Buck-Boost Converter Design Guide
Inverting & Non-Inverting Topologies, 4-Switch Design, SEPIC Comparison & Practical Design Strategies
DC-DC Converter
Design Guide
1. Introduction to Buck-Boost Converters
The buck-boost converter is a versatile DC-DC topology capable of producing an output voltage that can be either higher or lower than the input voltage — and, in the inverting variant, of opposite polarity. This flexibility makes buck-boost converters indispensable in battery-powered systems where the battery voltage spans both above and below the required output rail (e.g., a single Li-Ion cell at 2.7–4.2 V powering a 3.3 V rail), automotive systems with wide input transients, and any application where the input voltage range straddles the desired output.
There are three principal buck-boost variants: the classic inverting buck-boost (simplest, but produces negative output), the non-inverting 4-switch buck-boost (most popular in modern designs), and the SEPIC converter (a close relative, discussed in comparison). Each topology has distinct advantages and trade-offs in complexity, efficiency, component stress, and control strategy.
1.1 Comparison of Buck-Boost Topologies
| Topology | Vout Polarity | Switch Count | Efficiency | Control Complexity |
|---|---|---|---|---|
| Inverting Buck-Boost | Negative (inverted) | 1 switch + 1 diode | 75–90% | Simple (single-switch) |
| 4-Switch Non-Inverting | Positive (non-inverting) | 4 MOSFETs | 90–97% | High (mode transitions) |
| SEPIC | Positive | 1 switch + 1 diode | 80–92% | Moderate (RHPZ) |
| &Cuk | Negative | 1 switch + 1 diode | 78–90% | Moderate (RHPZ) |
1.2 Typical Applications
| Application | Input Range | Output | Topology Choice |
|---|---|---|---|
| Li-Ion to 3.3V Buck-Boost | 2.7–4.2 V | 3.3 V / 2 A | 4-Switch non-inverting |
| USB PD (5–20V) to 12V | 5–20 V | 12 V / 3 A | 4-Switch non-inverting |
| Negative Rail Generator | 5 V | -5 V / 0.5 A | Inverting buck-boost |
| Automotive 12V to 12V | 4.5–40 V (cold crank to load dump) | 12 V / 5 A | 4-Switch (wide Vin) |
2. Inverting Buck-Boost Converter
2.1 Topology and Operation
The classic inverting buck-boost uses a single switch (MOSFET), a single diode, a single inductor, and an output capacitor. The topology is deceptively similar to the buck and boost — the inductor is connected between the switching node and ground, with the diode reversed relative to the buck orientation.
State 1 — Switch ON (0 < t < DTs): The MOSFET conducts, connecting Vin across the inductor. The inductor current ramps up:
ΔIL(on) = Vin × D × Ts / L
The output capacitor supplies the load. The diode is reverse-biased.
State 2 — Switch OFF (DTs < t < Ts): The MOSFET turns OFF. The inductor current forward-biases the diode, transferring energy to the output. Because the diode is oriented toward ground from the negative output node, the output voltage is negative with respect to the input:
ΔIL(off) = |Vout| × (1 – D) × Ts / L
2.2 Steady-State Transfer Function
Key observation: When D < 0.5, |Vout| < Vin (buck region). When D > 0.5, |Vout| > Vin (boost region). At D = 0.5, |Vout| = Vin. The negative sign indicates polarity inversion.
2.3 Component Stresses
The inverting buck-boost imposes higher voltage stresses than either buck or boost topologies:
- MOSFET VDS(max): Vin + |Vout| (the switch must block the sum of input and output voltages)
- Diode PIV: Vin + |Vout|
- Average Inductor Current: IL(avg) = Iin + Iout = Iout × (1 + D/(1-D))
3. Non-Inverting 4-Switch Buck-Boost Converter
3.1 Topology Overview
The 4-switch (H-bridge) buck-boost converter is the most popular buck-boost implementation in modern power management ICs. It consists of four MOSFETs arranged in an H-bridge configuration with a single inductor. By controlling the switching pattern, the converter can operate in three distinct modes:
| Operating Mode | Vin vs. Vout | Active Switches | Behavior |
|---|---|---|---|
| Buck Mode | Vin > Vout | Q1, Q2 switch; Q3 = OFF, Q4 = ON | Equivalent to synchronous buck |
| Boost Mode | Vin < Vout | Q3, Q4 switch; Q1 = ON, Q2 = OFF | Equivalent to synchronous boost |
| Buck-Boost Mode | Vin ≈ Vout | All 4 switches active | Smooth transition; highest loss |
3.2 Mode Transition Smoothing
The transition region where Vin ≈ Vout is the most challenging. A well-designed controller implements a “buck-boost” transition mode where all four switches are actively switching within each cycle. This creates smooth transitions but incurs higher switching losses. Advanced controllers minimize time spent in this mode by using hysteresis in the mode selection threshold.
• Buck Mode: Vin > Vout + 0.5 V
• Buck-Boost Mode: Vout – 0.5 V ≤ Vin ≤ Vout + 0.5 V
• Boost Mode: Vin < Vout – 0.5 V
3.3 Efficiency Characteristics
The 4-switch buck-boost achieves its highest efficiency in buck-only or boost-only operation (90–97%). Efficiency drops in the buck-boost transition region due to all four switches switching, typically to 85–92%. Key efficiency drivers:
- Using low RDS(on) MOSFETs for both high-side and low-side positions
- Optimizing dead-time to minimize body diode conduction
- Selecting switching frequency to balance core losses and switching losses
- Diode emulation mode at light loads to prevent inductor current reversal
4. Buck-Boost vs. SEPIC: A Detailed Comparison
4.1 SEPIC Topology Overview
The Single-Ended Primary-Inductor Converter (SEPIC) is a close relative of the buck-boost family. It uses two inductors (or one coupled inductor), a series capacitor (the “DC-blocking” or “coupling” capacitor), a single switch, and a single diode. The key advantage: non-inverting output with single-switch simplicity.
The SEPIC transfer function is identical to the inverting buck-boost (magnitude):
4.2 Detailed Comparison
| Criterion | 4-Switch Buck-Boost | SEPIC | Inverting Buck-Boost |
|---|---|---|---|
| MOSFET Count | 4 | 1 | 1 |
| Inductor Count | 1 | 2 (or 1 coupled) | 1 |
| MOSFET VDS Stress | ≈ Vin or Vout | Vin + Vout | Vin + |Vout| |
| Peak Efficiency | 94–97% | 85–92% | 80–90% |
| RHPZ | Yes (in boost mode) | Yes (always present) | Yes (always present) |
| Output Ripple | Low (continuous) | Moderate (pulsating) | High (pulsating) |
| Control Complexity | High (mode transitions) | Moderate (RHPZ) | Low (RHPZ) |
| BOM Cost | Higher (4 FETs + driver) | Moderate (2 Ls + coupling C) | Lowest |
4.3 When to Choose Which
- 4-Switch Buck-Boost: Highest efficiency, best transient response, smallest solution size. Ideal for battery-powered devices needing maximum runtime, and applications where PCB area is at a premium. The go-to choice for modern portable electronics.
- SEPIC: Good balance of simplicity and performance with non-inverting output. Excellent for automotive and industrial applications where the input can be higher or lower than output, and where single-switch simplicity reduces BOM risk. The coupling capacitor also provides natural short-circuit protection.
- Inverting Buck-Boost: Lowest cost, simplest control. Best for generating negative rails from positive inputs, or in cost-sensitive applications where polarity inversion is acceptable (e.g., generating dual supplies for op-amps).
5. Design Example: 4-Switch Buck-Boost, 2.7–4.2 V to 3.3 V, 2 A
5.1 Specifications
| Input Voltage | Vin = 2.7–4.2 V (single Li-Ion) |
| Output Voltage | Vout = 3.3 V (±2%) |
| Output Current | Iout(max) = 2 A |
| Switching Frequency | fsw = 2 MHz (integrated controller) |
5.2 Operating Mode Analysis
At Vin = 4.2 V (fully charged) → Buck mode (Vin > Vout), Dbuck ≈ 3.3/4.2 ≈ 0.79.
At Vin = 3.3 V (mid-discharge) → Buck-Boost mode (Vin ≈ Vout).
At Vin = 2.7 V (near-empty) → Boost mode (Vin < Vout), Dboost ≈ 1 – 2.7/3.3 ≈ 0.18.
5.3 Inductor Selection
Design for the worst-case ripple (boost mode at minimum Vin: Vin(min) = 2.7 V, D = 0.18, IL(avg) = 2/(1 – 0.18) = 2.44 A):
6. Common Design Mistakes
Top 10 Buck-Boost Design Pitfalls
- Wrong Topology Selection: Using an inverting buck-boost when a non-inverting output is needed — then trying to “fix” it by swapping output connectors, creating ground loop issues.
- Ignoring Mode Transition Behavior: In 4-switch designs, the buck-boost transition region causes an efficiency dip and potential output voltage glitches if not properly managed. Verify behavior with swept Vin across the transition.
- Inadequate Bulk Capacitance: Both buck-boost and SEPIC topologies have pulsating output currents, requiring more output capacitance than an equivalent buck or boost.
- MOSFET Body Diode Conduction: In 4-switch designs, incorrect dead-time can cause body diode conduction in Q2 or Q4, increasing losses significantly at high frequencies.
- SEPIC Coupling Capacitor Ripple Current: The coupling capacitor in a SEPIC carries the full input current ripple. Underestimating its RMS current rating leads to overheating and premature failure. Use X7R MLCCs with adequate voltage derating.
- Feedback Divider Referenced to Wrong Ground: In inverting buck-boost, the feedback divider must be referenced to the negative output, not system ground. This is a common layout error.
- Overlooking Minimum On-Time: In buck mode at high Vin/Vout ratios, the required ON-time may be shorter than the controller’s minimum pulse width, causing pulse skipping and increased ripple.
- RHPZ Ignored in SEPIC: Like the boost, the SEPIC has a RHPZ that limits crossover frequency. Its frequency depends on both inductors and the coupling capacitor, making it more complex to predict.
- Startup Inrush Through the Body Diode: In 4-switch designs with a pre-biased output, startup can cause reverse current through the high-side body diode before the controller is active.
- No Input Disconnect in SEPIC: Like the boost, the SEPIC has a DC path from input to output when switching stops. For true shutdown, a load switch is needed.
7. Frequently Asked Questions
Q1: When should I use a 4-switch buck-boost vs. a SEPIC?
Choose 4-switch buck-boost when efficiency is paramount (>90% required), transient response is critical, or solution size must be minimized. Choose SEPIC when simplicity and cost matter more, or when single-switch reliability is preferred (fewer components to fail). SEPIC also provides inherent short-circuit protection via the coupling capacitor — a significant safety advantage in some applications.
Q2: Can I parallel buck-boost converters?
Yes, multiphase buck-boost converters are increasingly common for >10 A loads. However, the mode-transition logic becomes more complex: all phases must be in the same mode (buck, boost, or buck-boost) to prevent circulating currents. Some controllers force all phases into buck-boost mode whenever any phase would normally transition, sacrificing some efficiency for guaranteed current sharing.
Q3: How do I design the compensation for a 4-switch buck-boost?
The power stage transfer function changes dramatically between buck, boost, and buck-boost modes. Many integrated controllers implement internal compensation optimized for all modes. For discrete designs, compensate for the worst-case mode (typically boost mode at minimum Vin) and verify stability in all three modes. A single compensation network must provide adequate phase margin across all operating modes — this is a non-trivial design challenge.
Q4: What is the purpose of the SEPIC coupling capacitor?
The coupling capacitor (Cc) serves two critical functions: (1) it blocks DC while passing AC, allowing energy transfer from the input inductor to the output inductor, and (2) it provides galvanic isolation of the DC path, preventing a short circuit from input to output. In steady state, its voltage equals Vin. Its capacitance must be large enough that its voltage ripple is small (<5% of Vin), otherwise it affects the transfer function.
Q5: Can the inverting buck-boost be used to generate a positive output?
Only if you designate the “output” as ground and the “ground” as the negative rail. This creates a floating output where neither terminal is at system ground potential. While technically functional, this introduces severe layout challenges, EMC issues, and safety concerns. For positive outputs from a single switch, use a SEPIC instead.
Q6: Why do 4-switch buck-boost controllers need current sensing?
Current sensing is essential for three reasons in 4-switch designs: (1) cycle-by-cycle current limiting to protect the MOSFETs; (2) current-mode control to simplify compensation by reducing the power stage order; (3) smooth mode transitions — monitoring the inductor current enables seamless handoff between buck and boost modes without output voltage disturbances.
Q7: How does coupled-inductor SEPIC differ from discrete-inductor SEPIC?
A coupled-inductor SEPIC uses two windings on a single core, reducing component count and PCB area. If the coupling coefficient is high (k > 0.95), the input ripple current can be reduced to near zero by matching the turns ratio to the duty cycle. However, leakage inductance between windings creates voltage spikes that require snubbing. Discrete inductors avoid this but consume more board space.
Q8: What is the limit on buck-boost step-up/down ratio?
Practically, 4-switch buck-boost converters achieve voltage ratios from ~10:1 step-down to ~1:10 step-up with >85% efficiency. Beyond this, cascaded two-stage solutions or transformer-isolated topologies become more efficient. The inverting buck-boost and SEPIC are limited to ~5:1 in either direction due to higher component stresses and lower efficiency.
8. Conclusion
The buck-boost converter family provides the flexibility needed when input and output voltages cannot be ordered. The 4-switch non-inverting topology dominates modern battery-powered and USB PD applications with its high efficiency and compact implementation, while the SEPIC offers an attractive single-switch alternative where simplicity and reliability outweigh peak efficiency. The classic inverting buck-boost remains relevant for negative rail generation and cost-sensitive designs. Success in buck-boost design requires careful attention to mode transitions (in 4-switch designs), RHPZ compensation (in SEPIC and boost-mode operation), component voltage stresses (which can exceed Vin + Vout), and output ripple management (pulsating output currents demand more capacitance).