ADI Lab Circuit: 500V Common-Mode Voltage Current Monitor

Circuit Function and Advantages

The circuit shown in Figure 1 monitors current in a system and operates with positive high common-mode DC voltages up to +500 V with less than 0.2% error. The load current passes through a shunt resistor that is external to the circuit. The shunt resistor value is chosen so that the shunt voltage is approximately 500 mV at maximum load current.

Figure 1: High common-mode voltage current monitor (all connections and decoupling not shown)

When used with an external PNP transistor, the AD8212 can accurately amplify small differential input voltages in the presence of high positive common-mode voltages greater than 500 V.

Galvanic isolation is provided by the ADuM5402 quad-channel isolator. This is not only for protection, but also to isolate the downstream circuitry from high common-mode voltages. In addition to isolating the output data, the ADuM5402 digital isolator also provides isolated +3.3 V power to the circuit.

The AD7171 measurement results are provided as digital codes via a simple 2-wire SPI-compatible serial interface.

This combination of devices enables an accurate positive high voltage rail current sensing solution with low component count, low cost, and low power consumption.

Circuit Description

The circuit is designed for a full-scale shunt voltage of 500 mV at the maximum load current, IMAX. Therefore, the shunt resistor value is RSHUNT = (500 mV)/(IMAX).

The AD8212 process has a breakdown voltage limitation of 65 V. Therefore, the common-mode voltage must be kept below 65 V. By using external PNP BJT transistors, the common-mode voltage range can be extended to over 500 V, depending on the breakdown voltage of the transistors.

Figure 2: AD8212 high-voltage operation using an external PNP transistor

The AD8212 does not have a dedicated power supply. Instead, the device actually creates a 5 V supply by “floating” itself off the 500 V common-mode voltage using an internal 5 V series regulator, as shown in Figure 2. This regulator ensures that the most negative of all terminals, COM (pin 2), is always 5 V below the supply voltage (V+).

In this mode of operation, the supply current (IBIAS) of the AD8212 circuit is completely based on the supply voltage range and the selected RBIAS resistor value. For example, for V+ = 500 V and RBIAS = 500 kΩ,

IBIAS = (500 V −5 V)/RBIAS = 990 μA.

In this high voltage mode, IBIAS should be between 200 μA and 1 mA. This ensures that the bias circuitry is active, allowing the device to operate normally.

Note that the 500 kΩ bias resistor (5 × R2) is constructed from five individual 100 kΩ resistors. This is to provide protection against voltage breakdown across the resistors. Additional breakdown protection can be added by eliminating the ground plane directly below the resistor string.

The load current flowing through the external shunt resistor develops a voltage at the input of the AD8212. The internal amplifier A1 responds by causing transistor Q1 to conduct the necessary current through resistor R1 to equalize the potentials at the inverting and noninverting inputs of amplifier A1.

The current (IOUT) flowing through the emitter of transistor Q1 is proportional to the input voltage (VSENSE) and, therefore, is proportional to the load current (RSHUNT) flowing through the shunt resistor (ILOAD). The output current (IOUT) is converted to a voltage using an external resistor whose value depends on the input-to-output gain required in the application.

The transfer function of AD8212 is:

IOUT = gm × VSENSE

VSENSE = ILOAD × RSHUNT

VOUT = IOUT × ROUT

VOUT = (VSENSE × ROUT)/1000 gm = 1000 μA/V

The input sense voltage has a fixed range of 0 V to 500 mV. The output voltage range can be adjusted based on the ROUT value. A 1 mV change in VSENSE produces a 1 mA change in IOUT, which in turn produces a 1 mV change in VOUT when it flows through the 5 kΩ resistor.

In the circuit of Figure 1, the load resistor is 24.9 kΩ, resulting in a gain of 5. A full-scale input voltage of 500 mV produces an output of 2.5 V, which corresponds to the full-scale input range of the AD7171 ADC.

The AD8212 outputs are designed to drive high impedance nodes. Therefore, if interfacing with a converter, it is recommended that the output voltage across ROUT be buffered to ensure that the gain of the AD8212 is not affected.

Note that the supply voltage for the ADR381 and AD7171 is provided by the isolated power output (+3.3 VISO) of the ADuM5402 quad-channel isolator.

The reference voltage for the AD7171 is provided by the ADR381 precision band gap reference. The ADR381 has an initial accuracy of ±0.24% and a typical temperature coefficient of 5 ppm/°C.

Although both the AD7171 VDD and REFIN (+) can be powered by 3.3 V, using a separate reference provides greater accuracy. A 2.5 V reference can be selected to provide ample margin.

The input voltage to the AD7171 ADC is converted to an offset binary code at the output of the ADC. The ADuM5402 provides isolation for the DOUT data output, SCLK input, and PDRST input. Although an isolator is optional, it is recommended to protect downstream digital circuitry from high common-mode voltages during fault conditions.

The code is processed in a PC using the SDP hardware board and LabVIEW software.

The graph in Figure 3 shows how the circuit under test achieves less than 0.2% error over the entire input voltage range (0 mV to 500 mV). Also compared is the ADC output code recorded by LabVIEW and the ideal code calculated based on an ideal system.

Figure 3: Output and error vs. shunt voltage graph PCB layout considerations

In any circuit where accuracy is important, the power and ground return layout on the board must be carefully considered. The PCB should isolate the digital and analog sections as much as possible. This PCB is built using a 4-layer stackup with large ground and power layer polygons.

Common changes

There are several solutions available for high-side sensing of the positive supply, including IC solutions using sense amplifiers, difference amplifiers, or some combination of the two.

Figure 4: Bidirectional current sensing with positive common-mode voltage greater than +65 V

Figure 4 shows an alternative circuit that can be used when bidirectional current sensing is required for positive common-mode voltages greater than +65 V. By implementing another AD8212 in this configuration, the charge and load currents can be measured separately. Note that VOUT1 increases as ILOAD flows through the shunt resistor. VOUT2 increases as ICHARGE flows through the shunt resistor.

Circuit Evaluation and Testing

WARNING! HIGH VOLTAGE. This circuit contains potentially lethal voltages. Do not operate, evaluate, or test this circuit, or perform circuit board assembly, unless you are a trained professional who understands the operation of high voltage circuits. You must be familiar with this circuit and all necessary precautions for working with high voltage circuits before applying power.

This circuit uses the EVAL-CN0218-SDPZ circuit board and the EVAL-SDP-CB1Z system demonstration platform (SDP) evaluation board. The two boards have 120-pin mating connectors that allow for quick setup and evaluation of the circuit performance. The EVAL-CN0218-SDPZ board contains the circuit to be evaluated, as described in this note. The SDP evaluation board is used with the CN0218 evaluation software to capture data from the EVAL-CN0218-SDPZ circuit board.

Equipment Requirements

PC with Windows® XP, Windows Vista® (32-bit), or Windows® 7 (32-bit) with a USB port

EVAL-CN0218-SDPZ circuit evaluation board

EVAL-SDP-CB1Z SDP evaluation board

CN0218 Evaluation Software

Supply voltage: +6 V or +6 V wall wart

Shunt resistor with a maximum voltage of 500 mV at maximum load current

Electronic Load

Get Started

Load the evaluation software by placing the CN0218 Evaluation Software CD into the CD drive of the PC. Open "My Computer," locate the drive that contains the evaluation software CD, and open the Readme file. Follow the instructions in the Readme file to install and use the evaluation software.

Functional Block Diagram

See Figure 1 of this circuit note for a functional block diagram of the circuit and the EVAL-CN0218-SDPZ-SCH.pdf file for the circuit schematic. This file is included in the CN0218 Design Support Package

set up

The 120-pin connector on the EVAL-CN0218-SDPZ circuit board connects to the connector marked “CON A” on the EVAL-SDP-CB1Z (SDP) evaluation board. Nylon hardware should be used to secure the two boards firmly, using the holes provided at the ends of the 120-pin connector.

Place a shunt resistor (RSHUNT

test

Apply power to the +6 V power supply (or wall wart) connected to the EVAL-CN0218-SDPZ circuit board. Launch the evaluation software and connect the USB cable from the PC to the mini-USB connector on the SDP board.

Once USB communications are established, the SDP board can be used to send, receive, and capture serial data from the EVAL-CN0218-SDPZ board. As the electronic load is stepped through the board, data can be recorded at different load current values.