HCPL-7510 Datasheet, PDF (13/16 Pages) Agilent(Hewlett-Packard) – Isolated Linear Sensing IC

Current Sensing Resistors
The current sensing resistor
should have low resistance (to
minimize power dissipation),
low inductance (to minimize
di/dt induced voltage spikes
which could adversely affect
operation), and reasonable
tolerance (to maintain overall
circuit accuracy). Choosing a
particular value for the
resistor is usually a
compromise between
minimizing power dissipation
and maximizing accuracy.
Smaller sense resistance
decreases power dissipation,
while larger sense resistance
can improve circuit accuracy
by utilizing the full input
range of the HCPL-7510.
The first step in selecting a
sense resistor is determining
how much current the resistor
will be sensing. The graph in
Figure 18 shows the RMS
current in each phase of a
three-phase induction motor
as a function of average motor
output power (in horsepower,
hp) and motor drive supply
voltage. The maximum value
of the sense resistor is
determined by the current
being measured and the
maximum recommended input
voltage of the isolation
amplifier. The maximum sense
resistance can be calculated by
taking the maximum
recommended input voltage
and dividing by the peak
current that the sense resistor
should see during normal
operation. For example, if a
motor will have a maximum
RMS current of 10 A and can
experience up to 50%
overloads during normal
operation, then the peak
current is 21.1 A (=10 x 1.414
x 1.5). Assuming a maximum
input voltage of 200 mV, the
maximum value of sense
resistance in this case would
be about 10 mΩ. The
maximum average power
dissipation in the sense
resistor can also be easily
calculated by multiplying the
sense resistance times the
square of the maximum RMS
current, which is about 1 W in
the previous example. If the
power dissipation in the sense
resistor is too high, the
resistance can be decreased
below the maximum value to
decrease power dissipation.
The minimum value of the
sense resistor is limited by
precision and accuracy
requirements of the design. As
the resistance value is
reduced, the output voltage
across the resistor is also
reduced, which means that the
offset and noise, which are
fixed, become a larger
percentage of the signal
amplitude. The selected value
of the sense resistor will fall
somewhere between the
minimum and maximum
values, depending on the
particular requirements of a
specific design.
When sensing currents large
enough to cause significant
heating of the sense resistor,
the temperature coefficient
(tempco) of the resistor can
introduce nonlinearity due to
the signal dependent
temperature rise of the
resistor. The effect increases
as the resistor-to-ambient
thermal resistance increases.
This effect can be minimized
by reducing the thermal
resistance of the current
sensing resistor or by using a
resistor with a lower tempco.
Lowering the thermal
resistance can be accomplished
by repositioning the current
sensing resistor on the PC
board, by using larger PC
board traces to carry away
more heat, or by using a heat
sink. For a two-terminal
current sensing resistor, as the
value of resistance decreases,
the resistance of the leads
become a significant
percentage of the total
resistance. This has two
primary effects on resistor
accuracy. First, the effective
resistance of the sense resistor
can become dependent on
factors such as how long the
leads are, how they are bent,
how far they are inserted into
the board, and how far solder
wicks up the leads during
assembly (these issues will be
discussed in more detail
shortly). Second, the leads are
typically made from a
material, such as copper,
which has a much higher
tempco than the material from
which the resistive element
itself is made, resulting in a
higher tempco overall. Both of
these effects are eliminated
when a four-terminal current
sensing resistor is used. A
four-terminal resistor has two
additional terminals that are
Kelvin-connected directly
across the resistive element
itself; these two terminals are
used to monitor the voltage
across the resistive element
while the other two terminals
are used to carry the load
current. Because of the Kelvin
connection, any voltage drops
across the leads carrying the
load current should have no
impact on the measured
voltage.
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