A PPLICATION

AN978 revB

International Rectifier

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233 Kansas Street El Segundo CA 90245 USA

N OTE

HV Floating MOS-Gate Driver ICs

(HEXFET is a trademark of International Rectifier) Topics Covered:

Gate drive requirements of high side devices Block diagram of a typical MGD Bootstrap operation How to select the bootstrap components How to calculate the power dissipation in the MGD How to deal with negative transients Layouts and other guidelines How to isolate the logic from the power circuit How to boost the output current of an MGD to drive modules How to provide a continuous gate drive How to provide a negative gate driveDriving Buck converters Driving Dual-Forwards and switched reluctance industrial motor controllers Cycle-by-cycle current control by means of the SD pin Brushless and induction motor drives Push-pull and other low-side applications Driving a high-side P-Channel MOSFET How to drive thyristor gates Troubleshooting guidelines

1. GATE DRIVE REQUIREMENTS OF HIGH-SIDE DEVICES

The gate drive requirements for a power MOSFET or IGBT uti-lized as a high side switch (industrial drain connected to the high voltage rail, as shown in Figure 1) driven in full enhancement, i.e., lowest voltage drop across its terminals, can be summarized as follows:1.Gate voltage must be 10-15V higher than thedrain voltage. Being a high side switch, such gate voltage would have to be higher than the rail voltage, which is frequently the highest volt- age available in the system.2.The gate voltage must be controllable from thelogic, which is normally referenced to ground. Thus, the control signals have to be level-shifted to the source of the high side power device, which, in most applications, swings between the two rails.3.The power absorbed by the gate drive circuitryshould not significantly affect the overall effi- ciency.With these constraints in mind, several techniques are presentlyused to perform this function, as shown in principle in Table I. Each basic circuit can be implemented in a wide variety of con- figurations.
+ HIGH VOLTAGE RAIL GATESOURCE V

Figure 1. Power MOSFET in high sideconfiguration

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