Critical to getting a closed loop control system is accurate current measurement of the inverter. In this design, current measurement is done to sense the current through the inductor. Because the output is high voltage and the controller needs to remain isolated, the AMC3302 reinforced isolated current sensor is used to measure the resistor voltage drop. To keep system losses low, the AMC3302 has a ±50-mV input range. When compared to other devices with a typical input range of ±250 mV, the total power loss across the shunt is significantly reduced.

Sizing the shunt resistor for this design is a trade-off between sensing accuracy and power dissipation. A 2-mΩ shunt provides a ±50-mV output signal at the inverters approximate ±25-A output but also only generates 0.5 W of heat at full load. When choosing an actual device, select a high accuracy one to eliminate the need to calibrate each sensor path.

The voltage across the shunt resistor is fed into the AMC3302 isolated current sensor with integrated isolated bias power supply, which generates a differential output. This differential output is converted to a single ended output with a 1.65-V offset using an OPA4376 amplifier for measurement using the ADC present on the C2000™ MCU. Figure 2-11 shows the current sensing circuit.

Figure 2-11 Isolated Current Sensing With AMC3302