Performance Analysis of a New Single Stage ACDC Converter for High Power Applications

Performance Analysis of a New Single Stage ACDC Converter for High Power Applications

Deepthi B1, J Mohan2

1PG student( Power Electronics & Drives),Dept. of EEE, 2Assistant Professor,Dept. of EEE, Ranganathan Engineering College, Coimbatore, Tamil Nadu, India

Abstract Most of the power conversion equipment employs rectifiers that are made of diodes or thyristor to convert AC voltage to DC voltage before processing it. Such rectifiers produce very poor power factor with a large displacement factor and strong harmonic currents as these power converters absorb energy from the AC line only when the line voltage is higher than the DC bus voltage. Single phase PFC is used in the industry to convert ac mains to output dc voltage. This is the case with low power applications. But in high power applications, the ac-dc conversion consists of two steps-first ac is converted to an intermediate dc. This dc voltage is converted to the required value. The proposedconverter is a single stage converter with excellent input power factor, and with reduced number of semiconductor switches. This converter is a combination of the best features of both single stage and two stage converters. This converter consists of two controllers. One controller is used to control input power factor and the other one to regulate the output voltage. The operation of the proposed converter is confirmed with the results obtained from simulation.

Key Words:Power factor correction, ac-dc conversion, full bridge converter,PWM technique, single stage converters.

1. INTRODUCTION

   Power electronics is afieldthat deals with converting an available form of energy from a power source to the form required by a load. A power converter can be of semiconductor switches such as diodes, MOSFETs and IGBTs to achieve this power conversion. Different types of power converters are AC/DC converter, DC/DC converter, DC/AC inverter or AC/AC converter [3] depending on the application. Many types of power sources can be used for these converters, such as AC single phase, AC three-phase, DC source, battery, solar panel, or an electric generator.

   For high power applications, the ac-dc converters

   [13] are implemented with two converter stages. The first stage is an ac-dc front [10] end boost converter stage that converts ac input voltage to dc intermediate voltage .The second stage is a dc-dc converter that converts this intermediate voltage to a required voltage. The proposed converter combines both PFC correction and dc-dc conversion. This converter has reduced size and cost is low.

   Single stage converters are common for low power applications. But for high power applications, single stage full bridge converters are rarely used. This is because it is difficult for single stage converters to perform PFC [5] and dc-dc conversion simultaneously for wide load variations.

   There are two types of single stage converters-current fed and voltage fed. In current fed single stage, there is a large input boost inductor [2] which causes ripples in output. But voltage fed converters have a large energy storage capacitor connected across the input section do not have this problem.

   In two stage converters there are two controllers to control each section. That is each section has a controller to regulate its output. But the single stage converters consists of only one controller. Therefore there is no controller to regulate the dc bus capacitor voltage that is on the primary side of the main power transformer in the converter. Due to absence of this converter, the primary side dc bus voltage can vary significantly with line and load conditions and it is difficult to shape the input current. This is the problem with all the previously proposed converters for high power applications.

   There are many previously proposed converters like

   [7] resonant converters. But these converters have problems with designing, tuning of the resonant converters and with the size of magnetic components. There are many dc bus capacitor voltage reduction techniques. Each dc bus voltage reduction method tries to affect the dc bus energy equilibrium in some way and all these reduction techniques has it disadvantages also.

   These dc bus voltage reduction techniques include the use of very low values of output inductance, using auxiliary windings taken from the main transformer primary to extend the converters duty cycle , or operating the converter with a semi-continuous input current. Since one-controller single stage converters have no controller to actively shape the input current, simultaneous input PFC and dc/dc conversion can only be performed by keeping the converter duty cycle xed over the entire input line cycle. PFC in this case can only be ensured if the input inductor is designed to be sufficiently small so that the input current is discontinuous for all converters operating conditions and naturally bounded by a sinusoidal envelope. Such small input inductance values, however, restrict the amount of load that the converter can operate with as the input current peaks are extremely high at very high loads and low ac line input. As a result, most single- stage PWM converters have a maximum load of less than 1 kW.Several one-controller single stage converters must operate with non-standard control techniques in order to be able to perform input PFC and dc/dc conversion simultaneously, but these techniques may be extremely sophisticated for many power electronics engineers. For example, the converters proposed in [17][20] cannot be

   operated with conventional phase-shift PWM and, instead, must operate with PWM techniques that are unique to the converters. In addition to these drawbacks, many single stage converters have increased conduction losses.

   A new voltage fed PWM ac-dc converter is proposed. It can work with excellent input power factor, standard and widely used control methods, continuous input and output current. Here bridgeless converter is used in the input section which reduces conduction losses.

   The proposed converter is a combination of a single stage converter and a two stage converter. Performance approaches that of two stage converter and cost approaches that of single stage converter. In this converter there are two controllers-one to regulate the output voltage and one for the input section of the converter to regulate the dc bus capacitor voltage and actively shape the input current.

2. OPERATION OF THE PROPOSED CONVERTER

   The proposed converter combines an input section that consists of input inductors Lin1and Lin2 and rectifying diodes D1 and D2 with a dc-dc section. Components D1,Lin1and

   D2, Lin2 form a bridgeless input. Blocking diodes Db1 and Db2 are included to prevent any dc circulating current. A dc blocking capacitor, Cb1 is connected in series with the transformer primary.

   Of the proposed converters two independent controllers, one is used to regulate the voltage across primary side dc bus capacitor Cb by sending appropriate gating signals to S2 and S4.The gating signals of S1and S3 are complimentary signals of S2 and S4respectively. The other controller is used to regulate the output voltage by setting an appropriate phase shift between the gating signals of S2 and S4.Based on the output signals from each controller; some simple logic can be used to develop the appropriate gating signal for each converter switch. Although the output signals of the two controllers are combined at the end of the gating signal generation process, the two controllers are independent of each other from the point of view of control.

   Fig 2.1 Proosed converter

3. MODES OF OPERATION OF THE PROPOSED CONVERTER

   The converters typical modes of operation are as follows:

   Mode 1(t0< t < t1):During this mode, the body diodes of switches S1 and S4, DS1 and DS4, respectively, are conducting. A positive voltage of Vbus, is impressed across the transformer leakage inductance. This results in the

   transformer primary currentIprichanging with a slope of

   bus voltage. The transformer primary current in this mode is given by

   L

   ipri t = Ipri t0 + Vbus t t0 for (t0 < < t1) (2)

   lk

   WhereIpri(t0) is the transformer primary current at time t0. At time t1, the transformer primary current reaches zero. The output inductor current during this mode is given

   di pri = Vbus

   by

   (1)

   dt Llk

   iLo t = IL0 t0 + V 0 t t0 for t0 < < t1 (3)

   WhereLlk is the leakage inductance of the transformer referred to the primary side and Vbus is the dc

   L0

   Where Lo is the output inductor, Vo is the output dc voltage and ILo(t0) is the current in the output inductor at

   time t0.Moreover, a positive voltage of |VMsin SuTk| is impressed across input inductor Lin1, and its current also starts to rise during this mode. VM is the peak ac input voltage, Suis the angular frequency of the input ac voltage and Tkis the kth switching cycle under consideration.

   Mode 2(t1 < t < t2):Att = t1, the current through the transformer primary reduces to zero, reverses, and starts flowing through switches S1 and S4. The transformer primary current during this mode is given by

   Mode 5(t4< t < t5):The voltage at the transformer primary reduces to zero at time t4. During this mode, the currents in both the transformer leakage inductor and the output inductor start decreasing. The transformer primary current during this mode is given by

   N2L +L

   ipri (t) = Ipri (t4) + NV0 (t – t4) for (t4< t <t5) (10)

   0 lk

   and the output inductor current reflected to the transformer

   i t = Vbus t t for t

   < < t (4)

   primary is as follows:

   pri

   Llk 1 1 2

   iLo t = ILo (t4) + NV0 (t – t4) for (t4< t <t5)

   The output inductor current during this mode is given by

   N2L0 +Llk

   (11)

   iLo t = ILo (t1) +V0 t t1 for (t1 < < t2) (5)

   Mode 6(t < t < t ):At the beginning of this mode, at

   L0 5 6

   At the end of this mode, the current in the transformer primary equals the current in the output inductor reflected on the transformer primary so that the freewheeling of the output inductor current ends at time t2 such that

   Ipri t2 = I L0 t2 (6)

   t=t5, switch S4 is turned off. The currents in the transformer and the input inductor Lin start charging and discharging the output capacitors of switches S4 and S3, CS4 and CS3, respectively, to and from Vbus. At the end of this mode, the body diode of S3 starts conducting. The duration of this mode is small enough so that the following relations hold.

   Ipri (t5) Ipri (t6) and ILo (t5) ILo (t6)

   WhereILo(t2) is the output inductor current reflected to the transformer primary.

   Mode 3(t2 < t < t3):This is an energy transfer mode as energy is transferred from the dc bus to the output through the transformer. A positive voltage of Vbus is

   (12)

   Mode 7(t6< t < t7):During this mode, the body diodes of switches S2 and S3, DS2 and DS3, respectively, are conducting so that a negative voltage of Vbus is impressed across the transformer leakage inductance. This results in the transformer primary current Ipri changing with a slope of

   impressed across the series combination of the transformer

   di pri = – Vb

   (13)

   leakage inductor. The transformer primary current during this mode is given by

   2 2 3

   ipri t = Ipri t + Vbus NV0 t t2 for (t <t<t )

   N2L0 +Llk

   (7)

   And the output inductor current reflected to the transformer primary is given by

   dt Llk

   So that the transformer primary current in this mode is given by

   L

   ipri (t) = Ipri (t6) Vbus (t – t6) for (t6< t <t7) (14)

   lk

   WhereIpri (t0) is the transformer primary current at time t0.

   The output inductor current during this mode is given by

   i t = I t + Vbus NV0 t t for (t

   < < t )

   iLo (t) = ILo (t6) V0 (t- t6) for (t6< t <t7)

   L0 L0 2

   N2L0 +Llk 2 2

   3

   (8)

   L0

   (15)

   Where N is the turns ratio of transformer primary and secondary turns..

   Mode 4 (t3< t < t4):Att = t3, the gate pulse of switchS1 reduces to zero and initiates the turn-off of switch S1.The current in the transformer starts charging and dischargingthe output capacitors of switches S1 and S2, CS1 and CS2,respectively. At the end of this mode, thebody diode of S2 starts conducting. The duration of this modes small enough so that the following relations hold.

   Ipri (t3) Ipri (t4) and IL0(t3) IL0(t4) (9)

   Also, during this mode, a net negative voltage of (|VM sin SuTk| Vbus) is impressed across input inductorLin1, and its current also starts to fall during this mode.

   Mode 8(t7< t < t8):Att = t7, the current through the transformer primary reduces to zero, reverses, and starts flowing through switches S2 and S3. The output inductor current still continues its freewheeling, similar to Mode-7. The transformer primary current during this mode is given by

   i t = Vbus (t – t ) for (t < t <t ) (16)

   From t1 and t4, a positive voltage of Vbus is incident across

   pri

   Llk 7 7 8

   the magnetizing inductance LM.

   The output inductor current during this mode is as follows:

   L

   iLo (t) = ILo (t7) + V0 (t – t7) for (t7< t <t8) (17)

   0

   At the end of this mode, the current in the

   transformerprimary equals the current in the output inductor that is reflected to the transformer primary so that the freewheeling of the output inductor current ends at time t8 such that

   |Ipri (t8)| = ILo (t8) (18)

   Mode 9(t8< t < t9):This is another energy transfer mode as energy is transferred from the dc bus to the output through the transformer. A negative voltage of Vb is impressed across the series combination of the transformer leakage inductor and the equivalent output inductor

   ILo (t11 ) ILo (t12 ) where t12 = Tsw + t0 (24)

   During this mode, a net negative voltage of (|VM sin SuTk|

   Vbus) remains impressed across theinput inductor Lin1 and its current continues to fall during this mode.From t10 to t12, the primary current remains at (/2) (VbusTSw/LM).

4. CONVERTER DESIGN

   1. Minimum Input Inductor Value

      Since the proposed converter is an acdc PFC converter that operates with continuous input current, the minimum value of input inductor that will ensure that the input inductor current is continuous over the entire operating range can be determined using the same equations as those used for equations standard PFC converters.

      reflected to the primary side. This causes the currents in

      VM 2Lin

      (25)

      transformer primary (in the direction opposite to that in Mode-3) and the output inductor Lo to rise during this mode. The transformer primary current during this mode is given by

      IM Tsw

      Where VM is the peak input ac voltage and IM is the peak input ac current. This equation is applicable to any ac dc PFC converter operating with continuous input current.

      i ( t ) = I

      (t ) Vbus NV0 ( t – t ) (19)

      For the purpose of analysis, it can be considered to be a

      pri

      pri 8

      N2L0 +Llk 8

      lossless resistance such as

      R = VM = V2 M

      and the output inductor current reflected to the transformer primary is as follows:

      e IM

      2Po

      (26)

      N2L +L

      iLo (t) = ILo (t8) Vbus NV0( t-t8)for (t8< t <t9) (20)

      0 lk

      Mode 0(t9< t < t10):Att = t9, the gate pulse of switch S2 reduces to zero and initiates the turn-off of switch S2. At the end of this mode the body diode of S1 starts conducting so that S1 can be turned on with ZVS. The duration of this mode is smallenough so that the following

   2. DC Bus Capacitor Design

   The dc bus capacitor is designed in the same manner as it would be for a conventional two-stage converter. The minimum value of the dc bus capacitor to satisfy this condition for the given converter specifications can be calculated as

   b (27)

   C = Po 700 F

   4fline ,min V2 (V bus Il ×r ESR )

   relations hold.

   Ipri (t9) Ipri ( t10 ) and ILo (t9 ) ILo (t10 ) (21)

   Mode 11(t < t < t ):The voltage at the

   where

   bus

   l

   I = Po

   Vbus 2

   V bus

   min

   10 11

   transformer primary reduces to zero at time t10. During this mode, the currents in both the transformer leakage inductor (in the direction opposite to that in Mode-4) and the output inductor start decreasing. The transformer primary current during this mode is given by

   (28)

5. EXPERIMENTAL RESULTS

   The simulation is done with an input ac voltage

   N2L + L

   ipri (t) = Ipri (t10 )+ NV0 (t – t10 ) for ( t10 < t <t11 )

   0 lk

   Vin,ac

   = 85V. The simulation is done with:Lin1

   = Lin2

   = 80 H,

   (22)

   and the output inductor current reflected to the transformer primary is given by

   N2L +L

   iLo (t) = ILo (t10 ) + NV0 ( t-t10 ) for (t 10< t <t11 )(23)

   0 lk

   Mode 12(t11< t < t12):At the beginning of this

   mode, at t = t11, switch S3 is turned off. The current in the transformerstarts charging and discharging the output capacitorsof switches S3 and S4, CS3 and CS4, respectively, to andfrom Vbus. The duration ofthis mode is small enough so that the following relation should

   Ipri (t11 ) Ipri (12) and

   Lo = 2.5 H, Co = 20mF.

   The gating waveforms of all the converter switches are shown in Fig.6.1. It can be seen that the gating signals of each pair of switches S1 S2 and S3 S4 in a converter leg are complimentary to each other, and that the gating waveforms of S1 and S4 are displaced by a phase-shift.

   Fig 6.1 Gating Signals

   The input voltage waveform obtained is shown in Fig.6.2.It can be seen that the input voltage is 85V AC. A pure sinusoidal input supply is given to the circuit. The sinusoidal signal is shown in Fig 6.2through scope.

   Fig 6.2 Input voltage waveform

   The output voltage waveform obtained is shown in Fig.6.3. From the output waveform it can be seen that the output is pure DC. The output voltage=15.36V.

   Fig 6.3 Output voltage waveform

   The voltage across the inductance Lin 1 is shown in Fig.6.4. A voltage measurement is connected across the input inductance and the voltage is verified using scope.

   Fig.6.4 Voltage waveform across inductanceLin1

6. CONCLUSION

A new single stage ac-dc converter with two controllers is developed for high power applications. The proposed converter has reduced size, low cost and excellent input power factor. The complexity in designing the

converter for high power applications is also reduced .The new converter is implemented using two controllers-one is used to actively control input power factor and the intermediate bus voltage, the other is used control output voltage. This converter is a combination of both single stage and two stage converters. The bridgeless PFC topology removes the input rectifier conductionlosses and is ableto achieve higher efficiency.

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B.DEEPTHI presently pursuing M.E in Power Electronics & Drives, at Ranganathan Engineering College, Coimbatore. Her areas of interest are power electronic inverters & converters, power quality and FACTS

J.MOHAN presently working as Assistant Professor in the department of Electrical and Electronics Engineering at Ranganathan Engineering College,Coimbatore. His areas of interest are power electronic converter & inverters, AC& DC drives, power quality and Renewable energy.