SPRUHJ1I January 2013 – October 2021 TMS320F2802-Q1 , TMS320F28026-Q1 , TMS320F28026F , TMS320F28027-Q1 , TMS320F28027F , TMS320F28027F-Q1 , TMS320F28052-Q1 , TMS320F28052F , TMS320F28052F-Q1 , TMS320F28052M , TMS320F28052M-Q1 , TMS320F28054-Q1 , TMS320F28054F , TMS320F28054F-Q1 , TMS320F28054M , TMS320F28054M-Q1 , TMS320F2806-Q1 , TMS320F28062-Q1 , TMS320F28062F , TMS320F28062F-Q1 , TMS320F28068F , TMS320F28068M , TMS320F28069-Q1 , TMS320F28069F , TMS320F28069F-Q1 , TMS320F28069M , TMS320F28069M-Q1
The two shunt current measurement technique uses the principle of Kirchhoff's Current Law (KCL) that the sum of the currents into a single node equals zero. By measuring only two phase currents, the third is calculated with KCL. A circuit for the two shunt current measurement technique is shown in Figure 18-7.
The two shunt measurement circuit has an advantage over the single shunt circuit in that it can see circulating currents. Now all currents are measured only during switching state zero. Figure 18-8 shows an example of a switching waveform and where the ADC samples the current. The PWM for IA is almost 100% duty cycle and in this example causes the IA current to rise. The PWM for IB is about 50% duty cycle and its current stays at about zero amps for this period. Phase current can only be measured when that phase's lower switch is conducting. In the example, IA is measureable for a very short time while IB has a long time to view. When the measured phase is operating at PWMs near 100%, this is the inherent problem when using the two shunt technique. For the example when IA is sampled, the measured current signal has not yet stabilized giving an incorrect representation of the current signal. Another pointer to note about using the two shunt technique during motoring is that the current being measured is now bipolar. So zero amps is now represented as half of the ADC full scale.
As the duty cycle increases the time to measure voltage across the shunt resistor for the phase needs to be quicker. For example if a duty cycle of 95% were commanded with a 20 kHz PWM waveform there will be 2.5 µs of on-time for the measured phase. Ideally the slew rate of the op-amp should be 1/10 of the on-time or 0.25 µs. A full scale output voltage transition of the op-amp is 1.65 V. The minimum slew rate is calculated to be 6.6 V/µs. As the duty cycle increases even more, the slew rate must be increased to capture the signal properly. Although the two shunt current measurement technique lessens the op-amp's speed requirement as compared to the single shunt measurement, there is a duty cycle where the slew rate has to be very large. For the single and two shunt measurement techniques there is no way of getting around the need for a fast and expensive op-amp.