Power Electronics Lecture10 D C to D C

Power Electronics Lecture-10 D. C to D. C Converters (Choppers) Dr. Imtiaz Hussain Assistant Professor email: imtiaz. hussain@faculty. muet. edu. pk URL : http: //imtiazhussainkalwar. weebly. com/ 1

Power Electronic Interface • Power Electronics is an enabling technology providing the needed interface between the electrical source and electrical load. • The source and load often do differ in frequency, voltage amplitudes and number of phases. 2

Powering the information Technology • Most of the consumer electronics equipment supplied from the mains, internally needed very load dc voltages. • Figure shows the distributed architecture typically used in computers. • In which incoming voltage from the utility is converted into dc voltage (24 V). • This semi regulated voltage is distributed within the computer where on-board power supplies convert this 24 V into tightly regulated lower voltage. 3

Powering the information Technology • Many devices such as cell phones operates from low battery voltages. • However, the electronic circuitry requires higher voltages. • Thus necessitating a circuit to boost input dc to higher dc voltages 4

Introduction to D. C Converters (Choppers) • DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. • Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply. • They are also widely used in dc-motor drive applications. • Often input to these converters is an unregulated dc voltage, which is obtained by rectifying the line voltage.

Introduction Battery DC Unregulated AC Line Voltage 1 -Phase or 3 -Phase DC Regulated Uncontrolled Diode Rectifier Filter DC Unregulated D. C to D. C Converter vc D. C to D. C Converter System Load

Efficiency & Power Losses • High efficiency is essential in any power processing application. • The efficiency of a converter is • The power lost in converter is

Efficiency & Power Losses • Efficiency is a good measure of the success of a given converter technology. • With very small amount of power lost, the converter elements can be packaged with high density, leading to a converter of small size and weight, and of low temperature rise. • How can we build a circuit that changes the voltage, yet dissipates negligible power?

Efficiency & Power Losses • The various conventional circuit elements are illustrated in Following figure. • The available circuit elements fall broadly into the classes of resistive elements, capacitive elements, magnetic devices including inductors and transformers, semiconductor devices operated in the linear mode and semiconductor devices operated in the switched mode.

Types of dc-dc Converters • Types of D. C to D. C converters – AC Link Choppers – Linear Converters – Switch Mode – Magnetic – E. t. c

AC Link Choppers • First dc is converted to ac with the help of an inverter. • After that, AC is stepped-up or stepped-down by a transformer, which is then converted back to dc by a diode rectifier. • Ac link chopper is costly, bulky and less efficient as the conversion is done in two stages.

Simple dc-dc Converters • Let us now construct a simple dc-dc converter. The input voltage vg is 100 V. It is desired to supply 50 V to an effective 5Ω load, such that the dc load current is 10 A.

Resistive dc-dc Converters • Using Voltage divided rule.

Linear dc-dc Converters • Linear Mode dc-dc converter

Switch Mode dc-dc Converters

Conclusion • Capacitors and magnetic devices are important elements of switching converters, because ideally they do not consume power. • It is the resistive element, as well as the linear-mode semiconductor device, that is avoided. • Semiconductor in switch mode however dissipate comparatively low power in either states (ON and OFF). • So capacitive and inductive elements, as well as switchedmode semiconductor devices, are available for synthesis of high-efficiency converters.

Switch Mode D. C to D. C Converters • Switch-mode DC to DC converters convert one DC voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. • This conversion method is more power efficient (often 75% to 98%) than linear voltage regulation (which dissipates unwanted power as heat). • This efficiency is beneficial to increasing the running time of battery operated devices.

Switch Mode D. C to D. C Converters • PWM or PFM regulates the output dc voltage. • The power flow through these converters is only in one direction thus their voltages and currents remain unipolar.

Switch Mode D. C to D. C Converters • A Chopper is a high speed on/off semiconductor switch. • It connects source to load and disconnects source from load at very high speed. • In this manner a chopped dc voltage is obtained from a constant dc supply Vs is obtained.

Switch Mode D. C to D. C Converters • During the period Ton, the chopper is on and load voltage Vo is equal to source voltage Vs. • During the period Toff, the chopper is off and load current io flows through the freewheeling diode.

Switch Mode D. C to D. C Converters • The load voltage is given by • Thus the load voltage can be varied by varying the switching duty ratio D.

Control of D. C to D. C Converters • Average value of output voltage Vo can be controlled by opening and closing the semiconductor switch periodically. • The control strategies for varying duty ratio D are 1. Constant frequency system 2. Variable frequency system 22

Constant frequency system • In this scheme Ton is varied but frequency is kept constant. • Variation of Ton means adjustment of pulse width. Therefore, this scheme is called PWM scheme.

Constant frequency system (PWM) + Vo (desired) amplifier Vo (actual) Vcontrol Comparator Sawtooth Wave vst vcontrol t Switch Control Signal Ton Toff Ts Switch Control Signal

Variable frequency system • In this scheme Ton is kept constant but the frequency is varied.

Variable frequency system • Or Toff is kept constant but the frequency is varied.

Switch Mode D. C to D. C Converters • Types of Switch Mode D. C to D. C Converters – Step-Down (Buck) converter – Step-up (Boost) converter – Step Down/Up (Buck-Boost) converter

Step-Down Converter (Buck Converter) • As name implies a step-down converter produces a lower average output voltage than the dc input voltage Vd. • Its main application is in regulated dc power supplies and dc-motor speed control.

Step-Down Converter (Buck Converter) •

Step-Down Converter (Buck Converter) •

Step-Down Converter (Buck Converter)

Step-Down Converter (Buck Converter) • Continuous conduction mode – A buck converter operates in continuous mode if the current through the inductor (IL) never falls to zero during the commutation cycle.

Design procedure for Buck Converter • Calculate D to obtain required output voltage. • Select a particular switching frequency: – –preferably >20 KHz for negligible acoustic noise • Higher fs results in smaller L, but higher device losses. – Thus lowering efficiency and larger heat sink.

Design procedure for Buck Converter • Inductor requirement • C Calculation • Possible switching devices: MOSFET, IGBT and BJT. Low power MOSFET can reach MHz range.

Example-1 • A buck converter is supplied from a 50 V battery source. Given L=400 u. H, C=100 u. F, R=20 Ohm, f=20 KHz and D=0. 4. Calculate: (a) output voltage (b) output voltage ripple. Solution (a) output voltage 35

Example-1 (b) Output Ripple 36

Example-2 • A buck converter has an input voltage of 50 V and output of 25 V. The switching frequency is 10 KHz. The power output is 125 W. (a) Determine the duty ratio, (b) value of L to ensure continuous current, (c) value of capacitance to limit the output voltage ripple factor to 0. 5%. Solution (a) output voltage (b) Value of L 37

Example-2 • Resistance is calculated as • L must at least be 10 times greater than Lmin. 38

(c) Value of C Example-2 • Value of capacitance to limit the output voltage ripple factor to 0. 5% can be calculated using following equation. 39

Example-3 • Design a buck converter such that the output voltage is 28 V when the input is 48 V. The load is 8 Ohm. Design the converter such that it will be in continuous current mode. The output voltage ripple must not be more than 0. 5%. Specify the frequency and the values of each component. Suggest the power switch also. Solution: • First of all determine the switching frequency. • Then calculate the switching duty ratio 40

Example-3 • Now value of inductor can be chosen to ensure continuous conduction 41

Example-3 • Value of capacitance to limit the output voltage ripple factor to 0. 5% can be calculated using following equation. 42

Example-3 • Selection of Power semiconductor switch 43

Step-Up Converter (Boost Converter) • A boost converter (step-up converter) is a DC-to-DC power converter with an output voltage greater than its input voltage.

Step-Up Converter (Boost Converter) • When the switch is closed, current flows through the inductor in clockwise direction and the inductor stores the energy. • Polarity of the left side of the inductor is positive.

Step-Up Converter (Boost Converter) • When switch is opened, the output receives energy from the input as well as from the inductor. • Hence output is large.

Step-Up Converter (Boost Converter) S Closed S Open

Step-Up Converter (Boost Converter)

Step-Up Converter (Boost Converter) • Boost Converter Design • Minimum inductor value • Capacitor Value

Example-4 • The boost converter has the following parameters: Vd=20 V, D=0. 6, R=12. 5 ohm, L=65 u. H, C=200 u. F, fs=40 KHz. Determine (a) output voltage, (b) output voltage ripple. Solution (b) output voltage ripple (a) output voltage 50

Example-5 • Design a boost converter to provide an output voltage of 36 V from a 24 V source. The load is 50 W. The voltage ripple factor must be less than 0. 5%. Specify the duty cycle ratio, switching frequency, inductor and capacitor size, and power device. Solution • First of all determine the switching frequency. • Then calculate the switching duty ratio 51

Example-5 • Calculate the load resistance • Calculate inductor value to ensure continuous current • Inductance must be greeter than or equal to Lmin 52

Example-5 • Then calculate the Capacitor Value for ripple factor less than 0. 5% • While selecting the power device we must take into account the switching frequency 53

Example-5 • Selection of Power semiconductor switch 54

Buck-Boost Converter • The buck–boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. – If D>0. 5, output is higher – If D<0. 5, output is lower • Output voltage is always negative

Buck-Boost Converter • while in the On-state, the input voltage source is directly connected to the inductor (L). This results in accumulating energy in L. In this stage, the capacitor supplies energy to the output load.

Buck-Boost Converter • In Off-state, the inductor is connected to the output load and capacitor, so energy is transferred from L to C and R.

Buck-Boost Converter • In ON-state (Switch Closed)

Buck-Boost Converter • In OFF-state (Switch Opened)

Buck-Boost Converter • Steady state operation

Buck-Boost Converter

Example-6 • Determine the switching duty ratio of a buck-boost converter such that the output voltage is -28 V when the input is 100 V. The load is 1 Ohm. Design the converter such that it will be in continuous current mode. The output voltage ripple must not be more than 0. 5%. Specify the frequency and the values of each component. Suggest the power switch also. Solution: • First of all determine the switching frequency. • Then calculate the switching duty ratio 62

Example-6 • Then calculate the Capacitor Value for ripple factor less than 0. 5% • Value of inductor can be calculated as 63

Example-6 • While selecting the power device we must take into account the switching frequency 64

Switch mode vs Linear Power Supplies • One of the major applications of switch mode dc-dc converters is in switch mode power supplies (SMPs). • SMPs offer several advantages over linear mode power supplies. – Efficient (70 -95%) – Weight and size reduction • They also have some disadvantages – Complex design – EMI problems 65

Linear Mode Power Supply 66

Switch Mode Power Supply 67

To download this lecture visit http: //imtiazhussainkalwar. weebly. com/ END OF LECTURE-10 68