Buck-Boost Converter

BUCK - BOOST CONVERTER

A Buck-Boost Regulator produces an output voltage which may be less than or greater than the input voltage.

The polarity of output voltage is opposite to that of input voltage. So, this regulator is also known as inverting or flyback regulator.

Construction

Transistor Q₁ acts as switch. Diode is connected in series with the load. The inductor L is connected in parallel after the switch and before the diode. A capacitor C is connected in parallel with load. Fig. shows the circuit diagram of Buck-Boost converter.

Operation

The circuit operation is divided into 2 modes.

Mode 1

Q₁ is turned on and D₁ is reverse biased. The input current increases and flows through inductor L and transistor Q₁.

Mode 2

Q₁ is switched OFF.

i. Current flowing through inductor L will flow through L, C, D₁ and load.

ii. The energy stored in inductor L will be transferred to the load and the inductor current will fall until Q₁ is switched ON again in the next cycle.

The equivalent circuits for mode 1 and mode 2 are shown in Fig.5.40

For continuous load current, the steady state voltages and currents waveform of the buck-boost regulator is shown in Fig.5.41.

Assume inductor current rises linearly from I₁ to I₂ in time t₁.

The inductor current falls linearly from I₂ to I₁ in time t₂.

ΔI - Peak to peak ripple current of inductor L

Equate ΔI in equation (3) and (5)

Substitute t₁ = KT and t₂ = (1 - K) T

The average output voltage is

Assume lossless circuit,

The average input current is given by

where I_a → average output current

Switching period is given by

Substitute 

When transistor Q₁ is ON, the capacitor supplies the load current for t = t₁.

The average discharging current of the capacitor is I_C = I_a.

Peak-to-peak ripple voltage of capacitor is

Substitute

Advantages

1. A buck-boost regulator doesnot use a transformer and provides output voltage polarity reversal.

2. High efficiency.

3. Easy to implement short-circuit protection

4. Less expensive.

Disadvantages

1. Input current is discontinuous and high peak current flows through transistor Q₁.

2. Slow response to fast load steps.

3. As sensed voltage is negative, inverting op-amp is required for feedback and closed loop control.

4. No isolation between input and output.