Voltage Sensing

There are several methods of voltage sensing. The signals to be measured are around 800V DC for the DC bus, and 230V AC for the grid. The ADCs in the F28377S has an input range of 0V to 3.3V.

The simplest method to read the required voltage would be a voltage divider. Due to the large input resistance this would present to the ADC (large resistors selected to minimise loss power in the resistors), a unity gain buffer op-amp circuit should be used. This method works well however provides no isolation. Best practice would be to have an isolated input for anything connected to mains voltage.

There are several isolated methods:

  • Step-down transformer: only applicable to AC and can present phase shift and distortion.
  • Hall effect voltage transducer (resistor across the input voltage, hall effect sensor reads the current through this resistor). The currents required can be around 14mA which is 11.2W at 800V (very high power consumption for a sensing circuit).
  • Voltage divider (preferred), coupled with one of the following for isolation:
    • Isolation amplifier (preferred): There are both AC and DC isolation amplifiers. ACPL-C870ISO124. ISO121, ISO 122, ISO124. Need to check the inputs as they can be single-side or accept negative voltages. The outputs might also be bipolar or unipolar, so an op-amp(s) may be required to shift and amplify. These also require isolated supplies for the input (can be derived from the measured voltage, but better to use an isolated DC-DC converter from the control side).
    • Same as above but with analog linear optocoupler coupled with external op-amps. E.g. HCNR200LOC110. Discouraged!
    • External dedicated (SAR?) ADC fed from isolated supply which has SPI output fed through digital optocoupler. The microcontroller reads this isolated SPI signal.
    • External Sigma-Delta ADC isolated at the ADC itself, or with digital optocoupler, to Sigma-Delta demodulator on micronctoller.

Best solution is probably something like an ISO124. This can accept ±12V inputs with unity gain. This would then feed into a unity gain op-amp with filtering. This would then feed into a bipolar ADC such as the ADS8568. For 4 measurement channels, a single quad op-amp and quad ADC could be used.