acpl-c797 Avago Technologies, acpl-c797 Datasheet - Page 15

• Manufacturer Part Number: acpl-c797
• Description: Optically Isolated Sigma-delta Modulator
• Manufacturer: Avago Technologies

Table 11. Isotek (Isabellenhütte) four-terminal shunt summary
Note: Values in brackets are with a heatsink for the shunt
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 11; the maximum
current and motor power range for each of the PBV series
shunts are indicated. For shunt resistances from 50 m:
down to 10 m:, the maximum current is limited by the
input voltage range of the isolated modulator. For the 5
m: and 2 m: 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 dissipa-
tion capability by acting as a heat sink. Liberal use of vias
where the load current enters and exits the PC board is
also recommended.
15
Shunt Resistor
Part Number
PBV-R050-0.5
PBV-R020-0.5
PBV-R010-0.5
PBV-R0005-0.5
PBV-R002-0.5
Shunt
Resistance
m:
50
20
10
5
2
Tol.
%
0.5
0.5
0.5
0.5
0.5
Maximum
RMS Current
A
3
7
14
25 [28]
39 [71]
Shunt Connections
The recommended method for connecting the isolated
modulator to the shunt resistor is shown in Figure 19. V
of the ACPL-C797 is connected to the positive terminal of
the shunt resistor, while V
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 connec-
tion between the isolated modulator circuit and the gate
drive circuit should be the positive power supply line.
In some applications, however, supply currents flowing
through the power-supply return path may cause offset
or noise problems. In this case, better performance may
be obtained by connecting V
the shunt resistor with two conductors, and connecting
GND1 to the shunt resistor with a third conductor for the
power-supply return path, as shown in Figure 21. When
connected this way, both input pins should be bypassed.
To minimize electromagnetic interference of the sense
signal, all of the conductors (whether two or three are
used) connecting the isolated modulator to the sense
resistor should be either twisted pair wire or closely
spaced traces on a PC board.
Motor Power Range
120 V
hp
0.8-3
2-7
4-14
7-25 [8-28]
11-39 [19-71]
ac
– 440 V
IN
ac
– is shorted to GND1, with the
IN
+ and V
kW
0.8-3
2-7
4-14
7-25 [8-28]
11-39 [19-71]
IN
– directly across
IN
+