- This converter can be obtained by the cascade connection of two converters: the step-down converter and the step-up converter
- The output voltage can be higher or lower than the input voltage
- Used in regulated dc power supplies where a negative polarity output may be desired with respect to the common terminal of the input voltage
- The output to input voltage conversion ratio
- This allows V0 to be higher or lower than Vd
- When the switch is ON:
Diode is reversed biased
Output circuit is thus isolated
Inductor is charged
- When the switch is OFF:
the output stage received energy from the inductor
Continuous current conduction mode
- Parasitic elements are due to the losses associated with the inductor, capacitor, switch and diode
Example 2: Step-down (Buck) converter
The chopper below controls a dc machine with an armature inductance La = 0.2 mH. The armature resistance can be neglected. The armature current is 5 A. fs = 30 kHz. D = 0.8The output voltage, Vo, equals 200V.
(a) Calculate the input voltage, Vd
(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, ia(t), and the dc current, id(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 Vd = 14 V.
The output voltage V0 = 42 V.
Inductor L = 10 mH
Output resistor R = 1 Ω
Switching frequency fs=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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