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AN-641 APPLICATION NOTE

One Technology Way ? P.O. Box 9106 ? Norwood, MA 02062-9106 ? Tel: 781/329-4700 ? Fax: 781/326-8703 ? www.analog.com

A 3-Phase Power Meter Based on the

ADE7752

By Stephen English and Rachel Kaplan

INTRODUCTION This application note describes a high accuracy, low cost 3-phase power meter based on the ADE7752. The meter is designed for use in a Wye-connected 3-phase, 4-wire distribution system. The ADE7752 may be designed into 3-phase meters for both 3-wire and 4-wire service. This reference design demonstrates the key features of an ADE7752 based meter, and is not intended for production. The ADE7752 is a low cost single-chip solution for electrical energy measurement that surpasses the IEC 61036 Class 1 meter accuracy requirement. It typically realizes less than 0.1% error over a 500:1 current dynamic range for balanced polyphase loads. The chip contains a reference circuit, analog-to-digital converters, and all of the digital signal processing necessary for the accurate measurement of active energy. A differential output driver provides direct drive capability for an electromechanical counter, or impulse counter. A high frequency pulse output is provided for calibration. An additional logic output on the ADE7752, REVP, indicates negative active power on any phase or a possible miswiring. The ADE7752 data sheet describes the device’s functionality in detail and is referenced several times in this document. DESIGN GOALS Speci?cations for this Class 1 meter design are in accordance with the accuracy requirements of IEC 61036, and Indian Standards IS 13779-99. Tables I and II review the overall accuracy at unity power factor and at low power factor. Table I shows the speci?cations of the meter for both balanced loads and balanced lines. Table II addresses balanced polyphase voltages with a single-phase load. The meter was designed for an IMAX of 50 A/phase, an Ib of 5 A/phase, and a 100 impulses/kWh meter constant. The ADE7752 provides a high frequency output at the CF pin. This output is used to speed the calibration process and provide a means of quickly verifying meter functionality and accuracy in a production environment. CF is 16 times F1, F2, the frequency outputs. In this case, CF is calibrated

to 1600 impulses/kWh. The meter is calibrated by varying the attenuation of the line voltage using the resistor networks on each phase. Each phase to neutral voltage is 240 V. See the Channel 2 Input Network section. An additional speci?cation for this meter design is taken from IS 13779-99. The speci?cation states that the meter must work with only one phase active at 30% lower and 20% higher than the nominal line value. Table I. Accuracy Requirements (for a polyphase balanced load) Current Value1 0.05 Ib ? I < 0.1 Ib 0.1 Ib ? I < IMAX 0.1 Ib ? I < 0.2 Ib PF2 1 1 0.5 inductive 0.8 capacitive Percentage Error Limits3 Accuracy Class 1 Class 2 ±1.5% ±1.0% ±1.5% ±1.5% ±2.5% ±2.0% ±2.5%

NOTES 1The current ranges for speci?ed accuracy shown in Table I are expressed in accordance with IEC 61036, Table 15 percentage error limits, Section 4.6.1, p. 53. 2 Power factor (PF) in Table I relates to the phase relationship between the fundamental voltage and current waveforms. In this case, PF can be de?ned as PF = cos( ), where is the phase angle between pure sinusoidal current and voltage. 3 Accuracy is de?ned as the limits of the permissible percentage error. The percentage error is de?ned as:

Percentage Error = energy registered by meter –true energy

true energy

×100%

(1)

Table II. Accuracy Requirements* (for a polyphase meter with single-phase load) Current Value 0.1 Ib ? I < IMAX 0.2 Ib ? I < IMAX

* Accuracy

PF 1 0.5 inductive

Percentage Error Limits Accuracy Class 1 Class 2 ±2.0% ±2.0% ±3.0% ±3.0%

class for unbalanced load as de?ned in IEC 61036, Table 13, Section 4.6.1, p. 53, Edition 2.1.

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Figure 1 is a block diagram of a low cost, simple watthour meter using the ADE7752. It shows the three phases and how they are connected to the meter. Three current transformers sense the load current and convert the signals to a proportional voltage required by the ADE7752. The total energy is registered by a mechanical counter.

240V 240V 240V

An opto-isolator is provided on this meter, connected to the CF pin of the ADE7752. This allows calibration of the meter while isolating the calibration equipment from the line voltages. The instantaneous power and energy are calculated per phase, and the net active energy is accumulated as a sum of the individual phase energies inside the ADE7752. With the ABS pin set low, the sum represents the absolute values of the phase energies. With the ABS pin high, the ADE7752 takes into account the signs of the individual phase energies and performs a signed addition. In the meter described in this application note, ABS is set high. If negative active power is detected on any of the three phases, the REVP output LED of the ADE7752 is lit. This feature is useful to indicate meter tampering or to ?ag installation errors. The ADE7752 continues to accumulate energy despite the status of the REVP output pin. REVP will reset when positive power is detected again. The output of REVP and the CF pulse are synchronous. If more than one phase detects negative power, the REVP LCD remains lit until all phases detect positive power. An LED connected to the CF output of the ADE7752 displays the energy measured in impulses/kWh. The ADE7752 data sheet describes this operation in detail. The frequency outputs, F1 and F2, are used to drive the electromechanical counter. See the Design Equations section. This design has a startup current of 13.75 mA and a no-load threshold of 3.3 W. See the Starting Current section. DESIGN EQUATIONS The ADE7752 produces an output frequency that is proportional to the summed values of the three phase energies. A detailed description of this operation is available in the ADE7752 data sheet. To calibrate the meter, the inputs to the ADE7752 must be de?ned based on the equation:

MECHANICAL COUNTER N VAP VBP VCP ATTENUATION NETWORKS ANTIALIASING FILTERS 6 7 ANTIALIASING FILTERS 4 REVP 16 15 14 5

24 23

ADE7752

8 9

ANTIALIASING FILTERS 10 13 1

CF

LOAD

Figure 1. 3-Phase, 4-Wire/Wye Watthour Meter Block Diagram DETAILED DESCRIPTION The front end of the meter is made up of three pairs of voltage and current input networks. Each of the three line voltages is attenuated and ?ltered through identical antialiasing ?lters. See the Channel 2 Input Network section. The current channels’ signals are converted from current to a voltage through current transformers and burden resistors. The signals are then ?ltered by the antialiasing ?lter on each of the three phases, and the result is applied to the current inputs of the ADE7752. Each phase of the meter has a power supply associated with it. The power supply is shown in Figure 8. If power is lost in two of the three phases, the meter will continue to operate. Each phase has a corresponding LED that is on when the respective phase is active. A calibration network is associated with each of the three line voltages. These circuits use binary-weighted resistor values connected in series to set the amount of attenuation needed for each of the three input voltages. Having ±25% calibration ability to compensate for variations in the voltage reference and input ?lter components is recommended. See the Design Calculations section.

F 1, F 2 =

5.922 × (V1 × I1 + V2 × I2 + V3 × I 3 ) × F1?7 VREF

2

(2)

where: I is the differential rms voltage signal on respective current channels V is the differential rms voltage signal on respective voltage channels VREF is the reference voltage (2.4 V ± 8%) (V) F1–7 is one of ?ve possible frequencies selected by using the logic inputs SCF, S0, and S1. See Table II. The calculations for this meter design are shown in the Design Calculations section. ADE7752 REFERENCE Pin 12 of the ADE7752 can be used to connect an external reference. This design does not include the optional reference circuit and uses the ADE7752 internal reference.

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The on-chip reference circuit of the ADE7752 has a typical temperature coef?cient of 20 ppm/°C. Refer to the ADE7752 data sheet for graphs of typical performance characteristics over temperature. Current Transformer Selection The current transformer is the device used in this design for measuring load current. This sensor arrangement provides isolation because the line-to-line voltage differs by more than 498 V. Current transformers offer an advantage as current sensors because they do not contact the conductor, they handle high current and have low power consumption and low temperature shift. Figure 3 illustrates the application used in this design for each channel of the 3-phase meter. When selecting a current transformer, carefully evaluate linearity under light load. The CT performance should be better than the desired linearity of the meter over the current dynamic range. A current transformer uses the concept of inductance to sense current. A CT is made up of a coil wound around a ferrite core. The current-carrying wire is looped through the center of this winding, which creates a magnetic ?eld in the winding of the CT and a voltage output proportional to the current in the conducting wire. The properties that affect the performance of a given CT are the dimensions of the core, the number of turns in the winding, the diameter of the wire, the value of the load resistor, and the permeability and loss angle of the core material. When choosing a CT, consider the dc saturation level. At some high, ?nite value of current or in the presence of a high dc component, the ferrite core material exhibits hysteresis behavior and the CT can saturate. Manufacturers of CTs can specify this maximum level. The current range is calculated using Equation 3. where: R is the resistance of the burden resistor and the copper wire. represents the core losses. L is a parameter based on the permeability of the core, the dimensions of the core, and the square of the number of turns. The phase error caused by a particular CT should be measured with and compensated for by a low-pass ?lter before the ADC inputs. Phase mismatch between channels will cause energy measurement errors. See the Correct Phase Matching between Channels section. Low-pass ?lters are already required by the ADE7752 for antialiasing, and are covered in more detail in the Antialias Filters section. The corner frequency of these antialiasing ?lters on the current channels can be ?ne tuned by changing the components in the RC circuit in order to add additional compensation for CT phase shift. Channel 1 Input Network Figure 3 shows the input stage to Channel 1 of the meter. The current transformer has a turns ratio of 1500:1. The burden resistor is selected to give the proper input voltage range for the ADE7752, less than 500 mVPEAK. See the Design Calculations section. The additional components in the input network provide ?ltering to the current signal. The ?lter corner is set to 4.8 kHz for the antialias ?lters. See the Antialias Filters section.

PHASE A LOAD CURENT R82 R15 C16

IAP

R83 R17 C17 IAN

I MAX ≈

where:

ωN R

2 sec

B sat AFe

(3)

CURRENT TRANSFORMER

Figure 2. ADE7752 Phase A – CT Wiring Diagram The burden is center tapped so that external capacitive coupling may be reduced. The wires of the CT are twisted tightly to reduce noise. Channel 2 Input Network The meter is calibrated by attenuating the line voltage down to 70 mV. See the Design Calculations section. The line voltage attenuation is carried out by a resistor divider as shown in Figure 4. Phase matching between Channel 1 and Channel 2 is important to preserve in this network. Figure 4 shows the attenuation network for the voltage inputs. All three phases have the same attenuation network. The –3 dB frequency of this network, on Phase A for example, is determined by R75 and C21 because the sum of the other resistors in the network is much greater than R75. The approximate equation is shown in Figure 3.

R is the resistance of the burden resistor and the copper wire. AFe represents the dimensions of the core. Bsat is the value of the magnetic ?eld at which the core material saturates. Nsec is the number of turns in the CT. CTs may also cause a phase shift of the signal. A CT used for metering should have a linear phase shift across the desired current dynamic range. The phase error for a CT is derived using Equation 4.

tan? =

R cosδ ωL

(4)

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PHASE A 240V R80 R81 R75 R76 R66 R64 R68 R65 R70 R78 R79 R73 C21 VAP 70mV

is required. The ?rst measurement should be at the test current, Ib, with unity power factor and the second at low power factor (0.5 capacitive). The measurement error is processed according to the following equation:

f–3dB = (2

R75

C21)

Error =

CFPF = 0.5 –

CFPF = 1 2

CFPF = 1 2

(6)

The phase error is then:

Phase Error = –arcsin

Figure 3. Attenuation Network Because the ADE7752 transfer function is extremely linear, a one-point calibration (Ib) at unity power factor is all that is needed to calibrate the meter on each phase. If the correct precautions were taken at the design stage, no calibration is necessary at low power factor (PF = 0.5). CORRECT PHASE MATCHING BETWEEN CHANNELS Correct phase matching is important in energy metering applications because any phase mismatch between channels will translate into signi?cant measurement error at low power factor. The errors induced in the system at PF = 1 are minimal. A power factor of 0.5 with a phase error of as little as 0.5° will cause a 1.5% error in the power measurement. If current lags the voltage by 60° (PF = –0.5) and pure sinusoidal conditions are assumed, the power is easily calculated, on a single phase, as V rms I rms cos(60°). An additional phase error can be introduced to the overall system with the addition of antialiasing ?lters. Phase error ( e) is introduced externally to the ADE7752 (e.g., in the antialias ?lters). The error is calculated as

Error 3

(7)

For a single-pole RC low-pass ?lter, the phase lag is:

θ = – arctan(2πf × RC )

(8)

For example, if the antialias ?lters are single-pole lowpass ?lters with R = 1 k and C = 33 nF, the phase lag at 50 Hz is 0.59° according to Equation 8. If the measurements performed with this ?lter in place on the current and voltage phases show that the CT causes 1° phase error (using Equations 6 and 7), then the resistor value should be 2.68 k to give 1.59° total phase shift. Because there is generally minimal part-to-part variation for CTs, the same ?lters usually can be used in production on all three phases to compensate for the constant phase error. ANTIALIAS FILTERS As mentioned in the previous section, one possible source of external phase errors is the antialias ?lters on the input channels. The antialias ?lters are low-pass ?lters placed before the analog inputs of any ADC. They are required to prevent aliasing, a possible distortion due to sampling. Figure 4 illustrates the effects of aliasing.

ALIASING EFFECTS

%Error = cos(δ°) – cos(δ + φe ) / cos(δ°) × 100%

[

]

(5)

IMAGE FREQUENCIES

See Note 3 for Table I, where is the phase angle between voltage and current and e is the external phase error. With a phase error of 0.2°, for example, the error at PF = 0.5 inductive (60°) is calculated as 0.6%. As this example demonstrates, even a very small phase error will produce a large measurement error at low power factor. Current transformers often produce a phase shift between the current and voltage channels. To reduce the error caused at low power factor, the resistors in the antialias ?lter can be modi?ed to shift the corner frequency of the ?lter (in the current channel), introducing more or less lag. The antialias ?lters are described in detail in the next section. The phase error should be measured independently on each phase (A, B, and C). To calibrate the phase error on one phase of the meter, a two-point measurement

SAMPLING FREQUENCY

0

2

417 FREQUENCY (kHz)

833

Figure 4. Aliasing Effects Figure 4 shows how aliasing effects could introduce inaccuracies in an ADE7752 based meter design. The ADE7752 uses two - ADCs to digitize the voltage and current signals for each phase. These ADCs have a very high sampling rate, i.e., 833 kHz. Figure 4 shows how frequency components (indicated by the darker arrows) above half the sampling frequency (also known as the Nyquist frequency), i.e., 417 kHz, get imaged or folded back down below 417 kHz (indicated by the gray arrows). This will happen with all ADCs, regardless of the architecture.

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In the example shown, only frequencies near the sampling frequency, i.e., 833 kHz, will move into the band of interest for metering (0 kHz to 2 kHz). This fact allows the use of a very simple LPF (low-pass ?lter) to attenuate these high frequencies (near 833 kHz) and thus prevent distortion in the band of interest. The simplest form of LPF is the simple RC ?lter, which has a single pole with a roll off or attenuation of –20 dBs/decade. Choosing the Filter –3 dB Frequency In addition to having a magnitude response, ?lters also have a phase response. The magnitude and phase response of a simple RC ?lter (R = 1 k , C = 33 nF) are shown in Figures 5 and 6. Figure 5 shows that the attenuation near 900 kHz for this simple LPF is greater than 40 dBs. This is suf?cient attenuation to ensure that no ill effects are caused by aliasing.

0dB

frequency components to be aliased and cause accuracy problems in a noisy environment.

0

–20 PHASE (Degrees)

–40

–60

–80

–100 10

100

1k

10k

100k

1M

FREQUENCY (Hz)

Figure 6. RC Filter Phase Response

–0.4 (50Hz, –0.481)

–20dB –0.5 PHASE (Error)

(R = 900 , C = 29.7nF)

(50Hz, –0.594) (R = 1k , C = 33.0nF) –0.6

–40dB

–0.7 –60dB 10 100 1k 10k FREQUENCY (Hz) 100k 1M –0.8

(50Hz, –0.718) (R = 1.1k , C = 36.3nF)

Figure 5. RC Filter Magnitude Response The phase response can introduce signi?cant errors if the phase response of the LPFs on both current and voltage channels are not matched. This is true for all of the phases in which the desired (120°) phase shift between phases should be preserved. Phase mismatch can easily occur as a result of poor component tolerances in the LPF The . lower the –3 dB frequency in the LPF (antialias ?lter), the more pronounced these errors will be at the fundamental frequency component or line frequency. Even with the corner frequency set at 4.8 kHz (R = 1 k , C = 33 nF), the phase errors due to poor component tolerances can be signi?cant. Figure 7 illustrates this point. In Figure 6, the phase response for the simple LPF is shown at 50 Hz for R = 1 k ± 10%, C = 33 nF ± 10%. Remember, a phase shift of 0.2° can cause measurement errors of 0.6% at low power factor.This design uses resistors of 1% tolerance and capacitors of 10% tolerance for the antialias filters to reduce the likelihood of problems resulting from phase mismatch. Alternatively, the corner frequency of the antialias filter could be pushed out to 10 kHz to 15 Hz. The corner frequency should not be made too high, however, because doing so could allow high

45

50 FREQUENCY (Hz)

55

Figure 7. Phase Shift at 50 Hz Due to Component Tolerances Note that this risk is also why precautions were taken with the design of the calibration network on the voltage channels. The tolerance of the components used in these networks is low to prevent errors. CALIBRATING THE METER The meter is calibrated by setting the appropriate value to the S1 and S0 pins and by varying the gains of the voltage channels. The current channels are ?xed by the turns ration of the CT and the burden resistor. To ensure the proper output frequency, the meter is calibrated using CF. The gains of the voltage channels are varied to ensure that the product of the current and voltage channels (active energy) is calibrated to 1600 impulses/kWh. The voltage channel uses a resistor divider network to adjust the attenuation. The setup of this network is described in the Channel 2 Input Network and Design Calculations sections.

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DESIGN CALCULATIONS The goal of the design calculations is to achieve appropriate input signal levels for the Channel 1 and Channel 2 ADCs. Each channel requires a voltage input less than 500 mVPEAK. The input levels should be set up so that the full dynamic range of current results in a frequency output on F1, F2 that will drive the stepper motor counter. The frequency outputs can be calculated using Equation 1 set equal to the maximum output frequency, which corresponds to the nominal line voltage and maximum current. To use Equation 9 for calculating the Channel 2 input level, ?x the Channel 1 input level to some percentage of full scale and choose an appropriate F1–7 value from the ADE7752 data sheet. This F1–7 value should allow the voltage input to be less than 500 mVPEAK. Since the current channel requires a voltage input, a burden resistor is used to yield the calculated input level. The line voltage must be trimmed with a resistor network to the appropriate ADC input level. Calculate F1, F2, and CF The frequency for F1 and F2 can be calculated using Equation 9. Choose F1–7 from Table III. The expression can be evaluated to ?nd the corresponding input voltage. In this case, F1–7 is 4.77, so V = 0.105 V. Other values for F1–7 do not yield reasonable results for the voltage, i.e. some fraction of full scale rms input to allow for headroom; 105 mV is 30% of the full scale rms voltage input. The following calculation demonstrates that suf?cient headroom is achieved if the voltage input that results with the maximum line voltage (288 V rms from the IS Speci?cation) is below the full scale ADC input level (500 VPEAK or 353 V rms). For three phases at maximum power, where POWERMAX = 3 50 Arms 288 V rms = 43.2 kW 43.2 kW = 4320 impulses/H 1 H/3600S = 1.2 Hz

F1, F2MAX = 100 impulses/kWh = 4320 impulses/H

Plugging 1.2 Hz into Equation 9 and solving for the voltage with F1–7 = 4.77, as done in the previous calculation, the voltage input is 126 mV. This value is 36% of the full-scale rms voltage input. With F1–7 of 4.77 selected, suf?cient headroom has been achieved. Max F1/F2 is chosen as 1.83 Hz with S1 = 0 and S0 = 1. Table III in the ADE7752 data sheet shows the choices for Max F1/F2. The desired CF in this case is 1600 impulses/kWh. Knowing that CF = k ? F1, F2 = 1600 impulses/kWh = 16 100 impulses/kWh; SCF is chosen to be 1 so that the meter constant is 16 times that of the stepper motor ratio. Table III. F1–7 Frequency Selections and Max Output Frequency SCF 0 1 0 1 0 1 0 1 S1 0 0 0 0 1 1 1 1 S0 0 0 1 1 0 0 1 1 F1–7 1.27 1.19 5.09 4.77 19.07 19.07 76.29 0.60 Max F1/F2 (Hz) 0.49 0.46 1.95 1.83 7.33 7.33 29.32 0.23 Max CF (Hz) 78.19 3.66 312.77 29.32 117.30 58.65 234.59 3.67

F1,F2 =

5.922 × (V1 × I1 + V2 × I2 + V3 × I 3 ) × F1?7 VREF

2

(9)

Design parameters: Line voltage = 240 Vrms (each phase to neutral) VMAX = 240 V rms + 20% = 288 V rms Class 100 meter with IMAX = 50 A rms Meter constant = 100 impulses/kWh CF = 1600 impulses/kWh CT turns ratio (CTTRN) = 1500:1 There are 10 different choices of frequency output through SCF, S1, and S0 pins. To choose the proper frequency, the maximum F1, F2 frequency output using the line voltage and IMAX must be calculated. For three phases at maximum power, where POWERMAX = 3 50 A rms 240 V rms = 36 kW 36 kW = 3600 impulses/H 1 H/3600S = 1 Hz

F1, F2MAX = 100 impulses/kWh = 3600 impulses/H

Calculate RBURDEN At maximum current, the voltage input signal at the current channel is: IRMS/CTTRN = 50 A rms/1500 = 33.33 mA rms VIN = 500 mVPEAK or 353.6 mV rms; 60% V rms = 215 mV rms RBURDEN = 215 mV rms/33.33 mA rms = 6.45 Since the input signal is differential for each channel, the burden resistor is split in two to yield 3.23 ? 2.

At maximum current, the input signal at the current channel should be some fraction of full scale to allow headroom. Equation 1 can be used to choose an H frequency by fixing the current input to be 60% of full scale rms, 215 mV rms.

1 Hz =

5.922 × 3 × (V × 0.215) × F1–7

(2.4)

2

(10)

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Calculate Attenuation Network Each phase will have the same attenuation network. From Equation 9, V = 105 mV rms. The line voltage of 240 V must be trimmed to this value. The attenuation network is calculated to be 240 V/105 mV or an attenuation of 2285:1. POWER SUPPLY DESIGN The IEC 61036 speci?cation requires the meter to have a mean power consumption of 2 W or 10 VA for a polyphase meter. The IS speci?cation for the power supply is 1 W per voltage channel. Another key specification that relates to power supply design requires the power supply to operate with only one phase active at 70% of nominal. The line voltage may vary from –30% to +20%, in accordance with the Indian standard. The current drawn from the power supply at the regulator, VR1, output with no load is 9.75 mA. When the stepper motor engages (with a 10 A load applied), the current draw increases by 15 mA. The result is approximately 25 mA of current draw at 5 V. This is equivalent to 0.125 W of peak power consumption. The current draw of one phase of the meter is 10 mA, measured at the line input. At 240 V with all three phases running, the total power consumption of this design is 7.2 VA. Since the line voltage varies from 168 V to 288 V, a power supply that will work over this extended range is needed. This design uses a power supply based on three power transformers that transfer power rather than current or voltage. For this reason, as the line voltage decreases, the current increases, keeping the power used by the supply constant. Figure 8 shows a diagram of the power supply circuit. The supplies for this meter are three full wave recti?ed supplies connected in parallel through diodes. The output of this circuit is then ?ltered and regulated to 5 V. The MOV-Ferrite bead at the input to the power supply is used to minimize the effect of electrical fast transients. Large differential signals may be generated by the inductance of the PCB traces and signal ground. These large signals may affect the operation of the meter. The analog sections of the meter will ?lter the differential signal and minimize the effect on the duration of the pulse. The ferrite and capacitor create a low-pass ?lter before the MOV. In an EFT event, this ensures protection during the small time it takes for the MOV to turn on. For more information concerning this issue, see Application Note AN-559. An LED on each phase of the power supply indicates the status of power on that phase. Blocking diodes prevent the LED from lighting when the voltage to the phase is shorted. Without this diode, current ?ow from the other phases could light the indicator LED. At the output of the regulator, C12 and C2 ?lter ripple that could degrade the performance of the power supply.

L3 V1 240V PHASE A C1

T3

CR3

1k

L2 V2 240V PHASE B C2

T2

CR2 VR1 7805 1k C12 + VDD + 5V C2 –

L1 V3 240V PHASE C C24

T1

CR1

1k

Figure 8. Power Supply

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STARTING CURRENT The no-load threshold and start-up current features of the ADE7752 eliminate creep effects in the meter. A load that generates a frequency lower than the minimum frequency will not result in a pulse on F1, F2, or CF. The minimum output frequency is de?ned in the ADE7752 as 0.005% of the full-scale output frequency (F1, F2) for the F1–7 selection. For this meter, the minimum output frequency on F1, F2 is 9.15 10–5 Hz, and the meter constant is 100 impulses/kWh. (100 impulses/kWh)(1 H/3600)(P) = 9.15 10–5 Hz

The minimum load then becomes 3.3 W, which translates to 13.75 mA of startup current at 240 V. The IEC speci?cation for the no-load threshold, Section 5.6.4, states that the meter should not register a pulse for a speci?ed time with the voltage at 115% VREF and open circuit current. The no-load threshold described above for the ADE7752 ensures compliance with this speci?cation. The IEC speci?cation, section 4.6.4.3, for start-up current is 0.4% of Ib, or 40 mA with an Ib = 10 A. According to the speci?cation, the meter must start and continue to register current at this level. The design of the meter described in this application note meets this speci?cation by starting up at 13.75 mA, as calculated. ADE7752 REFERENCE DESIGN PERFORMANCE This reference design surpasses the IEC 61036 Class 1 accuracy requirements, as outlined in Section 4.6.1 of the IEC 61036 standard. The typical performance plots shown demonstrate the performance of this reference design against the IEC accuracy limit. Voltage and frequency variation tests were performed according to Section 4.6.2 of the IEC 61036 standard.

Figure 9. Final Implementation of ADE7752 Reference Design

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2.0 1.5 1.0 IEC LIMIT 0.5 ERROR (%) ERROR (%) –0.3 PHASE B ONLY –0.4 PHASE A ONLY –0.5 –1.0 IEC LIMIT –1.5 –2.0 0.1 –0.6 –0.7 0.1 0 PF = 1 –0.5 BALANCED LOAD PF = 1 IEC 61036 4.6.1 0 –0.1 –0.2 UNBALANCED LOAD PF = 0.5 IEC 61036 4.6.1 IEC LIMIT = 2%

PHASE C ONLY

1.0 CURRENT (A)

10

100

1.0 CURRENT (A)

10

100

TPC 1. Balanced Polyphase Load with Unity Power Factor

2.0 1.5 1.0 0.5 ERROR (%) PF = 0.8 CAP 0 –0.5 –1.0 IEC LIMIT –1.5 –2.0 0.1 PF = 0.5 IND BALANCED LOAD IEC 61036 4.6.1 IEC LIMIT

TPC 4. Unbalanced Load over Power Factor

0.8 IEC LIMIT 0.6 0.4 0.2 ERROR (%) 216V 0 –0.2 –0.4 –0.6 IEC LIMIT –0.8 0.1 264V BALANCED LOAD PF = 1 IEC 61036 4.6.2

1.0 CURRENT (A)

10

100

1.0 CURRENT (A)

10

100

TPC 2. Balanced Polyphase Load over Power Factor

0.30 0.25 0.20 0.5 ERROR (%) PHASE C ONLY PHASE B ONLY PHASE A ONLY ERROR (%) 0.15 0.10 0.05 0 –0.05 –0.10 0.1 UNBALANCED LOAD PF = 1 IEC 61036 4.6.1 IEC LIMIT = 2% 1.0 1.5

TPC 5. Voltage Variation ±10% from 240 V with Unity Power Factor

BALANCED LOAD PF = 0.5 IEC 61036 4.6.2

IEC LIMIT

0 264V (240V+10%) –0.5 216V (240V–10%) –1.0 IEC LIMIT –1.5 0.1

1.0 CURRENT (A)

10

100

1.0 CURRENT (A)

10

100

TPC 3. Unbalanced Load with Unity Power Factor

TPC 6. Voltage Variation ±10% from 240 V with Power Faction = 0.5 Inductive

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AN-641

0.6 0.4 BALANCED LOAD PF = 1 IEC 61036 4.6.2 0.25 IEC LIMIT 0.20 0.15 0.2 0.10 ERROR (%) 49Hz 0 51Hz –0.2 –0.05 –0.4 IEC LIMIT –0.6 0.1 1.0 CURRENT (A) 10 100 –0.10 –0.15 0.1 168V ERROR (%) 0.05 192V 0 288V BALANCED LOAD PF = 0.5 IS 13779:1999 11.2 IS LIMIT –30% = 3.5% IS LIMIT 20% = 2.1%

1.0 CURRENT (A)

10

100

TPC 7. Frequency Variation ±2% from 50 Hz with Unity Power Factor

0.8 IEC LIMIT 0.6 0.4 0.2 ERROR (%) 0 –0.2 –0.4 –0.6 IEC LIMIT –0.8 0.1 1.0 CURRENT (A) 10 100 BALANCED LOAD PF = 0.5 IEC 61036 4.6.2

TPC 9. Indian Standard Voltage Variation +20% and –30% from 240 V with Unity Power Factor

–0.1 BALANCED LOAD PF = 0.5 IS 13779:1999 11.2 IS LIMIT –30% = 5% IS LIMIT 20% = 3%

–0.2

–0.3 ERROR (%) 192V –0.4

51Hz 49Hz

–0.5 288V –0.6 168V

–0.7 0.1

1.0 CURRENT (A)

10

100

TPC 8. Frequency Variation ±2% from 50 Hz with Power Factor = 0.5 Inductive

TPC 10. Indian Standard Voltage Variation +20% and –30% from 240 V with Power Factor = 0.5 Inductive

–10–

REV. 0

REV. 0

825 1 4 TP13 TP14 3 NECP52501-1 2 DGND VDD R11 CR2 RED U1 R92 0 R91 0 R1 VDD C8 0.1 F K1 C4 22pF R12 1.02k Y1 10mHz C5 22pF C1 1 F DGND DGND C9 0.1 F DGND DGND C7 0.1 F

R2

R15 590

P1

R82 2.2

C16 0.056 F

AGND R3 0 C6 0.1 F

R83 2.2

AGND

C17 0.056 F

P2

P3 K2

R17 590 R16 590

R87 2.2 AGND

C15 0.056 F

AGND

AGND

R88 2.2

C14 0.056 F

P4

R14 590 R18 590 PHASE A

AGND

1 2 3 4 5 6 7 8 9 10 11 12

F1 CF F2 DGND S1 VDD S0 REVP IAP CLKOUT CLKIN IAN SCF IBP IBN ABS VAP ICP VBP ICN VCP AGND REF VN 1 T3 5 CR3 R10 0 3 4 R7 0 AGND PHASE B AGND AGND AGND 2 3 4 14VA AGND AGND PHASE C R13 590 C13 0.056 F 1 T5 5 CR5 2 3 4 6 7 8 14VA AGND 590 AGND 0 R55 R54 590 1.18k C20 0.056 F AC2 AC1 + – 1

R84 1.18k

24 23 22 21 20 19 18 17 16 15 14 13

P5 AGND + C3 0.1 F R9 0 R8 0 150MHz AGND R6 0 R50 AGND DGND C10 0.1 F 2 6 7 8 14VA 1 T4 5 VAL REFDES–U2 PACKAGE TYPE 9024

R89 2.2

C18 0.056 F

AC2 AC1

+ –

1

R86 1.18k

CR11

UA7BL05ACD VR1 VIN 2

C26 CR8 470 F

GND VOUT VDD C12 0.01 F CR4 6 7 8 AC2 AC1 + – 1

R85 1.18k CR10

AGND

R90 2.2 R4 825 RED CR1 DGND R28 R37 0 R34 2.32k 1.18k 590 0 R35 0 R36 R38 R39 R25 909k 200k 100k 49.9hk 25.5k 11.5k 5.11k 1M 0 R20 0 R21 0 R22 0 R23 0 R24 0 R33 R26 R32 R27 R31 R29 R30

AGND C19 0.056 F

Figure 10. Reference Design Schematic

R46 R40 0 R44 2.32k 0 R43 R41 R59 909k 200k 100k 49.9k 25.5k 11.5k 5.11k 1M 0 R45 0 R47 0 R49 0 R58 0 R52 0 R57 R60 R48 R50 R51 R53 R56 R42 R67 R80 909k 200k 100k 49.9k 25.5k 11.5k 1M 0 R66 0 R68 0 R70 0 R79 0 R73 R81 R69 R71 R72 R74 R77 0 R78 5.11k R61 0 R65 2.32k R62 0 R64 1.18k R63 0 R76 590 R75 590 C21 0.056 F

–11–

P6

R19 590

C2 10 F 25V AGND 2

CR7

C25 470 F

AGND

AGND

L1

PHASE C

TP7

150MHz

TP8

CR9

C22 0.01 F

V1

AGND

AGND

AGND

2

CR6

C11 470 F

AGND

L2

PHASE B

TP9

150MHz

P10

C23 0.01 F

V2

AGND

AGND

AGND

AGND

L3

PHASE A

P11

150MHz

P12

C24 0.01 F

V3

AGND

AGND

AGND

AGND

AN-641

AN-641

Figure 11. Reference Design Component Placement

–12–

REV. 0

AN-641

Figure 12. Reference Design PCB Layout

REV. 0

–13–

AN-641

IV. Bill of Materials # QTY 1 1 1 2 5 3 1 9 REFDES C1 C2 C3 C4, C5 C6, C7, C8, C9, C10 C11, C25, C26 C12 C13, C14, C15, C16, C17, C18, C19, C20, C21 3 3 3 3 1 3 1 12 C22, C23, C24 CR1, CR2, CR6, CR7, CR8 CR3, CR4, CR5 CR9, CR10, CR11 L1, L2, L3, L4 R4, R11 R12 R13, R14, R15, R16, R17, R18, R19, R36, R54, R55, R75, R76 3 3 3 3 3 3 3 3 3 3 6 R20, R45, R66 R21, R47, R68 R22, R49, R70 R23, R58, R79 R24, R52, R73 R25, R59, R80 R26, R60, R81 R33, R57, R78 R34, R44, R65 R35, R43, R64 R82, R83, R87, R88, R89, R90 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 RESR1206 R1206 R1206 R1206 R1206 R1206 R1206 R1206 R1206 R1206 R1206 R1206 200 k 100 k 49.9 k 25.5 k 11.5 k 909 k 1M 5.11 k 2.32 k 1.18 k 2.2 CRAD1024 LED DIODE RECT DIODE SGNL FERRITE BEAD RESR1206 RESR1206 RESR1206 DF045 1N4148 1806 R1206 R1206 R1206 150 MHz 825 1.02 k 590 L433 0.01 F, 250 V RED CAP CAPC1206 CAPC1210 C1206 C1210 470 F, 25 V 0.01 F, 50 V 0.056 F, 16 V Device CAPC080 C-D7343 CAPC3216 CAPC1206 CAPC1206 Package C0805 DCASE C3216 C1206 C1206 Value 1 F, 16 V 10 F, 6.3 V 10 F, 250 V 22 pF, 50 V 0.1 F, 50 V

–14–

REV. 0

AN-641

# QTY 38 REFDES R3, R5, R6, R7, R8, R9, R10, R92, R27, R28, R29, R30, R31, R32, R37, R38, R39, R40, R41, R42, R46, R48, R50, R51, R53, R56, R61, R62, R63, R67, R69, R71, R72, R74, R77, 3 3 12 R84, R85, R86 T3, T4, T5 TP1, TP2, TP3, TP4, TP5, TP6, TP7, TP8, TP9, TP10, TP11, TP12 2 1 1 3 1 1 3 1 1 TP13, TP14 U1 U2 V1, V2, V3 VR1 Y1 CT PCB CASE Connector NECP52501-1 AD7752 MOV UA78L05AILP XTALHC49 Current Transformer TO-226AA HC49 UA78L05AI 10 MHz CNLOOPTP DIP04 SO24 VAL PS2501-1 VAL RESR1206 Transformer Connector CNLOOPTP R1206 1.18 kW VAL ORG Device RESR1206 Package R1206 Value 0

REV. 0

–15–

? 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies.

–16–

E03613–0–4/03(0)

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