20-06-2012, 05:42 PM
SWITCHED MODE POWER SUPPLY
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INTRODUCTION:
SMPS i.e. Switch Mode Power Supply, before starting to introduce this topic we first have to know what power supplies are
Power Supply
A device for the conversion of available power of one set of characteristics to another set of characteristics to meet specified requirements.
Conventional series regulated linear supplies maintain a constant voltage by dissipating excess power in ohmic losses. The linear regulator can, therefore, tend to be very inefficient.
Switch mode power supplies uses a high frequency switch (in practice a transistor) with varying duty cycle to maintain the output voltage. The output variations caused by the switching are filtered out by a LC filter. These are current state of art in high efficiency.
Depending on the type of output voltage, power supplies can be categorized into two types, they are:
AC power supplies
DC power supplies
What is switch mode power supply?
SMPS are an extraordinary array of high frequency alternative. These are the switching regulators of high efficiency that can step up, down and invert the input voltage.
Why we go for SMPS?
Controlled dc supply can also be obtained from phase controlled rectifiers. But An AC to DC rectifier operates at supply frequency of 50 Hz (or 60 Hz). In order to obtain almost negligible ripple in the DC output voltage, physical size of the filter circuits required is quite large. This makes the DC power supply inefficient bulky and weighty.
On the other hand SMPS works like DC chopper. By operating the on/off switch very rapidly, AC ripple frequency rises which can be easily filtered by L and C filters circuits which are small in size and less weighty. It may therefore be inferred that it is the requirement of small physical size and weight that has led to the wide spread use of SMPS.
The output DC voltage is controlled by varying the duty cycle of the chopper by PWM of FM techniques. SMPS can be used as linear supplies to step down a supply voltage. Unlike a linear regulator, however, an SMPS can also provide a step up function and an inverted output function.
BACKGROUND:
ORIGIN OF SWITCHED MODE TECHNIQUES:
The origins of switched mode converters are linked with the developments in inverter circuitry. An inverter is a processor for generating AC from DC and is, therefore, a constituent of some forms of switched mode power supplies. The inverters like AC-DC, DC-DC etc were developed before the first transistors appeared and therefore, employed valves as switching elements, such as a push pull inverter described by Wagner and thereby came transistors.
The various forms of transistors switching circuits developed during the 1950s were categorized into three main groups by the end of the decade, namely
Ringing choke.
Self oscillating push pull and
Drive push pull converters.
DEVELOPMENTS OF SWITCH MODE TECHNIQUES:
The 1960s heralded the development of modern forms of switching regulators and switched mode power supplies. During the early 1960s three forms of non dissipative switching regulators were developed for low voltage DC to DC applications. They are the buck boost regulators. The buck regulator steps down the input voltage to a lower regulated output voltage.
The boost regulator steps up the input voltage to a higher regulated level. The buck booster regulator, also referred to as fly back regulator, is used to regulate a negative voltage at a level higher or lower than the positive input voltage. The method of regular control in all cases is achieved by varying the duty ratio of the electronic switch, most commonly by pulse width modulation.
The advances in electronics need for dc power supplies for use in Integrated circuits (ICs) and digital circuits has increased manifold. For such electronic circuits, NASA was the first to develop a light weight and compact switched mode power supply in 1960’s for use in its space vehicles. Subsequently, this power supply became popular and presently, annual production of SMPSs may be as high as 70% to 80% of the total number power supplies produced.