Step-Down/Step-Up (Buck-Boost) Converter

1. This converter can be obtained by the cascade connection of two converters: the step-down converter and the step-up converter
2. The output voltage can be higher or lower than the input voltage
3. Used in regulated dc power supplies where a negative polarity output may be desired with respect to the common terminal of the input voltage
4. The output to input voltage conversion ratio
   
5. This allows V₀ to be higher or lower than V_d
6. When the switch is ON:
   Diode is reversed biased
   Output circuit is thus isolated
   Inductor is charged
   
7. When the switch is OFF:
   the output stage received energy from the inductor

 

Continuous current conduction mode

1. Inductor current i_L flows continuously
2. Average inductor voltage over a time period must be zero
   Assuming a lossless circuit
3. Depending on the duty ratio, the output voltage can be either higher or lower than the input

Effect of parasitic elements

1. Parasitic elements are due to the losses associated with the inductor, capacitor, switch and diode
2. Parasitic elements have significant impact on the voltage transfer ratio

Example 2: Step-down (Buck) converter

The chopper below controls a dc machine with an armature inductance L_a = 0.2 mH. The armature resistance can be neglected. The armature current is 5 A. f_s = 30 kHz. D = 0.8

The output voltage, V_o, equals 200V.
(a) Calculate the input voltage, V_d
(b) Find the ripple in the armature current.
(c) Calculate the maximum and the minimum value of the armature current
(d) Sketch the armature current, i_a(t), and the dc current, i_d(t).

Example 3: Step-down (Buck) converter characteristics

A step-down dc-dc converter shown in the following figure is to be analyzed.

Assume in all calculations constant voltage over the series resistor R. The output capacitor C is large; assume no ripple in the output voltage. Rated output is 20 V and 25 A
(a) Calculate rated output power.
(b) Calculate equivalent load resistance.
(c) Calculate duty ratio D for rated output. The voltage across the series resistor R must be taken into consideration.

Example 4: Step-up (Boost) converter characteristics

A step-up dc-dc converter shown in the following figure is to be analyzed.

The input voltage V_d = 14 V.
The output voltage V₀ = 42 V.
Inductor L = 10 mH
Output resistor R = 1 Ω
Switching frequency f_s=10 kHz
(a) Duty ratio, switch on and off time.
(b) Plot inductor and diode voltages.

Comparison of Converters

Buck converter: step-down, has one switch, simple, high efficiency greater than 90%, provides one polarity output voltage and unidirectional output current

Boost converter: step-up, has one switch, simple, high efficiency, provides one polarity output voltage and unidirectional output current, requires a larger filter capacitor and a larger inductor than those of a buck converter

Buck-boost converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal.

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