LED news and insight from our technology editor Steve Bush, who has been messing with LED lighting for more years than he would care to remember.
Electronics Weekly’s guide to LED heatsinking
Check out the latest in the Electronics Weekly’s ‘big guides’ to LED technology – specifically, to LED heatsinking.
Steve Bush, our Technology Editor, begins:
In the early days of power LEDs, many lighting firms rushed out solid-state luminaries only to find them failing in service.
The problem was heatsinking. Manufacturers were used to filament bulbs that run hot and are cooled by radiation, and fluorescent tubes that have a huge surface area from which heat convects.
Conductive cooling, the mainstay of electronics, was a black art.
That was a decade ago now, but the danger of unreliable product is still here, particularly as cost has to be low and power LEDs are being squeezed into smaller and more demanding applications such as MR16 down-lighters.
What are the basics of LED cooling? “It is thermal management, you can do calculations from thermal resistance. We do those on a regular basis,” Lumileds application engineer Michiel Kruger tells Electronics Weekly.
Thermal resistance (R?), measured in kelvin per watt (K/W), is the traditional metric used to determine how hot the junction of a semiconductor will get.
You simply add the junction-to-mounting-face R? of the LED package to the R? of the heatsink mounting face to the ambient air. If there is a PCB in between, you add that R? as well.
From the total thermal resistance, you can back-calculate the junction temperature from the ambient air temperature at a given LED dissipation.
But even at this basic approximation level, there are complications.
“The real question is how to take temperature into account as this affects the performance of the LED, so it is not a straight-forward call,” says Kruger. “LED forward voltage [Vf] goes up with power, and Vf goes down when the die gets hot. With just thinking the LED dissipates 1W, you miss a number of factors. If you don’t do it correctly, you may over-design.”
So at the minimum, the simple approach becomes iterative as junction temperature affects light output and the input power has to be changed to correct this.
And this is with a heatsink of known characteristic, in a plentiful supply of ambient air.
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