It’s a problem that’s been around for as long as there have been battery-powered systems: you load the battery the wrong way, with the poles inverted, and create a reverse-polarity event. The system fails temporarily or is permanently damaged.
Purpose-built batteries, designed to fit the system they’re shipped with, help minimize the chances of incorrect insertion and reverse polarity, but tried-and-true, off-the-shelf batteries like AAA, AA, C, and D cell batteries, or even CR123, CR2 and lithium coin cells, are easy enough to get wrong.
In the past, designers have used mechanical structures to prevent electrical contact with the battery terminals if they’ve 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.
Mechanical solutions have persisted, though, despite these limitations, because they’ve been the only real options that designers can use to protect against improper battery installation. Electrical solutions, designed to protect against reverse-polarity events caused by inverted batteries, have been problematic.
Using series diodes typically isn’t an option, because of the voltage drop during normal operation. Using a diode-to-ground setup isn’t a very good idea either, since a reverse-polarity event can cause the batteries to discharge dangerously for extended periods of time and overheat the diode.
Discrete MOSFETs require complex structures and may not be optimized or specified to protect against reverse polarity. The critical specs to evaluate performance during a reverse-polarity event may be missing, and that can leave the designer in the uncomfortable position of having to draw estimates from the performance characteristics on the data sheet and guess about safe operating windows. Also, depending on how the MOSFETs are implemented, they may require a controller or other costly functions.
Multi-function ICs, which are sometimes equipped with circuitry that protects against reverse polarity, often add significant circuit complexity, since they are able to operate in a positive bias environment and then operate or survive in reverse polarity mode. As a result, multi-function ICs come with significant performance and/or cost penalties. Due to the cost/performance tradeoffs, typical implementations have fairly limited reverse-bias capability (-2 V or -6 V).
More recently, though, the emergence of dedicated reverse-polarity protection devices has given designers a more viable electrical option. Dedicated devices (like those supplied by Fairchild) represent one of the most cost-effective and highest-performance approaches for preventing reverse polarity and are a very good choice for battery-operated systems.
Figure 1. shows a circuit used to protect against reverse polarity using a dedicated device.
Figure 1. Protecting Against Reverse Polarity Using a Dedicated Device
It’s a simple setup that provides consistent, reliable protection. The design requires very little PCB space, minimizes voltage loss, and responds quickly and effectively during reverse-bias conditions.
Overall cost is good, too. A series Schottky diode is typically less expensive than a dedicated reverse-polarity protection device, but once operating currents begin to increase, the total cost of a Schottky-based method begins to go up as well. Given the cost/performance tradeoff, the dedicated reverse-polarity protection device is likely to be the most attractive electronic method.
People will continue to make mistakes with batteries, but the way designers protect against mishaps is likely to change. All things considered, dedicated reverse-polarity protection devices may, over time, fully replace cumbersome mechanical solutions.
For determining the best method for your particular design ,please go to http://edn.com/design/power-management/4433697/1/Protecting-against-reverse-polarity–Methods-examined–Part-1
Application Note: AN-9739 Reverse Battery Protection of Smart Switches
Reverse Polarity Protection Devices