Nobody wants their system to fail or, worse yet, burst into flames. But that’s what can happen if reverse polarity is allowed to do its damage.
Reverse polarity is the result of steady-state reverse bias or a negative transient. It’s a dangerous electrical condition that can be difficult to prevent once a system leaves the factory.
The risk of reverse polarity is a real threat in a wide range of 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.
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 the prevention of reverse polarity in mind. In some cases, the chargers have reversed electrical contacts or the polarity can be set by the user, which leaves room for error.
- Using the “Hot Plug” Feature of USB
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 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 plug” transactions. This transient can affect both the device being disconnected and the other devices on the rail. As charge currents increase, this problem only worsens.
- 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. Mechanical solutions can prevent electrical contact with the battery terminals if the battery has been inserted correctly, but these solutions require custom moldings and can be subject to contact fatigue over time.
- 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. Efforts to save energy, by moving to LEDs and CFLs, can create a problem where none existed before.
- Plugging the Device into the Power Supply of a Car (or Airplane, Train, etc.)
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.
Since there are so many ways to trigger a reverse-polarity event, it’s important that designers do what they can, before the system leaves the factory, to prevent reverse polarity from causing damage.
Application Note: AN-9739 Reverse Battery Protection of Smart Switches
Reverse Polarity Protection Devices
BENCHMarks™, Vol. 4, 2012: Flexible, Space-Saving Reverse Polarity Protectors