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Protecting Against Reverse Polarity: Which Method Is Right for You?

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.

Table 1

Ratings for Methods of Reverse Polarity Protection

Ratings for Methods of Reverse Polarity Protection

See the EDN article Protecting against reverse polarity: Methods examined, Part 1 for the details of these proper protection methods.

17 comments on “Protecting Against Reverse Polarity: Which Method Is Right for You?

  1. geek
    August 22, 2014

    @Richard: Interesting article. I agree that reverse polarity can really damage the electronic device, but do you not think that the damage caused by reverse polarity can be prevented by having a sensor or fuse within the device that can detect it righr away and break all connections instead of letting the current pass to the curcuit and allowing the circuit to be damaged.

  2. samicksha
    August 23, 2014

    I believe we can connect diode in reverse here which can resist the flow of current and further can help in protecting circuit from reverse polarity.

  3. vasanjk
    August 24, 2014

    Richard,

    You have outlined almost every possible cause for reverse polarity related issues.

     

    By far the most useful within the topologies used usually, those I have come across seems to be the TVS+PTC solution. Quite useful.

  4. dassa.an
    August 25, 2014

    @vasanjk: I haven't tried it yet but I think it's an ideal solution for my project. 

  5. amikolajczak
    August 25, 2014

    Every circuit is different, but yes, you can protect against reverse polarity with either switches or diodes.

    I think key thing is that most systems dont fail due to negative currents. Reverse voltage often turns on body diodes or embedded protection diodes. These are usually rated for negative transients.

    So its not the small transient that kills a system, its extended conduction of damage level currents. So what ever solution you use – your solution just has to stop and or block enough current and voltage to prevent damage.

    Exact current and voltage levels are very system dependent. That is why some people can get away with a PTC (that may take a mSec or even seconds to “trip” and after trip may leak a few hundred mA), while others need fast switches or diodes that can respond in nSec's.

  6. eafpres
    August 25, 2014

    Hi Richard.  One method that usually works:

    1) Obtain at least two of the device you are working with.

    2) Connect to power as you think it should be.

    3) If smoke, fire, loud noises, and lack of function ressult, use the 2nd device, reverse the polarity and return to step 2.

  7. Richard.Chung
    August 25, 2014

    @vasanjk. stay tuned. the future blog and future portion of the article does talk about the combination you describe.

  8. Steve Taranovich
    August 26, 2014

    @eafpres1—Haha—that's the reality of the situation with many designers. That's why I love Littelfuse tutorials and fundamentals they have been posting here on Planet Analog

  9. David Maciel Silva
    August 26, 2014

    Hello Samicksha,

    You're right, it works very well. But in some cases can damage the diode, due to high reverse voltage at the time of plug and play. Would like more information on the method presented in the table as active dedicated.

     

     

  10. geek
    August 30, 2014

     

    “Every circuit is different, but yes, you can protect against reverse polarity with either switches or diodes.”

     

    @amikolajczak: Is that a work around or a proper solution towards preventing against reverse polarity? Exactly what kind of switches or diodes can be used?

  11. SunitaT
    August 31, 2014

    @eafpres1, The procedure sounds practical enough  though I havent reallly tried out. Myb I should and get bto see the outcome :).

  12. SunitaT
    August 31, 2014

    Hi Richard, I'm impressed to see all the possible causes that I have in the recent past found out to cause reversed polarity not only outlined hre but also put into details. Whatsmore, there are others that I have come to learn of right now.

  13. SunitaT
    August 31, 2014

    To be frank, I have in several occasiona encountered reverse polarity simply because of plugging devices into power supply. Before making any connection we should always think about reverse polarity so as to  not to cause any damage to the devices.

  14. SunitaT
    August 31, 2014

     

    I think not only designers are impacted by reverse polarity issues consumers are also impacted by the reverse polarity issues. So consumers are also should be made aware about issues related to reverese polarity.

  15. Victor Lorenzo
    September 1, 2014

    Unfortunately most consumer electronics clients simplify things to only one check: Does the adapter plug fit into the device's power connector? If it does it SHOULD work, that is the assumption…. there it comes when the device overreacts and produces those not so beautifull fireworks and smoke.

  16. Richard.Chung
    September 1, 2014

    @eafpres1. your comments made me laugh. I do have empathy what it is like to have something blow up on the bench – confusion, frustration- possible delays to impending deadline. I wonder if engineers do what you are proposing, how would they react?

  17. RedDerek
    September 2, 2014

    I liked eafpres1's comment as well. Any bench work always starts out with a current limited supply.  If I expect 100ma draw at most, I'll set the current limit to 100ma. If the supply does not come up, I'll up the current limit about twice the expected and watch the output voltage. If it recovers, then I continue. If not, then I start touching devices, carefully, and find the hot spot to start the debug process.

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