Electronic Design

Power-MOSFET Gate Drivers

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What is a power-MOSFET gate driver?
It is a power amplifier that accepts a low-power input from a controller IC and produces the appropriate high-current gate drive for a power MOSFET. A gate driver is used when a pulse-width-modulation (PWM) controller cannot provide the output current required to drive the gate capacitance of the associated MOSFET. Gate drivers may be implemented as dedicated ICs, discrete transistors, or transformers. They can also be integrated within a controller IC. Partitioning the gate-drive function off the PWM controller allows the controller to run cooler and be more stable by eliminating the high peak currents and heat dissipation needed to drive a power MOSFET at very high frequencies.

What's the circuit model for a gate driver and power MOSFET?
Figure 1 shows the simplified model, including the parasitic components that influence high-speed switching, gate-to-source capacitance (CGS), the gate-to-drain capacitance (CGD), and drain-to-source capacitance (CDS). Values of the source inductance (LS) and drain inductance (LD) depend on the MOSFET's package. The other parasitic component is RG, the resistance associated with the gate signal distribution within the MOSFET that affects switching times.

What are the primary gate-driver design considerations?
An important attribute for the gate driver is its ability to provide sufficient drive current to quickly pass through the Miller Plateau Region of the power-MOSFET's switching transition. This interval occurs when the transistor is being driven on or off, and the voltage across its gate-to-drain parasitic capacitor (CGD) is being charged or discharged by the gate driver. Figure 2 plots total gate charge as a function of the gate-drive voltage of a power MOSFET. Total gate charge (QG) is how much must be supplied to the MOSFET gate to achieve full turn-on. It is usually specified in nanocoulombs (nC).

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