Lab 08 · Power Electronics Laboratory

DC-DC Converters

Buck · Boost · Buck-Boost · Duty Cycle Control · PWM

Dr. Mithun Mondal BITS Pilani
Home› Power Electronics› Lab› Lab 8: DC-DC Converters
§ 01

Aim & Tools

To study the operation of DC-DC converters (Buck, Boost, Buck-Boost) using MATLAB Simulink. The duty cycle D is varied from 20% to 80% and simulated output voltage is compared with theoretical values.

↓Buck (Step-Down)

Output voltage is lower than input. \(V_o = D \cdot V_{in}\). Used in voltage regulators, battery chargers.

↑Boost (Step-Up)

Output voltage is higher than input. \(V_o = V_{in}/(1-D)\). Used in PV inverters, LED drivers.

↕Buck-Boost

Output magnitude can be higher or lower, but polarity is inverted. \(V_o = D V_{in}/(1-D)\cdot(-1)\). Used in battery systems.

§ 02

Theory — Converter Equations

Buck Converter Output
\[V_o = D \cdot V_{in}\]
D = duty cycle = Ton/T. For D = 0.4 and Vin = 12 V: Vo = 4.8 V.
Boost Converter Output
\[V_o = \frac{V_{in}}{1-D}\]
For D = 0.4 and Vin = 12 V: Vo = 20 V. Note: output voltage increases as D approaches 1.
Buck-Boost Converter Output
\[V_o = -\frac{D}{1-D}\cdot V_{in}\]
Negative sign indicates inverted polarity. For D = 0.5: |Vo| = Vin (unity conversion with inversion).
§ 03

Simulation Procedure

  1. Construct the converter circuit in MATLAB Simulink.
  2. Use a Pulse Generator block to generate gate pulses for the IGBT switch.
  3. Configure the pulse generator: Period = switching period T; Pulse Width = D × T × 100%.
  4. Vary the duty cycle (Pulse Width) as per the observation table.
  5. Record simulated output voltage and compare with theoretical formula.
Pulse generator configuration
Fig. 1 — Pulse Generator configuration: set Pulse Width % to control duty cycle.
§ 04

Buck Converter

Buck converter Simulink model
Fig. 2 — Simulink model: Buck converter with IGBT, inductor, diode, capacitor, and R load.
Pulse generator at 40% duty cycle
Fig. 3 — Pulse generator configured for 40% duty cycle.

Buck Converter Observation Table

S.NoVin (V)Duty Cycle D (%)Vo,calculated (V)Vo,simulated (V)
1.20
2.40
3.60
4.80
§ 05

Boost Converter

Boost converter Simulink model
Fig. 4 — Simulink model: Boost converter with inductor, IGBT, diode, capacitor, and R load.

Boost Converter Observation Table

S.NoVin (V)Duty Cycle D (%)Vo,calculated (V)Vo,simulated (V)
1.20
2.40
3.60
4.80
§ 06

Buck-Boost Converter

Buck-Boost converter Simulink model
Fig. 5 — Simulink model: Buck-Boost converter (note inverted output polarity).

Buck-Boost Converter Observation Table

S.NoVin (V)Duty Cycle D (%)Vo,calculated (V)Vo,simulated (V)
1.20
2.40
3.60
4.80
§ 07

Results

For each converter, attach: (1) MATLAB Simulink circuit diagram (2) Simulated waveforms at 40% duty cycle: Source voltage and current, Output Voltage and Current, Voltage across IGBT (3) Completed observation tables comparing calculated vs simulated output voltages.