Common thermal considerations in LEDs include test point temperature and thermal power.

One characteristic typically associated with While it’s true that LEDs are cool relative to filaments found in incandescent and halogen lamps, they do generate heat within the semiconductor structure, so the system must be designed in such a way that the heat is safely dissipated. The waste heat white LEDs generate in normal operation can damage both the LED and its phosphor coating (which converts the LED’s native blue color to white) unless it’s properly channeled away from the light source.

A luminaire’s thermal design is specified to support continuous operation without heat damage and oftentimes separates the LEDs from temperature-sensitive electronics, which provides an important advantage over individual LED replacement bulbs.

Test point temperature
Test point temperature (Tc) is one characteristic that plays an important role during integration to determine the amount of heat sinking, or cooling, that the luminaire design requires. In general, the higher the Tc limit compared to worst-case ambient temperature (Ta), the more flexibility a luminaire manufacturer will have in designing or selecting a cooling solution.

The worst-case ambient temperature is usually 40ºC or higher, so a module with a low Tc rating (e.g., 65ºC) doesn’t have much headroom above the already hot ambient temperature. Trying to keep a module at Tc 65ºC when the Ta is 40ºC and dissipating 40W thermal power is very difficult to do with a passive heat sink, so a fan or other active heat sink will likely be required. On the other hand, a module with a Tc rating of 90º C or higher (while still meeting lumen maintenance and warranty specifications) has at least 50º C headroom over the ambient temperature and should be able to make use of a reasonably sized passive heat sink.

However, the higher you can push the test point on the LED module, the smaller the heat sink you need. It’s dependent on the Ta – if the module can’t withstand a high enough maximum temperature, it’s impossible to cool below Ta unless you have a refrigerated system, regardless of the size or effectiveness of the heat sink. Stretching the difference between Tc and Ta as much as possible will give you greater room to deviate from the norm and be creative in your heat sink selection.

From phosphor to where the heat sink is located, Xicato is driving Corrected Cold Phosphor to lower the resistance between the phosphor and the heat sink, without having to cool through the hot LEDs. Today, the module output is at 4000 lumens, which wouldn’t have been possible five years ago.

The bottom-line considerations with respect to test point temperature are really flexibility and cost. If a module with a high Tc rating is chosen, there will be more options for design and cost savings than are provided by a module with a low Tc rating, assuming the same power dissipation.

Figure 1: Xicato XSM module family sample passive heat sink matrix showing suitable module usage for a range of thermal classes.

Thermal power
Another key characteristic, thermal power (load) has always been a difficult number to deal with. LED module manufacturers don’t always provide the information required to calculate thermal power because this value can change based on such variables as lumen package, Color Rendering Index (CRI), correlated color temperature (CCT), etc. Cooling solutions are often rated for performance in terms of degrees Celsius per watt, which, unfortunately, necessitates calculating the thermal power.

To address this problem, Xicato has developed a “class system,” through which each module variation is evaluated and assigned a “thermal class.” With this system, determining the appropriate cooling solution is as simple as referencing the thermal class from the module’s data sheet to a matrix of heat sinks. FIGURE 1 is a sample passive heat sink thermal class matrix for the Xicato XSM module family.

Let’s take, as an example, a 1300 lumen module with a thermal class rating of “F.” According to the matrix, for an ambient condition of 40°C, the best choice of heat sink would be one that is 70 mm in diameter and 40 mm tall. Validation testing is still required for each luminaire during the design phase, as variations in trims, optics, and mechanical structures can affect performance. Looking at the example module, if a manufacturer were to design a luminaire around this class “F” heat sink and nine months later a new, higher-flux class “F” module were released, the same luminaire would be able to support the higher-lumen module without the need for additional thermal testing. The thermal-class approach supports good design practice, speeds development and product portfolio expansion, and provides a future-proof approach to thermal design and integration.

Most specification sheets cite an electrical requirement for the module and the lumen output. Electrical input is basically the voltage the module will require and the current needed to drive it; the product of these two variables is power. The problem with output is that it’s always displayed in lumens – a lumen is not a measure of power, but rather a unit that quantifies and draws optical response to the eye. It’s calibrated specifically on what the human eye sees, but there’s a quality of brightness that comes into play that can’t easily be tied back to electrical power. There’s no way to figure out exactly how much thermal power is being dissipated by the module – power “in” is measured in electrical energy (voltage × current), while power “out” is non-visible electromagnetic, visible electromagnetic, and thermal power. None of this is shown in datasheets.

This intangible factor creates a challenge – for most customers, a watt is a watt, but in reality, there are thermal watts, electrical watts and optical watts; not all are easily determined. The customer can attempt calculations – e.g., how to cool 10 thermal watts – but the fact is that people don’t generally think that way. Many customers don’t have engineers on staff, and those that do often use rough approximations to determine compatibility.

Xicato has defined modules that go up to Class U. The Tc rating, while independent of module flux package, is interrelated. Class A modules, in general, don’t need a heat sink; lower power modules usually achieve about 300 lumens. On the other hand, an XLM 95 CRI product is a Class U product that requires either a passive heat sink or an active heat sink. Once the module and heat sink have been selected and integrated into the luminaire, the next step is thermal validation, which Xicato performs for the specific fixture utilizing an intensive testing process that includes detailed requirements that must be met by the luminaire maker when submitting a fixture for validation (see Table 1 for a partial summary).

The validation is based not on lumens, but on the thermal class model, and the fixture rating is also based on thermal class, rather than wattage, because watts differ. With this approach, an upgrade can be made easily without having to do any retesting. •

JOHN YRIBERRI, Xicato, is the director of Global Application Engineering, Xicato, Inc., San Jose, CA. John joined Xicato in November of 2007 and was the Project leader for Xicato’s first LED platform- the Xicato Spot Module (XSM).