# Variable Voltage Power Supply Control Input Design Example

## 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: R1, R2, and R3. 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.

## 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:

• R1 = 317.8 KΩ
• R2 = 10.0 KΩ
• R3 = 19.9 KΩ

## 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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