AN-937 Datasheet, PDF (2/21 Pages) List of Unclassifed Manufacturers – Gate Drive Characteristics and Requirements for HEXFET

Index
AN-937 (v.Int)
When a voltage is applied between the gate and
source terminals, an electric field is set up within the
HEXFET^®. This field “inverts” the channel (Figure
2) from P to N, so that a current can flow from drain
to source in an uninterrupted sequence of N-type
silicon (drain-channel-source). Field-effect
transistors can be of two types: enhancement mode
and depletion mode. Enhancement-mode devices
need a gate voltage of the same sign as the drain
voltage in order to pass current.
Depletion-mode devices are naturally on and are
turned off by a gate voltage of the same polarity as
the drain voltage. All HEXFET^®s are enhancement-
mode devices.
SILICON GATE
CHANNEL
N
SOURCE
P
N
GATE OXIDE
SOURCE
METALLIZATION
INSULATING
OXIDE
N
All MOSFET voltages are referenced to the source
TRANSISTOR
DRAIN
DRAIN
TRANSISTOR
terminal. An N-Channel device, like an NPN
CURRENT
CURRENT
transistor, has a drain voltage that is positive with
respect to the source. Being enhancement-mode
DIODE CURRENT
devices, they will be turned on by a positive voltage
on the gate. The opposite is true for P-Channel
Figure 2. Basic HEXFET Structure
devices, that are similar to PNP transistors.
Although it is common knowledge that HEXFET^®transistors are more easily driven than bipolars, a few basic considerations
have to be kept in mind in order to avoid a loss in performance or outright device failure.
2. GATE VOLTAGE LIMITATIONS
Figure 2 shows the basic HEXFET^®structure. The silicon oxide layer between the gate and the source regions can be punctured
by exceeding its dielectric strength. The data sheet rating for the gate-to-source voltage is between 10 and 30 V for most
HEXFET^®s.
Care should be exercised not to exceed the gate-to-source maximum voltage rating. Even if the applied gate voltage is kept below
the maximum rated gate voltage, the stray inductance of the gate connection, coupled with the gate capacitance, may generate
ringing voltages that could lead to the destruction of the oxide layer. Overvoltages can also be coupled through the drain-gate
self-capacitance due to transients in the drain circuit. A gate drive circuit with very low impedance insures that the gate voltage
is not exceeded in normal operation. This is explained in more detail in the next section.
Zeners are frequently used “to protect the gate from transients”. Unfortunately they also contribute to oscillations and have been
known to cause device failures. A transient can get to the gate from the drive side or from the drain side. In either case, it would
be an indication of a more fundamental problem: a high impedance drive circuit. A zener would compound this problem, rather
than solving it. Sometimes a zener is added to reduce the ringing generated by the leakage of a gate drive transformer, in
combination with the input capacitance of the MOSFET. If this is necessary, it is advisable to insert a small series resistor (5-10
Ohms) between the zener and the gate, to prevent oscillations.
3. THE IMPEDANCE OF THE GATE CIRCUIT
To turn on a power MOSFET a certain charge has to be supplied to the gate to raise it to the desired voltage, whether in the
linear region, or in the “saturation” (fully enhanced) region. The best way to achieve this is by means of a voltage source, capable
of supplying any amount of current in the shortest possible time. If the device is operated as a switch, a large transient current
capability of the drive circuit reduces the time spent in the linear region, thereby reducing the switching losses.
On the other hand, if the device is operated in the linear mode, a large current from the gate drive circuit minimizes the
relevance of the Miller effect, improving the bandwidth of the stage and reducing the harmonic distortion. This can be better
understood by analyzing the basic switching waveforms at turn-on and turn-off for a clamped inductive load, as shown in Figures
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