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15
motor output power (in horsepower, hp) and motor drive
supply voltage. The maximum value of the shunt is deter-
mined 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 experi-
ence up to 50% overloads during normal operation, then
the peak current is 21.1 A (=10x1.414x1.5). 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 re-
sistance times the square of the maximum RMS current,
which is about 1 W in the previous example. 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 require-
ments of a specic design. When sensing currents large
enough to cause signicant heating of the shunt, the tem-
perature 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 resistance increases. This eect 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. For a two-terminal shunt, as the value of shunt
resistance 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
eective 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. 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 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. Several four-terminal shunts from Isotek (Isabel-
lenhütte) suitable for sensing currents in motor drives up
to 71 Arms (71 hp or 53 kW) are shown in Table 11; the
maximum current and motor power range for each of the
PBVseries shunts are indicated. For shunt resistances from
50mΩ down to 10mm, the maximum current is limited
by the input voltage range of the isolated modulator. For
the 5 mΩ and 2 mΩ shunts, a heat sink may be required
due to the increased power dissipation at higher currents.
When laying out a PC board for the shunts, a couple of
points should be kept in mind. The Kelvin connections to
the shunt should be brought together under the body
of the shunt and then run very close to each other to the
input of the isolated modulator; this minimizes the loop
area of the connection and reduces the possibility of stray
magnetic elds from interfering with the measured signal.
If the shunt is not located on the same PC board as the
isolated modulator circuit, a tightly twisted pair of wires
can accomplish the same thing. Also, multiple layers of
the PC board can be used to increase current carrying
capacity. Numerous plated-through vias should surround
each non-Kelvin terminal of the shunt to help distribute
the current between the layers of the PC board. The PC
board should use 2 or 4 oz. copper for the layers, resulting
in a current carrying capacity in excess of 20 A. Making
the current carrying traces on the PC board fairly large
can also improve the shunt’s power dissipation capabil-
ity by acting as a heat sink. Liberal use of vias where the
load current enters and exits the PC board is also recom-
mended.
Shunt Connections
The recommended method for connecting the isolated
modulator to the shunt resistor is shown in Figure 26.
VIN+ is connected to the positive terminal of the shunt
resistor, while VIN– is shorted to GND1, with the power-
supply return path functioning as the sense line to the
negative terminal of the current shunt. This allows a single
pair of wires or PC board traces to connect the isolated
modulator circuit to the shunt resistor. By referencing the
input circuit to the negative side of the sense resistor, any
load current induced noise transients on the shunt are
seen as a common-mode signal and will not interfere with
the current-sense signal. This is important because the
large load currents owing through the motor drive, along
with the parasitic inductances inherent in the wiring of
the circuit, can generate both noise spikes and osets that
are relatively large compared to the small voltages that
are being measured across the current shunt. If the same
power supply is used both for the gate drive circuit and
for the current sensing circuit, it is very important that the
connection from GND1 of the isolated modulator to the
sense resistor be the only return path for supply current