Current Sensing
Chip Resistors
www.yageo.com
About Yageo
Founded in 1977, the Yageo Corporation has become a
world-class provider of passive component services with
capabilities on a global scale, including production and sales
facilities in Asia, Europe and the Americas.
Yageo currently ranks as the world No.1 in chip-resistors,
No. 3 in MLCCs and No. 4 in ferrite products, with a
strong global presence: 21 sales offices in 15 countries, 9
production sites, 8 JIT logistic hubs, and 2 R&D centers
worldwide. Ferroxcube and Vitrohm, who produce ferrites
and leaded resistors, are also a part of the Yageo group.
We support our customers with extensive literature
including datasheets, brochures and application notes, which
are also available electronically on our website at:
www.yageo.com
Table of Contents
Introduction
.............................................................3
Current-Sensing Circuit Applications
.........4
Market Applications
.............................................6
Product Portfolio
...................................................7
Product Selection Tables
................................ 10
2
Introduction
Low Resistance, High Power for Current Sensing Applications
Current measurement is very important in power and instrumentation systems for circuit control, protection,
monitoring, and performance enhancement. Engineers in power supply and battery circuit designs need to consider
a give-and-take strategy between low resistance values to minimize power losses and sufficient voltage supplies to
avoid noises generated from the environments or particularly in switch mode power supplies.
Yageo’s current-sensing chip resistors are also fully compatible with today’s high volume pick-and-place assembly
systems. As such, they offer attractive, cost-effective solutions to designers of low voltage power supplies and
battery management systems. Featuring a comprehensive resistance range of 0.5 milli-ohms to 1 ohm (low-ohmic),
and available from 0.05 to 5 watts, they are not only applicable to battery packs, power supplies and converters, but
also suitable for use in diverse power control circuits of tablets, notebook computers and hard disks.
Yageo now offers three types of surface-mount (SMT) current-sensing chip resistors based on thick film, metal foil,
and metal plate technologies, with scalable product portfolios to meet the various demands of customers and their
applications.
Current Sensing Chip Resistors
Introduction
Main Features of Yageo’s Current-Sensing Chip Resistors
• Low resistance value from 1mΩ to 20 mΩ for minimizing power losses.
•
High power-rating from 0.05 to 5 watts.
•
Tight tolerance within 2% to exhibit actual current via voltage reading.
•
Low TCR to avoid measurement distortions. TCR ranges from 50 to 100ppm/ºC
for metal and 100 to 1500ppm/ºC for thick film current sensors.
•
Scalable off-the-shelf products in standard case sizes.
•
Compatibility with surface-mount assembly process.
•
RoHS/REACH-compliant & Halogen-free.
The low temperature coefficient of resistance (TCR) of Yageo's current-sensing chip resistors minimizes the
resistance change caused by self-heating and high temperature environments.
Thermal electromotive force (EMF) is also an important consideration. Thermal EMF is an important parameter of
the metal foil series of battery management circuits, and of current-sensor resistors. Thermal electromotive force
(EMF) of an Mn-Cu alloy is especially optimal with low EMF below µ0.03 uV/oC.
3
Current-Sensing Circuit Applications
Low-Ohmic Resistors in Power-Sensing
Current-sensor resistors are used in power sensing applications
such as sensing output current in power supplies and
automotive engine management systems. As shown in Figure 1, a
typical function for a current-sensor chip resistor is as a current-
sensor (Rsense). This generates the sensing voltage (Vs) for a
feedback control network through which an output current (Io)
passes. The sensing voltage triggers MOSFET switches, switching
them ON and OFF to regulate the duty factor of the current
passing through a choke (L).
The sensing voltage (Vs) is given by the simple relation:
Vs = Io x Rsense
This sensing voltage is generally set at around 100 mV both to save power and maintain satisfactory noise
immunity. To sense a 5 A average output current, Rsense must be 100 mV/5 A = 20 mΩ. The power dissipation will
then be:
P = Io2Rsense = 5 A x 5 A x 20 mΩ = 0.5 W
A current-sensor chip resistor with a power rating 1.0 W would then be recommended for this application to
provide an adequate safety margin.
Figure 1
Current sensor chip resistor in current
sensing application
Over-Current Detection
As a means to detect the current passing through the
transistor (see Figure 2), a resistor in series is added
between an emitter and a ground. This resistor shouldn't
emit smoke or catch fire even when the switching transistor,
subjected to a larger current, breaks down. In addition,
reduced parasitic inductance is required, particularly for high
frequency switching control. Recommended resistors with
low resistance are metal-plate types, like the PE-series.
DC/DC converter
Figure 3 on the right shows the current-detecting circuit
of a DC/DC converter. The voltage across the current-
detecting resistor is fed back to control the output power.
The resistance should be low to reduce power dissipation,
and the resistor should withstand a repeated rush current.
Furthermore, the self-inductance should be low for high-
frequency applications. Recommended types are PE-series
chip resistors. As for high frequency DC/DC converters,
metal-plate chip resistor, PE-series are the best fit.
Figure 2
Over-current protection circuit
Figure 3
DC/DC converter circuit
4
Tight Tolerance in Sensing Resistance
The magnitude of the output ripple depends on the
inductance of the choke - the higher the inductance,
the lower the ripple. A high inductance choke, however,
reduces the ability of the circuit to respond to high
frequency transients. Such a choke will also be physically
large, limiting the possibilities for miniaturization so
essential to modern mobile equipment.
2
Current Sensing Chip Resistors
0.04 = 4
A trade-off is therefore necessary between choke
peak current with a ripple of 0.04
volume and output current ripple. Experience indicates
that a ripple of 0.04 provides a good compromise in
this area. With this ripple value, the peak output current (Ipeak) is 2% greater than the average current (Iavg): Ipeak
= 1.02 x Iavg (Figure 4).
The voltage generated across the sensing resistor is used in a feedback network to trigger the power-switching
IC.To allow for variation in the characteristics of the power-switching IC, a safety margin for the sensing-voltage is
necessary. A -2% margin on sensing-voltage is usually taken for general applications.
Figure 4
Relationship between average output current and
Introduction
As mentioned earlier, the relation between current sensor resistance, feedback sensing voltage and output
current is given given by the formula: Rsense = Vs/Io. With an output ripple of of 0.04, a 4% (± 2%) deviation
on output current and a safety margin on the sensing voltage of -2%, the allowable deviation on (Rsense) is:
0.98 x V
s
0.98 x 1
o
V
s
0.85 x 1
o
If Vs= 100 mV and Io= 5 A, the allowable current sensor sensing resistance must lie in the range 19.2mΩ to 20.4mΩ.
Excellent Low TCR Values for Precision Applications
The above discussion does not, of course, take into account the effects of the temperature coefficient of
resistance (TCR) on current sensing applications. With a maximum deviation of 4% on output current and a safety
margin of 2% on sensing voltage, the maximum allowable deviation on sensing resistance is 6%. The limit on TCR
is then given by:
T.C.R.
So
TCR
0.06
1.06 Rsense
Figure 3 plots the allowable T.C.R. values required to maintain tolerance on sensing resistance within the
specified limit. TCR values of Yageo’s current-sensor chip resistors fall well within these allowed limits over the
temperature range 25°C to 155°C.
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