written 3.3 years ago by |
A non isolated (transformerless) topology of the buck-boost
converteris shown in Fig.l.a.
The converter consists of dc input voltage source VS,
controlled switch S, inductor L, diode D, filter capacitor C, and
load resistance R.
With the switch on, the inductor current increases while the
diode is maintained off. When the switch is turned off, the
diode provides a path for the inductor current.
Note the polarity of the diode that resuts in its current being
drawn from the output.
The buck-boost converterwaveforms are shown in Fig.l.b.
The condition of a zero volt-second product for the inductor in
steady state yields
V.DT=-V(-D)T-----()
Hence, the dc voltage transfer function of the buck-boost
converter is,
M =V,/V,=-D/(1-D)-----(2)
The output voltage Vo is negative with respect to the ground.
Its magnitude can be either greater or smaller (equal at
D=0.5) than the input voltage as the name of the converter implies.
When switch S is ON:
When switched on ON for a time tv the diode will be open
circuited since it does not allow currents in reverse direction
from input to output
Hence the Buck Boost converter can be redrawn as follows
During this state the inductor charges and the inductor
current increases. The current through the inductor is given as