HCPL-7860 View Datasheet(PDF) - HP => Agilent Technologies
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HCPL-7860 Datasheet PDF : 18 Pages
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PC Board Layout
The design of the printed
circuit board (PCB) should
follow good layout practices,
such as keeping bypass
capacitors close to the supply
pins, keeping output signals
away from input signals, the
use of ground and power
planes, etc. In addition, the
layout of the PCB can also
affect the isolation transient
immunity (CMR) of the
isolated modulator, due
primarily to stray capacitive
coupling between the input
and the output circuits. To
obtain optimal CMR
performance, the layout of the
PC board should minimize any
stray coupling by maintaining
the maximum possible distance
between the input and output
sides of the circuit and
ensuring that any ground or
power plane on the PC board
does not pass directly below
or extend much wider than the
body of the isolated modulator.
Shunt Resistors
The current-sensing shunt
resistor should have low
resistance (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
particular 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 18 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 recommended
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 x 1.414
x 1.5). Assuming a maximum
input voltage of 200 mV, the
maximum value of shunt
resistance in this case would
be about 10 mΩ.
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 18. Motor Output Horsepower vs. Motor
Phase Current and Supply Voltage.
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
resistance 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
requirements 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
particular requirements of a
specific design.
When sensing currents large
enough to cause significant
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
resistance 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
resistance 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.
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