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Tutorial: Boost Converter

Design and simulate a boost (step-up) DC-DC converter.

Difficulty: Beginner (⅓) | Duration: 30 minutes

Circuit file: resources/tutorials/2xx_dcdc_converters/202_boost_converter/boost_simple.ipes

Learning Objectives

  • Understand boost converter operation
  • Calculate output voltage and component values
  • Simulate and verify the design
  • Analyze voltage and current waveforms

Theory

Topology

The boost converter steps up DC voltage:

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

Operating Principle

Switch ON (0 < t < D·T): Inductor charges from V_in, diode is reverse-biased, capacitor supplies load.

\[V_L = V_{in}, \quad \frac{di_L}{dt} = \frac{V_{in}}{L}\]

Switch OFF (D·T < t < T): Inductor discharges through diode to load and capacitor.

\[V_L = V_{in} - V_{out}, \quad \frac{di_L}{dt} = \frac{V_{in} - V_{out}}{L}\]

Key Equations

Voltage conversion ratio (CCM):

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

Minimum inductance for CCM:

\[L_{min} = \frac{D(1-D)^2 R}{2f_{sw}}\]

Output voltage ripple:

\[\frac{\Delta V_{out}}{V_{out}} = \frac{D}{R C f_{sw}}\]

Inductor current ripple:

\[\Delta i_L = \frac{V_{in} \cdot D}{L \cdot f_{sw}}\]

Design Example

Specifications

Parameter Value
Input voltage 12 V
Output voltage 24 V
Output power 24 W
Switching frequency 100 kHz
Max voltage ripple 1%

Calculations

Duty cycle:

\[D = 1 - \frac{V_{in}}{V_{out}} = 1 - \frac{12}{24} = 0.5\]

Load resistance:

\[R = \frac{V_{out}^2}{P_{out}} = \frac{24^2}{24} = 24\ \Omega\]

Minimum inductance:

\[L_{min} = \frac{0.5 \times 0.5^2 \times 24}{2 \times 100k} = 15\ \mu H\]

Choose L = 100 uH (well above minimum for low ripple).

Output capacitor:

\[C = \frac{D}{R \times \frac{\Delta V}{V} \times f_{sw}} = \frac{0.5}{24 \times 0.01 \times 100k} = 20.8\ \mu F\]

Choose C = 47 uF.

Simulation

Setup

  1. Open boost_simple.ipes
  2. Verify parameters match the design
  3. Set simulation: T_end = 2 ms, dt = 50 ns

Run

Press F5 and open the SCOPE to view:

  1. Output voltage - Should settle to ~24V
  2. Inductor current - Triangular waveform, always positive (CCM)
  3. Switch voltage - Pulses to V_out when OFF

Expected Waveforms

  • Output voltage: 24V with < 1% ripple
  • Average inductor current: I_out / (1-D) = 2A
  • Inductor current ripple: V_in x D / (L x f_sw) = 0.6A

Exercises

Exercise 1: Duty Cycle Variation

Change D from 0.3 to 0.7 in steps of 0.1. Plot V_out vs D and compare with theory.

Exercise 2: CCM/DCM Boundary

Reduce the load power (increase R) until the inductor current touches zero. This is the CCM/DCM boundary.

Exercise 3: Component Sizing

Reduce L to 15 uH (minimum). Observe increased ripple. Then try 5 uH to see DCM operation.

Important Notes

Boost Converter Limitations

  • Output is always higher than input (V_out > V_in)
  • As D approaches 1, efficiency drops due to parasitic resistances
  • Practical limit: V_out < 4-5x V_in
  • Input current is continuous (good for solar/battery applications)

Next Steps