SLVSEE8B November 2019 – May 2021 DRV8899-Q1
The power loss due to the PWM switching frequency depends on the slew rate (tSR), supply voltage, motor RMS current and the PWM switching frequency. The switching losses in each H-bridge during rise-time and fall-time are calculated as shown in Equation 6 and Equation 7.
Both tRISE_PWM and tFALL_PWM can be approximated as VVM/ tSR. After substituting the values of various parameters, and assuming 105 V/µs slew rate and 30-kHz PWM frequency, the switching losses in each H-bridge are calculated as shown below -
The total switching loss (PSW) is calculated as twice the sum of rise-time (PSW_RISE) switching loss and fall-time (PSW_FALL) switching loss as shown below -
The rise-time (tRISE) and the fall-time (tFALL) are calculated based on typical values of the slew rate (tSR). This parameter is expected to change based on the supply-voltage, temperature and device to device variation.
The switching loss is inversely proportional to the output slew rate. 10 V/µs slew rate will result in approximately ten times higher switching loss than 105 V/µs slew rate. However, lower slew rates tend to result in better EMC performance of the driver. A careful trade-off analysis needs to be performed to arrive at an appropriate slew rate for an application.
The switching loss is directly proportional to the PWM switching frequency. The PWM frequency in an application will depend on the supply voltage, inductance of the motor coil, back emf voltage and OFF time or the ripple current (for smart tune ripple control decay mode).