Recently the demand for even lower no load power requirements in the application has meant that flyback topology is being used more widely in mobile phone chargers.SMPS i.e. Switch Mode Power Supply, before starting to introduce this topic we first have to
know what power supplies are
another set of characteristics to meet specified requirements.
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:
Ø AC power supplies
Ø 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
Ø Ringing choke.
Ø Self oscillating push pull and
Ø 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.
TYPES OF CONTROLLED DC VOLTAGE:
DC-DC converters are widely used in regulated power supplies and in DC motor
drive applications. The input to these converters is an unregulated DC voltage, which is
obtained by rectifying the line voltage and therefore it will fluctuate due to changes in
the line voltages. However in the DC-DC converters, the average DC output voltage must
be controlled to equal a desired level.
There are two methods of obtaining the controlled DC output voltage at a
desired level. They are:
· Multiple switch topologies
· Alternative topologies
MULTIPLE SWITCH TOPOLOGIES:
The main disadvantage of the single topologies is the need for the high voltage
blocking capacity of the transistor switch (twice the DC input voltage), especially when
operating from a rectified AC mains supply. Also the single switch topology is not an
ideal solution for higher power converters, where the current rating of the transistor
switch needs to be much greater. Therefore another group of isolated converters
utilizing more than one switch can be identified. The three multiple switch topologies,
i.e.
· Half bridge
· Full bridge
· Push-pull converters.
All are buck derived due to nature of switching involving pulsating input current
and non-pulsating current and also having an identical ideal voltage gain of the forward
converter.
ALTERNATIVE TOPOLOGIES:
In multiple switch converters, transistor switching overlap can occur which could
cause catastrophic failure to converter by effectively applying a short circuit to the same
supply source. To eliminate this problem an alternative topology is Weinberg push-pull
converter, which is inherently self protecting when there is any possibility of component
imbalance or conduction overlap.
In all basic switched mode topologies, the finite duration of the switching
transition will cause high peak pulse power dissipation in the device. This produces
degradation in converter efficiency and worst of all, can lead to transistor destruction
during the turn off transition due to the inherent BJT second breakdown phenomenon.
Therefore the greatest amount of research into alternative switched mode topologies
has been in the field of resonant converters. These converters have tuned circuits as
part of the power conversion stage and exhibit sinusoidal voltages or currents, so
leading to transistor switching transitions at the ideal conditions.
Classification of smps:
Ø Based on the type of input and output waveforms:
· AC in DC out: rectifier, off-line converter input stage.
· DC in DC out: voltage converter or current converter, DC to DC converter.
· DC in AC out: inverter.
· AC in AC out: frequency changer, cyclo converter.
Ø Based on circuit topology:
· Non isolated topology: Non-isolated converters are simplest, with the three basic types using a single inductor for energy storage. In the Voltage relation column, D is the duty cycle of the converter, and can vary from 0 to 1. Vin is assumed to be greater than zero; if it is negative, negate Vout to match.
ü Buck
ü Boost
ü Buck-Boost
· Isolated topology: All isolated topologies include a transformer, and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio.
FORWARD CONVERTER: The forward converter is a DC/D converter that uses transformer windings to boost the voltage and provide galvanic isolation for the load. It is more energy efficient.
The extra winding of a forward converter’s transformer ensures that at the start of switch conduction, the net magnetization of the transformer core is zero. If there were no extra winding, then after few cycles the transformer core would magnetically saturate, causing the primary current to rise excessively, so destroying the switch (i.e., transformer). The diode on the secondary that is connected between the 0V line and the junction of the inductor and rectifying diode is often called the ‘flywheel diode’.
CONTROL METHODS: In the majority of convertor Topology application, PWM is used for controlling the convertors output voltage through feedback control of the switching transistors.
Other forms of control are becoming increasing popular. One such technique is current mode control which utilizes the switching transistor current as a control parameter and has the benefit of providing an inherently more stable closed loop response. Another control method finding favor with power supply designers is fed forward which improves the transient load and line response of mains driven power supplies.
SMPS 'Hiccup' Mode: In switch-Mode Power Supplies the 'hiccup' mode is often used for limiting output current. If an overload occurs, the circuit turns off. After an interval it comes on - has a look, as it were; if the overload is still present, it immediately goes off again. In some designs, this happens a few times, and the supply then shuts down permanently until the overload is removed and the circuit reset.
SMPS HOLD UP: Most offline switchers are designed to maintain a steady output over a few cycles of lost mains input. This can be achieved by sizing the input capacitor such that its voltage will not fall significantly during the power interruption. The time period over which the SMPS is capable of maintaining an output when mains power is lost is frequently known as 'hold up time'.
SWITCHED MODE TOPOLOGY APPLICATIONS: The switched mode power supply market is now well established within the electronic sector, with a large number of power supply manufacturers worldwide providing a wide range of units for the commercial and military markets. The main endures system for switched mode supplies, or computers, both large main frame and smaller, personnel and word processor, and the various telecommunications systems. A typical system often requires a number of output voltages from its power supply the therefore the majority of the power supplies tend to be multiple output forms typically power supply voltages or +5 V for bipolar logic, +2V, -5V for ECL Logic, +12V for
C MOS Logic, +12V, +15V for operational amplifiers and +24V for DC motors such as disc drivers. The topologies and control methods use to achieve the desired output voltages in the various power ranges tends to vary from manufacturer to manufacturer. In general switching regulators are usually used as secondary regulators on multiple output units, isolated single ended configuration are; used in low power single or multiple output AC to DC convertors and multiple switched apologies are used for higher output power application. Also; used as secondary regulators in some multiple output power supplies are linear regulators, mainly 3 terminals integrated circuits in low current outputs and magnetic amplifiers for higher current outputs.
ADVANTAGES OF SMPS:
There are three main advantages of switching power supplies. They are
1. Switching elements operate as a switch by avoiding their operations in the active region. A significant reduction in power loss is thus achieved. This results in a higher efficiency (70%-90%).
2. Since high frequency transformer is used the size and weight of switching supplies is significantly reduced.
3. SMPS is less sensitive to input voltage variations.
DISADVANTAGES OF SMPS:
1. SMPS has higher output ripple and its regulation is worse
2. SMPS is source of both electromagnetic and radio interference due to high frequency switching
3. Control of radio frequency noise requires the use of filters on both input and output of SMPS. The advantage possessed by SMPS for outweigh they are short comings. This is the reason for the vide spread popularity and growth.
APPLICATIONS OF SMPS:
Ø Switched-mode PSUs in domestic products such as personal computers often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with rated frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V.
Ø Compact Fluorescent Lamps use a simple form of boost converter to generate the required 1200 V ignition and 600 V for sustained operation from the mains.
Ø More on aircraft electric power: Avionics, Airplane ground support.
Ø In the case of TV sets, for example, an excellent regulation of the power supply can be shown by using a variac. For example, in some TV-models made by Philips, the power supply starts when the voltage reaches around 90 V. From there, one can change the voltage with the variac, and go as low as 40 V and as high as 260 V (a peak voltage of 260×sqrt (2) ≈ 360 V p-p), and the image will show absolutely no alterations.
Ø Most modern desktop and laptop computers also have a voltage regulator module -- a DC–DC converter on the motherboard to step down the voltage from the power supply or the battery to the CPU core voltage, which is as low as 0.8 V for a low voltage CPU to 1.2–1.5 V for a desktop CPU as of 2007.
FUTURE SCOPE:
Most commercial switch mode power supplies in the market today operate in the range 10 KHz to 50 KHz. There is now growing trend in research work and new power supply designs in increasing the switching frequencies upwards to 100 KHz and above. The reason being to reduce even further the overall size of the power supply in line miniaturization trends in electronic and computer systems. MOSFETs inherit lack of storage and fall time affects when turned off.
Therefore MOSFETs are now increasingly replacing BJTs in new designs operating at much higher frequencies. But still the intrinsic characteristics of the MOSFET produce a large on resistance which increases excessively when the devices breakdown voltage is raised. Therefore, power MOSFET is only useful up to voltage ratings of 500V. Another new device likely to displace the BJT in many high power applications is the insulated gate transistor (IGT). This device combines the low power drive characteristic of MOSFET with the low conduction losses and high blocking voltage characteristic of the BJT.
Therefore the device is highly suited to high power, high voltage applications.
In future, more and more integrated power devices will be introduced so simplifying board layout and reducing component count.
The driving force in every manufacturers design will always be the combined component and production costs. Therefore, any new device or topology will have to justify its implementation based on mainly commercial criteria.
CONCLUSION:
A Switching mode power supply is a power supply that provides the power supply function through low loss components such as capacitors, Inductors and Transformers and the use of switches that are in one of the two states, on or off. The advantage is that the switch dissipates very little power in either of this two states and power conversion can be accomplished with a minimal power loss, which equates to high efficiency. SMPS, Designs relay upon the efficiency of a switch to control amount of power with relatively little losses.
The primary advantage of the switching mode power supplies is then can accomplish power conversion and regulations at 100% efficiency given ideal parts. All power losses are due to less than ideal parts and power loss in the control circuitry.
Bibliography:
1) POWER ELECTRONICS BY Dr. P. S. BHIMBRA
2) POWER ELECTRONICS BY MUHAMMAD H. RASHID
WEBSITES:
No comments:
Post a Comment
leave your opinion