LED Characteristics That Require Attention
＜Depend on Junction Temperature＞
Changes in Characteristics With Respect to Temperature (Luminosity, Wavelength, Forward Voltage)
Some LED characteristics will vary depending on chip temperature (Tj: the junction temperature of the light-emitting block), which includes ambient temperature and LED heat generation during operation.
Typical characteristics changes are explained below.
Generally, when Tj rises the light intensity decreases. This is due to increased recombination of electrons and holes that do not contribute to light emission.
The emission wavelength changes based on temperature variations in the same way that luminosity changes. Primarily, the semiconductor bandgap varies depending on temperature, resulting in a change in wavelength.
The amount of wavelength change will differ depending on the material, but with InGaAlP LEDs the change will occur towards longer wavelengths, with ?d varying by 0.1nm/°C based on temperature rise. In applications with restrictive temperature requirements, it is necessary to consider the wavelength change within the set's guaranteed operating temperature range.
Forward Voltage (VF)
Except in special cases, changes in VF are caused by variations in the emission wavelength and semiconductor bandgap. When the temperature rises VF decreases by 2mV/°C. VF change is an important consideration in circuit design.
When the LED operates at constant current VF change should pose no serious problems as a circuit constant. However, at constant voltages VF will drop as the temperature rises, causing an increase in current.
As the current rises Tj will continue to increase, resulting in a further drop in VF until equilibrium is reached. In contrast, as low temperatures VF increases, causing the current to drop, which may make it difficult to obtain the required luminosity during constant voltage operation.
LEDs inherently possess distributed characteristics and variations during manufacturing. For this reason, minimum values have been established for luminosity rank and even electrical characteristics.
As a result, it is necessary to take these variations into account in optical and circuit designs. For example, before any VFfluctuations occur due to temperature, variations are present based on a particular distribution.
Therefore, if there is insufficient design margin, when the VF change is large it is important to consider whether the desired characteristics can be obtained taking into account temperature variations. Depending on the circuit and set characteristics it may be necessary to narrow the width of characteristics variations. In this case it is important to determine whether it's possible to consider and support the introduction of special standards.