Page 110 - Rashid, Power Electronics Handbook

96 I. Batarseh

L D where K oX is the oxide relative dielectric constant, E denotes
0
4 the free-space dielectric constant as equal to
8:854 10 ÿ12 F=m; and T oX is the oxide thickness layer as
3 given by data in Table 6.1.
Finally, the diffusion and junction region capacitances
between the bulk-to-channel (drain and source) are modeled
by C BD and C BS across the two diodes. Because for almost all
S power MOSFETS, the bulk and source terminals are connected
1
together and at zero potential, diodes D BD and D BS do not
have forward bias, thereby resulting in very small conductance
5
values, that is, small diffusion capacitances. The small signal
model for MOSFET devices is given in Fig. 6.29.
0
EXAMPLE 6.4. Figure 6.30a shows an example of a soft-
switching power factor connection circuit that has two
MOSFET. Its PSPICE simulation waveforms are shown
FIGURE 6.27 Example of a power electronic circuit that uses a power
in Fig. 6.30b.
MOSFET.
Table 6.2 shows the PSPICE code for Fig. 6.30a.

In the saturation (linear) region we have 6.8 Comparison of Power Devices
2
C GS ¼ L C oX þ C GSO
3 W
As stated earlier, the power electronic range is very wide, from
C GB ¼ C GB0 L ð6:48Þ
hundreds of milliwatts to hundreds of megawatts and thus it is
C ¼ C
GD GD0 very dif®cult to ®nd a single switching device type to cover all
Where C oX is the per-unit-area oxide capacitance given by power electronic applications. Today's available power devices
have tremendous power and frequency rating range, as well as
K E
oX 0
C ¼ diversity. Their forward current ratings range from a few
oX
T
oX amperes to a few kiloamperes, their blocking voltage rating
d
i
Drain D
i
D C
R D BD
C
r BD
D
C gd d' g BD
C
gd i
BD
+ V - + - +
GD V
BD
g V '
m gs g
Gate Bulk(Substrate) +g V ' o i
i V i mb bs b
DS DS i G
i B - V ' +
G BS
- - V BS + C gs
C
gs
i
BS
+ V ' - S' g
+ - GS BS
V
GS
C R
r BS S C
S BS
C C
gb gb
i i S
S
FIGURE 6.28 Large-signal model for the n-channel MOSFET. FIGURE 6.29 Small signal equivalent circuit model for MOSFET.