HCPL-7860 datasheet(15/18 Pages) HP

15 / 18 page

Figure 18. Motor Output Horsepower vs. Motor

Phase Current and Supply Voltage.

MOTOR PHASE CURRENT - A (rms)

535

440

380

220

120

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 m

Ω.

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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