Page 106 - Rashid, Power Electronics Handbook

92 I. Batarseh

Integrating both sides from t to t with n ðt Þ¼ I =g þ V , As the drain-source voltage starts increasing, the device starts
m
Th
0
2
GS 2
we obtain the following expression for v ðtÞ, leaving the on-state and enters the saturation (linear) region.
GS
During the transition time the device exhibits large voltage

I 0 ÿðtÿt 2 Þ=t and current simultaneously. At higher drain-source voltage
v ðtÞ¼ þ V Th e ð6:36Þ
GS
g m values that approach the avalanche breakdown it is observed
that power MOSFET suffers from a second breakdown
Hence the gate current and drain-to-source current are given phenomenon. The second breakdown occurs when the
by, MOSFET is in the blocking state (off) and a further increase
in v DS will cause a sudden drop in the blocking voltage. The

ÿ1 I 0 ÿðtÿt 2 Þ=t source of this phenomenon in MOSFET is caused by the
i ðtÞ¼ þ V Th e ð6:37Þ presence of a parasitic n-type bipolar transistor as shown in
G
R g
G m Fig. 6.24.
i ðtÞ¼ g V ðe ÿðtÿt 2 Þ=t ÿ 1Þþ I e ÿðtÿt 2 Þ=t ð6:38Þ The inherent presence of the body diode in the MOSFET
DS
0
Th
m
structure makes the device attractive for applications in which
The time interval between t t < t is obtained by evaluat-
2 3 bidirectional current ¯ow is needed in the power switches.
ing v ðt Þ¼ V , at which the drain current becomes
GS 3 Th Today's commercial MOSFET devices have excellent high
approximately zero and the MOSFET turn-off. As a result,
operating temperatures. The effect of temperature is more
we have
prominent on the on-state resistance as shown in Fig. 6.25.
As the on-state resistance increases, the conduction losses
v ðt Þ¼ V Th also increase. This large v limits the use of the MOSFET
GS 3
DSðOWÞ
I 0 ÿðt 3 ÿt 2 Þ=t
¼ þ V Th e
g
m
Solving for Dt 32 ¼ t ÿ t , we obtain i
2
3
D
I
cmax Max power
I 0
Dt 32 ¼ t ÿ t ¼ t ln 1 þ ð6:39Þ Current limit (P cmax )
2
3
V g
Th m
Second
For t > t , the gate voltage continues to decrease exponentially breakdown limit
3
to zero, at which the gate current becomes zero and C SOA
GD Voltage limit
charges to ÿV . Between t and t , I discharges to zero as
DD 3 4 D v
shown in the equivalent circuit Fig. 6.22d. CE,max
The total turn-off time for the MOSFET is given by
v DS
t off ¼ Dt þ Dt þ Dt þ Dt 43 FIGURE 6.23 Safe operation area (SOA) for MOSFET.
21
32
10
Dt þ Dt 32 ð6:40Þ
21
Drain
The time interval that most affects the power dissipation are
Dt 21 and Dt . It is clear that in order to reduce the MOSFET
32
t on and t off times, the gate-drain capacitance must be reduced.
Readers are encouraged to see the reference by Baliga [1] for
detailed discussion on the turn-on and turn-off characteristics
of the MOSFET and to explore various fabrication methods.
npn BJT
6.6.4 Safe Operation Area Gate
The safe operation area (SOA) of a device provides the current
and voltage limits the device must be able to handle to avoid
destructive failure. Typical SOA for a MOSFET device is
shown in Fig. 6.23. The maximum current limit while the
device is on is determined by the maximum power dissipation,
Source
R
P diss;ON ¼ I DSðONÞ DSðONÞ ð6:41Þ FIGURE 6.24 MOSFET equivalent circuit including the parasitic BJT.