HCPL-7860 Datasheet, PDF (27/28 Pages) Agilent(Hewlett-Packard) – Isolated 15-bit A/D Converter

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 A_rms (71 hp or
53 kW) are shown in Table 9; 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 modula-
tor; 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 modula-
tor to the shunt resistor is shown
in Figure 21. V_IN+ (pin 2 of the
HPCL-7860) is connected to the
positive terminal of the shunt
resistor, while V_IN- (pin 3) is
shorted to GND1 (pin 4), with the
power-supply return path func-
tioning 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 connec-
tion 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 con-
nection between the isolated
modulator circuit and the gate
drive circuit should be the
positive power supply line.
Table 9. Isotek (Isabellenhütte) Four-Terminal Shunt Summary.
Shunt
Maximum
Shunt Resistor ^Resistance
Tol. RMS Current
Part Number
mΩ
%
A
Motor Power Range
120 V_ac-440 V_ac
hp
kW
PBV-R050-0.5
50
0.5
3
0.8-3
0.6-2
PBV-R020-0.5
20
0.5
7
2-7
1.4-5
PBV-R010-0.5
10
0.5
14
4-14
3-10
PBV-R005-0.5
5
0.5
25 [28]
7-25 [8-28]
5-19 [6-21]
PBV-R002-0.5
2
0.5
39 [71]
11-39 [19-71] 8-29 [14-53]
Note: Values in brackets are with a heatsink for the shunt.
1-286