LED components and their drive electronic components produce a lot of heat, and heat dissipation can be maintained, and the optimal light output can be maintained, and the LED work life can be extended. Although the passive cooling of the radiator is sufficient to meet a typical LED application demand, high-brightness, high-power LED lamps should also consider using active cooling.
Active forced airflow cooling will increase the complexity of the lamp, but in terms of the life and performance of the LED lamps, it has great advantages, far exceeds these shortcomings.
This article will discuss the relationship between fever and LED performance. Then, this article will introduce two forced airflow cooling modules: a fan and another synthetic jet. Finally, we will also discuss the forced airflow LED cooling example in automotive applications.
How to affect LED performance
There must be as much as possible to keep the LED cool, there are three main reasons. The first reason is related to the radiation power of the LED, which changes the power and the functional relationship with the junction temperature. Higher ambient temperature can lead to higher junction. As the junction temperature increases, the forward voltage and lumen output of the LED will fall (Figure 1).
Second, LEDs work within the maximum rated tonal temperature, which may trigger a variety of stress mechanisms, will significantly shorten the LED life. Third, the high ending will reduce the color color index (CRI) of the LED. CRI is a technical indicator based on objective industry standards to measure color "authenticity".
Of course, good thermal management has other advantages. For example, efficient heat dissipation can minimize replacement frequencies, thereby reducing the total cost of the LED lighting system. Good thermal management also allows the luminaire to operate at a higher power level, reducing the number of LEDs needed to achieve specific brightness.
Passive and active cooling
In most cases, the designer can choose a passive or active cooling. Passive cooling uses natural air conductivity and convection, usually in a radiator, speeding up the heat dissipation of these two modes. The heat sink can solve heat management problems for low LMD LED luminaires. However, only relying on the heat sink and the LED luminaire of 75 W or 100 W power is dissipated. Even if the radiator is designed specifically for this application, the designer will also find that it is too large, costly, inconvenient in mechanical.
In order to effectively design for high flow applications, we may need an active cooling system to diffuse the heat generated by the LED components and drive electronic components. In addition to the heat sink, the active cooling also includes a subsystem, typically a fan, forhaizing air blowing heat sink.
A large advantage of active cooling is that the volume and size of the heat sink reduce up to two-thirds. This reduces the cost of the heat sink and reduces the size of the entire system. It also makes the transformation more feasible.
The ideal objective application of active cooling is the board (COB) LED, which includes multiple LEDs, mounted on the same PC board. LEDs used in COB are chips and are not used in traditional packaging. Therefore, their installation only takes a smaller space and provides more operational convenience for the forced airflow. When used for a COB device, the active cooling fan can effectively reduce the temperature to 90 ˚F / 36˚C, which is much lower than the operating temperature threshold 120˚c of the typical COB device.
Forced air flow module selection consideration
The fan designed for cooling LEDs must have extremely low power consumption, which can be installed in a small space, and the expected service life is at least comparable to the electric lamp itself (the service life provided by the working principle of modern fan has reached several hundred thousand hours). The fan can force air to enter the system, forcing air to flow through the heat sink, more efficiently dissipated, effectively reducing the ambient temperature. The axial flow fan is commonly used, and the air enters and leaves the same axial direction.
Manufacturers typically use performance curves to describe the characteristics of the fan, the curve draws the relationship between the airflow and the static pressure, in an inch or millimeter water column. The air volume in these fans is relatively high, and the air pressure is relatively low.
Active cooling solution is also very efficient, many of these solutions are only 0.5 watts. The efficiency of the fan blades will change, and there is a functional relationship with the pneumatic load. The peak efficiency of the axial flow fan is usually present at approximately one-third of the pressure points of the highest pressure. In order to calculate the fan efficiency in accordance with the ratio of the input power and output power, the following formula can be derived:
Fan efficiency = Pout / Pin
Fan input power (pin) = pin (wat) = V x i
Fan output power (Pout) or aerodynamic (using metric unit) = pout = air pressure (unit is m3 / sec) X air flow (Pasca)
Use standard units, formulas become:
Pout (Watt) = (Air Pressure (inch Water)) X Air Flow (CFM)) / 8.5
Forced airflow options
Now let's look at some fans, develop their purpose is to keep the LEDs at the best level. Sunon's LED cooling module LA004-024A83DY is four times higher than that of passive cooling (Figure 2). The fan uses a patented DR Maglev (magnetic levitation) motor to eliminate the typical fan motor usually exist and shake. Maglev technology has zero friction characteristics because there is no contact between the shaft and the bearing. Sunon's fan also complies with US Energy Star Noise Standard MR16, and the noise level measured at 12 inch water column pressure does not allow more than 24 dBA. The fan weight is 194 grams, the speed is 2200 rpm (+/- 10%), and the overall size is only 86 x 45.4 mm, and the power consumption is only 0.26 watts.
All major components of Sunon's Maglev fan products are made of plastic to provide optimal insulation resistance and electrostatic discharge (ESD) performance. The fan has sufficient flexibility to be installed in a smaller space, and the expected life is usually 50,000 hours. The average non-fault time (MTTF) is 70,000 hours at a temperature of 60 ˚C and a relative temperature of 65%.
Aavid Thermalloy's SYNJET cooling products use another active cooling technology, also designed for LED modules and arrays, which use oscillating film sheets to generate a constant air flow through the heat sink sheet. Such a fanless hair dryer is a flexible film capable of "breathing". It slowly inhaled the air, then quickly discharil, blowing through the heat sink, forming an efficient convection cooling.
At work, it uses an electromagnetic actuator to generate a driving force to allow film oscillation. As the film is oscillated inside the cavity, a synthetic jet is generated at a nozzle around the cavity. These airflow rapid pulses (usually 30 to 200 pulses per second) will increase heat from the heat source, but only less air, allow 75 W and 100 W LED incandescent lamps to be replaced.
For example, the use of SYNJET COOLER Model Spars-CM012-002 is to cool, spotlights, track lights, chandeliers, and PAR 30 luminaire (or smaller) cool. Its maximum hot power data is 32 W, a diameter of 75 mm, and the power supply is 12 watt direct power, provides level or PWM control selection. Its noise is as low as 18 dBA.
Cool car LED headlights
As technology continues to extend from the luxury model to mainstream vehicles, automobiles equipped with LED headlights is also increasingly popular. For car headlights, it is specified that each lamp must reach the brightness of 750 lumens, so a number of LEDs with high output are required to meet this standard.
High brightness is generated on very small planes, so that the chip has high local thermal stress, and the LED junction temperature may far exceed the maximum value of 125 ˚C. Even in energy-saving LED headlights, due to multiple LED chips inside the lamp housing, the total dissipation heat must be reached high. Unless the waste heat generated by the module can be distributed, the chip can be shortened to 15,000 to 30,000 hours in the cooling environment of waste heat.
Regulation requires the latest LED headlights only as a self-sufficient device. Even if there is only one LED failure, the entire headlight must also be replaced because the replacement of a single LED component cannot be performed. Therefore, the entire cooling system must be placed inside the headlights (Fig. 3). Since the space inside the headlight is small, the size of the heat sink is limited, and thus the air convection is reduced.
Designers must also take into account various climatic factors, including humidity, salt content, and all air suspension particles. The fan built-in on the headlight can accurately blow the wind to the area that must be dissipated, or the area where air is required.
The example of the latter case is to condense when moisture aggregates in a non-close-closed headlights. Compared to the traditional halogen lamp, due to the low level of waste, if the air flows into the lamp housing, these moisture will only evaporate, except for the main cooling function, the fan can also assume this task.
Therefore, for automotive headlights, we need to be able to resist vibration, impact, extreme temperatures in a closed environment. Typical working temperature range requirements are between -40 ° C to + 120 ° C.
EBM-PAPST engineers have accepted this challenge, and they developed a DC axial fan model 622m. These fans have compact and robust advantages, especially suitable for meetings of LED headlight technology. It has a weight of 0.085 kg, a size of 60 x 60 x 25 mm, a rotational speed of 4550 rpm, a service life at 40 ° C at a temperature of 77,500 hours, at the maximum service life of 30,000 hours.
As in other most lighting systems, in the car, the cooling fan must minimize the noise level. Remember that EBM-PAPST's designers placed the 622M model fan in the closed acoustic isolation ring and reduced the net noise below 7 dB. In contrast, the background noise in the quiet office is usually approximately 40 dB, the noise of medium rainfall is 50 dB, and people typically have a sound below 12 dB.
in conclusion
Forced gas flow is an effective way for LED thermal management. Especially for high-brightness LEDs, the active cooling has many advantages, including reduces the dimensions, lower overall costs, increasing light output, and extending the LED service life.
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