TI's gate drivers use a pull-up and pull-down structure in the output stage in order to drive a power device. The pull-up structure supplies the source current to turn on the power device, and the pull-down structure supplies the sink current to turn off the power device. Traditional gate drivers supply the source and sink currents through a single output pin; however, some gate drivers feature split outputs, which have separate pins for sourcing and sinking currents. Source current takes the “turn-on” path and sink current takes the “turn-off path”. Figure 1 shows the output stage of a gate driver with a single output, while figure 2 shows the output stage of a gate driver with split outputs.
Figure 1: Gate driver output stage with single output
Figure 2: Gate driver output stage with split outputs
For power devices, rise time, fall time, and turn-on and turn-off delays are typically different. Gate resistances are used to control these characteristics. A lower gate resistance on the turn-off path allows for a faster turn-off and can also help prevent false turn-on caused by fast switching and Miller current. Likewise, a lower gate resistance on the turn-on path can result in faster turn-on, which reduces switching losses. Careful consideration of the system design and behavior is needed when adjusting the gate resistances.
A gate driver with a single output can use a diode in parallel with the gate resistor to adjust the turn-on and turn-off path separately. Figure 3 shows the orientation of the diode D1 for faster turn-off. When the gate driver sources current, the diode blocks the current flow in the path with no resistance. This causes the source current to go through resistor R1 only, making the turn-on gate resistance 2.2Ω. When the gate driver sinks current, the diode permits the current flow and ideally reduces the turn-off gate resistance to 0Ω. A resistor can be placed in series with the diode to adjust the turn-off path as needed. For a faster turn-on, simply reverse the diode orientation.
Figure 3: Using a diode for a faster turn-off in a gate driver with single output
A gate driver with split outputs gives the benefit of separately controlling the turn-on and turn-off path without a diode. The removal of diode saves PCB space for better layout and eliminates parasitics that come with the diode, both of which are important in fast switching applications such as GaN. The bill-of-materials cost is also reduced. Figure 4 shows how a gate driver with split outputs can be used to achieve a faster turn-off compared to turn-on.
Figure 4: Using a gate driver with split outputs
In some instances, it may be beneficial for a gate driver IC to not include split outputs. Split outputs take up two pins in the IC package, which could require other features of the gate driver to be removed to maintain the same package size. Otherwise, a bigger package may take up more PCB space, lower power density, and cost more.
