16
Shunt Connections
The recommended method for connecting the isolated
modulator to the shunt resistor is shown in Figure 17.
V
IN+
(pin 2 of the HPCL-7560) is connected to the posi-
tive terminal of the shunt resistor, while V
IN-
(pin 3) 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 cur-
rent 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 im-
portant that the connection from GND1 of the isolated
modulator to the sense resistor be the only return path
for supply current to the gate drive power supply in
order to eliminate potential ground loop problems. The
only direct connection between the isolated modulator
circuit and the gate drive circuit should be the positive
power supply line.
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 cur-
rent. 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 (Isabellen-
hütte) suitable for sensing currents in motor drives up
to 71 Arms (71 hp or 53 kW) are shown in Table 3; the
maximum current and motor power range for each
of the PBV series shunts are indicated. For shunt resis-
tances from 50 mW down to 10 mW, the maximum cur-
rent is limited by the input voltage range of the isolated
modulator. For the 5 mW and 2 mW 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 pos-
sibility 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 capability by acting as a heat
sink. Liberal use of vias where the load current enters
and exits the PC board is also recommended.
