Project 2 Final System Design and Performance Report. Triple Output Power Supply

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1 Taylor Murphy & Remo Panella EE /12/18 Project 2 Final System Design and Performance Report Triple Output Power Supply Intro For this project, we designed a triple output power supply using switch mode power supply regulators. The main component of this circuit is the LM2576-ADJ, which is a step down converter. This converter allows us to take an unregulated DC voltage and step it down to create a regulated voltage in an efficient way. This converter has an internal regulator so it can provide line and load regulation. All that is really needed to create a regulated voltage output with this converter is a diode, inductor, and resistive feedback network. The initial goal of this project was to create a power supply capable of supplying 0 20 V, 0 (-20) V, and 0 6 V and use a microcontroller to control an LCD screen. The microcontroller aspect was scrapped due to time constraints and difficulty so the final design is a switch mode power supply capable of supplying voltages near the initial goals. We aren t able to produce voltages below 1.23 V, because this is the reference voltage that the regulator uses. In order to power the circuit, we are using a wall transformer. Essentially, this circuit has 5 stages: Power Supply, 5 V output, 0 20 V Output, 0 (-20) V, and 0 6 V. This design of the circuit hit a few snags but in the end, we were able to create a triple output power supply using switch mode converters.

2 Circuit Schematic 2700 uf

3 Schematic Details Power Supply: The circuit is powered off a 16.5 VAC wall transformer. This is then passed through a full wave rectifier and a smoothing capacitor. This provides us with an unregulated DC voltage. This is then used with the converters to form the triple output power supply uf 5 V Output: This section regulates the input voltage to a steady 5 V. The adjustable converter allowed us to set up a voltage divider with the feedback pin to regulate the output to a constant 5 V. This output was meant for the microcontroller and LCD screen.

4 0 20 V Output: This section takes the unregulated voltage and converts it to a DC voltage in the range of 1.23 to 24 V. The voltage output is controlled by adjusting the potentiometer. 0 6 V Output: This section takes the unregulated voltage and converts it to a DC voltage in the range of 1.23 to 9 V. The voltage output is controlled by adjusting the potentiometer.

5 0 (-20) V Output: This section is used to track the voltage from the 0 20 V output section and invert it to get a negative voltage. The voltage output is controlled by adjusting the potentiometer of the 0 20 V Output section. The voltage output can be fine-tuned by using the potentiometer in this section. Design Details This main focus behind this design was to provide steady regulated DC voltages at various ranges. To do this, we had to start with AC voltage from a wall plug. We used a 16.5 VAC wall transformer to step down the 120, 60 Hz wall voltage. We then passed this through a full wave rectifier and a large capacitor to smooth this voltage and obtain an unregulated DC voltage. This voltage is then stepped down in the many various stages. At the input to every stage, there is an input capacitor of 100 uf. This is required by the converter to maintain stability. The output from each stage is in the buck configuration with a diode, inductor, and capacitor. The lower voltage stages use a 68 uh inductor while the larger stages use 100 uh inductors. Also in every stage but the inverting one, the ground and On/Off pins are grounded. The On/Off pin needs to be grounded or else the converter is put into shutdown mode. The 5 V output stage used a resistor network of a 2.2 kohm and a 6.8 kohm resistors to provide

6 feedback to the converter. The equation to determine this was V OUT = V REF (1 + R 2 R 1 ) with the reference voltage for the converter being 1.23 V. This provides us with a theoretical output of V. This output would have been used to power the microcontroller and LCD screen but since we didn t include those, it can be used as a steady 5 V output. The 0 6 V and 0 20 V output stages follow a similar principle to the 5 V stage except instead of fixing the resistor values, we use potentiometers. The 0 20 V stage used a 100 kohm potentiometer with a 2.2 kohm resistor while the 0 6 V stage uses a 10 kohm potentiometer and a 1.5 kohm resistor to provide the feedback network. These values were chosen because it allows us to reach our desired output range and even slightly exceed it. The 0 (-20) V stage operates slightly differently. By not connecting the ground pin, On/Off pin, diode, and capacitor to ground, the circuit bootstraps the ground pin to the negative output voltage and provides us with negative voltage. This setup will just invert whatever the input voltage is so we connected it to the output of the 0 20 V stage. This essentially acts like the track function on the lab power supplies. So in order to vary the voltage of the negative stage, you use the potentiometer in the positive stage. This allows for large changes while the potentiometer in the negative stage allows for small adjustments (range of 500 mv). Below is an image of the prototype circuit.

7 Circuit Performance In order to test the circuit, we first tested it with the lab power supply as the voltage source for each stage. We tested each stage separately and then as a whole. We found the circuit performed well and was able to meet the voltage ranges we had set. We then included the power stage with the wall transformer and rectifier. This didn t introduce any issues with the circuit. We tested to see the output ranges for all the various stages. The 5 V stage stays at a constant V. The 0 6 V stage output is able to be varied from V. the 0 20 V stage output is able to be varied from V. The 0 (-20) V stage output is able to be varied from 0 (-23.1) V. There is a slight difference between the voltage at the positive and negative 20 V stages, meaning it doesn t track perfectly. This can be fixed through by adjusting

8 the potentiometer of the 0 (-20) V stage to fine tune this output and make it match the positive voltage stage. All of the stages have very little ripple, the largest ripple we saw was around 10 mv. Also, when adjusting from a higher voltage to a lower voltage, there is a slight delay between adjusting the potentiometer and the output being changed. This isn t of too much concern though. Overall, these tests met our expectations for the circuit. Conclusion Overall, this circuit worked well for the goal in mind. We didn t include a microcontroller or LCD screen but all that would have done is show the output voltages. The reason of not including these were time constraints and difficulty. The microcontroller and LCD would be powered by the 5 V stage. Doing some research on how the microcontroller would work though is it would use its reference voltage to compare it to the voltage it was reading from the various stages. There would then be a calculation that could create a data output to the LCD and display the voltage of the various stages. The circuit works well but is a little tricky to set voltages to something exact unless you are patient. The voltages are very sensitive to tweaks in the potentiometer. So when setting a voltage, it is best to adjust the potentiometer very slowly to obtain the desired voltage. This circuit is a nice tool to use as a power supply at home if you don t have a big fancy power supply. You will need to use it with a voltmeter in order to see what voltage you are setting it at before connecting it to an actual circuit. Overall, this project turned out well and is an efficient power supply.