DC-DC Converters-Buck,Boost and Buck Boost

Switching regulators

1. Switching Regulators DC-DC power converters are widely used in regulated switch-mode dc power supplies and dc motor drive applications. The input to the DC-DC converter is an unregulated dc voltage Vs. The converter produces a regulated output voltage Vo, having a magnitude that differs from Vs ie . unregulated dc is converted to regulated dc at a desired voltage level. Here , self-commutated devices like power BJT, power MOSFET, IGBT etc are used instead of thyristors . It is used for low and medium power applications whereas thyristorised choppers are used for high power applications.

2. Different DC-DC converters are 1. Step-down (buck) converter 2. Step-up (boost) converter 3. Step-down/step-up (buck-boost) converter 4. Cuk converter 5. Full-bridge DC-DC converter

3. Buck Converter A buck converter produces a lower average output voltage V o than the dc input voltage V s . It is a step down converter. Continuous conduction mode is considered where the inductor current is assumed to flow continuously with a linear rise and fall. Its main application is in regulated dc power supplies and dc motor speed control .

4. MODES OF OPERATION MODE 1:Switch on During the interval when the switch is ON, the diode becomes reverse biased. T he input current rises and provides energy to the load as well as to the inductor and the inductor gets charged. MODE 2: Switch off During the interval when the switch is off, the inductor current flows through the diode, transferring some of its stored energy to the load . Inductor current falls until switch is turned on again in next cycle. The filter capacitor at the output is assumed to be very large so that V o (t ) = V o . For a buck converter, average inductor current I L is equal to the average inductor current I o since the average capacitor current in steady state is zero .

5. Problem :- A buck converter has a switching frequency of 10kHz. It is used to obtain a fixed dc output voltage of 48V across 1Ω resistor when fed from a 192V dc source. The filter inductor has an inductance of 200μH. Determine the duty ratio, average load current, average inductor current, peak to-peak ripple current, minimum and maximum inductor currents. Assume continuous conduction mode

6. Boost Converter A boost converter produces a higher average output voltage V o than the dc input voltage V s . It is a step up converter. Continuous conduction mode is considered where the inductor current is assumed to flow continuously with a linear rise and fall. Its main application is in regulated dc power supplies and dc motor speed control .

7. MODES OF OPERATION MODE 1:Switch on When the switch is ON, the diode is reverse biased, thus isolating the output voltage. T he input current rises and charges the inductor . Capacitor discharges to maintain the load. MODE 2: Switch off When the switch is OFF, the output stage receives energy from the inductor as well as from the input. Inductor current falls until switch is turned on again in next cycle. The filter capacitor at the output is assumed to be very large so that V o (t ) = V o .   LdI   V s -V    

8. Expression for average output voltage

9. Buck Boost Converter In buck-boost converters, the output voltage is either higher or lower than the input voltage; but the polarity of the output voltage is reversed with respect to the input voltage . Output voltage is negative. It is a step up/step down converter. Continuous conduction mode is considered where the inductor current is assumed to flow continuously with a linear rise and fall. Its main application is in regulated dc power supplies and dc motor speed control .

10. MODES OF OPERATION MODE 1:Switch on When the switch is ON, the diode is reverse biased, thus isolating the output voltage. T he input current rises and charges the inductor . Capacitor discharges to maintain the load. MODE 2: Switch off When the switch is OFF, the energy stored in the inductor is transferred to the output through diode. No energy is supplied by the input during this interval . Inductor current falls until switch is turned on again in next cycle. The filter capacitor at the output is assumed to be very large so that V o (t ) = V o .   LdI   V    

11. Expression for average output voltage