The I-V characteristics of LEDs illustrate the current through it for different values of applied voltage.
The objective of any current limiting technique is to maintain the current through the LED within the chosen value.
In a constant current LED driver, the voltage of the power supply is varied within a range to limit the current drawn to a specified value.
In any LED, it is necessary to limit the forward current to ensure long life and good performance
LEDs are rapidly becoming the face of modern lighting systems. LEDs are simply diodes that emit light when forward biased. The current flows from the anode to the cathode of the LED and outputs light. Their light output covers all the visible light frequencies. Some LEDs emit infrared and ultraviolet light. In any LED, it is necessary to limit the forward current to ensure long life and good performance. When designing an LED circuit, you might wonder how to limit current to LED. In this article, we will discuss current limiting techniques in LED circuits.
Forward Voltage and Forward Current of LEDs
When an LED is forward biased or energized, the current flows from anode to cathode. In forward biased conditions, electron-hole recombination takes place in the semiconductor junction of the LED. The electron-hole recombination in LEDs emits energy in the form of light. The transition of electrons from a higher energy band to a lower energy band gives spontaneous emissions in the form of light output. Under reverse-biased conditions, no light output can be achieved from LEDs. The color of light output from LEDs under forward biased conditions depends on the semiconductor compounds used in it.
LEDs are current-dependent devices with the forward voltage and forward current dependent on the semiconductor materials. Like signal diodes, LEDs are characterized by their forward voltage and forward current. Typically, the forward voltage is between 1.2 to 3.6 V, and the forward current is between 10 to 30 mA. The forward current to voltage (I-V) characteristics are an important curve in any LED datasheet. Let’s examine the LED I-V characteristics to understand why there is a need to limit the LED current.
I-V Characteristics of LEDs
The I-V characteristics of an LED illustrate the current going through it for different values of applied voltage. The forward LED current is plotted as a function of voltage. The LED I-V characteristics follow a non-linear pattern, which is against Ohm’s law. According to Ohm’s law, the current and voltage share a linear relationship. In such linear I-V characteristics, the current drawn by the device increases with increasing voltage. For example, resistors are linear circuit components that obey Ohm’s law. In resistors, the ratio of voltage to current always equals the resistance value.
This is not the case with LEDs; LEDs are not governed by Ohm’s law. Until the voltage applied across the LED reaches the forward voltage, the current drawn is very low. When the applied voltage crosses the forward voltage value, the current increases exponentially with increasing voltage. The excess current flowing through LEDs can instantly destroy them, so it is important to limit the excessive current flow.
How to Limit Current to LED
In any LED circuit, it is important to limit the current. From the I-V characteristics, the LED forward current value for giving a particular light intensity can be obtained. The objective of any current limiting technique is to maintain the current through the LED within the chosen value or limit. Below are a few ideas on how to limit current in LED circuits.
Parallel LED circuits
Paralleling the same color LED is seen in lighting circuits. The current drawn by each branch is dependent on the internal resistance. This method is useful only when the same color LEDs are paralleled. When the colors are mixed, the LEDs with the least internal resistance draw more current, leading to a short lifespan or even burnout.
Maintain the forward voltage value given in the datasheet across the LED
The applied voltage from a regulated DC power supply can be adjusted such that it maintains the voltage across the LED to the forward voltage range. This may affect the light intensity. If the user is satisfied with the light intensity output, the voltage regulated to a forward voltage value is an easy method.
Constant current LED drivers
LED drivers are power supplies dedicated for LED applications. There are two types of LED drivers, namely the constant voltage LED driver and constant current LED driver. In a constant current LED driver, the voltage of the power supply is varied within a range to limit the current drawn to a specified value.
Current limiting resistors
Current limiting resistors are connected in series with the LED. The current limiting resistors can be inserted in both series and parallel LED circuits. The LED current limiting resistors maintain the current at a specified level through an LED that is within the safe forward current values given in the LED datasheet. The current limiting resistor value is determined from the known parameters, such as thesupply voltage (Vsupply), forward current (ILED), and forward voltage (VLED) of the LED. The current limiting resistor (RLED) value can be given by the equation:
When you are clear about the forward current value for providing the required light intensity output, then figuing out how to limit current to LED is straightforward. You can design current limited LED circuits using Cadence’s PCB design and analysis tools.
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