Buck-Boost Converter Design Guide — Complete Step-by-Step Tutorial

Master non-inverting buck-boost DC-DC converter design with practical formulas, component selection tables, worked examples, and expert tips for efficient power supply development.

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Quick Answer

A buck-boost converter is a DC-DC switching regulator that can step up (boost) or step down (buck) an input voltage to produce a regulated output. The output voltage V_OUT is determined by the duty cycle D: V_OUT = −V_IN × D/(1−D) for the inverting topology, or V_OUT = V_IN × D/(1−D) for the non-inverting 4-switch topology. Buck-boost converters are essential in battery-powered systems where the battery voltage can be above or below the required output (e.g., 3.0–4.2V Li-Ion to 3.3V rail).

How a Buck-Boost Converter Works

A buck-boost converter combines the operating principles of both buck and boost converters into a single topology. During the switch-on phase, the inductor stores energy from the input. During the switch-off phase, the inductor releases energy to the output capacitor and load. Unlike pure buck or boost converters, the buck-boost can produce an output voltage that is either higher or lower than the input voltage, making it uniquely versatile.

Operating Modes

Mode	Condition	Behavior	Duty Cycle Range
Buck Mode	V_IN > V_OUT	Steps voltage down; inductor current continuous	D < 0.5
Boost Mode	V_IN < V_OUT	Steps voltage up; inductor current continuous	D > 0.5
Buck-Boost Mode	V_IN ≈ V_OUT	Smooth transition; all 4 switches active	D ≈ 0.5

Core Design Formulas

Duty Cycle

For inverting buck-boost:
D = |V_OUT| / (V_IN + |V_OUT|)

For non-inverting (4-switch):
D = V_OUT / (V_IN + V_OUT)

Inductor Selection

L = (V_IN × D) / (ΔI_L × f_SW)

Where:
ΔI_L = 0.2–0.4 × I_OUT(max) × (V_OUT / V_IN)

Critical Inductance (CCM/DCM Boundary)

L_crit = (V_IN × D × (1−D)) / (2 × I_OUT × f_SW)

Output Capacitor

C_OUT ≥ (I_OUT(max) × D) / (f_SW × ΔV_OUT)

Component Selection Guide

Component	Key Parameter	Design Rule	Trade-off
Inductor (L)	Inductance, I_SAT	L ≥ L_crit; I_SAT ≥ 1.3 × I_L(peak)	Higher L = lower ripple, larger size
MOSFET Switches	R_DS(on), V_DS(max), Q_G	V_DS ≥ 1.2 × (V_IN + V_OUT)	Lower R_DS(on) = higher efficiency, higher cost
Output Capacitor	Capacitance, ESR, Voltage Rating	ESR < ΔV_OUT / ΔI_L	Ceramic = low ESR; Electrolytic = high capacitance
Input Capacitor	Capacitance, RMS Current Rating	I_RMS ≥ I_OUT × √(D/(1−D))	MLCC + bulk electrolytic combo recommended
Schottky Diode (inverting only)	V_R, I_F(avg)	V_R ≥ V_IN + V_OUT; I_F ≥ I_OUT	Lower V_F = better efficiency

Worked Example 1: 5V to 12V Boost (Buck-Boost Mode)

Specifications: V_IN = 5V, V_OUT = 12V, I_OUT = 1A, f_SW = 500kHz, ΔV_OUT = 120mV (1%)

Step 1: Calculate Duty Cycle

D = 12 / (5 + 12) = 12 / 17 = 0.706 (70.6%)

Step 2: Calculate Inductor Ripple Current

ΔI_L = 0.3 × 1A × (12/5) = 0.72A

Step 3: Calculate Inductance

L = (5V × 0.706) / (0.72A × 500,000) = 3.53 / 360,000 = 9.8μH
Select standard value: 10μH

Step 4: Output Capacitor

C_OUT ≥ (1A × 0.706) / (500,000 × 0.12V) = 0.706 / 60,000 = 11.8μF
Select: 22μF ceramic (X7R, 25V) with 2 × 10μF for margin

Step 5: Peak Inductor Current

I_L(peak) = I_OUT × (1 + D) / (1−D) + ΔI_L/2
= 1 × 1.706/0.294 + 0.36 = 5.80 + 0.36 = 6.16A

Note: Peak inductor current (6.16A) is significantly higher than the 1A output current. Always verify inductor saturation current rating exceeds I_L(peak) with margin.

Worked Example 2: 12V to 5V Buck (Buck Mode)

Specifications: V_IN = 12V, V_OUT = 5V, I_OUT = 2A, f_SW = 400kHz

D = 5 / (12 + 5) = 0.294
L = (12 × 0.294) / (0.3 × 2 × (5/12) × 400,000) = 3.528/30,000 = 117.6μH
Select: 100μH or 150μH standard

Worked Example 3: Li-Ion Battery to 3.3V (Buck-Boost Transition)

Specifications: V_IN = 3.0–4.2V (Li-Ion), V_OUT = 3.3V, I_OUT = 500mA, f_SW = 1MHz

V_IN	Mode	Duty Cycle	Required L	I_L(peak)
4.2V	Buck	0.440	8.2μH	1.05A
3.7V	Buck-Boost	0.471	7.8μH	1.19A
3.0V	Boost	0.524	7.0μH	1.45A

Selected Inductor: 10μH, I_SAT ≥ 2.0A

Efficiency Optimization

Technique	Impact	Implementation
Synchronous Rectification	+3–5% efficiency	Replace diode with low-R_DS(on) MOSFET
Pulse-Skipping at Light Load	+10–15% at <10% load	Enable PFM mode in controller IC
Low-ESR Ceramic Capacitors	+1–2% efficiency	Use X7R/X5R MLCCs, avoid Y5V
Optimized Dead-Time	+1–3% efficiency	Minimize shoot-through; 10–30ns typical
Gate Drive Optimization	+2–4% efficiency	Use strong gate drivers; minimize Q_G

5 Common Mistakes in Buck-Boost Design

Mistake 1: Underestimating Peak Inductor Current
Designers often assume I_L(peak) ≈ I_OUT. In buck-boost, peak current can be 3–6× higher than output current.

Correct: Always calculate I_L(peak) = I_OUT/(1−D) + ΔI_L/2 and choose inductor with I_SAT ≥ 1.3 × I_L(peak).

Mistake 2: Neglecting Right-Half-Plane Zero (RHPZ)
Buck-boost converters have a RHPZ that limits control loop bandwidth. Ignoring it causes instability.

Correct: Set crossover frequency f_C < f_RHPZ/5 where f_RHPZ = (1−D)² × R_LOAD / (2π × L × D).

Mistake 3: Insufficient Input Capacitance
Buck-boost input current is discontinuous with high RMS ripple, causing EMI and voltage sag.

Correct: Calculate input RMS current: I_IN(RMS) = I_OUT × √(D/(1−D)) and choose capacitors rated for this ripple.

Mistake 4: Inadequate PCB Layout
Long traces in the power loop create parasitic inductance, causing voltage spikes and EMI.

Correct: Minimize the hot loop area (V_IN → switch → inductor → GND). Keep traces short and wide. Use a 4-layer board with dedicated power plane.

Mistake 5: Wrong Operating Mode (CCM vs DCM)
Forcing CCM at light load wastes power; accidentally entering DCM at heavy load causes voltage sag.

Correct: Design inductor for CCM at I_OUT(min). Use forced-PWM or auto-PFM/DCM transition depending on application noise requirements.

FAQ

1. What is the difference between inverting and non-inverting buck-boost converters?

The inverting buck-boost uses a single inductor and produces a negative output voltage relative to input ground. The non-inverting (4-switch or SEPIC-based) topology produces a positive output voltage. Non-inverting designs are preferred for most modern applications due to simpler system integration, though they require more switches. The 4-switch H-bridge buck-boost is the most popular non-inverting topology today.

2. How do I calculate the duty cycle for a buck-boost converter?

For a non-inverting buck-boost: D = V_OUT / (V_IN + V_OUT). For example, with V_IN = 5V and V_OUT = 12V, D = 12/(5+12) = 0.706 or 70.6%. Note that D = 0.5 when V_IN = V_OUT, D < 0.5 in buck mode, and D > 0.5 in boost mode.

3. Why is my buck-boost converter efficiency lower than expected?

Common causes: (a) inductor DC resistance (DCR) losses — use a larger core or lower DCR part; (b) MOSFET switching losses — reduce f_SW or choose lower Q_G FETs; (c) diode forward voltage in non-synchronous designs — switch to synchronous rectification; (d) operating in DCM at heavy load — increase inductance to stay in CCM. Target 85–92% efficiency for well-designed buck-boost converters.

4. Can I use a buck-boost converter for battery-powered devices?

Yes, buck-boost converters are ideal for battery-powered devices. A single Li-Ion cell (3.0–4.2V) can power a 3.3V rail through the entire discharge curve. This is the primary use case for buck-boost converters in portable electronics, IoT sensors, and wearables. The TPS630xx and LTC353x families are popular IC choices for this application.

5. What is the Right-Half-Plane Zero and why does it matter?

The RHPZ is a unique characteristic of boost and buck-boost converters operating in CCM. It introduces a phase lag while increasing gain, making the control loop inherently unstable at high frequencies. The RHPZ frequency is f_RHPZ = (1−D)² × R_LOAD / (2πLD). Compensate by setting the crossover frequency below f_RHPZ/5 to ensure stability.

6. Should I use CCM or DCM for my buck-boost design?

Use CCM for: outputs above 1–2A, noise-sensitive applications, and when predictable EMI performance is needed. Use DCM for: light loads (<100mA), ultra-low standby power, and when simpler compensation is desired. Many modern ICs support automatic CCM/DCM transition (pulse-skipping or burst mode) for optimal efficiency across load ranges.

7. How do I select the right buck-boost controller IC?

Key criteria: (1) Input voltage range must cover your entire operating range; (2) output current capability with margin; (3) integrated vs. external FETs — integrated is simpler, external allows higher power; (4) features: soft-start, UVLO, thermal shutdown, power-good flag; (5) switching frequency — higher = smaller passives, lower = better efficiency. Popular families: TI TPS630xx (integrated), LTC3789 (external FETs, high power), MP28164 (compact 4-switch).

8. What PCB layout guidelines should I follow for buck-boost converters?

Critical layout rules: (1) Minimize the high di/dt loop area — place input capacitors as close as possible to the power switches; (2) use a solid ground plane on layer 2; (3) keep the switch node (LX) trace as short and wide as possible to minimize radiated EMI; (4) separate power ground and signal ground, connecting at a single star point; (5) place the feedback network away from noisy switch nodes; (6) use Kelvin sensing for output voltage feedback when load regulation is critical.

Related Calculators

Reference: Fundamentals of Power Electronics (Erickson & Maksimovic), Texas Instruments SLVA535, Analog Devices AN-1120.