PPTC devices protect high-brightness LED lighting systems

As lighting technology has moved from power-hungry incandescent lamps to cold cathode fluorescent lamps (CCFLs) and now light-emitting diodes (LEDs), it has become clear that while end users are willing to pay a higher cost for greener lighting, they also have an inherent expectation that longer life and greater reliability will be a net benefit to their investment.

In meeting these expectations, LED design engineers must consider a variety of variables that affect product performance and life. From power management to power density to overvoltage and overtemperature protection, the uniqueness of LED technology brings new challenges that are unrelated to older technologies.

With improved chip design and materials, LED technology has advanced rapidly, enabling it to become brighter, more energy-efficient, longer-lasting light sources, and able to be applied in a wider range of applications. Despite the increasing popularity of the technology, it remains a fact that excessive heat and improper application can significantly affect LED life and performance.

High brightness LEDs (HB LEDs) are energy-efficient, cost-effective devices that enable the next generation of lighting solutions. From architectural lighting to automotive lighting to backlighting of various display devices and new consumer electronics (such as flash in camera phones), the application of HB LED lighting will continue to grow.

Overcurrent Conditions in HB LED Lighting Systems

LED light output varies with chip type, packaging, efficiency of each wafer batch, and other variables. LED manufacturers use terms such as high brightness to describe the density of LEDs. HB LED drivers can be powered by linear or switching power supplies. Linear drivers are most suitable when the supply voltage is slightly greater than the load voltage, and resistors are used to limit their current. Switching power supplies are also often used because they are more efficient.

Typically, a current sensing resistor provides feedback to a current regulation controller to monitor the current supplied to the HB LED. Another alternative solution is to use a polymer positive temperature coefficient (PPTC) device to limit the current flowing through the LED.

Figure 1. Typical current protection design for HB LED lighting

As shown in Figure 1, a PPTC device is one of a series of elements in a circuit. Usually the resistance of the PPTC device is less than the rest of the circuit, and has little or no effect on normal circuit performance. However, once an overcurrent condition occurs, the device increases resistance (trips) and reduces the current in the circuit to a value that any circuit element can safely carry. This change is caused by the rapid increase in device temperature caused by the I2R heating principle.

The device will remain in its tripped or latched state until the fault is removed. Once the power to the circuit is reapplied, the PPTC device will reset and allow current to flow again, allowing the circuit to resume normal operation. When PPTC devices are unable to prevent a fault from occurring, they will react quickly to limit the current to a safe level to help prevent subsequent damage to downstream devices. In addition, their miniaturized form factor makes them easy to use in space-constrained applications.

Over-temperature protection for HB LED lighting

Unlike traditional lighting, thermal management of HB LEDs is an important design consideration because they are extremely heat sensitive. To improve reliability and operating life, the PN junction cannot reach the conduction temperature. Since PPTC devices are thermally activated, any change in the temperature around the device will affect its performance. As the temperature around the device increases, less energy is required to trip the device, so it can clamp and reduce the current value.

Working Principle of PPTC Device

PPTC circuit protection devices are made of a semi-crystalline polymer and conductive particles. At normal temperatures, these conductive particles form a low-resistance network structure within the polymer. However, if the temperature rises to the switching temperature (Tsw) of the device, whether this condition is caused by high current or by rising ambient temperature, the crystalline material in the polymer will melt and become amorphous. The volume increase that occurs during the melting stage of the crystalline phase causes the conductive particles to separate under the action of hydraulic forces, and causes a huge nonlinear increase in the resistance value of the device, as shown in Figure 2.

Figure 2. PPTC device protection circuitry switches from a low resistance state to a high resistance state in response to an overcurrent or overtemperature condition.

Typically, the resistance value will increase by three or more orders of magnitude. The increased resistance value can reduce the amount of current flowing under fault conditions to a lower steady-state level, thereby protecting the equipment within the circuit. The PPTC device will remain in the latched (high resistance) state until the fault is removed and the circuit power is disconnected; after the conductive composite cools down and recrystallizes, the PPTC device will return to the low resistance state.

Under normal working conditions, the heat generated or dissipated by the PPTC device is in a relatively low temperature equilibrium state, as shown in point 1 in Figure 3. When the ambient temperature remains unchanged and the current flowing through the device increases, the heat generated by the device will also increase. If the increased current is small, the heat generated can be dissipated into the environment, and the device will stabilize at a higher temperature, as shown in point 2 in Figure 3.

Figure 3. Typical operating curve of PPTC device

Conversely, if the ambient temperature rises instead of the current increasing, the device will stabilize at a higher temperature, possibly reaching point 2 again as shown in the schematic. Point 2 may also be the result of the combined effect of current and temperature increase. Further increases in current, temperature, or both will cause the device to heat up and reach a temperature where the resistance increases rapidly, as shown in point 3 in the figure. This is the so-called inflection point at the lower end of the curve. Any further increase in current or ambient temperature will cause the device to generate heat faster than it can dissipate heat to the environment, causing its temperature to rise rapidly.

In this stage, a very small temperature change will produce a very large resistance increase, as shown between points 3 and 4 in Figure 3. This is a normal operating region when the PPTC device is tripped. The increase in resistance causes a corresponding decrease in the current flowing through the circuit.

Because the temperature change between points 3 and 4 is small, this relationship will continue until the device reaches the upper inflection point on the curve at point 4. As long as the externally applied power supply voltage remains at this level, the device will remain latched in the tripped state. Once the external voltage is removed and the power cycle is initiated, the PPTC device will reset to the low impedance state and the circuit will return to normal operation.

Figure 4. Circuit status before and after PPTC device tripping

Figure 4 illustrates the circuit for protecting a HB LED lighting system before and after a PPTC trip. The figure shows how the current is reduced after a trip, protecting the circuit from damage caused by overcurrent and overtemperature conditions.

Complies with Class 2 power safety standards

Using a Class II power supply in a lighting system can be an important factor in reducing costs and increasing flexibility. Intrinsically limiting power sources (such as transformers, power supplies, or batteries) may include protection devices as long as they do not rely on the output limitations of the Class II power supply.

A non-self-limiting power supply, by definition, has a discrete external protection device that automatically interrupts the output when the current or energy output reaches a predetermined value.

A wide variety of circuit protection devices can protect Class II power supplies used in LED lighting applications. Figure 5 illustrates the operation of a coordinated protection strategy that uses an MOV on the AC input and a PolySwitch PPTC device on the output circuit branch to help manufacturers meet the overload test requirements for switches and control gear in Section 35.1 of the UL1310 specification.

Figure 5. Schematic diagram of coordinated protection of the second type of power supply

Summarize

Resettable PPTC devices have demonstrated effectiveness in a variety of HB LED lighting system applications. Like traditional fuses, they limit current after exceeding the rated value. However, unlike traditional fuses, PPTC devices can be reset after the fault is removed and the power is re-closed. Because it is thermally activated, it can prevent damage to the circuit under over-temperature conditions. This unique feature can help designers improve the reliability and life expectancy of lighting systems, as well as reduce the number of components and reduce design complexity.

As with any circuit protection strategy, the effectiveness of a solution will depend on the unique design considerations of each individual circuit layout, board type, specific components and specific application.