Design & Construction of Switched Mode Power Supplies

Written By: Sachin Mehta

University of Nevada, Reno

Abstract: This laboratory experiment—conducted over a several week period provided us with an insight into key elements of electrical engineering and power regulation. Using specific circuits we were able to vary the outputs, such as the voltage, that can be used in a variety of different types of problems. For example, when a piece of electronics needs a certain voltage input, but a higher voltage is the only one available—a buck converter can assist in giving the CD player, or whatever it might be the proper power. This experiment allowed us to maintain a knowledge of circuits and an understanding of power regulation.

Procedure/Results:

Part 1: Completed

Part 2: In order to complete this laboratory experiment, we needed to be able to apply circuit theory relating to power conversion and regulation. Among the various types of converters, the first portion of this lab was to design a dc-dc step down converter with certain specifications that are listed in Table 1 below. This portion of the laboratory experiment used electrical engineering and circuit theory in order to determine the correct voltage, current, and power ratings for the various components that we needed to implement.

Table 1: DC-DC Step Down Specifications input voltage (V) 10

output voltage (V) 5output current (mA) 250

output voltage ripple (mV) 50

Part 3: The following calculations involve various parameters from above that give rise to certain specifications that we were then able to input and use in the construction of the DC-DC step down converter. In addition: the saturation voltage, the forward voltage, and frequency were taken from the MC34063 and 1N5819 datasheets. They are 1.0 V, 0.6 V, and 100 kHz respectively.

ton

toff

=V out+V F

V ¿(min)−V sat−V out

= 5+0.610−1−5

=1.4V

t on+ toff=1f= 1100kHz

=10 µs

t off=ton+t off

t on

t off

+1= 10µs1.4+1

=4.17×10−6 s

t on=10µs−4.17×10−6=5.833µs

CT=(4×10−5 ) t on=(4×10−5 ) (5.833×10−6 )=233.33 pF

I pk ( swtch )=2× I out=(2 ) (250mA )=500mA

RSC=0.3

I pk (swtch )

= 0.3500mA

=0.6Ω

Lmin=V ¿−V sat−V out

I peak

× t on=10−1−5500mA

×5.833µs=46.7µH

CO=I peak (t on+ I off )8V ripple ( pp)

=500mA ×10µs8×50mV

=12.5µF

V out=1.25(1+ R2

R1)=5V

The component ratings for the DC-DC step down converter are now depicted in Table 2 below.

Table 2: Component Ratings Component Type Vmax Imax Iavg Pmax

Transistor 10 500 250 5Inductor 10 500 250 5Resistor 10 500 250 5

Capacitor 10 N/A N/A 5Diode 10 500 250 5

Part 4: Using Mouser.com, it was possible to determine real parts that could be used for the inductor, diode, and capacitor.

The inductor that would be used in the design: Cooper Bussmann, Part #: 04-SDH2812-470-R

Diode that would be used in the design: Vishay Semiconductors, Part #: 78-V1OWM100 M3/I

Capacitor used in design: Kemet, Part #: 80-C1812X102JDG

Part 5: This portion of the laboratory experiment asked for circuit design and implementation using the program known as MultiSim. This software has uses among the engineering community in order to simulate various circuits, allowing for study before buying all the real

physical hardware, capacitors, voltage sources, etc. For this section of the experiment, the circuit designed with MultiSim can be seen below in Fig. 2.

Figure 2: DC-DC Step Down Converter MultiSim Circuit Schematic

When running simulations of the circuit, outputs were obtained that looked very reasonable in regards to what results we were supposed to have obtained.

Figure 3 shows the inductor voltage output from the DC-DC step down converter that was developed in MultiSim.

Figure 3: MultiSim Output of Inductor Voltage

On the other hand, Fig. 4 clearly shows the spikes in the plot of current in response to time of the inductor in the DC-DC step down converter simulation. The voltage rises and decreases in a similar way to a spike shape form.

Figure 4: Transient Analysis of Inductor Current in DC-DC Step Down Converted from MultiSim

Figure 5, 6, 7, and 8 depict the transient analyses of the output voltage, output current, transistor voltage, and diode voltage respectively.

Figure 5: Output Voltage of DC-DC Step Down Converter

Figure 6: Output Current of DC-DC Step Down Converter from MultiSim

Figure 7: Transistor Voltage from DC-DC Step Down Converter

Figure 8: MultiSim Output of DC-DC Step Down Converter’s Diode Voltage

Part 6: After simulation of the step down converter was completed using MultiSim, the next part of the laboratory experiment called for obtaining a resistive load for the circuit and implementing the circuit on a breadboard. Waveforms were then obtained for both the discontinuous and continuous modes of the DC-DC step down converter. The continuous mode voltage and current outputs of the different components can be seen below in Fig. 9 to Fig. 14.

Part 7:

Figure 9: Inductor Voltage in Continuous Mode

Figure 10: Inductor Current in Continuous Mode

Figure 11: Continuous Mode Output Current

Figure 12: Continuous Mode Diode Voltage

Figure 13: Transistor Voltage in Continuous Mode

Figure 14: Continuous Mode Output Voltage

In order to complete this part of the laboratory experiment, discontinuous mode outputs of the DC-DC Step Down Converter were acquired. They can be seen below in the next figures.

Figure 15: Discontinuous Mode Inductor Current

Figure 16: Discontinuous Mode Output Voltage

Figure 17: Output Current of the Discontinuous Mode

Figure 18: Inductor Voltage from Discontinuous Mode of DC-DC Step Down Converter

Figure 19: Transistor Voltage of Discontinuous Mode

Figure 20: DC-DC Step Down Converter Discontinuous Mode Diode Voltage

Part 8: In order to measure the waveforms of the inductor current, as well as the output current many methods could be implemented. One process would be to place a 1 Ω resistor in series with the inductor or by using a 1 Ω load to determine current. Then, dividing the newly obtained voltage of the load or inductor component by that resistance (the 1 Ω resistance) gives the output desired. This would result in the current (in amperes) through either the inductor, or the load in question.

Part 9: Determining the efficiency, η, for the DC-DC step down converter was accomplished by comparing the output and input current and voltage. It is known that the ideal relationship between power, voltage, and current is defined as Eq. (1) shows below.

P=V ×I (1)

Table 3: Measurements of DC-DC Step Down Converter Output & Input Current & VoltageInput Voltage

(V) Input Current

(A)Power

(W)Output Current

(A)Output

Voltage(V)Power Out

(W)Efficiency

(%)10 0.154 1.54 0.236 5.12 1.21 78.510 0.143 1.43 0.231 5.18 1.2 83.710 0.13 1.3 0.215 5.16 1.11 85.310 0.115 1.15 0.191 5.15 0.98 85.510 0.102 1.02 0.164 5.17 0.86 84.610 0.86 0.86 0.143 5.17 0.74 8610 0.72 0.72 0.117 5.22 0.61 84.810 0.54 0.54 0.94 5.19 0.49 90.310 0.4 0.4 0.75 5.18 0.39 97.1

Plotting the data from above resulted in an efficiency graph of the DC-DC Step Down Converter. Fig. 21 shows the Output Current vs. Efficiency.

Figure 21: Output Current vs. Efficiency

It is important to note that the plot above clearly demonstrates that as the output current increases, efficiency of the converter decreases at a somewhat steady rate and ceases at nearly 80%. Note, that this also means that as the load resistance increases the efficiency, η, decreases—which can have major consequences in operation. Efficiency of any circuit is desired to be as a high percentage as possible, but cannot always be achieved.

Part 10: The data discovered from Table 3 is reminiscent of the plots from earlier in the report (Fig. 3 to Fig. 8), which implies correct measurements were obtained and the DC-DC step down

converter was developed concisely and properly for the laboratory experiment. The fact is that the figures and data from prior in the report reflect a somewhat more ideal environment with the MultiSim software. On the other hand, the efficiency curve shown in Fig. 21 was obtained from data taken from a physical circuit, implemented on a breadboard. Therefore, this data had sources of error stemming from equipment calibration (or lack thereof), component tolerances, and human error. This laboratory experiment being part of student curriculum meant that the equipment and components that we were given to use were not of the best and most ideal nature and accuracy. If this was research material, or along those lines, better diodes, capacitors, etc. would have been implemented—resulting in a more specific and precise plot of efficiency. In addition, the MultiSim simulation outputs used a MOSFET and diode that were overall different parts than what was used in the bread-boarded circuit. This is a major reason as to why simulation results did not completely reflect the measured results. In addition, switching overvoltages were present in the DC-DC step down converter that was developed and built. In order to ‘bypass’ this error it would have been better to use such capacitors in a parallel setup with the other components. This would have resulted in a somewhat filter effect that would polish and reduce ripples on the voltage.

Part 11: This portion of the laboratory experiment, onward, focused on the design and analysis of another type of converter: DC-DC Step Up Converter. Different specifications were implemented for this converter design than were used in the previous step-down configuration. These specifications are as follows:

However, similarly to before, the saturation voltage, forward voltage, and frequency were taken from the MC34063 and 1N5819 datasheets. They are 0.45 V, 0.6 V, and 100 kHz respectively. The following section details the mathematical calculations that were made to obtain some of the circuit parameters.

ton

toff

=V out+V F−V ¿

V ¿(min)−V sat−V out

=10+0.6−55−0.45

=1.23V

t on+ toff=1f= 1100kHz

=10 µs

t off=ton+t off

t on

t off

+1= 10µs1.23+1

=4.48×10−6 s

t on=10µs−4.83×10−6=5.52µs

CT=(4×10−5 ) t on=(4×10−5 ) (5.52×10−6 )=220.70 pF

I pk ( swtch )=2× I out (t on

t off

+1)=(2 ) (250mA )(1.23+1)=0.446 A

RSC=0.3

I pk (swtch )

= 0.3446mA

=0.672Ω

Lmin=V ¿−V sat−V out

I peak

× t on=10−1446mA

×5.517µs=56.3 µH

CO=(9)I peak(t on+ I off )8V ripple ( pp)

=( 0.1mA ×5.517 µs50mV

)(9)=99.3 µF

V out=1.25(1+ R2

R1)=10V

Part 12: This portion of the laboratory experiment required determining the voltage, current, and power ratings for all of the necessary components for the step-up converter. Table 4 below depicts these ratings.

Table 2: Component Ratings for Step-Up ConverterComponent Type Vmax Imax Iavg Pmax

Transistor 10 100 50 1Inductor 10 100 50 1Resistor 10 100 50 1

Capacitor 10 N/A N/A 1Diode 10 100 50 1

Part 13: Simulation of this converter with MultiSim can be seen below where the circuit schematic depicts the configuration of the DC-DC step up converter. The circuit shows a MOSFET (metal oxide semiconductor field-effect transistor) and a Schottky diode among other types of circuit components. The schematic diagram of the DC-DC step up converter is depicted in Fig. 22.

Figure 22: DC-DC Step Up Converter MultiSim Circuit Schematic

For this circuit converter, simulations were configured using MultiSim as prior in the report and are shown in the next several figures. For example, Fig. 23 can be viewed as the waveform voltage across the inductor.

Figure 23: Inductor Voltage

Figure 24: Current through Inductor

Figure 25: Output Voltage

Figure 26: Output Current

Figure 27: Transistor Voltage

Figure 28: Diode Voltage

Part 14: A resistive load was designed for the step-up converter and it was then implemented on the breadboard as a physical circuit. Actual components were used such as various passive circuit components and equipment such as the function generator. This allowed the design to be able to tested, studied, and analyzed—and was done next in the laboratory experiment procedure.

Part 15: The DC-DC Step Up Converter was implemented and analyzed—with the resulting waveforms shown amongst the next diagrams and figures. The following set of waveforms are outputs of the continuous conduction mode of the inductor.

Figure 29: Inductor Voltage in Continuous Mode

Figure 30: Continuous Mode of Inductor Current

Figure 31: Continuous Mode Output Voltage

Figure 31: Output Current from the Continuous Mode

Figure 32: Continuous Mode Diode Voltage

Figure 33: Continuous Mode Transistor Voltage

Part 16: In order to measure a parameter of output current and inductor current, various processes could have been implemented in the experiment. This would happen in the most ideal way if a simple resistor was placed in series with the inductor from the circuit in Fig. 22.

Then performing division of the inductor would result in the parameter of ‘inductor current’ in ampere units. In order to analyze the DC-DC Step Up Converter in a different way of thinking, another means of reflection was performed and the efficiency of this step-up converter was determined. The output voltage was measured in response to the input voltages, in addition to the current using the relationship from Eq. (1). This showed that the voltage is related seamlessly to both current through and the power. By providing an input voltage and varying the current—measurements and data were obtained that are depicted below in Table 3.

Table 3: Efficiency Data from DC-DC Step Up Converter

Figure 34: Efficiency Plot of DC-DC Step Up Converter

Part 17: The measurements from the efficiency curve that is shown above shows that the data are reflections of the simulation results shown prior in the report. Differences between these data and the plots that were obtained from the MultiSim simulation result from the fact that

occur the simulated outputs are from MultiSim, which uses exact input voltages and component parameters, at the very least. Essentially, these simulations arise from a more ‘ideal’ environment—not a complete ideal—but closer than the efficiency curve data. The plot that is shown in Fig. 34 has data points that stem from environmental factors affecting the experiment and therefore the results that were obtained. A method to improve this could be to implement a higher quality type of equipment pieces—and more precise components like the diodes, transistors, and capacitors. Improving the condition of switching overvoltages would be to simply add capacitors in a parallel scheme with the components from the DC-DC Step Up Converter that were already inputted in place in the circuit.

Summary: This laboratory experiment used and implemented various aspects of engineering processes and provided an in depth insight into a major part of many electrical systems. The switching regulator essentially boils down to the conversion of what is available into the needed or desired portion. In terms of this laboratory experiment, given a voltage—the requirement is to decrease that voltage, or increase it. Which one is chosen all depends on the type of work that is being performed, what equipment is available, and the methodology. The DC-DC Step Up Converter, or Boost Converter, uses the ideology that is the output voltage is increased, the available output current must decrease. These converters use MOSFETs or some sort of power switching process that stems from speed and cost considerations for choice. The components that are used and implemented in the DC-DC Step Down Convert, or Buck Converter, are all those that are used in the Boost and are just rearranged in design. Both simulations and actual circuits were analyzed over the two week period of this laboratory experiment which showed that there is differences and sources of error among the DC-DC converters that were implemented on breadboards. A major source of error that can be seen in the efficiency curve data stems from the fact that the components that were being used had tolerances and were not precise in their parameters. What is meant by this is that the 100 Ω resistor that was used in the Boost Converter circuit was, in fact, 99.2 Ω. This was determined using one of the pieces of equipment in the laboratory room. However, these equipment pieces are not being used for highly sensitive research so their calibration was most likely not completed. Lastly, human error is always a suspicious portion that could take hold of an experiment and affect data negatively. Although the circuit design was performed with a close eye, there is always a possibility that error was made by me or my partner. This laboratory experiment conducted studies of both the Buck and Boost Converter types and provided a quick, but thorough view into their operations and mechanisms of action. With these considerations, I am now more confident in a very important portion of electronics engineering—making me a better versed engineer.