432 7 AC/DC Converters–Rectifiers
The condition that the boost converter works in continuous mode is that the average
value of the current through the inductance L is greater than a half of its changes, i.e.,
IG [ DiG ;
2
ð7:77Þ
VG DiG VG
IG ¼ Re ; 2 ¼ 2L DðtÞT ;
where VG is the average value of the voltage vg. From (7.76) and (7.76) the con-
dition for the CCM work of converter is obtained
VG [ VG À vVg T ) Re\ T À 2L vgÁ : ð7:78Þ
Re 2L 1 1À
V
The right side of expression (7.78) has the maximum value if vG = VSM, so DCM
will be reached if
Re [ T À 2L vSM Á : ð7:79Þ
1 À V
If Re has a value between the values given in Eqs. (7.78) and (7.79) the boost
converter will operate in DCM if vG is small, or if vG is approaching VSM the
converter will work in CCM. It should be noted that for the discontinuous mode the
effective resistance of the boost converter Reff ¼ 2L does not necessary coincide
TD2 ðtÞ
with the emulated resistance Re, and in this case is necessary to modify the
inequalities which define the boundary between the continuous and the discontin-
uous modes.
Also, some other topologies of DC/DC converters giving the appropriate ratio 1:
M(D(t)), such as buck-boost, Ćuk, SEPIC, or their variations with pulse transformer
in some designs, can be used for the realization of the rectifier with circuit for power
factor correction.
7.9 Active Rectifier
Inactive rectifier techniques, diodes and thyristors are replaced with actively con-
trolled switches such as transistors, usually MOSFET or IGBT, or with fully
controlled thyristor units such as GTO or IGCT.
Active rectifiers are often used for power factor correction, so the input current
follows the input voltage, thereby eliminating the input reactive currents, and
eventually increasing the efficiency of the system.
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