Free Inverting Buck-Boost Converter Calculator

Understanding the Inverting Buck‑Boost Converter

An inverting buck‑boost converter (also referred to as a buck‑boost converter with inverted topology) is a class of DC‑DC converter that can either step up or step down an input voltage while simultaneously reversing its polarity. This means a positive input yields a negative output, and a negative input yields a positive output—a useful property in many power‑supply designs where polarity inversion is required.

The free online buck boost duty cycle calculator described here is designed to help engineers and hobbyists quickly determine two essential values for circuit design: the duty cycle and the inductance. By entering the input voltage, output voltage, switching frequency, and desired ripple current, the calculator returns the necessary duty cycle and inductance. It can also work backward, allowing you to solve for any parameter if the others are known.

How Does the Inverting Buck‑Boost Topology Work?

The circuit stores energy in an inductor during the on‑time interval and transfers that energy to the output with opposite polarity during the off‑time. Its basic configuration includes a switching transistor (typically a MOSFET), a diode, an inductor, and input/output capacitors. The topology is similar to that of a flyback or forward converter, but the key difference is the inversion of voltage polarity.

Duty Cycle and Operating Modes

The duty cycle $D$ for an inverting buck‑boost converter is given by the ratio of the output voltage magnitude to the sum of the input and output magnitudes:

$$D = \frac{|V_{out}|}{|V_{out}| + |V_{in}|}$$

Here, absolute values are used because polarity is handled separately; the calculator ignores sign to focus on magnitude. The duty cycle determines whether the converter is in buck mode or boost mode:

• Buck mode when $D < 0.5$ → $|V_{out}| < |V_{in}|$
• Boost mode when $D > 0.5$ → $|V_{out}| > |V_{in}|$

The on‑time interval $T_{on}$ is then derived from the duty cycle and switching frequency $f_{sw}$:

$$T_{on} = \frac{D}{f_{sw}}$$

Inductance Calculation

Once the duty cycle is known, the required inductance $L$ can be computed using:

$$L = \frac{V_{in} \cdot D}{\Delta I_{ripple} \cdot f_{sw}}$$

where $\Delta I_{ripple}$ is the maximum allowable peak‑to‑peak ripple current. A larger inductance reduces ripple current but increases the size and cost of the component. This inverting buck boost inductance relationship is key to balancing performance and practical design constraints.

Worked Example

Consider the following parameters:

• Input voltage $V_{in} = 18\ \text{V}$
• Output voltage $V_{out} = 12\ \text{V}$
• Switching frequency $f_{sw} = 200\ \text{kHz}$
• Ripple current $I_{ripple} = 200\ \text{mA}$

Using the duty‑cycle formula:

$$D = \frac{12}{12 + 18} = 0.40 \quad (40\%)$$

Since $D < 0.5$, the converter operates in buck mode. The inductance is then:

$$L = \frac{18 \times 0.40}{0.2 \times 200 \times 10^{3}} = 180\ \mu\text{H}$$

The calculator accepts the same inputs in any order: you could start with a target duty cycle and ripple to find the required frequency, or use inductance and duty cycle to infer the input voltage.

Design Considerations and Trade‑offs

The inverting buck‑boost topology offers several advantages: high efficiency, compact size, and the ability to produce an inverted output. However, designers should be aware of potential drawbacks such as electromagnetic interference (EMI) and increased control complexity at extreme duty cycles. Careful selection of the switching frequency and inductor value is essential to balance ripple, efficiency, and component size.

Using the Tool in Your Design Flow

Whether you are prototyping a new buck boost circuit or refining an existing DC‑DC converter design, this dc dc converter calculator provides a fast, reliable way to perform iterative what‑if analyses. By adjusting the input voltage, output requirement, frequency, and ripple specification, you can evaluate how each parameter influences the duty cycle and inductance—and ultimately choose the optimal components for your application.