Update : January 03, 2024
Power MOSFETs are voltage-controlled devices used as switching elements in power circuits, motor drivers, and various systems. The gate is the electrical isolation control terminal for each device, with the other terminals being the source and drain. To operate the MOSFET, a voltage is typically applied to the gate (relative to the source or emitter).
Specialized drivers are employed to apply voltage to the gate of power devices and provide driving current. Gate drivers are crucial for turning power devices on and off, charging the gate of power devices to reach the final on-state voltage (VGS(ON)), or discharging the gate to reach the final off-state voltage (VGS(OFF)).
To achieve the transition between these two gate voltage levels, a loop involving gate drivers, gate resistances, and power devices introduces some power losses.
Today, high-frequency converters for medium to low power applications primarily utilize gate voltage-controlled devices like MOSFETs. For high-power applications, Silicon Carbide (SiC) MOSFETs are commonly used due to their fast switching characteristics, requiring higher driving currents.
Gate drivers are not only applicable to MOSFETs but also extend to emerging devices like Silicon Carbide (SiC) FETs and Gallium Nitride (GaN) FETs. They act as power amplifiers, accepting power input from controller ICs and generating substantial current to drive the gate of power switching devices.
But why use a microcontroller to drive power transistors? To better answer this question, let's consider large-scale applications.
Switching power supplies are at the core of almost every modern electrical system.
Devices plugged into wall outlets can utilize switch-mode power supplies for power factor correction and generating DC power rails.
Automotive systems employ switch-mode power supplies to maintain systems such as batteries, motors, and chargers.
Grid infrastructure requires efficient conversion of DC power from solar panels to transfer energy to DC storage systems and AC grids.
Given the plethora of topologies and increasing complexity in applications with high-power transistor arrays, modern switch-mode power supplies typically use microcontrollers or other ASICs to coordinate their switches to meet precise timing requirements.
This poses challenges as most microcontroller outputs are not optimized for driving power transistors. The following summarizes the reasons for using gate drivers:
Gate drivers function to turn power devices on and off rapidly, minimizing losses. To avoid Miller effect or slow switching at certain loads, the driver must establish the off-state with lower impedance than the on-state of the transistor on the gate. Negative gate drive margin plays a vital role in reducing these losses.
This is the inductance shared by the gate driver current loop and the output current loop. Negative gate drive margin combined with source lead inductance directly impacts the switch speed under load, contributing to the source inductance degradation effect (source lead inductance couples the output switching current back to the gate drive, slowing down the gate drive).
Gate drivers apply a voltage signal (VGS) between the gate (G) and source (S) of the power MOSFET while providing a large current pulse.
Broadly speaking, gate drivers fall into two categories: non-isolated gate drivers and isolated gate drivers. Most non-isolated gate drivers for high-voltage operation are half-bridge drivers.
Half-bridge drivers are designed to drive power transistors stacked in a half-bridge configuration. They have two channels: low-side and high-side. The low-side is a relatively simple buffer, usually sharing the same ground point as the control input. The high-side is carefully designed and referenced to the half-bridge's switching node, allowing the use of two N-channel MOSFETs or two IGBTs.
The switching node should transition rapidly between the high-voltage bus and power ground, providing an opportunity to power the high side through a bootstrap circuit using the same power as for the low side.
A high-voltage level translator must be included to convey whether the output should be high or low. This translator, with typically minimal leakage current in the range of a few microamps or less, adds some propagation delay for sufficient noise filtering.
Additionally, the low-side driver must match the longer delay of the high-side driver. Non-isolated gate drivers for high-voltage operation have limitations. First, they cannot exceed the silicon process limit since they are all on the same silicon die.
Most non-isolated gate drivers have a working voltage not exceeding 700V. Second, the level translator must endure the pressures of high-voltage operation and convey the output status in high-noise environments, usually adding some propagation delay for noise filtering.
The low-side driver then needs to match the longer delay of the high-side driver. Third, non-isolated gate drivers for high-voltage operation lack flexibility. Many complex topologies exist that require multiple outputs to switch above or below the control common level.
An increasingly common feature in modern gate drivers is the integration of an isolation layer between the input and output circuits. These devices use one silicon die for control signals and another for output drive signals, physically isolating them with distance and insulating materials.
While control signals can pass through the isolation layer in various ways, unlike non-isolated gate drivers, the isolation layer prevents any significant leakage current from flowing from one side to the other.
Because one input die can be isolated from multiple output dies, and the output dies can be isolated from each other, the output common terminal can freely shift upward from the input common terminal or other output common terminals until it reaches the limit of isolation technology.
In contrast to non-isolated gate drivers with inflexible level translators and predetermined output roles, isolated gate drivers can have outputs referenced to any node in the circuit and can be constructed as single-channel or dual-channel devices. The limits of isolation technology far exceed the silicon process limits of non-isolated gate drivers, providing isolation layers with withstand voltages exceeding 5 kV. Besides raising the voltage limit and flexibility, isolated gate drivers can be used to achieve faster and more robust operation.
There are many reasons for using isolation. Many applications require isolated power supplies due to regulatory requirements, and isolated gate drivers can be used to simplify system architecture.
At times, the strength of the isolation layer can enhance the system's resilience against surges, lightning, and other abnormal events that could potentially damage the system.
In other cases, by flexibly using the isolation layer, the design of the topology can be simplified without the need for additional signal converters or level converters, such as an inverted buck/boost.
Even in traditional half-bridge applications where isolation is not strictly required, isolated gate drivers can surpass non-isolated gate drivers with superior propagation delay, higher driving force, and better tolerance to high-voltage transients."
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