Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 7

In practice, capacitors are often connected to the output of a rectifier to filter out the voltage
ripples and inductors are often adopted to smooth the load current.

1.2.1.2 Phase-controlled Rectifiers

Diode rectifiers can only provide fixed output voltages. In order to obtain a variable DC output
voltage, thyristors, which can be turned on by applying a firing pulse when forward biased,
can be adopted to form phase-controlled rectifiers. The output voltage of a phase-controlled
rectifier can be changed by varying the firing angle of the thyristors. Phase-controlled rectifiers,
often with an efficiency above 95%, are widely used in many industrial applications, especially
in variable-speed drives.
Figure 1.6 shows a phase-controlled single-phase full-bridge rectifier and its operation.
During the positive half-cycle of the input voltage, thyristors T 1 and T 2 are forward biased
and the input voltage is passed to the load through T 1 and T 2 after they are fired. Because of
the large inductive load, thyristors T 1 and T 2 continue conducting even when the input voltage
becomes negative. Similarly, during the negative half-cycle of the input voltage, thyristors T 3
and T 4 are forward biased and the voltage is rectified and passed to the load after they are
fired. Thyristors T 1 and T 2 are forced to turn off when they are backward biased and the load
current is transferred from T 1 and T 2 to T 3 and T 4 . It can be seen that from the firing angle α to

π v s and input current i s are positive so the energy flows from the source to

the load. However, during the period from

, the input voltage

π to π + α , the input voltage v s is negative and the

i o

T 1 T 3

+

i s

L

~

v s v o

R

-

T 4 T 2

(a) Topology

v o

v s

0 π 2π 3π 4π 0 π 2π 3π 4π

ωt/rad ωt/rad

(b) Input and output voltages

(c) Line current

Figure 1.6 Phase-controlled rectifier with a large inductive load when α = π/ 3