MPPT Interface - Meeting Notes

Owned by Titus Chow
Oct 03, 2017

Meeting Notes - September 30, 2017

Interface Board Isolation

The one of the requirements of designing the interface board is how to isolate the high voltage environment (i.e. MPPT) from the low voltage environment (i.e. microcontrollers). The main challenge is the isolation of analog to digital converter, which measures the high voltage analog signal and converts it to low voltage binary digital signal. There are two options of isolating the ADC: 

  • Analog Isolation
    • Interfere the the analog measurement
    • Isolation amplifier requires power supply on both the input-side (MPPT) and output-side (Interface broad)
  • Digital Isolation
    • Digital signal is immune to the measurement interference
    • The supply voltage of the ADC must be isolated
    • Optocouplers can be used to isolate the signal

Voltage Sensing

Use a voltage divider to map the high voltage (0-30v) to a low voltage (0-5v) for the ADC

Current Sensor

There are two ways to sense the current: direct and indirect

  • Direct 
    • Place a shunt resistor with known resistance in the circuit
    • Measure the voltage of the shunt resistor and calculate the current based on Ohm's law
    • Typically amplifier is used to amplify and an ADC is used to measure to the voltage
    • There are current shunt monitor chips that provide I2C communication
    • Require isolation
  • Indirect
    • Use Hall effect sensor to measure the current
    • Does not require additional isolation
    • Example: ACS722LLCTR-40AU-T

Cost

The cost of the interface board should be cheap, because we will build 12 interface boards.

Polarity 

MPPTs might output a slight amount of negative voltage, thus, we nee to use a diode to ensure the unipolarity or use ICs that can operate in bipolarity.

Potential ADC Chip

ADS7924

Power Supply

ADCs, current sensing monitors, temperature sensors, isolation amplifiers and other ICs need a small amount of current to operate. Due to the isolation requirement, supplying power to the ICs is tricky. There are few options of supplying the power to the ICs.

  1. Supply power from the high-voltage side (i.e. MPPT)
    1. Use ICs with wide supply voltage range
    2. Use voltage regulator?
  2. Supply power from the low-voltage side (i.e. boards)
    1. Use a DC-DC converter to create an isolation barrier 
    2. DC-DC converters are expensive
  3. Supply power from the high-voltage and low-voltage sides
    1. Only needed if we plan to use isolation amplifier
    2. It is really complicated
  4. Use AA battery to power the interface board