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Protect Your LED Lighting Circuits
6/12/2010Guest columnist Faraz Hasan, marketing and business development manager at Tyco Electronics Circuit Protection Products says that LEDs can require overcurrent protection despite built-in safety features in driver circuits.
LED light manufacturers are wrestling with the fact that without adequate thermal management, heat can degrade the LED’s lifespan and affect colour output.
Also, since LED drivers are typically semiconductor devices, they can fail short. This means fail-safe backup overcurrent protection may be required.
It is accepted that the optical behaviour of an LED varies significantly with temperature. The amount of light emitted by the LED decreases as the junction temperature rises and, for some technologies, the emitted wavelength changes with temperature. If drive current and junction temperature are not properly managed, the LED’s efficiency can drop quickly, resulting in reduced brightness and shortened life.
Power line coupled transients and surges can also reduce LED lifespan, and many LED drivers are susceptible to damage resulting from improper DC voltage levels and polarity. LED driver outputs may also be damaged or destroyed by short circuits.
Most LED drivers include built-in safety features, including thermal shutdown, as well as open and short LED detection. However, additional overcurrent protection devices may be needed to help protect ICs and other sensitive electronic components.
LEDs are driven with a constant current, with the forward voltage varying from less than 2V to 4.5V, depending on the colour and current. Older designs relied on simple resistors to limit LED drive current, but designing an LED circuit based on the typical forward voltage drop as specified by a manufacturer can lead to overheating of the LED driver.
Overheating may occur when the forward voltage drop across the LED decreases to a value significantly less than the typical stated value. During such an event, the increased voltage across the LED driver can result in higher total power dissipation from the driver package.
LED applications use power conversion and control devices to interface with various power sources, such as the AC line, a solar panel or battery power, to control power dissipation from the LED driver.
Protecting these interfaces from overcurrent and overtemperature damage is frequently accomplished with resettable polymeric positive temperature coefficient (PPTC) devices.
Protection systems
Figure 1 illustrates a coordinated protection scheme for switch-mode power supplies and LED driver inputs and outputs.
As shown on the left-hand side of the figure, PPTC device (eg Tyco’s PolySwitch) can be installed in series with the power input to help to protect against damage resulting from electrical shorts, overloaded circuits or customer misuse.
A metal oxide varistor (MOV) placed across the input helps to provide overvoltage protection in the LED module.
The device may also be placed after the MOV. Many equipment manufacturers prefer protection circuits combining resettable overcurrent protection devices with upstream fail-safe protection.
In this example, R1 represents a ballast resistor used in combination with the protection circuit.
LED drivers may be susceptible to damage resulting from improper DC voltage levels and polarity. Outputs may also be damaged or destroyed by an inadvertent short circuit. Powered ports are also susceptible to damaging overvoltage transients, including ESD pulses.
The right side of the figure shows a coordinated circuit protection design for an LED driver and bulb array.
Use of a clamping diode on the driver input can reduce the need for significant heat sinking.
This polymer-protected zener diode (eg Tyco’s PolyZen) can help to provide transient suppression, reverse bias protection, and overcurrent protection in a single package.
A second PPTC device on the driver output can be used to help to protect against damage caused by inadvertent short circuits or other load anomalies.
