fs6611 Fortune Semiconductor Corporation, fs6611 Datasheet - Page 15

• Manufacturer Part Number: fs6611
• Description: Energy Metering Ic With Impulse Output
• Manufacturer: Fortune Semiconductor Corporation

Rev. 1.1
Fig12 Real Power to Impulse Converter
As can be seen in Fig12, the frequency output CF is
seen to vary over time, even under steady load
conditions. This frequency variation is primarily due
to the cos(2ωt) component in the instantaneous real
power signal. The output frequency on CF can be up
to 2048 times higher than the frequency on F1 and
F2. This higher output frequency is generated by
accumulating the instantaneous real power signal
over a much shorter time while converting it to a
frequency. This shorter accumulation period means
less averaging of the cos(2ωt) component. As a
consequence, some of this instantaneous power
signal passes through the digital to frequency
converter. This will not be a problem in the
application. When CF is used for calibration
purposes, the frequency should be averaged by the
impulse counter. This will remove any ripple. If CF is
measuring energy, e.g., in a microprocessor based
application, the CF output should also be averaged
to calculate power. Because the outputs F1 and F2
operate at a much lower frequency, more averaging
of the instantaneous real power signal is carried out.
The result is a greatly attenuated sinusoidal content
and a virtually ripple-free frequency output.
13.7
The easiest way to interface the FS6611 to a
micro-Controller is to use the CF high frequency
output with the output frequency scaling set to
2048×F1, F2. This is done by setting SCF = 0 and
S0 = S1 = 1 (see Table Ⅳ). With full-scale AC
signals on the analog inputs, the output frequency
on CF will be approximately 5.57 kHz. Fig13
illustrates one scheme that could be used to digitize
the output frequency and carry out the necessary
averaging mentioned in the previous section.
v ( t)
i( t)
Ins tantaneous Real Pow er Signal
Interfacing the
Controller for Energy Measurement
V ×
2
I
DSP
0
( Fr eq Domain)
ω
FS6611
2
C F
F 1
F 2
ω
F1
CF
F req
to a Micro-
T ime
T ime
Fig13 Interfacing the FS6611 to MCU
As shown, the frequency output CF is connected to
an MCU counter or port. This will count the number
of pulses in a given integration time that is
determined by an MCU internal timer. The average
power proportional to the average frequency is given
by:
Average Frequency = Average Real Power =
The energy consumed during an integration period is
given by:
Energy=Average Power×Time=
For the purpose of calibration, this integration time
can be 10 to 20 seconds to accumulate enough
pulses to ensure correct averaging of the frequency.
In normal operation, the integration time can be
reduced to one or two seconds depending, for
example, on the required update rate of a display.
With shorter integration times on the MCU, the
amount of energy in each update may still have
some small amount of ripple, even under steady
load conditions. However, over a minute or more,
the measured energy will have no ripple.
13.7.1
Calculating and displaying power information will
always have some associated ripple that will depend
on the integration period used in the MCU to
determine average power and also the load. For
example, at light loads, the output frequency may be
10Hz. With an integration period of two seconds,
only about 20 pulses will be counted. The possibility
of missing one pulse always exists, since the
FS6611 output frequency is running asynchronously
to the MCU timer. This would result in a
one-in-twenty
measurement.
Fr equenc y
A v erage
* REV P m us t b e u s e d i f th e m e te r i s bi d i r e c ti o na l
C F
F S6611
o r d i r e c ti o n o f e ne r gy fl ow i s ne e d e d
Power Measurement Considerations
REV P*
C F
(or
Fr equenc y
5%)
Ripple
error
Counter
UP/ DO W N
Time
Co u n te r
Time r
M CU
in
×Time=Counter
Ti m e
the
Counter
Timer
FS6611
<
±
power
10%
15/20