Circuit Operation
A fully-controlled rectifier circuit contains only controlled-rectifiers,
whereas a semi-controlled rectifier circuit is made up of both controlled
and uncontrolled rectifiers. Due to presence of diodes, free-wheeling operation
takes place without allowing the bridge output voltage to become negative.
In a semi-controlled rectifier, control is effected only for positve output
voltage, and no control is possible when its output voltage tends to become
negative since it is clamped at zero volt. This page describes the operation
of a single-phase half-controlled rectifier.
A semi-controlled full-wave bridge rectifier can be configured in a few ways.
They are shown below.
The circuit in Configuration 1 contains two SCRs and two diodes. When source
Vin is positive, SCR S1 can be triggered at a firing
angle called a and then current flows out of the
source through SCR S1 first, then through the load and returns
via diode D3. If
then SCR S1 and diode D3 conduct during a
< wt < p. When p
< wt < 2p, Vin is negative and
SCR S2 is normally triggered when wt = p
+ a. During p < wt
< (p + a) , the output
of the bridge circuit would have been negative if we had used a fully-controlled
bridge rectifer and if the current flow was continuous. But here we have two
diodes D3 and D4 instead of two SCRs. When the output
of the bridge tends to becomes negative just after wt exceeds p,
diode D4 tends to get forward-biased and it starts conducting.
Then diode D3 is reverse-biased and it stops conducting. During
p < wt < (p +
a) , the devices in conduction are SCR S1
and diode D4 and the output of the bridge is clamped at zero, assuming
that the on-state drops across devices in conduction is zero. During ( p
+ a) < wt < 2p , the devices in conduction
are SCR S2 and diode D4. SCR S2 and diode
D3 would conduct during 0 < wt < a
.
The circuit in configuration 1 has SCRs as the devices in the top-half and
diodes as the devices in the bottom-half. Instead, it it is possible to use
SCRs as the devices in the bottom-half and diodes as the devices in the top-half.
It is also possible to build a semi-controlled full-wave bridge rectifier
as shown by the circuit in configuration 2.
The behaviour of the circuit is the same as described earlier. In this circuit,
SCR S1 and diode D3 conduct during a
< wt < p. During p
< wt < (p + a)
, the devices in conduction are diodes D3 and D4 and
the output of the bridge is clamped at zero. During (p
+ a) < wt < 2p
, the devices in conduction are SCR S2 and diode D4.
Diodes D3 and D4 would conduct during 0 < wt <
a.
Yet another configuration is available for semi-controlled bridge rectifier,
as shown by the circuit in configuration 3.
In this circuit, SCRs S1 and S3 conduct during a
< wt < p. During p
< wt < (p + a)
, the device in conduction is diode D and the output of the bridge is clamped
at zero. During (p + a)
< wt < 2p , the devices in conduction are
SCRs S2 and S4. Diode D would conduct during 0 <
wt < a .
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