Anahtarlamalı güç kaynağı ve güç faktörü düzeltme devresinin tasarımı
Başlık çevirisi mevcut değil.
- Tez No: 55613
- Danışmanlar: PROF.DR. NEJAT TUNÇAY
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 69
Özet
ÖZET Bu tezin konusu yüksek verimli, çıkış gerilim dalgalanması az bir anahtarlamalı güç kaynağının ekonomik olarak tasarlanmasıdır. Gücün artmasıyla anahtarlamalı güç kaynaklan sağladıkları yüksek verim, boyut ve ağırlıklarının az olması gibi üstünlüklerinin yanında, ekonomik olarak da doğrusal güç kaynaklarına oranla çok üstünlük sağlamaktadırlar. Çıkış gerilimi 48 V doğru gerilim olan akım kontrollü bir anahtarlamalı güç kaynağı seçilmiştir. Anahtarlamalı güç kaynaklarında giriş doğru gerilimi genel olarak bir tamdalga doğrultucu ve bir kondansatör üzerinden sağlanmaktadır. Gücün artmasıyla kaynaktan çekilen akım şeklinin ideal şeklinden uzaklaşması sonucu giriş güç faktörü azalmaktadır. Bu çalışmada güç kaynağı girişine bir güç faktörü düzeltme devresi tasarlanarak, kaynağın şebekeden çekeceği faydalı güç miktarı artırılacaktır.
Özet (Çeviri)
SUMMARY For many electronic systems there is a need to have a regulated dc supply. At first, the linear regulator was used to meet this requirement. The linear regulator contains a mains transformer and a series dissipative regulator. Because of these drawbacks, dimensions and weight of supply will be high and also efficiency of the supply will be only around 30 to 60 %. These disadvantages of the linear regulator limit its range of application. Typical efficiencies of switching supply are between 70 to 90 %. Furthermore the switching regulator operates the power device in the on and off states. Thus the sizes of the power transformer, filter components and of course the cost are extremely reduced in comparison to the linear regulator. All of these advantages make the switching power supply more appropriate choice than the linear regulator. However the switching power supply has also some drawbacks, but they can be eliminated. First the switching power supply is more complicated than linear regulator. Because of the high switching frequency, generated noise not only affects its output but also distorts into the ac input supply. This problem can be eliminated by inserting additional filters to the supply, but of course the cost will be increase. The transient response is the time required for the output voitage to return within its regulator limits when a sudden change is occured either in the line voltage or current. For linear regulator this transient response is in a range of microseconds, though it is miliseconds for switching regulators. Linear supplies are chosen for low power levels and also used in circuits where a quiet supply voltage is necessary such as audio or interface circuits. Because of its high efficiency, low size and cost, the switching power supply is used in most applications. The non-isolated switching supplies have very limited use, they have only one output. The output range is also limited by the input and the duty cycle. Transformer isolated supplies don't have this drawbacks and addition of transformer provides some advantages as follows: VI- The energy passes through a ferrit material prior the reaching output. This transformer isolation provides a dielectric barrier in event of a semiconductor failure. - By selecting the transformer turns ratio, we can provide wide range of difference between input and output. - Multiple outputs are easily obtained by adding more secondary windings to the transformer. There are some disadvantages with transformers because of their size, weight and power loss and also voltage spikes can be generated due to leakage inductance. There are five types of transformer isolated converters: - Flyback converter - Forward converter - Push-pull converter - Half-bridge converter - Full bridge converter Flyback converter is the simplest converter of all the isolated converters. All of the output power of flyback has to be stored in the core, during the transistor on time. This means that the core size and cost will be much greater than the other topologies. The secondary inductance of the transformer is in series with the diode when the switch is turned off. This means that there is no filter requirement in the output circuit. in the converters the output capacitor is only supplied switch of time. In order to achieve low output ripple, very large capacitors must be used. Output current has higher peak values than a forward converter at equal power, and flyback converters have much higher output ripples than the other topologies. These drawbacks limit its application range up to 200 W. For the applications having low cost but multiple outputs, flyback will be appropriate choice. Forward converter transfers the energy directly to the output through the inductor during the“on”time the switch. Output current is always continuous. This current is very easy to smooth, and the requirements for the output capacitor is lower than flyback's. VI!Transformer and core is smaller than flyback's. The smaller transformer and output filter capacitor requirements means that the forward converter is suitable for use at higher output power levels than the flyback converter can attain. The major problem of this topology is removing the core magnetisation energy by the end of each switching cycle. This can be eliminated by adding a reset winding with clamp diode to the primary. In this thesis, forward converter with two switches was chosen and designed as an example. The voltage across the switch is damped to Vinput, allowing the use of more efficent, faster devices. The magnetisation reset is achieved through the two clamped diodes. By this way reset winding was eliminated. The major advantage of the forward converter is the very low output ripple that can be achieved for relatively small sized L.C. components.. Push-pull converter have a core operating symmetrically. This means more smaller transformer sizes and provides higher output power levels than the forward converters and also multiple otuputs easily. Major problem of this topology is flux symmetry inbalance occured due to unequal“on”time of the switches. Half-bridge converters are used for higher power applications in the 500 W to 1000 W range. Voltage rating of power switches are equal to maximum D.C. input voltage. The bridge circuits have excellent transformer utilisation, very low output ripple and high output power capabilities. Input capacitors have large size. The top switch must also have isolated drive. Full bridge converter is a higher power version of the half bridge and provides the highest output power level of any of the converters discussed. In order to regulate the output voltage, a control feedback loop is required. Two types of control are commonly used: - Voltage-mode control - Current-mode control In a conventional voltage-mode controlled regulator, the feedback voltage is used to control the duty cycle of the converter. In a current-mode controlled regulator, the feedback voltage is used to control only the output current of the converter. viiiCurrent- mode control has a second control loop added to the voltage feedback loop. Outer loop senses DC output voltage, inner loop (second loop) senses peak inductor current and keeps them constant. Adding the second loop results in a number of performance advantages including improved transient response, a simpler and more easily designed control loop than voltage-mode control. Peak current sensing will automatically flux balancing in push-pull switching regulators. Additionally, by using this method, limiting the peak value of the power switches can be provided easily. A switching power supply is a simple dc to dc transformer. The dc voltage input to the converter is typically sourced from the full wave bridge rectified ac mains voltage capacitor input ripple filters are used to regulate the dc voltage. The conventional capacitor filtered power supply extracts a very nonsinusoidal current waveshape from the ac line. This waveshape creates a high harmonic load on the power line and this harmonic load is not contributing to the useful power of the supply. Fig.1. Graetz circuit. Figure 2 illusrates the waveform of current drawn from the A.C. power supply by a conventional capacitor filtered D.C. power supply. There are several significiant drawbacks of poor power factor. Without power factor control, useful power capability of power supply will be low. international standards specify maximum limits for the amount of current allowed at each of the harmonics of the line frequency. IXFig.2. Voltage and current waveforns. Power factor correction can be achieved by using a boost converter prior to the actual power supply. An active power factor corrector must control both the input current and the output voltage. The current loop is programmed by rectified line voltage so that the input to the converter will appear to be resistive.
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