LED Overall Construction

Outside of the semiconductor diode, there are several other key components of an LED that are required in order for it to function. These include the leadframe made up of the post and anvil, the reflective cavity, wire bond and the epoxy lens or case. Certain LED designs may include additional parts or have further sophistication, but all contain these basic parts. Below is a detailed list of each of these components.

Leadframe – Outside of the semiconductor die, the leadframe is the heart of an LED chip. This consists of an anvil, which is negatively charged and holds the semiconductor material itself, and the post, which is positively charged and contains the wire bond which provides current into the die. These two components of the leadframe do not physically touch, and are only connected via the wire bond.

Reflective Cavity – This is a reflective material that surrounds the semiconductor die, directing all light outwards towards the lens. It is usually many times larger than the die itself.

Wire Bond – This is the tiny filament of wire that runs from the post to the center of the semiconductor die, providing it current.

Epoxy Lens/Case – This provides protection and structural stability to the LED unit, rigidly affixing all components in place. It offers a degree of impact protection, as well as significant vibration resistance, which is critical for industrial or high performance applications.

Sub-Types

All Light-Emitting Diodes are built on the same basic principle and components. However, there are some significant differences in the design between these different technologies, which are detailed in the following diagrams.

Standard Diode – This is the most basic form of LED, and also the oldest. It involves a relatively straightforward circuit consisting of an anvil and a post, with a wire bond electrically connecting the post to the semiconductor material in the anvil. All of these components are encased in an epoxy resin lens/housing, with anode and cathode connections ready for easy soldering to a board.

SMD LED – Short for “Surface Mount Device”, these LEDs are unique in that instead of being individual parts that are manually soldered to a board, they are actually mounted onto the board itself. One of the biggest advantages of this design is that the LED mount acts as a heat sink, allowing higher current flow and higher efficiency, generating more light.

COB LED – Standing for “Chip on Board”, this is an evolution of the SMD design. In this design, the LED chip is mounted directly to the circuit board using thermal adhesive. This allows for further efficiency in cooling, due to the direct contact between the semiconductor die and the board. The increase in cooling efficiency over SMD designs allows even greater efficiency and performance.

Supporting Components

While the semiconductor diode is the central piece of a LED light , there are other components required to make it function properly. LEDs operate on a relatively low voltage, normally in the range of 3 to 3.6 volts. While this works great in low voltage circuits and equipment such as mobile phones and other battery powered devices, the higher voltage provided by household power is not suitable for LEDs on their own.

Not only is household power in the 120 to 240 volt range, it is also alternating current (AC) which is incompatible with LEDs that require direct current (DC). This requires the use of conversion circuitry which can transform household line AC voltage to DC voltage in a usable range for LEDs. There are several components of this circuitry which are listed below:

Input Fuse – This is a vital component to prevent catastrophic disintegration in the case of a short circuit or overcurrent failure. Input fuses are required by fire safety codes, with the option of using printed circuit board conductors or traditional fuses for this purpose.

Input Transient Protection – This is to protect against outside electromagnetic fields, such as that experienced during an electrical storm or lightning. Movistors are typically used for this purpose, and are particularly important for high performance LEDs that have tighter voltage margins.

Bridge Rectifier – Because LEDs operate on direct current and not alternating current, a rectifier is required. Bridge rectifiers are used for this purpose, as they provide full wave rectification by utilizing both halves of the incoming waveform for the greatest efficiency possible. Additionally, unlike other types of rectifiers, these do not require a centre tapped transformer which would be ill suited to LED applications.

Capacitor Smoothing – Because the waveform that comes from a bridge rectifier consists of a constant series of half waveforms that rise and fall between peak voltage and zero voltage, a capacitor is required to smooth out the voltage coming into the circuit.

DC-DC Converter – The drivers for LEDs are pretty straightforward and consist of a low cost DC-DC converter. This provides a constant source of current for the LEDs, allowing them to function.

Heat Sink – Modern LED applications are considerably more demanding than those of simple indicator lights of the past, requiring what are known as High-Power LEDs (HP-LED). These generate a considerable amount of heat, requiring a heat sink to properly dissipate heat in order to prevent damage.

Applications Beyond Lighting

An interesting application not often considered for LEDs is in the communications realm. One of the unique characteristics of light-emitting diodes is that they cycle on and off millions of times per second, very high data bandwidth can be achieved. This means that they can be used for communication purposes, effectively becoming wireless routers to transmit information to and from various devices.

As long as the communicating devices are in a well lit area with a clear field of view, this can be a highly effective method of communication, taking the place of a high speed wifi internet connection while using a fraction of the energy. In fact, this is the exact principle of how fiber optic cables work – communication via light transmission. The one downside to this is that unlike RF transmission such as wifi, the signal will not be able to penetrate or pass through objects or barriers such as walls, furniture, etc.