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Tutorial: Buck-Boost Topologies

Explore converter topologies that can both step up and step down voltage.

Difficulty: Intermediate (⅔) | Duration: 30 minutes

Circuit files: - resources/tutorials/2xx_dcdc_converters/203_buck_boost/buckBoost_simple.ipes - resources/tutorials/2xx_dcdc_converters/203_buck_boost/cuk_simple.ipes - resources/tutorials/2xx_dcdc_converters/203_buck_boost/sepic_simple.ipes

Topologies Covered

Topology Output Polarity Output Range Continuous Input Current
Buck-Boost Inverted 0 to -inf No
Cuk Inverted 0 to -inf Yes
SEPIC Non-inverted 0 to +inf Yes

Inverting Buck-Boost

Topology

    Vin ──[FET]──●──[LLLLL]──●── Vout (negative)
                 │            │
               [D]          [C]  [R]
                 │            │    │
                GND ──────────●────┘

Voltage Ratio

\[V_{out} = -\frac{D}{1-D} \times V_{in}\]
D V_out/V_in Mode
0.25 -0.33 Step-down
0.50 -1.00 Unity
0.75 -3.00 Step-up

Inverted Output

The basic buck-boost produces a negative output voltage relative to input ground.

SEPIC Converter

Topology

The SEPIC (Single-Ended Primary-Inductor Converter) provides non-inverted output:

         L1            C_coupling     D
    Vin──[LLLLL]──●────||────●──|>|──●── Vout
                  │          │       │
                [FET]      [L2]    [C]  [R]
                  │          │       │    │
                 GND────────●───────●────┘

Voltage Ratio

Same as buck-boost magnitude, but non-inverted:

\[V_{out} = \frac{D}{1-D} \times V_{in}\]

Advantages

  • Non-inverted output
  • Continuous input current (good for battery/solar)
  • Can step up or step down

Disadvantages

  • Two inductors and coupling capacitor
  • Higher component count than buck-boost
  • Pulsating output current

Cuk Converter

Topology

         L1        C_coupling        L2
    Vin──[LLLLL]──●──||──●──[LLLLL]──●── Vout (negative)
                  │       │           │
                [FET]   [D]         [C]  [R]
                  │       │           │    │
                 GND─────●───────────●────┘

Voltage Ratio

\[V_{out} = -\frac{D}{1-D} \times V_{in}\]

Advantages

  • Continuous input AND output current
  • Low ripple at both ports
  • Can couple L1 and L2 on same core

Design Guidelines

Coupling Capacitor (SEPIC/Cuk)

\[C_{coupling} = \frac{I_{out}}{f_{sw} \times \Delta V_C}\]

Typically 1-10 uF ceramic. Must handle RMS ripple current.

Inductors

\[L_1 \geq \frac{V_{in} \times D}{f_{sw} \times \Delta i_{L1}}\]
\[L_2 \geq \frac{V_{out} \times (1-D)}{f_{sw} \times \Delta i_{L2}}\]

Simulation

Comparing Topologies

  1. Open each circuit file
  2. Set identical specs: V_in = 24V, D = 0.5, f_sw = 100 kHz
  3. Run and compare output voltage, ripple, and waveform shapes

What to Observe

Measurement Buck-Boost SEPIC Cuk
Output polarity Negative Positive Negative
Input current Pulsating Continuous Continuous
Output current Continuous Pulsating Continuous

Exercises

Exercise 1: Step-Up/Step-Down Transition

Sweep D from 0.2 to 0.8. Identify the crossover point where |V_out| = V_in.

Exercise 2: SEPIC vs Buck-Boost

Compare efficiency of SEPIC and buck-boost at D = 0.5, 0.3, and 0.7.

Exercise 3: Coupled Inductors

For the Cuk converter, try coupling L1 and L2 (k = 0.95). Observe the effect on ripple.

Next Steps