The circuit shown in Figure 1 is a fully isolated current sensing circuit with its own isolated power supply. The circuit is extremely robust and can be installed close to the sense resistor for accurate measurements with minimal noise pickup. The output is a single 16 MHz bit stream from a sigma-delta modulator, processed by a DSP through a sinc ^{3 digital filter.}

This circuit is ideal for AC current monitoring in solar photovoltaic (PV) converters, where peak AC voltages may be as high as hundreds of volts and currents may vary from a few mA to 25 A.

The circuit uses a 1mΩ sense resistor to measure peak currents up to ±25 A through 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 Σ-Δ modulator. Higher currents can be measured by reducing the gain of the AD8639 accordingly (up to ±50 A or ±100 A) to ensure maximum benefit from the full-scale input range of the AD7401A.

A current of ±25 A through a 1 mΩ resistor develops a voltage of ±25 mV. This voltage is then amplified by the AD8639 to ±250mV and input to the AD7401A. The differential inputs of the AD7401A act as the difference amplifier in a pass three op amp instrumentation amplifier configuration.

With a typical offset voltage of only 3 μV, a drift of 0.01 μV/°C, and a noise of 1.2 μV pp (0.1 Hz to 10 Hz), the AD8639 is ideal for applications where dc error sources must be minimized. Virtually zero drift characteristics over the entire operating temperature range can bring great benefits to solar panel applications. 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 current measurement current to prevent any leakage current from entering this sensitive low voltage area. BAT54 Schottky diodes protect the input of the AD8639 from transient overvoltage and ESD.

A single-pole RC filter (102 Ω, 1 nF) has a differential-mode bandwidth of 1.56 MHz, which reduces bandwidth noise at the input of the AD7401A.

The Σ-Δ modulator requires a clock input from an external source, such as a DSP processor or FPGA. Possible clock frequencies range from 5 MHz to 20 MHz, with the circuit shown in Figure 1 using 16 MHz. The modulator's extremely powerful unit stream output can be processed directly by a sinc ³ filter, where the data is converted into an ADC word.

Both AC and DC information can be analyzed with the AD740x device, so not only the AC performance can be monitored, but also any DC injection that may be present in the system. In solar applications, DC injection is critical because if too much DC current is injected into the grid, the result can be saturation of any transformer in its path, so DC current must be limited to the low microampere range.

A key advantage of using AD740x devices is that they can be very close to the actual AC current path, whereas a DSP or FPGA may be some distance away or even on another board in the system. In this way, by minimizing EMI/RFI effects, the result is improved overall system accuracy.

Security is provided by a 20 µm polyimide film isolation barrier. More information on these and the various certifications can be found in the relevant data sheets. The AD7401A operates up to an 891 V unipolar range, or a 565 V bipolar range across the isolation barrier, as shown in Table 1.

¹ refers to the continuous voltage amplitude across the isolation barrier. See the AD7401A data sheet for details.

Power configuration

The ADuM6000 is a 5 V isolated dc-to-dc converter that provides 5 V dc power to the isolation barrier through an internal 625 kHz PWM. This current is rectified and filtered at the isolated side of the isolation barrier.

The AD8639 op amp supply is regulated to ±2.5 V for better noise performance. The +2.5 V is provided by the low-noise ADP121 low-dropout regulator, which is driven by an isolated +5 V supply.

The ADM8829 switched-capacitor voltage inverter is driven from an isolated +5 V supply to produce a −5 V output voltage, which is regulated to −2.5 V by an ADP7182 negative linear regulator.

principle

The AD7401A is a second-order Σ-Δ modulator. The on-chip digital isolation uses ADI's iCoupler® ^technology to convert analog input signals into a high-speed 1-bit data stream. The analog modulator continuously samples the analog input signal, eliminating the need for external sample-and-hold circuitry. The AD7401A is powered by a 5 V power supply and can input a differential signal of ±250 mV (full scale ±320 m). The input information is contained in the output data stream in the form of data stream density, which can have a maximum data rate of 20 MHz. Reconstruct the original information through appropriate digital filters. The processor side (non-isolated) can use either 5 V or 3 V power supply (VDD2).

Current measurement in solar applications requires isolated measurement techniques. The AD7401A is one of many Analog Devices products that enable such applications with AC measurements. This type of isolation is based on i Coupler ^® technology.

Current transformers are an alternative method of isolation known as galvanic isolation.

This article describes the typical performance of current measurement modules designed by Analog Devices using the AD7401A and ADuM6000 devices.

Solar photovoltaic (PV) inverter system applications

Solar photovoltaic inverters convert electrical energy from solar panels and efficiently deliver it to the utility grid. The power from the solar panel is basically a DC source, which is converted into AC and fed into the utility grid in a certain phase relationship with the grid frequency with extremely high efficiency (95% to 98%). The conversion can be single-stage or multi-stage, as shown in Figure 2. The first stage is usually a DC-DC conversion, where the low voltage and high current output of the solar panel is converted into high voltage and low current. The purpose of this is to boost the voltage to a level that is compatible with the grid's peak voltage. The second stage usually converts DC voltage and current to AC voltage and current, usually using an H-bridge circuit. (See the ADI article " Integrating solar photovoltaic power generation systems into smart grids with isolation technology ").

Previous solar PV inverters were simply modules that dumped electrical energy into the utility grid. Solar inverters for new designs focus on safety, grid integration and cost reduction. To this end, solar PV inverter designers are considering new technologies not used in existing solar inverter modules to improve performance and reduce costs as much as possible.

In this circuit, DSP controls the DC-DC converter and DC-AC converter. The utility grid is generally connected via relays. AC current measurement is implemented by the AD7401A, which measures the current output to the grid, typically 25A.

A solar PV inverter system may or may not have an isolation transformer at the output (for cost-saving reasons), but if there is no transformer, the solar PV inverter must measure the DC component of the output current. This current is called DC injection, and its value is critical to the operation of the circuit. Too much DC is injected into the grid and the result may be saturation of any transformer in the DC path. DC injection current must be limited to low mA values. In this application, both tasks must be completed. From this, cost savings can be realized, since alternatives such as Hall effect current sensors may require two devices: one for the high current range and one for the low current range.

AD7401A Offset Performance

The offset of the AD7401A in the current measurement module is measured over the entire temperature range up to 125°C. The results are shown in Figure 3 and meet the specifications in the AD7401A data sheet. The maximum measured offset change with no current flowing in the shunt resistor is approximately ±20 mA over the entire temperature range (temperature range: −40°C to +125°C).

The voltages applied during the test are as follows:

• VDD_ISO = 5 V
• VDD_FPGA = 3.3 V
• MCLKIN = 16 MHz (EVAL-CED1Z, using Altera FPGA, 256 decimation rate).
• VIN = 6 V @ 62 mA (current sensing module input supply voltage).

Linearity performance

The linearity of the module was analyzed at current conditions up to ±28A. As shown in Figure 4, linearity less than ±0.2% can be achieved after calibration. The voltages specified in the previous section were used in the analysis. Figure 4 shows both full-scale error and absolute error analysis, defined as follows:

Full-scale error = (Vshunt _– Vcalculated ₎ / Vfull _-scale
Absolute error = (Vshunt _– Vcalculated ₎ / _Vshunt

in

Vshunt = current in the precision shunt resistor (measured with DVM) _{Vcalculated = calculated current from the ADC output (AD7401A) Vfullscale} = full _{- scale} current range of the module ( 28 A).

The advantage of using the absolute error method is that the error can be analyzed at low measurement ranges, where the error performance is more prominent. This is important for solar applications because DC injection can be measured in the low current range.

SINC3 filter performance

The AD7401A is specified at a extraction rate (DR) of 256, but the device can be used at other extraction rates. When DR = 256, the response of the 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.

For higher decimation rates, the sinc3 filter response improves significantly. The response of the sinc3 filter is shown in Figure 7 when DR = 1024, where the data rate is 15.6 kHz. At this point, the system's noise performance has improved, as shown in Figure 8, but the data rate has been reduced.

Layout considerations

Special care should be taken when designing printed circuit board (PCB) layouts, which must comply with relevant radiation standards. For board layout recommendations, see the AN-0971 application note . An example of this layout is shown in Figure 9. The key to the layout is to ensure there is good overlap between layer 3 (floating layer) and layer 2 (ground layer). This simple overlap can significantly reduce the radiation in the system. Figure 10 shows a top view of the PCB layout, and Figure 11 shows a photo of the actual circuit board.

ADI's isolated ADC and isoPower devices meet the needs of the solar industry and provide new technologies for power systems. Using this technology improves the performance of grid integration compared to conventional methods used in today's solar inverters.