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
An example of the three shunt current measurement circuit is shown in Figure 18-9. The three shunt current measurement technique is very robust and surprisingly can be cost effective even when compared to using a single or two shunt measurement technique. First, with the single and two shunt circuits over-modulation is difficult to achieve. Second, higher priced fast slew rate op-amps must be selected for the one and two shunt techniques. The three shunt technique can bounce sampling between current signals, selecting two out of three phases each period, which allows for long times for the current signals to settle. If large current measurement windows are possible, then much slower and cheaper op-amps can be used. For example, Figure 18-10 shows three PWM switching signals and what shunt resistor will be sampled. As can be seen, there is plenty of time for the current signal to stabilize.
With three shunts the op-amp slew rate can be well under 1 V/µs. There are many advantages to having slower amplifier circuits for current measurement. First cost will be less. Second a slower amplifier will have a lower bandwidth to pick up noise. Third in many of the current amplifier circuits there is crosstalk between the phase measurements. What will happen is a spike from the C phase switching will show up in the A phase measurement. A slow amplifier circuit will attenuate the crosstalk signal.