15
PC Board Layout
The design of the printed circuit board (PCB) should
follow good layout practices, such as keeping bypass
capacitors close to the supply pins, keeping output
signals away from input signals, the use of ground and
power planes, etc. In addition, the layout of the PCB can
also aect the isolation transient immunity (CMR) of the
isolated modulator, due primarily to stray capacitive
coupling between the input and the output circuits. To
obtain optimal CMR performance, the layout of the PC
board should minimize any stray coupling by maintain-
ing the maximum possible distance between the input
and output sides of the circuit and ensuring 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.
Shunt Resistors
The current-sensing shunt resistor should have low
resistance (to minimize power dissipation), low induc-
tance (to minimize di/dt induced voltage spikes which
could adversely aect operation), and reasonable toler-
ance (to maintain overall circuit accuracy). Choosing a
particular value for the shunt is usually a compromise
between minimizing power dissipation and maximiz-
ing 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. The rst step in selecting a shunt is
determining how much current the shunt will be sens-
ing. The graph in Figure 18 shows the RMS current in
each phase of a three-phase induction motor as a func-
tion of average motor output power (in horsepower, hp)
and motor drive supply voltage. The 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 maximum 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 op-
eration, 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 shunt resistance in this case would
be about 10 mW.
The maximum average power dissipation in the shunt
can also be easily calculated by multiplying the shunt
resistance times the square of the maximum RMS cur-
rent, which is about 1 W in the previous example.
Figure 18. Motor Output Horsepower vs. Motor Phase Current and Supply
Voltage.
15
5
40
15 20 25 30
25
MOTOR PHASE CURRENT - A (rms)
10
30
MOTOR OUTPUT POWER - HORSEPOWER
5 350
0
440
380
220
120
10
20
35
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 oset and noise,
which are xed, become a larger percentage of the
signal amplitude. The selected value of the shunt will
fall somewhere between the minimum and maximum
values, depending on the particular requirements of a
specic design.
When sensing currents large enough to cause signi-
cant heating of the shunt, the temperature coecient
(tempco) of the shunt can introduce nonlinearity due to
the signal dependent temperature rise of the shunt. The
eect increases as the shunt-to-ambient thermal resis-
tance increases. This eect can be minimized either by
reducing the thermal resistance of the shunt or by us-
ing 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.
For a two-terminal shunt, as the value of shunt resis-
tance decreases, the resistance of the leads becomes a
signicant percentage of the total shunt resistance. This
has two primary eects on shunt accuracy. First, the ef-
fective 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 is-
sues 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 ma-
terial from which the resistive element itself is made,
resulting in a higher tempco for the shunt overall. Both
of these eects are eliminated when a four-terminal
shunt is used. A four-terminal shunt has two additional
terminals that are Kelvin-connected directly across the
