AN2317 - Application

External Circuit Design

13/27

According to

Equation 12 and Equation 13 on page 12, the filter’s magnitude and phase

response can be seen in

Figure 7 and Figure 8 on page 14.

●

When f = 50Hz:

Equation 14

and

Equation 15

●

When f = 60Hz:

Equation 16

and

Equation 17

Assume that the current lags the voltage by a phase angle,

δ. After an anti-aliasing filter, a

phase error (

ϕ) is introduced into the STPM01. The power factor (PF) error is calculated as:

Equation 18

When,

δ = –60° (PF = –0.5), and

f = 50Hz,

according to

Equation 14, a phase error,

Equation 19

This indicates that even a small phase error will translate into a significant measurement

error at a low power factor. Thus correct phase matching is required in this situation.

ϕ

0.35°

–

=

A

u

1

≈

ϕ

0.43°

–

=

A

u

1

≈

error

PF

δ

cos

δϕ

+

()

cos

–

δ

cos

----------------------------------------------- 100percent

⋅

=

error

PF

60

–

°

()

cos

60°

–

0.35°

–

()

cos

–

60

–

°

()

cos

---------------------------------------------------------------------------------- 100percent

1percent

=

⋅

=