Inverting Buck-Boost Converter Calculator
Estimate ideal duty cycle and converter currents for a DC-DC supply that produces an output voltage with reversed polarity.
Inverting converter operating point
Enter output-voltage magnitude as a positive value; the circuit output polarity is negative.
About inverting buck-boost converters
An inverting buck-boost converter is a switched-mode DC-DC topology that creates an output voltage whose polarity is opposite to its input. During the switch-on interval, the input source stores energy in an inductor while the diode isolates the output. During the switch-off interval, the inductor reverses its terminal voltage and transfers energy through the diode to the output capacitor and load. By changing the fraction of each switching period spent on, the circuit can produce an output magnitude either lower or higher than the input voltage.
For an ideal converter operating in continuous conduction, output-voltage magnitude divided by input voltage equals duty cycle divided by one minus duty cycle. Rearranging gives duty cycle = output magnitude divided by the sum of input and output magnitudes. A 12 V input and 5 V output magnitude therefore require an ideal duty cycle of about 29.41 percent. The actual controller may need a slightly different duty cycle because semiconductor drops, winding resistance, switching loss, and control overhead are not part of the ideal voltage conversion equation.
Output current equals output power divided by output-voltage magnitude. Average input current is estimated from power balance after efficiency: input current = output power divided by input voltage and efficiency as a decimal. The average inductor current shown by this calculator follows the continuous-conduction output relation, where output current equals inductor current multiplied by one minus duty cycle. Ripple current rides on this average and must be calculated from inductance, switching frequency, applied voltage, and operating interval before selecting an inductor.
Efficiency has a major effect on input current and thermal design even though it does not alter the ideal duty-cycle result displayed here. Lost power becomes heat in the switch, diode or synchronous rectifier, inductor, capacitor equivalent resistance, and control circuitry. Current ratings need margin above calculated averages because peak and RMS values are higher. Startup, current limiting, discontinuous conduction, minimum on-time, and transient response can also determine whether a proposed operating point is practical.
Use this calculator for first-pass topology evaluation rather than final component selection. Confirm the chosen controller supports an inverting configuration, the required duty-cycle range, and all absolute maximum pin voltages. Derive inductor value and ripple, output-capacitor ripple current, switch stress, diode reverse voltage, compensation, and thermal limits from the controller data sheet. Remember that the output node is below input ground, which affects probes, connectors, feedback networks, and downstream circuits. Never connect a ground-referenced oscilloscope clip in a way that shorts the negative output.
Inverting converter examples
| Design target | Calculated operating point | Interpretation |
|---|---|---|
| 12 V to -5 V, 10 W, 90% efficient | 29.4118% duty, 0.9259 A input | Output current is 2 A and average inductor current is 2.8333 A. |
| 5 V to -12 V, 6 W, 80% efficient | 70.5882% duty, 1.5 A input | The boost-like operating point uses a relatively high duty cycle. |
| 24 V to -24 V, 24 W, 92% efficient | 50% duty, 1.087 A input | Equal voltage magnitudes produce an ideal 50 percent duty cycle. |
How to estimate converter operation
- Enter the positive input supply voltage.
- Enter the desired negative output as a positive voltage magnitude.
- Provide required output power and expected efficiency.
- Select Calculate Converter to estimate duty cycle and average currents.
Inverting buck-boost FAQ
Why is output voltage entered as a positive magnitude?
The topology itself establishes negative output polarity. Using magnitude keeps duty-cycle and current calculations clear and positive.
Can this calculator size the inductor?
No, inductance also depends on switching frequency and allowable ripple. Use the reported average current as one input to a complete component design.
Why does efficiency not change ideal duty cycle?
The duty relation comes from ideal inductor volt-second balance. Losses affect required input power and real control margin rather than that first-order ratio.
What is continuous conduction mode?
In continuous conduction, inductor current never falls to zero within a switching period. The equations used here assume that operating mode.
Is average current enough to rate the switch?
No, switches and inductors must withstand peak and RMS current with transient margin. Detailed design must include ripple, saturation, temperature, and fault conditions.