AN2317 - Application

External Circuit Design

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1.1.2

Secondary current sensing

The secondary channel uses shunt resistor structure (see

Figure 4). The 420µW shunt

resistor is used to maximize the use of the dynamic range of the current sensing circuit.

However, there are some important considerations when selecting a shunt structure for

energy metering applications.

●

The power dissipation in the shunt must be minimized.

The maximum rated current for this design element is 20A, so the maximum power

dissipated in the shunt is calculated as follows:

●

The higher power dissipation may make it difficult to manage the thermal issues.

Although the shunt is manufactured from manganin material, which is an alloy with a

low thermal resistance, an apparent error may occur when it reaches a high

temperature.

●

The shunt should be able to resist the shortage of the phase circuit.

This reduces the shunt resistance is much as possible.

The design values used are:

–

–

Ib = 2A, and

–

Shunt resistance = 420µ

Ω.

The remaining design elements calculated from these values are as follows:

–

Voltage across shunt:

–

Mains power dissipation:

–

Error:

20A

()

2

420

μΩ 168mW

=

×

2A

420

μΩ 0.00084V

=

×

220V

2A

0.44kW

=

×

1.68

10

3

–

×

0.44

10

3

–

×

100percent

0.0004percent

=

×

⁄