As long as it is a battery-powered system, this problem has always existed: you mistakenly load the battery, and you reverse the positive and negative terminals, resulting in reverse polarity events. The system is temporarily faulty or permanently damaged.

Custom batteries designed to fit the system they are assembled with can help minimize the chances of incorrect insertion and reverse polarity, but proven and reliable off-the-shelf batteries such as AAA, AA, C, and D cells, as well as CR123, CR2, and button lithium batteries, are also vulnerable to failure.

In the past, designers used mechanical structures to avoid electrical contact with battery terminals if the battery was not inserted correctly. But mechanical solutions are far from perfect. They usually require special machining because the spring contacts require a well-controlled mechanical component tolerance to ensure good contact when the battery is inserted correctly, but no contact when not inserted correctly. These narrow tolerances can lead to long-term stability problems, as necessary springs and contacts can bend or malfunction. Even with normal use, the cycle of normal insertion can cause contact fatigue and, over time, limit reliability.

But despite these limitations, mechanical solutions have always existed because they are the only practical options available to designers to prevent incorrect battery installation. Electrical solutions designed to prevent reverse polarity events caused by inverting batteries have been controversial.

Series diodes are usually not chosen because of the voltage drop during normal operation. Using a diode ground setting is also not a very good idea, as reverse polarity events can cause the battery to discharge dangerously for a long time and overheat the diode.

Discrete MOSFETs require complex structures and may not be optimized or specifically used to prevent reverse polarity. Critical specifications for evaluating performance during reverse polar events may be lost, and this may cause designers to have to derive estimates from performance characteristics on data sheets and guess safe operating time periods, which is worrisome. And, depending on how MOSFETs are applied, they may require a controller or other costly features.

Multifunctional ics are sometimes equipped with circuits that prevent reverse polarity, which often significantly increases the complexity of the circuit because they are able to operate in a positive bias environment and then operate in reverse polarity mode or not be damaged. As a result, multifunctional ics come with significant performance and/or cost costs. Typical implementations have relatively limited reverse bias capabilities (-2 V or -6 V) due to the cost performance tradeoff.

The special reverse polarity protection device is an effective way to prevent the wrong insertion of the battery

Recently, however, the advent of dedicated reverse polarity protection devices has provided designers with more viable electrical alternatives. Dedicated devices, such as those supplied by Fairchild, represent one of the most cost-effective and performance-efficient ways to prevent reverse polarity and are an excellent choice for battery-powered systems.

Figure 1. Shows a circuit that prevents reverse polarity using a dedicated device.

New ways to prevent battery insertion by mistake (Electronic Engineering Album)

Figure 1: Use of dedicated devices to prevent reverse polarity

This simple setup provides continuous and reliable protection. The design requires minimal PCB space, minimizes voltage losses, and responds quickly and efficiently under reverse bias conditions.

The overall cost is also good. Series Schottky diodes are generally cheaper than dedicated reverse polarity protection devices, but once the operating current starts to increase, the total cost of Schottky-based methods starts to rise. Due to the cost performance tradeoff, dedicated reverse polarity protection devices are likely to be the most attractive electronic method.

People will continue to make mistakes with batteries, but the way designers prevent minor mishaps will most likely change. When all is considered, dedicated reverse polarity protection devices may over time completely replace complex mechanical solutions.

The "fuse and TVS" approach also has some limitations. The fuse must have a series resistance to function, and there must be enough resistance to activate if protection is to be provided. This resistance causes some power loss in the system and may cause the fuse to heat up and heat cycle during normal operation. Unfortunately, many fuse technologies with the lowest resistance are also the most susceptible to fatigue. Carefully select fuses with longer uptime.

In an overvoltage event, when the diode shunt to the ground and pulls enough current to lower the voltage voltage, diode matching can be a problem. If the size is not appropriate, the diode may overheat the board without opening the fuse or the diode may fail before the fuse is opened. This can lead to downstream thermal events where the power consumption is not enough to open the fuse, and therefore the purpose of using the fuse in the first place cannot be achieved.

If reverse bias reset is required, TVS can be used with a positive temperature coefficient (PTC) resistor (or thermistor). However, the matching requirements are more stringent and the size increases, so there is a need to verify system-level protection in reverse bias events.

Special reverse polarity protection device

A better way to add reverse bias reset is to use a dedicated reverse polarity protection device (Figure 2).

New Ways to prevent the wrong insertion of batteries (Part 2) (Electronic Engineering Album)

Figure 2. Dedicated reverse polarity protection device method

Because dedicated devices are designed to protect reverse bias or reverse polarity, operating power consumption is kept extremely low. Series resistance reflects application requirements, and unlike with PTC resistors, fuses, or series diodes, series voltage drop is not a requirement for normal operation.

By taking advantage of the possibility of extremely low resistance, you can minimize power consumption and voltage drop as needed. This is beneficial for efficiency and device size, as the need for package power consumption is optimized. The static current is kept at extremely low levels. Additional features such as overvoltage protection can be integrated into the device to further maximize protection while minimizing its cost.

The combination of all these features makes a dedicated device one of the easiest and most cost-effective methods to implement. It is also an ideal way to upgrade battery-powered devices that use mechanical solutions to avoid reverse polarity caused by improper battery installation.

Optimal protection method for reverse polarity in systems less than 100mA

Low-current systems - those that operate at less than 100 mA or 200 mA - cover a wide range of applications, from security systems and fire alarms to systems for building automation, public address and data networks.

These include many different work environments, and designers cannot always predict how and where their systems will be used. Depending on the situation, the system may be exposed to adverse electrical conditions such as steady-state reverse bias or negative transients, which can cause reverse polarity events and damage the system.

The result can be as simple as an electrical failure, but if the situation is serious, it can lead to a fire. Therefore, it is not uncommon for designers to add circuits to prevent the negative effects of reverse polarity.

There are various ways to achieve this, but for low-current applications, its efficiency is generally less of an issue. As long as the system can withstand power consumption and the operating voltage drop is associated with the parties method, two simple methods of series PN or Schottky diode can be used to achieve this purpose.

Series PN diode

If the design can accept a large series voltage drop (±1 V), or may have high voltage reverse transients (> 200 V), then using a series PN diode is a good choice. Figure 2 provides an example of the design. This is a simple, low-cost solution that provides fast blocking, resettable functionality, and high breakdown voltages.

New ways to prevent battery insertion by mistake (Electronic Engineering Album)

Figure 2: Series diode method

This diode consumes the least power, so less heat sink is required, and the cost is lower. As long as the device does not overheat during normal operation or possible failure conditions, the system will function properly.

Even so, the solution is not suitable for every design. The cost advantage quickly disappears as the operating current rises. Moreover, at higher current, the greater the power consumption, the larger and more expensive the final required diode, the need to use better thermal conductivity of the package and heat dissipation structure.

In addition, in low-voltage systems (≤5V), the diode voltage drop may require additional downstream booster circuits, making what is expected to be a low-cost approach actually expensive.

Therefore, it is important to remember these points before using the PN diode method.

Series Schottky diode

A similar but more widely used approach is to use a series Schottky diode instead of a series PN diode. The voltage drop is a little lower (±0.6 V) and the design consumes less power.

Figure 3 shows the Schottky diode setup. This configuration provides excellent blocking, simple design import, and low cost. It is also reconfigurable and may support relatively high breakdown voltages (> 200 V).

New ways to prevent battery insertion by mistake (Electronic Engineering Album)

Figure 3: Series Schottky diode method

The lower voltage drop reduces the thermal management requirements associated with conventional PN diodes, which may enable smaller and less costly packages.

Still, care is needed, as the pressure drop may still be too high for many applications. Moreover, although Schottky diodes have a wider operating range than series PN diodes, the best applications for this method are applications that use currents below 200 mA and have higher voltages (>5 V).

conclusion

No matter which method is used, the two main aspects of voltage drop and power consumption should be considered. Assuming that both parameters are within acceptable ranges, both methods effectively protect low-current systems at low cost from damage that can result from reverse polar events. If voltage drop or power consumption is an issue, consider active solutions such as Fairchild‘s FR devices.

The "fuse and TVS" approach also has some limitations. The fuse must have a series resistance to function, and there must be enough resistance to activate if protection is to be provided. This resistance causes some power loss in the system and may cause the fuse to heat up and heat cycle during normal operation. Unfortunately, many fuse technologies with the lowest resistance are also the most susceptible to fatigue. Carefully select fuses with longer uptime.

In an overvoltage event, when the diode shunt to the ground and pulls enough current to reduce the supply voltage, diode matching can be a problem. If the size is not appropriate, the diode may overheat the board without opening the fuse or the diode may fail before the fuse is opened. This can lead to downstream thermal events where the power consumption is not enough to open the fuse, and therefore the purpose of using the fuse in the first place cannot be achieved.

If reverse bias reset is required, TVS can be used with a positive temperature coefficient (PTC) resistor (or thermistor). However, the matching requirements are more stringent and the size increases, so there is a need to verify system-level protection in reverse bias events.

Special reverse polarity protection device

A better way to add reverse bias reset is to use a dedicated reverse polarity protection device (Figure 2).

New Ways to prevent the wrong insertion of batteries (Part 2) (Electronic Engineering Album)

Figure 2. Dedicated reverse polarity protection device method

Because dedicated devices are designed to protect reverse bias or reverse polarity, operating power consumption is kept extremely low. Series resistance reflects application requirements, and unlike with PTC resistors, fuses, or series diodes, series voltage drop is not a requirement for normal operation.

By taking advantage of the possibility of extremely low resistance, you can minimize power consumption and voltage drop as needed. This is beneficial for efficiency and device size, as the need for package power consumption is optimized. The static current is kept at extremely low levels. Additional features such as overvoltage protection can be integrated into the device to further maximize protection while minimizing its cost.

The combination of all these features makes a dedicated device one of the easiest and most cost-effective methods to implement. It is also an ideal way to upgrade battery-powered devices that use mechanical solutions to avoid reverse polarity caused by improper battery installation.