PM9904BPD Ver la hoja de datos (PDF) - South African Micro Electronic Systems

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PM9904BPD

Evaluation Board for the SA9904B Energy Metering IC

South African Micro Electronic Systems 

PM9904BPD

CIRCUIT DESCRIPTION

ANALOG SECTION

The analog (metering) interface described in this section is

designed for measuring 3 x 230V/80A with precision better

than Class 1.

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 temperature effects.

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

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

86.4mV across each resistor at rated conditions.

Current Sense Resistors

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.

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

where:

I = Line current / CT Ratio

The three current channels are identical so R1 = R2 = R3 =

R4= R5 = R6.

sames

Voltage Divider

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

a 1MW resistor.

The following equation is used to calculate the 14V voltage

drop:

RA = R22 + R23 + R24 +R25

RB = R8 || R13

Combining the two equations gives:

(RA + RB) / 230V = RB / 14V

A 24kW resistor is chosen for R13 and a 1MW resistor is used

for R8.

Substituting these values result in:

RB = 23.44kW

RA = RB x (230V / 14V - 1)

RA = 361.6kW

Resistor values of R22, R24 are chosen to be 82kW and

resistors R23 and R25 is chosen to be 120kW each.

The three voltage channels are identical so

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

calculated as follows:

C = 1 / ( 2 x p x Mains frequency x R5 x tan (Phase shift angle))

C = 1 / ( 2 x p x 50Hz x 1MW tan (0.18 degrees ))

C = 1.013µF

V1In

V1 Out

CT1

TZ76

R1

R26

2.7k

2.7R

R2

GND

2.7k

Figure 10: Current input configuration

Pin 19

Pin 18

L1 R22

R23

V1In

82k

120k

J3

R24

R25

82k

120k

C5 R8

R13 1u 1M

24k

GND

Figure 11: Mains voltage divider

Pin 17

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