Delay and Dead Time in Integrated MOSFET Drivers

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1 Introduction

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 (t_pd). The analog drivers insert automatic internal dead time (t_dead) to avoid cross conduction of MOSFETs and shoot-through currents.

Figure 1-1 Propagation Delay Timing in Integrated MOSFET Drivers

However, propagation delay and dead time can change based on many factors:

Direction of current into or out of the phase
Additional dead time from the MCU PWM inputs
Delay Compensation to minimize duty cycle distortion

This application note investigates how each factor affects driver delay and dead time in integrated MOSFET drivers.

2 Direction of Current Into or Out of the Phase

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.

Figure 2-1 Sinusoidal Current Control Waveforms

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.

Figure 2-2 DRV8316 Dead Time and Propagation Delay Specifications for 200 V/μs Slew Rate

2.1 INHx Rising, INLx Falling, Current is Going Out of the Phase

When current is going out of the phase, propagation delay and driver dead time is determined by whether INHx is rising or falling.

In Figure 2-3, when INLx goes low (green), current is momentarily pulled through the body diode (purple) of the low-side (LS) FET to continue sourcing current out of OUTx (red). The duration the body diode of the LS FET conducts is the dead time. When the body diode stops conducting (dead time is over), the high-side (HS) FET begins to conduct (blue).

Figure 2-3 Current Switching With INHx Rising, INLx Falling, and Current Out of OUTx

Note how the current direction is opposite internally between the body diode of the LS FET and the conduction path of the HS FET. To reduce a large change of current, the device waits until the LS body diode fully conducts and then turns on the HS FET, which lengthens propagation delay to the typical or maximum value as specified in the data sheet.

The example waveform in Figure 2-4 shows INHx rising and INLx falling and current is going out of the phase (positive current in green) for the DRV8316 with a slew rate of 200 V/µs and sinusoidal commutation. The dead time and propagation delay typical values in the DRV8316 data sheet specifications are 500 ns and 700 ns, respectively, with maximums included as well. Note how internal dead time is included into the propagation delay.

Figure 2-4 Waveforms of Dead Time and Propagation Delay When INHx is Rising, INLx is Falling, and Current Flows Out of OUTx