Buck-boost converter duty cycle calculator

Enter both values as positive magnitudes in volts, even though the physical output rail is negative relative to the input reference. The calculator applies the continuous-conduction ideal transfer relationship, reports the duty cycle as both a fraction and percentage, and also shows the requested voltage ratio. It is useful for first-pass topology selection and design checks before losses, component limits, and control margins are included.

Enter voltage magnitudes for an inverting supply

Use the input voltage available at the converter and the magnitude of the required output voltage. For example, a design converting a positive 12 V source into a negative 5 V rail should be entered as 12 for input_voltage and 5 for output_voltage. Do not enter -5, because the equation already represents an inverting topology and works with absolute magnitudes. Both numbers must be finite and greater than zero. The calculator assumes they use the same unit, and the published form labels them in volts. The result duty_cycle is a fraction between zero and one, while duty_cycle_percent is the same value multiplied by 100. A result of 0.294118 therefore means approximately 29.4118 percent of each switching period is on. This distinction matters when moving the result into a controller data sheet, simulator, spreadsheet, or firmware configuration, since those destinations may expect either a normalized fraction or a percentage. The returned voltage_ratio is |Vout| divided by |Vin| and provides a quick check on the requested conversion level.

Understand the ideal buck-boost equation

For an ideal inverting buck-boost converter operating in continuous conduction mode, the magnitude relationship is |Vout| / |Vin| = D / (1 - D). Rearranging that expression for duty cycle gives D = |Vout| / (|Vin| + |Vout|). The calculator evaluates this rearranged form directly, which avoids an iterative solver and produces a deterministic answer. If the requested output magnitude equals the input magnitude, the ideal duty cycle is 0.5. A smaller output magnitude produces a duty below 0.5, and a larger output magnitude produces a duty above 0.5. The formula describes the steady-state conversion ratio of the ideal power stage; it does not predict switching frequency, ripple, inductor current, peak switch stress, efficiency, or transient response. Continuous conduction also means the inductor current is assumed not to fall to zero during a switching period. If a real design enters discontinuous conduction at its intended load, a different transfer relationship applies and this ideal result should not be treated as the operating duty cycle.

Turn the result into a practical starting point

Treat the calculated duty cycle as a first-pass target, not as a complete converter design. Real switches, diodes, inductors, capacitors, traces, and current-sense elements introduce voltage drops and losses, so a regulated converter normally requires a somewhat different operating duty ratio. Input voltage also varies in most systems. Calculate the duty cycle at minimum, nominal, and maximum input voltage while holding the desired output magnitude constant; those three results reveal the range the controller must support. Then compare that range with the controller's minimum on-time, maximum duty-cycle limit, switching frequency, and compensation requirements. Verify semiconductor voltage and current ratings separately, because an inverting buck-boost switch can experience combined input and output stress. Inductor ripple and peak current must also be calculated for the chosen frequency and inductance. Finally, confirm the design with a component-aware simulation and bench measurements across load, temperature, and supply tolerance. The API price is $0.002 per request, while the browser calculation uses the same deterministic core for quick interactive checks.