Buck-Boost Converter

A buck-boost converter can step down or step up the input DC voltage based on the duty cycle. They are frequently used in battery-powered devices where the battery voltage starts above and drops below the required operating voltage.

Core Concept

The circuit produces an inverted output voltage relative to the input. When the switch is ON, the input source stores energy in the inductor while the diode is reverse-biased.

The capacitor alone feeds the load. When the switch turns OFF, the inductor reverses its polarity to maintain current flow.

This forward-biases the diode, transferring the inductor energy to the capacitor and load. The output voltage magnitude matches the input at a 50 percent duty cycle.

Key Formulas

Voltage ratio Vo / Vin = -D / (1 - D). Critical inductance Lc = ((1 - D)^2 * R) / (2 * fs). Inductor ripple current dI = (Vin * D) / (L * fs).

Given:\nVin = 15 V, Duty cycle D = 0.5.\n\nFormula:\nVo = -Vin * (D / (1 - D))\n\nSteps:\nD / (1 - D) = 0.5 / 0.5 = 1\nVo = -15 * 1 = -15 V\n\nFinal Answer:\nThe output voltage is -15 V, effectively acting as an inverter.

Exam Tip: GATE trap: The basic buck-boost converter always produces an inverted negative output voltage. The switch must withstand a voltage stress equal to Vin plus the absolute value of Vo.

Performance Parameters

• Voltage Gain: Can be lesser than, equal to, or greater than 1.
• Input Current: Discontinuous, requiring input filtering.
• Output Current: Discontinuous, leading to high output voltage ripple.
• Switch Voltage Stress: Highest among the basic converters, equal to Vin + |Vo|.
• Polarity: Output is strictly inverted with respect to the input.

Quick Revision

• Can step up or step down voltage.
• Output voltage is negative.
• Inductor is placed in parallel.
• Both input and output currents are discontinuous.
• Useful for wide input voltage ranges.
• Exam trap: Failing to account for the negative sign when using the voltage gain formula in design problems.