Buck-Boost Converter Dr. M. Gulam nabi alsath associate Professor/ece

• Unit 5 : Power Amplifiers & DC-DC Converters

• Buck-Boost Converter
• Dr. M. GULAM NABI ALSATH
• Associate Professor/ECE

• Buck-Boost Converter

• Cascaded combination of Buck and Boost converter
• Output voltage is less than or greater than the input voltage
• By construction, the output voltage polarity is opposite to the input voltage polarity
• Therefore this converter is also called as Inverting Converter
• Has two modes of operation based on the switching actions.
• Based on the current continuity in inductor current, it can also be operated in continuous or discontinuous mode.

• Circuit Diagram

• Mode 1: Switch Closed

• Note: Vs is Vin

• Mode 1

• Switch State: ON (0 ≤ t ≤ ton)
• Diode is reverse biased
• Voltage across the inductor is nearly the input voltage
• Inductor current raises linearly from I1 to I2
• The corresponding inductor side voltage and energy stored are given by

• Mode 2: Switch Open

• Mode 2

• Switch State: OFF (ton ≤ t ≤ T)
• Diode is forward biased
• Voltage across the inductor reverses and starts to discharge the energy
• Current in the inductor decreases linearly from I2 to I1.
• This inductor current charges the capacitor which will deliver energy to the load in the absence of Vin
• The average output voltage is given by

• Equation for Output Voltage

• Under steady state conditions, the ripple current during ON state is equal to the ripple current during the OFF state.

• Remember!

• The output voltage for BUCK is
• The output voltage for BOOST is
• The output voltage for BUCK-BOOST is

• Inductor Current

• For a lossless circuit,

• Inductor current is similar to boost’s current

• Capacitor Voltage

• Capacitor Voltage is similar to Boost Circuit

• Waveforms