Power Semiconductor Switches
Diodes
iD
iD
+
v
D
-
Vrated
R
vD
0
Reverse blocking
region
Figure 1: A diode circuit
Figure 2: i - v characteristics of a diode
iD
vD
0
Figure 3: Idealized characteristics of a
diode
i
At turn-on, the diode can be
considered an ideal switch because it turns
on rapidly compared to the transients in the
power circuit. However at turn-off, the
diode current reverses for a reverse recovery
time trr as shown in Figure 4 before falling
to zero. In many circuits, this reverse
current does not affect the converter
characteristic and so the diode can also be
considered as ideal during the turn-off
transient.
D
t rr
Q rr
t
Figure 4: Diode turn-off
1
-2-
Various types of diodes are available:
1.
Schottky diodes: Used where a low forward voltage drop (typically 0.3 v) is needed
in very low output voltage circuits. Reverse blocking voltage capability is 50 - 100
V.
2.
Fast-recovery diodes: Used in high frequency circuits where a small reverse
recovery time is needed. At power levels of several hundred volts and several
hundred amperes, such diodes have trr ratings of less than a few microseconds.
3.
Line-frequency diodes: The on-state voltage is designed to be low. trr is usually
larger which is acceptable for line-frequency applications. These diodes are
available with blocking voltage ratings of several kilo volts and current ratings of
several kilo amperes. They can be connected in series and parallel to satisfy any
voltage and current requirement.
Thyristors
A
i
A
+
v
G
i
G
K
AK
A thyristor can be triggered into the on-state by applying a
pulse of positive gate current for a short duration provided
that the device is in its forward blocking state. The forward
voltage drop in the on-state is only a few volts. Once the
device begins to conduct, it is latched on and the gate
current can be removed. The thyristor cannot be turned off
by the gate, and thyristor conducts as a diode. The thyristor
turns off only when the anode current tries to go negative
under the influence of the circuit in which the thyristor is
connected.
Figure 5: A thyristor
2
-3-
iA
on-state
Reverse breakdown
voltage
off-state
vAK
0
Forward breakdown
voltage
Reverse blocking
region
Figure 6: i - v characteristics of a thyristor
iA
on-state
off-to-on
vAK
0
Reverse
blocking
Forward
blocking
In reverse bias at voltages
below the reverse breakdown
voltage, only a negligibly small
leakage current flows in the
thyristor. Usually the thyristor
voltage ratings for forward and
reverse blocking voltages are the
same.
The thyristor current
ratings are specified in terms of
maximum rms and average
currents.
Figure 7: Idealized characteristics of a thyristor
Turn-off time interval (tq)
The time which must elapse after the forward current through the thyristor has
ceased before forward voltage may again be applied without turn-on is called the "turn-off
time". When conduction ceases, a high concentration of charge carriers still exists in the
neighbourhood of the center junction of the thyristor, and until this concentration has been
sufficiently reduced by recombination, it is not possible to apply a forward voltage without
conduction immediately taking place. During tq a reverse voltage must be maintained
across the thyristor and only after this time is the device capable of blocking a forward
voltage without giving into its on-state.
3
-4In addition to voltage and current ratings, turn-off time tq, and the forward voltage
drop, other characteristics that must be considered include the rate-of-rise of the current
(di/dt) at turn-on and the rate-of-rise of voltage (dv/dt) at turn-off.
Bipolar Junction Transistors
C
A sufficiently large base current results in the
device being fully on. This requires that the control
circuit provide a base current that is sufficiently large
so that
I
IB > C
hFE
iC
+
B
iB
+
v CE
v BE
-
-
where hFE is the dc current gain of the device.
E
Figure 8: A NPN transistor
iC
on
off
0
vCE
BJTs are current-controlled devices and base
current must be supplied continuously to keep them in
the on-state. The dc current gain hFE is usually only 5
- 10 in high power transistors and so these devices are
sometimes connected in a Darlington or triple
Darlington configuration to achieve a larger current
gain. One of the main disadvantages of Darlington
configurations are their slower switching speeds.
Voltage ratings are up to 1400 V and current
ratings of a few hundred amperes.
Figure 9: Idealized
characteristics of a
NPN transistor
Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs)
N-channel MOSFET
4
-5D
MOSFETs
are
voltage-controlled
devices. MOSFETs require the continuous
application of a gate-source voltage of
approximate magnitude in order to be in the onstate. No gate current flows except during the
transitions from on to off or vice versa when the
gate capacitance is being charged or discharged.
The switching times are very short, being in the
range of a few tens of nanoseconds to a few
hundred nanoseconds depending on the device
type.
iD
+
v DS
G
+
vGS
-
S
Figure 10: An N-channel MOSFET
MOSFETs are available in voltage ratings
in excess of 1000 V but with small current ratings, and with up to 100 A at small voltage
ratings.
iD
v GS = 7 V
6V
on
5V
off
4V
v
DS
0
Figure 11: i - v characteristics of an N-channel MOSFET
iD
on
off
0
v DS
Figure 12: Idealized characteristics of an
N-channel MOSFET
5
-6GATE-TURN-OFF THYRISTORS (GTOs)
The GTO can be turned on by a short-duration gate current pulse, and once in the
on-state, the GTO may stay on without any further gate current. The GTO can be turned
off by applying a negative gate-carthode voltage and therefore, causing a sufficiently large
negative gate current to flow. This negative current need only flow for a few
microseconds but it must have a very large magnitude, typically as large as one-third the
anode current being turned off.
A
iA
i
A
+
on
v
AK
G
i
-
vAK
off
G
0
K
Forward
blocking
Reverse
blocking
Figure 13: A Gate-turn-off
thyristor
Figure 14: Idealized characteristics of a GTO.
iA
on-state
Turn-off
Reverse breakdown
voltage
Turn
-on
off-state
vAK
0
Reverse blocking
region
Forward breakdown
voltage
Figure 15: i-v characteristics of a GTO.
6