Datasheet

17
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-788J.
The first step in selecting a sense
resistor is determining how
much current the resistor will be
sensing. The graph in Figure 28
shows the rms current in each
phase of a three-phase induction
motor as a function of average
motor output power (in horse-
power, 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 maxi-
mum 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 resis-
tive element itself is made,
resulting in a higher tempco
overall.
Figure 28. Motor output horsepower vs.
motor phase current and supply voltage.
MOTOR OUTPUT POWER – HORSEPOWER
0
0
MOTOR PHASE CURRENT – A (rms)
40
5202535
20
35
30
25
15
10
5
10 15 30
440
380
220
120