Free Inverting Buck-Boost Converter Calculator
Size power inductors, calculate switch duty cycle, peak inductor current, and switch voltage stress (Vin + |Vout|) for negative voltage power supplies.
⚡ Converter Operating Parameters
Inductor Saturation Rating: ≥ 3.65 A (Peak Current)
• MOSFET Voltage Rating: Select ≥ 35V to 40V (Must include 1.5× safety margin).
• Diode Reverse Breakdown: Must withstand 24.5V reverse bias during switch conduction.
• Average Inductor Current: 3.06 A (Equal to I_in + I_out!).
The Voltage Stress Trap: Unlike buck or boost converters where switch voltage stress equals $V_{in}$ or $V_{out}$, the inverting buck-boost switch experiences the SUM of both rails: $V_{stress} = V_{in} + |V_{out}|$. Running a 24V input to -12V output exposes the switch to over 36V!
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Principles of the Inverting Buck-Boost Converter
The inverting topology generates a negative output voltage rail with a magnitude that can be higher, lower, or equal to the positive input voltage.
Governing Equations (Continuous Conduction Mode)
Average Inductor I: I_L(avg) = I_in + I_out = I_out / (1 - D)
Inductor Ripple: ΔI_L = Ripple_Ratio × I_L(avg)
Inductance Formula: L = (V_in × D) / (f_sw × ΔI_L)
Peak Switch Current: I_pk = I_L(avg) + (ΔI_L / 2)
Switch Voltage Stress: V_switch = V_in + |V_out| + V_f
Why the Inductor Carries Both Input & Output Current
In an inverting converter, the inductor connects between the switch node and ground. It stores energy from the input during $t_{on}$, and transfers that energy to the output capacitor during $t_{off}$. Therefore, its average DC current equals the sum of both input and output currents, requiring significantly heavier wire and larger magnetic cores than a standard buck regulator of equal power.
Frequently Asked Questions
Can I invert +5V to -5V with a standard buck converter IC?
Yes! Many standard buck ICs (like TI TPS54302 or LM2596) can be wired in the inverting buck-boost configuration by connecting the IC ground pin to the negative output rail (-Vout) instead of system ground. Note that the IC’s maximum voltage rating must exceed Vin + |Vout|.
Why does the inverting buck-boost have a Right-Half-Plane Zero (RHPZ)?
Like the boost converter, energy is only delivered to the output during the switch OFF time. A sudden increase in load demand causes the duty cycle to widen, temporarily robbing the output of recharge time. This creates a Right-Half-Plane Zero in the loop gain transfer function, restricting maximum crossover bandwidth.
Does an inverting converter require an isolated ground?
No! The input and output share a common 0V ground plane. The output terminal itself is simply biased below system ground at -12V or -5V.