Signs of the Times - September 2012 - 34

LIGHTING TECHNIQUES
By Marcus Thielen

Marcus Thielen is a physicist and lighting-industry consultant from Duisburg, Germany.

Keep Cool!
Heat-sink calculations for LED signs

As page 24), in ST’sare trending reported July issue (see LEDs
more and more towards high-power versions with considerable light output for use in “classical” lighting applications. While four years ago, single LEDs had a maximum power level of approximately 5-7 watts (W), today we’re seeing devices with up to 180W per thumbnail-size chip. Because most electrical energy in an LED still is converted into heat, the need for heat control is obvious. Most LED luminaires shown at Germany’s “light+building” show had cooling fins or even active cooling fans. All LEDs are semiconductors, and light generation occurs in a very narrow area – the “junction” – where different kinds of semiconductor material are joined. Light generation, and most of the heat generation, occurs here. An LED will be immediately destroyed if this junction gets hotter than 221oF (or, in very few cases, 293oF) during operation. Components in outdoor LED signs should be able to withstand a continuous ambient operating temperature of 150oF (based on average conditions inside channel letters and lightbox signs from north to south Europe; standards will soon be established).

But how can a signmaker ensure the component specs aren’t exceeded? Typical data-sheet text offers only an open-liability waiver: “Thermal design of the end product is of paramount importance.” Calculation Exact calculations of an LED’s thermal operating parameters are rarely possible. No standard parameters are provided in datasheets, so any calculations – or better estimating – depends on the data available. In most cases, the maximum, permissible junction temperature is given; sometimes, there is also the case temperature or the LED’s operating (ambient) temperature. Heat dissipation from an LED setup involves multiple steps - first from the junction to the case, then from the case to the heatsink (or mounting plate), then to the ambient air. In a signbox or channel letter, it’s questionable how well the hot ambient air inside can dissipate the heat to the outside. Here’s a calculation of the heatsink necessary for a 15W LED, where the data sheet states a maximum case temperature of Tc=185oF. The EN regulation states an ambient temperature of Ta=150oF, so the temperature difference between the

LED case and the ambient air must not exceed 20K. (Note: For formal correctness, we use the absolute Kelvin scale for temperature differences, and degrees Farenhreit for the convenience of common values. A 1Kº difference equals 9/5oF (or 1.8oF). Further, for ease of reading, we use n-1 instead of dividing 1/n. Also, for dimensions, we stay metric to avoid fractional conversions.) The “thermal resistance” of a heatsink or part is usually given in Kelvins per Watt (KW-1), which is the temperature rise on the hot side over the cold side (kept at constant temperature) for a given power to be conducted. To dissipate Pv=15W at a 20K difference, the maximum thermal resistance Rth of a heatsink is Rth = 20 K/15W = 1.3 KW-1. For a heatsink with higher thermal resistance, for example, R th = 2 KW-1, the LED temperature could be calculated by the formula Tc= Ta+(Rth*Pv), so the smaller heatsink would give Tc = 150oF + (2 KW-1 *15 W)*1.8 = 203oF, which is more than the 185oF the LED can take. A burnt LED will result. Calculations become more complicated when the LED isn’t mounted directly on a heatsink, but multiple materials with different thermal resistances are connected, and these simply provide the total thermal resistance. So, if we mount an LED – for which the manufacturer has stated an internal thermal resistance from the junction to the case of Rth,j-c = 2.5 KW-1 on a heatsink with R th,c-a = 2 KW-1 – the total thermal resistance from the junction to the environment is

Outdoor LED luminaire: LEDs are mounted directly to a massive aluminium heatsink, which forms the outer enclosure. 34 SIGNS OF THE TIMES / SEPTEMBER 2012 / www.signweb.com

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Signs of the Times - September 2012

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