LEDs are an extremely versatile and compact alternative to the traditional incandescent bulb with the added ability to extend to applications where incandescent lighting was not possible. Savings in long-term operation and energy costs while utilizing a robust package are just a few of the advantages that have made LEDs popular. These benefits do not come without overhead, nor does reliable design necessarily have to be complicated. One of the key design challenges for LED applications is adequate temperature and thermal management to ensure overall reliability and long product life, especially in high-power LED modules. Current limiting power resistors provide an efficient solution, and Ohm's Law assists in determining the optimal component selection in a straightforward way.
We will provide the various factors that influence a designer's choice of thermal management solution for high-power LED modules, and will introduce the technology of thermal design at a system level. A sample schematic and circuit analysis will be used for reference throughout to illustrate the impact of these factors, and will demonstrate how this solution matches the design goals for power, reliability and operational lifespan of LED modules. An alternate method will also be presented that shows how controlling current provides effective voltage regulation.
HIGH-POWER LED MODULES AND THERMAL CHALLENGES
Thermal challenges involved in implementing an LED module design are typically electrical and environmental in nature. Current, temperature fluctuations, and case temperature each affect the LED circuit. Due to the inherently low internal resistance of an LED, an external solution is necessary to limit the amount of current an LED module is exposed to. If the current is limited improperly, an LED will burn up rather quickly, resulting in lower system reliability and possible costly repairs and warranty returns. The reliability of an LED module also can be impaired dramatically with an increase in temperature. Power LED modules require an alternate thermal solution because they often are used in products where a cooling system is not possible, such as in portable battery-powered equipment.
LED lighting designs provide a much longer lifespan than traditional incandescent light bulbs as long as the temperature is controlled adequately. An important issue designers must contend with in LED module thermal design is case temperature.Similar to other semiconductors such as power transistors, power LEDs can have high power ratings and are intolerant of high case temperatures. High case temperatures can reduce the component's Mean Time Between Failure (MTBF) rate, and, thus affect the design's overall reliability and lifespan.
Temperature fluctuations are an operational hazard of unmanaged LED modules, which have an inherent risk of damage. If the current through an LED junction exceeds its rated value, the LED can overheat easily, possibly causing an overheated junction, melting of the plastic LED housing and damage to the internal bonding of the device. Thus, it is critical to include some type of thermal control in LED modules to ensure the current does not exceed the module's specified limits.
Three factors determine the limits to which the operating current of an LED must be controlled:
- Application using the LED
- Individual specification of the LED
- Specifications of the entire LED system
High-power LED modules are increasingly being used in portable applications where the design specifications require control of the component surface temperature. Some specifications stipulate the specific temperature limits of the components used in the LED modules to match regulatory guidelines and to further help protect LEDs from additional sources of heat that could reduce the lifespan of the devices.So it is imperative that designers understand the individual specifications for each LED so that adequate thermal management can be applied.
To control the energy within LED modules, the use of a single resistor to limit the forward current is an effective solution. An example of a typical LED system in an industrial or automotive application is used for illustration purposes. As illustrated in Fig. 1, the system consists of a string of two LEDs powered by a standard low cost regulator. Throughout this example the supply voltage is 12 V, each LED has a forward voltage of 2.0 V, and diode current is 0.3 A. The values of each parameter will change based on thermal factors taken into account.
Circuit analysis determines key parameters of the resistor in the circuit, which includes voltage, resistor value, and power. Equation (1) calculates the voltage drop of the resistor based on two diodes with 2 V voltage drop each. Equation (2) uses Ohm's Law to calculate the resistance value based on the voltage of the resistor and the current in the circuit. With the voltage rating and resistance value known, the final calculation is to find the required power rating of the resistor, shown in Equation (3).
The resistor will have an 8.0V drop, resulting in a 26.6Ω resistor that dissipates 2.4W.
TEMPERATURE AND POWER
The above example assumes a constant system temperature. However, any increase in the ambient temperature will affect the voltage to drop because the LED forward voltage is temperature dependent. If an LED has a temperature coefficient of -5 mV/°C and the ambient temperature inside the LED module increases by 50 °C, then the forward voltage will drop by 0.25 V for a VDIODE of 1.75 V as calculated in Equation (4). If there are two LEDs in the string then both are taken into account for a total drop of 0.5 V.
Designers also must consider the production variations in the LED forward voltage, which could be ±0.3 V. Based on this variation, the power dissipated in the same 28.33 Ω resistor could be even higher. The modified diode voltage is calculated in Equation (8) and substituted in Equation (9) to obtain the voltage of the resistor. Finally, the power rating of the resistor is calculated in Equation (10).
To avoid permanent damage to the LEDs, special consideration must be given to these factors during the design of an LED module.
Use of a regulator is another method to control the current in an LED module. Voltage regulation circuits require LEDs to be driven below their maximum current (as a safety precaution for changes in forward voltage). This often requires adding LEDs to the string to meet the system specifications, which can increase costs. However, utilizing constant current control rather than a voltage regulator can be a more cost-effective solution because a linear constant current regulator requires very few external components.
Fig. 2 shows the same LED string with a constant current voltage regulator. In this example, the output from the chip is 12 V. A reference voltage provided by a power resistor enables a constant 12 V output and constant current of 0.3 A with feedback through the LED string.
Equation (11) assumes a voltage drop across the diodes due to production variations in the forward voltage. The resistor ensures constant current within rated limits is supplied, and its voltage, resistance and power parameters are calculated in Equations (12) to (14).
Using a 30 Ω resistor that can dissipate 2.6 W is suggested. This method provides a simple, reliable and cost-effective solution.
IMPROVEMENTS IN TECHNOLOGY FOR THERMAL CONTROL
Technology advancements are providing increasingly available and necessary solutions to operational and environmental challenges. To meet the parameters of a system, such as the design limits and maximum temperature of a resistor when it is under load, new materials and advances in manufacturing techniques have contributed to the availability of cost-effective components such as surface mount resistors capable of handling very high power levels without generating high temperatures.
Next-generation power resistors meet the thermal management needs in high power LED modules. Certain models can dissipate 20 W on an infinite heat sink. On a standard FR4 printed circuit board a copper-based resistor can dissipate 3W to 4W. Some power resistor designs utilize insulated metal substrates (IMS), which are thermally conductive materials that enable the resistor to dissipate even more power while keeping the surface temperature of the printed circuit board at an acceptable 85-90 °C.
To create added reliability in LED applications, it is critical for designers to consider the thermal requirements at the onset of an LED module design. Inserting a high-power resistor that can limit current and dissipate power is an easy-to-implement solution for standalone designs or combined with a regulator for more complex designs. It is important to consider the temperature and voltage variations that are defined in each LED component's specification. The challenges specific to the environment and application may also dictate the technology required.