Table of Contents
What Is an Inverting Buck-Boost Converter?
An inverting buck-boost converter is a DC-DC power converter topology that produces an output voltage with opposite polarity to the input. Unlike a standard buck (step-down) or boost (step-up) converter, the inverting buck-boost can produce an output voltage magnitude that is either higher or lower than the input voltage, making it extremely versatile for power supply design.
The circuit uses a single inductor, a switching transistor (typically a MOSFET), a diode, and output capacitor. During the switch ON time, energy is stored in the inductor from the input. During the switch OFF time, the inductor releases its energy to the output, which due to the circuit topology results in a negative output voltage relative to the input ground rail.
This topology is widely used in applications requiring a negative supply rail from a positive input, such as operational amplifier circuits, analog signal processing, and LCD bias voltage generation.
Key Formulas
Where D is the duty cycle, fsw is the switching frequency, and ΔIL is the peak-to-peak inductor ripple current.
Operating Modes
| Mode | Condition | Behavior |
|---|---|---|
| Buck Mode | |Vout| < Vin | D < 0.5, steps down voltage magnitude |
| Boost Mode | |Vout| > Vin | D > 0.5, steps up voltage magnitude |
| Unity | |Vout| = Vin | D = 0.5, equal magnitude inversion |
| CCM | IL > 0 always | Continuous conduction, higher efficiency |
| DCM | IL reaches 0 | Discontinuous conduction at light loads |
Component Selection Guide
- MOSFET: Must handle Vin + |Vout| voltage stress and peak inductor current. Choose RDS(on) low enough to minimize conduction losses.
- Diode: Schottky diodes are preferred for low forward voltage drop. Must withstand the same voltage stress as the MOSFET.
- Inductor: Select an inductor with saturation current above the peak current. Core material should be suitable for the switching frequency.
- Output Capacitor: Electrolytic or ceramic capacitors sized for output ripple voltage. ESR contributes to output voltage ripple.
Frequently Asked Questions
Why is the output voltage negative?
The inverting topology connects the inductor so that when the switch turns off, the inductor's flyback voltage drives current through the diode into the output capacitor in a direction that creates a negative voltage with respect to the input ground. This is inherent to the circuit topology, not a defect.
What is the maximum practical duty cycle?
While theoretically D can approach 1, practical converters limit the duty cycle to about 0.8-0.85 to allow sufficient off-time for the inductor to transfer energy. Very high duty cycles also lead to high peak currents and poor transient response.
How does switching frequency affect the design?
Higher switching frequencies allow smaller inductors and capacitors, reducing board space. However, switching losses increase proportionally, reducing efficiency. A typical range is 100 kHz to 2 MHz for modern converters. The optimal frequency balances component size against efficiency requirements.