Page 104 - Rashid, Power Electronics Handbook

90 I. Batarseh

Notice that the MOSFET sustains high voltage and current
simultaneously during intervals Dt 21 and Dt . This results in
32
large power dissipation during turn-on, which contributes to
overall switching losses. The smaller the R , the smaller Dt 21
G
and Dt 32 become.
6.6.3 Turn-off Characteristics

To study the turn-off characteristic of the MOSFET, we will
consider Fig. 6.17b again by assuming the MOSFET is on and
in steady state at t > t with the equivalent circuit of Fig.
0
6.18(e). Therefore, at t ¼ t we have the following initial
0
conditions:
v ðt Þ¼ I r
DS 0
D DSðONÞ
v ðt Þ¼ V GG
GS 0
t ðt Þ¼ I 0
DS 0
ð6:29Þ
i ðt Þ¼ 0
0
G
v ðt Þ¼ V GG
0
c GS
v ðt Þ¼ V ÿ I r
C GD 0 GG 0 DSðONÞ
At t ¼ t , the gate voltage v GG ðtÞ is reduced to zero as shown
0
in Fig. 6.21a. The equivalent circuit at t > t is shown in Fig.
0
6.22a.
We assume that the drain-to-source remains constant while
C GS and C GD are discharging through R as governed by the FIGURE 6.21 Turn-on switching waveforms.
G
following relations,
ÿn
i ¼ G ¼ i þ i
G C GS C GD
R G
is ®xed, that is, I ¼ I . From the input transfer character-
dv GS dv GD D 0
¼ C þ C istics, the value of v , at which I ¼ I , is given by
GS GD GS D 0
dt dt
I 0
As v DS is assumed constant, then i becomes v GS ¼ þ V Th ð6:32Þ
G
g m
ÿv GS
i ¼
G
R The time interval Dt 10 ¼ t ÿ t can be obtained easily by
0
1
G ð6:30Þ
dv GS setting Eq. (6.31) to Eq. (6.32) at t ¼ t . The gate current
1
¼ðC GS þ C GD Þ during the t t < t is given by
dt 2 1
V
Hence, evaluating for v GS for t t , we obtain i ¼ÿ GG ÿ e ÿðtÿt 0 Þ=t ð6:33Þ
0
G
R G
ÿðtÿt 0 Þ=t
v ðtÞ¼ v ðt Þe ð6:31Þ
GS
GS 0
Because for t ÿ t the gate-to-source voltage is constant and
2
1
equals v ðt ¼ I =g þ V as shown in Fig. 6.21b, then the
Where GS 1 0 m Th
entire gate current is being drawn from C GD and
v ðt Þ¼ v GG
GS 0
dv GD dðv GS ÿ v Þ dv DS
DS
t ¼ðC GS þ C GD ÞR G i ¼ C GD dt ¼ C GD dt ¼ÿC GD dt
G

As v GS continues to decrease exponentially, drawing current ¼ v ðt Þ ¼ 1 I 0 þ V Th
GS 1
from C , it will reach a constant value at which drain current R G R G g m
GD