A gate driver is a power amplifier that takes a low-power input from a controller IC and generates the appropriate high-current gate drive for a power device. The design and performance of gate driver circuitry are becoming increasingly important as the requirements for power electronics continue to rise.
Modern power electronics systems are built around power semiconductor devices. As switching elements in switched-mode power supplies (SMPS), universal power supplies (UPS), and motor drives, these systems employ many gated semiconductor devices such as ordinary transistors, FETs, BJTs, MOSFETs, IGBTs, and others. Power electronics technology has generally evolved in lockstep with power semiconductor devices.
In the power electronics industry, power level requirements and switching frequency are increasing. Metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs) are two of the most widely used and efficient semiconductor devices in most applications for medium to high power switching power supplies.
The gate of a MOSFET or IGBT is the device’s electrically isolated control terminal. These devices’ other terminals are source and drain or emitter and collector. To operate a MOSFET/IGBT, a voltage is typically applied to the gate that is relative to the device’s source/emitter. The gate terminal must be made positive with respect to its source/emitter in order to drive these switching devices into conduction.
The parasitic capacitances between the three terminals, namely gate-to-source (Cgs), gate-to-drain (Cgd), and drain-to-source (Cds), influence the switching behavior of power devices. These capacitances are typically non-linear and vary with bias voltage. Charging the gate capacitor turns on the power device and allows current to flow between its drain and source terminals, while discharging it turns off the device and a large voltage is blocked across the drain and source terminals.
A power device’s gate voltage does not increase unless its gate input capacitance is charged, and the power device does not turn on until its gate voltage reaches the gate threshold voltage (Vth). The Vth of a power device is defined as the minimum gate bias required to create a conduction path between its source and drain regions.
When using a power device as a switch, a voltage sufficiently greater than Vth should be applied between the gate and source/emitter terminals.
Gate Drivers for Power Electronics
The output of a logic IC can never be used to drive the gate of a power switch in high power applications (PWM controller).
Charging the gate capacitance would take an inordinate amount of time, most likely longer than the duration of a switching period, due to the low current capabilities of these logic outputs. To apply a voltage and provide drive current to the power device’s gate, dedicated smart gate drivers must be used. This could be a driver circuit, with dedicated ICs, discrete transistors, or transformers as components. It can also be built into a PWM controller integrated circuit.
A gate driver is a power amplifier that accepts a low power input from a controller IC and generates the correct high current gate drive for a power device. When a PWM controller is unable to provide the output current required to drive the gate capacitance of the associated power device, it is used.
The gate driver circuit is an essential component of power electronics systems. Many smart gate drivers are used to drive power semiconductor devices and serve as an important interface between the high-power electronics and the control circuit.
The output of DC-DC converters or SMPS is primarily determined by the behavior of gate driver circuits, which means that if the gate driver circuit fails to properly drive the gate of a power device, the DC-DC converter output will be incorrect. As a result, when designing power electronic converters, the gate driver circuit is critical.
Types of Gate Drivers
High-Side-Low-Side Drivers — Used to power two switches connected in a bridge configuration (both floating & ground referenced switches).
Low-Side Drivers — These are used to power ground-connected switches (low side switches).
Gate Driver Isolation
For both functional and safety reasons, gate drive circuits for power inverters and converters frequently require electrical isolation. To avoid shock hazards, regulatory and safety certification agencies require isolation. It also safeguards low voltage electronics against damage caused by faults in the high power side circuit and human error on the control side.
The electrical separation between different functional circuits in a system prevents a direct conduction path between them and enables individual circuits to have different ground potentials. Signal and power can still be transmitted between isolated circuits via inductive, capacitive, or optical means. Many power device applications (for example, converters requiring high power density and efficiency) necessitate the use of an isolated gate drive circuit.
There are high and low switches in power converter topologies such as half-bridge, full-bridge, buck, two-switch forward, and active clamp forward, for example, because low side drivers cannot be used directly to drive the upper power device. Because the upper power devices’ source and emitter are not at ground potential, an isolated gate drive optocoupler is required (floating).
The source terminal of switch 1 can be floating anywhere from ground to DC bus potential in a simple bridge topology structure with a driving circuit like the one shown here. As a result, driving high side switches necessitates the use of two components:
Floating supply — used to power any circuitry associated with this floating midpoint potential.
Level shifter — used to send the PWM control signal to the floating driver circuitry.
In general, there are two popular methods for implementing isolated gate drivers: magnetic (via gate drive transformers) and optical (using a gate drive optocoupler).