When designing drive circuits for GaN devices designers must consider the maximum allowable gate voltage, the gate threshold voltage, and the body diode voltage drop. For an enhancement-mode GaN device, such as the GS66516B, the 6V gate-source voltage is half that of a MOSFET, making the generation of the turn-on / turn-off voltages and currents far simpler. The forward-voltage drop of the transistor’s intrinsic body diode is about 1 volt higher than comparable silicon MOSFETs. As the temperature coefficient is lower, compensation circuit requirements are also simpler.
GaN devices are generally faster than MOSFETs with comparable values of RDS(ON). In fact, turn-on times are typically four time faster while turn-off times are twice as fast. At the system level this delivers benefits although additional care is needed dynamic performance of the driver as dV/dt rates can exceed 100V/ns. There is a distinct possibility of creating a shoot-through condition during the transition phase which can affect the efficiency of the system.
Adjusting the gate drive pull-up resistor can achieve the fastest transition time without creating other loss-creating effects. By optimizing this resistance overshoot and ringing are also diminished thereby avoiding false turn-on/ / turn-off and also significantly reducing EMI. To further reduce high-frequency LC-ringing in practical applications designers may wish to add ferrite beads in series with the gate and also consider an RC ‘snubber’ across the gate-source path.
Figures 2 and 3 give a graphical overview of a GaN transistor’s turn-on / turn-off performance and shows the topics to consider for reliable operation.
The turn-on and turn-off performance can be individually optimised for GaN devices with a low threshold voltage by splitting the pull-up / pull-down connections in the driver, thereby permitting the insertion of a discrete resistor.
As well as optimizing stability, tweaking the gate turn-on / turn-off resistor ratio will also ensure the highest level of drive performance. A typical value for the turn-on resistor is in the range 10 to 20Ω. Too large a value will reduce the dV/dt slew rate at turn-on, thereby reducing switching speeds and increasing power losses. Gate oscillation and losses from the Miller-effect can be issues if the slew rate is too high. To achieve fast and robust pull-down with minimal impedance the turn-off resistance should be approximately 10% of the turn-on resistance.
Selecting a gate driver
TI’s LMG1205 gate drive IC is designed to address most of the issues that arise when driving GaN devices, while being flexible enough to allow designers to make design adjustments to switching speed and other parameters to suit the selected switching device. The LMG1205 has been optimized for use with enhancement-mode GaN switches and can be used in synchronous buck, boost, or half-bridge topologies where it will drive both the high-side and low-side switches with independent inputs for each side, allowing the ultimate flexibility.
The low propagation delay (typically 35ns) is matched to 1.5ns between channels thereby avoiding shoot-through issues and ensuring high efficiency. The LMG1205 features split-gate outputs to allow the current to be individually optimized and can source 1.2A / sink 5.0A to prevent undesired turn-on during rapid transitions.
There are several comparable devices that were designed specifically for GaN use and are alternatives to the LMG120G. These include the Silicon Labs Si827x series, Analog Devices ADuM4223A/B family and Maxim’s MAX5048C.
However, existing low frequency MOSFET drivers can be used in place of dedicated GaN drivers, provided the performance and features meet the needs of the GaN device.
As well as selecting the correct gate driver and implementing the required drive circuitry, all of the usual considerations for high speed circuits still apply. For example, attention must be paid to the layout, keeping the driver physically close to the switching device to minimize stray and undesired coupling. In some applications a kelvin-source connection can be used to minimize common-source inductance. In other applications there may be a benefit to using power supply rails that are galvanically isolated.
Wide bangdgap devices such as 600V GaN power transistors are now commercially available, making the performance benefits attainable for all designers. The faster speeds attainable with GaN devices mean that a good understanding of basic high-frequency analog is needed. However, to take full advantage of the new devices, designers have to carefully select the driver IC and ensure that the associated circuitry is designed to ensure proper switching.