SPST (single-pole single-throw) switches
SPST switch, with
voltage and current
polarities defined
Buck converter
with SPDT switch:
L
1
iL(t)
+
1
+
i
2
+
–
Vg
C
R
V
–
v
–
with two SPST switches:
0
iA
A
L
iL(t)
+
+ vA –
All power semiconductor
devices function as SPST
switches.
Fundamentals of Power Electronics
+
–
Vg
2
–
vB
+
B
iB
C
R
V
–
Chapter 4: Switch realization
Realization of SPDT switch using two SPST switches
G
G
G
G
A nontrivial step: two SPST switches are not exactly equivalent to one
SPDT switch
It is possible for both SPST switches to be simultaneously ON or OFF
Behavior of converter is then significantly modified
—discontinuous conduction modes (chapter 5)
Conducting state of SPST switch may depend on applied voltage or
current —for example: diode
Fundamentals of Power Electronics
3
Chapter 4: Switch realization
Quadrants of SPST switch operation
1
+
i
Switch
on state
current
A single-quadrant
switch example:
v
ON-state: i > 0
–
OFF-state: v > 0
0
Switch
off state voltage
Fundamentals of Power Electronics
4
Chapter 4: Switch realization
Some basic switch applications
Singlequadrant
switch
switch
on-state
current
Currentbidirectional
two-quadrant
switch
switch
off-state voltage
switch
on-state
current
Voltagebidirectional
two-quadrant
switch
Fundamentals of Power Electronics
switch
on-state
current
switch
off-state
voltage
switch
on-state
current
Fourquadrant
switch
switch
off-state
voltage
5
switch
off-state
voltage
Chapter 4: Switch realization
4.1.1. Single-quadrant switches
1
+
i
Active switch: Switch state is controlled exclusively
by a third terminal (control terminal).
Passive switch: Switch state is controlled by the
applied current and/or voltage at terminals 1 and 2.
v
–
0
SCR: A special case — turn-on transition is active,
while turn-off transition is passive.
Single-quadrant switch: on-state i(t) and off-state v(t)
are unipolar.
Fundamentals of Power Electronics
6
Chapter 4: Switch realization
The diode
• A passive switch
i
• Single-quadrant switch:
1
+
• can conduct positive onstate current
on
i
v
off
v
–
0
Symbol
instantaneous i-v characteristic
Fundamentals of Power Electronics
7
• can block negative offstate voltage
• provided that the intended
on-state and off-state
operating points lie on the
diode i-v characteristic,
then switch can be
realized using a diode
Chapter 4: Switch realization
The Bipolar Junction Transistor (BJT) and the
Insulated Gate Bipolar Transistor (IGBT)
BJT
C
1
i +
• An active switch, controlled
by terminal C
i
v
• Single-quadrant switch:
on
–
off
0
IGBT
v
• can block positive off-state
voltage
1
i +
C
v
–
0
• can conduct positive onstate current
instantaneous i-v characteristic
Fundamentals of Power Electronics
8
• provided that the intended
on-state and off-state
operating points lie on the
transistor i-v characteristic,
then switch can be realized
using a BJT or IGBT
Chapter 4: Switch realization
The Metal-Oxide Semiconductor Field Effect
Transistor (MOSFET)
• An active switch, controlled by
terminal C
i
on
1
i +
C
• Normally operated as singlequadrant switch:
v
off
v
–
on
(reverse conduction)
0
• can conduct positive on-state
current (can also conduct
negative current in some
circumstances)
• can block positive off-state
voltage
Symbol
instantaneous i-v characteristic
Fundamentals of Power Electronics
9
• provided that the intended onstate and off-state operating
points lie on the MOSFET i-v
characteristic, then switch can
be realized using a MOSFET
Chapter 4: Switch realization
Realization of switch using
transistors and diodes
Buck converter example
iA
A
L
iL(t)
+
+ vA –
Vg
+
–
–
vB
B
C
R
V
+
Switch A: transistor
iB
–
Switch B: diode
iA
SPST switch
operating points
Switch A
on
iB
Switch B
iL
on
Switch A
Switch B
off
off
Vg
vA
Switch A
Fundamentals of Power Electronics
–Vg
iL
vB
Switch B
10
Chapter 4: Switch realization
Realization of buck converter
using single-quadrant switches
iA
+
vA
L
–
+
Vg
–
vB
+
+
–
iL(t)
vL(t)
–
iB
iA
Switch A
on
iB
Switch B
iL
on
Switch A
Switch B
off
off
Vg
Fundamentals of Power Electronics
vA
–Vg
11
iL
vB
Chapter 4: Switch realization
4.1.2. Current-bidirectional
two-quadrant switches
• Usually an active switch,
controlled by terminal C
i
1
i
on
(transistor conducts)
+
C
v
–
0
BJT / anti-parallel
diode realization
Fundamentals of Power Electronics
v
off
on
(diode conducts)
instantaneous i-v
characteristic
12
• Normally operated as twoquadrant switch:
• can conduct positive or
negative on-state current
• can block positive off-state
voltage
• provided that the intended onstate and off-state operating
points lie on the composite i-v
characteristic, then switch can
be realized as shown
Chapter 4: Switch realization
Two quadrant switches
switch
on-state
current
i
1
+
on
(transistor conducts)
i
v
off
v
switch
off-state
voltage
–
0
Fundamentals of Power Electronics
on
(diode conducts)
13
Chapter 4: Switch realization
MOSFET body diode
i
1
i
on
(transistor conducts)
+
off
v
C
v
–
on
(diode conducts)
0
Power MOSFET
characteristics
Fundamentals of Power Electronics
Power MOSFET,
and its integral
body diode
14
Use of external diodes
to prevent conduction
of body diode
Chapter 4: Switch realization
A simple inverter
iA
+
Vg
+
–
Q1
D1 vA
–
v0(t) = (2D – 1) Vg
L
iL
+
+
Vg
+
–
Q2
D2 v
B
–
C
R
v0
–
iB
Fundamentals of Power Electronics
15
Chapter 4: Switch realization
Inverter: sinusoidal modulation of D
v0(t) = (2D – 1) Vg
v0
Sinusoidal modulation to
produce ac output:
Vg
D(t) = 0.5 + Dm sin (ωt)
D
0
0.5
1
–Vg
The resulting inductor
current variation is also
sinusoidal:
iL(t) =
Vg
v0(t)
= (2D – 1)
R
R
Hence, current-bidirectional
two-quadrant switches are
required.
Fundamentals of Power Electronics
16
Chapter 4: Switch realization
The dc-3øac voltage source inverter (VSI)
ia
Vg
+
–
ib
ic
Switches must block dc input voltage, and conduct ac load current.
Fundamentals of Power Electronics
17
Chapter 4: Switch realization