PM9904BPD
ssaammeess
CIRCUIT DESCRIPTION
ANALOG SECTION
VoltageDivider
Referring to figure 11 the connections for the voltage sense
input for one phase is shown. The current into the A/D
converter (IVP) is set 14µARMS at nominal mains voltage. This
voltage sense input saturates at approximately 17µARMS. A
nominal voltage current of 14µA allows for 20% over driving.
Each phase voltage is divided down by a voltage divider to
14V. The current into the voltage sense input is set at 14µA via
a1MW resistor.
The analog (metering) interface described in this section is
designed for measuring 3 x 230V/80A with precision better
thanClass1.
The most important external components for the SA9904B
integrated circuit are the current sense resistors, the voltage
sense resistors and the bias setting resistor. The resistors
used in the metering section are of the same type to minimize
any temperatureeffects.
The following equation is used to calculate the 14V voltage
drop:
RA=R22+R23+R24+R25
RB=R8|| R13
Combiningthetwoequationsgives:
(RA+RB)/ 230V=RB/ 14V
A 24kW resistor is chosen for R13 and a 1MW resistor is used
forR8.
Bias Resistor
Pin VREF (SA9904B pin 15) is connected to Vss via R7 which
determines the on chip bias current. With R7=47kW optimum
conditions are set. VREF does not require any additional
circuitry.
CT Termination Resistor
Substitutingthesevaluesresult in:
RB=23.44kW
RA=RBx(230V/ 14V-1)
RA=361.6kW
The voltage drop across the CT termination resistors should
be at least 16mV at rated current (Imax). The on-board CT's
have low phase shifts and have a ratio of 1:2500. Each CT is
terminated with a 2.7W resistor resulting in a voltage drop of
86.4mVacrosseachresistorat ratedconditions.
Resistor values of R22, R24 are chosen to be 82kW and
resistorsR23andR25ischosentobe120kW each.
CurrentSenseResistors
Referring to figure 10 the resistors R1 and R2 define the
current level into the SA9904B’s current sense inputs (phase
one IIP1 and IIN1). The resistor values are selected for an
input current of 16µA into the current inputs at rated
conditions.
Thethreevoltagechannelsareidenticalso
R14=R16=R17=R18=R20=R22=R24=82kW and
R15=R17=R19=R21=R23=R25=120kW
The capacitors C3, C4 and C5 is used to compensate for
phase shifts between the SA9904’s voltage sense inputs and
current sense inputs. The on-board CT's were characterized
and found to have a constant phase shift of 0.18 degrees. The
value of the phase shift compensation capacitors were
calculatedasfollows:
According to equation described in the Current Sense inputs
section of the datasheet:
R1 = R2 = (I / 16µA) x RSH / 2
= 80A /2500 / 16µA x 2.7W/ 2
= 2.7kW
C= 1/ (2x pxMainsfrequencyxR5xtan(Phaseshift angle))
C= 1/ (2x px50Hzx1MWtan(0.18degrees))
C= 1.013µF
where:
I = Line current / CT Ratio
The three current channels are identical so R1 = R2 = R3 =
R4=R5=R6.
J3
R1
CT1
Pin 19
Pin 18
V1In
C5
R22
82k
R23
R24
82k
R25
R8
1M
L1
2.7k
V1In
Pin 17
R26
120k
120k
2.7R
R13
24k
1u
R2
V1 Out
TZ76
2.7k
GND
GND
Figure 11: Mains voltage divider
Figure 10: Current input configuration
8/22
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