2. BJT Switching Performance.
When base current is applied, a transistor does not
turn on instantly because of the presence of internal
capacitances.
The various switching waveforms of an npn power
transistor with resistive load between collector and
emitter.
When input voltage vB up to base circuit is made - V₂
at to , junction is reverse biased EB. VBE = -V2 the
transistor is off, iB = Ic = 0 and vCE =VCC.
3. BJT Switching
Performance
At time t , input voltage v
₁ B
is made + V1 and iB rises to
IB1 as shown in Fig.
After t , base emitter
₁
voltage vBE begins to rise
gradually from - V and
₂
collector current Ic begins to
rise from zero and
collector-emitter voltage
VCE starts falling from its
initial value Vcc.
After some time delay td
called time delay time, the
collector current rises to 0.1
Ics, VCE falls from Vcc to 0.9
Vcc and vBE reaches VBES
4. BJT Switching
Performance
This delay time is
required to charge the
base-emitter capacitance
to VBES =0.7V.
Thus, delay time td is
defined as the time
during which the
collector current rises
from zero to
0.1 Ics and collector-
emitter voltage falls
from Vcc to 0.9 Vcc .
5. After delay time td,
collector current rises
from 0.1 Ics to 0.9 Ics and
VCE fall from 0.9 Vcc to
0.1 Vcc in time tr .
This time tr is known as
rise time which depends
upon transistor junction
capacitances.
Rise time tr , is defined as
the time during which
collector current rises
from 0.1 Ics to 0.9 Vcs and
collector-emitter voltage
falls from 0.9 Vcc to 0.1
Vcc. This show that total
turn -on time ton = td +tr.
6. In case transistor is to be
turned off, then input voltage
vB and input base current iB
are reversed. At time t2, input
voltage vB to base circuit is
reversed from V1 to - V2.
At the same time, base
current changes from IB1 to -
IB2 as shown in Fig. Negative
base current IB2 removes
excess carriers from the base.
The time ts required to
remove these excess carriers
is called storage time and
only after ts , base current IB2
begins to decrease towards
zero.
7. Transistor comes out of
saturation only after ts.
Storage time ts, is usually
defined as the time during
which collector current falls
from Ics to 0.9 Ics and
collector emitter voltage vCE
rises from VCES to 0.1 VCES.
Negative input voltage
enhances the process of
removal of excess carriers
from base and hence reduces
the storage time and therefore,
the turn-off time.
After ts collector current
begins to fall and
collector-emitter voltage
starts building
up.
8. Time tf called fall time, is
defined as the time during
which collector current drops
from 0.9 Ics to 0.1 Ics and
collector-emitter voltage rises
from 0.1 Vcc to 0.9 VCC.
Sum of storage time and fall
time gives the transistor
turn-off time toff i.e . toff = ts
+tf.
The various waveforms
during transistor
switching are shown in
Fig. In this figure, tn =
conduction period of
transistor, to = off period,
T = 1/f is the periodic
time and f is the
switching frequency.
9. POWER MOSFETS
• A metal-oxide-semiconductor field-effect transistor (MOSFET)
is a recent device developed by combining the areas of field-
effect concept and MOS technology.
• A power MOSFET has three terminals called drain (D), source
(S) and gate (G) in place of the corresponding three terminals
collector, emitter and base for BJT. The circuit symbol of
power MOSFET is as shown in Fig.
10. Here arrow indicates the direction of electron flow. A BJT
is a current controlled device whereas a power MOSFET is
a voltage-controlled device.
As its operation depends upon the flow of majority carriers
only, MOSFET is a unipolar device. The control signal, or base
current in BJT is much larger than the control signal (or gate
current) required in a MOSFET.
11. • Power MOSFETs are of two types; n-channel
enhancement MOSFET and p-channel enhancement
MOSFET. Out of these two types, n-channel
enhancement MOSFET is more common because of
higher mobility of electrons. As such, only this type of
MOSFET is studied in what follows.
• An insulating layer of silicon dioxide (SiO2) is grown on
the surface. Now this insulating layer is etched in order
to embed metallic source and drain terminals. A layer of
metal is also deposited on SiO2, layer so as to form the
gate of MOSFET in between source and drain terminals.
Structure of Power MOSFET
13. • When gate circuit is open junction n+ region below drain
and p-substrate is reverse baised by input voltage
VDD.Therefore no current flow between drain to source.
• When gate is made positive with respect to source an
electric field is established . Eventually induced negative
charges in the p-substrate below Sio2 layer are formed
thus causing the p layer below gate to become induced
n-layer.
• These negatives called electrons form n –channel
between two n+ region and current can flow from drain
to source.
14. • If VGS is made more positive induced N-
channel become more deep and there fore
more current can flow from D to S.
• This shows that drain current in enhanced by
the gradual increase of Gate voltage that’s
why its called enhancement type of MOSFET.
15. • Its mian disadvantage is large ON state
resistance thus large conduction losses
17. V-I Characteristics:Transfer characteristics (Input
Characteristic):
• This characteristic shows the variation of drain current ID as a
function of gate-source voltage VGS..Threshold voltage VGST is
an important parameter of MOSFET.
18. • VGST is the minimum positive voltage between
gate and source to induce n-channel. Thus, for
threshold voltage below VGST device is in the
off-state. Magnitude of VGST is of the order of
2 to 3 V. Thus the voltage below VGST, device
is in off state.
19. Output characteristics.
• Output characteristics, indicate the variation of drain current
ID, as a function of drain-source voltage VDS, with gate-source
voltage VGS as a parameter. For low values of VDSthe graph
between ID-VDS is almost linear; this indicates a constant value
of on-resistance RDS =VDS /ID.
20. For given VGS if VDS is increased, output characteristic is
relatively flat, indicating that drain current is nearly constant.
A load line intersects the output characteristics at A and B.
Here A indicates fully-on condition and B fully-off state.
PMOSFET operate as a switch either at A or at B just like
BJT.
21. Switching characteristics
(switching losses)
The switching characteristics
of a power MOSFET are
influenced to a large extent
by the internal capacitance of
the device and the internal
impedance of the gate drive
circuit.
At turn-on, there is an initial
delay tdnduring which input
capacitance charges to gate
threshold voltage VGST. Here
tdnis called turn-on delay
time.
22. Switching characteristics
(switching losses)
There is further delay tr called rise
time, during which gate voltage
rises to VGSP, a voltage sufficient
to drive the MOSFET into on
state. During tr drain current rises
from zero to full-on current ID .
Thus, the total turn-on-time is
ton = tdn + tr The turn-on time can
be reduced by using low-
impedance gate-drive source.
23. As MOSFET is a majority
carrier device, turn-off process is
initiated soon after removal of
gate voltage at time t . The turn-
₁
off delay time, tdf is the time
during which input capacitance
discharges from overdrive gate
voltage V to V
₁ GSP.The fall time
tf is the during which input
capacitance discharges from VGSP
to threshold voltage. During tf
drain current falls from ID to
zero. So when VGS ≤ VGST,
PMOSFET turn-off is complete.
Switching waveforms for a
power MOSFET are shown in
24. Application of PMOSFET.
• PMOSFETs find applications in high-frequency
switching applications, varying from a few watts to
few kWs. The device is very popular in switched-
mode power supplies and inverters. These are, at
present available with 500 V, 140 A ratings.
• Comparision of PMOSFETs with BJT.
• The three terminals in a PMOSFET are designated as
gate, source and drain. In a BJT, the corresponding
three terminals are base, emitter and collector. A
PMOSFET has several features different from those
of BJT. These are outlined below:
30. INSULATED GATE BIPOLAR TRANSISTOR (IGBT)
• IGBT has been developed by combining into it the best
qualities of both BJT and PMOSFET.
• Thus an IGBT possesses high input impedance like a
PMOSFET and has low on-state power loss as in a BJT.
• Further, IGBT is free from second breakdown problem
present in BJT.
• All these merits have made IGBT very popular amongst
power-electronics engineers.
• IGBT is also known as metal oxide insulated gate
transistor (MOSIGT), conductively-modulated field effect
transistor (COMFET) or gain-modulated FET (GEMFET).
• It was also initially called insulated gate transistor (IGT).
32. The circuit shows the various parameters pertaining to IGBT
characteristics.
Static I-V or output characteristics of an IGBT (n-channel type)
show the plot of collector current IC, versus collector-emitter
voltage VCE for various values of gate-emitter voltages VGE1 ,
VGE2 etc.
These characteristics are shown in Fig.
IGBT Characteristics: Static I-V Characteristics:
Circuit Diagram of IGBT Static I-V Characteristics
35. When VGE
is less than the threshold voltage VGET
, IGBT is in the off-
state.
36. Switching
Characteristics:
Switching
characteristics of an IGBT
during turn-on and turn-
off are sketched in Fig.
The turn-on time is
defined as the time
between the instants of
forward blocking to
forward on-state.
Turn-on time is
composed of delay time
tdn and rise time tri.e. ton =
tdn+tr
37. The delay time is
defined as the time for
the collector-emitter
voltage to fall from VCE
to 0.90 VCE.
Here VCE is the initial
collector-emitter voltage.
Time tdn may also be
defined as the time for
the collector current to
rise from its initial
leakage current ICE to 0.1
IC. Here IC is the final
value of collector
current.
38. The rise time tr is the
time during which
collector-emitter voltage
falls from 0.9 VCE to0.1
VCE. It is also defined as
the time for the collector
current to rise from 0.1 Ic
to its finalvalue Ic. After
time ton the collector
current is IC and the
collector-emitter voltage
falls tosmall value called
conduction drop = VCES
where subscript S
denotes saturated value.
39. The turn-off time is
somewhat complex. It
consists of three intervals:
(i) delay time, tdf(ii) initial
fall time, tf1and (iii) final
fall time, tf2; i.e. toff=tdf+tf1
+tf2 . The delay time is
thetime during which gate
voltage falls from VGE to
threshold voltage VGET.
AS VGEfalls to VGETduring
tdf, the collector current
falls from Ic to 0.9 Ic. At
the end of tdf, collector-
emittervoltage begins to
rise.
40. The first fall time tf1is
defined as the time during
which collectorcurrent falls
from 90 to 20% of its initial
value Ic, or the time during
which collector-
emittervoltage rises from
VCES to 0.1 VCE.
The final fall time tf2 is the
time during which collector
current falls from 20 to
10% of Ic, or the time
during which collector-
emitter voltage rises from
0.1 VCE to final value VCE
see Fig.
42. Application of IGBTs
IGBTs are widely used in medium power applications
such as dc and ac motor drives, UPS systems, power
supplies and drives for solenoids, relays and
contactors. Though IGBTs are somewhat more
expensive than BJTs, yet they are becoming popular
because of lower gate-drive requirements, lower
switching losses and smaller snubber circuit
requirements IGBT
