* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The input current for a buck power converter is discontinuous due to
Radio transmitter design wikipedia , lookup
Nanogenerator wikipedia , lookup
Index of electronics articles wikipedia , lookup
Analog-to-digital converter wikipedia , lookup
Power MOSFET wikipedia , lookup
Schmitt trigger wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Television standards conversion wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Operational amplifier wikipedia , lookup
Wilson current mirror wikipedia , lookup
Surge protector wikipedia , lookup
Voltage regulator wikipedia , lookup
Current source wikipedia , lookup
Coupon-eligible converter box wikipedia , lookup
Integrating ADC wikipedia , lookup
Current mirror wikipedia , lookup
Power electronics wikipedia , lookup
Opto-isolator wikipedia , lookup
Overview of the Buck Converter Chapter 3 CHAPTER 3 OVERVIEW OF THE BUCK CONVERTER 3.1 INTRODUCTION In the buck converter the average output voltage is less than the input supply voltage. The buck converter is shown in Fig.3.1. The buck converter is similar to the step down converter. The inductance and capacitance are the filtering components. Due to the freewheeling diode the output voltage and current are maintained ripple free. Fig. 3.1. Circuit diagram of buck converter The most common power converter topology is the buck power converter, sometimes called a step down power converter. Power supply designers choose the buck power converter because the output voltage is always less than the input voltage in the same polarity and is not isolated from the input. The buck regulator circuit is a switching regulator, as shown in figure. It uses an inductor and a capacitor as energy storage elements so that energy can be transferred from the input to the output in discrete packets. The advantage of using switching regulators is that they offer higher efficiency than linear regulators. The one disadvantage is noise or ripple, the ripple will need to be minimized 7 Overview of the Buck Converter Chapter 3 through careful component selection. A requirement of the design is to have high current slew rate (up to 930 A/μs) to increase switching speed of microprocessor from one state to the other but this causes voltage drop spikes at the processor power supply. To achieve high current slew rate the inductor Lo should be as small as possible. This in turn while achieving faster transient response will cause the output voltage ripple to increase. To reduce output voltage ripple, the switching frequency should be increased but this lowers efficiency. This means that the selection of the switching devices will be an important issue. The output voltage ripple can also be reduced by increasing the output capacitance, this means a large capacitor in practical design. The input current for a buck power converter is discontinuous due to the power switch. The output current for a buck power converter is continuous because the output current is supplied by the output inductor/capacitor combination, the output capacitor never supplies the entire load current for continuous inductor current mode operation. 3.2 OPERATION OF BUCK CONVERTER 3.2.1 CONTINUOUS CONDUCTION MODE Mode 1: Fig. 3.2. Operation of buck converter in Mode 1 Continuous inductor current mode is when current flows continuously in the inductor during the full switching cycle. A buck converter operating in continuous conduction mode has two unique switching states during each switching cycle. The first state corresponds to 8 Overview of the Buck Converter Chapter 3 the case when the switch is ON. The equivalent circuit is shown in Figure 3.2. In this state, the current through the inductor rises, and the energy stored in it increases, during this state the inductor acquires energy. Vi Lo di L I Vo Lo Vo dt DT (2.1) When the switch is closed, the diode is in the OFF state. The diode is there so there will always be a current source for the inductor. Mode 2: Fig. 3.3. Operation of buck converter in Mode 2 The second state is when the switch is OFF and the diode is ON. The equivalent circuit is shown in Figure 2.3 In this state, the inductor current free-wheels through the diode and the inductor supplies energy to the RC network at the output. The energy in the inductor falls in this state. 0 Vo Lo di L I Vo Lo dt (1 D)T 9 (2.2) Overview of the Buck Converter Chapter 3 When the switch is open, the inductor discharges its energy. When all of its energy has discharged, the current falls to zero and tends to reverse, but the diode blocks conduction in the reverse direction. In the third state both the diode and the switch are OFF, in this state the capacitor discharges its energy and the inductor is at rest with no energy stored in it. Combining both the equations 2.1 and 2.2 we can get the equations as below, Vi Vo Vo DT (1 D)T , Lo Lo Vo = D x Vi Fig. 3.4. Buck operating waveforms 10 (2.3) Overview of the Buck Converter Chapter 3 3.2.2 DISCONTINUOUS CONDUCTION MODE The inductor current flows into the output capacitor and load resistor combination. The average current flowing in the output capacitor is always zero so the buck converter load current is the average of the inductor current. When the load current is decreased below a critical level, i.e. half the inductor ripple current, the inductor current will be zero for a portion of the switching cycle. In a non-synchronous buck converter, if the inductor current attempts to fall below zero, it cannot due to the unidirectional current flow in the freewheeling diode, and just stays at zero until the start of the next switching cycle. This operating mode is called discontinuous conduction mode. A buck converter operating in discontinuous conduction mode has three unique switching states during each switching cycle as opposed to two states for continuous conduction mode. If the load current is decreased further, the circuit is put into discontinuous mode. This condition is shown in Figure 2.6 Fig. 3.5. Discontinuous Mode 3.3 CONCLUSION Thus the Buck converter and its operation and different kinds of modes has been presented in this chapter. The disadvantages of the buck converter has been overcome with the help of the Interleaved buck converter(IBC). 11