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For integrated MOSFET drivers, data sheets define propagation delay as the time it takes for changing input logic edges INHx and INLx (whichever changes first if MCU dead time is added) to change the half-bridge output voltage (OUTx) as Figure 1-1 shows. It includes an input deglitch delay, an analog driver, and a comparator delay resulting in a propagation delay time (tpd). The analog drivers insert automatic internal dead time (tdead) to avoid cross conduction of MOSFETs and shoot-through currents.
However, propagation delay and dead time can change based on many factors:
This application note investigates how each factor affects driver delay and dead time in integrated MOSFET drivers.
When commutating with sinusoidal control, all three half-bridges are switching using synchronous PWM inputs with varying duty cycles and 120 degrees out of phase from each other. This results in smooth sinusoidal phase current (Figure 2-1), which means that the current direction of each phase is going into or out of the motor output pins (OUTx) of each phase. Depending on the direction of the current, the propagation delay can vary depending on whether the high- and low-side inputs (INHx and INLx) of the phase are rising or falling and the direction of current at that instant in time.
This application report describes four scenarios of synchronous inputs switching, direction of current from OUTx, and how they can affect propagation delay and dead time using the DRV8316 integrated MOSFET BLDC motor driver. An assumption is made that there is no additional MCU dead time and INHx and INLx are synchronous PWM inputs. Furthermore, a fixed output slew rate of 200 V/µs is assumed and the propagation delay and dead time to the values mentioned in the data sheet in Figure 2-2 is compared.