SLPS598C May 2017 – January 2018 CSD88584Q5DC
The CSD88584Q5DC power block has the ability to switch at voltage rates greater than 1 kV/µs. Special care must be then taken with the PCB layout design and placement of the input capacitors; high-current, high dI/dT switching path; current shunt resistors; and GND return planes. As with any high-power inverter operated in hard switching mode, there will be voltage ringing present on the switch nodes U, V, and W. Switch-node ringing appears mainly at the HS FET turnon commutation with positive winding current direction. The U, V, and W phase connections to the BLDC motor can be usually excluded from the ringing behavior since they are subjected to high-peak currents but low dI/dT slew-rates. However, a compact PCB design with short and low-parasitic loop inductances is critical to achieve low ringing and compliance with EMI specifications.
For safe and reliable operation of the three-phase inverter, motor phase currents have to be accurately monitored and reported to the system microcontroller. One current sensor needs to be connected on each motor phase winding U, V, and W. This sensing method is best for current sensing as it provides good accuracy over a wide range of duty cycles, motor torque, and winding currents. Using current sensors is recommended because it is less intrusive to the VIN and GND connections.
However, for cost sensitive applications, current sensors are generally replaced with current sense resistors.
Breaking the high-current flow path from the source terminals of the power block to GND by introducing the RCS current shunt resistors introduces parasitic PCB inductance. In the event the switch node waveforms exhibits peak ringing that reaches undesirable levels, the ringing can be reduced by using the following ringing reduction components:
With a switching frequency of 20 kHz on the three-phase inverter, the power dissipation on the RC snubber resistor is 80 mW per channel. As a result, 0805 package size for resistors Rs1, Rs2, and Rs3 is adequate.