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LAB3 - Applications and Operating Principle

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LAB3 - Applications and Operating Principle

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Applications

Buck-boost converters are typically used when an input voltage source varies and may be either above or below the required output voltage at different times.

Operating principle

The principle of the buck–boost converter is as follows:

  • While in the On state, the input voltage source is directly connected to the inductor (L).
    This results in accumulating energy in L.
    In this stage, the capacitor supplies energy to the output load.
  • While in the Off state, the inductor is connected to the output load and capacitor, so energy is transferred from L to C and R.

Compared to the buck and boost converters, the characteristics of the buck–boost converter are mainly:

  • The polarity of the output voltage is opposite to that of the input.
  • The output voltage can vary continuously from 0 to -∞ (for an ideal converter).
    The output voltage ranges for a buck and a boost converter are respectively 0 to  and to -∞.



Figure 16: Buck-Boost Converter

With:

  • : Input voltage
  • : Input current
  • : Output voltage
  • : Output current
  • : Inductor current
  • : Inductor voltage

Continuous Conduction Mode

In continuous conduction mode, the buck-boost converter assumes two states per switching cycle.

State 1

The On state is when switch S is closed.



Figure 17: State 1 of a Buck-Boost Converter

For the subinterval 1

When the switch pictured above is closed, the voltage across the inductor is:


From figure 17 above : 

State 2

The Off state is when the switch S is opened.



Figure 18: State 2 of a Buck-Boost Converter

Therefore, the energy stored in L increases during On-state and then decreases during the Off-state.
L is used to transfer energy from the input to the output of the converter.

 For the subinterval 2:

When the switch is opened, the diode is forward biased.
Current flows through the diode and the voltage across the inductor is:

From figure 18 above: