ACPL-796J-560E Datasheet, PDF (14/16 Pages) AVAGO TECHNOLOGIES LIMITED – Optically Isolated Sigma-Delta Modulator

Shunt Resistors
The current-sensing shunt resistor should have low re-
sistance (to minimize power dissipation), low inductance
(to minimize di/dt induced voltage spikes which could
adversely affect operation), and reasonable tolerance (to
maintain overall circuit accuracy). Choosing a particu-
lar value for the shunt is usually a compromise between
minimizing power dissipation and maximizing accuracy.
Smaller shunt resistances decrease power dissipation,
while larger shunt resistances can improve circuit accuracy
by utilizing the full input range of the isolated modulator.
The first step in selecting a shunt is determining how
much current the shunt will be sensing. The graph in
Figure 19 shows the RMS current in each phase of a three-
phase induction motor as a function of average motor
output power (in horsepower, hp) and motor drive supply
voltage. The maximum value of the shunt is determined
by the current being measured and the maximum rec-
ommended 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 experience up to 50%
overloads during normal operation, then the peak current
is 21.1 A (= 10 × 1.414 × 1.5). Assuming a maximum input
voltage of 200 mV, the maximum value of shunt resistance
in this case would be about 10 m:.
40
440
35
380
220
30
120
25
20
15
10
5
0
0 5 10 15 20 25 30 35
MOTOR PHASE CURRENT - A (rms)
Figure 19. Motor Output Horsepower vs. Motor Phase Current and Supply.
The maximum average power dissipation in the shunt
can also be easily calculated by multiplying the shunt
resistance 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 resis-
tance 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 require-
ments of the design. As the shunt value is reduced, the
output voltage across the shunt is also reduced, which
means that the offset and noise, which are fixed, 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 par-
ticular requirements of a specific design.
When sensing currents large enough to cause signifi-
cant heating of the shunt, the temperature coefficient
(tempco) of the shunt can introduce nonlinearity due to
the signal dependent temperature rise of the shunt. The
effect increases as the shunt-to-ambient thermal resis-
tance increases. This effect can be minimized either by
reducing the thermal resistance of the shunt or by using
a shunt with a lower tempco. Lowering the thermal resis-
tance 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 signifi-
cant percentage of the total shunt resistance. This has two
primary effects on shunt accuracy. First, the effective resis-
tance 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 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.
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