Overcurrent Protection
By Khagendra Thapa, Systems Engineer, Zetex Semiconductors, U.K.; and Chris White, UK and Ireland Sales Manager,
Raychem Circuit Protection, a business unit of Tyco Electronics
W
here 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 PolySwitchdevice
is “flat” in its hold-current range. Hold current (IH) is the
maximum current the device will pass without interruption at 20oC in still-air conditions. As the device reaches its
definite trip current (IT), it will change from a low-resistance
state to a high-resistance state. The value of IT is generally
twice the value of IH.
Because the resistance of the PolySwitchdevice is relatively
constant up to IH, 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
Once a fault condition is removed and the PolySwitch
device temperature drops, the device returns to a lowresistance value, restoring normal circuit operation.
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 PolySwitchdevice 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 IH 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 PolySwitchdevice is defined by R1MAX.
At 20°C, the value will be less than R1MAX unless the device has
tripped or the load current is beyond the value of IH.
Using the R1MAX and IH of a PolySwitch device, the maximum VSENSETRIPSTAR
can then simply be calculated:
START
T
VSENSETRIPSTART = R1MAX × IH .
And for an overcurrent tripped condition, IT and R1MAX
can be used to approximate VSENSETRIPSTART :
VSENSETRIP ≈ R1MAX × I T .
If the maximum VSENSE voltage is to be exceeded, a 10-k
resistor should be placed in series with the VSENSE- input of the
ZXCT1030. This is because when the PolySwitch trips, the
differential voltage across the VSENSE pins of the current monitor could exceed the device ratings and damage the ZXCT
device. The choice of sense voltage indication will depend
5
Resistance (�)
4
3
IH =0.75 A
2
VIN =10 V
1
0.0
0.0
0.2
0.4
0.6
ILOAD (A)
0.8
1.0
1.2
Fig. 1. The resistance of the nanoSMDC075F PolySwitch device remains
essentially constant up to the holding current ( IH ).
Power Electronics Technology April 2006
50
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POWER DESIGN
on system requirements. The output
PolySwitch
voltage (VOUT) of the ZXCT1030 or
device
VIN
ZXCT1021 is VSENSETRIP ×10, where 10
is the typical signal gain offered by
VSENSE+
VSENSE- RLIMITING = 10 k�
the current monitors.
ZXCT1030
The ZXCT1030 has an onboard
VCC
5-V VCOMPSUPPLY
comparator and voltage reference. A
VOUT
reference level can be set on the nonRCOMP
inverting input of the comparator
Load
to establish a trigger level to switch
CompOUT
+
Comp
the comparator output. VOUT is inIN
VREF OUT
ternally connected to the inverting
Control
input of the comparator. Once VOUT
exceeds the desired threshold set via
CompIN , the comparator output is
pulled low. The comparator output
stage is an open-collector output. Fig. 2. An overload early warning circuit can be implemented by monitoring a PolySwitch
This can be connected to the supply device with a current monitor.
rail via a pull-up resistor, typically
10 kΩ.
USB port protection, where a PolySwitch nanoSMDC075F
The output signal from the comparator can be connected
or nanoSMDC150F device will help provide single or dual
to a microcontroller to inform the system that excessive
port protection, while the Zetex ZXCT1030 will provide an
current is flowing into the load. This allows the system to
error flag to denote that an overcurrent event is imminent,
make an intelligent decision, such as saving data in nonvolaand/or has occurred depending on the trigger level set at the
tile memory. A typical application area for this function is
ZXCT1030 comparator input.
PETech
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51
Power Electronics Technology April 2006