Reliability Design Reference of High Brightness LED Lighting System

With the development of lighting technology from extremely power consuming incandescent lamp to cold cathode lamp tube (CCFL) and then to today's light emitting diode (LED) lamp, it can be clearly seen that while end users are willing to pay higher costs for greener lighting, they also have an internal expectation that longer service life and higher reliability will be the net benefit of their investment.

While meeting these expectations, led design engineers must consider various changing factors that affect the efficiency and life of their products. From power management to power density, to overvoltage and overtemperature protection, the uniqueness of LED technology brings a variety of new challenges unrelated to older technologies.

With the improved chip design and materials, LED technology has developed rapidly, promoting its rapid development to brighter, more efficient and energy-saving light sources with longer service life, and can be applied in a wider range. Despite the increasing popularity of technology, there is still a fact that excessive heat and inappropriate applications will significantly affect the life and efficiency of LEDs.

High brightness LED (HB LED) is an energy-saving and high cost-effective device, which can ensure the next generation of lighting solutions. From architectural lighting to automotive lighting to backlight of various display devices, and new consumer electronics (such as flash in camera phones), the application of HB LED lighting continues to grow.

Overcurrent in Hb LED lighting system

LED light output varies with wafer type, package, efficiency of each wafer batch, and other variables. LED manufacturers use terms such as high brightness to describe the density of LEDs. The HB LED driver can be powered by a linear or switched power supply. When the supply voltage is slightly greater than the load voltage, the linear driver is the most suitable, and the resistance will be used to limit its current. Switching power supplies will also be used frequently because they are more efficient.

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

As shown in Figure 1, a PPTC element is one of a series of elements in a circuit. Generally, the resistance of PPTC element is less than the rest of the circuit, which has little or no impact on the normal circuit performance. However, once an overcurrent condition occurs, the element will increase the resistance (trip) and reduce the current in the circuit to a current value that any circuit unit can safely carry. This change is caused by the rapid rise of element temperature caused by I2R heating plateau.

Figure 1: current protection design for HB LED lighting.

The element will remain in its tripped or locked state until the fault is eliminated. Once the power supply connected to the circuit is closed again, the PPTC element will reset and allow the current to start flowing again, so as to restore the normal operation of the circuit. When PPTC components cannot prevent the occurrence of faults, they will react quickly and limit the current to a safe level to prevent subsequent damage to downstream components. In addition, their miniaturized shape makes them easy to use in space limited applications.

Unlike traditional lighting, Hb LED is very thermal sensitive, and its thermal management is an important design consideration. In order to improve reliability and service life, the PN interface cannot allow the increase of conduction temperature. Since the PPTC element adopts hot start, any change in the temperature around the element will affect its efficiency. As the temperature around the element increases, less energy requires the element to trip, so it can clamp the current value and reduce it.

Working principle of PPTC components

PPTC circuit protection element is made of semi crystalline polymer and conductive particles. At normal temperature, these conductive particles form a low resistance network structure in the polymer. However, if the temperature rises to the switching temperature (TSW) of the element, whether this condition is caused by high current or by the rise of ambient temperature, the crystalline material in the polymer will melt and become amorphous. The volume increase in the melting phase of the crystal phase will lead to the separation of conductive particles under the action of hydraulic force, and the resistance value of the element will grow greatly.

Typically, the resistance value will increase by 3 or more orders of magnitude. The increase of resistance value can reduce the amount of current flowing under fault conditions to a lower steady-state level, so as to protect the equipment in the circuit. Before troubleshooting and circuit power disconnection, PPTC elements will remain in the latch (high resistance) state; After the conductive composite is cooled and recrystallized, the PPTC element will return to the low resistance state.

Under normal working conditions, the heat generated or lost by PPTC element is in a relatively low-temperature equilibrium state, as shown in point 1 in Figure 2. When the ambient temperature is constant and the current flowing through the element increases, the heat generated by the element will also increase. If the increased current is insignificant and the heat generated can be lost to the environment, the element will stabilize at a higher temperature, as shown in point 2 in Figure 3.

Figure 2: PPTC element protection circuit changes from low resistance state to high resistance state in response to overcurrent or overtemperature.

Figure 3: typical working curve of PPTC element.

On the contrary, if the ambient temperature rises instead of the current increases, the element will stabilize at a higher temperature and may reach the second point in the tableland diagram again. The second point may also be the result of the increase of current and temperature. With the further increase of current, temperature or the combination of the two, the element will rise and reach the temperature at which the resistance increases rapidly, as shown in point 3 in the figure, which is the so-called inflection point at the low end of the curve. Any further increase in current or ambient temperature will cause the element to generate heat faster than it loses heat to the environment, causing its temperature to rise rapidly.

In this stage, a very large resistance value will increase with a very small temperature change, as shown between points 3 and 4 in the figure. This is a normal working area when PPTC element trips. An increase in resistance results in a corresponding decrease in the current flowing through the circuit.

Because the temperature change between points 3 and 4 is very small, this relationship will remain until the element reaches the upper inflection point of point 4 on the curve. As long as the externally applied power supply voltage remains at this level, the element will always be locked in the tripping state. Once the applied voltage is disconnected and the power loop is started, the PPTC element will reset to the low resistance state and the circuit will return to the normal working state.

Figure 4 illustrates the circuit for protecting HB LED lighting system before and after PPTC trip. This figure shows how the current is reduced after tripping, so as to protect the circuit from damage caused by overcurrent and overtemperature.

Figure 4: circuit status before and after PPTC element tripping.

Comply with class 2 power supply safety standards

Using the second type of power supply in a lighting system can become one of the important factors to reduce cost and improve flexibility. Power supplies that are inherently restrictive, such as transformers, power supplies or batteries, may include protective elements as long as they do not depend on the output limits of the second type of power supply.

Non self limiting power supply, according to its definition, has a discrete external protection element. When the current and energy output reach a predetermined value, it will automatically interrupt the output.

A wide variety of circuit protection elements can protect class II power sources for LED lighting applications. Fig. 5 illustrates the working principle of a cooperative protection strategy. It adopts a mov on the AC input and a PolySwitch PPTC element on the output circuit branch? The manufacturer shall meet the overload test requirements for switches and control devices in sub section 35.1 of UL1310 specification.

Figure 5. Schematic diagram of cooperative protection of class II power supply

Summary of this paper

Self resetting PPTC elements have shown effectiveness in diversified HB LED lighting system applications. Like conventional fuses, they limit the current after exceeding the rated value. However, unlike traditional fuses, PPTC elements can be reset after troubleshooting and power reclosing. Due to its hot start, it can prevent the damage caused by over temperature. Can this unique function? The designer can improve the reliability and average life of the lighting system, reduce the number of components and reduce the design complexity.

As with any circuit protection strategy, the effectiveness of a solution will depend on various special design considerations for each different circuit layout, board type, specific components and specific applications. Te circuit protection works with OEM manufacturers to select and implement the best method.

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