Tutorial
Gate Driver ICs
Gate driver ICs find use are power amplifiers for power MOSFETs in power supply applications. Inputs to these gate driver ICs are typically logic levels from PWM ICs. Outputs can be single-ended or dual synchronous rectifier drive. MOSFETs require 1.0A to 2.0A drive to achieve switching efficiently at frequencies of hundreds of kilohertz. This drive is required on a pulsed basis to quickly charge and discharge the MOSFET gate capacitances. Fig. 1 shows the MOSFET capacitances associated with a gate driver IC output driving the input of a power MOSFET.
A simple example will explain the gate drive requirements and also show that the Miller effect, produced by drain-source capacitance, is the predominant speed limitation when switching high voltages. A MOSFET responds instantaneously to changes in gate voltage and will begin to conduct when its gate threshold is reached and the gate-to-source voltage is 2.0V to 3.0V; it will be fully on at 7.0 V to 8.0 V.
Many manufacturers now provide logic level and low threshold voltage MOSFETs that require lower gate voltages to be fully turned on. Gate waveforms will show a porch at a point just above the threshold voltage that varies in duration depending on the amount of drive current available and this determines both the rise and fall times for the drain current.
The popularity of recent processor ICs has created a demand for regulation at low voltages with high currents. Because of the forward voltage drop required for rectifier diodes, even Schottky rectifiers, most dc-dc converter circuits employ synchronous rectification to achieve higher efficiencies.
Synchronous rectification presents a new problem to the MOSFET drive circuit designer: shoot-through current. Because the circuit must turn on and off both the power switch and the rectifier switch, a low impedance may be presented to the input voltage source during switching transitions. This low transition impedance can cause a shoot-through current to be conducted through both the power switching MOSFET and the synchronous rectifier MOSFETs. High shoot-through currents result in higher EMI generation, more noise on the input voltage source, lower efficiency and reduced reliability of the dc-dc converter circuit.
Some manufacturers of control ICs specify a minimum amount of non-overlap, or dead-time. That is, there is a minimum time at the switching transitions (two per operating cycle) where both MOSFETs are turned off. Defining a dead time prevents the problem of shoot-through current, but reduces circuit efficiency from its optimum value. During the dead time, the switching inductor causes current to continue to flow through the rectifier MOSFET body diode. To improve efficiency, you can shunt the low-side synchronous rectifier MOSFET with a Schottky diode.
Where no dead time is specified, the drive design must be done carefully to prevent shoot-through current. Some drive circuits have gate resistors with anti-parallel diodes to slow MOSFET turn-on time while minimizing the turn-off time.
When using an IC controller, shunt the output drive with a Schottky rectifier if necessary, to prevent transients from pulling the drive pin voltage below -0.3V.
Increased switching frequency of PWM controller ICs reduces power converters volume and cost, so fast rise and fall times are necessary to minimize switching losses. Discrete solutions can achieve reasonable drive capability, but implementing delay and other housekeeping functions necessary for safe operation can become cumbersome and costly. In contrast, the gate drive IC can overcome these limitations.
The lifetime and performance of a driver is basically determined by the drive power requirements of the load, the thermal characteristics of the driver package and its cooling method. The driv