## Introduction

An engineer in my group is learning Mathcad and he asked me if could I show him how to use Mathcad to design the control input for a variable voltage power supply. After looking at the problem, I decided this would be a nice test case for my first use of Mathcad Prime 2.0.

## Background

Let's begin the exercise by looking at how we will be controlling the output voltage of the power supply. Take a close look at Figure 1.

On the left-hand side of Figure 1, I show the control input for standard fixed voltage power supply. For this case, the power supply sets the output voltage (labeled OUT) to a value that will maintain a voltage on the feedback pin (labeled FB) of 1.23 V. On the right-hand side of Figure 1, I show the control input for a variable voltage supply. In this case, I sum the output voltage from Digital-to-Analog Converter (DAC) onto the FB pin along with a scaled version of the output voltage. As I increase the DAC voltage, the output voltage must drop to maintain a 1.23 V level on the FB pin. Similarly, a decrease in the DAC voltage means the output voltage must increase to compensate.

The design requires that I select three resistor values: *R _{1}*,

*R*, and

_{2}*R*. There is a bit of algebra associated with determining the values of these resistors. It turns out that I saw a designer using trial and error to determine these values. Since we have a tool like Mathcad available, I thought this problem would make a nice demonstration of the power available in a computer algebra system.

_{3}## Analysis

### Requirements

The requirements are pretty basic:

- The maximum power supply output voltage is 60 V.
- The minimum power supply output voltage is 20 V.
- The maximum DAC voltage is 2.5 V.
- The minimum DAC voltage is 0 V.
- The feedback pin must be maintained at 1.23 V.

### Calculation

Figure 2 shows my Mathcad Prime 2.0 worksheet.

This analysis shows that my three resistor values are:

*R*= 317.8 KΩ_{1}*R*= 10.0 KΩ_{2}*R*= 19.9 KΩ_{3}

## Conclusion

This project worked out well. It was a good exercise for Mathcad Prime 2.0. I liked the fact that it let me use units in the numerical solver. The interface was a fairly straightforward extension of the Mathcad 15.0 interface. I will continue to try Mathcad Prime on further exercises.

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thanks for providing brief description about Mathcad,it is really helpful for everyone.

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