Transcription
Flyback ConverterProject report submitted in partial fulfillment of the requirements ofBachelor of TechnologyBy,Anurag Gupta120070029Guide: Professor Mukul C. ChandorkarDepartment of Electrical EngineeringIndian Institute of Technology, BombayApril, 2016
Table of ContentsTable of Contents . 21Introduction . 41.1Buck-Boost converter . 41.1.11.2Flyback converter . 51.2.123Principle of operation . 5Principle of operation . 5Flyback converter for Modular Multilevel converter . 62.1Rating. 72.2TNY279 Functional description . 72.3TNY279 Operation . 82.4Feedback circuit . 92.5Current limit state machine . 102.6Schematic . 112.7PCB layout . 132.8Testing . 142.9Application . 172.9.1Pre-charging of module capacitors . 182.9.2Design modification . 202.9.3Challenges . 21Flyback converter for powering Nixie tubes . 223.1Nixie tubes . 223.2Rating. 223.3Multi-output flyback converter . 223.3.1Motivation. 223.3.2Design specification . 223.3.3Transformer design . 253.3.4Results . 273.4USB powered flyback converter . 283.4.1Design. 293.4.2Transformer design . 29
3.4.33.54Results . 30Conclusion . 32Reference . 33
1 IntroductionFlyback converter (Figure 1) is a dc-dc converter topology derived from buck-boost converter(Figure 2) with inductor split up to form a transformer for galvanic isolation between inputand output. Section 1.1 describes the working of buck-boost converter followed bydescription of flyback converter in Section 1.2.Figure 1: Flyback converterFigure 2: Buck-Boost converter1.1 Buck-Boost converterA buck-boost converter has an output voltage that is either greater than or less than inputvoltage depending on duty cycle of switching pulse. Its voltage gain expression is given byπππ’π‘π· πππ1 π·πππ’π‘ : ππ’π‘ππ’π‘ π£πππ‘πππ πππππ π πππππππ‘ππ πΆ1πππ : πΌπππ’π‘ π ππ’πππ π£πππ‘πππ π1π·: ππ’π‘π¦ ππ¦πππ ππ π π€ππ‘πβπππ ππ’ππ π
1.1.1 Principle of operationLet us assume that buck-boost converter is operating in continuous conduction mode (CCM)for analysis. During steady state, voltage Vout appears across the capacitor (C1) and a non-zeroaverage current flows through the inductor (L1). The basic operation of buck-boost convertercan be understood by analyzing the two states of switch (Q1).When the switch (Q1) turns ON, input voltage is directly connected to inductor L1, ignoringthe on state resistance of switch, and diode D1 gets reverse biased. This leads to rise in currentthrough inductor governed by expressionπππ πΏ ( )ππ‘π: ππππ‘πππ πππππ π ππππ’ππ‘πππΏ: ππππ’π ππ ππππ’ππ‘πππππ: πΌπππ’ππ‘ππ ππ’πππππ‘When the switch (Q1) turns OFF, current through inductor cannot immediately die down tozero, hence, diode (D1) starts conducting due to Faradayβs law of electromagnetic inductionproviding a path for inductor (L1) to charge the output capacitor (C1).To derive voltage gain expression, we can use the condition that average voltage acrossinductor should be equal to zero (or else the inductor will burn). During ON state, ππΏ1 πππand during OFF state, ππΏ1 πππ’π‘ . Applying average voltage criteria, we getπππ π· π πππ’π‘ (1 π·) π 0π: ππππ ππππππ ππ π π€ππ‘πβπππ ππ’ππ π πππ’π‘π· πππ1 π·1.2 Flyback converterAs explained earlier, flyback converter is obtained by replacing inductor with transformer ina buck-boost converter. Corresponding voltage gain expression for flyback converter isπππ’π‘ π2π· ππππ1 1 π·π1 : ππ’ππππ ππ π‘π’πππ ππ π‘βπ πππππππ¦ π πππ ππ π‘ππππ πππππππ2 : ππ’ππππ ππ π‘π’πππ ππ π‘βπ π ππππππππ¦ π πππ ππ π‘ππππ ππππππ1.2.1 Principle of operationWe can analyze the two states of switch (Q1) for deriving the voltage gain expression in amanner similar to buck-boost converter.When the switch Q1 turns ON, input voltage appears across the primary side of transformer,thereby, increasing the energy stored in magnetizing inductance πΏπ of transformer. Because
of the shown dot polarities in Figure 1, negative voltage appears across the diode D1 and itdoes not conducts. During this state, capacitor (C1) satiates the current demand of load.When the switch Q1 turns OFF, current stored in Lm cannot instantaneously die down to zero.Hence, diode (D1) starts conducting because of the Faradayβs law of electromagneticinduction and transfer of energy from inductor to output capacitor (C1) takes place.Figure 3 illustrates the voltage and current waveform for ON and OFF state of switch (Q1). πΌπin the plot represents peak value of current through primary side of transformer (T1).Figure 3: Primary voltage, primary current, secondary current and output voltage waveform forPWM switching of flyback converterTo derive voltage gain expression, we can apply average voltage criteria on the primary sideof transformer (T1) to getπππ π· π πππ’π‘ π1 (1 π·) π 0π2πππ’π‘ π2π· ππππ1 1 π·2 Flyback converter for Modular Multilevel converterDuring first part of the project, a flyback converter which takes rectified input from an ACpower supply and produces a regulated output voltage was designed as shown in Figure 4. Afull bridge rectifier followed by a smoothing capacitor was used to obtain unregulated DCsupply for the flyback converter. Further, a transformer with turn ratio of 10:1, designed byWurth Electronik, and TNY279 switch plus controller IC from Power Integration was used forgalvanic isolation and output regulation respectively. Section 2.1 specifies the rating of flyback
converter followed by functional description and operation of TNY279 in Section 2.2 andSection 2.3 respectively. Design of feedback loop is discussed in Section 2.4 followed by tersedescription of current limit state machine feature of TNY279 switch in Section 2.5. Towardthe end, Section 2.6 and 2.7 covers the schematic of implemented design and its PCB layoutin Eagle.Figure 4: Flyback converter with TNY279 controller IC2.1 RatingInput: 85-265 VAC, 3.15 A.Output: 15 V, 1 A.2.2 TNY279 Functional descriptionFigure 5 and Figure 6 shows the package and functional block diagram of TNY279 controllerIC used for the design of flyback converter. Pin EN/UV, BP/M, D and S representsenable/under-voltage, bypass/multifunction, drain and source respectively.Figure 5: TNY279 package (Source: Power Integrations)
Figure 6: TNY279 functional block diagram (Source: Power Integrations)During ON state, current flows from D to S. BP/M is used to decouple internal power supplyand to decide global limiting value of current from drain to source by appropriate choice ofcapacitor between BP/M and S. An internal current limit state machine adaptively adjusts thelocal current limit for different loads. EN/UV pin decides the state of switch based on feedbackfrom the output voltage. It can also be used to detect under-voltage on the input side andshut down the MOSFET.2.3 TNY279 OperationDuring normal operation, input circuitry at EN/UV consists of a low impedance sourcefollower set at 1.2 V. If current through this terminal exceeds the threshold value of 115 Β΅A,a logic 1 is generated at the output of this circuitry otherwise a logic 0 is generated. Based onthe output of this logic, generated at the rising edge of internally generated 132 kHz signal,state of the switch is controlled. If logic 1 is sampled on the rising edge, MOSFET is turned offotherwise itβs turned on. During the cycle when MOSFET is turned on, drain current keepsincreasing and MOSFET is turned off as soon as this currents reaches the drain-source currentlimit as shown in Figure 7. Note that this current limit is updated by current limit statemachine based on previous cycles and is explained later.
Figure 7: TNY279 switching waveform (Source: Power Integrations)2.4 Feedback circuitUnlike PWM mode, TNY279 uses on/off method to regulate output voltage using externalfeedback circuitry. In a typical implementation, reverse breakdown voltage of zenerconnected in series with optocoupler LED decides the regulated output voltage as shown inFigure 8. When output voltage exceeds the target regulated value, LED starts to conduct andoptocoupler pulls the EN/UV pin to zero leading to turning off of switch. To set a regulatedoutput voltage of 15 V, zener diode (ZD1) with reverse breakdown voltage of 15 V was chosenfor the design. Resistance (R3) precludes damage to optocoupler by circumscribing thecurrent flowing through LED.Figure 8: Feedback circuit for TNY279
2.5 Current limit state machineThe current limit state machine reduces the current limit β for comparison with drain currentwhen MOSFET is in on state β when output is connected to light load. This increases thefrequency of switching and allays the associated audible noise due to magnetostrictionphenomenon in transformer. The state machines observes the past switching cycles ofMOSFET to determine the load condition and updates current limit in discrete steps. Figure 9and Figure 10 represents the state machine adaptation to different load conditions.Figure 9: Variation in drain current limit for moderately heavy load (Source: Power Integrations)Figure 10: Variation in drain current limit for very light load (Source: Power Integrations)
2.6 SchematicEagle 7.3 was used to design schematic (Fig. 8) for designed flyback converter.Figure 11: Eagle schematic layout for flyback converterOverall schematic can be understood by understanding its subparts as illustrated in Figure 12:Full bridge rectifier followed by pi filter - Figure 14. Subpart corresponding to Figure 12represents a full bridge rectifier followed by pi filter to generate unregulated DC supply. F1, afuse of rating 3.15 π΄, breaks supply to circuit in the event of a fault. LED1 is meant to indicateon/off state of input. IN4007, with rating of 700 V RMS voltage, was chosen for AC rectificationkeeping in mind the maximum voltage across diodes.Figure 12: Full bridge rectifier followed by pi filter
Figure 13 represents the snubber circuit on the primary side of transformer to prevent voltagespike, during transition of states. Use of zener clamp and parallel RC optimizes both EMI andenergy efficiency.Figure 13: Snubber circuit for primary windingRemaining subpart of the schematic represents a DC-DC flyback converter topology as shownin Figure 14. Additional circuitry like 3.6 MΞ© resistance facilitates under voltage protection;additional bias winding on transformer provides overvoltage protection in the event of openfeedback loop faults; indicator LED indicates the state of output.Figure 14: DC-DC flyback converter
In this project, transformer ratio of 10:1 is used for designing a flyback converter with 15 Vregulated output and 1 π΄ current rating. Therefore, reverse breakdown voltage of zenerdiode (in this case 15 V), connected between optocoupler input and output voltage, plus theoptocoupler LED forward drop should be such that when the output exceeds 15 V, currentshould flow in the LED of optocoupler. This would result in current greater than 115 Β΅A tosink from EN/UV pin of TNY279 switch, turning the MOSFET off. Note that transformer ratioof 10:1 was chosen in accordance with zener clamping circuit. As per design criteria of zenerclamping circuit, if clamping voltage of zener diode is around 150 V, output voltage of 12 Vwhen reflected on primary would be close to but less than 150 V.2.7 PCB layoutEagle 7.3 was used for designing PCB layout as shown in Fig. 12. Basic considerations whiledesigning the layout show in Figure 15 were as follows: Minimizing distance between positive and negative terminals of AC source to reducestray inductance.Minimum separation of 2 cm between ground plane of input and output.Large distance between input and output connector for safety.Figure 15: Board layout of the flyback converter
2.8 TestingIn order to characterize output voltage regulation, input voltage sweep from 0 - 230 V (r.m.s)was carried out using variac for resistive load of 16 Ξ©, 25 Ξ©, 40 Ξ©, 55 Ξ©, 80 Ξ©, 148 Ξ©, 200 Ξ©and no load. Multimeter was used to record input r.m.s voltage and output dc voltage. Errorin regulated output voltage is 2% which is within acceptable limit for the application of IGBTgate driver. Figure 16 - Figure 23 shows the plot of output voltage for input voltage sweepwith different load conditions.Figure 16: Voltage sweep of flyback converter (No load)Figure 17: Voltage sweep of flyback converter (Rload 200 Ξ©)
Figure 18: Voltage sweep of flyback converter (Rload 148 Ξ©)Figure 19: Voltage sweep of flyback converter (Rload 80 Ξ©)
Figure 20: Voltage sweep of flyback converter (Rload 55 Ξ©)Figure 21: Voltage sweep of flyback converter (Rload 40 Ξ©)
Figure 22: Voltage sweep of flyback converter (Rload 25 Ξ©)Figure 23: Voltage sweep of flyback converter (Rload 16 Ξ©)2.9 ApplicationFlyback converter, in this section, was designed for the purpose of driving IGBT gate fromoutput of module capacitor voltage in Modular Multilevel Converter (MMC). A MMC is a
power electronic device which can generate as many level of output voltage as the numberof modules in one leg. Functional diagram of a three phase MMC is illustrated in Figure 24.Figure 24: Three phase Modular Multilevel Converter (Source: Modular Multilevel Converter,Modulation and Control, Sreejith M.R.)2.9.1 Pre-charging of module capacitorsOne of the challenge associated with MMC is that of pre-charging its module capacitors. Useof auxiliary power supply makes the process cumbersome and expensive. Therefore, activeresearch is undergoing in an attempt to pre-charge module capacitor from main power supplyitself. It is achieved in two stage: in the beginning, an uncontrolled pre-charging of modulecapacitors is initiated through diodes of MOSFET present in series with capacitor (ref. Figure25 and Figure 26). After a certain threshold voltage is achieved, flyback converters attachedat the output of module capacitors are employed for controlled pre-charging using sortingalgorithm. Flyback converter was chosen for this low power application because of itsrequirement for less no. of components.
Figure 25: Half bridge cell of a MMC (Source: Modular Multilevel Converter, Modulation and Control,Sreejith M.R.)Figure 26: Full bridge cell of a MMC (Source: Modular Multilevel Converter, Modulation and Control,Sreejith M.R.)The problem with abovementioned technique for controlling gate drives is unstable voltageimbalance across module output due to negative resistance characteristics of flybackconverter. Say, a small voltage imbalance of Ξπ takes place across module 1 and module 2 oflimb 1. This creates a voltage π Ξπ and π Ξπ at the output of module 1 and module 2respectively. Because of this small perturbation, more current will be drawn by flybackconverter from module 1 (negative resistance characteristics of flyback converter) and lessercurrent will be drawn from module 2. This leads to further deterioration of voltage difference.
One way to circumnavigate the problem is by producing unregulated output in the range of30-40 V from flyback converter followed by a linear regulator such as 7815. Section 2.9.2discusses two possible types of modification in the existing flyback converter foraccommodating this feature.2.9.2 Design modificationA crude way to generate desired deregulation at output is to add a P-MOSFET in series withTNY279 switch and an N-MOSFET in parallel to this configuration as shown in Figure 27.Reverse breakdown voltage of feedback zener diode (ZD1) is increased to 35 V. During startingphase, P-MOSFET (Q1) remains ON providing a path for TNY279 to bring up the output voltageto 35 V. An external logic immediately turns off the P-MOSFET at this instant and the NMOSFET (Q2) starts to regulate output voltage in the range of 30 β 40 V using hysteresiscontrol. Logics for MOSFET Q1 and Q2 are not shown in figure for the sake of clarity.Figure 27: Modified version 1 of flyback converter feedback controlThe only drawback of this technique is increase in the number of components leading to highcost of setup.A second technique is proposed herein which eliminates the need of a P-MOSFET, a majorcontributor to the total cost of previous setup. BP/M pin of TNY279 IC which provides theutility of overvoltage protection is exploited here. When the required output of 35 V isachieved, BP/M is shorted to pin S by use of an external N-MOSFET, thereby, shutting downthe IC. From then on, an external logic based on output voltage feedback, roughly regulatesthe output in the range of 30 β 40 V. Although total number of components for the setupremains the same, total cost reduces significantly because of replacement of P-MOSFET withN-MOSFET which is lot cheaper. Figure 28 illustrates the schematic of second idea.
Figure 28: Modification version 2 of flyback converter feedback control2.9.3 ChallengesAlthough design modifications in previous subsection eliminates the problem of instability, itis inefficient due to use of linear regulator for high power application. Therefore, steps arebeing undertaken to utilize a modified version of sorting algorithm for control.
3 Flyback converter for powering Nixie tubes3.1 Nixie tubesA nixie tube, or cold cathode display, is an electronic device for displaying numerals or otherinformation using glow discharge. It operates on 180 π, 0.001 β 0.002 π΄ input power source.Here, in this section, we will design various dc-dc converter for powering nixie tube anddiscuss their advantages and disadvantages.3.2 Ratingπππ : 180 ππΌππ : 2 ππ΄3.3 Multi-output flyback converter3.3.1 MotivationA flyback converter with multi-output terminals is proposed which takes input from 220 VACpower supply and generates output of 180 V and 5 V at its two output terminal. 180 V is usedto power nixie tube whereas 5 V supplies power to micro-controller. In order to regulateoutput, 5 V output terminal is fed back to control circuit as nixie tubes are more tolerable toripples in voltage than micro-controller. Figure 29 represents the topology for multi-outputflyback converter.Figure 29: Multi-output flyback converter3.3.2 Design specificationπππ : 220 ππ΄πΆπππ’π‘1 : 180 π, 2 ππ΄ (π ππππππππ¦ π€ππππππ)πππ’π‘2 : 5 π, 2 π΄ (π‘πππ‘ππππ¦ π€ππππππ)
ππ : 100 ππ»π§3.3.2.1 Continuous conduction modeTo begin with the design of flyback converter letβs assume N1 : N2 320 : 181 (becausecapacitor voltage on primary side of transformer 220 2 320 and we have assumed 1V diode drop on secondary side), N1 : N3 320 : 6 and D 0.5 as a rule of thumb for CCM.Peak value of secondary current comes out to be 0.008 A (refer waveform in Figure 30) usinglaw of energy conservation for one cycle,πππ’π‘ (π‘ππππ ππππππ) π(πππππ) π ππππ (πππ’π‘ 1) (1 π·) ππ· (1 π·) πππ’π‘ ππππ 181 0.5 1 0.5 180 0.00222 πΌπ ππππ 0.008 π΄Let us consider 10 % current ripple in magnetizing inductance. From this we can obtain thatvalue of inductance reflected on secondary side of transformer,πππ’π‘ 110 (1 π·) π 0.008πΏ100 πΏ 1.13 π»1.13 H of inductance will make the size of converter bulky, hence, we cannot proceed withour design in CCM.Figure 30: Voltage and current waveform for flyback converter in CCM3.3.2.2 Discontinuous conduction modeGiven the infeasible value of magnetizing inductance obtained in CCM, letβs start with 1 mHas suitable value for magnetizing inductance in DCM. Assuming, as before, turn ratio π1 : π2
320: 181 and π1 : π3 320: 6 (keeping in mind 1 V drop across output diode). Fromππequation π πΏ ππ‘ we obtain,ππ π· π πΌππππππΏπππ : πππππππ¦ π πππ π£πππ‘ππππΏπ : ππππππ‘ππ§πππ ππππ’ππ‘ππππ ππ πππππππ¦ π ππππΌπππππ : ππππ π£πππ’π ππ πππππππ¦ ππ’πππππ‘π·: π·π’π‘π¦ ππ¦πππAnd from law of conservation of energy we get,ππ πΌπππππ π· 181 0.002 6 22Dividing above two equations we obtain value of peak secondary current,πΌπππππ 0.5 π΄Substituting πΌπππππ in any of the two equations we get,π· 0.15625Note that obtained value of duty cycle is applicable for full load condition only. Therefore, aclosed loop control is mandatory for variable load. Figure 31 represents typical waveforms forDCM operation at steady state. Similarly, capacitance values can be obtained by satisfying thespecification of 1% ripple in voltage.Figure 31: Voltage and current waveform for flyback converter in DCM
3.3.3 Transformer designNow that basic parameters for the design have been derived, we can focus on the design oftransformer. One of the popular method in literature for the design of high frequencytransformer is area product approach. We will use the same procedure for our design. Weknow thatππΞπ΅ π΄π π1 ππ‘π· ππ· π π1 π1πππ₯ ΞB AcΞπ΅: πΆβππππ ππ πππ’π₯ ππππ ππ‘π¦ (0.3 π πππ βππβ πππππ’ππππ¦ ππππ)π1 π1 Similarly,π·1 πΞπ΅ π΄ππ·1 ππ3 π3πππ₯ Ξπ΅ π΄ππ·1 : πΉππππ‘πππ ππ π ππ¦πππ πππ π€βππβ ππ’π‘ππ’π‘ πππππ ππ ππ πππππ’ππ‘πππ πππππ2 π2πππ₯ In order to get a successful design, our windings should fit in the given window area.π. π. πΎπ€ π΄π€ π1 π1 π2 π2 π3 π3 π1 πΌ1πΌ2πΌ3 π2 π3 π½π½π½πΎπ€ : πππππππ ππππ‘ππ (0.4 πππ π‘ππππ ππππππ πππ πππ)πΌ1 : π . π. π. π£πππ’π ππ πππππππ¦ ππ’πππππ‘πΌ2 : π . π. π. π£πππ’π ππ π ππππππππ¦ ππ’πππππ‘πΌ3 : π . π. π. π£πππ’π ππ π‘πππ‘ππππ¦ ππ’πππππ‘π½: πΆπ’πππππ‘ ππππ ππ‘π¦ (3 π΄ ππ2 )π΄π€ : ππππππ€ ππππ ππ ππππSubstituting value of N1, N2 and N3 from previous equation, we getπΎπ€ π΄π€ π½ Ξπ΅ π΄π ππ π1πππ₯ π· πΌ1 π2πππ₯ π·1 πΌ2 π3πππ₯ π·1 πΌ3Divide and multiply R.H.S. with average current of each term πΎπ€ π΄π€ π½ Ξπ΅ π΄π ππ π1πππ₯ π· πΌ1ππ£π πΌ1πΌ1ππ£π πΎπ€ π΄π π½ Ξπ΅ ππ ππ1 πΌ1πΌ1ππ£π π2πππ₯ π·1 ππ2 π΄π : π΄πππ πππππ’ππ‘ (π΄π π΄π€ )πππ : π΄π£πππππ πππ€ππ ππ’π‘ππ’π‘ ( π 1, 2, 3)πΌ2πΌ2ππ£ππΌ2ππ£π πΌ2πΌ2ππ£π ππ3 π3πππ₯ π·1 πΌ3πΌ3ππ£ππΌ3ππ£π πΌ3πΌ3ππ£π
For DCM operation mode, we can substitute the value on RHS to obtain14 π·4 (1 π·)πΎπ€ π΄π π½ Ξπ΅ ππ (ππ2 ππ3 ) ( )π33π: ππππππππππ¦ ππ ππππ£πππ‘ππ ( 90 %)πΌπππ 4 π· πππ π·πΆπ οΏ½οΏ½3On substituting values in equation above, we get π΄π 538 ππ4 . Standard table formagnetic characteristics of ferrite core recommends use of EE20/10/5 core with π΄π 1490 ππ4 , π΄π€ 47.8 ππ2 , π΄π 31 ππ2. Now, we need to calculate R.M.S. values forcurrent waveforms.2π·π1ππππΌ1 ( π‘) 0.117 π΄π 0πΏππΌ2 π1 πΌ 0.207 π΄π2 1πΌ3 π1 πΌ 6.24π3 1Using R.M.S. value obtained in previous step, we can re-verify that our winding fits in thewindow area.π. π. π1 πΌ1πΌ2πΌ3 π2 π3 πΎπ€ π΄π€ 0.4 47.8 19.12π½π½π½π1 π1πππ₯ π· π320 0.16525 10 5 57ΞB Ac0.3 31 10 6181 57 323206π3 32 1181π2 57 0.1170.2076.24 32 1 10.927333As 10.927 19.12, winding will comfortably fit into available window area. We now needto find air gap required to achieve required inductance usingπ΄ππΏπ ππ 0.12 ππππ π0 π 2 ππ : π΄ππ πππ πππππ‘β
The last step is to find standard wire gauge for each winding and then the design process iscomplete.π1 πΌ1 0.039 ππ2 πππΊ 343π2 πΌ2 0.069 ππ2 πππΊ 313π3 πΌ3 2.08 ππ2 πππΊ 1533.3.4 ResultsFlyback converter designed in this section was simulated for full load in Simulink. Figure 32depicts the Simulink model for the multi-output flyback converter. Rectified 220 VAC inputwas fed to the input of converter and a closed loop control was implemented using PIcontroller block with 5 V as the reference voltage. Parameters πΎπ and πΎπΌ were set to 1 and 15respectively after successive trial and error method. Output error was given to a PWMgenerator block for generating switching pulses for MOSFET. Resulting voltage waveforms fortwo output terminals are shown in Figure 33 and Figure 34 and input current waveform atsteady state is shown in Figure 35. Peak value of 5 A input current during steady stateindicates that our design was correct.Figure 32: Simulink model for multi-output closed-loop flyback converter
Figure 33: Simulation waveform for 5 V output terminalFigure 34: Simulation waveform for 180 V output terminalFigure 35: Input current waveform for DCM operation at steady state3.4 USB powered flyback converterIn the early part of this project, idea of using USB port (5 V, 0.5 A power supply) to power nixietube was proposed using multi stage converter. If we were to use a single stage boostconverter, it would require 97.2 % duty cycle (D) for its steady state operation. Practically it isnot advisable to go beyond 90 % duty cycle, hence, a multi-stage design using boost converterwas proposed. It consisted of two cascaded boost converter: first stage boosts the inputvoltage from 5 V to 30 V followed by second stage which boosts it to 180 V. It had its own
demerits in terms of high input current and absence of galvanic isolation between its inputand output. Further, operation in continuous mode required large inductance value. In orderto tackle these problem, a flyback converter design operating is proposed. Again the problemof high inductance value exists if we were to operate in CCM, so, operational design in DCMis made in following section. Added advantage of powering3.4.1 DesignBased on the design principle followed in Section 3.3.2.2, we can apply law of energyconservation to obtain,π· π· 181 0.0022 πΏπ π·2 πΏπ 2896ππ ππ To obtain πΏπ , use the maximum input current limitation. Letβs keep πΌπππππ 0.3 π΄ to be onsafer side i.e. to avoid current overshoot to exceed 0.5 A during transient. More on this willbe discussed in results section. Therefore,ππ π· π 0.3πΏπ π· 6000 πΏπDividing above two equations, we obtainπ· 2896 0.483 πππ πΏπ 80.5 ππ»60003.4.2 Transformer designRefer to Section 3.3.3 for detailed derivation of transformer design. We can use the resultobtained to get area product for transformer using14 π·4 (1 π·)πΎπ€ π΄π π½ Ξπ΅ ππ ππ2 ( )π33 π΄π 0.623 17.3 ππ436000Standard table for magnetic characteristics of ferrite core recommends use of EE20/10/5 corewith π΄π 1490 ππ4 , π΄π€ 47.8 ππ2 , π΄π 31 ππ2. Now, we need to calculate R.M.S.values for current waveforms.2π·π1ππππΌ1 ( π‘) 0.21 π΄π 0πΏππΌ2 π1 πΌ 5.81 ππ΄π2 1Using R.M.S. value obtained in previous step, we can re-verify that our winding fits in thewindow area.
π. π. π1 πΌ1πΌ2πΌ3 π2 π3 πΎπ€ π΄π€ 0.4 47.8 19.12π½π½π½π1 π1πππ₯ π· π5 0.483 10 5 3ΞB Ac0.3 31 10 6π2 181 3 1095 3 0.210.00581 109 .42133As 0.421 19.12, winding will comfortably fit into available window area. We now need tocalculated air gap required to achieve given inductance usingπ΄ππΏπ ππ 4.35 10 3 ππππ π0 π 2 ππ : π΄ππ πππ πππππ‘βThe last step is to find standard wire gauge for each winding and then the design process iscomplete.π1 πΌ1 0.07 ππ2 πππΊ 303π2 πΌ2 0.00194 ππ2 πππΊ 4533.4.3 ResultsFlyback converter designed was simulated for full load in Simulink. Figure 36 depicts theSimulink model for the designed flyback converter. 5 V input was fed from a dc source and aclosed loop control was implemented using PI controller block parameters πΎπ and πΎπΌ set to.0001 an
In this project, transformer ratio of 10:1 is used for designing a flyback converter with 15 V regulated output and 1 current rating. Therefore, reverse breakdown voltage of zener diode (in this case 15 V), connected between optocoupler input and output voltage, plus the optocoupler LED forward drop should be such that when the output exceeds .
