What is LED Driver and how does it work?

LED stands for “Light Emitting Diode” and is a semiconductor light source. By applying voltage, its electrons are activated and the LED begins to emit a beam.

LED Working Principle

LEDs start to emit light by transmitting from 10 mA and 1.5 V on average. The emissivity of light emitting diodes decreases with time. When the emitting power drops to half of the normal power, the diode is economically dead. The average life of an LED is 100000 hours.

The light intensity emitted by the LED increases in direct proportion to the current passing through it. However, this increase is linear up to a certain value of the current, then it bends. If the current supplied to the diode exceeds the point where the linearity breaks down, called the threshold value, the diode will overheat and break down.

LED Selection Criteria

6 factors should be considered in the selection of LED lighting systems.

  • LED Index and Cover (Bind)
  • Color Rendering Index
  • Color Temperature
  • Cooling System
  • Light Emission Angle
  • Luminous Flux Output (Efficiency Factor)

LED Index and Cover

LED Array is the most important part of LED selection and is a process that determines color and light output as well as optical array. While some chips are a single circuit, some may be part of a group in a sheath called a “Bind”. The version, color and brightness of each individual circuit may be different, the LEDs in the “Bind” are powered by the same driver and behave exactly the same.

LED chips are placed in a silicon circuit, and a circuit can contain thousands of LED chips. Silicon circuits with LED chips are called PCB (Printed Circuit Board, “Printed Circuit”). LED lighting systems are created with different indexes and “Bind” patterns.

Color Rendering Index

The success of displaying the colors of the objects illuminated by light sources is determined by the “CRI”, that is, the “Color Rendering Index”, or, in other words, by the “Ra”, that is, the “Color Separation Index”. The theoretical maximum value of CRI is 100. Light sources are classified as medium if their color separation index is between 50-70, good if 70-90, and excellent color separation index between 90-100. Color discrimination indices of lamps and efficiency factors are inversely proportional to each other.

Cooling System

Optimal heat dissipation is key to the quality of LED performance. Poor heat dissipation causes loss of efficiency, shortening of life and deterioration of color transition. For temperature, 4 basic elements need to be considered; ambient temperature, heatsink material, driver current, “Bind” or arrangement of LEDs.

There are two types of cooling systems for heat removal, active cooling and passive cooling. In active cooling, there is a fan system and an extra power is spent. Passive cooling is usually done with a metal core.

Light Emission Angle

The luminous flux spreads around 1800, with lenses this light is limited to the desired range. The light emission angle is defined as the angle at which 50% of the light emanating from the LED is located. We should choose the appropriate angle according to the feature of the place we will illuminate.

Luminous Flux Output (Efficiency Factor)

Efficiency Factor expresses lamp efficiency in lumens per watt. Although the current flowing through the LEDs is constant, the efficiency factors decrease with increasing ambient temperature due to the characteristic features of the diodes. This decrease can vary between 0.3% and 0.7% for each degree, depending on the type of material the LEDs are made of.

LEDs have advantages over incandescent lamps such as lower energy consumption, longer life, robustness, smaller dimensions, fast switching, higher durability and reliability. They require more stable current and heat management compared to compact fluorescent lamps that give the same light output.

led driver working principle

LED Drivers

As the use of LEDs became widespread, LED drivers, which are a component of the LED system, began to gain more importance.

LEDs are low voltage light sources that require an optimal operating current or a constant DC voltage. Each of the LEDs used for lighting requires a DC voltage of 2-4 V and a current of several hundred mA, but because they are connected in series, high voltage is required. In addition, the light source must be protected against line voltage fluctuations during operation. Changes in voltage can cause a disproportionate change in current, which changes the light output. The light output of the LED is proportional to the current and is defined for a certain current range. If the current exceeds the manufacturer’s recommended range, the brightness of the LEDs may increase, but their light output can drop rapidly due to temperature rises inside the device, causing the LEDs to have a shorter lifespan. The lifetime of LEDs is defined as the point at which the light output decreases by 30%.

The LED driver is like the ballast in fluorescent and HID systems. LEDs require a device that will convert AC voltage to DC voltage and regulate the current flowing through the LED during operation. While the driver converts 220 V 50 Hz AC voltage to low voltage DC voltage, it also protects the LEDs from line surges.

LED drivers are usually of constant voltage types such as 10V, 12V and 24V. Some drivers are specially designed and only run a specific group of LED arrays. Drives are generally compact and operate with high efficiency. It features remote control of the power supply and is Class 2 isolated for safe operation.

Using Resistor as current limiter

In some applications, current-limiting circuits such as resistors can be offered as cheaper alternatives to constant-current circuits. However, this has many drawbacks. First of all, resistors generate fugitive energy due to heat. The heat produced by the resistors must be dissipated. In addition, there is no protection that will balance the light output in the supply voltage and prevent the LEDs from being damaged by high voltage in the protections made with only resistors. Therefore, it is not recommended for permanent applications. A very small change in voltage causes a disproportionately large change in current.

In addition, the application voltage to achieve the desired light output varies depending on the LED size, material and temperature. When the LED temperature increases, the voltage decreases and the current increases (Figure 23). Increasing the current causes additional heating at the junction. If the current is not limited, the junction will deteriorate due to heat. This phenomenon is called thermal runaway. By driving LED light sources with a regulated constant current power supply, problems such as voltage variations and light output variation and shortening of life due to these changes can be eliminated. Therefore, constant current drivers are recommended as the power source of LED lighting systems.

What is a junction?

In the diode, there is a junction, also called a planar metallurgical contact, between p and n type materials. It is the separator joint region between the emitter-base-collector triplets in transistors.

Light Output

The light output of LED light sources increases with increasing drive current. But the yield is affected by it. Figure 24 shows this relationship. Normally LED lamps have a current value specified in their data sheet. This current value is a reference point for other technical information. Higher than recommended drive current will cause overcurrent and low lumens.

Temperature Increase Effect

The performance characteristics of an LED are given at an operating temperature of 25 0C. However, since LEDs generally operate above 25 0C, this value is only taken as a reference value. The light output of the LED source decreases as the temperature of the LED junction increases. Ambient temperature also affects the light output of LEDs.

Dimming and Color Changing Feature

Drivers have features such as dim the light, change its color and turn it on sequentially. LEDs can be easily added to control circuits. Many drivers are equipped with systems such as 0-10 V control circuits, occupancy sensors, architectural and dramatic lighting controls, building and lighting automation.

Drivers are also used to change colors or provide sequences. This happens by dimming some of the colors in a set of colored LEDs. In another method, the driver works in conjunction with a color sequencer, taking an LED output with 10 V or 24 V output, which can be mixed into a three source output, usually red, blue, and green, which can be mixed in a very wide color range.

Considerations in LED Driver Selection

One of the biggest problems encountered in LED Lighting Systems is driver overload. Significant voltage drop occurs at the end groups of the chain if many more LED groups are connected in series than the values ​​for which the driver was designed.

Another major problem is the problem due to incorrect drive voltage. When the wrong driver voltage is applied, the LED will either not work at all or operate at higher current levels than designed. The installer needs to compare the drive voltage with the load voltage. For example, if an LED circuit with a load of 10 V is fed with a 12 V driver, the LED life will be noticeably reduced.

One of the characteristics of LED drivers is the quality of the DC output voltage. Drivers must be able to operate in such a way as to maintain a constant DC current that will provide the highest light output of the LEDs without excessive stress. In addition, power losses and voltage drops in the connection are also important.

Finally, attention should be paid to the ambient temperature. Most riders are already indoors and in a dry location, while those outdoors need to be in a protective box. It is wrongly believed that LEDs produce little or no heat. Attention should be paid to the mounting elements and ventilation conditions that allow the heat load to be discharged.