Silicon carbide and gallium nitride switching devices are key components in power supply circuits. Although these devices exhibit superior performance in terms of inherent characteristics such as operating speed, high voltage, current handling, and low power consumption, designers often focus solely on these devices and frequently overlook the associated drivers.
The function of gate driver
Effective power supply circuits consist of more than just fixed components like SiC and GaN MOSFETs. The gate driver, a separate component located before the electronic switches, ensures optimal energy transfer for efficient driving. Simply transmitting a square or rectangular wave directly to the gate terminals of the components is insufficient. The drive signal must be properly arranged to provide the correct potential, ensuring the oscillation is optimal for the different components. This helps minimize parasitic elements and reduce power losses as much as possible. Therefore, designers must execute projects based on circuit design and consider the terminal load. This involves researching and developing high-quality gate drivers capable of efficiently driving power components.
Inefficient drivers not only lead to significant power losses but also frequently cause improper synchronization, resulting in abnormal circuit operation and even MOSFET damage. These devices are voltage-controlled, with their gates serving as control terminals, electrically isolated from the device. To activate a MOSFET, a voltage must be applied to this terminal by a specific driver.
The gate terminal of a MOSFET can be considered a nonlinear capacitor. By applying charge to the gate capacitor, the device is activated, allowing current to flow between the drain and source terminals. Conversely, discharging the capacitor switches it to an "off" state. To activate a MOSFET, a voltage greater than the threshold voltage (VTH) needs to be supplied between the gate and source terminals. This value is the minimum required to charge the capacitor and allow the MOSFET to turn on. Typically, digital systems such as microcontrollers (MCUs) are not powerful enough to directly activate the device. Therefore, an interface (i.e., a driver) is always needed to connect the control logic to the power switch.
Gate drivers are primarily used as level shifters. However, gate capacitors do not have the ability to charge instantaneously; they require a certain amount of time to reach their maximum charge. During this brief period, the device operates with a large current and voltage, generating a significant amount of electrical energy as heat. Unfortunately, this energy remains unused, representing a depletion of power. To minimize switching time and allow for rapid charging of the gate capacitor, state transitions must be very rapid, requiring large current transients. The behavior of a silicon carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) as an electronic switch is described below. It illustrates the key signals at different points during transient events.
The "V (pulse)" signal at the top represents the PWM wave that powers the system. This is an ideal rectangular signal with a frequency of 100 kHz in this example. This is a perfect signal.
The “V(gate)” signal represents the actual signal at the gate terminal. As you can see, its trend is irregular because the gate capacitance is not linear, and its voltage reaches its maximum value after a few minutes, which is the time required for the capacitor to charge to its maximum capacity. This interval is determined by the time constant RC, which is approximately 150 ns in this example.
The "I (Load)" signal represents the current flowing through the load and drain terminals. Initially, this signal is low when the MOSFET is turned on, and then reaches its maximum level when the MOSFET is turned off. This sequence repeats indefinitely. Note that the switching is not immediate and instantaneous, but follows the switching of the gate voltage.
The “V (drain)” signal shows the trend of VGS voltage change. Obviously, it is inversely related to the current and always follows the charging speed of the gate capacitance.
Finally, the power dissipation of the MOSFET (VDS × ID) is shown, exhibiting a high, detrimental peak value corresponding to the rising and falling edges of the drive signal. This is the power loss, a factor that the gate driver must minimize as much as possible.
