PPTC overcurrent protection components for portable electronic devices, chargers and power adapters-EEWORLD

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Introduction Portable electronic devices, such as cell phones, PDAs, and laptops, have special overcurrent protection requirements. These devices are typically powered (charged) by AC/DC power adapters, which convert mains or unregulated DC power into a suitable low-voltage DC power supply. As more and more people begin to purchase power adapters and so-called universal chargers on the aftermarket, the possibility of using incompatible or faulty power adapters for portable devices has greatly increased. Since the voltage, polarity, and current of the power adapter may not match the circuit specifications of the device, it will cause damage to the device and even pose a safety hazard. Due to their small size, blown fuses were once widely used in portable electronic devices. However, with the advent of the new micro series, the size of resettable circuit protection components is no longer a bottleneck. And, since most faults in such devices are transient in nature, the use of resettable circuit protection components can enable manufacturers to avoid the high cost of repairing small faults. Overcurrent damage caused by incompatible power adapters can be effectively avoided by connecting a polymer positive temperature coefficient (PPTC) component in series with the power interface. In addition, if some other components are connected in parallel, such as Zener diodes, instantaneous voltage limiting diodes or arc suppression circuits, it can also prevent overvoltage. 2 Battery Charging Circuit Protection Figure 1 is a typical battery charging circuit. In it, the AC power is converted into a suitable voltage through the AC/DC power adapter to charge the battery pack. The PPTC component and the overvoltage protection component work together to complete the following tasks: 1) Provide overcurrent protection against excessive current that may damage the FET and the battery pack. 2) When the polarity is reversed, the PPTC acts to limit the overcurrent generated by the forward conduction of the Zener diode. 3) When the overvoltage component provides protection against voltage overload, the PPTC component limits the conduction current to protect the overvoltage device.

Figure 1 Typical battery charging circuit protection

PPTC components can also be used in the ports of devices connected to battery packs to protect against overcurrent damage caused by the use of faulty or mismatched battery packs. Battery pack output protection can also provide ideal protection when powering various hands-free car call devices or headsets.

3 AC/DC power adapter protection

AC/DC power adapters are widely used in battery charging and provide low-cost DC power for various consumer appliances. Using separate AC/DC power adapters can simplify the design of terminal equipment and facilitate passing various certification standards.

Various power adapters have their own safety and reliability requirements, including short-circuit current limiting and transformer coil overheating protection. If the coil exceeds the temperature specified by the insulation material, the insulation layer may be damaged and cause the transformer to short-circuit, or even cause a fire. 　　

Although the overcurrent protection effect of PPTC components is very significant, its own temperature decay characteristics have received little attention. In fact, this feature is very suitable for providing effective protection in the event of overheating faults.

Some transformer coils can have overheating protection functions through built-in thermal fuses. However, since thermal fuses are one-time components, they are not suitable for preventing instantaneous faults, such as output short circuits or input voltage fluctuations. The CPTC (ceramic PTC) component has the disadvantage of high impedance, which will cause excessive power loss under normal working conditions. Of course, by improving the heat resistance level of the insulating material, further protection measures can be eliminated, but the direct result is that the cost of the transformer is greatly increased.

Figure 2 shows the overheat protection characteristics of the PPTC component used for the secondary coil of the transformer. In the experiment, the output end of the linear power adapter was artificially short-circuited, and because of the existence of the coil resistance, the output current was about 1A. At this time, the temperature of the secondary coil began to increase. When it reached 100°C, the PPTC component was activated under the combined effect of temperature and current, thereby limiting the current of the secondary coil and preventing the coil temperature from rising further.

Figure 2 Overheat protection characteristics of the secondary coil of the PPTC transformer

When selecting PPTC components, it is necessary to consider the maximum load current that can pass, the maximum ambient temperature (usually 45°C), and the maximum operating time determined by the transformer temperature rise rate.

4CLA short-circuit protection

Due to the variability of the automotive environment and the need to power various precision electronic devices, CLAs (car cigarette lighter power adapters) must operate over a wide temperature range and charging conditions. Therefore, CLAs often experience short-circuit faults and fuse burnout. Usually, these situations are caused by overcurrent, charger circuit failure, or reverse charging.

Connecting an overcurrent protection device to the CLA input can prevent such faults and possible damage. The specific protection requirements of this device are determined by the load current of the terminal equipment and the fault sensitivity of the CLA power conversion circuit. Usually, at the input of the CLA, an overvoltage protection device, such as a transient voltage limiting diode, is used to provide overvoltage protection; combined with a self-resetting protection component, such as a PPTC component, a "one-plug-in" solution can be provided to protect the circuit and reduce warranty repairs.

Figure 3 shows a typical CLA circuit diagram. In Figure 3, a PolySwich self-resetting component and a transient voltage limiting (TVS) diode provide input protection. IC is used to control DC/DC step-down converter. The protection measures of PPTC alone or in combination with TVS can prevent damage caused by the following four faults.

Figure 3 Typical CLA circuit protection

1) Load overcurrent fault in the car phone causes excessive current, which will cause the PPTC to jump to a high impedance state until the fault is eliminated.

2) Automotive circuit protection when the converter fails If the converter or control IC fails, the short-circuit current will cause the PPTC to jump to a high impedance state to protect the car's circuit and fuse.

3) Engine start-up moment When the engine starts, a transient peak voltage will be generated, which is usually suppressed by the TVS diode. However, if the transient voltage value is too large, it may exceed the tolerance of the TVS diode. If PPTC is used, it can act and limit the current before the excessive current damages the TVS, thereby protecting the TVS.

4) Polarity inversion If the polarity of the car battery is inverted, the TVS diode will be forward-conducted, and the excessive current will pass through the PPTC, causing it to jump to a high impedance state, thereby protecting the TVS and limiting the reverse voltage flowing through the converter to the level of the TVS forward voltage drop.

When selecting PPTC components, the maximum load current that may pass, the maximum ambient temperature, and the maximum action time required to prevent damage to other components must be considered. Based on these parameters, you can choose a pin-type or surface-mount resettable device.

5 Conclusion

PPTC components have shown excellent performance in a wide range of circuit protection applications and have UL, CSA and TV safety certifications. PPTC provides reliable, resettable protection for devices, effectively reducing huge warranty returns and greatly improving customer satisfaction.

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