Calculate output voltage for ideal inverting buck-boost DC-DC converters: Vout = -Vin × (D / (1 - D)). This is an idealized continuous-conduction model. Real converters have losses, switching ripple, and parasitic elements. As duty cycle approaches 1, output voltage approaches infinity (ripple and losses dominate in practice).
Last updated: March 2026
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An inverting buck-boost converter is a DC-DC converter that produces an output voltage with opposite polarity to the input. It can both buck (step down) and boost (step up) the voltage magnitude, controlled by the duty cycle of the switching signal.
The output voltage is always negative relative to the input ground. As duty cycle increases, the output magnitude increases. This topology is commonly used in power supplies and battery charging circuits where voltage inversion is needed.
Determine the DC input voltage (Vin) that will be supplied to the converter in volts.
Duty cycle is the ratio of ON time to total switching period (between 0 and 0.99 or 0-99%). This controls the conversion ratio.
Use the standard inverting buck-boost formula: Vout = -Vin × (D / (1 - D))
The negative sign indicates output polarity reversal. Higher duty cycles produce larger magnitude outputs. Duty cycle > 0.5 boosts; < 0.5 bucks.
Ensure duty cycle stays below 0.99 to avoid instability. Check component ratings match the calculated output voltage and current requirements.
Given:
Step 1: Calculate Duty Cycle Ratio
Step 2: Apply Inverting Buck-Boost Formula
Result: The output voltage is -18 V (inverted and boosted from 12 V input). The negative polarity indicates the output is referenced opposite to the input ground.
Why is the output voltage negative?
The inverting buck-boost topology uses a configuration where the output is polarity-inverted relative to the input. The negative sign indicates this 180-degree phase reversal in voltage polarity.
What happens if duty cycle equals 1?
If D = 1.0, the denominator (1 - D) becomes zero, making the output voltage infinite. This is why duty cycle must stay below 0.99 to avoid instability and component damage.
How does duty cycle affect output magnitude?
Higher duty cycles produce larger output magnitudes (more boost effect). Lower duty cycles produce smaller magnitudes. The relationship is exponential—doubling D doesn't double the output, it increases it more dramatically.
When would I use an inverting buck-boost converter?
These converters are used when you need a negative voltage supply from a positive input, or when you need both buck and boost capability in a compact design. Common in battery-powered systems and dual-rail power supplies.
What's the difference between buck-boost and inverting buck-boost?
Regular buck-boost maintains the same polarity (positive stays positive). Inverting buck-boost reverses polarity (positive becomes negative). The formulas differ: buck-boost uses D/(1-D), inverting uses -D/(1-D).
How do I select components for a given duty cycle?
Choose inductors and capacitors based on ripple current/voltage specifications at your calculated duty cycle and switching frequency. Higher duty cycles require more ripple filtering. Use FET and diode ratings 20-30% above calculated voltage/current.
What switching frequency should I use?
Typical range: 100 kHz to 1 MHz. Higher frequencies reduce component size but increase losses. Lower frequencies reduce switching losses but require larger filter components. Consider EMI, losses, and PCB layout constraints.
Can I stack inverting buck-boost converters?
Yes, multiple stages can be cascaded to achieve greater voltage conversion ratios or to create isolated outputs. Each stage adds complexity and efficiency loss, so typically limit to 2-3 stages maximum.
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