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LED lighting requires precise power and thermal management systems


Release time: 2018-08-21 17:27:00.000

LED technology has made rapid development, and chip design and material improvements to promote the development of brighter, more durable light source, light source applications are expanding. The increasing awareness of energy saving has also accelerated the popularity of LED lighting technology. Today, LED lamps are rapidly replacing traditional lighting methods with the following advantages: 1) low energy consumption, and the power consumption of new bulbs ranges from 0.83 watts to 7.3 watts; 2) long life, LED bulbs can be used for 50000 hours; 3)LED bulbs are heat-resistant, Shock-resistant, ready-to-use, very suitable for applications that require frequent switching operations; 4) Help meet safety and green initiatives; 5) The LED bulb is not hot to the touch and does not contain mercury; 6) It can be fully dimmed. When dimming, the LED bulb will not change color, while the incandescent lamp will yellow when dimming; 7) No frequency interference, no ballast resistor that interferes with radio and TV signals.

Although this technology is becoming more and more popular, LED lighting manufacturers continue to overcome the problem of high thermal sensitivity of LED light sources. Excessive heat or improper application will greatly reduce the performance of the LED light source.


 

comparison of thermal conductivity

A light fixture using a 60W incandescent bulb produces approximately 900 lumps of illumination and must dissipate 3 watts of heat by conduction. Only 12 LEDs are needed to achieve the same 900 lumen illumination using a typical DC LED as the light source. Assuming that VF (forward voltage) is 3.2V and current is 350mA, the input power of the lamp can be calculated as follows:

Power = 12 x 3.2V x 350mA = 13.4W.

In this case, about 20% of the input power is converted to light and 80% to heat. This depends on a variety of factors, and heating may be related to underlying irregularities as well as phonon emission, sealing, and materials.

Of the total heat generated by LEDs, 90% is transferred by conduction. To dissipate heat from the LED junction, conduction is the primary pathway for heat conduction, as convection and radiation account for only about 10% of the total heat transfer. For example, an LED may convert nearly 10.72 watts of heat (13.4 x 0.8). Of these, 9.648 watts (10.72 x 0.9) are transmitted or migrated from the LED junction by conduction.

Obviously, LED light sources require precise power and thermal management systems, because the electrical energy provided to the LED is mostly converted into heat compared to other light sources. Without proper thermal management, this heat can affect LED lifetime and color output. At the same time, since LED driving devices are silicon devices, they will soon fail. This means that a fail-safe standby overcurrent protection device must be provided.

junction temperature effect

The optical performance of LEDs varies significantly depending on the temperature. The amount of light emitted by an LED decreases as the junction temperature increases, and for some technologies, the emission wavelength also changes with temperature. If the drive current and junction temperature are not properly controlled, the LED efficiency will drop rapidly, resulting in reduced brightness and shortened life.

Another LED characteristic related to junction temperature is forward voltage. If only a simple bias resistor is used to control the drive current, VF will decrease with increasing temperature and the drive current will increase. This can lead to thermal collapse, especially for high power LEDs, and component failure. It is common practice to accelerate heat conduction by mounting the LED on a metal core PCB to control the junction temperature.

Wire coupling transients and inrush currents also reduce LED life, and many LED drivers are easily damaged by DC voltage and polarity errors. The output of the LED driver may also be damaged or destroyed by a short circuit. Most LED drivers have built-in safety features, including thermal shutdown and LED open and short circuit detection. However, additional overcurrent protection devices may be required to protect integrated circuits (ICs) and other sensitive electronic components.
 

LED driver input and output protection

LEDs are driven by constant current, and their forward voltage is between less than 2V and 4.5V, which is related to color and current. Older designs rely on simple resistors to limit the LED drive current, but designing LED circuits based on the typical forward voltage drop specified by the manufacturer can cause the LED driver to overheat.

This may occur when the forward voltage drop across the LED drops to a level well below the typical specified value. In such events, the increased voltage across the LED driving device may result in higher overall power dissipation from the driving device package, thus adversely affecting performance or lifetime.

Currently, most LED applications utilize power conversion and control devices connected to various power sources, such as AC wires, solar panels, or batteries, to control the power dissipation of the LED driver. These interfaces are protected from over-current and over-temperature damage, often with resettable polymer positive temperature coefficient (PPTC) devices. A PolySwitchLVR device can be connected in series with the power input to prevent damage due to electrical short circuit, circuit overload or user error. In addition, a metal oxide varistor (MOV) placed on the input also contributes to overvoltage protection within the LED module.

The Poly Switch LVR can also be placed after the MOV. Many equipment manufacturers prefer a combination of protection circuits and resettable Poly Switch devices with upstream fail-safe protection. In this example, R1 is a ballast resistor used in combination with the protection circuit.

The LED driving device is easily damaged by the DC voltage and the wrong polarity. The output can also be damaged or destroyed by a harmful short circuit. Live ports are also susceptible to transient damage due to overvoltage, including ESD pulses.

Collaborative Protection Scheme for Input and Output of LED Driver

The figure above represents a typical circuit protection design for LED drivers and light bulb arrays. The PolyZen device on the driver input provides designers with the simplicity of a traditional clamp diode while avoiding the need for a large number of heat sinks. The device facilitates transient suppression, reverse bias protection, and overcurrent protection in a single, small package.

The Poly Switch device on the driver output helps prevent damage from unwanted shorts or other load anomalies. In order to give full play to the role of the Poly Switch device, it can be thermally connected to the metal core circuit board or LED heat sink. In addition, the PESD device juxtaposed with the LED can also be protected against electrostatic discharge (ESD).

 

 

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LED lighting requires precise power and thermal management systems

LED technology has made rapid development, and chip design and material improvements to promote the development of brighter, more durable light source, light source applications are expanding. The increasing awareness of energy saving has also accelerated the popularity of LED lighting technology. Today, LED lamps are rapidly replacing traditional lighting methods with the following advantages: 1) low energy consumption, the power consumption of new bulbs ranges from 0.83 watts to 7.3 watts; 2) long life, LED bulbs can be used for 50000 hours; 3)LED bulbs are heat-resistant, shock-resistant, ready-to-use, very suitable for applications that require frequent switching operations; 4) help to meet safety and green initiatives; 5

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