Derating LEDs: Understanding Forward
Current vs. Ambient Temperature
Figure 1 and Figure 2 show an example where the current is derated to account for temperature. The technical data of Toshiba LED usually shows the relationship between the allowable current value and the temperature lamps. Please refer to this information when planning a new design.
The following example describes how to reduce the temperature during design:
Figure 1 – Specified by design (22 mA) exceeds permissible value at high temperatures
Figure 2 – Specified by design (22 mA) is within permissible value at high temperatures
LED Lamp
An LED lamp is to be used at an ambient temperature of +25°C.
To obtain the required luminosity, the LED lamp current must be set to 20 mA.
The guaranteed operating temperature range for the equipment is -10° to +70°C.
If the current is derated for temperature as shown in Figure 1, at +70°C the LED lamp current must be set to at or less than 12 mA. Hence a design which yields an LED lamp current of 22 mA is not permitted. However , some other types of LEDs which emit the same color have their current derated to take temperature into account, as shown in Figure 2. Since those LEDs can maintain a current of about 27 mA at a temperature of +70°C, they satisfy the above design conditions.
Derating LEDs for Longevity
Characteristically, the luminous intensity of LEDs gradually decreases over the life of the LED. The rate at which the luminous intensity falls varies according to the material used and the forward current at which the LED is driven. The larger the current, the greater the diminution of luminosity. Thus, when setting the forward current of the LED lamp, do not determine the setting solely from the temperature characteristics, as in the above example, but consider also longevity characteristics. An effective way of improving the longevity characteristics is to set the forward current of the LED lamp relatively low.
