Safety-enhanced-high-step-up-DC-DC-converter-for-ac-photovoltaic-module-applications Chapter 2 Dc-dc Converters 12 Page

8/13/2019 Safety-enhanced-high-step-up-DC-DC-converter-for-ac-photovoltaic-module-applications Chapter 2 Dc-dc Converter

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CHAPTER 2

DC-DC CONVERTERS

2.1 INTRODUCTION

The DCDC converters are widely used in regulated switch mode dc supplies

and in dc motor drive applications. The input to these converters is often an

unregulated dc voltage, which is obtained by rectifying the line voltage and therefore

it will fluctuate due to changes in the line voltage magnitude. Switch mode DC-DC

converters are used to convert the unregulated dc input into a controlled dc output at a

desired voltage level.

2.2 TYPES OF DC-DC CONVERTERS

These converters are very often used with an electrical isolation transformer in

the switch mode dc power supplies and almost always without an isolation

transformer in case of dc motor drives. The following types of DC-DC converters are

known:

1) Step-down (buck) converter2) Step-up (boost) converter3) Step-down/Step-up (buck-boost) converter4) Cuk converter5) Full-bridge converter

Another type of DC-DC converter is the ac link DC-DC converter. In this

DC-DC converter, 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 dc

by a diode rectifier. As the conversion is in two stages, dc to ac and then ac to dc, this

type of converter is costly, bulky and less efficient.

Fig 2.1Block Diagram of DC-DC Converter


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A DC-DC converter is a static device that converts fixed dc input voltage to a

variable dc output voltage directly. A DC-DC converter may be thought of as dc

equivalent of an ac transformer since they behave in an identical manner. As DC-DC

converters involve onstage conversion, these are more efficient.

DC-DC converters are now being used all over the world for rapid transit

systems. These are also used in trolley cars, marine hoists, forklift trucks and mine

haulers. The future electric automobiles are likely to use DC-DC converters for their

speed control and braking. DC-DC converter systems offer smooth control, high

efficiency, fast response and regeneration.

The power semiconductor devices used for a DC-DC converter circuit can be

power BJT, power MOSFET, GTO or force commuted thyristor. These devices, in

general, can be represented by a switch with an arrow. When the switch is off, no

current can flow. When the switch is on, then the current flows in the direction of

arrows only. The power semiconductor devices have on-state voltage drops of 0.5 V

to 2.5 V across them. For the sake of simplicity, this voltage drop across these devices

is neglected. As stated above, a DC-DC converter is dc equivalent to an ac

transformer having continuously variable turns ratio. Like a transformer, a DC-DC

converter can e used to step down or step up the fixed dc input voltage. A DC-DC

converter is a high speed on/off semiconductor switch. It connects source to load and

disconnects the load from source at a fast speed. In this manner, a converted load

voltage is obtained from a constant dc supply. DC-DC converter is represented by a

switch inside a dotted rectangle, which may be turned on or turned off as desired.

Fig 2.2DC-DC Converter


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During the period T on, the DC-DC converter is on and load voltage is equal

to source voltage Vs. During the interval T off, DC-DC converter is off, load current

flows through the freewheeling diode FD.

Fig 2.3Waveforms for DC- DC Converter

As a result, load terminals are short circuited by FD and load voltage is

therefore zero during T off. In this manner, a dc-dc converted voltage is produced at

the load terminals. The load current is continuous. The average voltage Vois given by

Vo= {T on/ (T on+ T off)} Vs

T on= on-time; T off= off-time

T = T on+ T off

The above equation shows that load voltage is independent of load current. The above

equation can be written as

Vo = f * T on* V s

F = 1/T

2.2.1 Buck converter

In this circuit the transistor turning ON will put voltage Vinon one end of the

inductor. This voltage will tend to cause the inductor current to rise. When the

transistor is OFF, the current will continue flowing through the inductor but now


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flowing through the diode. We initially assume that the current through the inductor

does not reach zero, thus the voltage at Vxwill now be only the voltage across the

conducting diode during the full OFF time. The average voltage at Vxwill depend on

the average ON time of the transistor provided the inductor current is continuous.

Fig 2.4Buck Converter

Fig 2.5Voltage and current changes

To analyse the voltages of this circuit let us consider the changes in the

inductor current over one cycle. From the relation

the change of current satisfies

( ) ( )


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For steady state operation the current at the start and end of a period T will not

change. To get a simple relation between voltages we assume no voltage drop across

transistor or diode while ON and a perfect switch change. Thus during the ON time

Vx=Vinand in the OFF Vx=0. Thus

( ) ()

This simplifies to

( )

and defining "duty ratio" as

The voltage relationship becomes Since the circuit is loss less and

the input and output powers must match on the average Vo* Io= Vin* Iin. Thus the

average input and output current must satisfy Iin=D IoThese relations are based on

the assumption that the inductor current does not reach zero.

2.2.2 Boost converter

Fig 2.6Boost converter circuit


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In a boost converter, the average voltage Vo, is greater than the input voltage.

A boost converter is also called a step up converter. As the figure suggests, a large

inductor L in series with source voltage Vs is essential. When the switch is on, the

closed current path is as shown and inductor stores energy during T on period. When

the switch is off, as the inductor current cannot die down instantaneously, this current

is forced to flow through the diode and load for a time T off. As the current tends to

decrease, polarity of the EMF induced in l is reversed. As a result, voltage across the

load, given by Vo=Vs + L (di/dt), exceeds the source voltage V s. In this manner, the

circuit acts as a step up converter and the energy stored in L is released to the load.

2.3 CONTROL OF DC-DC CONVERTERS

In dc-dc converters, the average dc output voltage must be controlled to equal

a desired level, though the input voltage and the output load may fluctuate. Switch

mode dc-dc converters utilize one or more switches to transform dc from one level to

another. In a dc-dc converter with a given input voltage, the average output voltage is

controlled by controlling the switch on and off durations. One of the methods for

controlling the output voltage employs switching at a constant frequency and

adjusting the on duration of the switch to control the average output voltage. In this

method, called pulse-width modulation (PWM) switching, the switch duty ratio,

which is defined as the ratio of the on duration to the switching time period, is varied.

The other control method is more general, where both the switching frequency

(and hence the time period) and the on duration of the switch are varied. This method

is used only in dc-dc converters utilizing force-commutated thyristors. Variation in

the switching frequency makes it difficult to filter the ripple components in the input

and output waveforms of the converter.

In the PWM switching at a constant switching frequency, the switch control

signal, which controls the state (on or off) of the switch, is generated by comparing a

signal level control voltage v control with a repetitive waveform. The control voltage

signal generally is obtained by amplifying the error, or the difference between the

actual output voltage and its desired value. The frequency of the repetitive waveform

with a constant peak establishes the switching frequency. This frequency is kept

constant in a PWM control and is chosen to be in a few kilohertz to a few hundred

kilohertz range. When the amplified error signal which varies very slowly with time

relative to the switching frequency, is greater than the saw-tooth waveform, the switch


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control signal becomes high, causing the switch to turn on. Otherwise, the switch is

off. In terms of V control and the peak of the saw-tooth waveform, the switch duty

ratio can be expressed as

2.4 PROPOSED SYSTEM

The proposed converter has several features: 1) The connection of the two

pairs of inductors, capacitor, and diode gives a large step-up voltage-conversion ratio;

2) the leakage-inductor energy of the coupled inductor can be recycled, thus

increasing the efficiency and restraining the voltage stress across the active switch;

and 3) the floating active switch efficiently isolates the PV panel energy during non

operating conditions, which enhances safety. The operating principles and steady-

state analysis of the proposed converter are presented.

Fig 2.7Potential difference on the output terminal of non floating switch

micro inverter

Photovoltaic (PV) power-generation systems are becoming increasingly

important and prevalent in distribution generation systems. A conventional centralized

PV array is a serial connection of numerous panels to obtain higher dc-link voltage for

main electricity through a dcac inverter. Unfortunately, once there is a partial

shadow on some panels, the systems energy yield becomes significantly reduced. An

ac module is a micro inverter configured on the rear bezel of a PV panel this


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alternative solution not only immunizes against the yield loss by shadow effect, but

also provides flexible installation options in accordance with the users budget.

Many prior research works have proposed a single-stage dcac inverter with

fewer components to fit the dimensions of the bezel of the ac module, but their

efficiency levels are lower than those of conventional PV inverters.

2.5 OPERATING PRINCIPLES OF THE PROPOSED CONVERTER

The simplified circuit model of the proposed converter is shown in Fig. 2.8.

The coupled inductor T1 is represented as a magnetizing inductor Lm, primary and

secondary leakage inductors Lk 1 and Lk 2 , and an ideal transformer. In order to

simplify the circuit analysis of the proposed converter, the following assumptions are

made.

Fig 2.8Polarity definitions of voltage and current in proposed converter

1) All components are ideal, except for the leakage inductance of coupled

inductor T1, which is being taken under consideration. The on-state resistance RDS(ON)

and all parasitic capacitances of the main switch S1are neglected, as are the forward

voltage drops of diodes D1D3.2) The capacitors C1C3are sufficiently large that the voltages across them

are considered to be constant.

3) The ESR of capacitorsC1 C3 and the parasitic resistance of coupledinductor T1 are neglected.

4) The turns ratio n of the coupled inductor T1windings is equal to N2/N1.


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2.5.1 Continuous Conduction Mode Operation

The operating principle of continuous conduction mode (CCM) is presented in

detail. The current waveforms of major components are given in Fig. 2.9. There are

five operating modes in a switching period. The operating modes are described as

follows.

Fig 2.9Some typical waveforms of proposed converters at CCM operation.

Mode I [t0 , t1]: In this transition interval, the magnetizing inductor Lm

continuously charges capacitor C2through T1when S1is turned ON. The current flow

path is shown in Fig. 2.10; switch S1and diode D2are conducting. The current iLmis

decreasing because source voltage Vincrosses magnetizing inductor Lmand primary

leakage inductor Lk1; magnetizing inductor Lm is still transferring its energy through


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coupled inductor T1to charge switched capacitor C2, but the energy is decreasing; the

charging current iD2 and iC2 are decreasing. The secondary leakage inductor current

iLK2is declining as equal to iLm/ n. Once the increasing iLk1equals decreasing iLmat t

= t1, this mode ends.

Fig 2.10Mode I of proposed converter in Continuous Conduction Mode

Operation

Mode II[t1 , t2]: During this interval, source energy V in is series connected with

N2 , C1 , and C2 to charge output capacitor C3and load R; meanwhile magnetizing

inductor Lm is also receiving energy from Vin.

Fig 2.11Mode II of proposed converter in Continuous Conduction Mode

Operation

The current flow path is shown in Fig. 2.11, where switch S 1 remains ON, andonly diode D3 is conducting. The iLm, iLk1, and iD3are increasing because the V in is


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crossing Lk1, Lm, and primary winding N1; Lmand Lk1are storing energy from V in;

meanwhile Vin is also serially connected with secondary winding N2 of coupled

inductor T1, capacitors C1, and C2, and then discharges their energy to capacitor C3

and load R. The iin , iD3 and discharging current |iC1|and |iC2|are increasing. This

mode ends when switch S1is turned OFF at t = t2.

Mode III [t2 , t3]: During this transition interval, secondary leakage inductorLk2

keeps charging C3when switch S1is OFF.

Fig 2.12Mode III of proposed converter in Continuous Conduction ModeOperation

The current flow path is shown in Fig. 2.12, where only diode D1 and D3 are

conducting. The energy stored in leakage inductor Lk1 flows through diode D1 to

charge capacitor C1 instantly when S1 is OFF. Meanwhile, the energy of secondary

leakage inductorLk2is series connected with C2to charge output capacitor C3and the

load. Because leakage inductance Lk1 and LK2 are far smaller than Lm, iLk2 rapidly

decreases, but iLm is increasing because magnetizing inductor Lm is receiving energy

fromLk1. Current iLk2decreases until it reaches zero; this mode ends at t = t3.

Mode IV[t3, t4]:During this transition interval, the energy stored in magnetizing

inductorLmis released to C1and C2simultaneously. The current flow path is shown in

Fig. 2.13. Only diodesD1andD2are conducting. Currents iLk1and iD1are continually

decreased because the leakage energy still flowing through diodeD1keeps charging

capacitor C1. TheLmis delivering its energy through T1andD2to charge capacitor C2.


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The energy stored in capacitor C3is constantly discharged to the loadR. These energy

transfers result in decreases in iLk1and iLmbut increases in iLk2. This mode ends when

current iLk1is zero, at t = t4.

Fig 2.13Mode IV of proposed converter in Continuous Conduction Mode

Operation

Mode V[t4 , t5]:During this interval, only magnetizing inductor Lmis constantly

releasing its energy to C2. The current flow path is shown in Fig. 2.14, in which only

diodeD2is conducting. The iLm is decreasing due to the magnetizing inductor energy

flowing through the coupled inductor T1to secondary windingN2andD2continues to

charge capacitor C2. The energy stored in capacitor C3is constantly discharged to the

load R. This mode ends when switch S1 is turned ON at the beginning of the next

switching period.

Fig 2.14Mode V of proposed converter in Continuous Conduction Mode

Operation


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2.5.2 Discontinuous Conduction Mode Operation

The detailed operating principles for discontinuous conduction mode (DCM)

operation are presented in this section. Fig. 2.15 depicts several typical waveforms

during five operating modes of one switching period. The operating modes are

described as follows.

Fig 2.15Some typical waveforms of proposed converters at DCM operation.

Mode I [t0 , t1]: During this interval, source energy V in is series connected

with N2 , C1, and C2 to charge output capacitor C3 and load R; meanwhile,

magnetizing inductor Lm is also receiving energy from Vin. The current flow path is

shown in Fig. 2.16, which depicts that switch S1 remains ON, and only diode D3 is

conducting. The iLm, iLk1, and iD3are increasing because the Vinis crossing Lk1, Lm,

and primary winding N1; Lmand Lk1are storing energy from Vin; meanwhile, Vinalso


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is serially connected with secondary winding N2of coupled inductor T1 , capacitors

C1, and C2; then they all discharge their energy to capacitor C3and load R. The iin, iD3

and discharging current |iC1| and |iC2| are increasing. This mode ends when switch S1

is turned OFF at t = t1.

Fig 2.16Mode I of proposed converter in Discontinuous Conduction Mode

Operation

Mode II [t1 , t2]: During this transition interval, secondary leakage inductor

Lk2keeps charging C3when switch S1is OFF. The current flow path is shown in Fig.

2.17, and only diode D2and D3are conducting.

Fig 2.17Mode II of proposed converter in Discontinuous Conduction Mode

Operation

The energy stored in leakage inductor Lk1 flows through diode D1 to charge

capacitor C1 instantly when S1 is OFF. Meanwhile, the energy of secondary leakage


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inductor Lk2 is series-connected with C2 to charge output capacitor C3and the load.

Because leakage inductance Lk1 and LK2 are far smaller than Lm, iLk2 decreases

rapidly, but iLm is increasing because magnetizing inductor Lm is receiving energy

from Lk1. Current iLk2reduces down to zero, and this mode ends at t = t2.

Mode III[t2, t3]: During this transition interval, the energy stored in coupled

inductor T1 is releasing to C1 and C2. The current flow path is shown in Fig. 2.18.

Only diodes D1and D2are conducting. Currents iLk1and iD1are continually decreased

because leakage energy still flowing through diode D1keeps charging capacitor C1.

The Lmis delivering its energy through T1and D2to charge capacitor C2.The energy

stored in capacitor C3 is constantly discharged to the load R. These energy transfers

result in decreases in iLk1and iLmbut increases in iLk 2. This mode ends when current

iLk1reaches zero at t = t3.

Fig 2.18Mode III of proposed converter in Discontinuous Conduction Mode

Operation

Mode IV [t3 , t4]: During this interval, only magnetizing inductor Lm is

constantly releasing its energy to C2. The current flow path is shown in Fig. 2.19, and

only diode D2 is conducting. The iLm is decreasing due to the magnetizing inductor

energy flowing through the coupled inductor T1 to secondary winding N2 and D2

continues to charge capacitor C2. The energy stored in capacitor C3 is constantly

discharged to the loadR. This mode ends when current iLmreaches zero at t = t4.


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Fig 2.19Mode IV of proposed converter in Discontinuous Conduction Mode

Operation

Mode V[t4 , t5]:During this interval, all active components are turned OFF;

only the energy stored in capacitor C3is continued to be discharged to the loadR. The

current flow path is shown in Fig. 2.20. This mode ends when switch S1is turned ON

at the beginning of the next switching period.

Fig 2.20Mode V of proposed converter in Discontinuous Conduction Mode

Operation

2.6 SUMMARY

This chapter discusses the introduction about DC-DC converter, different

types of DC-DC converters and control of DC-DC converters. Here the proposed

converter and its continuous conduction mode of operation and discontinuous

conduction mode of operations are explained.

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Safety-enhanced-high-step-up-DC-DC-converter-for-ac-photovoltaic-module-applications Chapter 2 Dc-dc Converters 12 Page