Power-MOSFET Gate Drivers

July 19, 2004

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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About the Author

Sam Davis 2

Sam Davis was the editor-in-chief of Power Electronics Technology magazine and website that is now part of Electronic Design. He has 18 years experience in electronic engineering design and management, six years in public relations and 25 years as a trade press editor. He holds a BSEE from Case-Western Reserve University, and did graduate work at the same school and UCLA. Sam was the editor for PCIM, the predecessor to Power Electronics Technology, from 1984 to 2004. His engineering experience includes circuit and system design for Litton Systems, Bunker-Ramo, Rocketdyne, and Clevite Corporation. Design tasks included analog circuits, display systems, power supplies, underwater ordnance systems, and test systems. He also served as a program manager for a Litton Systems Navy program.

Sam is the author of Computer Data Displays, a book published by Prentice-Hall in the U.S. and Japan in 1969. He also authored the book Managing Electric Vehicle Power. He is also a recipient of the Jesse Neal Award for trade press editorial excellence, and has one patent for naval ship construction that simplifies electronic system integration.

You can also check out additional articles on his other author page.