CN0280: Robust Completely Isolated Current Sense Circuit with Isolated Power Supply for Solar Photovoltaic Converters PDF

Circuit Note
CN-0280
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0280.
AD7401A
Isolated Sigma-Delta Modulator
AD8639
Auto-Zero, Rail-to-Rail Output Dual Op Amp
ADuM6000
Isolated, 5kV, DC-to-DC Converter
ADM8829
Switched-Capacitor Voltage Inverter
ADP121
150mA, Low Quiescent Current, CMOS
Linear Regulator
ADP7104
20V, 500 mA, Low Noise, CMOS LDO
ADP7182
−30V, 200mA, Low Noise, Negative Linear
Regulator
Robust Completely Isolated Current Sense Circuit with Isolated Power Supply for
Solar Photovoltaic Converters
EVALUATION AND DESIGN SUPPORT
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit in Figure 1 is a completely isolated current sensor
with an isolated power source. The circuit is highly robust and
can be mounted close to the sense resistor for accurate
measurements and minimum noise pickup. The output is a
single 16 MHz bit stream from a sigma-delta modulator that is
processed by a DSP using a sinc3 digital filter.
The circuit is ideal for monitoring the ac current in solar
photovoltaic (PV) converters where the peak ac voltage can be
several hundred volts, and the current can vary between a few
mA and 25 A.
CIRCUIT DESCRIPTION
The circuit uses a 1mΩ sense resistor to measure peak current
up to ±25 A using a dual AD8639 low offset amplifier. The gain
of the amplifier is set to 10 to take advantage of the full scale
range of the AD7401A sigma delta modulator. Higher currents
can be measured (up to ±50 A or ±100 A) by simply lowering
the gain of the AD8639 accordingly to ensure that the full scale
input range of the AD7401A is used to the maximum advantage.
With a typical offset voltage of only 3 μV, drift of 0.01 μV/°C,
and noise of 1.2 μV p-p (0.1 Hz to 10 Hz), the AD8639 is well
suited for applications in which dc error sources must be
minimized. The solar panel application benefits greatly from
nearly zero drift over the operating temperature ranges. Many
systems can take advantage of the rail-to-rail output swing
provided by the AD8639 to maximize signal-to-noise ratio (SNR).
A guard ring is used around the inputs of the current
measurement circuit to prevent any leakages from entering this
sensitive low voltage area. The BAT54 Schottky diodes protect
the inputs of the AD8639 from transient over-voltages and ESD.
The single pole RC filter (102 Ω, 1 nF) has a differential mode
bandwidth of 1.56 MHz and reduces the wideband noise at the
input of the AD7401A.
The sigma-delta modulator requires a clock input from an
external source like a DSP processor or FPGA. The clock
frequency can range from 5 MHz to 20 MHz, and a frequency
of 16 MHz was used for the circuit in Figure 1.The highly
robust single bit stream output of the modulator can be
processed directly by a sinc3 filter where the data can be
converted to an ADC word.
The ±25 A current through the 1 mΩ resistor creates a voltage
of ±25 mV. This is then amplified by the AD8639 to ±250mV
and input to the AD7401A. The differential input of the AD7401A
acts as the difference amplifier in the traditional three op amp
instrumentation amplifier configuration.
Rev. 0
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whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)
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CN-0280
Circuit Note
13kΩ
40kΩ
+2.5V_ISO
−2.5V_ISO
5V_ISO
+2.5V_ISO
BAT54S
0.1µF
±25A
51Ω
1/2
AD8639
10Ω
VDD1 VDD1
10Ω
VDD2
GND
51Ω
MDAT
1nF
CLOCK
100Ω
AD7401A
91kΩ
27kΩ
82pF
51Ω
1/2
–2.5V_ISO
47kΩ
VIN
VOUT
GND2
ADP7104
GND2
GND
−2.5V_ISO
+2.5V_ISO
+2.5V_ISO
5V
VISO
IN
ADP121-2.5
0.1µF
10µF
VISO
GND
10kΩ
40kΩ
10kΩ
10µF
ADP7182
GND
OUT
10µF
0.1µF 47µF
RCSEL
ADuM6000
IN
ADM8829
RCIN
0.1µF
10µF
CAP+ CAP− GND
4.7µF
VDD1
VSEL
−5V_ISO
ADJ
VOUT
VIN
VDD1
GNDISO
GND1
GNDISO
GND1
AGND_ISO
10857-001
OUT
−2.5V_ISO
3.3V
5V
5V_ISO
10µF
DSP
ADJ
VIN−
GND1
BAT54S
DATA
MCLKIN
RF
AD8639
10µF
ADP7104
±250mV
91kΩ
RG
10kΩ
VIN
VOUT
47µF
VIN+
RF
1mΩ
SENSE
RESISTOR
6V TO 20V
ADJ
5V
DGND
Figure 1. Isolated Current Sense Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
Both ac and dc information can be analyzed using the AD740x
devices, thus not only monitoring ac performance but also any
dc injection that may be present in the system. DC injection is
important in solar applications because too much dc current
injected into the grid may saturate any transformers in its path;
therefore the dc current must be limited to the low milliampere
range.
A key advantage using the AD740x devices is that they can be
very close to the actual ac current path, while the DSP or FPGA
can be a distance away or even on another board in the system.
This increases the accuracy of the overall system by minimizing
the effects of EMI/RFI.
Table 1. Maximum Continuous Working Voltage1 for AD7401A
Parameter
AC Voltage, Bipolar
Waveform
AC Voltage, Unipolar
Waveform
Max
565
Unit
V peak
891
V peak
DC Voltage
891
V
1
Constraint
50-year minimum
lifetime
Maximum CSA/VDE
approved working
voltage
Maximum CSA/VDE
approved working
voltage
Refers to continuous voltage magnitude imposed across the isolation
barrier. See the AD7401A data sheet for more details.
Safety is accomplished using the isolation barrier of a 20 µm
polyimide. Further information on this and the various approvals
can be obtained from the relevant datasheets. The AD7401A
can operate to voltages up to 891 V unipolar range or 565 V
bipolar range, across its isolation barrier as shown in Table 1.
Rev. 0 | Page 2 of 7
CN-0280
SOLAR
PANEL
DC-TO-DC
CONVERSION
DC-TO-AC
CONVERSION
~
=
The ADuM6000 is a 5 V isolated dc-dc converter which
operates using an internal 625 kHz PWM to drive the 5 V dc
power across the isolation barrier. This is rectified on the
isolated side of the barrier and filtered.
AC CURRENT
MEASURMENT
USING SHUNT
DC LINK
=
=
AD7401A
The AD8639 op amp supplies are regulated to ±2.5 V for better
noise performance. The +2.5 V is supplied by the low noise
ADP121 low dropout regulator, which is driven from the +5 V
isolated supply.
2-WIRE
CONNECTION
SOLAR
PANEL
DC-TO-DC
CONVERSION
Current transformers offer alternative methods of isolation
known as galvanic isolation.
This document describes the typical performance of a current
measurement module designed by Analog Devices using the
AD7401A and the ADuM6000 devices.
Solar Photovoltaic (PV) Inverter System Application
A solar PV inverter converts power from a solar panel and transfers
this power to the utility grid efficiently. Power from the solar
panel, which is essentially a dc current source, is converted to ac
current and fed onto the utility grid in phase with the frequency
of the grid, and to a very high efficiency level (95% to 98%). The
conversion can take one or more stages as shown in Figure 2.
The first stage is typically a dc-to-dc conversion, where the low
voltage and high current of the solar panel output is converted
to high voltage and low current. The reason for this is to raise
the voltage to a level compatible to the peak voltage of the grid.
The second stage typically converts the dc voltage and current
to ac voltage and current, typically using an H-Bridge circuit.
(See Isolation Technology Helps Integrate Solar Photovoltaic
Systems onto the Smart Grid, Analog Dialogue article).
AC CURRENT
MEASURMENT
USING SHUNT
DC LINK
=
Current measurement in solar applications requires isolated
measurement techniques. The AD7401A is just one of many
Analog Devices products that offer such isolation applications
in ac measurements. This type of isolation is based on iCoupler®
Technology.
~
=
Theory
The AD7401A is a second-order, sigma-delta (Σ-Δ) modulator
that converts an analog input signal into a high speed, 1-bit data
stream with on-chip digital isolation based on Analog Devices,
Inc., iCoupler® technology. The AD7401A operates from a 5 V
power supply and accepts a differential input signal of ±250 mV
(±320 mV full scale). The analog modulator, eliminating the
need for external sample-and-hold circuitry, continuously samples
the analog input. The input information is contained in the
output stream as a density of ones with a data rate up to 20 MHz.
The original information is reconstructed with an appropriate
digital filter. The processor side (non-isolated) can use a 5 V or
a 3 V supply (VDD2).
DC-TO-AC
CONVERSION
=
AD7401A
ANALOG DEVICES DSP
2-WIRE
CONNECTION
SINGLE PHASE AC GRID
An ADM8829 switched capacitor voltage inverter is driven by
the isolated +5 V and generates a −5 V output which is regulated to
−2.5 V using the ADP7182 negative linear regulator.
ANALOG DEVICES DSP
L N
10857-002
Power Supply Configuration
SINGLE PHASE AC GRID
Circuit Note
Figure 2. Solar PV Inverter System
Previous solar PV Inverters were simply modules that dumped
power onto the utility grid. Solar inverters for new designs
focus on safety, grid integration and cost reduction. To achieve
this, Solar PV inverter designers are looking to new technology,
some previously unused in existing solar inverter modules, to
improve their performance and potentially reduce cost.
In the circuit the DSP controls the dc-to-dc converter and the
dc-to-ac converter. The connection to the utility grid is typically
made with a relay. The ac current measurement is performed by
the AD7401A which measures current output to the grid,
typically 25A.
Solar PV inverter systems may or may not have an isolation
transformer at the output, mainly due to cost savings, but
without a transformer, the solar PV inverter must measure the
dc component of the output current. This current is known as
dc injection, and its value is critical to the operation. Too much
dc injection onto the grid may saturate any transformers in the
path. The dc injection current must be limited to the low mA.
Both of these tasks can be accomplished by the circuit in this
application. Thus, cost savings can be accomplished because
alternative methods like Hall effect current transducers may
need two devices: one for the high current range and one for the
lower current range.
Offset Performance of AD7401A
The offset of the AD7401A in the current measurement module
was measured over temperature up to 125°C. The results are
shown in Figure 3 and are in line with the data sheet specifications
of the AD7401A. The maximum variation of offset measured
over the temperature range with no current flowing in the shunt
was approximately ±20 mA from −40°C to +125°C.
Rev. 0 | Page 3 of 7
CN-0280
Circuit Note
The following voltages were applied during the test.
0.5
0.4
• VDD_ISO = 5 V
0.3
• VDD_FPGA = 3.3 V
• VIN = 6 V @ 62 mA (Current sense module input supply
voltage).
ERROR (%)
0.2
• MCLKIN = 16 MHz (EVAL-CED1Z with Altera FPGA ,
256 Decimation Rate).
0.1
0
–0.1
–0.2
0.05
–0.3
0.04
–0.4
0
0.02
2
4
6
8
10
12
14
16
18
20
22
24
28
26
AMPS
0.01
Figure 4. AD7401A Linearity Performance
0
SINC3 Filter Performance
–0.01
The AD7401A is specified for a decimation rate (DR) of 256.
However, it is possible to use this device at higher decimation
rates. For a DR = 256, the response of a sinc3 filter is shown in
Figure 5 where the output data rate is 62.5 kHz, and the FFT
noise floor is shown in Figure 6.
–0.02
–0.03
10
20
30
40
50
70
80
90 105
TEMPERATURE (°C)
0
Figure 3. AD7401A Module Offset
–10
–20
The linearity of the module with currents up to ±28A was
analyzed. As seen in Figure 4, Linearity of <±0.2% can be
achieved after calibration. The voltages specified in the previous
section were applied during the analysis. Figure 4 shows both
full scale error and absolute error analysis defined as follows:
–30
AMPLITUDE (dB)
Linearity Performance
–40
–50
–60
–70
Full-Scale Error = (VSHUNT – VCALC) / VFULLSCALE
–80
Absolute Error = ( VSHUNT – VCALC) / VSHUNT
–90
where
VSHUNT = Current in the precision shunt( measured with a
DVM)
VCALC = Calculated current from the output of the ADC
(AD7401A)
VFULLSCALE = Full-scale current range of the module (28 A).
–100
0
20
80
40
60
FREQUENCY (kHz)
100
120
10857-005
0
10857-003
–0.04
–0.05
–40 –30 –20 –10
Figure 5. Sinc3 Filter Response for Decimation Rate = 256,
Output Data Rate = 62.5 kHz
0
–20
–40
AMPLITUDE (dB)
The advantage of using the absolute error method is to analyze
the errors in the lower range of measurement where errors can
be highlighted. This is important for solar applications where dc
injection can be measured in the low current range.
–60
–80
–100
–120
–140
0
5
10
20
15
FREQUENCY (kHz)
25
31.246
10857-006
AMPS IN SHUNT/PRIMARY
–0.5
10857-004
AD7401 ABSOLUTE ERROR
AD7401 FULL-SCALE ERROR
0.03
Figure 6. 16K-Point FFT Showing Noise Floor for Decimation Rate = 256,
Output Data Rate = 62.5 kHz
Rev. 0 | Page 4 of 7
Circuit Note
CN-0280
For higher decimation rates, the sinc3 Filter response is greater
improved. For a DR = 1024, the response of a sinc3 filter is
shown in Figure 7 where the data rate is 15.6 kHz This improves
the noise performance of the system as shown in Figure 8,
however at a reduced data rate.
0
–20
AMPLITUDE (dB)
–40
0
–10
–20
–80
–100
–30
–40
–120
–140
0
–60
–70
1
2
4
5
3
FREQUENCY (kHz)
7
6
7.811
10857-008
–50
Figure 8. 16K-Point FFT Showing Noise Floor for Decimation Rate = 1024,
Output Data Rate = 15.6 kHz
–80
Layout Considerations
–90
–100
0
20
80
40
60
FREQUENCY (kHz)
100
120
Special care must be taken during printed circuit board (PCB)
layout to meet emissions standards. See the AN-0971
Application Note for board layout recommendations. An example
of such a layout is shown in Figure 9. The key to the layout is to
ensure a good overlap between Layer 3 (Floating Plane) and
Layer 2 (Ground) .This simple overlap keeps emissions to a
minimum in the system. Figure 10 shows the top view of the
PCB layout, and Figure 11 shows a photo of the actual board.
10857-009
Figure 7. Sinc3 Filter Response for Decimation Rate = 1024,
Output Data Rate = 15.6 kHz
10857-007
Figure 9. 4-Layer Board Example
10857-010
AMPLITUDE (dB)
–60
Figure 10. Proposed Layout for Current Measurementt
Rev. 0 | Page 5 of 7
Circuit Note
10857-011
CN-0280
Figure 11. Photo of Current Measurement Board
Analog Devices isolated ADCs and isoPower devices meet the
needs of the Solar Industry, and provides new technology
advances required for Power systems. This leads to improved
performance in grid integration using this new technology from
the conventional methods used in solar inverters today.
Functional Block Diagram
A functional block diagram of the test setup is shown in Figure 12.
POWER
SUPPLY
+6V TO +20V
AT 200mA
COMMON VARIATIONS
+
I+
The AD8638 op amp is a single version of the AD8639.
Other members of the AD7401A sigma-delta modulator family
include the AD7400 that includes a 10 MHz on-chip clock.
DC SOURCE
100V AT 28A
6.5 DIGIT DVM
AND CALIBRATED
SHUNT
I−
POWER
SUPPLY
+7V AT 2A
COM
TB2 TB1
EVAL-CN0280-EB1Z
EVALUATION
BOARD
J7
DATA
J4
GND
EVAL-CED1Z
CONVERTER
EVALUATION AND
DEVELOPMENT
BOARD
J12
J1
CLOCK
GND
5V
GND
CIRCUIT EVALUATION AND TEST
Equipment Needed
PC
• DC source capable of 28A output at 100V to simulate source.
• 6.5 digit DVM and calibrated shunt to measure input current
• EVAL-CN0280-EB1Z evaluation board
• 6 V, 200 mA power supply
• 7 V, 2 A power supply
• EVAL-CED1Z converter evaluation and development board
software.
• Example code for implementing the sinc3 filter can be found
in the AD7401A data sheet.
Rev. 0 | Page 6 of 7
Figure 12. Test Setup Functional Diagram
10857-012
USB
Circuit Note
CN-0280
LEARN MORE
Data Sheets and Evaluation Boards
CN-0280 Design Support Package:
http://www.analog.com/CN0280-DesignSupport
AD7401A Data Sheet
CN-0183 Circuit Note, A Novel Analog-to-Analog Isolator Using
an Isolated Sigma-Delta Modulator, Isolated DC-to-DC
Converter, and Active Filter, Analog Devices.
ADuM6000 Data Sheet
AD8639 Data Sheet
Defining Smart Grids and Smart Opportunities, IMS Research,
February 28, 2012.
Cantrell, Mark. Application Note AN-0971, Recommendations
for Control of Radiated Emissions with isoPower Devices.
Analog Devices.
Chen, Baoxing, John Wynne, and Ronn Kliger. High Speed
Digital Isolators Using Microscale On-Chip Transformers,
Analog Devices, 2003.
ADM8829 Data Sheet
ADP121 Data Sheet
ADP7104 Data Sheet
ADP7182 Data Sheet
REVISION HISTORY
10/13—Revision 0: Initial Version
Chen, Baoxing. iCoupler® Products with isoPower™ Technology:
Signal and Power Transfer Across Isolation Barrier Using
Microtransformers, Analog Devices, 2006.
Chen, Baoxing. “Microtransformer Isolation Benefits Digital
Control.” Power Electronics Technology. October 2008.
Ghiorse, Rich. Application Note AN-825, Power Supply
Considerations in iCoupler® Isolation Products, Analog
Devices.
Krakauer, David. “Digital Isolation Offers Compact, Low-Cost
Solutions to Challenging Design Problems.” Analog
Dialogue. Volume 40, December 2006.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Murname, Martin, Isolation Technology Helps Integrate Solar
Photovoltaic Systems onto the Smart Grid, Analog Dialogue,
Analog Devices, 46-09, September 2012.
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CN10857-0-10/13(0)
Rev. 0 | Page 7 of 7
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