AN-937 Datasheet, PDF (1/21 Pages) List of Unclassifed Manufacturers – Gate Drive Characteristics and Requirements for HEXFET

Index
AN-937 (v.Int)
Gate Drive Characteristics and Requirements for
HEXFET^®s
Topics covered:
Gate drive vs base drive
Enhancement vs Depletion
N vs P-Channel
Max gate voltage
Zener diodes on gate?
The most important factor in gate drive: the impedance of the gate drive circuit
Switching 101 or Understanding the waveforms
What happens if gate drive impedance is high? dv/dt induced turn-on
Can a TTL gate drive a standard HEXFET^®?
The universal buffer
Power dissipation of the gate drive circuit is seldom a problem
Can a C-MOS gate drive a standard HEXFET^®?
Driving HEXFET^®s from linear circuits
Drive circuits not referenced to ground
Gate drivers with optocouplers
Gate drive supply developed from the drain of the power device
Gate drivers with pulse transformers
Gate drivers with choppers
Drive requirements of Logic Level HEXFET^®s
How fast is a Logic Level HEXFET^®driven by a logic circuit?
Simple and inexpensive isolated gate drive supplies
A well-kept secret: Photovoltaic generators as gate drivers
Driving in the MHz? Use resonant gate drivers
Related topics
(Note: Most of the gate drive considerations and circuits are equally applicable to IGBTs. Only MOSFETs are mentioned for the
sake of simplicity. Special considerations for IGBTs are contained in INT-990)
1. GATE DRIVE VS BASE DRIVE
The conventional bipolar
transistor is a current-driven
device. As illustrated in
Figure 1(a). a current must
be applied between the base
and emitter terminals to pro-
duce a flow of current in the
collector. The amount of a
drive required to produce a
given output depends upon
the gain, but invariably a
current must be made to flow
into the base terminal to
produce a flow of current in
the collector.
CURRENT
IN BASE
I_B
PRODUCES
CURRENT
IN COLLECTOR
I_C
VOLTAGE
AT GATE
+
+
+
PRODUCES
CURRENT
IN DRAIN
I_D
CURRENT
SOURCE
VOLTAGE
SOURCE
(a) Bipolar Transistor
(b) HEXFET
Figure 1. Bipolar Transistor is Current Driven, HEXFET is Voltage Driven
The HEXFET^®is fundamentally different: it is a voltage-controlled power MOSFET device. A voltage must be applied between
the gate and source terminals to produce a flow of current in the drain (see Figure 1b). The gate is isolated electrically from the
source by a layer of silicon dioxide. Theoretically, therefore, no current flows into the gate when a DC voltage is applied to it -
though in practice there will be an extremely small current, in the order of nanoamperes. With no voltage applied between the
gate and source electrodes, the impedance between the drain and source terminals is very high, and only the leakage current
flows in the drain.
To Order