Electronic Design

Eric Mendenhall

I design power electronics circuits and systems, focused on audio power amplifiers and switch-mode power supplies (SMPS) under 10 kW. I've been employed in the engineering field for almost 20 years. My career path has always centered around circuit design, but my skills have expanded to include many aspects of product design.

In the course of a typical project, I will design linear and switching circuits, along with all the high-frequency magnetics needed. The circuit designs always include an analysis of the control systems, as they are very important to amplifiers and SMPS. Because any power design requires a thermal design as well, I do a theoretical and empirical thermal analysis. The product must pass stringent conducted and radiated emissions requirements. I run these tests and modify the design accordingly. At high frequencies the pc-board layout is critical, so I design and redesign this as well. Finally, the product must pass safety testing requirements, which is another element I must consider.

After I received my BSEE in 1986, my first design job was at BGW Systems, a manufacturer of professional audio power amplifiers in the 1- to 2-kW range. In those days, the industry was just starting to move away from class B output stages in favor of higher-efficiency topologies such as class G and class H. The industry had not yet adopted switching power supplies. I recall a 1.8-kW class B amplifier that weighed 88 lb.

I realized that SMPS and power factor correction (PFC) would be key in the next generation of products and began studying them after work. I attended Dr. Cuk's SMPS classes at Cal Tech and began an MSEE program at Cal State, focusing on SMPS and linear circuits. After completing my MSEE and designing a few BGW amplifiers and subassemblies, I joined QSC Audio Products. There I designed the SMPS, with PFC, for the most powerful amplifier in the industry, a 9-kW class H product weighing in at 59 lb. Then I moved on to Alesis Studio Electronics, designing lower-power amplifiers and SMPS.

Partially due to the long commute, my home laboratory grew until I had better equipment there than in the office, so my next career move was into consulting. See my lab at www.audiopowerelectronics.com. A key benefit of consulting was that I was well supported by the parent company staff, which allowed me to focus on my core competencies. With that focus came remarkable productivity and job satisfaction. Also, there is a certain sense of fairness from hourly wages, especially during 24/7 workaholic frenzies. It additionally meant that I was never lacking good equipment. I simply bought everything I needed. During that time, I designed products or subsystems for JBL Professional, Harman Multimedia, and Condor DC Power Supplies before joining Line 6, a guitar product manufacturer. There I believe we put the first SMPS in a guitar amplifier.

Later I joined Gibson Labs to design a line of professional sound reinforcement class G audio power amplifiers with SMPS and PFC in the 1- to 3-kW range, weighing 29 lb. The design incorporates several interesting ideas, three of which brought me patents. One involves PFC size reduction, another optimal thermal management, and the third amplifier output stage protection. I'm very proud of my work. You can check it out at http://labs.gibson.com/amplifiers.html.

As the industry moves toward higher efficiencies and lower weight, class D is becoming increasingly accepted. This kind of amplifier is really a switch-mode power supply reproducing an audio signal. Class D in combination with SMPS is the ultimate in efficiency. Theoretically, there are no intentionally dissipative linear elements, as all losses are due to parasitics. I'm sure these will be central to the next phase of my career—somewhat ironic for a linear circuit designer!

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