Page 97 - Rashid, Power Electronics Handbook

6 The Power MOSFET 83

of device structure, geometry, and bias voltages. During turn- In power electronics, the aim is to use power-switching
on, capacitors C gd and C must be charged through the gate, devices that operate at higher and higher frequencies. Hence,
gs
hence, the design of the gate control circuit must take into the size and weight of output transformers, inductors, and
consideration the variation in these capacitances. The largest ®lter capacitors will decrease. As a result, MOSFETs are now
variation occurs in the gate-to-drain capacitance as the drain- used extensively in power supply design that requires high
to-gate voltage varies. The MOSFET parasitic capacitance is switching frequencies, including switching and resonant mode
given in terms of the device data sheet parameters C , C , power supplies and brushless dc motor drives. Because of its
iss
oss
and C rss as follows: large conduction losses, its power rating is limited to a few
kilowatts. Because of its many advantages over BJT devices,
C gd ¼ C rss modern MOSFET devices have received high market accep-
C ¼ C ÿ C rss tance.
iss
gs
C ¼ C oss ÿ C rss
ds
where C rss is the small-signal reverse transfer capacitance; C iss 6.6 MOSFET Regions of Operation
is the small-signal input capacitance with the drain and source
terminals shorted; and C oss is the small-signal output capaci- Most MOSFET devices used in power electronics applications
tance with the gate and source terminals shorted. are of the n-channel, enhancement type, like that shown in
The MOSFET capacitances C , C and C are nonlinear
gs gd ds Fig. 6.6a. For the MOSFET to carry drain current, a channel
and a function of the dc bias voltage. The variations in C
oss between the drain and the source must be created. This occurs
and C are signi®cant as the drain-to-source voltage and the
iss when the gate-to-source voltage exceeds the device threshold
gate-to-source voltage each cross zero. The objective of the
voltage V . For v GS > V , the device can be either in the
Th
Th
drive circuit is to charge and discharge the gate-to-source and
triode region, which is also called ‘‘constant resistance'' region,
gate-to-drain parasitic capacitances to turn on and off the or in the saturation region, depending on the value of v .For
DS
device, respectively. given v , with small v DS (v DS < v GS ÿ V ), the device oper-
Th
GS
C ates in the triode region (saturation region in the BJT), and for
gd larger v ðv > v ÿ V ), the device enters the saturation
DS DS GS Th
region (active region in the BJT). For v < V , the device
GS Th
turns off, with drain current almost equal to zero. Under both
regions of operation, the gate current is almost zero. This is
C
ds why the MOSFET is known as a voltage-driven device and,
therefore, requires simple gate control circuit.
The characteristic curves in Fig. 6.6b show that there are
three distinct regions of operation labeled as triode region,
saturation region, and cut-off region. When used as a switch-
C ing device, only triode and cut-off regions are used, whereas,
gs
when it is used as an ampli®er, the MOSFET must operate in
(a)
the saturation region, which corresponds to the active region
G in the BJT.
S S The device operates in the cut-off region (off-state) when
v GS < v , resulting in no induced channel. In order to operate
Th
SiO 2 the MOSFET in either the triode or saturation region, a
channel must ®rst be induced. This can be accomplished by
n +
n + applying gate-to-source voltage that exceeds v , that is,
Th
C gs
p
C C
ds g n - v GS > V Th
d
n +
Once the channel is induced, the MOSFET can operate in
either the triode region (when the channel is continuous with
no pinch-off, resulting in drain current proportional to the
D channel resistance) or the saturation region (the channel
(b)
pinches off, resulting in constant I ). The gate-to-drain bias
D
FIGURE 6.9 (a) Equivalent MOSFET representation including junc- voltage (v GD ) determines whether the induced channel enters
tion capacitances; and (b) representation of this physical location. pinch-off or not. This is subject to the following restriction.