Page 99 - Rashid, Power Electronics Handbook

6 The Power MOSFET 85

a positive constant MOSFET parameter. The term ð1 þ lv Þ 6.6.1 MOSFET Switching Characteristics
DS
is added to the i equation in order to account for the increase Because the MOSFET is a majority carrier transport device, it
D
in i due to the channel-length modulation. Here i is given is inherently capable of high frequency operation [5–8].
D
D
by
However, the MOSFET has two limitations:
2 1. high input gate capacitances; and
i ¼ kðv GS ÿ V Þ ð1 þ lv Þ saturation region ð6:10Þ
D
DS
Th
2. transient=delay due to carrier transport through the
drift region.
From the de®nition of the r given in Eq. 6.11, it is easy to
0
show that the MOSFET output resistance can be expressed as As stated earlier, the input capacitance consists of two compo-
follows: nents: the gate-to-source and gate-to-drain capacitances. The
input capacitances can be expressed in terms of the device
1 junction capacitances by applying the Miller theorem to Fig.
r ¼ ð6:11Þ
0 6.15a. Using the Miller theorem, the total input capacitance
lkðv GS ÿ V Þ
Th
C , seen between the gate-to source, is given by
in
If we assume that the MOSFET is operating under small signal C ¼ C þð1 þ g R ÞC ð6:12Þ
condition, that is, the variation in v on i vs v is in the in gs m L gd
GS D GS
neighborhood of the dc operating point Q at i and v as
D GS
shown in Fig. 6.13. As a result, the i current source can be The frequency responses of the MOSFET circuit are limited by
D
represented by the product of the slope g and v as shown in the charging and discharging times of C . The Miller effect is
in
m GS
Fig. 6.14. inherent in any feedback transistor circuit with resistive load
that exhibits a feedback capacitance from the input and
output. The objective is to reduce the feedback gate-to-drain
resistance. The output capacitance between the drain-to-
source C does not affect the turn-on and turn-off
i D ds
MOSFET switching characteristics. Figure 6.16 shows how
C gd and C vary under increased drain-source v Ds voltage.
Slope=gm
gs
Q In power electronics applications, power MOSFET are
I D operated at high frequencies in order to reduce the size of
C gd
G
D
V Th
V GS v GS
FIGURE 6.13 Linearized i D vs v GS curve with operating dc point (Q).
+
V
gs r
g m V gs
C O
gs -
G D
+
S
(a)
v g v r
gs m gs O D
G
- +
Vgs r
g m V gs
C O
(1+g m R L )C gd gs -
S
(b)
S
FIGURE 6.14 Small signal equivalent circuit including MOSFET FIGURE 6.15 (a) Small signal model including parasitic capacitances.
output resistance. (b) Equivalent circuit using Miller theorem.