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Posted on 02 July 2019

# Isolated Voltage Sensor Expands Input Range by 10 Times

First generation optical isolation amplifiers have an input range of ±200 mV. This is ideal for current sensing, but input dynamic range should be wider for high-voltage sensing applications. The new ACPL-C87x input voltage range of 2 V is 10 times wider than previous generation isolation amplifiers. With a 1 GΩ input impedance, signal source loading errors are minimized. Rated at 5000 V double protection isolation, the ACPL-C87X is an ideal device for isolated voltage sensing.

By Hong Lei Chen, Product Manager, Avago Technologies

Many industrial and home appliance applications require isolated voltage sensing for safe, robust system control and protection. These applications include motor drives, solar inverters, and uninterruptable power supplies (UPS). The voltage sensor must accurately measure the DC link high voltage and provide galvanic isolation between the hazardous high voltage side and the low voltage controller side, which is accessible to system operators. The isolated voltage sensor can also be used in isolated temperature sensor designs and 0-10 V analog control protocols. In these applications, the voltage sensor must linearly and accurately measure the temperature or control signal and send it across the isolation barrier thus providing safety insulation and eliminating ground loops.

To meet these functionality and performance requirements, optically isolated Avago HCPL-7800A/7800/7840 devices have been used, but these isolation amplifiers were specifically designed for shunt-based current sense applications, where an input voltage range of ±200 mV (full scale ±320 mV) [1, 2] is adequate. Low-level signals in high-voltage sensing applications pose significant limitations because of the low dynamic range. Wider dynamic range isolation amplifiers are needed.

The ACPL-C87B, ACPL-C87A and ACPL-C870 optical isolation amplifiers were designed with a nominal input range of 0 to 2 V [3] to serve voltage sensing applications. This is an expansion of 10 times compared to the 0 to 200 mV input range of previous generation HCPL-7840 devices. Besides the improved input voltage range, the new isolation amplifier feature a high 1 GΩ input impedance compared to the 500 kΩ input impedance of the HCPL-7840 series; this is a 2,000 times improvement. The very high input impedance drastically reduces the error caused by loading the signal source.

### How the Optical Isolation Amplifier Works

Functional blocks of the ACPL-C87X are shown in Figure 1. First the isolation amplifier senses the input voltage (single-ended analog signal) and converts it to a digital bit stream. The bit stream is then transmitted across the optical coupling pair consisting of an LED and a photodetector. This optical signal path provides the electrical insulation barrier. Because the transmitted signal is optical rather than electrical, it is immune to magnetic fields and electrical noise. The photodetector recovers the optical signal and converts it back to an electrical signal, which is decoded and filtered to reproduce an analog output signal. The output voltage, provided in differential mode for better common mode noise rejection, is proportional to input voltage with unity gain.

The isolation amplifier is available in three gain accuracy options: ±0.5% (ACPL-C87B), ±1% (ACPL-C87A) and ±3% (ACPL-C870). Their stretched SO-8 package is 30% smaller than a DIP-8 package. All amplifiers have a double protection rating of 5000 VRMS/1 min per the UL 1577 safety standard. The 1230 VPEAK maximum working voltage specification per IEC/EN/DIN EN 62019-5-5 ensures circuits on the low voltage side are not damaged by hazardous high voltages.

Using the ACPL-C87X as an isolated voltage sensor is straightforward. Select resistors to form a voltage divider to scale down the voltage signal to be measured to a level within the sensor input range. With an integrated isolation and sensing circuit, the application circuit is significantly simplified compared to alternative solutions that employ separate devices to perform sensing and isolation functions.

Typical applications include:

• Isolated voltage sensing in AC and servo motor drives
• Isolated DC-link voltage sensing in solar inverters, wind turbine inverters
• Isolated sensor interfaces
• Signal Isolation in data acquisition systems
• General purpose voltage isolation

### Overvoltage and Undervoltage

Detection In motor drives, the DC-link voltage must also be continuously kept under control. Under certain operating conditions, a motor can act as a generator and deliver a high voltage back into the DC-link through the inverter’s power device and/or the recovery diodes. This high voltage is added to the DC-link voltage, and the IGBTs are stressed by a very high - and potentially damaging - surge voltage. Using the ACPL-C87X to monitor the DC-link, as shown in Figure 2, overvoltage conditions are easily detected.

In this circuit, a voltage divider formed by resistors R1 and R2 scale down the DC bus voltage to match the 0 V to 2 V input voltage range of the ACPL-C87X. The overvoltage range threshold can be set as high as 2.4 V, 20% higher than the nominal voltage.

An undervoltage condition caused by an overload on the power grid or loss of a power phase is a common condition. This circuit can also detect an undervoltage condition, thanks to the wide input range of the ACPL-C87x series and its low linearity error, as low as 0.05%. For example, a designer may set a threshold 20% lower than nominal DC-link voltage as an undervoltage condition. This sets a 1.6 V input voltage as the lower limit.

A detailed circuit with the ACPL-C87x is shown in Figure 3. Given that the ACPL-C87x’s voltage sensor nominal input voltage for VIN is 2 V, a user needs to choose resistor R1 according to Equation 1:

For example, if VL1 is 600 V and R2 is 10 kΩ, then the value of R1 is 2990 kΩ.

Choosing resistors is flexible. One method is to combine several resistors to match the target value; for example, 2 MΩ, 430 kΩ and 560 kΩ resistors in series make 2990 kΩ exactly. A VIN of 2 V corresponds to a VL1 of 600 V. However, in the cases that VL1 is not 600V, specific resistance values might be difficult to find. Another method is to round up the target value to a convenient value, for example 3 MΩ, to make resistor selection easier. In such cases, the scaling relationship may need fine tuning. In the same example with a VL1 of 600 V, R1 of 3 MΩ, and R2 of 10 kΩ, VIN is solved to be 1.993 V.

The down-scaled input voltage is filtered by the anti-aliasing filter formed by R2 and C1, with corner frequency of 159 kHz , and then sensed by the ACPL-C87X.

(The value of R1 is usually much larger than R2, therefore neglected in calculation)

The galvanically isolated differential output voltage (VOUT+ - VOUT-) is proportional to the input voltage. The OPA237, configured as a difference amplifier, converts the differential signal to a single-ended output. This stage can also be made to amplify the signal, and, if required, low-pass filter the signal to limit bandwidth. In this circuit, the difference amplifier is designed for a gain of one with a low-pass filter corner frequency of 15.9 kHz. Resistors R5 and R6 can be changed for a different gain. The bandwidth can be reduced by increasing the capacitance of C4 and C5. The isolated output voltage VOUT, which is linearly proportional to the line voltage on the high voltage side, can be safely connected to the system microcontroller. With the ACPL-C87X gain of 1, the overall transfer function is simply:

or

(The value of R1 is usually much larger than R2, therefore neglected in calculation.)

### Tradeoff Measurement Accuracy for Lower Power Dissipation

The input stage of the application circuit in Figure 3 can be simplified as shown in Figure 4. R2 and RIN, the input impedance of the ACPLC87x, create a current divider that results in an additional measurement error component that will add on top of the amplifier gain error. With the assumption that R1 and RIN have a much higher value than R2, the resulting loading error can be estimated to be R2/RIN.

With an RIN of 1 GΩ for the ACPL-C87x, the loading error is negligible for R2 values up to 1 MΩ, where the error is approximately 0.1%. Though this error is small, it can be reduced by lowering R2 to 100 kΩ (error of 0.01% approximately), or 10 kΩ (error of 0.001% approximately). However with a lower R2, the higher power dissipation in the resistive divider string must be examined, especially for high voltage sensing applications. For a 0.01% loading error with an R2 of 100 kΩ measuring 600 V DC across L1 and L2, the resistive divider string consumes only 12 mW, assuming a VIN of 2 V. If R2 is reduced to 10 kΩ to reduce the error to 0.001%, the power consumption increases tenfold to 120 mW. In energy efficiency critical applications, such as photovoltaic (PV) inverters and battery-powered applications, this trade-off between measurement accuracy and power dissipation is a useful design option.

In addition, ACPL-C87X features a shutdown mode, which can be activated with a high level logic input on the shutdown pin. In this mode, the IDD1 supply current is reduced to only 15 μA, making it ideal for battery-powered devices and other power-sensitive applications.

### Isolated Temperature Sensing using a Thermistor

Thermistors are widely used to measure temperature. Galvanic isolation between the potential of the thermistor and that of the system analog-to-digital converter is often required, especially when the thermistor is mounted near high voltages or in electrically noisy or poorly grounded environments. A lack of isolation can impair safety and induce electromagnetic interference (EMI).

A simple isolated temperature sensor circuit is shown in Figure 5. RT1 and R2 form a voltage divider from the floating, constant 5 V voltage source that also powers the voltage sensor. Choose RT1 and R2 so that the voltage fed into the ACPL-C87X isolation amplifier does not exceed the full-scale range of 2.46 V. The high impedance input terminal of the ACPL-C87X allows a relatively high resistance of R2 without causing a significant loading error. Select the resistor and capacitor values after reviewing the thermistor manufacturer data sheet.

Conclusion

Many applications require isolated high-voltage sensing. Designed specifically for high-voltage sensing, new generation optically isolated amplifiers, such as the ACPL-C87X, make monitoring and system protection circuits more accurate and easier to design.

References
• HCPL-7840 Isolation Amplifier Data Sheet. Avago Technologies, AV02-1289EN. (http://www.avagotech.com/pages/en/optocouplers_plastic/)
• HCPL-7800A/7800 Isolation Amplifier Data Sheet. Avago Technologies, AV02-0410EN. (http://www.avagotech.com/pages/en/optocouplers_plastic/)
• ACPL-C87B/C87A/C870 Precision Optically Isolated Voltage Sensor Data Sheet. Avago Technologies, AV02-3563EN. (http://www.avagotech.com/pages/en/optocouplers_plastic/)

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## One Response

1. opusensembe says:

Can the circuit on figure 3 using ACPL-C87X be used to measure rectified positive AC voltages between L1 and L2 (ground) in the range of 3400V peak? Or does the 1230VPEAK maximum working voltage in ACPL-C87X limit that application?

I am assuming that the 1230VPEAK apply to voltages seen at VIN and GND1. Is this correct?

Putting it in other way, What is the maximum L1 voltage that can be measured with this circuit assuming VL2 = 0VDC(ground)?

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