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San José State University College of Engineering/Electrical Engineering EE174: Analog Peripheral for Embedded Systems Section 01, Fall 2016 LAB3 DC-DC Converter Boost Converter Switched mode supplies can be used for many purposes including DC to DC converters. Often, although a DC supply, such as a battery may be available, its available voltage is not suitable for the system being supplied. For example, the motors used in driving electric automobiles require much higher voltages than could be supplied by a battery alone. The answer to this problem is to use fewer batteries and to boost the available DC voltage to the required level by using a boost converter. The DC input to a boost converter can be from many sources as well as batteries, such as rectified AC from the mains supply, or DC from solar panels, fuel cells, dynamos and DC generators. Fig. 3.2.1 illustrates the basic circuit of a Boost converter. However, in this example the switching transistor is apower MOSFET, both Bipolar power transistors and MOSFETs are used in power switching, the choice being determined by the current, voltage, switching speed and cost considerations. The rest of the components are the same as those used in the buck converter illustrated in Fig. 3.1.2, except that their positions have been rearranged. VOUT = VIN / (1 – D) Boost converter Operation Fig 3.2.2 illustrates the circuit action during the initial high period of the high frequency square wave applied to the MOSFET gate at start up. During this time MOSFET conducts, placing a short circuit from the right hand side of L1 to the negative input supply terminal. Therefore a current flows between the positive and negative supply terminals through L1, which stores energy in its magnetic field. There is virtually no current flowing in the remainder of the circuit as the combination of D1, C1 and the load represent a much higher impedance than the path directly through the heavily conducting MOSFET. Fig. 3.2.3 shows the current path during the low period of the switching square wave cycle. As the MOSFET is rapidly turned off the sudden drop in current causes L1 to produce a back e.m.f. in the opposite polarity to the voltage across L1 during the on period, to keep current flowing. This results in two voltages, the supply voltage VIN and the back e.m.f.(VL) across L1 in series with each other. This higher voltage (VIN +VL), now that there is no current path through the MOSFET, forward biases D1. The resulting current through D1 charges up C1 to VIN +VL minus the small forward voltage drop across D1, and also supplies the load. Fig.3.2.4 shows the circuit action during MOSFET on periods after the initial start up. Each time the MOSFET conducts, the cathode of D1 is more positive than its anode, due to the charge on C1. D1 is therefore turned off so the output of the circuit is isolated from the input, however the load continues to be supplied with VIN +VL from the charge on C1. Although the charge C1 drains away through the load during this period, C1 is recharged each time the MOSFET switches off, so maintaining an almost steady output voltage across the load. The theoretical DC output voltage is determined by the input voltage (VIN) divided by 1 minus the duty cycle (D) of the switching waveform, which will be some figure between 0 and 1 (corresponding to 0 to 100%) and therefore can be determined using the following formula: VOUT = VIN / (1 – D) General Description The MCP16251/2 is a compact, high-efficiency, fixed frequency, synchronous step-up DC-DC converter. This family of devices provides an easy-to-use power supply solution for applications powered by either one-cell, two-cell or three-cell alkaline, NiCd, NiMH, one-cell Li-Ion or Li-Polymer batteries. A low-voltage technology allows the regulator to start up without high inrush current or output voltage overshoot from a low voltage input. High efficiency is accomplished by integrating the low-resistance N-Channel boost switch and synchronous P-Channel switch. All compensation and protection circuitry are integrated to minimize external components. MCP16251/2 operates and consumes less than 14 µA from battery, while operating at no load (VOUT = 3.3V, VIN = 1.5V). The devices provide a true disconnect from input to output (MCP16251) or an input-to-output bypass (MCP16252), while in shutdown (EN = GND). Both options consume less than 0.6 µA from battery. Output voltage is set by a small external resistor divider. Two package options, SOT-23-6 and 2 x 3 TDFN-8, are available. Required Task: Build the Boost converter circuit using Microchip MPC16251 as shown above Obtain the Line Regulation: Vary the input voltage from 1.5V to 3.0V and plot the output voltage as the function of the input voltage for a constant output load. Configure the on board evaluation module to generate a regulated output voltage of 3V, and observe the waveforms mentioned in Figure 2.19-2.24 and compare with the simulation results. Vary the input voltage for a regulated output voltage of 3V and observe the change in the duty cycle of the PWM waveform. Use Table below to record the readings. Compare the readings with simulation results and plot the graph between the input voltage and duty cycle. Is the plot linear? Vary the input voltage for a fixed load and observe the output voltage. Use Table below for taking the readings for line regulation Observe the inductor voltage and output voltage. Observe output ripple voltage. References: http://www.learnabout-electronics.org/PSU/psu32.php http://ww1.microchip.com/downloads/en/DeviceDoc/25173A.pdf