Why Package Devices Are King

Posted: March 11, 2009 in General Commentary
Tags: , , , , ,
render21

Under that LED die lies a reflector surface and a thermal slug to manage the loads these powerful products demand.

There is one universal truth in all solid-state components, LEDs included – HEAT is the enemy. There is no escaping the loss in performance and life that results from heat within an LED device. The reason packaged High Brightness LED devices do as well as they do comes down to one key factor – thermal management. Inside those innocuous looking devices is a slug of copper that conducts heat from the LED and delivers it to the PCB, or MCPCB directly. This is considerably more effective than the 5mm (and similar through hole design) products, which have virtually no heat sink beyond the legs and one small bit of metal under the reflector cup. The illustrations here show this in better detail.

5mm2

The relative lack of thermal control (other than the standoff legs and a small metal bar under the reflector) limits these through hole devices to very low loads or intermittant use.

The standard 5mm, or through hole style LED is just fine for what it was invented for – loads of a fraction of a watt, and use for indicators that were operated intermittently. It’s when this design is loaded with high output LED die driven at higher currents, then operated continuously, that causes these things to fall apart. Thermal design is all about staying ahead of the heat gain. Fail that and the microchip can’t survive. First lumen output suffers, then ultimately the die begins to break down. In the worst cases, the bond wires melt or break from thermal expansion and contraction.

HB Packaged LED devices are designed specifically to manage the additional heat of high current operation and continuous duty that general illumination requires.

In addition, this thermal conductor design allows luminaire engineers to draw heat away from the LEDs through metal circuit boards, heat sinks, and other designs that pull heat from that internal slug and distributes it. This cannot be done effectively on through hole LEDs (5mm or others), as the stand off legs simply cannot conduct heat effectively from the die inside.

HB package devices also include considerably more reflector area and optical control around the LED die. This amplifies the performance of the die inside, as well as producing control from more precise placement of the LED inside luminaire optical systems.

As a general rule, surface mount LED devices should be held to elss than 1/4 watt, and driven at no more than 10 to 30 mA. Common package devices using high quality LED die and proper internal and external thermal management can reach power levels up to 5 watts at drive currents of 700 mA. At the extreme end, LED packages have been deployed that exceed 200W, while delivering reasonable life and lumen depreciation – although these devices require expert engineering development and are generally very specific in application. Try any of this with a through hole device and the result will be a little pop and  barely visible flash as it expires.

Comments
  1. I will have to try this out. Thanks for the tip.

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