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Power Supply Design ComparisonTim Chou
Oregon State University
ECE 441 November 21, 2005
There are many different types of power supplies to electronic circuits. For our senior design project (Hackleman Vox Communicator), it is important that the right type of power supply is selected. One power supply design might be more practical to implement in our project than another power supply design. Some of the things to consider when choosing a power supply are efficiency, circuit complexity, cost, size and the precision of the output voltage regulation. This document will propose a power supply of best fit for our senior design project based on the descriptions of four different power supplies. The four types of power supplies that will be compared include the unregulated, linear regulated, switching regulated and ferroresonant regulated power supply.
Unregulated power supplies are uncommon these days, but they can be used to convert AC to DC voltage. The general design of an unregulated power supply starts with transformer which steps the AC voltage down using the transformers turns ratio. An example of this voltage drop might be from 120 Volts to 20 Volts. This voltage drop would require a transformer with a turns ratio of 6 to 1. After going through the transformer, the AC voltage continues to alternate in polarity. To acquire a constant output the voltage polarity needs to be positive. A full wave bridge rectifier may be used to convert the negative voltage to positive pulsating DC. A large capacitor is then used to smooth the pulsating DC voltage and an inductor can be used to filter the rippling effects. Once the signal goes through filtering components, the signal becomes an unregulated constant DC output. Using an unregulated power supply has its advantages and disadvantages. These power supplies are simple in design, reliable and inexpensive. Furthermore, the power supplies generally produce less heat and are more efficient than regulated power supplies with similar output voltage. Although these power supplies are cheap and inexpensive, they are unpopular because of the DC output voltage dependence on the AC input. Variation or voltage spikes from the AC input can give an unwanted DC voltage output value. Also, ripple output proves to be a problem with unregulated power supplies. Ripple voltages can be as high as 10%.
Linear regulated power supplies are occasionally used to help reduce the problem of input dependence and ripple voltage. The design of a regulated power supply is very similar to unregulated, but a linear regulator is included in the end of the design. The linear regulator keeps the output voltage constant. The difference between the output DC voltage and the minimum input voltage is known as the dropout voltage. If the input is above the dropout voltage, the regulator turns on and the voltage drop across the regulator is turned into heat. If input is below the dropout voltage, the regulator does not act. Thus, the DC output is relatively constant and independent of the AC input. One of the main disadvantages of a linear power supply is its low power efficiency. The power efficiency of a linear power supply is roughly 40% where as the power efficiency for an unregulated power supply and a switching regulated power supply is approximately 85%. Linear power supplies can also be larger in size. Some of the advantages of using a linear power supply include low output ripple, low noise and better regulation than both the unregulated and switching power supplies.
Switching regulated power supplies are the most common type of power supply used today. The main difference between the linear power supply and the switching power supply is how the voltage is regulated. A switching power supply uses pulse-width modulation (PWM) to control the DC voltage output. Here, a switching transistor turns on and off rapidly creating an high and low DC voltage. This voltage is converted by a transformer into a high-frequency AC voltage. Then, the signal goes through abridge rectifier and large capacitors which creates the output. The switching power supply regulates voltage by monitoring the DC output, and the circuit adjusts the duty cycle to keep the output constant. The switching is generally applied in three ways with different effective power ranges. The flyback circuit configuration is a suitable design for 100 watt supply. The forward converter configuration is another application of switching for 80-200 watt power supplies. Finally, the center-tapped push-pull circuit (half-bridge push-pull circuit) is the most complex type and has a range of 150-600 watts. There are several reasons to the current popularity of switching power supplies. One reason is the small size. The switching power supply is smaller than the regulated power supply because the components that operate at 20 kHz frequencies are smaller than the components that operate at 60 Hz. Linear power supplies operate at the 60 Hz rangeand consequently have larger components. With more and more electronic devices reducing in size, a switching power supply is an appealing option. Also, the switching power supply runs at lower a temperature which leads to better power efficiency than linear power supplies. Some of the consequences of using a switching power supply are noise in their output and EMI/RFI emission due to the switching transients. These power supplies are also more complicated in the circuit design, and they are generally more expensive.
Ferroresonant regulated power supplies are another possible option for choosing a power supply. The ferroresonant power supply is unique in that the voltage regulation takes place inside the transformer by core saturation means. The cost to build a ferroresonant power supply is less than the linear or switching power supply because itdoesnt require a linear regulator or components for PWM. The downside of using a ferroresonant power supply is the poor precision of the voltage regulation compared to the linear and switching power supply. The voltage regulation of the ferroresonant power is supply is not much different the unregulated power supply. And, the ferroresonant power supply is heavier than the rest. After reviewing the advantages and disadvantages of the four power supplies, a linear regulated power supply could be the best fit for the design project. It provides the most precise voltage regulation unlike the unregulated and ferroresonant power supply. Voltage regulation is important in this design project, because the microprocessor we will be using will most likely require a constant DC voltage. The switching power supply is suitable option for the design project, but the circuit complexity may take too much time to design. The unregulated, linear regulated, switching regulated and ferroresonant regulated power supply all differ in how each regulates voltage. Each has its own benefits and drawbacks. The linear regulated power supply encompasses benefits more in line with the senior design project constraints of complexity and voltage regulation precision. As the project progresses and modification take place the type of power supply design may change. But, knowing the different design options will prove to save time later in the future.
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http://classes.engr.oregonstate.edu/eecs/fall2005/ece441/groups/g16/white_papers/white_paper_tim.pdf 
