HCPL-7520 datasheet(12/15 Pages) HP

12 / 15 page

Current Sensing Resistors

The current sensing 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 resistor is usually a compromise

between minimizing power

dissipationandmaximizingaccuracy.

Smaller sense resistance

decreases power dissipation,

while larger sense resistance

can improve circuit accuracy by

utilizing the full input range of

the HCPL -7520.

The first step in selecting a sense

resistor is determining how much

current the resistor 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 sense resistor

is determined by the current

being measured and the maximum

recommended input voltage

of the isolation amplifier. The

maximum sense resistance can be

calculated by taking the maximum

recommended input voltage

and dividing by the peak current

that the sense resistor 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 sense resistance in this

case would be about 10 m

Ω.

The maximum average power

dissipation in the sense resistor

can also be easily calculated by

multiplying the sense resistance

times the square of the maximum

RMS current, which is about 1 W

in the previous example. If the

power dissipation in the sense

resistor is too high, the resistance

can be decreased below the

maximum value to decrease

power dissipation. The minimum

value of the sense resistor is

limited by precision and accuracy

requirements of the design. As

the resistance value is reduced,

the output voltage across the

resistor 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 sense

resistor 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 sense resistor, the

temperature coefficient (tempco)

of the resistor can introduce

nonlinearity due to the signal

dependent temperature rise of the

resistor. The effect increases as

the resistor-to-ambient thermal

resistance increases. This effect

can be minimized by reducing the

thermal resistance of the current

sensing resistor or by using a

resistor with a lower tempco.

Lowering the thermal resistance

can be accomplished by

repositioning the current sensing

resistor 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

current sensing resistor, as the

value of resistance decreases, the

resistance of the leads become a

significant percentage of the total

resistance. This has two primary

effects on resistor accuracy.

First, the effective resistance of

the sense resistor 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 leads

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 overall. Both

of these effects are eliminated

when a four-terminal current

sensing resistor is used. A

four-terminal resistor 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.

Figure 18. Motor output horsepower vs. motor

phase current and supply voltage.

MOTOR PHASE CURRENT – A (rms)

535

440

380

220

120

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