Design And Construction Of A Switch Mode Power Supply

1.0 INTRODUCTION
1.1 BACKGROUND OF THE STUDY
A power supply is an essential part of every electronic device. It converts an AC power line voltage to a steady state DC output voltage as required by all electronic circuits. Power supplies can be categorized into two types: linear and Switch Mode.

Linear power consists of a transformer, rectifier, and filtering and regulator circuits. The rectifiers are used to convert 50/60Hz AC voltage to a pulsating DC. The filters are used to convert the pulsating DC to a smooth voltage. Finally, the voltage regulators are used to produce a constant output voltage, irrespective of the variations in the ac line voltage or by the circuit loadings. The output voltage is sampled by an error amplifier block which compares it with a reference signal and generates an error signal on the basis of comparison [3]. The error signal is applied to a series pass element which alters its resistance accordingly and thus regulates the output voltage. Hence, the series pass element absorbs any changes in the input voltage and any dynamic changes in the output voltage due to the load changes, within its designed tolerance band [1]. The size of the components, such as the transformers and output filters, are very bulky in these supplies. Linear power supplies are not suitable for smaller modern electronics system because of their high power loss, low power density and bulky size.

The SMPS converts the available unregulated AC or DC input to a regulated DC output. The input is taken from the AC mains and then rectified, filtered and fed to a high frequency DC-DC converter. SMPS can be operated within the kHz to MHz frequency range. The increased switching frequencies cause a decrease in the size of the energy storage elements, such as the capacitors, inductors and transformers, in an almost linear manner [3].

This work is aimed at building a Switch Mode power supply of 12v,/5amps.

1.2 PROBLEM STATEMENT
Switch Mode power supply was invented to solve problems observed in linear power supply such as low frequency and efficiency. Linear DC regulated power supplies (“linear supplies”) alter the output voltage to the required value through the power frequency (50/60 Hz alternating current) voltage transformer [8]. After being lowered (or raised) to the appropriate AC voltage value, the waveform is rectified, filtered, and stabilized by a voltage regulation circuit, and is then output as a regulated direct current (DC). These types of power supplies have a common characteristic; the voltage transformer works in the linear range [7]. The noise on the input is not amplified on the output. This makes for a quiet and stable output. But, there are drawbacks. The large static loss of the regulating element requires a large heat sink to cool the device and the physical size of a transformer that works in the power line frequencies (50/60 Hz) is large and heavy.  Switching power supply was invented which does not operate at normal power line frequencies (50/60 Hz) but rather at considerably higher frequencies (several KHz to several MHz) [13]. In a switching power supply, the regulating device operates in the saturation or cut-off area as opposed to the linear power supply regulator which operates in the less efficient linear range. Switching power supply has Small size, lighter weight, and higher efficiency (can reach nearly 99%). It also have larger output ripple, RFI/EMI interference from pulsed power, higher output voltage if failure occurs [14].

1.3 AIM AND OBJECTIVES OF THE STUDY
This work is aimed at building a Switch Mode power supply of 12v,/5amps. The objectives are:

• To build the system prototype
• To overcome challenges seen in linear power supply
• To increase efficiency of power supply used in electronics.

1.4 SIGNIFICANCE OF THE STUDY
This work shall serve as a means of learning to how to build an efficient and high frequency power supply which can be used to power our appliances.

The study will also serve as an avenue of knowing the difference between linear power supply and Switch Mode power supply.

1.5 SCOPE OF THE STUDY
The scope of this work covers building a Switch Mode power supply incorporates a switching regulator to convert electrical power efficiently. It is mainly used for obtaining a controlled dc power supply as output. It is used to convert power (voltage) using switching devices that are turned on and off alternatively at high frequencies. It uses storage components like inductors or capacitors to supply power when the switching device is in its non-conduction state (off-state). SMPS possesses high efficiency and is widely used in various electronic equipment such as computers, battery chargers, and other sensitive equipment requiring a stable and efficient power supply.