Sıfır gerilimde anahtarlamalı, faz kaydırmalı rezonant PWM kontrollü, tam köprü DC-DC dönüştürücü tasarımı ve uygulanması
Design and application of zero voltage switching, phase shifted resonant PWM controlled, full bridge DC-DC converter
Yazar: AKİF HAKKI POLAT
Danışman: DOÇ. DR. ÖZGÜR ÜSTÜN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Bu çalışmanın konusu; tam köprü DC-DC dönüştürücülerin gelişmiş bir versiyonu olan faz kaydırmalı rezonant pwm tekniği ile kontrol edilen, yüksek verimli tam köprü DC-DC dönüştürücü tasarımı ve uygulamasının yapılmasıdır. 
Tezin asıl konusu olan tam köprü dönüştürücü, DC giriş gerilimi ve DC çıkış gerilimine sahiptir. Tezin uygulanması esnasında kolaylık sağlaması açısından ve yüksek güçte ve gerilimde laboratuvar tipi DC güç kaynaklarının çokça kullanılmaması sebebiyle, tasarlanan devrenin girişi AC şebekeden bir güç faktörü düzeltici yükseltici dönüştürücü ile beslenmektedir. Bu sayede şebekeden çalışma esnasında çekilen akımın düşük harmonik değerlerine sahip olması ve şebeke tarafında bozucu etkisinin minimum düzeye inmesi sağlanmaktadır. Yükseltici dönüştürücünün çıkışı olan yüksek DC gerilim ise sıfır gerilimde anahtarlamalı tam köprü DC-DC dönüştürücünün giriş katını oluşturmaktadır. Her iki devrenin de kontrol katının beslemesi flyback tipi bir DC-DC dönüştürücü ile sağlanmaktadır. Flyback dönüştürücü, yükseltici dönüştürücünün çıkışı ve dolayısıyla SGA tam köprü dönüştürücünün girişindeki yüksek DC gerilimden beslenmekte ve yapısı gereği izole çıkış gerilimlerini oluşturmaktadır. Kontrol entegreleri, flyback DC-DC dönüştürücü tarafından oluşturulan izole beslemelerin gerekli noktalara referanslanmasıyla beslenmektedir.
Tasarlanan tam köprü DC-DC dönüştürücü 1900 W gücündedir ve 380 V DC giriş geriliminden beslenmektedir. 57.6 V çıkış gerilimi ve 33 A çıkış akımı değerlerine sahiptir. Çıkış gerilimi, özellikle dört adet kurşun-asit akünün boost şarj gerilimine eşit olacak şekilde seçilmiştir. Bu sayede devrenin çıkışı hem bir DC gerilim kaynağı olarak kullanılabilmekte, hem de akü şarj cihazı olarak kullanılabilecek şekilde bir akım limiti fonksiyonu eklenmesine imkan  sağlamaktadır. Çıkış gerilimi, giriş geriliminden izoledir. İzolasyon ise tam köprü DC-DC dönüştürücü topolojisi sayesinde bir yüksek frekanslı trafo ile sağlanmaktadır. 
DC gerilim önce yüksek frekanslı trafo üzerinde 75 kHz frekansında bir kare dalga AC gerilime dönüştürülmekte, trafonun sekonderinde ise dönüştürme oranında indirgenmiş bu gerilim, yüksek frekansta çalışabilen güç diyotları aracılığıya doğrultulmaktadır. Doğrultulduktan sonra ise bir bobin ve DC elektrolitik kondansatörler vasıtasıyla oluşturulan bir alçak geçiren filtreden geçirilerek tam DC gerilim çıkışta oluşturulmaktadır. Bu devrede kullanılan yüksek frekanslı trafo sayesinde hem gerilim dönüşümü kolayca giriş ve çıkış gerilimleri arasında yapılabilmekte hem de izolasyon istenen uygulamalarda doğal olarak izolasyon sağlanmaktadır.

Abstract:
In this thesis project, design and application of an advanced version of the classical full bridge DC-DC converter is realized. Unlike its classical counterpart, phase shifted resonant pwm control technique is used to achieve zero voltage turn on of the power switches of the converter. In the classical full bridge DC-DC converter conventional dc pwm technique is used to regulate the output voltage and the turn on transition of the power switches are made with voltage on the switch. This kind of switching is called "hard switching". In the phase shifted full bridge converter, switches start conduction with lower or zero voltage on them as compared with the "hard switching" version. This type of switching is called "soft switching". By the utilization of the soft switching technique the efficiency of the converter is improved and electromagnetic interference effects of the power converter is also minimized.
The power stage of the thesis project is realized by two main converters. These are power factor corrected boost converter and the phase shifted full bridge converter. The main focus of this thesis is the phase shifted full bridge converter but for the sake of the implementation easiness the PFC boost converter is used as a preregulator. DC bus voltage is created by the PFC boost converter. The full bridge converter is operated from that DC bus voltage. In the laboratory tests, it is usually hard to find high voltage and high power DC power sources. PFC boost converter's regulated output voltage and high power factor sinusoidal input current features are utilized during the laboratory tests of the designed converter. Also big amount of field applications, phase shifted full bridge converters are implemented with a preregulator.
Supply voltages of the control stage is created by a flyback converter, which is fed from the DC bus voltage. Flyback converter's outputs are also isolated and referenced to the appropriate ground points for the analog control chips.
All the converter circuits used in this thesis project are controlled by analog control chips which are specially designed to control the power stage of its converter topology. The control chips used in this project are;
● UCC3895, Texas Instruments for Phase Shifted Full Bridge Converter
● UC3854, Texas Instruments for PFC Boost Converter
● FAN6754, Fairchild for Flyback Power Supply Unit
Using an analog controller is advantageous in power supply designs if avaliable because of the continuous analog sampling structure helps the designer to easily control the transient conditions and dynamic response in high switching frequencies. Zero voltage switching let the power supply is operated at higher frequencies and at high frequencies digital control structure's sampling time and instruction cycles of the calculations may not be enough to operate. The only disadvantage of the analog controller is the lack of flexibility in some applications. Changing the control parameters or adapting the parameters to the circuit operation in different operating conditions is hard to implement in analog control applications.
The circuit operation starts with the mains voltage applied to the PFC boost converter input. The voltage is rectified with the full wave rectifier stage. PFC boost converter is inactive at that instant so the full wave rectifier charges the three 470 uF 450 V DC bus electrolytic capacitors at the output of the PFC boost converter. In the first energization these capacitors are at zero voltage and if not limited very high currents can be drawn from the AC mains. To limit the peak currents two series NTC resistors is located series with the rectifier input. When the DC voltage exceeds 250 V the flyback converter starts to operate from the DC bus. When the flyback converter output voltages are created PFC boost converter control chip UC3854 starts to operate and the DC voltage is boosted to 380 V. The flyback converter creates the all necessary voltages to the control chips and the control circuits. 
DC bus voltage is observed by an electronic control circuit formed with an opamp comparator. Comparator circuit compares the divided DC bus voltage with a reference voltage level. When the DC bus voltage reaches the appropriate voltage level the output of the comparator changes its position. When the DC bus voltage exceeds 350 V the comparator changes its output from high to low and the input NTC resistors are shunted with a power relay. In the continuous operation this relay improves the whole circuit efficiency by eliminating the NTC resistors conduction power losses.
DC bus voltage is formed by PFC boost converter and phase shifted full bridge converter is fed from that DC bus voltage. When the flyback converter output voltage is created all the control chips are ready to operation but the operation sequence of the coverters is important with the appropriate voltage levels. The phase shifted full bridge converter design calculations are made with the minimum voltage of 370 V DC. Below that voltage high frequency power transformer turns ratio could not be enough to create the output voltage of the converter. Because of this limitation DC bus observer comparator opamp output is also used to control the enable input of the phase shifted full bridge control chip UCC3895. Below the 350 V DC bus voltage, control chip is kept disabled and the full bridge stage can not starts to operate. The full bridge operation is starts when the comparator opamp output changes from high to low. This signal means PFC stage and DC bus is voltage is OK and the full bridge operation can start. By this control function, full bridge stage is started its operation with the appropriate DC input voltage. After all the power circuits starts their operation, power flows from the mains to the DC load at the output of the full bridge.
Designed full bridge circuit's ouput nominal power value is 1900 W. The output voltage is 57.6 V and the output current is 33 A at full load. DC voltage output value is selected intentionally to accomodate four series connected lead-acid batteries boost voltage level. In this project converter is designed as a DC voltage source but the output voltage level permits to use this circuit as a battery charger with small modifications. With a controlled current limiting circuit according to the charge characteristic, converter can be configured as a battery charger. The working principle of the power stage will be as same as in voltage source mode of operation.
Phase shifted full bridge converter power stage consists of four power Mosfets, one high frequency power transformer, one resonant choke and four fast power diodes. Switching frequency is 75 kHz. Because of this level of frequency and also considering the nominal power level of 1900 W, the most appropriate selection is to use power Mosfets as switching components. The high frequency power transformer is designed with ferrite core in E type geometry. Transformer secondary is centertapped and the four diodes are used in half wave rectifier topology in two parallel configuration. Resonant choke is series connected to the high frequency transformer primary side to increase the leakage inductance of the transformer to achieve zero voltage transition of the power switches. Resonant coke is designed with a toroid powder core with the calculated inductance value.
In the output filter section, one high current choke and four parallel output DC electrolytic capacitors are used. Output lowpass filter choke is designed with ferrite core. Because of the continuous DC magnetic bias in the output choke, an air gap is created between the two core halves. Air gap prevents the high permeability core material from going into saturation. The output filter capacitors are paralelled to lower the ESR value and also to handle the output choke rms ripple current value at the switching frequency. Triangular current superimposed on the dc output current is filtered by the output parallel connected capacitors.
In all the three converters, current mode control is used. PFC boost converter control chip UC3854 permits the use of the average current mode control. By the use of average current mode control, low input current harmonic distortion value is achieved. Phase shifted full bridge converter control chip UCC3895 offers three control method such as voltage mode control, peak current mode control and average current mode control. Peak current mode control is selected in the phase shifted full bridge converter control. By the use of this control method transformer can be constructed without airgap, because control method prevents transformer going into saturation in case of volt-second unbalance. Without an airgap transformer turns number can be limited to a small number to provide adequate magnetizing inductance. Also the fringing field effects around the airgap is eliminated. Also with the current mode control dynamic response is improved as compared with average current mode control and voltage mode control. In case of using voltage mode control an air gap must be created between core halves. 
One disadvantage of the peak current mode control is sensitivity to the electromagnetic noise. In that case, pcb design is very critical, compared to the other control methods. But the advantages such as improved dynamic response and peak current limiting capability make this control an appropriate selection.
PCB design is very critical in this type of high power converters. High currents are switched at high frequencies. All the power and control loops are seperated in the pcb to minimize the interference of the power signals to the control signals. Control chips power and control planes are seperated and the references are connected by a single connection.