As lighting technologies transition from power-hungry incandescents to coldcathode fluorescent lamps (CCFLs), and now to light-emitting diodes (LEDs), it is clear that while end-users are willing to pay more for greener light there is an inherent expectation that longer life and improved reliability will be the net benefit of that investment.
In addressing these expectations, design engineers must consider a wide range of variables which influence the performance and lifetime of their product. From power management to power density to overvoltage and overtemperature protection, the uniqueness of LED technology presents new challenges that are not associated with older technologies.
LED technology has advanced rapidly, with improved chip designs and materials facilitating the development of brighter, more energy-efficient, and longer-lasting light sources that can be used in a wide spectrum of applications. In spite of the technology’s growing popularity, it remains a fact that excessive heat or inappropriate applications can dramatically affect LED lifetime and performance.
Resettable polymeric positive temperature coefficient (PPTC) devices have demonstrated their effectiveness in a variety of LED lighting applications. Like traditional fuses, they limit current after specified limits are exceeded. However, unlike fuses, PPTC devices have the ability to reset after the fault is cleared and the power is cycled.
A variety of overvoltage protection devices including metal oxide varistors (MOVs), electrostatic discharge (ESD) surge protection devices, and polymer-enhanced Zener diodes can be used in a coordinated scheme with PPTC devices to help improve LED performance and reliability.
LED Driver Input and Output Protection
LEDs are driven with a constant current, with the forward voltage varying from less than 2 V to 4.5 V, depending on the color 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.
Today, most LED applications utilize 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 PPTC devices.
The PPTC device has a low-resistance value under normal operating currents. In the event of an overcurrent condition, the device “trips” into a high-resistance state. This increased resistance helps protect the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady-state level. The device remains in its latched position until the fault is cleared. Once power to the circuit is cycled, the PPTC device resets and allows current flow to resume, restoring the circuit to normal operation.
While PPTC devices cannot prevent a fault from occurring, they respond quickly, limiting current to a safe level to help prevent collateral damage to downstream components. Additionally, the small form factor of PPTC devices makes them easy to use in space-constrained applications.
Figure 1 illustrates a coordinated protection scheme for switch-mode power supplies (SMPSs) and LED driver inputs and outputs. SMPSs offer the size, weight, and energy-saving advantages required for consumer electronics, and have continued to replace linear-regulators in many applications. However, because SMPSs lack the inherent resistance of prior-generation designs, they often require more robust circuit protection. PPTC devices can help manufacturers meet UL60950-1/LPS (Limited Power Source) requirements for SMPSs and help improve equipment safety and reliability.
As shown on the left-hand side of the figure, a PPTC device, such as a PolySwitch™ device, can be installed in series with the power input to help protect against damage resulting from electrical shorts, overloaded circuits or customer misuse. Additionally, an MOV placed across the input helps provide overvoltage protection in the LED module.
The PPTC device may also be placed after the MOV. Many equipment manufacturers prefer protection circuits combining PPTC devices with upstream fail-safe protection. In this example, R1 is a ballast resistor used in combination with the protection circuit.
The right side of Figure 1 shows a coordinated circuit protection design for an LED driver and bulb array. A PolyZen™ device placed on the driver input offers designers the simplicity of a traditional clamping diode while obviating the need for significant heat sinking. This polymer-protected precision Zener diode helps provide transient suppression, reverse bias protection, and overcurrent protection in a single, compact package.
As shown in Figure 1, a PolySwitch PPTC device on the driver output can help protect against damage caused by inadvertent short circuits or other load anomalies. To fully leverage the PolySwitch device, it can be thermally bonded to the metal core circuit board or LED heat sink. To help prevent damage caused by an electrostatic discharge (ESD) event, ESD protection devices, such as low-capacitance (typically 0.25 pF), small-form-factor PESD devices can be placed in parallel with the LEDs.
Meeting Class 2 Power Supply Safety Standards
Utilizing a Class 2 power source in a lighting system can be an important factor in reducing cost and improving flexibility. Inherently limited power sources — a transformer, power supply, or battery — may include protective devices as long as they are not relied upon to limit the output of the Class 2 supplies.
Non-inherently limited power sources, by definition, have a discrete protective device that automatically interrupts the output when the current and energy output reaches a prescribed limit.
A variety of circuit protection devices can help protect Class 2 power sources for LED lighting applications. Figure 2 illustrates how a coordinated circuit protection strategy, employing an MOV on the AC input and a PolySwitch device on an output circuit branch, can help manufacturers meet the requirements of UL1310 paragraph 35.1 overload test for switches and controls.
A coordinated circuit protection scheme can help LED lighting designers reduce component count, provide a safe and reliable product, comply with regulatory agency requirements, and reduce warranty and repair costs. Although specific standards address various fault protection needs, it is always good practice to apply protection devices as close as possible to the chip sets’ I/O and Vcc pins, as board traces may be susceptible to conducted transients.
As with any circuit protection scheme, the effectiveness of a solution will depend on the individual layout, board type, specific components, and unique design considerations. TE Circuit Protection works with OEM customers to help identify and implement the best approach.
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