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Pspice ile D.A./D.A rezonans çeviricilerinin analizi

Başlık çevirisi mevcut değil.

  1. Tez No: 66859
  2. Yazar: KÜRŞAT TANRIÖVEN
  3. Danışmanlar: PROF. DR. R. NEJAT TUNÇAY
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Elektrik-Elektronik Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 149

Özet

ÖZET Bu çalışmada d.a./d.a. rezonans çeviricilerin avantajları ve dezavantajları incelenmiştir. Günümüzde bilgisayar, elektronik, uzay programları ve bir çok alanda kullanılan cihazlarda ufak yapılı yüksek verimli güç kaynaklarına olan İhtiyaç gittikçe artmaktadır. Anahtar modunda çalışan çeviricilerin boyutlarını (endüktans, kondansatör ve transformatör boyutlarını azaltmak) azaltmak için çalışma frekansını artırmak gerekmektedir. Ancak çeviricide bulunan güç anahtarlarındaki kayıplar frekans artışı ile doğru orantılı olarak artmaktadır. Rezonans çeviricilerde LC rezonansı ile (seri LC, paralel LC, seri-paralel LCC....) anahtarlarda yumuşak anahtarlama elde edilir. Yumuşak anahtarlamanın temel ilkesi anahtar içinden geçen akım veya uçlarındaki gerilim sıfır olduğunda iletime geçirmektir. Elde edilen bu yumuşak anahtarlama ile anahtarlama güç kayıpları azalmakta ve çeviricinin verimi artmaktadır. Bu çalışmada birinci bölümde rezonans çeviriciler ile elde edilen faydalar belirtildi. İkinci bölümde geleneksel rezonans çeviriciler üzerinde duruldu. Üçüncü bölümde sıfır akım ve gerilim anahtarlama metodları incelendi. Dördüncü bölümde seri rezonans çeviricilerin analizi, dizaynı için gerekli denklemler ve çalışma durumları incelendi..Beşinci bölümde paralel rezonans çeviricinin analizi ve harmonik etkisine göre çalışma bölgesi belirlenmiştir. Altıncı bölümde PWM çeviricilerde yumuşak anahtarlama teknikleri sunuldu. Yedinci bölümde bu çeviricilerin karşılaştırmaları yapılmıştır. X

Özet (Çeviri)

SUMMARY Resonant dc-dc converters are increasingly becoming a viable option as SMPS (Switch Mode Power Supplies) for variety of industrial applications. Their important advantages of low losses, reduced EMI (Electromagnetic Interference) and low noise, make them very suitable for high frequency, low to medium power dc-dc conversion. It is known that classical power supplies offer the advantages of simplicity, sturdiness and high reliability. But, for obvious reasons, they present major drawbacks whenever the size and weight of a power supply is critical. In the last decade, SMPS have offered clean solutions to these drawbacks. However, a further reduction of weight and size of SMPS calls for higher operating frequency. This means that, the power switches are severely stressed and commutation losses are tremendously increased. Recently, it is believed that the exploitation of LC resonance at commutation is offering the adequate solutions to the above mentioned limitations of SMPS. Figure 1 Basic structure of conventional resonant converters. Figure 1 illustrates the basic structure of conventional resonant converters. In figure 2 waveforms are shown for a resonant converter operating above resonance. In resonant converter the half bridge applies a square wave of voltage to the resonant circuit, and due to the filtering action of the resonant circuit, approximate sine waves of current are present in the resonant inductor Lr. The fact that the circuit is operating above resonance can be deduced from the fact that the current delivered to the resonant circuit (that is, the current in Lr ) is lagging the voltage applied to the resonant circuit ( that is, the fundamental component of the square wave applied by the half bridge circuit ). The current carried by the power switch is a 180 section of this sine wave of current as illustrated in figure 2. From figure 2 it is seen that no turn on losses exist in the switch because its inverse diode carries current and the voltage YIacross the switch is zero before the switch conducts forward current. Note that the inverse switch current is caused by the opposite switch turning off. For example, if the bottom switch in the half bridge turns off, the current that was in this JIMP Figure 2 Ideal resonant converter waveforms. switch is transiently maintained by the inductive action of the resonant indictor, which forces the current to come up through the upper switch in the inverse directions (that is, through its inverse parasitic diode). Note also that once the current reverse due to the resonant action of the circuit, the inverse diode which was conducting has a turn-off time tq equal to the forward VTTconducting time of the power switch before forward voltage is applied to the diode. This fact results in no switching stresses being applied to the diode, and, in fact, the inverse parasitic diode associated with power switches are of sufficient speed to be useful even at circuit operating frequencies of hundreds of kHz. Therefore, a main advantage of operating above resonance for the resonant converters is that there are no diode or switching losses and the diode can be of medium speed. However, that to achieve those advantages the switches must switch off current and are therefore subject to turn-off switching losses. However, lossless snubbers can simply be implemented by placing small snubber capacitors directly across the switch devices. No snubber discharge resistors are needed. This can be done because the capacitor never discharged by turning the switch on but rather is discharged by turning opposite switch in the half bridge. For example, when a bottom switch turns off a capacitor which is placed directly across the upper switch will be discharged by the load current. Note also that considerable switching losses in power switches operating at higher frequencies are actually due to storing energy in the switch drain-source and drain-gate capacitance and then discharging these capacitances internally (and losing the associated energy) the next time the switch turns on. This loss can be significant at higher voltages and frequencies. By operating the resonant converters above resonance, this loss is eliminated. That is, the energy stored in any capacitance directly across the device is returned to the dc source by virtue of the opposite switch turning off. In addition, the output and input filter sizes are minimised because the frequency is limited to a known lower limit (in operation below resonance the frequency is lowered to control output, and therefore the filters must be designed for the lowest frequencies encountered)., All of the aforementioned advantages are lost if the converter is operated below resonance. That is, below resonance operating results in switch turn-on switching losses, diode switching losses (high-speed diodes are needed), energy stored in device capacitances is discharged and lost internal to the switch's and the input and output filters must be designed for the minimum switching frequency. Switch turn-off does occur in a lossless manner when operating above resonance, this is not a major argument for operating the converter below resonance. For all of these reasons, it is felt that operation of resonant converters above resonance is proper choice for most power supply applications operating at high frequencies. The switches in a resonant converter create a square-wave ac waveform from the dc source. Inductors and capacitors then remove the unwanted harmonic from the square-wave. As the difference in frequency between the fundamental component and the lowest harmonic (the third) of the square-wave is so small, a resonant LC circuit tuned to approximately the switching frequency is used, rather than a simple low-pass filter, to remove harmonics from the fundamental. Hence the name resonant converter. The tuned filter can be very selective if its Q is high enough to give good peaking in its impedance versus frequency characteristic. Because the network composed of the resonant filter and the external ac system has a reactive impedance at all but its resonant frequency, the switches in a resonant converter must be able to transfer energy in both directions. Therefore implement each switch to either carry bipolar current or block a bipolar voltage. Thus we can use also a resonant converter designed for average power flow from dc system to ac system to transfer energy in the other direction should the application require it. XIIIThe resonant converter's semiconductor devices can have significantly lower switching losses than those of the semiconductor devices in a high-frequency dc/dc or dc/ac converter. The energy lost when a device Q switches on or off is 'ou.afr Eioss = JVgdt o where ton,0fr is the time it takes the semiconductor device to turn on or off, that is, the rise or fall time of its current and/or voltage. A resonant converter can designed to have one of the switch variables remain near zero during this time, resulting in low switching losses. Not all resonant topologies have this feature. For those that do, the reduced dissipation allows us to use certain devices in a resonant converter at a higher switching frequency than possible otherwise in a high-frequency switching converter. This advantage is often the basis, for making a decision to use the resonant converter topology. Unfortunately, in return for the lower switching losses, the semiconductor devices are subjected to higher on-state currents and off-state voltages, giving them higher stress parameters than they would have in a nonresonant topology operating at the same power level. Therefore more expensive devices are often required, and higher conduction losses usually result. There are two approaches to the resonant converter- one dual of the other. In the first, the switches create a square wave of voltage that is applied to a series resonant circuit. This is called a series resonant converter. In the second, the switches create a square wave of current that is applied to a parallel resonant circuit, resulting in the parallel resonant converter. The advantages of operating resonance principles is summed below 1. The device switching loss at both turn on and off disappear, thus giving high inverter efficiency. 2. The device heating is low due to conducting loss only; therefore, heat sinking or cooling requirement is low. 3. The inverter can be operated without snubbers. 4. All the above factors make the inverter size small and at reduced cost. Lower 5. size and smaller heat dissipitation open up the possibility of inverter integration at higher power levels. 6. The device reliability is improved because there is no stress due to excursion in an active area. 7. The EMI problem is less severe because resonant voltage pulses have lower dv/dt than those of a stress-switched inverter. 8. For motor drive systems, the acoustic noise will be very small because of high frequency. 9. The machine insulation is less stressed because of lower dv/dt resonant voltage pulses. Yrv

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