fs7755 Fortune Semiconductor Corporation, fs7755 Datasheet - Page 11

• Manufacturer Part Number: fs7755
• Description: Energy Metering Ic With Impulse Output
• Manufacturer: Fortune Semiconductor Corporation

Rev. 1.0
13. Detail Description
1
The two ADCs digitize the voltage signals from the current and voltage transducers. These ADCs are 16-bit
second order sigma-delta with an oversampling rate of 895 kHz. This analog input structure greatly simplifies
transducer interfacing by providing a wide dynamic range for direct connection to the transducer and also by
simplifying the anti-aliasing filter design. A programmable gain stage in the current channel further facilitates
easy transducer interfacing.
The real power calculation is derived from the instantaneous multiplication of the current and voltage signals. In
order to extract the real power component (i.e., the DC component), the instantaneous real power signal is a
low pass filter output. Fig5 illustrates the instantaneous real power signal. This scheme correctly calculates
instantaneous real power for non-sinusoidal current and voltage waveforms at all power factors. All signal
processing is carried out in the digital domain for superior stability over temperature and time.
2
Theroy of Operation
The low frequency output of the FS7755 is generated by accumulating this real power information. This
low frequency inherently means a long accumulation time between output pulses. The output frequency is
therefore proportional to the average real power. This average real power information can, in turn, be
accumulated (e.g., by a counter) to generate real energy information. Because of its high output frequency
and shorter integration time, the CF output is proportional to the instantaneous real power. This is useful
for system calibration purposes that would take place under steady load conditions.
Power Factor Considerations
The method used to extract the real power information from the instantaneous power signal (i.e., by low
pass filtering) is still valid even when the voltage and current signals are not in phase. Fig6 displays the
unity power factor and a power factor = 0.5 conditions, i.e., current signal lagging the voltage by 60
assume the voltage and current waveforms are sinusoidal, the real power component of the instantaneous
power signal (i.e., the DC term) is given by:
This is the correct real power calculation.
i(t), v(t)
V
V
V
V
2 P
2 N
1 N
1 P
x1 , x8 , x1 6, x 32
P GA
i(t ) & v (t) Sig nal
Ins t ant aneous
A D C
A D C
Time
V ×
P(t)
2
⎛ × I
⎜
⎝
I
V
2
In st ant aneou s Re al
⎞
⎟
⎠
DSP
Power Signa l
×
cos
( )
60
Time
0
F2
CF
F1
Fig5
Signal Flow Diagram
FS7755
0
. If we
11/22