Page 100 - Rashid, Power Electronics Handbook

86 I. Batarseh

Capacitance +V DD
C
gs

L
O D
C
gd
i
D
C
GD
Voltage

R
FIGURE 6.16 Variation of C gd and C gs as a function of v DS . G
i
the magnetic components. In order to reduce the switching G
losses, power MOSFET are maintained in either the on-state +
v GG - C GS
(conduction state) or the off-state (forward-blocking) state.
It is important we understand internal device behavior,
which leads to an understanind of the parameters that
govern the device transition from the on-state and off-states. (a)
To investigate the on- and off-switching characteristics, we
+V
consider the simple power electronic circuit shown in Fig. DD
6.17a under inductive load. The ¯yback diode D is used to
pick up the load current when the switch is off. To simplify the
analysis we will assume the load inductance is a large enough
I
L that the current through it is constant as shown in Fig. O D
0
6.17b.
i
6.6.2 Turn-On Characteristics
D
Let us assume initially that the device is off and that the load C
G
current I , ¯ows through D as shown in Fig. 6.18a, v GG ¼ 0. D
0
The voltage v DS ¼ V DD and i ¼ i .At t ¼ t , the voltage v GG
G
D
0
is applied as shown in Fig. 6.19a. The voltage across C GS starts
charging through R . The gate-source voltage, v GS controls the
G
¯ow of the drain-to-source current i . Let us assume that for R G
D
t t < t , v GS < V , that is, the MOSFET remains in the i
0
1
Th
cut-off region with i ¼ 0, regardless of v . The time interval G
D
DS
ðt 1; t Þ represents the delay turn-on time needed to change v + C G
0
2
C GS from zero to V . The expression for the time interval GG - S
Th
Dt ¼ t ÿ t can be obtained as shown next.
10 1 0
The gate current is given by
(b)
v GG ÿ v GS
i ¼ FIGURE 6.17 (a) Simpli®ed equivalent circuit used to study turn-on
G
R G and turn-off characteristics of the MOSFET; and (b) simpli®ed equivalent
¼ i þ i ð6:13Þ circuit.
c GS c GD
dv GS dðv ÿ v Þ
D
G
¼ C ÿ C
GS GD then i is given by
dt dt G
where v G and v D are gate-to-ground and drain-to-ground i ¼ C dv GS þ C dv GS ¼ðC þ C Þ dv GS ð6:14Þ
voltages, respectively. As we have v ¼ v , v ¼þV , G GS dt GD dt GS GD dt
G GS D DD