Advancements in High Power Current Sense Resistors by sdfgsg234

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									   Higher Power Dissipation Advancements in Resistive Type Current
                              Sensors

As power supply technology advances, so does the need for accurate current sensing
methods; for low and medium power applications, low resistance value current sense
resistors are used almost exclusively. In higher power applications, often more exotic
methods of current sensing are used such as hall effect devices and other devices which use
the principle of the magnetic field developed around a current carrying conductor.

However, due to the simplicity, ease of use, and overall low cost of resistor type current
sense methods, designers often prefer to use them over other technologies. In recent years,
advancements in resistor technology, packaging and lower resistance values with increased
current handling capability have allowed their use in increasing numbers in power supply
applications.

Resistors in current sense applications use the basic principle that current flowing through
a resistor generates a voltage across it in accordance with Ohms Law. In order to minimize
power lost, resistance values are chosen so as to be as low as possible, yet will generate a
sufficient voltage to be immune from background and stray noise (which can be significant
in many environments and also be self generated in switching type power supplies).
Traditionally, surface mount type current sense resistors were relegated to 1-2 watts power
dissipation which prevented their use in all but the lower power and lower current sensing
applications. As newer techniques are developed to manage the heat generated on the
board by a surface mount component, designers have more options in their power supply
designs.




Resistor Types and Technologies
Resistors are relatively simple devices and in their simplest form, can consist of defining a
copper trace on the PC board with a given size and shape. Although quite cost effective
for small power dissipation applications, this technique isn’t widely used except in very
low precision applications. Copper material used in the manufacture of PC boards has a
high temperature coefficient of resistance of approximately +0.4% per degree C, and other
factors relating to circuit board etching and manufacturing result in resistors made with this
technique to typically be capable of no better than a ±40 to 50%. In addition, for resistors
with large power ratings, the amount of board area needed to minimize TCR effects and to
dissipate the power needed, make this technique impractical.
Precision resistors are typically made from four basic techniques: thick film, wirewound,
thin film and composition types. For current sense applications, the first two methods are
overwhelmingly used (thin film and composition types, with few exceptions, aren’t
capable of achieving resistance values in the milliohm range that is needed for most current
sensing applications).

Figure 1 below shows a typical thick film current sense resistor (1 watt power rating)
which is manufactured similarly to standard value chip resistors. For this type resistor, a
flat ceramic base or substrate is screen printed with a paste made of fine particles of metal
and glass powder. The conductor material is typically a silver alloy containing palladium
or platinum, and the resistor element for very low resistance values is often an alloy
containing approximately 60% palladium and 40% silver. After printing, the thick film
resistor is fired at high temperature to sinter the metal and glass particles. This type of
construction is capable of resistance values down to a few milliohms and is low cost, but is
usually limited to only a watt or two of power dissipation. Because the construction is
basically a leadless (non compliant) termination, scaling this method up to larger sizes (and
higher power dissipation) is limited as larger sizes than the 2512 configuration can
experience problems with solder joint failures if the board is subject to flexing or
temperature cycling.




                Fig. 2. Thick Film Current Sense Resistors (2512 Package)

However, by using a side termination, as shown in IRC’s LRF3W style component on the
same photograph, the power dissipation can be increased to 3 watts in what is traditionally
a 1 watt size package. Part of the increased power handling capability of the LRF3W part
can be attributed to its internal construction which uses copper conductors and
copper/nickel alloy resistors. In addition, this configuration is much less affected by TCE
mismatch of the resistor to the board, and the wider solder terminations allow much larger
connections to the resistor, which is beneficial when conducting larger amounts of current.
These materials and the internal construction serve to spread the heat generated internally,
allowing more efficient power dissipation in a smaller size package.
               Fig. 3.   LRF3W “1225” Size Resistor Rated 3 watts @70C

Wirewound resistors, the other main construction technology for making current sense
resistors (which at very low resistance values aren’t really “wound”, but instead are
typically stamped from flat metal resistor alloy), are used in applications down to sub
milliohm values. The metals typically used for wirewound resistors are usually made from
one of the following resistance alloys: cupron (an alloy of copper and nickel), manganin
(nickel, copper, and manganese) or nichrome (nickel and chromium). These metal alloys
have characteristics of low resistance change with temperature (TCR), high stability, even
at elevated temperature, and cost effectiveness.

Package configurations for surface mounted current sense resistors
The traditional SMT package for wirewound resistors basically consists of a flat stamped
resistor alloy with solderable terminations attached to each end. This configuration is
simple and reliable, but is also limited to relatively small package sizes and
correspondingly low power dissipation. One variation of this type, the OARS Series from
IRC, is formed so that the resistor element is elevated off of the board, significantly
reducing the amount of heat transferred by conduction to the board. With an outline size
of 0.81” X 0.275”, this arrangement better takes advantage of any airflow that might be
across the board, further reducing its operating temperature and offers an inherent degree
of compliancy with the PC board. This type of component is shown below, alongside its
thermal “footprint”. A companion part, the OARS-XP (XP=”extended power”) is
available with power ratings to 5 watts.
   Fig. 4. (ULR-Traditional Flat Metal Strip Resistor, OARS configuration, and OARS
                            Thermal Footprint (at 4.5 Watts)


Another common package is the molded SMT wirewound, consisting of a wirewound or
metal element molded into an epoxy package with compliant metal leads for soldering.
This package has the advantages of power dissipation up to 3.5 watts, low resistance
values (typically to 1 milliohm or less) and compliant terminations. However, beyond 3.5
watts, this construction makes it difficult to dissipate the extra heat generated.




Fig. 5. Molded wirewound surface mount package



Other Power SMT Configurations
For high power current sense applications, there are additional package configurations
which may be considered. The TO-263 (or D2-Pak) is available down to approximately 10
milliohms and has a power dissipation rating of typically 20 watts, when used with a
heatsink. Extremely high power dissipation can be had in a SOT-227 package. With this
package, power ratings up to 100 watts or more are available and usually in a Kelvin (four
terminal connection). Of course with this part, a heat sink is also necessary.
Fig. 6. Very High Power Current Sense Packages: TO-263-20 watts (left) and SOT-227-
100 watts (right)

Four Terminal vs. Two Terminal Connections
Current sensing resistors, because of their very low resistance values, are susceptible to
errors in voltage measurement due to the resistance of the connection to the board. As a
result, minute variations in the amount of solder used, trace layout, etc., can affect the
measured voltage when using any 2 terminal resistor. The solution to this problem is to
use a 4 terminal connection (also known as a Kelvin connection). This type of connection
minimizes voltage measurements error by separating the power and voltage sensing
terminals. Special 4 terminal resistors are available for optimum accuracy, but acceptable
performance from 2 terminal resistors can usually be obtained if circuit traces are designed
properly. Figure 5A below illustrates how the potential for errors with a 2 terminal
connection can occur. Fig. 5B shows a modified Kelvin (4 terminal) connection for PC
board traces which can significantly increase current sensing accuracy when using two
terminal resistors. The four terminal connection separates the current carrying traces from
those used for voltage sensing. By eliminating current flow in the voltage sensing
terminals, the associated voltage drop in it is eliminated, increasing the accuracy of the
measured voltage.

                                         Measured Voltage
                   R test                                           R test
                      leads                                             leads


                              R solder joint R element R solder joint

Fig. 7A. Current flowing through the solder joint resistance affects the measured voltage.
Fig. 7B. Modified Kelvin Connection on the board allows accurate sensing of voltage on
two terminal resistors.


Special Considerations for Using Current Sensing Resistors
In current sensing applications, often the sense voltage is quite low (typically millivolts).
As a result, the materials used in current sensing resistors are often selected by their
manufacturer to exhibit a low thermal EMF. (Thermal EMF is the thermocouple effect
caused by different metals used in the resistor’s construction). Unequal temperatures
found across the resistor, which are always present to some degree, coupled with the
different resistor materials, produce a thermal EMF error, which adds to the total error in
sensing the voltage across the current sense resistor.

Conclusion
Because current sense resistors have specific performance requirements that are separate
from other resistor applications, it is important for design engineers to be knowledgeable
of the state of the technology, what is available, and how to use them.


Tom Morris
Applications Engr. Mgr.
T.T. Electronics-IRC
4222 S. Staples St.
Corpus Christi, TX 78411
Ph 361-985-3151

								
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