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The use of Zetex current monitors with PolySwitch™
overcurrent device
Khagendra Thapa, Systems Engineer, Zetex Semiconductors
Introduction
Zetex current monitors work by measuring the small voltage across a low value sense resistor.
This signal is translated to either a voltage or current output and then used in many applications
to measure, control or protect the system. The sense device is normally a precision low ohm
resistor. It is possible to make use of the small finite value resistance of a PolySwitch™ device as
the sense element.
PolySwitch devices are used where resettable over-current protection is required and can be used
as a more practical solution than fuses. They are thermally operated devices with a finite trip time.
The PolySwitch has a low ohmic resistance in its normal conduction condition that is ideal for use
with a Zetex current monitor over a wide range of currents from hundreds of mA to tens of amps.
This application note discusses the applications, considerations and limitations of using a
PolySwitch device in conjunction with Zetex current monitor IC’s to monitor current and give a
pre-warning of an over-current trip event.
Applications
The advantages of a PolySwitch device include the ability to reset automatically. Once a fault
condition is removed and the device temperature drops (the I2R loss related to fault current is no
longer present), the device will return to a low resistance value to allow normal operation.
Nuisance trips can also be avoided as the operation of the PolySwitch device is based around the
temperature of the device. If a short duration event occurs, eg the inrush current for an inductive
load, the PolySwitch device will not trip.
Applications where overcurrent protection is a critical feature include battery packs, motor
control and lighting applications. (see Figure 1).
Polyswitch
device
VIN
D1
Hall
C1
1uF
Motor controller IC
Diode
GND
Figure 1
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Typical application circuit utilizing a PolySwitch device
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Current measurement utilizing a PolySwitch device
Figure 2 illustrates a typical PolySwitch device inserted between the VSENSE+ and VSENSEterminals of a ZXCT1021 current monitor. The PolySwitch device provides a path for the current
to flow into and protect if excessive current is drawn from the load.
The resistance of the PolySwitch is 'flat' in its hold current (IH) range. IH is the maximum current
the device will pass without interruption at 20°C in still air conditions.
As the device reaches its trip current (IT) the device will change from a low resistance state to a
high resistance state.
Polyswitch
device
VIN
10k
Limiting
resistor
VSENSE+
VSENSE-
Load
ZXCT1021
GND
VOUT
Figure 2
The use of a PolySwitch device with a current monitor device
The increase in resistance of the PolySwitch device can be orders of magnitude, dependent upon
the device selected and the supply voltage. Figure 3 shows a typical change in resistance for a
given load current.
1.0
VIN=10V
Resistance (⍀)
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Iload (A)
Figure 3
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Typical resistance change of RUE120 with a change in load current
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The resistance of the PolySwitch device is relatively constant up to the device's maximum hold
current IH. Prior to IH being reached, the voltage developed across the device rises ‘linearly’ as
load current increases, providing the device ambient temperature is constant.
The PolySwitch device will remain in a high impedance state until either the power is removed or
the load current (ILOAD) is reduced such that ILOAD x VIN < PD, where PD is the power dissipated by
the selected PolySwitch device in its tripped state (as defined in the Raychem PolySwitch data for
each device).
Current measurement accuracy using a PolySwitch is not assured. The PolySwitch device
impedance tolerance is much wider and will swamp the current monitors accuracy tolerance. Use
with a comparator as a pre-warning of an impending trip event is more appropriate.
For more detailed information on the operation of PolySwitch device please refer to Glossary.
Limiting resistor
Consider the circuit in Figure 1. If the PolySwitch device trips and changes to a high impedance
state, the supply voltage will develop across the PolySwitch device. If the maximum VSENSE
voltage is to be exceeded, a 10k resistor should be placed in series with the VSENSE- input. This is
because when the PolySwitch trips the differential voltage across the VSENSE pins of current
monitor could exceed the device ratings and damage the ZXCT device.
Early warning overload/overcurrent
In applications where overcurrent or fault conditions occur, it is possible, utilizing a PolySwitch
device and a current monitor, to provide a pre-warning signal that a trip is about to occur.
Figure 4 shows how a PolySwitch device can be used in conjunction with a ZXCT1030 to provide
an early warning flag that either an overload current or current surge is occurring.
The PolySwitch device is placed in series with the supply rail and load. It is selected to remain in
a low impedance state for a required amount of current flow for the load. The hold current (IH) is
the critical parameter to ensure this.
Polyswitch
device
VIN
To load
10k
Limiting resistor
SENSE +
VCC
SENSE -
Current
monitor
5V V COMP_SUPPLY
VOUT
R COMP
Load
COMP_OUT
+
C OMP_IN
V REF_OUT
Control
Figure 4
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Typical application circuit for early warning flag
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Once a fault condition occurs, the current flow through the device activates a change in
impedance. This increase in the temperature initiates the resistance change. The rapid change of
resistance can be seen in Figure 3. The rate of change of resistance is determined by the
magnitude of the current.
The maximum operating (non-trip state) resistance value (measured 1 hour after a trip event for
through-hole devices or 1 hour after re-flow for surface-mount devices, unless otherwise stated
on the device datasheet) of a PolySwitch device is R1MAX. This means that at 20°C the value will
be never be higher than the stated value unless the device has tripped or load current is beyond
maximum hold current IH.
Using R1MAX and the maximum hold current IH of a PolySwitch, a maximum VSENSE trip level
(start of trip process) can be calculated (for ambient at 20°C).
Note:
Hold current (IH) gives the maximum current level where the device will hold in a normal
operating state at ambient 20°C.
Definite trip current (IT) gives current level above which a device will definitely trip at ambient
20°C.
In between IH and IT the device is in transition period with increased impedance and device may
reach tripped state.
For indication of start of an overcurrent condition (IH) and R1MAX can be used to calculate VSENSE
(trip start):
VSENSE (trip start) = R1MAX x IH
For the overcurrent tripped condition (IT) and R1MAX can be used to approximate VSENSE (trip):
VSENSE (trip) R1MAX x IT
Choice of VSENSE depends upon the application.
As the PolySwitch device impedance increases with the temperature, the temperature effect
needs to be taken into account to obtain the R1MAX for the maximum normal operating ambient
temperature in the above equations. Also, the impedance tolerance of the PolySwitch device
needs to be considered.
Please refer to the glossary section ‘Resistance of PolySwitch devices’ and ‘Tyco Electronics
Raychem Circuit Protection’ web sites for PolySwitch device impedance changes and tolerances.
The PolySwitch datasheets and handbook give the RMIN, RMAX and R1MAX and thermal derating
curves for hold and trip currents for the purpose. (RMIN is the minimum resistance of the device
as supplied at 20°C, unless otherwise specified. RMAX is the maximum resistance of the device as
supplied at 20°C, unless otherwise specified):
The output voltage VOUT of the ZXCT1030 will be VSENSE(trip) x 10.
The ZXCT1030 current monitor IC has an on-board comparator and voltage reference. A reference
level can be set on the non-inverting input of the comparator to set a trigger level to switch the
comparator output. VOUT is connected to the inverting input of the comparator. Once VOUT
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 10k⍀.
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The output signal from the comparator can be the input to a microcontroller to inform the system
excessive current is flowing into the load. This allows the system to make an intelligent decision
on a required action, eg save data into non-volatile memory.
The choice of PolySwitch device depends on the application and the maximum current the device
is required to pass without interruption at ambient temperature conditions. This suggests at
higher ambient temperatures, hold current (IH) stated at 20°C needs to be temperature de-rated.
The derating information is provided in the Raychem PolySwitch databook. As an example, a
single USB current can be up to 500mA for 0°C to 70°C (for commercial temperature range). Based
on this, the hold current for a single-port USB has to be 500mA at 70°C. From Raychem
PolySwitch data, one of the choices of PolySwitch can be nanoSMDC075.
Conclusion
The use of PolySwitch overcurrent devices alongside current monitor ICs allows current to be
measured giving an indication of the level of current flow into a load. The limitations of this are
based around the thermal activation process of the PolySwitch device and the impedance
tolerance.
Combining a ZXCT1030 and a PolySwitch together provides a circuit solution, which gives early
warning to a system that an overload condition is occurring or has already occurred. This allows
a system to react before the current is restricted into the load. A system that shuts down with no
early warning can result in loss or corrupted data. A typical application area for this is USB port
protection, where a device such as the nanoSMDC075F or nanoSMDC150F will provide single or
dual-port protection whilst the Zetex ZXCT1030 will provide an error flag to denote that an
overcurrent event has occurred.
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Glossary
PolySwitch overcurrent device
PolySwitch PPTC (Polymeric Positive temperature Coefficient) devices are protection devices for
circuits, often called resettable fuses. They are placed in circuit as a series element; allowing
current to flow into a circuit under normal operation and protect a circuit if an over-temperature
or over current condition occurs.
When an overcurrent condition occurs in circuit, the PolySwitch device changes from a low
resistance state to a high resistance state, restricting the amount of a current flow into a load. A
PolySwitch device is a non-linear thermistor. The device's resistance increases either due to its
internal I2R heating effect or an increase in the ambient temperature, providing a mechanism for
circuits to be protected from overload currents, overtemperature events and short circuits.
Once the current increases above IH towards definite trip current IT, the resistance is non linear.
For a given device (with fixed heat transfer coefficient, operating temperature and a constant
ambient temperature) the device operates in a constant power state and its resistance is
proportional to the square of the voltage across the device. Therefore the sense voltage will be of
a non-linear value. Beyond IT the device is in a fully tripped state and remains in 'relatively
constant' high impedance. The output of the current monitor will give a voltage gain of ten times
the sense voltage, but this will no longer be directly proportional to the load current:
VOUT = VSENSE x 10
Resistance of PolySwitch devices
The activation process of a PolySwitch device is temperature. Figure 5 shows the operating curve
for a PTC device. This gives inherent limitations if the device is to be utilized as a current
measurement device. As current passes through the device it will begin to heat due to the I2R
losses.
The energy balance of polymeric PTC PolySwitch device is described by the equation below
(ref: Tyco Electronics Raychem Circuit Protection web pages):
mCp(⌬T/⌬t) = I2R- U (T-TO)
Where:
m
= Mass of the PolySwitch device
Cp
= Heat capacity of the PolySwitch device
⌬t
= Change in time
⌬T
= Change in the PolySwitch device temperature
I
= Current flowing through the device
R
= Resistance of the device
U
= Overall heat-transfer coefficient
T
= Temperature of the device
TA
= Ambient temperature
The I2R generated heat is lost to the environment at the rate U (T-TO). Any heat not dissipated to
the environment raises the PolySwitch device temperature. If heat generated and heat lost to
environment by the PolySwitch device is in balance the above equation simplifies to:
I2R = U (T-TO)
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This describes the energy balance during normal operations points A to C in Figure 5. Between
points A and C the resistance of the device can change by either current change, environment
temperature change, or both. Within these points the resistance of the device is able to stabilize
if there is no further increase in current or temperature.
Log Resistance (⍀)
D
A
B
C
Temperature (°C)
Figure 5
Temperature versus log resistance of PolySwitch device
At point C, a further increase in current temperature, ambient temperature, or both will cause the
device to reach a temperature whereby its resistance rapidly increases. For increased current
beyond IH, the rate of I2R heat generated is greater than the rate of heat loss from PolySwitch
device heating it up rapidly. (Points C to D in Figure 5), thus resulting in a large increase in
PolySwitch device impedance. Temperature change between point 3 and 4 is small and can
therefore assume (T-TO) to be constant resulting in PolySwitch device constant power state:
I2R= V2/R= U (T-TO)
Where U and (T-TO) are considered constant for operation between points C and D.
It is for this reason the thermal derating curve will need to be considered alongside the hold
current and the trip current.
The change in impedance of the PolySwitch device for change in ambient temperature while
allowing constant current can be obtained using the above equations along with thermal derating
curves for IH and IT, PolySwitch device impedance at 20°C.
Reference:
Circuit Protection Databook: Tyco Electronics Raychem Circuit Protection, 2004
Tyco Electronic Circuit protection web site: www.circuitprotection.com
Note:
The content of this application note was first produced in Power Electronics Magazine, April 2006
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