Metal Oxide Varistor, MYH Series, surge protective device

Circuit-protection Strategies for Improving LED Lifetime and Reliability

Excess heat and short-circuit failures drive the need for circuit protection in LED-based lighting. To combat these failures, temperature-sensitive protection devices team up with overvoltage protection to help ensure LED performance and reliability. By Faraz Hasan, Tyco Electronics' Circuit Protection Business Unit - EDN

LED luminaires require precise power and heat management because LEDs convert most of the electrical energy they receive into heat rather than light. Without adequate thermal management, this heat can degrade the LED's life span and affect colour output. Also, LEDs can fail short because they are silicon devices, so they may require fail-safe backup in the form of overcurrent protection.

Resettable PPTC (polymeric-positive-temperature-coefficient) circuit-protection devices have demonstrated their effectiveness in a variety of LED-lighting applications. Like traditional fuses, they limit current after they exceed specified limits. However, unlike fuses, PPTC devices can reset after the fault clears and the power cycles.

You can use a variety of overvoltage-protection devices, including MOVs (metal-oxide varistors), ESD (electrostatic-discharge) surge-protection devices, and integrated overcurrent/overvoltage devices, in a coordinated scheme with PPTC devices to help improve LED performance and reliability.

Heat conduction

A lighting fixture using a 60W incandescent light bulb produces approximately 900 lumens of light and must dissipate 3W of heat through conduction. In comparison, using typical dc LEDs as the light source to achieve the same 900 lumens would require about 12 LEDs.

Assuming a forward voltage of 3.2V and current of 350 mA, you can calculate the input power to the LED fixture as 12×3.2V×350 mA=13.4W. In this scenario, approximately 20% of the input power converts into light, and approximately 80% converts into heat, depending on various heat-generation and other factors that relate to substrate irregularities, phonon emissions, binding, and materials.

Of the total heat an LED generates, 90% transfers through conduction. To dissipate heat from the junction of an LED, conduction is the principal channel of transfer because convection and radiation account for only about 10% of overall heat transfer. For example, an LED may convert almost 10.72W - 13.40W×0.80 - of heat. Of that amount, conduction transfers or removes 9.648W, or 10.72W×0.90, of heat from the junction.

Temperature effect

The optical behaviour of an LED varies significantly with temperature. The amount of light an LED emits decreases as the junction temperature rises, and, for some technologies, the emitted wavelength changes with temperature. If you do not properly manage drive current and junction temperature, the LED's efficiency can quickly decrease, resulting in reduced brightness and shortened life.

Another LED characteristic, relating to junction temperature, is the forward voltage of the LED (Figure 1). If you use only a simple bias resistor to control the drive current, forward voltage drops, and the drive current increases as temperature rises. This situation can lead to thermal runaway, especially for high-power LEDs, and cause the component to fail. It is common practice to control junction temperature by mounting the LEDs on metal-core PCBs (printed-circuit boards) to provide rapid heat transfer.

Power-line coupled transients and surges can also reduce LED lifetimes, and many LED drivers are susceptible to damage resulting from improper dc voltage levels and polarity. Short circuits can also damage or destroy LED-driver outputs. Most LED drivers have built-in safety features, including thermal shutdown and open- and short-LED detection. However, additional overcurrent-protection devices may be necessary to help protect ICs and other sensitive electronic components.

I/O protection

A constant current drives LEDs, and their forward voltage varies from less than 2V to 4.5V, depending on the colour and current. Older designs rely on simple resistors to limit LED-drive current, but designing an LED circuit using the typical forward-voltage drop that the manufacturer specifies can cause the LED driver to overheat. Overheating may occur when the forward-voltage drop across the LED decreases to a value that is 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 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. Designers frequently protect these interfaces from overcurrent and overtemperature damage by using resettable PPTC devices. These devices have low-resistance values 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 clears. Once power to the circuit cycles, the PPTC device resets and allows current flow to resume, restoring the circuit to normal operation.

Although 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 applications that have space constraints.

Some figures can illustrate a coordinated protection scheme for an SMPS (switch-mode power supply) and LED-driver inputs and outputs. You can install a PPTC device in series with the power input to help protect against damage resulting from electrical shorts, overloaded circuits, or customer misuse. Additionally, you can place an MOV across the input to help provide overvoltage protection in the LED module. You can also place the PPTC device after the MOV. Many equipment manufacturers prefer protection circuits combining resettable PPTC devices with upstream fail-safe protection. In this example, R1 is a ballast resistor that works with the protection circuit.

And it shows a coordinated circuit-protection design for an LED driver and an LED array. A thermally protected precision zener diode can help protect circuits from damage from overvoltage and overcurrent fault conditions. Placing such a device on the driver input offers designers the simplicity of a traditional clamping diode and obviates the need for using a significant amount of heat sinking. To fully leverage the PPTC device, you can thermally bond it to a metal-core PCB or LED heat sink. To help prevent damage from an ESD event, you can place small-form-factor, low-capacitance - typically, 0.25-pF - ESD-protection devices in parallel with the LEDs.

Designers typically use MOVs for transient-overvoltage suppression in ac line-voltage applications. New thermally enhanced MOVs help protect a variety of low-power systems against damage from overcurrent, overtemperature, and overvoltage faults, including lightning strikes, ESD surges, loss of neutral, incorrect input voltage, and power induction.

Under normal operating conditions the ac line voltage you apply to an MOV should not exceed the device's maximum ac root-mean-square voltage rating, and, if the transient energy does not exceed the MOV's maximum rating, short events clamp to a suitable voltage level. However, a sustained abnormal overvoltage or limited-current condition, such as a loss of neutral, may cause the MOV to go into thermal runaway. Designers frequently protect the MOV from thermal overheating by placing a thermal-cutoff device in series with the MOV. A typical line-voltage transient-protection scheme may also incorporate an overcurrent-protection element, such as a fuse, to protect the system from damage from an overcurrent overload that exceeds a predetermined level.

Standard unprotected MOVs are typically 275V-ac rms for a universal input-voltage range. In a loss-of-neutral condition, they can overheat - with negative consequences - even if you use a fuse or power resistor upstream. The MOV includes a PPTC element to help prevent thermal runaway, maintaining varistor surface temperature at less than 150°C. In the event of an overvoltage transient, such as a loss-of-neutral event, the PPTC element heats up, trips, and goes into a high-resistance state, helping to reduce the risk of MOV-device failure.