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Switch Mode Power Supplies Maxx Coral 4/3/2015 Switch Mode Power Supplies Abstract This application note explains the basic theory on the principles of how switch mode power supplies operate. This article is for the engineer who has minimal experience with switch mode power supplies. A brief discussion is given to the advantages and disadvantages of a linear regulator versus a switch mode power supply. The principles of a switch mode power supply will be learned and can then be applied to choosing and design a supply. Example circuit schematics along with figures and simulation data will enhance an engineer's knowledge of the operation of switch mode supplies. The strengths and weaknesses of different switch mode power supply solutions will be shown. Introduction Most electronic circuit designs that are created need a voltage rail to be increased, decreased or regulated for the operation of the integrated circuit (IC) to perform in a correct and reliable fashion. With the endless different solutions that are currently present, it can be difficult to know what the best option for your current design application. Every solution will have different efficiencies, transient responses, output regulation, deliverable power and cost and it is the engineer who in the end needs to make a finally decision based on the many different variables. Linear Regulator The first question you need to ask yourself is which power converter is right for your design? Linear regulators can simply a power converter design. Some linear regulators are as simple as a three pin IC, with an input and output ripple capacitor as seen in Figure 1. Figure 1: Linear Regulator Schematic The simplicity of a linear regulator does not come without a trade-off. Relative to a switch mode power supply, a linear regulator is less efficient. The regulators usually operates around 25% efficient when your output voltage is about half your input voltage. The other 75% of the power is dissipated in thermal energy. This dissipation of thermal energy leads to larger IC package size and required heat sinks to keep the IC operating in a proper temperature range. When space on a printed circuit board (PCB) is critical to a design, a linear regulator as a power convertor is not the best choice. When your output voltage is near the input voltage, the efficiency does increase. Another drawback of a linear regulator is it only works when a larger DC voltage needs to be stepped-down to a lesser DC voltage. Thus any application requiring a step-up in voltage cannot use such a regulator. There are applications where linear regulators provide greater performance for a power converter than their switching counterparts. For low power designs that are not space limited, and not thermally constrained, a linear regulator can be a simple low cost solution. The cost of components and design time are greatly reduced. With linear regulators no external inductor is needed which leads to less electromagnetic interference (EMI) issues for other nearby circuit elements. EMI is less likely to be a problem. Since the linear regulators do not have a switching frequency there is no switching related electromagnetic radiation being generated by the IC. Finally, linear regulators have faster transient responses due to their internal feed-back loop. Switch Mode Power Supply What if linear regulators will not work in your design? Then consider using a switch mode power supply. A switch mode power supply (SMPS) are generally chosen by designer for their increased efficiency. SMPS efficiency can be greater than 90%, which means smaller package optimizing the space on a PCB. With only 10% of the power lost to thermally energy, heat sinks can be eliminated from the design again saving space and cost. There are three main topologies for SMPS and they are a buck converter, boost converter, and buck/boost converter which is sometimes called inverting regulator. Theory of Operation A SMPS has the ability to convert a DC input voltage to different DC output voltage values, depending on how the four main discrete elements are arranged. The main elements in a SMPS are an inductor, a capacitor, a diode, and a transistor. As mentioned previously, these four elements can up the three main topologies used in industry and they are a buck converter, boost converter, and buck/boost converter. Simplified examples of these topologies can be seen in Figure 2. Figure 2: Switch Mode Power Supply Topologies The SMPS operates by controlling the path of current that charges the inductor, differently than the path of current that is discharged from the inductor. The transistor being used as a switch being able to change its impedance to high and low states and the diode works well together to achieve the unique current flow of each topology. A microcontroller of some sort would be connected to the transistor to control the switching of its state. The microcontroller would pulse width modulate the base of a BJT or the gate of a MOSFET to create a square wave. This square wave in turns controls the duty cycle and switching frequency of the SMPS. The switching controls the ramping of current into and out of the inductor, which in turns is related directly to the output voltage. Current that is stored in the inductor during the charging cycle, is pumped into the load capacitor and resistor when the transistor's state changes causing the inductor's discharging cycle. When the inductor is charging and not delivering power to the load resistance, the output capacitor holds the output voltage near the desired voltage. Since the capacitor cannot hold the DC voltage output completely constant, a ripple voltage appears across the load. This ripple voltage is directly dependent on switching frequency, duty cycle, peak inductor current and the capacitor's capacitance value. When a minimal ripple voltage is required a low pass filter can filter out the high frequency ripple voltage that is riding on the DC voltage, leaving a near perfect DC output voltage. An interesting phenomenon occurs in a SMPS, since the inductor is changing between absorbing power and delivering power. Since the current cannot change direction instantaneously in an inductor, one will see the voltage across its terminals will change polarity instantaneously, as seen in Figure 3. Figure 3: Inductor Voltage and Current Graphs Buck Converter When a design specification requires that the output voltage be less than the input voltage, an engineer would want to choose a buck converter topology, as seen in Figure 2. Boost Converter When a design specification requires that the output voltage be greater than the input voltage, an engineer would want to choose a boost converter topology, as seen in Figure 2. Buck-Boost Convert or Inverting Regulator When a design specification requires that the output voltage be greater in magnitude than the input voltage yet an inverted sign, an engineer would want to choose a buck-boost converter topology, as seen in Figure 2. Summary In summary linear regulators and switch mode power supplies both have their own place in many different electronic applications. Linear regulators are simple to use and easy to quickly test with, but have a draw back when it comes to efficiency and package space. Switch mode power supplies offer a versatile variety of different output voltages, but are more costly to design and buy. SMPS also will radiate more electromagnetic noise which can be problem for nearby circuitry. Engineers and designers need to look at the given design specifications and make an informed decision on which power converter is right for their design. References 1. Corporation, Linear Technology. AN140 - Basic Concepts of Linear Regulator and Switching Mode Power Supplies (n.d.): n. pag. Basic Concepts of Linear Regulator and Switching Mode Power Supplies. Oct. 2013. Web. 2. "An Introduction to Switch-Mode Power Supplies." - Application Note. N.p., n.d. Web. 02 Apr. 2015. 3. Dr. Gregory M. Wierzba "ECE-402 Application of Analog Integrated Circuits" - Spring 2014 .
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