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Texas Instruments Design Considerations for Dual DRV425 Bus Bar Application Application notes
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Design Considerations for Dual DRV425 Bus Bar
Application
Scott Vestal, Magnetic Sensing Products
This document is a supplement to the Bus Bar Theory
of Operation Application Report (SLOA237) and the
DRV425 Bus Bar App Magnetic Field Calculator
(SBOC480).
There are several different ways to sense current. The
majority of applications are based on measuring the
voltage across a shunt resistor. This approach is
difficult at high currents (>100A) and/or high voltages
(>100V). Magnetic field based current sensing is a
common solution for designs extending beyond these
levels. In a magnetic field based solution, the
measured magnetic field (B) is proportional to the
current (I) and inversely proportional to the distance (r)
from the current carrying conductor (Ampere’s Law),
as shown in Figure 1.
I
µ I
B= 0
2Œr
r
B
the hole. Since the magnetic field is perpendicular to
the current flow, a hole provides an amplification of the
magnetic field inside of the hole as the current flows
around it. The size of the hole needs to be as small as
possible but at least larger than the width of the PCB
designed for the dual DRV425 devices. The smaller
the hole, the larger the magnetic field generated by
each side of the cutout. The magnetic fields around
each side of the hole are elliptical in shape and are in
opposite direction to each other inside the cutout. The
magnitude of the magnetic fields generated by each
side of the cutout will be 0 in the center of the cutout
and increase as you move toward the edges of the
cutout. The magnitude of each magnetic field will be
largest in the y-axis inside the cutout. This allows each
DRV425 device in the vertical PCB orientation to see a
larger magnitude of the desired magnetic field. Each
devices axis of sensitivity is oriented in opposite
directions on the PCB enabling a doubling of the
desired magnetic field to be measured. Another benefit
of this orientation is it reduces or cancels the effect of
any stray magnetic fields since they only flow in one
direction inside the cutout.
y
BL
x
Figure 1. Ampere's Law
z
Measuring high current through a bus bar can be
accomplished using two (2) DRV425 Integrated
Fluxgate Magnetic Field Sensors placed in a cutout in
the center of a bus bar. As the current is split into
equal parts around the cutout, a magnetic field
gradient is generated around each side of the cutout.
The magnetic field lines present inside the cutout flow
in opposite directions. The DRV425 device has a
maximum magnetic field sensing range of 2mT.
System level considerations need to be investigated
when designing with this implementation as to not
exceed this maximum range. The performance of the
design is affected by the cutout configuration, DRV425
device PCB layout orientation and location of
stray/interfering magnetic fields.
(Top)
I
BL
I/2
I
I
I
BR
I/2
BR
B
L
BR
(End)
I
(Side)
I
I
Figure 2. Recommended Configuration
This configuration, however, does not work for all
systems. Looking at all magnetic field influences of the
design becomes critical for optimal performance.
Below we will investigate possible reasons for
selecting a different configuration.
The TI recommended implementation is a hole placed
in the center of a bus bar with the DRV425 devices
axis of sensitivity oriented vertically on the PCB, as
shown in Figure 2. When a hole is placed in the center
of a bus bar the current is split into equal parts around
SBOA185 – March 2017
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Design Considerations for Dual DRV425 Bus Bar Application Scott Vestal, Magnetic
Copyright © 2017, Texas Instruments Incorporated
Sensing Products
1
www.ti.com
Hole vs Slot
BStray1
y
A hole is recommended due to the amplification of the
magnetic field as the current wraps around the hole.
Because stray magnetic fields do not get any
amplification due the hole, this configuration provides
a better signal-to-noise level. Here are two reasons for
using the alternative slot configuration.
BStray2
x
I
1. Low current/small bus bar. A slot configuration will
allow for a narrower opening when using the
vertical PCB device layout orientation if the bus bar
is not large enough to accommodate a hole. A
smaller opening creates a larger magnetic field
differential seen by the dual DRV425 devices.
2. Large current/large bus bar. Since the slot does not
have the amplification effect of the hole, a slot will
produce a smaller magnetic field for the same
current and cutout width.
BStray1
BStray2
DRV425-1
Axis of
Sensitivity
Vertical vs Horizontal
Vertical and horizontal describe the PCB layout
orientation of the axis of sensitivity of the fluxgate
sensor internal to the DRV425. In the vertical layout,
the axis of sensitivity for each DRV425 device are
placed in the y-axis. Each device measures the y-axis
component of any magnetic field. Similarly, in the
horizontal layout, the axis of sensitivity for each
DRV425 device are placed in the x-axis and measures
the x-axis component of any magnetic field.
Interestingly, the final values for the desired differential
magnetic field seen by each device in both orientations
is very similar. However the larger values seen by
each DRV425 device inside the cutout in the vertical
PCB layout orientation is the reason why it is the
recommended PCB layout. Here is a reason for using
a horizontal PCB device layout orientation.
1. Large stray magnetic fields in the y-axis. As shown
in Figure 3. The magnitude of the stray magnetic
fields are seen entirely by each DRV425 device
before being subtracted out. While the difference of
the 2 DRV425 devices may be small, there is a
possibility that the total magnetic field seen by one
or both DRV425 devices may exceed the 2mT
magnetic field range limit. This would result in an
invalid measurement. Since the strength of the
magnetic field is inversely proportional to the
distance, the spacing from stray field sources (like
another bus bar) needs to be evaluated at the
system level to ensure saturation of either DRV425
does not occur.
DRV425-2
Figure 3. Stray Field
Understanding the system level influences for any
magnetic field measurement is critical to achieve the
best performing system. Stray magnetic fields cannot
be eliminated but their influence can be minimized.
Alternative Device Recommendations
Based on system requirements, alternate devices are
available that can provide the needed performance
and functionality. The DRV421 provides a closed loop
isolated current measurement with an external core
and compensation coils. The AMC1305 provides onboard isolation using an external shunt resistor.
Table 1. Alternative Device Recommendations
Device
Optimized Parameters
Performance Trade-Off
DRV421
Precision Integrated
Fluxgate Sensor
Requires External Core
and Compensation Coils
AMC1305x
High Precision,
Reinforced Isolated
Delta-Sigma Modulator
Slightly Higher Cost
Table 2. Adjacent Tech Notes
2
SBOA179
Integrated, Current Sensing Analog-toDigital Converter
SBOA168
Monitoring Current for Multiple Out-ofRange Conditions
SBOA160
Low-Drift, Precision, In-Line Motor Current
Measurements With PWM Rejection
Design Considerations for Dual DRV425 Bus Bar Application Scott Vestal, Magnetic
Sensing Products
Copyright © 2017, Texas Instruments Incorporated
SBOA185 – March 2017
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