An LED is a device that emits light when electrically biased. Similar to any electronic component, LEDs also have electrical parameters that need to be taken into consideration when designed into a system.
Electrical Characteristics of LEDs: LED Fundamentals
Introduction LED-Chip Reflector A Light Emitting Diode (LED) is Wire Bond Mold a device that emits light when electrically biased. Lead frame Similar to any electronic component, component LEDs also have electrical parameters that need to be taken into consideration Silicone when designing with LEDs. Bond Wire LEDs are very similar to Chip standard diodes and most of the electrical characteristics of standard diodes also apply to Cavity LEDs. 0.25 0 25 mm Here is a simple picture to show how LEDs are constructed. constructed Printed Circuit Board (PCB)
How does an LED emit light? n-Crystal n Cr stal p-Crystal p Crystal + Metallic Contact - + Epitaxy Layer Depletion zone Substrate - Electrons Holes LED chip‘s PN junction is biased in a forward direction; Free charge is forced (overcome Vf) into the depletion zone; Electrons recombine with holes, and some of these recombinations emit light; The color of the light is based on the material selection, which directly affects the forward voltage of the LED.
Electrical parameters of an LED The following electrical parameters should be taken in to consideration when designing with LEDs. 1. Vf and current: the I-V curve of the LED will have these information 2. Pulse d 2 P l and surge current of th LED t f the 3. Reverse current and/ or reverse voltage 4. Junction temperature (Tj) a. a reduction in Vf due to Tj b. shift in color due to Tj c. flux degradation due to Tj 5. 5 Recommended PCB foot print foot-print
Forward Voltage - Vf Similar to standard diodes, in LEDs, nothing happens until a threshold voltage is reached. Once the threshold is reached, current through the LED rapidly increases with increasing voltage. Due to this behavior the preferred method to drive the LED is with constant current. As it can be seen from the graph on the right, nothing happens until the threshold voltage of ~2.75V. Once the 2.7V is reached, current th h d t through th LED h the increases exponentially with slight increase in voltage.
LED current (forward current) LED current is one of the key parameters as it determines the amount of light that the LED puts out, the forward voltage of the LED, and the color or wavelength shift when the LEDs is driven, in a particular design, at a different current than the binning current. The Vf of an LED varies slightly depending on the LED current. As LEDs are driven current using constant current, if the system has a resistor type current regulation, an accurate Vf should be used to calculate the resistor value. The color shift due to different LED current also determines what dimming methodology to be utilized in a system, if the system requires some kind of dimming. If color shift is due to analog dimming, (where LED DC current is varied to achieve different dimming levels), is not acceptable, PWM (Pulse Width Modulation) dimming should be utilized. LED current also determines the efficacy of the LED as well as the system efficacy.
LED current (forward current) … Shown on the right is the relative flux vs LED current. Since the binning current for this LED is 350mA, the flux at 350mA is x 1 in a relative graph. When the LED current is 700mA the flux 700mA, will be ~1.74 times that of the flux at 350mA. Wh d i i an LED system, th LED When designing t the current will determine the total flux/ light output of the system, along with some other key parameters of the system.
LED current (forward current) … Efficacy of an LED with respect to LED current is shown on the right. Th efficacy of an LED decreases as The ffi f d LED current is increased. It is required to consider this phenomenon when designing an LED system as this will impact the overall system efficacy.
LED current (forward current) … There will be a slight color shift due to LED current, if the LED current is different from the binning current. As it can be seen in the graph on the right, there will be no shift at 350mA because that is the binning current. At 700 A one should expect t see a Cy 700mA, h ld t to C shift of ~0.0075 and a Cx shift of ~0.003 on the CIE 1931 diagram. This particular characteristics of an LED will eventually determine the dimming methodology, if the system requires some kind of dimming.
Pulse and Surge current Surge current is the absolute maximum non- DC current th t th LED can h dl Th t that the handle. The maximum surge current and the definition of it should be taken in to consideration when designing with LEDs. The definition of surge can be represented as: t < 50mS, D=0.016, and Ts=25°C where Ts is the solder point temperature. The frequency and the duty cycle of the pulse current is very important and should be considered during system design. Also, note that the definition of pulse can vary at different solder point temperatures temperatures.
Reverse current/ voltage – IR / VR Reverse current and/ or the reverse voltage of an LED is one of the critical parameters to be considered when designing with LEDs LEDs. Most LEDs are not designed to be operated in the reverse direction. Also, because of how the protection device within an LED is oriented (see below), care should be taken when the LEDs are placed in anti-parallel manner. Since LEDs are not designed for reverse operation, negative spikes within the circuit should be taken into account to ensure the LEDs are properly operated.
Junction temperature - Tj The junction temperature of the LED is a key factor of the life of an LED. In terms of electrical characteristics of an LED, junction temperature plays a role on the forward voltage of the LED (Vf), pulsed current, flux reduction, and color shift. As demonstrated in the graph, Vf reduces when Tj increases. This should be considered when using a resistor to regulate LED the current. current
Junction temperature – Tj … The graph on the right shows the flux reduction when Tj increases. Even though this may not be considered an electrical parameter, it will impact the electrical parameters indirectly. Flux degradation at higher Tj can be compensated with LED current and when the LED current is changed many of the changed, other electrical parameters of an LED are impacted. For this and other reasons such as Vf drop and color shift, Tj should be taken into consideration when finalizing other electrical parameters.
Junction temperature – Tj … As demonstrated in the chart, the color shift due to Tj may be significant and needs to be taken in to consideration during the design g g process.
PCB footprint The PCB footprint may not be considered an electrical parameter, but is included here because it can impact the electrical characteristics. Shown on the right is the recommended footprint for OSRAM s OSRAM’s OSLON package package. Proper footprint is required for proper thermal management of the LED and ease of assembly, including correct placement and reflow of the LED.
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