Cirrus Logic AN336 User Manual
AN336
CS5463: Using the Epsilon Register & AFC Function in
Reactive Power and Line Frequency Measurements
1. INTRODUCTION
In the CS5463, average reactive power (Qavg) is generated by averaging the voltage multiplied by the current with
a 90° phase shift between them. The 90° phase shift is realized by a digital filter. This filter will give exactly -90°
phase shift across all frequencies, and utilizes Epsilon (
curate Qavg result over line-frequency variation, the Epsilon register has to be updated in accord with the line frequency.
ε) to achieve unity gain at the line frequency. To attain ac-
I[n ] from I - H P F
V[n] from V -HPF
π × Epsilon
Figure 1. Average Reactive Power Data Flow
-90° Phase
Shift
N
Σ
÷N
Q[n]
Q
AVG
2. THE RELATIONSHIP BETWEEN THE EPSILON REGISTER AND LINE
FREQUENCY
The value in the Epsilon register is the ratio of the line frequency to the output word rate (OWR).
Epsilon = Line Frequency / OWR
By default, Epsilon is set as 0x1999F or 0.0125 decimal for 50 Hz line frequency and 4000 Hz OWR. If the
CS5463 is used in different line and OWR frequencies, Epsilon must be reconfigured accordingly. For example, if the line frequency is 60 Hz and OWR is 4000 Hz, Epsilon should be reconfigured as
60 / 4000 = 0.015.
3. AFC - AUTOMATIC FREQUENCY CALCULATION
Epsilon can be calculated automatically by the CS5463 by setting the AFC bit in the Operational Mode
(Mode) register. The Frequency Update bit (FUP) in the Status register is set every time the Epsilon register
has been automatically updated.
To achieve 0.1% resolution, the Epsilon update period is fixed at 1000 zero-crossings or 10 seconds for
50 Hz line frequency. In other words, the delay between the line frequency change and the correct Epsilon
output will be around 20 seconds (worst case) for a 50 Hz system.
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DECEMBER '08
AN336REV1
AN336
In power meter applications, there are strict requirements for the limit of reactive power error due to line frequency variation. The AFC function can keep the Epsilon register automatically up dated with line frequency
changes to ensure accurate reactive power calculations.
If the measurement delay is not important to the application, the AFC function can be used to measure the
line frequency as well.
Line Frequency = Epsilon × OWR
4. PRECAUTIONS WHEN USING THE AFC IN THE CS5463 REVISION D
In revision D of the CS5463, the HPF settling time has been increased to 3000 samples o r 3000/OWR seconds (see the CS5463 Technical Bulletin). However, this settling time was not taken into account by the
automatic Epsilon calculation. This may make the 1
abled before HPF has settled. See Figure 2.
HPF s ettling tim e
1st Upda te P e rio d
Initial Ep s ilo n
(By default or preset)
st
Epsilon and Qavg results inaccurate if the AFC is en-
nd
Upda te P er io d
2
Inaccu r a te Epsilon
& Qavg
3rd Update Period
Accurate Epsilon
& Qavg
Enable AFC
Start of Continuous
Conversion
Figure 2. HPF Settling Time – AFC Enabled Before Start of Conversion
To solve this problem, the AFC should be enabled at least 3000 /OWR seconds after the continuous conver sion has
started. See Figure 3.
HPF settling time
nd
2
Upda te P er io d
Accurate Epsilon
& Qavg
3rd Upda te Period
Accurate Epsilon
& Qavg
Start of Continuous
Conversion
1st Update Period
Initial Ep s ilo n
(By default or preset)
Enable AFC
Figure 3. HPF Settling Time – AFC Enabled After HPF Has Settled
5. CONCLUSION
The Epsilon register plays an important role in the CS5463 reactive power and line frequency calculations.
To avoid inaccurate measurements, the Epsilon register should be preset based on the line fr equency, and
the AFC should be enabled with some delay after the start of continuous conversions to allow the CS5463
to automatically track any line frequency variation.
2 AN336REV1
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