Estimated reading time: 7 minutes

Tim's Takeaways: The Misadventures of High Voltage and Other Related Problems with Power

If you’ve read my column before, you know how much of a fan I am of aviation, especially when it comes to older airplanes. You can imagine how ecstatic I was when, 11 years ago, my wife gave me the greatest gift of all: a half-hour ride on a fully restored WWII B-17 Flying Fortress. This plane was the real deal, folks. A four-engine heavy bomber stuffed with gun turrets, narrow and cramped crew areas, and the cold hard metal of unforgiving hardware that could give you a serious bruise on the forehead if you weren’t paying attention. From wingtip to wingtip, the “Liberty Belle” was saturated with a rich ambiance of history that emanated from every one of her nearly 400,000 rivets that held this aircraft together. One by one, the four 1,200 horsepower Wright Cyclone engines came to life, and together with the other passengers and crew, we took off on that warm and sunny day from the airport in Hillsboro, Oregon.

There were a couple of things that happened during the trip that you might find interesting. For example, I really did poke my head out of the open upper hatch in the radio room. Maybe sticking my head out of an airplane in a 120-mph slipstream wasn’t the brightest thing I’ve ever done (I did take my glasses off first), but I got a great picture out of it. At another point in the trip I was in the nose of the plane peering through the Norden bombsite at the city below me. Even though I knew that the dummy bombs were not real, and they were permanently attached so they couldn’t be dropped, (plus the bomb bay doors were closed, and the bomb release mechanism wasn’t even electrically connected), I still couldn’t make myself flip the bomb salvo switch. I think that it just had gotten a little too real for my imagination at that point, and I didn’t want to take a chance on blowing up the city.

There were plenty of other interesting things about that trip such as exploring the aircraft, playing with the radio equipment, training the guns around to get my sights on imaginary enemy aircraft, and enjoying the trip as the pilots flew us across the countryside. Oh, and just for excitement, about halfway through the trip, the pilots announced that the plane had suffered a problem in the electrical system. They didn’t give us much in the way of details, but I did hear the phrase “a battery exploded,” which resulted in the electrical system being knocked out on the left side of the plane. Thankfully this didn’t affect how the plane was flying, as an older plane like this didn’t rely as much on electrical systems for flight as a modern plane would. However, there was one big problem that was unavoidable: the pilot was unable to lower the left side main landing gear wheel and the tail wheel from the cockpit. As you can guess, without all of the landing gear down and locked, our landing was undoubtedly going to end up in a crash.

Understanding Higher Voltage
Ever since that flight I’ve often wondered about that electrical problem, and the mental image of a battery exploding somewhere in the internal workings of the B-17 is a very intriguing one to say the least. Did the battery suffer an internal short and failure, or did a mechanical anomaly cause something to get crossed up resulting in a more exciting short with sparks and smoke? Maybe the incident was due to an over-voltage problem somewhere, or just maybe the battery actually did explode? Whatever was the root cause of the problem, it is obvious that our plane experienced a serious electrical power failure that was unplanned and a surprise for everyone on board. With that flight serving as an example of the consequences of an electrical failure, it has motivated me to understand more about the different requirements of power and higher voltages in my own small world of circuit board design.

Most PCB designers are aware of the unique power requirements for the garden-variety digital design. Power supply component placement must be kept tight to keep the trace routing as short as possible for lower inductance and to reduce the potential of generating noise. The connections between these parts should be routed with wide traces on one layer only, avoiding the use of vias, except for the vias that extend into the ground plane to help with thermal dissipation. And it’s also important to keep these power circuits isolated from sensitive digital and analog circuitry for the best signal and power integrity of the design. However, what designers are less familiar with is how to work with high-voltage circuitry, which is increasing in importance as more high-voltage design work is being done.

High-voltage circuitry can be found in everything from computers and consumer products to large industrial equipment, and designers must learn the intricacies required to lay them out. For instance, higher voltages can arc between exposed metal conductors on a circuit board if those leads, pads, traces, or other areas of metal are too close to each other. With the unexpected arcing of high-voltages also comes the breaking down of the different insulating barriers in and on a circuit board. Eventually this continued arcing can create an actual short between the exposed conductors, creating a potentially dangerous situation that can cause a lot of damage.

Clearance and Creepage
To eliminate the potential of arcing in high-voltage circuits on their boards, it is essential for PCB designers to understand both clearance and creepage spacing rules in their design. While clearance is the direct space between two metal conductors from a line-of-sight perspective, creepage is the space between the two conductors that follows the physical contours of the circuit board. These clearances must be adhered to for the board to avoid problems with its high-voltage circuitry. Even placing high-voltage components on opposite sides of the board may cause a problem if the parts end up violating the allowed creepage spacing distance.

There are many other aspects of high-voltage circuits that PCB designers also need to understand in order to successfully lay out these boards. For instance, which board materials are better for applications of high-voltage, what copper weight should be used, and how can that copper be distributed throughout the board to ensure its manufacturability? Fortunately, this edition of Design007 Magazine is focused on high-voltage design, and there are numerous experts weighing in here that can offer some help.

But Did We Crash?
Now, back to my crippled B-17 flight. If you’ve ever watched the movie “Memphis Belle,” you know that B-17s were equipped with manual cranks for lowering the landing gear in case of an emergency. During war time, these planes were always getting shot up and losing their electrical or hydraulic systems, and the cranks were used more often than you might expect. The crew on our plane was able to crank down the landing gear manually without too much difficulty, allowing us to land normally. And with the time it took to crank the wheels down, plus the need to burn off some fuel for safety’s sake, our trip extended much longer than the half hour it was originally scheduled for. As you can guess, this didn’t generate any complaints from me. In fact, I tried to get the flight engineer to let me have a go at cranking the gear down myself so I could claim partial credit for “landing” the plane. Sadly, he flatly denied my request, mumbling something about insurance liability or some such nonsense. Pity.

And with that our trip was all but over except for one last detail. You see, without a functional electrical system, the warning bell we were told to listen for didn’t ring as it was supposed to when it was time to strap in for landing. A couple of us were still in the nose of the plane wondering why the airport was getting so big in the window when the crew realized we didn’t know that the Liberty Belle was on its final approach for landing. Once they told us to run back to our seats (which isn’t easy in that plane), we barely had enough time to buckle up before the wheels plopped safely back down onto Hillsboro’s runway. With several fire trucks following us “just in case,” we taxied to our parking spot where the engines were finally shut down and our trip was safely concluded. And although some of my fellow passengers were experiencing different levels of anxiety due to our in-flight emergency, I must confess that I had the time of my life.