Power electronics digital notes b. Tech III year

Buck boost regulator 
A simple buck converter can only produce voltages lower than the input voltage, and a boost converter, 
only voltages higher than the input. To provide voltages over the complete range a circuit known as a 
buck-boost converter is required. 
There are many applications where voltages higher and lower than the input are required. In these 
situations a buck-boost converter is required. 

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Buck-Boost Converter basics 
The buck-boost DC-DC converter offers a greater level of capability than the buck converter of boost 
converter individually, it as expected it extra components may be required to provide the level of 
functionality needed. 
There are several formats that can be used for buck-boost converters: 

+Vin, -Vout:
This configuration of a buck-boost converter circuit uses the same number of 
components as the simple buck or boost converters. However this buck-boost regulator or DC-DC 
converter produces a negative output for a positive input. While this may be required or can be 
accommodated for a limited number of applications, it is not normally the most convenient 
format. 
 
Figure: 3.27 circuit diagram of buck boost regulator
 

When the switch in closed, current builds up through the inductor. When the switch is opened the 
inductor 
supplies 
current 
through 
the 
diode 
to 
the 
load. 
Obviously the polarities (including the diode) within the buck-boost converter can be reversed to 
provide a positive output voltage from a negative input voltage. 

+Vin, +Vout:
The second buck-boost converter circuit allows both input and output to be the 
same polarity. However to achieve this, more components are required. The circuit for this buck 
boost converter is shown below.