HCPL-7560 Datasheet, PDF (16/18 Pages) Agilent(Hewlett-Packard) – Optically Isolated Sigma-Delta Modulator

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.
Both of these effects 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 (Isabellenhütte)
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 resistances from 50
mΩ down to 10 mΩ, 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
fields 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.
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 positive
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
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 flowing through
the motor drive, along with the
parasitic inductances inherent
in the wiring of the circuit,
can generate both noise spikes
and offsets 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 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.