Technical Note

VISHAY DALE
www.vishay.com
Power Metal Strip® Resistors
Technical Note
Components and Methods for Current Measurement
By Bryan Yarborough
PURPOSE
Current sensing is used to perform two essential circuit functions. First, it is used to measure “how much” current is flowing in
a circuit, which may be used for power management in a DC/DC power supply to determine essential peripheral loads to
conserve power. The second function is to determine when it is “too much,” or a fault condition. If current exceeds safe limits,
then a software or hardware interlock condition is met and provides a signal to turn off the application, as in a motor stall or
short circuit condition in a battery. It is essential to choose the appropriate technology with a robust design to properly withstand
the extreme conditions that can exist during a fault. The appropriate component performing the measurement function would
need to sustain an accurate voltage signal as well as prevent damage to the printed circuit board.
MEASUREMENT METHODS
A signal to indicate the “how much” condition and the “too much” condition is available in a variety of different measurement
methods, as listed below:
1. Resistive (direct)
a. Current sense resistors
2. Magnetic (indirect)
a. Current transformer
b. Rogowski coil
c. Hall effect device
3. Transistor (direct)
a. RDS(ON)
b. Ratio-metric
Each has advantages that make it an effective or acceptable method for current measurement, but also has tradeoffs that can
be critical to the end reliability of the application. They can also be classified into two main categories of measurement methods:
direct or indirect. The direct method means that it is connected directly in the circuit being measured and that the measurement
components are exposed to the line voltage, whereas the indirect method provides isolation that may be necessary for design
safety.
1. Resistive
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TECHNICAL NOTE
Current Sense Resistor
The resistor is a direct method of current measurement that has the benefit of simplicity and linearity. The current sense resistor
is placed in line with the current being measured and the resultant current flow causes a small amount of power to be converted
into heat. This power conversion is what provides the voltage signal. Other than the favorable characteristics of simplicity and
linearity, the current sense resistor is a cost-effective solution with a stable temperature coefficient of resistance (TCR) of
< 100 ppm/°C or 0.01 %/°C and does not suffer the potential of avalanche multiplication or thermal runaway. Additionally,
low-resistance (< 1 m is available) metal alloy current sense products offer superior surge performance for reliable protection
during short circuit and overcurrent events.
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Components and Methods for Current Measurement
2. Magnetic
Current Transformer
A current transformer (Fig. 1) provides three key advantages: isolation from line voltage; lossless current measurement; and a
large signal voltage that can provide noise immunity. This indirect current measurement method requires a changing current such as an AC, transient current, or switched DC - to provide a changing magnetic field that is magnetically coupled into the
secondary windings. The secondary measurement voltage can be scaled according to the turns ratio between the primary and
secondary windings. This measurement method is considered “lossless” because the circuit current passes through the copper
windings with very little resistive losses (Fig. 2). However, a small amount of power is lost due to transformer losses from the
burden resistor, core losses, and primary and secondary DC resistance.
SECDCR
I
1:N
IInput PRI
DCR
RDCR
RBurden
I
I
V=
x RBurden
N
N
(
)
ICore
Fig. 1 - Ideal current transformer circuit
N2
RCore IMag
LMag RBurdon
N2
ITransformed
Fig. 2 - Current transformer loss components
Rogowski Coil
The Rogowski coil (Fig. 3) is similar to a current transformer in that a voltage is induced into a secondary coil that is proportional
to the current flow through an isolated conductor. The difference is that the Rogowski coil is an air core design as opposed to
the current transformer that relies upon a high-permeability core, such as a laminated steel, to magnetically couple to a
secondary winding. The air core design has a lower inductance to provide a faster signal response and very linear signal voltage.
Because of its design, it is often used as a temporary current measurement method on existing wiring such as a handheld meter.
This could be considered a lower-cost alternative to the current transformer.
RS = 20
t1
Trim 1
e
Trim 2
e
Hgnd
Rejustor
IM
Rogowski
coil
A
Rj1
1 kΩ
Rj2
5 kΩ
Zin
Vin
Amplifier
Integrator
16-QFN
4 mm x 4 mm
(a)
2
M2
1
Ac
Generator
Rf
Power amp
Zin
Coil under test
Fig. 3
Vout (t) = 1/τ∫Vin
Analog input
module
Rf
Reference coil
Switch
AC amplifier
LP filter
(b)
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TECHNICAL NOTE
Rload
Cf
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Components and Methods for Current Measurement
Hall Effect
When a current-carrying conductor is placed in a magnetic field (Fig. 4), a difference in potential occurs perpendicular to the
magnetic field and the direction of current flow. This potential is proportional to the magnitude of the current flow. When there
is no magnetic field and current flow exists, then there is no difference in potential. However, when a magnetic field and current
flow exist, the charges interact with the magnetic field causing the current distribution to change, which creates the hall voltage
(Fig. 5).
The advantage of hall effect devices is that they are capable of measuring large currents with low power dissipation. However,
there are numerous drawbacks that can limit their use, such as non-linear temperature drift requiring compensation, limited
bandwidth, low-range current detection requiring a large offset voltage that can lead to error, susceptibility to external magnetic
fields, ESD sensitivity, and high cost.
I
I
V=0
VH = V
B
Fig. 4 - Hall effect principle, no magnetic field
Fig. 5 - Hall effect principle, magnetic field present
3. Transistor
RDS(ON) - Drain-to-Source On-Resistance
Transistors are considered a “lossless” overcurrent detection method since they are standard control components to the circuit
design and no further resistance or power dissipating devices are required to provide a control signal. Transistor datasheets
provide the on-resistance for the drain-to-source (RDS(ON)) with a typical resistance in the m range for power MOSFETs. This
resistance consists of several components that begin with the leads (Fig. 6) connecting to the semiconductor die through the
resistance that makes up the numerous channel characteristics. Based on this information, the current passing through the
MOSFET can be determined by ILoad = VRDS(ON) / RDS(ON).
Each constituent of the RDS(ON) contributes to measurement errors that are due to minor variations in the resistances of the
interface regions and TCR effects. The TCR effects can be partially compensated by measuring temperature and correcting the
measured voltage with anticipated changes in resistance due to temperature. Often times the TCR for MOSFETs can be as large
as 4000 ppm/°C, which is equivalent to a 40 % change in resistance for a 100 °C rise. Generally, this measurement method
provides a signal with approximately 10 % to 20 % accuracy. Depending on the accuracy requirements, this may be an
acceptable range for providing overcurrent protection.
Metal
Si02
Load
ID
D
ID
N+
G
Gate
VDS
N
VGS +
VGS
+ VS
P
N+
Source
S
Fig. 6 - Simple model of an n-channel enhancement type MOSFET
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TECHNICAL NOTE
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Components and Methods for Current Measurement
Ratio Metric - Current Sense MOSFETs
The MOSFET consists of thousands of parallel transistor cells that reduce the on-resistance. The current sensing MOSFET uses
a small portion of the parallel cells and connects to the common gate and drain, but a separate source (Fig. 7). This creates a
second isolated transistor; a “sense” transistor. When the transistor is turned on, the current through the sense transistor will
be a ratio comparable to the main current through the other cells.
Depending on the transistor product, the accuracy tolerance range can vary from as low as 5 % to as wide as 15 % or 20 %.
This is generally not suitable for current control applications that typically require 1 % measurement accuracy, but is intended
for overcurrent and short circuit protection.
VBAT
Load
Gate Drive
ISENSE
Isf
KELVIN /
SOURCE
R1
VSENSE RSENSE
R2
- OUT
+
VOUT
Power Ground
Isf = SenseFET Current
Fig. 7
MEASUREMENT
METHOD
ACCURACY
ISOLATION
EMI (TAMPER
RESISTANCE)
ROBUST
SIZE
COST
High
No
High
High
Small
Low
Low
No
Moderate
Moderate
Small
Low
Moderate
No
Moderate
Moderate
Small
Moderate
Current Transformer
High
Yes
Moderate
High
Large
Moderate
Rogowski coil
High
Yes
Moderate
High
Large
Moderate
Hall effect
High
Yes
High
Moderate
Moderate
High
RESISTIVE (DIRECT)
Sense resistor
TRANSISTOR (DIRECT)
RDS(ON)
Ratio metric
MAGNETIC (INDIRECT)
4. Resistor Technology Options
I will begin the explanation of the impact of resistive technology with the least robust technology for current sense applications
and progress through to the most robust technology.
Thin Film: These devices are not typically used for current sense applications, but are included in this discussion to provide
breadth to the topic. Generally these resistive products are for precision applications because of their resistive layer ranges from
0.0000012" to 0.000004" thick. They are quite surge-tolerant in the appropriate application, but are not designed for the high
currents typically associated with the applications mentioned in this article.
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TECHNICAL NOTE
Resistor Technologies
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Components and Methods for Current Measurement
Vishay Thick Film Resistor Products: The RCWE and RCWL are typically 0.0005" to 0.002" thick, which is nearly 100 times
that of thin film. The increased thickness equates to a greater mass that is better able to carry the relatively high currents and
dissipate the heat across the substrate, as well as better able to manage transients. Another advantage of thick film products
is their flexibility; standard resistance values are available across a wide resistance range because of laser trimming and film
composition. A tradeoff for thick film vs. thin film is that thick film is not capable of the very tight tolerances of thin film products.
Vishay resistor products: RCWE, RCWL
Vishay Power Metal Strip® Resistor Products: The WSLP, WSL3921, WSL5931, WSR, WSHM, WSBS, and WSMS... (bulk
alloy) have the greatest surge capability because of their large current-carrying mass. They are available in resistance values as
low as 0.000005  with low TCR and are the best choice for high-current power supplies or where fault conditions can result
in extreme currents. These products do not have as wide of a resistance offering as thick film resistors because the resistor
alloy has limited resistivity and minimum alloy thickness to reach high range values.
Vishay Power Metal Strip resistor products: WSL, WSLT, WSLP, WSR, WSHM2818, WSBS, WSMS...
Power Metal Strip product overview (see “related information” table)
5. Product-Specific Features
Four-Terminal or Kelvin Construction
High-current applications require very low resistance values to minimize power loss, while providing an appropriate voltage
signal high enough to exceed the noise floor. The low ohmic values, typically < 25 m, often times benefit from a four-terminal
device that reduces measurement errors by separating the current flow from the voltage sense locations. This reduces two
types of errors: contact resistance as the part is mounted to the board, and the high temperature coefficient effects from the
amount of in-circuit copper (3900 ppm/°C). The WSK, WSK0612, WSL2726, WSLP2726, WSLT2726, WSL4026, WSLP4026,
WSLT4026, and WSL3637 all feature four-terminal constructions.
WSK0612
WSK2512
WSL3637
WSL2726
WSL4026
The following illustration from the Power Metal Strip product overview (see “related information” table) shows the pin
designations for a proper Kelvin connection.
E1
I1
E1
I1
E2
I2
I2
(E1 & E2 Voltage Connections, I1 & I2 Current Connections)
High Temperature
Hall effect and transistor current sense measurement can be adversely affected by high temperatures that can introduce
non-linear measurement errors, as well as compromise the long-term measurement stability. Increased temperature can affect
active devices by increasing the availability of charge carriers, whereas a resistor solution is entirely based on the fixed
metallurgical properties. Vishay Dale resistors use proprietary manufacturing processes that deliver products that are capable
of long-term stable operation at temperatures up to 275 °C. The high-temperature capability also enables a design to function
at higher rated power for the same temperature than other resistor manufacturers or comparable products; for a similar design
rated temperature.
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Components and Methods for Current Measurement
Power Metal Strip High-Temperature Capability
100
90
WSL
80
73 %
Power (%)
70
WSLT & WSR
60
50
45 %
40
Thick Film
30
30 %
20
10
0
0
25
50
75 100 125 150 175 200 225 250 275
70
Temperature (˚C)
Thermal Performance
There are four key advantages of the resistor construction in thermal design:
1. Degradation of the PCB Material - Standard FR4 PCB material is only rated to 130 °C; a typical power resistor that is against
the board could cause damage to the material during power excursions or reduce the upper temperature performance of
the circuit. An elevated current sense prevents damage to the circuit material and can permit the solder joint to run cooler,
such as with the WSL2726 or WSL4026.
The WSL provides a low profile, but still provides the board clearance that protects the PCB from hot spot exposure as
indicated by the following images.
WSL elevated
construction
The elevated design of the WSL2726 and WSL4026 is unique among typical current sense resistors because it protects the
circuit board from direct exposure to hot spot high temperatures and places the hot spot into the available airstream, which
dissipates the maximum amount of heat energy to the air instead of the PCB.
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TECHNICAL NOTE
2. Deterioration in Performance of Nearby Power or Semiconductor Components - A portion of heat will be dissipated to the
air instead of the PCB, which can positively affect the performance of nearby heat-affected devices. These effects may
include lifetime rating, power handling, LED luminous output lifetime, accuracy … or more simply put, reliability.
Additionally, the low thermal EMF (< 3 μV/°C) characteristics of the Power Metal Strip product (see “related information”
table) assures that nearby power- / heat-generating components will minimize potential error that can be introduced from
thermal gradients across the resistor. Standard thick film resistors have a typical thermal EMF of 40 μV/°C to 50 μV/°C, and
when multiplied by a 100 °C temperature increase can introduce as much as 5 mV of error, which could exceed allowable
measurement circuit error limits.
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Components and Methods for Current Measurement
3. Coefficient of Thermal Expansion (CTE) Mismatch - The all-metal welded construction of the Power Metal Strip series
minimizes solder joint stress that is a result of a CTE mismatch between the resistor and the circuit board. The cyclic stresses
that result from the CTE mismatch due to a lifetime of thermal cycling can lead to fatigue stress cracks in the solder joint
that can compromise long-term reliability. The mismatch is a result of the FR4 circuit board material having a CTE of
approximately 18 ppm/°C, while ceramic has a CTE of 5 ppm/°C, which is more rigid and less able to expand with the circuit
board material.
The Power Metal Strip series all-metal welded construction with a CTE of approximately 13 ppm/°C is more compatible with
the 18 ppm/°C CTE of the FR4 material, therefore stress due a CTE mismatch is significantly reduced. This leads to longterm reliability, which is the reason why the Power Metal Strip series has been designed into applications used in automotive,
military, and industrial markets since 1993.
4. Solder Joint Stress - The elevated construction of the WSL2726 and WSL4026 provides a greater ability to flex, which
reduces the stress generated by differences in thermal expansion coefficients between the heat-producing metal and the
dissipating circuit board material. Surface-mount ceramic resistors that are flat to the circuit board have a different
coefficient of expansion from the PCB material, which applies shear forces to the solder joint that can lead to failure or
changes in performance. In high thermal cycling applications, compliant terminations are preferred to other similar all-metal
construction parts because of this flexibility feature.
Ceramic Chip Resistor
WSLP2726
The figures above illustrate how the forces on each part are dissipated as a result of differences in the thermal expansion
coefficient between the current sense resistor and the circuit board materials.
RELATED INFORMATION
www.vishay.com/doc?49581
Video:
Power Metal Strip® Resistor Thermal
EMF (Product Demo)
www.vishay.com/videos/resistors/power-metal-strip174-resistor-power-coefficient-product-demo
Pulse Handling Capabilities
of Vishay Dale Wirewound
Resistors
www.vishay.com/doc?49076
Shunts, Current Shunts, and
Current-Sensing Resistors
www.vishay.com/doc?49159
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TECHNICAL NOTE
Product Overview:
Power Metal Strip® Surface-Mount
Current Sensing Resistors