Download Components and Methods for Current Measurement

designfeature
Bryan Yarborough
Applications Engineer, TT Electronics
Components and Methods for
Current Measurement
C
urrent 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 to make decisions about turning off
peripheral loads to conserve power or to return operation to
normal limits. A second function is to determine when it is a
“too much” or a fault condition. If current exceeds safe limits, a
software or hardware interlock condition is met and provides a
signal to turn off the application, perhaps a motor in a stalled condition or short circuit. It is essential to choose the appropriate technology with the necessary robustness to properly withstand the extreme conditions that can exist during a fault.
A signal to indicate the “how much” condition and the “too much” condition is
available in a variety of measurement methods:
1. Resistive (Direct)
l
RDCR
1:N
a. Current Sense Resistors
b. Inductor DC resistance
RBurden
I/N V= (I/N x RBurden)
2. Magnetic (Indirect)
a. Current Transformer
b. Rogowski Coil
Ideal current transformer circuit
c. Hall Effect Device
Fig. 1. In the ideal current transformer ac current passes through the copper wind- 3. Transistor (Direct)
a. RDS(ON)
ings with very little resistive losses.
b.
Ratio-metric
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Each method has advantages for current measurement, but also comes with
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.
Current measurement components and methods must
provide an accurate output
signal as well as preventing
damage to the associated
printed circuit board.
Current Sense Resistor
Rogowski Coil
Fig. 2. Rogowski coil is an air core design
that has a lower inductance providing a faster
response.
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 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, the existence of
low resistance (< 1 mΩ is available) metal alloy current sense products offer superior surge performance for reliable protection during short circuit and overcurrent
events.
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Drain
l
Gate
VH = V
VGS
VDS
VGS
Hall effect principle, magnetic field present
Fig. 3. Hall-Effect devices are capable of measuring large currents.
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Inductor DC resistance
The DC resistance of an inductor can also be used to
provide a resistive current measurement. This method is
considered “lossless” because of the low resistance value of
the copper, typically < 1 mΩ and because it is providing a
secondary use of an existing component. In higher current
applications; a 30 amp current would provide a 30 mV
signal for a 1 mΩ resistance value. This method has two
drawbacks; first copper has a high TCR (temperature coefficient of resistivity) of approximately 3900 ppm, which
causes the resistance value to increase by 39% for a 100°C
rise above room temperature. Because of this high TCR,
the temperature must be monitored and compensated to
provide an acceptable current measurement. The second
drawback is the variance in the resistance of the copper
due to dimensional changes that occur due to the conductor being wider or thinner from one lot to the next.
Current Transformer
A current transformer’s three key advantages are that it
provides isolation from the line voltage, provides lossless
current measurement, and the signal voltage can be large
providing a measure of 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 (Fig. 1). 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. However, a small amount
of power is lost due to transformer losses from the burden resistor, core losses, and primary and secondary DC
resistance.
Rogowski Coil
The Rogowski coil is similar to a current transformer in that
Metal
D
N+
G
N
+
+
P
VS
SiO2
N+
S
Source
B
Load
ID
ID
Fig. 4. Power MOSFET’s on-resistance provides current sensing capability
a voltage is induced into a secondary coil that is proportional
to the current flow through an isolated conductor. The
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exception
is that the Rogowski coil, (Fig. 2), 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 providing 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 alternate to the current transformer.
Hall Effect
When a current carrying conductor is placed in a magnetic
field, as shown in Fig. 3, 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 exists the
charges interact with the magnetic field, causing the current
distribution to change, which creates the Hall voltage.
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
VBAT
Load
Gate drive
ISENSE
R2
KELVIN/SOURCE
R1
VSENSE
RSENSE
–
+
OUT
VOUT
Power ground
Isf = senseFET current
Fig. 5. SenseFET uses a small portion of its parallel MOSFET cells to sense current.
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currentmeasurement
Fig. 6. (a) Shows a four-terminal
resistor for current sense applications. (b) Shows two pad designs,
(1) with isolated pad regions, and
(2) with a plated through-hole.
Pad Design 1
lel cells and connects to the common Gate and Drain, but
separate Source. 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%
or as wide as 15% - 20%. This
is not suitable for current control
applications that typically require
1% measurement accuracy, but is
intended for overcurrent and short
circuit protection.
Pad Design 2
Resistor technology benefits
use, including non-linear temperature drift requiring compensation, limited bandwidth, low range current detection
requiresTTelec_F6b.ai
a large offset voltage that can lead to error, susceptibility to external magnetic fields, and high cost.
Transistors
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
mΩ range for power MOSFETs (Fig. 4). This resistance
comprises several components that begin with the leads
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 error that is 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
change 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
100°C rise. Generally, this 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.
Ratio Metric Current Sense MOSFETs
The MOSFET consists of thousands of parallel transistor cells that reduce the on-resistance. The current sensing
MOSFET shown in Fig. 5 uses a small portion of the paral-
30 Power Electronics Technology | January 2012
Thin Film, are not typically used for
current sense applications, but is included in this discussion
to provide breadth to the topic. Generally these resistive
products are for precision applications because of the resistive layer ranges from 0.000001 in. to 0.000004 in. thick.
They are quite surge-tolerant in the appropriate application,
but are not designed for the high currents typically associated with the applications mentioned here.
(a)
(b)
OAR5
(2.94 Watts)
Side View
OAR5
(5.15 Watts)
Side View
OAR5
(5.15 Watts)
Side View
Fig. 7. Thermal images illustrating the isolation performance of the OAR and
OARS product.
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Fig. 8. Differences in the thermal expansion coefficient between the metal
materials of the current sense resistor and the circuit board materials dissipate
forces on each part.
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Thick Film, is typically 0.0005 to 0.002 in. 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 the Thick Film products is the flexibility to request off standard resistance values because of
the process efficiency of laser trimming. The tradeoff is that
these products are not as capable of the very tight tolerances
of Thin Film products.
Foil, has a larger cross section still and is a uniform resistive alloy, which is different from the Thick Film technology
that is resistive materials suspended in a glass matrix. By
comparison the foils tend to withstand larger surge transients as compared to the previous versions. The principle
advantage with this technology is the low range ohmic values with low TCRs.
Bulk alloy, this resistor technology has the greatest surge
tolerance because of its large current carrying mass. It
is available in resistance values as low as 0.000 5 Ω with
low TCR. This tends to be 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 the Thick Film products, because
the resistor alloy has limited resistivities to reach high range
values, as well as needing the mechanical strength to tolerate
process handling.
Product Specific Features
High--current applications require the resistance value to
be very low to minimize power losses and yet provide the
necessary signal level to provide a voltage signal high enough
to exceed noise levels. These low ohmic values often times
need a four-terminal connection to reduce errors that may
result due to the contact resistance that occurs when the
part is mounted to the board.
The CSL (Fig. 6a) offers four terminals by design, but
other standard surface mount devices can benefit from
a four-terminal pad design. These parts offer physically
separated connection points for current and voltage, which
reduces contact related measurement error. In the case of
the CSL the current will flow through the inner pins and
www.powerelectronics.com January 2012 | Power Electronics Technology
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currentmeasurement
voltage is sensed on the
Measurement
Method
outer pins and is recommended for best
accuracy with the
Sense Resistor
LRF3W to be configInductor DC resistance
ured as a cross flow
arrangement with curRDSon
rent on opposing diaRatio Metric
metrically corners (e.g.,
pin 2 to pin 3).
Current Transformer
The pad layout (Fig.
6b) creates separate
Rogowski Coil
regions for measuring
Hall Effect
signal voltage from the
current carrying portion, which reduces error. Pad Design 1
illustrates one method that creates an isolated pad region within the pad layout, but
this design may reduce the pad area below
necessary limits to carry high currents
through the copper trace. Pad Design
2 uses a plated through hole to connect
under the pad and connects to an internal
or outer trace for measurement; this maximizes the pad space to carry current to
the resistor. The contact point places the
signal line as close to the current channel as
possible; minimizing measurement error.
Accuracy
Isolation
EMI (Tamper
Resistance)
Robust
Size
Cost
High
No
Low
No
High
High
Small
Low
Moderate
High
Small
Low
Low
No
Moderate
Moderate
Small
Low
Moderate
No
Moderate
Moderate
Small
Moderate
High
Yes
Moderate
High
Large
Moderate
High
Yes
Moderate
High
Large
Moderate
High
Yes
High
Moderate
Moderate
High
Resistive (Direct)
Transistor (Direct)
Magnetic (Indirect)
formance. Observe the temperature of the
solder joint with respect to the hot spot.
These temperatures are based on reaching
thermal equilibrium, however in the application these results may be extended to
be considered as the thermal performance
characteristics for an overcurrent protection condition. The FR4 will not exceed its
temperature rating, though extreme circuit
conditions exist.
Solder Joint Stress
The OARS product family’s elevated and
curved construction permits the resistor
Fig. 9. LRF3W from TT electronics uses side to flex, reducing the stress generated by
differences in thermal expansion coeffiThermal Isolation
terminations to achieve a 3W rating.
cients between the heat producing metal
The OARS (Open Air Resistor Surface
and the dissipating circuit board material. Surface mount
Mount) is a unique design that elevates the hot spot of
components that are flat and parallel to the circuit board
the resistive material well above the circuit board material.
will apply shear forces to the solder joint that can lead to
This places the hottest region of the part into the available
failure or changes in performance. In high thermal cycling
airstream, which dissipates the maximum amount of heat
applications, the OARS has been preferred to other simienergy to the air instead of the PCB.
lar all metal construction parts because of this flexibility
This provides a two key advantages for thermal design,
feature (Fig. 8).
which affects the PCB material and the other neighboring
power or semiconductor components. Typical FR4 PCB
The LRF3W (Fig. 9) from TT electronics provides several
material is only rated to 130°C; a power resistor that is
design benefits derived from its 1225 aspect ratio with the
traditionally against the board could cause damage to the
termination along the long side of the component. The side
material during power excursions or reduces the upper
termination extends the power rating to 3 Watts, eliminates
temperature performance limits of the circuit. An elevated
the need to reduce the circuit traces as required by the tracurrent sense prevents damage to the circuit material and
ditional 2512 footprint, and reduces solder joint stresses due
permits the solder joint to run cooler. The second benefit
to differences in the temperature coefficient of expansion
by dissipating the heat to the air instead of the PCB is the
between the ceramic and PCB material. The 1225 aspect
improved performance of nearby heat-affected devices.
reduces the distance between the center / hot spot region of
These effects may include lifetime rating, power handling,
the part and the thermally dissipative circuit board material.
luminous output, accuracy, and reliability.
This permits a high 3W rating and reduces the stresses on
The thermal images shown in Fig. 7 help illustrate the
the solder joint due to the differences in temperature coefficients of expansion between the ceramic substrate and the
isolation performance of the OAR and OARS product.
thermal mass of the board material.
These tests were conducted on FR4 board material with no
ambient airflow; airflow would improve system thermal per-
32 Power Electronics Technology | January 2012
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