LED lighting requires precise thermal management to prevent heat degradation and maintain optimal performance. A coordinated circuit protection strategy using devices like PolySwitch, MOVs, and PolyZen can help protect sensitive components from overcurrent, overvoltage, electrostatic discharge, and thermal runaway. These protection devices allow manufacturers to reduce costs while improving reliability and meeting various safety standards.

1. Circuit Protection Considerations for LED Lighting Matthew Williams, Global Applications Engineering Manager Tyco Electronics Circuit Protection Business Unit

2. LED Lighting Increasingly Popular As government agencies, industry and consumers look for ways to reduce energy costs, light-emitting diode (LED) lighting technology is expected to boom. Low energy consumption Long service life Durability Help meet safety and green initiatives Fully dimmable No frequency interference

3. Thermal Management Requirements LED luminaires require precise power and heat management systems, since most of the electrical energy supplied to an LED is converted to heat rather than light. Without adequate thermal management, this heat can degrade the LED’s lifespan and affect color output. Since LED drivers are silicon devices, they can fail short. This means fail-safe backup overcurrent protection may be necessary.

4. Heat Conduction Comparisons A fixture using a 60W incandescent light bulb produces approximately 900 lumens of light and must dissipate 3 Watts of heat via conduction. Using typical DC LEDs as the light source to achieve the same 900 lumens would require about 12 LEDs Assuming a V F (forward voltage) of 3.2V and current of 350mA, the input power to the fixture could be calculated as: Power = 12 x 3.2V x 350mA = 13.4W In this scenario approximately 20% of input power is converted to light and 80% to heat. This is dependent on various factors

5. Heat Conduction Comparisons Of the total heat generated by the LED, 90% is transferred via conduction. To dissipate heat from the junction of an LED, conduction is the principal channel of transfer since convection and radiation only account for about 10% of overall heat transfer.

6. Junction Temperature Effect The optical behavior of an LED varies significantly with temperature. V F drops as junction temperature rises and the drive current increases. This can lead to thermal runaway, causing the component to fail. Typical solution to controlling junction temperature is to mount the LEDs on metal core PCBs to provide rapid heat transfer.

7. Other Circuit Design Challenges Power line coupled transients and surges can also reduce LED lifespan 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. Additional overcurrent protection devices may be needed to help protect integrated circuits (ICs) and other sensitive electronic components.

8. LED Driver I/O Protection LEDs are driven with a constant current. Older designs relied on simple resistors to limit LED drive current. If forward voltage drop across the LED decreases to a value significantly less than the typical stated value the driver may overheat. New systems utilize power conversion and control devices to control power dissipation from the LED driver. Protecting these interfaces from overcurrent and overtemperature damage is frequently accomplished with resettable PPTC devices.

9. Circuit Protection Solution for Switch Mode Power Supplies PolySwitch PPTC device installed in series with power input helps protect against damage caused by electrical shorts, overloaded circuits, or customer misuse. MOV placed across the input helps provide overvoltage protection. PolySwitch device may also be placed after the MOV. R1 is a ballast resistor.

10. Other Circuit Protection Considerations LED drivers may be susceptible to damage resulting from improper DC voltage levels and polarity. Outputs may be damaged or destroyed by an inadvertent short circuit. Powered ports are also susceptible to damaging overvoltage transients, including ESD pulses.

11. Coordinated Circuit Protection PolyZen device on the driver input helps provide transient suppression, reverse bias protection and overcurrent protection in a compact package. PolySwitch device on driver output helps protect against damage caused by short circuits or other load anomalies. PolySwitch device can be thermally bonded to circuit board or LED heat sink. PESD devices in parallel with LEDs help protect against electrostatic discharge damage.

12. PolySwitch Device – How it Works Resettable PPTC devices are composed of semi-crystalline polymer and conductive particles. If temperature rises above the device’s switching temperature the crystallites melt and become amorphous The increase in volume during melting of the crystalline phase separates the conductive particles resulting in a large non-linear increase in the resistance of the device.

13. PolySwitch Device – How it Works The resistance typically increases by three or more orders of magnitude. 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 (high resistance) position until the fault is cleared and power to the circuit is cycled. Conductive composite cools and re-crystallizes, restoring the PPTC to a low resistance state in the circuit and the affected equipment to normal operating conditions.

14. PolyZen Device – How it Works A low resistance, precision Zener diode is thermally coupled to a PPTC “thermal switch.” Extended overvoltage or reverse bias conditions will cause the PPTC to “trip” as the diode begins to heat up. A “trip event” causes the PPTC to transition from a low to high-resistance state, helping protect downstream electronics by generating a series element voltage drop and preventing thermal runaway of the Zener diode.

15. 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.

16. Circuit Protection Options Coordinated circuit protection strategy employs a metal oxide varistor (MOV) on the AC input and a PolySwitch device on an output circuit branch. This method can help manufacturers meet the requirements of UL 1310 paragraph 35.1 overload test for switches and controls.

17. AC Mains LED Lighting Protection Metal Oxide Varistors (MOVs) are typically used for transient overvoltage suppression in AC line voltage applications where lightning strikes, inductive load switching, or capacitor bank switching may cause transient overvoltage events. A sustained abnormal overvoltage/limited current condition may cause the MOV to go into thermal runaway resulting in overheating, outgassing and possibly fire. Protecting the MOV from thermal overheating is frequently accomplished with a thermal cut-off (TCO) device. Design may also incorporate a fuse to protect the system from damage caused by an overload that exceeds a predetermined level.

18. 2Pro Integrated Device The 2Pro device combines PPTC technology with an MOV component into one thermally-protected package. Because the PPTC element is in series with the MOV, additional overcurrent protection may not be required. This approach helps manufacturers meet industry requirements, such as IEC61000-4-5 and IEC60950, and helps reduce component count and optimize board space. The 2Pro device helps protect against damage caused by both overcurrent and overvoltage damage.

19. Integrated Overcurrent/Overvoltage Solution 2Pro device’s PPTC element helps prevent thermal runaway, maintaining varistor surface temperature at less than 150°C. In the event of an overvoltage transient the PPTC element of the 2Pro device heats up, trips and goes into a high resistance state, helping to reduce the risk of MOV device failure. Typical lighting application utilizing a 2Pro device for low-power AC/DC flyback converter protection

20. Summary A coordinated circuit protection scheme can help designers reduce component count, provide a safe and reliable product, and comply with regulatory agency requirements. Resettable PPTC devices help protect against damage caused by both overcurrent and overtemperature faults in LED lighting applications. MOV overvoltage protection devices help manufacturers meet a number of safety agency requirements. PolyZen and 2Pro hybrid devices help provide overcurrent and overvoltage protection in a single device. PESD devices provide exceptionally low capacitance in a small form factor.