that any ground or power plane
on the PC board does not pass
directly below or extend much
wider than the body of the
isolated modulator.
maximum value of the shunt is
determined by the current being
measured and the maximum
recommended input voltage of
the isolated modulator. The
maximum shunt resistance can be
calculated by taking the maxi-
mum recommended input voltage
and dividing by the peak current
that the shunt 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
temperature coefficient (tempco)
of the shunt can introduce
nonlinearity due to the signal
dependent temperature rise of the
shunt. The effect increases as the
shunt-to-ambient thermal
resistance increases. This effect
can be minimized either by
reducing the thermal resistance
of the shunt or by using a shunt
with a lower tempco. Lowering
the thermal resistance can be
accomplished by repositioning
the shunt on the PC board, by
using larger PC board traces to
carry away more heat, or by
using a heat sink.
Shunt Resistors
The current-sensing shunt
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 shunt is
usually a compromise between
minimizing power dissipation and
maximizing accuracy. Smaller
shunt resistances decrease power
dissipation, while larger shunt
resistances can improve circuit
accuracy by utilizing the full
input range of the isolated
modulator.
(=10x1.414x1.5). Assuming a
maximum input voltage of
200 mV, the maximum value of
shunt resistance in this case
would be about 10 mΩ.
For a two-terminal shunt, as the
value of shunt resistance
decreases, the resistance of the
leads becomes a significant
percentage of the total shunt
resistance. This has two primary
effects on shunt accuracy. First,
the effective resistance of the
shunt 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 lead 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 for
the shunt overall.
The maximum average power
dissipation in the shunt can also
be easily calculated by multiply-
ing the shunt resistance times the
square of the maximum RMS
current, which is about 1 W in
the previous example.
The first step in selecting a shunt
is determining how much current
the shunt will be sensing. The
graph in Figure 22 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
If the power dissipation in the
shunt is too high, the resistance
of the shunt can be decreased
below the maximum value to
decrease power dissipation. The
minimum value of the shunt is
limited by precision and accuracy
requirements of the design. As
the shunt value is reduced, the
output voltage across the shunt 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 shunt will
fall somewhere between the
40
440
35
30
25
20
15
380
220
120
Both of these effects are elimi-
nated when a four-terminal shunt
is used. A four-terminal shunt 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
10
5
minimum and maximum values,
depending on the particular
requirements of a specific design.
0
0
5
10 15
20 25
30 35
MOTOR PHASE CURRENT – A (rms)
When sensing currents large
enough to cause significant
heating of the shunt, the
Figure 22. Motor Output Horsepower
vs. Motor Phase Current and Supply
Voltage.
terminals are used to carry the
load current. Because of the
Kelvin connection, any voltage
1-285
HP [ HEWLETT-PACKARD ]
