Anahtarlamalı güç kaynağı tasarımı

Abstract

Düzensiz ve dalgalı bir giriş geriliminden, çekilen güçle oran tılı olarak, bir transistoru (veya birden fazla transistürleri) yük sek frekansta anahtarlayarak, çıkışta düzenli ve sınırlar içinde dal- galanmalı, yüksek verimli kaynaklar "Anahtarlamalı Güç Kaynakları (AGK)" olarak adlandırılırlar. önce endüktif ve kapasitif olarak iki ayrı sınıfa Ayrılırlar. Kapasitif olanlar daha çok düşük güçlerde, alçak girişten daha yüksek gerilimler elde etmek için kullanılır. Kullanımı daha yaygın olan endüktif AGK* lar trafolu ve tek çıkışlı endüktif olarak ikiye ayrılırlar. Trafolu olanlar, farklı gerilimlerde birden fazla çıkış verebilir ve aynı zamanda girişe göre ve farklı çıkışların arasında toprak yalıtkanlığı sağlarlar. Endüktif AGK' lar giriş gerilimine göre çıkışta; daha düşük (Buck), daha yüksek (Boost) ve ters polarite (Buck-Boost)' de geri lim verebilen tiplere ayrılırlar. Trafolu olanların ;(1) Geri dönüşlü (Flyback), (2) îleri yönde (Forward), (3) Puş-pul (Push-pull), (O Yarım köprü (Half -bridge), (5) Tam Köprü (Full-bridge) gibi farklı yapıları vardır. Geri dönüş (flyback) çeviriciler tasarımı en basit, puş-pul ise en zor olanıdır. En az eleman geri dönüşlü çeviricide, en fazla puş-pul ve tam-köprü- de kullanılır. Puş-pul ve tam köprüde sürücü devre tasarımı en kar maşıktır. Çıkış gürültüsü en az ileri (forward) tiptedir.. Devre ekonomisi bakımından ileri çeviriciler en çok kullanılandır. Geri dönüşlü (flyback) çeviriciler ucuz olmasına karşılık, çıkışta yüksek dalgalanma olması ve büyük çekirdekli trafo kullanılması yüzünden 20D vat1 lık güçlerin üzerinde pek tercih edilmezler. Yüksek güçlerde puş-pul ve tam köprü daha uygundur. Tasarımda, ekonomi ve gürültü belirleyici faktörler olduğu için ileri yönde çevirici seçilmiştir. Fiziksel olarak gerçekleş tirilen tasarım, laboratuvarda yapılan testlerde istenen bütün koşul ları sağlamıştır. Tasarımda anahtarlama elemanı olarak bir yüksek gerilime dayanıklı iki kutuplu (bipolar) transistor ile seri bağlanan düşük gerilimli bir MDSFET transistor kullanılmıştır. Anahtarlama MDSFET ile yapılmış ve anahtarlama frekansı olarak seçilen 25 kHz' de iyi netice alınmıştır. Güç kaynağının tasarımında kısa devreye karşı korumak için yeni (orjinal) bir devre tasarlanmıştır. Sonuç olarak tasarımdan beklenen yüksek verim (%8S), düşük gürültü, hat ve yük regülasyonu gibi bütün özellikler gerçekleştirilmiş ve cihaz haline getirilen AGK bunları sağlamıştır.

Switch Mode Power Supply (SMPS) is the subject matter of the thesis. SMPS converters are used to obtain a very well-regulated DC supplies from unregulated inputs by switching transistors at high frequencies (recently hundred kilohertz possible). Switching transistors are powered from unregulated supply at the input. Switching time, that is called duty cycle, depends on the frequency, output load, input unregulated supply. The term converter refers to the section from unregulated supply to the output. If SMPS is working directly from the mains supply, then it is called "off -line" it is easy to obtain unregulated supply by just rectifying and filtering the mains input. Switching transistors, high power rated, very fast and high voltage breakdown rated, are powered from the mains. SMPS find wide use because of their high efficiency (8D%), small size and low cost. Their small size comes from switching at high frequencies. SMPS' s are investigate first in two groups; capacitive and inductive. Capacitive types are used at very low power levels to obtain higher voltages from smaller inputs. Inductive SMPS1 s are divided into two types: 1. Single ended inductive 2. Transformer types. Those equipped with a transformer can give multiple outputs and ground isolation between the input and output, also between the outputs. To the contrary, single-ended inductive types give only one single output, and provide no ground isolation between the input and output. In the first type (single ended), an inductance is used for energy storage, in the latter a transformer does the function of the inductor. Single-ended types are also classified depending on their output voltage. If the output voltage is less than the input un regulated supply, then it is called Buck (step down). If the output is greater than the input, then it is Boost (step-up) and when the polarity of the output voltage is the opposite to the input, then it is called Buck-Boost. (X ITransformer types are: 1) Flyback 2) Forward 3) Push-pull *0 Half-bridge 5) Full-bridge Flyback converters are simplest to design, push-pull and full-bridge are the hardest. Least number of components are used in flyback. Push-pull and Full-bridge are the most complex. The noise at the output voltage is minimum in Forward Converters. Forward converters are used widely because of their lowest cost. Although flyback converters are cheap, they are being used above 2DD watts power outputs because of the great amount of ripple at the output and the necessity of using very big transformer cores. Full-bridge Bnd push-pull types are preferred at high power-levels. However, up to 2 kilowatts, good performances achieved with forward type SNIPS* s. It came out to be the optimum solution to use forward type converter because the noise requirements are severe and the required power is about 1 kilowatt. This rectifier which is the subject of this thesis has been designed to be used in telecommunications environment where the noise has utmost importance. Design is realized around a PbIM control integrated circuit. This IC included duty cycle control, reference voltage, oscillator and an undervoltage lockout circuit in it. As main switching component, a high voltage bipolar transistor is connected in cascode with a low-voltage MOSFET transistor which forms to a BIMOS configuration. This configuration gives high speeds and increases the breakdown voltage rating of bipolar to v*rRn. Switching is done through the gate of MOSFET. All protection measures are taken against high and low mains voltages, high output voltage, loading at the output and short circuit conditions. Specifically, short circuit protection design is original as far as it is known. When the output is short-circuited, rectifier still continues to supply 15 A rated current. This feature is important when rectifier is used to change high capacity batteries. Soft-start feature is also implemented in the design not to create severe transient conditions. This will also improves the reliability. This rectifier was designed to meet some a priori requirements t like 88 percent efficiency, noise levels and some transient conditions IXIIlike soft-start, line and load regulations. It met all the requirements, even BB percent efficiency which is quite high. Up_to-date technology is used in the design, like proportional base drive, BIMDS configuration, latest version of PUM chips. Propor tional base drive improved the efficiency a lot. Alsa necessary precautions are taken net to let switching noise to pasB to the mains, or from the mains to the rectifier by connecting a passive two-way filter right at the input- As a conclusion, it can be said that the design fulfilled all the requirements, although they are hard to meet. Also the latest ideas like proportional base drive, BIMOS switch are implemented successfully in the design. A new way of short circuit protection features the design. In the following part, brief descriptions about different topologies of power supplies, their usages, merits and drawbacks are given. 1. Linear Regulator: a) Post regulator fed from a line regulated secondary of a multi-output switching supply. b) Very reliable, noise free, low output ripple, very rare power transiBtor failure as no "second breakdown problems". c) Poor efficiency-35% for 5 volt outputs, 50% at 10 V output. 2. Push Pull: a) Provides multi-output line regulated output with a master both line and load regulated, others slaves only line regulated. b) Provides twice the power from a given core than is obtainable from a forward converter. c) "Off" transistor voltage rating must be not less than 2.5 times maximum DC input. Hence marginally safe in off line converters operating from Vac = 115 V. Not usable in off line converters operating from 220 Vac input. d) Especially useful in low voltage DC/DC converters. e) Flux unbalance problem occurring for unequal Dn time in the two power transistors can saturate transformer and destroy transistors. Bi-polar transistor push-pull must have trick circuits to avoid flux unbalance. (xii:)NF) Push-pull with Power Mosfeta can be designed inexpenaively to avoid the flux unbalance problem. g) Very desirable topology for 10 tû 15D watts output from DC sources of 36 V to 60 0 when used with Power Mosfets. 3- Single Ended Forward Converter: a) Frequently used up to output powers of 15D to 200 watts in off line regulators fed from 115 V/ac. Off transistor voltage rating must be not less than 2.5 times maximum DC input. b) Wot usable as off line converter fed from Vac = 220 V. c) No flux unbalance problem. d) Less expensive than push-pull as it uses only one as compared to two transistors for the push-pull. e) Must have power transistor of twice the peak current rating of the transistor used in a push-pull at equal powers. f) Output rectifier diodes reverse voltage stress is twice that that of the diodes in a push-pull for the same output DC voltage. k. Two Half Cycle Interleaved Forward Converter: a) Twice the output power of a single forward converter. But requires two transformers, two output inductors. b) No flux unbalance problem. c) Output rectifier reverse voltage stress is half that of the single forward converter. d) Off voltage stress Dn power transistor is still 2.5 times maximum DC input voltage. e) Maximum usage- up to 500 watts in aff line converter fed from Vac = 115 V. Not usable as 220 V off line converter. 5. Double Ended Single Half Cycle Mode Forward Converter: a) Voltage stress on "off" transistors is equal to maximum DC input- not twice that as for the single ended forward converter.. (xiii)b) Widely used in off line converters fed from 220 Uac or in applications whose operation must be either 114 Uac or 22D Uac. c) Used often at output powers up to 250 watts. 6. Flyback: a) Inexpensive and widely used topology for outputs up to 115 watts. Inexpensive as compared to single ended forward converter, because each output secondary requires only a filter capacitor rather than an inductor plus a filter capacitor as for the forward converter. b) Can be designed for peak "off" voltage stress less than twice maximum DC input.. But penalty is higher peak tran sistor current. Still not usable in off line converter from Uac = 220 U. c) Peak transistor current is twice that in a forward converter of equal power. d) High output ripple and high ripple current rating required for output capacitor- resulting from high triangular peak output current. 7. Half Bridge: a) Best alternative for output powers above 150 watts. Good choice up to and equal to 750 watts. b) Maximum voltage stress on off transistor is equal to maximum DC input rather than twice that in the forward converter. c) Usable in off line converters fed from 220 Uac or in converters which must operate either from 115 Uac or 220 Uac. d) Compared to double ended forward converter, its secondary output ripple frequency is twice that of the former. Hence smaller output filter components, lower voltage stress on rectifier diode. e) More than twice the output power available from a single core than when the same core is used in a forward converter. B. Full Bridge: a) Best choice for output powers above 750-800 watts. (xiv)b) Requires k power transistors. c) Can be used in off line converters from 22D Vac. 9. SCR Resonant Converter: a) Beat choice for output powers above 1 kid. Can be used up to about 2 or even k kid with SCR S731D. b) Lower RFI than with a full bridge because of sinusoidal transistor currents. c) SCR is a very rugged device- far less prone to failure than a power transistor because no second breakdown..