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

1 Introduction

Portable electronic devices, such as cell phones, PDAs, and laptops, have special overcurrent protection requirements. These devices are generally powered (charged) by AC/DC power adapters, which convert mains or unregulated DC power into suitable low-voltage DC power. 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.

Once widely used in portable electronic devices, blown fuses were small. However, with the new micro series, the size of resettable circuit protection components is no longer a limiting factor. And, since most faults in such devices are transient in nature, resettable circuit protection components can help manufacturers avoid the high cost of repairing small faults.

By connecting a polymer positive temperature coefficient (PPTC) element in series with the power interface, overcurrent damage caused by incompatible power adapters can be effectively avoided. In addition, if other components are connected in parallel, such as Zener diodes, instantaneous voltage limiting diodes or arc suppression circuits, overvoltage protection can also be achieved.

Figure 1 Typical battery charging circuit protection

2Battery charging circuit protection

Figure 1 is a typical battery charging circuit. In this circuit, AC power is converted to a suitable voltage through an 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 battery pack.

2) When the polarity is reversed, the PPTC acts to limit the overcurrent caused by the forward conduction of the Zener diode.

3) When the overvoltage component provides protection against voltage overload, the PPTC component limits the on-current to protect the overvoltage device.

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

3AC/DC power adapter protection

AC/DC power adapters are widely used in battery charging and to 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.

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

Figure 3 Typical CLA circuit protection

虽然PPTC元件的过电流保护效果非常显著，但其本身所具有的温度衰减特性却鲜受关注，事实上，这种特征非常适合在过热故障时提供有效保护。

Some transformer coils can have overheat protection by 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. CPTC (ceramic PTC) components have 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 insulation material, further protection measures can be eliminated, but the direct result is a significant increase in transformer costs.

Figure 2 shows the overheat protection characteristics of the PPTC element 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 around 1A. At this time, the temperature of the secondary coil began to increase, and when it reached 100°C, the PPTC element 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.

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 changing environment of the car and the need to power various precision electronic devices, CLAs (car cigarette lighter power adapters) must operate in a wide range of temperature and charging conditions. Therefore, CLAs often experience short circuit failures 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 the damage that may result. The specific protection requirements of such a device are determined by the load current of the terminal equipment and the fault sensitivity of the CLA power conversion circuit. Usually, an overvoltage protection device such as an instantaneous voltage limiting diode is applied to the input of the CLA to provide overvoltage protection; combined with a self-resetting protection component such as a PPTC component, a "one-plug-and-go" 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 element and a transient voltage limiting (TVS) diode provide input protection. The IC is used to control the DC/DC buck converter. The protection measures of PPTC alone or in combination with TVS can prevent damage caused by the following four faults.

1) A car phone with a load overcurrent fault 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 vehicle's circuits and fuses.

3) When the engine starts, a transient peak voltage will be generated when the engine starts, 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 conduct in the forward direction, and the excessive current passing through the PPTC will cause it to jump to a high-resistance state, thereby protecting the TVS and limiting the reverse voltage flowing through the converter to the TVS forward voltage drop level.

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

5 Conclusion

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