03-09-2012, 12:07 PM
DESIGN AND IMPLEMENTATION OF SWITCHED MODE POWER SUPPLY USING PWM CONCEPTS
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ABSTRACT
Switched Mode Power Supply (SMPS) is the most prevailing architecture for DC power supply in modern systems, primarily for its capability to handle variable loads. Apart from efficiency the size and weight of the power supplies is becoming a great area of concern for the Power Supply Designers. In this thesis an AC to DC converter SMPS circuit, having a power MOSFET for switching operation and a PWM based Feedback circuit for driving the switching of the MOSFET, is designed and simulated in NI MULTISIM circuit design environment. Further the same circuit is Hardware implemented and tested using NI ELVIS Suite.
In this design the line voltage at 220V/50Hz is taken as input, this voltage is stepped down, rectified and passed through filter capacitor to give an unregulated DC voltage. This unregulated voltage is chopped using a MOSFET switch, driven by PWM feedback signal, to control the output voltage level. An Isolation Transformer is used to isolate the DC output from input supply. The transformer output is again rectified by the high frequency Diode bridge rectifier and is filtered using a capacitor to give the regulated DC output. A Voltage regulator is connected to give the precise voltage output.
INTRODUCTION
Power Electronics is the art of converting electrical energy from one form to another in an efficient, clean, compact, and robust manner for convenient utilisation. The never ending drive towards smaller and lighter product poses serious challenges for power supply designers.
The aim of the project is to design, test and implement a switched mode power supply (SMPS) circuit for AC to DC conversion, having a power MOSFET for switching operation and a PWM based feedback circuit to drive the MOSFET switch using NI MULTISIM circuit design environment and NI ELVIS Breadboard.
THERORITICAL BACKGROUND
Power Supplies:
A power supply is a component, subsystem, or system that converts electrical power from one form to another; commonly from alternating current (AC) utility power to direct current (DC) power. The proper operation of electronic devices ranging from personal computers to military equipment and industrial machinery depends on the performance and reliability of DC power supplies.
Power supplies are circuits that generate a fixed or controllable magnitude dc voltage from the available form of input voltage. Integrated-circuit (IC) chips used in the electronic circuits need standard dc voltage of fixed magnitude. Many of these circuits need well-regulated dc supply for their proper operation.
SMPS
The prevailing DC power supply architecture in most modern systems is the Switch-Mode Power Supply (SMPS), which is known for its ability to handle changing loads efficiently. The power signal path of a typical SMPS includes passive, active, and magnetic components. The SMPS minimizes the use of lossy components such as resistors and linear-mode transistors, and emphasizes components that are (ideally) lossless: switch-mode transistors, capacitors, and magnetic.
Like a linear power supply, the switched mode power supply too converts the available unregulated ac or dc input voltage to a regulated dc output voltage. However in case of SMPS with input supply drawn from the ac mains, the input voltage is first rectified and filtered using a capacitor at the rectifier output. The unregulated dc voltage across the capacitor is then fed to a high frequency dc-to-dc converter. Most of the dc-to-dc converters used in SMPS circuits have an intermediate high frequency ac conversion stage to facilitate the use of a high frequency transformer for voltage scaling and isolation. The high frequency transformer used in a SMPS circuit is much smaller in size and weight compared to the low frequency transformer of the linear power supply circuit.
PWM Concepts
PWM or Pulse Width Modulation is the modulation technique where frequency and Amplitude of the pulse signal is not varied and the pulse width or the duty cycle is varied to encode the information. A common use of PWM is to control the average current or Voltage input to a device.
In this project the PWM Signal is generated as feedback control signal for driving the switching of the MOSFET switch. The output Voltage is taken as the reference level. A sine wave is generated using LMH6622MA OPAMP and 555 Timer. This sine wave signal is compared with the reference voltage using LM311 comparator, and the pulse width of the PWM is determined by this comparison. This switching period determines the voltage at the output. Thus if there is any change at the output Voltage, the corresponding change in the PWM pulse width will nullify its effect and the output voltage will be restored to desired value.
CONCLUSION
Suitable components were selected and tested for desired performance. Functional verification was performed on combined circuit of the selected components for open loop network both in NI MULTISIM and on NI ELVIS. PWM based feedback network was successfully designed tested and implemented both in NI MULTISIM and in Hardware using NI ELVIS Suite. The Design and implementation of desired SMPS circuit was successfully completed.