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Forward Current vs Ambient Temperature
Figures 1 and 2 show an example of in which the current is derated to take temperature into account. Toshiba technical data for LED lamp usually shows the permissible current values against temperature. Refer to this information when planning a new design.
The following example describes how to derate for temperature when designing:
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, set the LED lamp current 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 12 mA or less. 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 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 the LED is driven at. 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.