Learn the Art and Science of Switching Power Supply Design
My first design project after becoming a newly minted electrical engineer in 1975 was the HP 9878A I/O Expander for the HP 9825A desktop computer. I was given the responsibility of designing all of the electronic aspects of the project and worked in tandem with Jerry Nichols, the HP engineer in charge of the project’s mechanical design.
In concept, the I/O Expander was very much like one of today’s USB hubs. Take a computer’s I/O port and “expand” it into several ports. The HP 9878A I/O Expander’s job was to take one I/O slot and transform it into seven slots. As the I/O cards all had code names based on spices – Parsley, Sage, and Rosemary – the code name for the I/O expander project immediately became “Spice Rack.” Spice Rack was introduced in 1976 (Figure 1).
Figure 1: The HP 9878A I/O Expander (circa 1976) used linear power regulation for its 5, 12, and -12 volt power supplies because switching regulator design was still an art form back then. (Photo credit: Steve Leibson)
Part of that design job involved supply power to the I/O cards plugged into the expander’s seven slots. Each card could draw a maximum of 500 milliamps (mA) from the I/O slot’s regulated 5 volt rail, and 100 mA from the regulated 12 volt and -12 volt rails. So, the power supply in the I/O expander needed to provide 3.5 amps at 5 volts (in addition to the 5 volt supply current required by the I/O expander’s internal circuitry), and no more than 700 mA for the regulated 12 and -12 volt power supplies.
The HP 9878A’s 12 volt power supply requirements were easily met by using the new (for the time) µA7812 and µA7912 three-terminal regulators, but there was no easy choice for the regulated 5 volt power supply, which needed to provide nearly 4 amps. After some quick research, I chose the µA723 adjustable regulator IC in a ten pin TO-5 package, which required several additional support components and an external 2N3055 NPN power transistor in a big, beefy TO-3 package.
Aside: The legendary Bob Widlar designed the µA723 adjustable regulator IC in 1967 and it has proven remarkably durable. Like many of Widlar’s analog IC designs, the µA723 regulator is still widely available, even today. Not too shabby for an IC that’s half a century old.
The three-terminal regulators and the 2N3055 power transistor were bolted to an aluminum panel that was solidly attached to the HP 9878A’s cast aluminum chassis for heat sinking, while the board mounted µA723 adjustable regulator got a little clip-on heat sink. Heat dissipation was never a problem with this design.
The µA7812 and µA7912 three-terminal regulators might still be the right choices today, even after nearly 45 years, but the µA723 adjustable regulator and external 2N3055 NPN pass transistor are no longer the choice I’d make. To minimize excessive waste heat, the power supply design in the HP 9878A required a custom wound power transformer so that the unregulated DC power rail was just a little higher than the regulated 5 volt rail. However, back then, HP had its own transformer shop and custom wound transformers were no big deal. Now it’s a different story: I would not use a linear power supply for this task if I designed it today.
Back in 1975, switching power supply design was still in its infancy. In fact, it was more art than science and there were few ICs to help design switching power supplies. In truth, there were none. Today, there’s a lot more science and there are plenty of switching power regulators available in IC form, but I still think that the design of good, reliable, switching power supplies still remains very much an art form. As with all art, artists must first learn the rules. Only then can they break the rules, and only when it’s to their advantage to do so.
There are many resources to help you learn the art and science of switching power supply design. Some of the best resources are evaluation kits and associated coursework developed by Texas Instruments (TI) for its Power Management Lab Kit (PMLK) series. The series includes:
- The PLMK Buck EVM covering the LM3475 and TPS54160 step down switching regulator ICs
- The PLMK Boost EVM covering the LM5122 and TPS55340 step up switching regulator ICs
- The PLMK LDO (low drop out) EVM covering the TPS7A4901 and TPS7A8300 positive, adjustable LDOs
Each of these three products includes an evaluation board and a textbook respectively covering buck and boost switching power supply design and low dropout regulators (LDOs). You may need LDOs even for switching power supply design to derive multiple low current, regulated supply voltages from the regulated switching supply.
All of these TI textbooks include experiments based on the evaluation boards. By the time you have worked through all of the experiments in these three PMLK workbooks, you’ll know a lot of power supply science. I really wish I’d had these resources 45 years ago.