Electronics Calculators
Inverting Buck-Boost Converter Designer
Complete inverting buck-boost DC-DC converter design tool. Calculate duty cycle, inductor, output capacitor, MOSFET and diode ratings, switching losses, and efficiency. Produces a negative output voltage from a positive input.
Inverting Buck-Boost Designer
Inverting DC-DC · CCM/DCM · Full loss analysis · Interactive schematic
LIVE
Input Parameters
V
V
A
kHz
%
%
MOSFET / Diode / Cap
mΩ
V
nC
%
mΩ
V
Key Results
◆ CCM
Duty Cycle D
—%
D ideal
—
D practical
—
Voltage gain |M|
—
Input current
—
L recommended
—
μH
L minimum
—
μH
C minimum
—
μF
IL average
—
A
IL peak
—
A
ΔIL ripple
—
A
Schematic
Inductor
L minimum (CCM)
—
μH
L recommended (1.5×)
—
μH
I_L peak
—
A
I_L rms
—
A
ΔI_L ripple
—
A
Energy stored
—
μJ
Output Capacitor
C minimum
—
μF
ΔV ESR
—
V
Total ripple
—
V
Cap I_rms
—
A
ΔVout (cap)
—
V
V rating ≥
—
V
MOSFET (Q1)
V_DS stress
—
V
I_D peak
—
A
I_D rms
—
A
R_ds(on)
—
mΩ
P_cond
—
W
V_DS rating ≥
—
V
Diode (D1)
V_R (reverse)
—
V
I_F avg
—
A
I_F peak
—
A
P_diode
—
W
Component Selection Summary
| Component | Parameter | Minimum | Recommended | Status |
|---|
Efficiency
Efficiency
— %
—
60%70%80%90%98%
P_out
—
W
P_in
—
W
P_loss total
—
W
η estimate
—
%
Output/Loss ratio
—
×
Loss Breakdown
MOSFET Conduction (I²·Rds)—
MOSFET Switching (Q·V·f)—
Diode Conduction (Vf·I)—
Inductor DCR (est.)—
Cap ESR (I²·ESR)—
Detailed Loss Table
| Source | Formula | Power (W) | % of Total | Tip |
|---|
Efficiency (Loss tab)
Efficiency
— %
—
60%70%80%90%98%
Inductor Current iL(t)
Inductor Current iL(t)
Gate Signal
Diode Current iD(t)
Period T
—
μs
ON time D·T
—
μs
OFF time (1-D)·T
—
μs
Duty Cycle
D_ideal = |Vout| / (Vin + |Vout|)
D_prac = |Vout| / (Vin×η + |Vout|×(1-η))
t_on = D / fsw
t_off = (1−D) / fsw
Unlike buck/boost, D depends on both Vin and |Vout|. At equal Vin and |Vout|, D = 0.5. Duty cycle rises with larger output-to-input ratios.
Voltage Conversion
|Vout| = Vin × D / (1−D) (ideal)
Vout is NEGATIVE: Vout = −Vin×D/(1−D)
D = |Vout| / (Vin + |Vout|)
Output polarity is always inverted relative to input. Unlike buck and boost, both step-down and step-up operation are possible by varying D.
Min Inductance (CCM)
L_min = Vin × D × (1−D)²
/ (2 × Iout × fsw)
or (Vin×D) / (2 × ΔiL × fsw)
Inductor stores all the transferred energy — it connects to GND only during Q1 ON. Use 1.5× L_min for reliable CCM margin.
Inductor Current
I_L_avg = Iout / (1−D)
ΔiL = Vin × D / (L × fsw)
I_L_peak = I_L_avg + ΔiL/2
I_L_vly = I_L_avg − ΔiL/2
Average inductor current equals Iout/(1−D) — higher than Iout, especially at large D. I_L_peak drives saturation rating of the inductor.
Output Capacitor
C_min = D × Iout / (fsw × ΔVout)
ΔV_esr = ESR × ΔiL
ΔV_total = ΔV_cap + ΔV_esr
The capacitor must supply load current during the entire ON phase (while the diode is reverse biased). Low-ESR ceramic or polymer caps minimise ESR ripple.
CCM / DCM Boundary
L_crit = Vin×D×(1−D)²/(2×Iout×fsw)
I_crit = Vin×D/(2×L×fsw) = I_L_avg/2
CCM if I_L_avg > I_crit (L > L_crit)
At the boundary, inductor valley touches zero. Below this the converter enters DCM — output rises for the same duty cycle.
MOSFET Stress
V_DS = Vin + |Vout| (= Vin/(1-D))
I_D_rms = sqrt(D) × I_L_rms
P_cond = I_D_rms² × Rds(on)
P_sw = Qg × Vgs × fsw
V_DS is the sum of input and output voltages — higher than in a buck or boost alone. Select MOSFET with V_DS rating ≥ 1.3 × (Vin + |Vout|).
Diode Stress
V_R = Vin + |Vout|
I_avg = Iout
I_pk = I_L_peak
P_D = Vf × Iout
Diode reverse voltage equals Vin + |Vout| — same as the MOSFET. Use a Schottky with low Vf. Average diode current equals load current Iout.
How an Inverting Buck-Boost Converter Works
An inverting buck-boost converter produces a regulated negative output voltage from a positive input voltage using a single MOSFET, a freewheeling diode, an inductor, and an output capacitor. During the ON phase, Q1 closes and connects the input voltage across the inductor — current ramps up, storing energy. The output capacitor alone supplies the load. During the OFF phase, Q1 opens and the inductor’s collapsing field forward-biases D1, transferring the stored energy to the output capacitor and load. The polarity is inverted because the inductor’s bottom terminal (connected to GND during ON) is now the output terminal (connected through D1 to the negative rail).
The fundamental voltage relationship is |Vout| = Vin × D / (1 − D). At D = 0.5, |Vout| = Vin (unity gain). Below 0.5, the converter steps down; above 0.5, it steps up — but always with an inverted polarity. This makes it unique among non-isolated converters.
Key design consideration: Both the MOSFET and diode must withstand V_DS = Vin + |Vout|, which is higher than either voltage alone. For a 24 V to −12 V converter, the switch stress is 36 V. Always derate by 1.3× minimum, giving a 47 V rated device for this example.
Frequently Asked Questions
What is the duty cycle formula for an inverting buck-boost converter?
Ideal: D = |Vout| / (Vin + |Vout|). For a 24 V input to −12 V output: D = 12 / (24 + 12) = 0.333 (33.3%). Unlike the buck (D = Vout/Vin) or boost (D = 1 − Vin/Vout), both Vin and Vout influence D equally.
How do I calculate the minimum inductor for an inverting buck-boost?
L_min = Vin × D × (1−D)² / (2 × Iout × fsw). Example: 24 V in, 12 V out (D=0.333), 1 A load, 200 kHz: L_min = 24 × 0.333 × 0.444 / (2 × 1 × 200000) = 17.8 μH. Use 1.5× margin → 27 μH. Inductor saturation current must exceed I_L_peak = Iout/(1−D) + ΔiL/2.
Why are MOSFET and diode voltage ratings higher than in a buck or boost?
In a buck, V_DS = Vin. In a boost, V_DS = Vout. In an inverting buck-boost, V_DS = Vin + |Vout| because when Q1 is OFF, the inductor voltage adds to Vin across the switch. A 24 V → −12 V converter needs 36 V rated devices (plus 30% derating margin = 47 V). This is the main limitation at high voltages.
Can the inverting buck-boost step up and step down?
Yes. At D = 0.5, |Vout| = Vin (unity gain). D below 0.5 gives |Vout| less than Vin (step-down). D above 0.5 gives |Vout| greater than Vin (step-up). This flexibility makes the inverting buck-boost useful in battery-powered systems where the input rail can be above or below the desired output magnitude — but always note the output is negative.
Calculations are theoretical estimates. Actual performance depends on component parasitics, PCB layout, thermal management and control loop design.
CalcEngines.com — Free Engineering Calculators
CalcEngines.com — Free Engineering Calculators