Not All LED tubes are the same

Why some LED lamps do not last 50,000 hours?

Fig. 1: Potential LED failures due to poor system designs

A typical LED Luminaire or an LED system consists of many components ranges from LED die, LED package, LED component assembly, thermal interface from PCB to heat sink, electronic driver to drive the LED, diffuser, integrated thermal management to dissipate the heat.

The reliability of the final system depends on how the components are integrated, protected and how heat is dissipated. A poorly designed lamp affect the light quality of the lamp. Light quality is related to color temperature, glare, CRI or characteristics affecting the ergonomics of the lamps.

Poorly designed product and cheap components could bring catastrophic failures due to high trapped heat, high flickering and high total harmonic distorting affecting other appliances. The light decay or color shift may only be detected over time.

Other failures points could be contributed due to poor installation. For example, enclosing a LED lamp inside a plastic enclosure.

As consumer, it is tough to mitigate as there is no means to measure. This is the LED design dichotomy. Consumers demand cheaper products and there are too many failure points to mitigate.

The purpose of this blog is to create awareness on the many other parameters to consider when buying LED lamps.

How is the LED heat generated and dissipated?

Fig.1 shows the characteristic of LED components vs temperature. The top left picture shows that the LED light output deteriorates with temperature. The LED junction temperature could increase due to over current, heat not conducted away or LED working inside a high ambient temperature enclosure resulting in high light decay or color shift over time or catastrophic failure due to thermal runaway.

The bottom left picture shows for every 11 deg increase, the LED lifetime (L70) drops by 60% and the light output drops by 20%. Many uses LED components with LM79 aging information; however, due to poor design or installation, heat is usually trapped and not dissipated. Is the specification for the LED component or the lamp?

Some manufacturers even provide more than 10 years warranty. Some marketing that Glass tube or plastic tubes is cold to touch; thus, LED does not generate heat. In fact, only 20% of the power is converted to light and majority to heat.

So, why is the glass or plastic tube cold to touch?

Heat is dissipated by conduction, convection or radiation. If the heat source is not thermally connected, heat will be trapped.

In the case of a cold T8 tube, plastic and glass are bad heat conductor. LED component is thermally connected to a metal core PCB (MCPCB) using solder and the MCPCB is interfaced to the heat sink using thermal interface material. Due to cost pressure, the MCPCB which is a good heat sink is replaced with fiber glass PCB. There is almost no thermal conduit between the PCB and the Glass or plastic cover resulting in Heat being trapped inside and the housing feels cold to touch. This could lead to a thermal runaway; the higher the heat, the high the current demand until something breaks. Some good examples are LED traffic lights with random LED component failures. Same goes for lighting.

The decision process involves many departments from engineering, purchasing, etc. It is always the cost that breaks the deal.

In retrospect, the cost of a recall or replacing the faulty lamp is more than the initial cost of the lamp.

Lighting Decision Criteria Reference

Light Quality

Fig. 2: Light Quality Characteristics as a reference.

Summary:

The Quality of Light is dictated by the price of the lamp. The efficiency of the lamp is dictated by the price of the lamp. The life and reliability of the lamp is dictated by the price of the lamp. With good marketing, consumer might be easily confused.