Analog Meets Digital Part 5: Power Supplies

Designing analog electronics hardware is less orderly and predictable than digital electronics design. In Part 5 of this series, Andrew focuses on one of the least glamorous but most critical components of electronic circuits—their power supplies. He presents real-world examples, tips, and pitfalls of mains power, offline linear and switching supplies, USB power delivery, and disposable and rechargeable lithium batteries.

- By Andrew Levido

All electronic circuits require a power supply, so it's a part of circuit design with which we have probably all been involved. It is not the most exciting part of developing a new project, so it does not always get the attention it deserves. Unfortunately this lack of attention can occasionally return to bite us in the rear end. I would wager that most of you have chased down circuit problems that were ultimately traced to the power supply.

The design of power supplies for electronic projects is not one topic we can work through methodically, as we have for some of the other areas I have covered in the previous four parts of this article series [1,2,3,4]. Instead, I am going jump around a little and share some real-world examples of power supply circuits I have designed. Along the way I will give some design tips and point out some potential pitfalls.

MAINS POWER

The first step in designing a power supply is to define and characterize the power source. The domestic electricity mains—the AC power supplied by the electrical grid, and used either directly by appliances or converted to low-voltage DC for electronics—is a common power source for many electronics projects, so let's start there.

In most parts of the world, the mains is nominally 220V, 230V or 240V at 50Hz. In the US and a few other countries, it is 110V at 60Hz, and in Japan, it is 100V and 50Hz or 60Hz, depending on location. These voltages are nominal, and it is not uncommon for the specifications to permit variations of up to ±15% over the these values.