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

Circuit function and advantage

The current in the circuit monitoring system shown in Figure 1 can operate at a positive high common-mode DC voltage of up to +500 V with an error of less than 0.2%. The load current is passed through a shunt resistor external to the circuit. The shunt resistor value should be chosen so that the shunt voltage is approximately 500 mV at maximum load current.

Figure 1: High Common Mode Voltage and Current Monitor (all connections and decoupling not shown)

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

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

The measurement results of the AD7171 are provided in digital form via a simple two-wire SPI-compatible serial interface.

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

Circuit description

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

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

Figure 2: AD8212 uses a high-voltage mode of operation with an external PNP transistor

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

In this mode of operation, the supply current (IBIAS) of the AD8212 circuit is based entirely on the supply voltage range and the selected RBIAS resistance 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 circuit is active, allowing the device to function properly.

Note that the 500 kΩ bias resistor (5 &TImes; R2) consists of five separate 100 kΩ resistors. This is to provide protection against resistance voltage breakdown. 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 generates a voltage at the input of the AD8212. Internal amplifier A1 responds by causing transistor Q1 to conduct the necessary current through resistor R1 to equalize the potential at the inverting and non-inverting inputs of amplifier A1.

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

The transfer function of AD8212 is:

IOUT = gm &TImes; VSENSE

VSENSE = ILOAD &TImes; RSHUNT

VOUT = IOUT &TImes; 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. When VSENSE changes by 1 mV, a 1 mA change can be made on IOUT, and when the latter flows through a 5 kΩ resistor, a 1 mV change is produced at VOUT.

In the circuit shown in Figure 1, the load resistance is 24.9 kΩ, so a full-scale input voltage with a gain of 5.500 mV produces a 2.5 V output, which corresponds to the full-scale input range of the AD7171 ADC.

The AD8212 output is designed to drive high impedance nodes. Therefore, if interfacing to the converter, it is recommended to buffer the output voltage across ROUT 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 supply output (+3.3 VISO) of the four-channel isolator ADuM5402.

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

Although the AD7171 VDD and REFIN(+) can both operate from a 3.3 V supply, using a separate reference provides higher accuracy. A 2.5 V reference can be selected to provide sufficient headroom.

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

The code is processed in the 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). In addition, the ADC output code recorded by LabVIEW and the ideal code calculated based on the ideal system are compared.

Figure 3: Output and error vs. shunt voltage

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