DC-DC Power Converter Design & Implementation

CONTENTS

Chapter1 Introduction
1.1 Project Background
1.2 Project Objectives
1.3 Project Scopes & Methodology
1.3.1 Methodology
1.3.2 Works Schedule
1.3.3 Procedure
1.3.4 Equipment and Software
1.4 Outline of Thesis

Chapter 2 Basic Principle of Power Converter
2.1 Switching Mode Regulator & Chopper
2.1.1 Chopper Circuit
2.2 PWM Step Down Operation
2.3 Switching Losses in MOSFET
2.3.1 Turn-On Switching Loss Model
2.3.2 Turn-Off Switching Loss Model

Chapter 3 Buck Converter Design
3.1 Introduction about Buck Converter
3.1.1 Zero-Voltages-Switching Resonant Converters
3.1.2 Switch Configurations for ZVS Resonant Converters
3.1.3 ZVS Circuit Mode Operation
3.1.3 Why to go for ZCS
3.1.4 Zero-Current-Switching Resonant Converters
3.1.5 Switch Configurations for ZCS Resonant Converter
3.2 Buck Design……
3.2.1 Evolution of Buck Converter…
3.2.2 Purpose of different components in the Buck Converter
3.2.3 ZCS Circuit Mode Operation
3.3 Control Scheme & Strategies
3.3.1 Time Control Ratio (TCR)
3.3.2 Current Limit Control (CLC)

Chapter 4 Hardware Design
4.1 Simulation Result by using pspice Software
4.2 Block Diagram
4.3 Power Circuit & Control Circuit
4.4 Circuit ZCS Buck Converter Schematic Diagram

Chapter 5 Implementation & Test
5.1 Work & Implementation
5.2 Hardware Testing
5.3 PCB Layout Project Contraction Consideration…
5.4 Observations & Calculation
5.5 Calculation & Mathematical Analysis Mode of Operation

Conclusion

References

Acknowledgement

Appendix A

Appendix B

Appendix C

Chapter 1 Introduction

This chapter describes the project background, objectives, scopes, methodology, and Summary of the thesis. In the project background, it briefs the description of the buck converter and controller as well as the objectives and the scopes. Lastly, outline of this thesis is given in this chapter.

1.1 Project Background

Direct current to direct current (DC-DC) converters are power electronics circuits that converts direct current (DC) voltage input from one level to another. DC-DC converters are also known as switching converters, switching power supplies or switches. DC-DC converters are important in portable device such as cellular phones and laptops.

Why do we need DC-DC converter? For example, when we want to use a device with low voltage level, if we connected the device such as laptop or charger directly to the rectified supplied from the socket at home, the device might not functioning properly or it might be broken due to over current or overvoltage. Therefore to avoid unnecessary damage to the equipments and devices, we would need to convert the voltage level to suitable voltage level for the equipments to function properly. In this project, the configuration of DC-DC converter chosen for study was buck configuration. Buck converter converts the DC supply voltage to lower DC output voltage level. The buck converter is suitable for low power application due to the low voltage level at the output.

1.2 Project Objectives

The main objective of this project is to design a buck converter to convert the input DC voltage to lower DC output voltage level for low power applications to solve the problem of buck converter. The converter uses switching scheme operates the switches such as MOSFET in Cut-off and saturation region to reduce power loss across the transistor or switch. The output voltage level is then regulated by the control circuit and power circuit to desired output voltage level as in the design specification.

1.3 Project Scopes & Methodology

The scopes of this project are:

- Study the operation of buck converter.
- Study the operation of PWM step down techniques.
- Simulation of buck converter, control circuit and power circuit by using PSpice, Protel DXP 2004 and MATLAB software’s.
- Simulation of buck converter frequency response using PSpice software.
- Design the buck converter power stage circuit.
- Design the controller and compensator circuit.
- Testing and calibration of the completed buck converter to confirm the actual response with the theoretical predictions.
- Observation of waveforms from the different Test point of the converter.

The design specification is based on low power applications. The circuit is simulated by using PSpice software to obtain the desired output voltage to giving fix input value.

1.3.1 Methodology

This chapter discusses on methodology and procedures as well as equipments and software’s used in the entire work process. The methodology describes how the flows of the project and procedures topic describes how the project was divided into two phases (Final year project 1 and 2) and the works involved in each phase. The work schedule topic mentioned about the use of Gantt charts for the project schedules and the equipment and software topic describes about the equipments and software’s used when the project was carried out.

Fig1.1 below shows that the first step in this project was to design the circuit components value for buck converter configuration. During this step, the components value was calculated using established equations and formulas. Then models of buck converter and its controller were built and simulated using PSpice, and Protel DXP 2004 software’s. The output voltage and frequency response of the power circuit and control circuit were analyzed and compared with the earlier theoretical predictions

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Fig1.1 Project Methodology

Next, when the simulation results had been confirmed to be approximately the same with the predictions, the power circuit and control circuit are assembled on the PCB. The PCB circuits were built using Protel DXP 2004. The circuits were then printed on the PCB before they undergo the etching process. After the circuits had been etched properly, the hardware’s of buck converter power circuit and its controller 555 Timer on the PCB platforms were obtained. Then the components were placed and soldered on the PCBs to complete the hardware’s of buck converter power circuit and Control Circuit.

Lastly, the hardware’s were tested in the lab to ensure that they function as the desired buck converter in the earlier design process. Any flaws detected on the hardware’s were fixed immediately. Several numbers of tests were carried out during this step in order to make the hardware’s to operate properly and accurately.

1.3.2 Works Schedule

Gantt chart is used to organize works schedules and to simplify the projects outline for project 1 and project 2. Project 1 consists of works plan on designing and simulating the buck converter and control circuit using PSpice software while project 2 works plan emphasizes on constructing the hardware and thesis writing as describe in 1.3.3 procedure shows Fig1.2.

1.3.3 Procedures

The procedures involved in this project are as shown in the Figure-1.2.During final year project (FYP) 1; the works were focused on designing the buck converter control circuit and control Circuit. It was started with choosing a title for the FYP 1, and ends with completed design of the converter’s power stage, first seminar and report on the FYP 1 works. In FYP 2, the works were focused on constructing the hardware’s of buck converter power Circuit and Control circuit it ends with final FYP 2 and the submitting of completed thesis to the supervisor and the faculty.

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Figure1.2 Flow charts showing the works flow

1.3.4 Equipment and Software

The equipment used in this project consists of hardware components and also software program to carry out the circuit simulation. The software’s used for circuit simulation were PSpice, and Protel DXP 2004. A buck converter circuit was simulated using actual components’ value to simulate the output voltage response and compared it with the desired response in the earlier design. During hardware construction phase, Protel DXP 2004 and MATLAB were used to construct the PCB circuits for buck converter power circuit and its controller and to calculate the components’ value for both circuits respectively.

The components used to construct the hardware of buck converter and 555 Timer controllers were capacitors, resistors, inductors, diodes, MOSFET and etc. The values chosen were based on the specifications in the earlier design phase as mentioned in Chapter 4.

1.4 Outline of Thesis

This thesis consists of 5 chapters. In the first chapter, it discusses Methodology and Equipments of project. It gives introduction, background, project objects and scopes. In Chapter 2, literature reviews and theories on power converter and switching converter are discussed & PWM technique and step down operations and switching power losses in MOSFET are discussed while buck converter design, introduction about buck converters, buck design & control scheme and strategies are discussed in chapter 3.

In the chapter 4, Hardware design has included Zero voltages switching resonant converter & also include Zero current Switching resonant converters are discussed in detailed. Chapter 5, Topic covers Implementation & test. it gives the result information of software and hardware which is implemented in PCB along with observation of waveforms & mathematically analysis of Mode of operation. The suggestions and conclusions obtained upon successfully completing this project are given. Finally, the last part in the thesis provides the conclusions, references, Acknowledgement and appendices used in the project as well.

Chapter 2 Basic principle of Power Converter

2.1 Switching Mode Regulators & Chopper

DC converters can be used as switching-mode regulators to convert a dc voltage, normally unregulated to a regulated dc output voltage. The regulation is achieved by PWM at a fixed frequency and the switching device is normally BJT, MOSFET, or IGBT. The output of dc converters contains harmonics and the ripple content is normally reduced by an LC filter.

Switching regulators are commercially available as integrated circuits. The designer can select the switching frequency by choosing the values of R and C of frequency oscillator. As a rule of thumb, to maximize efficiency, the minimum oscillator period should be about 100 times longer than the transistor switching time; for example, if a transistor has a switching time of 0.5μs, the oscillator period would be 50μs, which gives the maximum oscillator frequency of 20 kHz. This limitation is due a switching loss in transistor.

The transistor switching loss increases with the switching frequency and as a result the efficiency decreases. In addition, the core loss of inductors limits the high-frequency operation. Control voltage is obtained by comparing the output voltage with its desired value. The reference voltage can be compared with a saw-tooth voltage to generate the PWM control signal for the dc converter. There are three basic topologies of switching regulators.

- Buck regulators
- Boost regulators
- Cuk regulators

Furthermore, depending upon the direction of current and voltage flows, dc converters can be classified into five types:

- First quadrant converters
- Second quadrant converters
- First and second quadrant converters
- Third and fourth quadrant converters
- Four-quadrant converters

2.1.1 Chopper Circuits

Many industrial applications require power from DC sources. Several of these applications, however, perform better in case these are fed from variable DC voltage sources. Examples of such DC system are subway cars, trolley buses, battery-operated vehicles, battery charging etc. From an AC supply systems, variable DC output voltage can be obtained through the use of phase controlled converters or motor-generator sets. The conversion of fixed DC voltage to an adjustable DC output voltage through the use of semiconductor devices can be carried out by the use of two types of DC to DC converters mentioned below.

(1) AC link chopper

In the ac link chopper dc is first converted to ac by an inverter (dc to ac converter), ac is then stepped-up or stepped-down by a transformer which is then converted back to a dc by a diode rectifier. As the conversion is in two stages, dc to ac and then ac to dc, the link chopper is costly, bulky and less efficient.

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Fig2.1 AC Link Chopper

(2) DC Link Chopper

A chopper is a static device that converts fixed dc input voltage to a variable dc output voltage directly. A chopper may be thought of as dc equivalent of an ac transformer since they behave in an identical manner. As choppers involve one stage conversion, these are more efficient.

Choppers are now being used all over the world for rapid transit systems. These are also used in trolley cars, marine hoists etc. The future electric automobiles are likely to use choppers for their speed control and braking. Chopper systems offer smooth control, high efficiency, fast response and regeneration. The power semiconductor devices used for a chopper circuit can be force-commutated thyristor, power BJT, power MOSFET, GTO or IGBT. Like the transformer, a chopper can also be used to step-down or step-up the fixed input voltage.

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Fig.2.2 DC Link Chopper

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Fig2.3 Representation of power semiconductor device

2.2 PWM Step Down operation

The principle of step down operation is explained as follows. When a switch SW, known as the chopper is closed for a time t1, the input voltage Vs appears across the load. If the switch remains off for a time t2, the voltage across the load is zero. The waveforms for the output voltage and load current shown below. The converter switch can be implemented by using a (1) power bipolar junction transistor (BJT), (2) power metal oxide semiconductor field effect transistor (MOSFET) (3) gate turn-off thyristor (GTO), or (4) insulated-gate bipolar transistor (IGBT). The practical devices have a finite voltage drop ranging from 0.5 to 2V, and for simplicity we neglect the voltage drop of these power semiconductor devices.

The average output voltage is given by

= (1/T)*dt = = f = (k )

And the average load current, = /R = k /R

Where T is the chopping period; k = /T is the duty cycle of chopper; f is the chopping frequency.

The effective input resistance seen by the source is

= / = / (k /R) = R/k

Which indicates that the converter makes the input resistance Ri as a variable resistance of R/k.

The duty cycle k can be varied from 0 to1 by varyingt₁, T or f. Therefore the output voltage can be varied from 0 to by controlling k, and the power flow can be controlled.

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Fig2.4 chopper Circuit

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Fig2.5 waveforms for the output voltage

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Fig2.6 waveforms for the load current

2.3 Switching Losses in MOSFET

The switching loss of power MOSFET is a major contributing factor towards the total power loss in high frequency power converters. Calculation of switching losses occurring in a MOSFET is a relatively difficult task .It is because the complex switching behavior of MOSFET’s are difficult to model.

The nonlinear characteristics arise due to the parasitic junction capacitance and inductance present in the MOSFET. It is comparatively easy to find out the switching losses by referring the parameters from the datasheet. A commonly used formula for estimating the MOSFET drain to source switching loss is given by:

=1/2 ( + ) f+1/2Coss f

Where ID , VD , and f are the load current, input voltage, and switching frequency while tON and tOFF are the MOSFET turn-on and turn-off times, respectively. Assuming a linear transition of iDS and vDS , the first term of (1) simply calculates the switching power loss as the area below iDS and vDS during the transition periods. The second term of (1) is often referred to as the output capacitance loss term.

The motive of including the second loss term is to account for the loss of energy stored in the output capacitance that is internally dissipated through the MOS channel in the form of Joule heating during MOSFET turn-on. COSS is the output capacitance of the MOSFET and given by: COSS = CGD + CDS

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Fig2.7 typical switching waveforms of power MOSFET with an inductor load

However the disadvantage of this method is that it predicts the magnitude of turn-on and turn-off loss as same. In a real converter operating at a high switching frequency, the model is highly inaccurate since turnoff loss is much larger due to parasitic inductances. In addition, the inductor ripple current decreases the current at turn-on and increases the current at turn-off, which further reduces turn-on switching loss and increases turn-off switching loss.

Hence, a separate approach is presented here to calculate the switching losses in a MOSFET. The proposed model uses the piecewise linear approximations of the switching waveforms. Turn-on switching loss occurs during Tr(rise time) and turn-off switching loss occurs during Tf(fall time).

The key to the model is prediction of the turn-on current I ON, the rise and fall times Tr and Tf, the reverse recovery current, I rr, the magnitude of the rising current slope Δ ids/ Δ t, and the current drop Δ i 1 f when vds 1 rises to V in at turn-off. The goal of the proposed model is to calculate the switching loss with respect to load current, driver supply voltage, driver gate current, and total circuit inductance in a simple manner. The MOSFET parasitic capacitances are required in the model. They are estimated using the effective values, using datasheet specification values for V ds 1 spec , Crss 1 spec , and Ciss 1 spec .

2.3.1 Turn-On Switching Loss Model

By definition, P ON is derived using the simple integral, representing the average power over one switching period: = dt= 1/6 f

Fig2.8 Piecewise Linearization approach during Ton

2.3.2 Turn-Off Switching Loss Model

The circuit waveforms and knowledge of the circuit operation are used extensively in order to derive the turn-off loss P OFF. The turn-off transition consists of two intervals T 1 f and T 2 f. During T 1 f, the Miller capacitor Cgd 1 is discharged while vgs 1 remains at V pl OFF, and ids 1 is assumed to remain constant.

During this interval, vds 1 increases from zero to V in . Therefore, from the geometry, the turn-off power loss P 1OFF, during T 1 f, is give

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Fig2.9 Piecewise Linearization approach during Toff