This is a guest article by Adrian Mikolajczak of Fairchild Semiconductor.
Reverse polarity, which is the result of a steady-state reverse bias or a negative transient, can cause serious problems in an electrical system. Most silicon devices are not designed to withstand negative polarity events. If not protected against, it can lead to total electrical failure or, if the condition is serious enough, create a fire. The risk of reverse polarity is a real threat in a wide range of very popular applications, including mobile electronics, battery-powered systems, devices that connect to an automotive power supply, DC-powered toys, products with barrel-jack connectors, or any DC device subject to negative hot-plug or inductive transients. Systems that support USB connectivity and/or USB charging are particularly susceptible.
What causes reverse polarity?
There are several things that can trigger a reverse polarity event. By and large, these things are beyond the control of the manufacturer to prevent, and that makes it important for designers to add protection to the system itself, before it leaves the factory. Having onboard protection against reverse polarity helps preserve the system and can reduce the number of returns resulting from damage caused by external factors.
Here are some of the most common causes of reverse polarity:
Using an after-market charger or power supply produced by a third party. There's a growing market for third-party chargers, and not all of them are designed with prevention of reverse polarity in mind. For example, there are chargers available with multiple power tips, including barrel jacks, USB connectors, or “custom” phone connectors, and, in some cases, the chargers have reversed electrical contacts or the polarity can be set by the user. That means the user can create a reverse-polarity problem during plugin, by producing a negative voltage source or, worse yet, a negative voltage source applied to the device being powered. As an example, in the case of USB, a quick search of local electronics shops revealed two third-party chargers with the potential for the user to invert the power tip and generate a reverse-polarity event.
Using the “hot-plug” feature of USB. More and more devices are being plugged into the USB bus. In the early days of USB, many designers thought that, since the USB power source was controlled by the USB specification and because the USB connector is keyed, reverse polarity was a thing of the past. This turned out not to be the case. As the number of devices using the bus goes up, the power consumption and current-draw limits go up, too. For USB 2.0, for example, the current limit for a bus device is 0.5 A, but for USB 3.0, it is 0.9 A and, with USB charging, 1.5 A.
The reality is that there is a serious possibility of reverse polarity with USB, and high-volume system manufacturers continue to push the industry for new, cost-effective protection solutions. The market response can be seen in the specifications of front-end ICs that sit on the USB bus of mobile phones and other devices. Historically, these ICs were rated to withstand only -0.3 V, but today, due to pressure from OEMs, many of these ICs are now being rated to -2 V or even -6 V.
The convenience of being able to connect or disconnect mobile devices while the bus is live means that “hot-plug” transactions are on the rise, as are the volume and amplitude of hot-plug transients. These inductive transients can swing the bus to a reverse-polarity condition. Although these swings tend to be short, they can be significant in amplitude. Voltage rail swings in excess of ±20 V have been measured during hot disconnect. This transient can affect both the device being disconnected and the other devices on the rail. As charge currents increase, this problem only worsens. The evolving environment makes robust, on-board protection a growing priority for system designers working with USB.
Using incorrectly inserted batteries. A battery-powered system can malfunction just because the batteries have been inserted incorrectly, with their poles inverted. This is especially true for devices that use traditional form factors like AAA, AA, C, and D cell batteries, or CR123, CR2, or lithium coin cells. In the past, the solution has been to provide a mechanical structure that prevents electrical contact with the battery terminals if the battery has been inserted incorrectly.
But mechanical solutions are far from perfect. They often require special tooling because the spring contacts require well controlled mechanical assembly tolerances to assure proper contact when the battery is inserted correctly, but no contact when it's not. These tight tolerances can result in long-term reliability issues, since the necessary springs and contacts can bend or fail. Even normal use, with regular insertion cycles, can cause contact fatigue and, over time, limit reliability.
Using the wall outlet in a developing country. There are still places in the world where the electrical infrastructure has few protection requirements and, as a result, the power supply can transmit large transients down the line. The interior wiring can make matters worse. In the past, traditional incandescent lights helped absorb and suppress transient energy on the power line, but new formats like LED and CFL don't have the same suppression characteristics. The move to save energy by switching to more efficient lighting technologies can have the negative side-effect of creating a problem where none existed before. Since the surge environment worldwide continues to evolve, and there's no way of knowing where the end product will be used, or with what third-party chargers and power supplies, integrating strong reverse-polarity protection can improve reliability and provide peace of mind.
Plugging the device into the power supply of a car (or airplane, train, etc.). Transportation-based power supplies, like those used in automobiles, airplanes, trains, and even mopeds or motorcycles, are notoriously “dirty.” The starter, or other electric motor, can pull hundreds of amps with large current surges, inductive spikes, and negative transients. In many cases, the power adapter in a transportation power supply includes reverse-polarity protection, but there are exceptions, especially in low-cost replacements. The unsuspecting user may cause a reverse-polarity event simply by plugging the device into a car's lighter jack, not realizing that the jack can cause a device failure.
What should designers do to prevent reverse polarity?
Since there are so many ways to trigger a reverse polarity event, it's important that designers do what they can to prevent reverse polarity from damaging their system. There are several ways to do this, and each method has its tradeoffs.
We compared a number of electronic solutions and evaluated them for cost and effectiveness. Our rated categories included solution cost, associated design cost or penalties (such as voltage drop, power consumption, and board space), and protection level (for steady state and the ability to protect against transient reverse bias). We graded devices on the familiar scale of “A” being the best and “F” being the worst. Grades were averaged on an equal scale to create a composite final grade. The results are given in Table 1.
See the EDN article Protecting against reverse polarity: Methods examined, Part 1 for the details of these proper protection methods.