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.