Page 279 - Mechatronics with Experiments

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ELECTRONIC COMPONENTS FOR MECHATRONIC SYSTEMS 265
diode is conducting, the current across the load and diode are
V Z
i = ; V ≥ V Z (5.98)
L
S
R L
V − V Z
S
i = ; V ≥ V Z (5.99)
S
s
R S
i = i − i ; V ≥ V Z (5.100)
S
S
Z
L
When the Zener diode is not conducting,
V < V Z (5.101)
L
i = 0.0 (5.102)
Z
i = i S (5.103)
L
R ⋅ i = V < V Z (5.104)
L
L
l
The power dissipated on a Zener diode is
[ ]
(V − V ) V Z
Z
S
P = i ⋅ V = − ⋅ V (5.105)
Z Z Z Z
R R
S L
which must not exceed the power rating of the diode. The main design parameters of a
diode are the maximum power dissipation capacity (P Z,max , i.e., 0.25 W to 50 W) and the
reverse breakdown voltage (V , i.e., 5 V to 100 V range).
Z
Thyristors Thyristors (also called silicon controller rectifiers (SCR)) are a three-
terminal device made of three junctions (junctions of PNPN) using two p-type and two
n-type semiconductor materials (Figure 5.11c). The three terminals are anode (A), cathode
(K), and gate (G). The input–output relationship in steady-state is the voltage (v ) and
AK
current (i ) between A and K where this relationship is a function of the gate current (i ).
AK G
SCR is a controllable diode. The gate is used to turn ON the SCR, but it cannot be used
to turn it OFF. SCR turns OFF when the i goes to zero, then the gate can be used to
AK
turn it ON again. In order to turn ON the SCR, it must be forward biased (V ≥ V (i ))
AK FB G
plus the gate current must be above the latching current specification (i > i G,min ), the
G
minimum amount of current required to turn ON the SCR. The forward voltage drop V FB
is in the range of 0.5V to 1.0 V. The current conducted by the SCR can range between
100 mA to 100 A. The power dissipated across the SCR is the voltage drop across it times
the conducting current,
P SCR = V drop ⋅ i AK < P max (5.106)
By controlling the timing of the gate current while the SCR is forward biased, a con-
trolled portion of a forward alternating voltage can be conducted. This is used in speed
control of DC motors from an AC source. If SCR is forward biased suddenly (i.e.,
dV ∕dt > 50 V∕μs), it may turn ON even though base current is not applied. To reduce
AK
such undesirable effects, a low pass passive RC-circuit (also called a snubber circuit)is
used in parallel with the SCR. In addition, when a SCR is turned ON, the rate of current
change, di(t)∕dt, may be very large. In order to reduce that, the load should have inductance
that is larger than a minimum required value. If the inductance is very low, the rate of
current change can be too large.
Triacs Triac is equivalent to two SCRs connected in reverse-parallel configuration
(Figure 5.11d). While an SCR can be turned ON only in the forward biased direction, a
triac can be turned ON in both directions. Typical voltage drop across a triac is about 2 V
while it is conducting. This voltage times the load current determines the energy dissipation