LEDs, Heat, and You

How Much Heat?

A 100W tungsten filament bulb is about 10% efficient, meaning it only generates 10W of light energy for every 90W of heat energy.  That “light energy” is what designers need to match with other sources to get an equivalent amount of light, which is why high-efficiency bulbs can call themselves equivalent to a 100W bulb while only actually consuming a fraction of that energy.  While LEDs themselves can be extremely efficient, other components of the system like the driver or lens tend to reduce the efficiency of an LED light bulb to about 70%.  That is an incredibly energy saving over the 10% efficient filament bulb, but still means that about 30% of the power consumed by the bulb gets emitted as heat rather than light. 

How Does That Matter?

When a filament bulb generates heat, that heat aids the efficiency of the bulb by lowering the electrical power required to heat the filament to the point of emitting light.  LEDs emit light (and heat) based on the PN junction of the diode, which becomes less efficient as heat is introduced.  The forward voltage of the LED is indicative of the size of the energy gap an electron must achieve before it can be absorbed into the other side of the diode junction and emit light.  When heat is introduced, the electrons start at a higher energy level, and the voltage of the LED decreases.  LEDs are constant current devices and are not (typically) driven in a way that allows them to compensate for this voltage drop by drawing more current.  The LEDs just fall backwards along their V-I curve and emit less light as the temperature increases.

Why Not Use Constant Voltage?

Most LED drivers are constant current and limit the LEDs as described above.  It seems like a good solution would be to move to constant voltage control, which would allow the LEDs to draw more current as the voltage decreases.  This works mathematically, but actually creates a positive feedback loop in the diode junction that causes the LED to draw more and more current, generating more and more heat until it burns up. Even if the power supply has a current limit, constant voltage supplies are not designed to actually regulate at that limit and will likely stop functioning once their limit is exceeded.  One way or another, something in your system will fail. 

Can We Compensate?

The compromise between a thermal runaway situation and dimming LEDs is to use temperature-sensitive driver chips.

参阅相关产品

CL25N8-G

Microchip Technology LED 驱动器 查看

These drivers are often placed in series with each string of LEDs in a fixture to balance parallel strings and compensate the allowed current based on fixture temperature.  These chips will limit the current to a range that is deemed safe for the LEDs to prevent runaway but also appropriately increase the current to compensate for lower voltages.  While some power supplies claim to compensate for temperature, that is only valuable in a situation where the ambient temperature fluctuates.  The temperature of concern is typically very localized to the LEDs and can only be properly regulated by a chip that is on the same board and in the same situation as the LEDs themselves. 

The easiest answer is usually to allow proper heat sinking for the LED board and not allow the junction temperature to reach the point where LED dimming is noticeable. 

Without a comparison point, our eyes typically do not detect dimming until the light source is at 50% of the original brightness, so things have to get pretty hot for the average application to be affected by the dimming effect.  If you use thermal vias, generous copper pours, metal heat sinks, and fans where appropriate, the typical temperature rise of an LED system should not practically affect your output.  If you do notice the fixture dimming, be sure to let it cool down before grabbing it to investigate – it will be hot!