Thermal management for LEDs

LED driver ICs with smart over temperature protection help in increasing lifetime and optimizing system cost of LED lighting systems
 LEDs offer an enormous potential to save energy and enhance the quality of light in indoor and outdoor lighting applications. In addition, LEDs enable a service free, long lifetime of lighting fixtures which allow a significant advantage for total cost of ownership calculations. In order to benefit from the long lifetime of LED lighting fixtures, it has to be considered that the LED lifetime is a function of the LED junction temperature. The fact that LEDs enable new compact lighting designs increases even more the necessity of putting high emphasis on thermal management of LED lighting solutions.  System manufacturers are addressing thermal challenges by designing systems with appropriate heat sinks, high-thermal conductivity enclosures and other advanced thermal design techniques. Generally, these manufacturers do not consider the LED driver IC as a control component in the thermal system. The use of an LED driver IC with intelligent over temperature protection provides an additional control mechanism that can increase the lifetime of LED light sources significantly, ensuring the rated lifetime and reducing the incidence of defective products. Depending on the lighting manufacturer and application, useful lifetime for LED lighting products range from about 20,000 hours to more than 50,000 hours compared to less than 2,000 hours for incandescent bulbs. The intelligent thermal protection helps also reduce system cost by enabling system integrators to design the heat sink with lower safety margin.
Despite the very high efficiency of LEDs compared to other light sources, a large portion of the energy used to drive LEDs is converted to heat. Unlike other light sources like incandescent bulbs, LEDs generate little or no infrared radiation. That’s why the generated heat must be conducted from the LED die to the underlying circuit board and heat sinks, housings or luminaries frame elements. If this heat transfer system has design or manufacturing flaws, or the system experiences extremely high temperature conditions the LEDs, the driver IC or other heat sensitive components like electrolytic capacitors can be damaged.
The patent pending smart thermal management technology included in the latest LED driver ICs can help protect the system from thermal failure resulting in improved LED system designs.
LED lifetime depends on temperatureHigh-quality LEDs are in general robust devices that can operate in excess of 100,000 hours, when properly operated. The lifetime of LEDs is directly related to the junction temperature to which they are exposed, increasing temperatures can reduce the operating lifetime significantly.
Different approaches for thermal protectionThe thermal design is often focused on the heat sink and PCB design, while the opportunities for thermal management by the LED driver IC and driving circuit are not considered. Intelligent over temperature protection by the LED driver IC can increase the lifetime of LED light sources significantly. 
Temperature protection with LED driver ICs can be implemented in a variety of ways. Some LED driver devices include a sense pin to which an external temperature sensor may be attached. Different temperature sensing devices, including diodes, on-chip sensors, positive temperature coefficient (PTC) or negative temperature coefficient (NTC) thermistors can be used in LED lighting applications to protect the LEDs from overheating. NTC thermistors are often the preferred choice for temperature sensing and control in many of these applications, primarily because of their small package sizes and attractive price/performance ratio. But accuracy, response time and thermal gradient depend on the way the NTC thermistor is mounted. Once the temperature is accurately sensed, the response to over temperature must be considered.  The typical response is to trigger an abrupt turn off of the current to the LEDs when a critical temperature has been detected.  These devices then can “restart” when the temperature is reduced or wait until a power cycle to restart.
Smart over temperature protection implemented at the LED driver ICInfineon Technologies has a granted patent which describes a circuit arrangement of a power supply unit for LED current control including a temperature sensor and a trigger circuit to provide a temperature dependent LED output current to protect the LEDs from overheating.
Leveraging the key idea of the above mentioned patent to use the LED driver IC as a thermal sensor Infineon Technologies has developed pending patent technology to protect the LEDs with a slope dimming characteristic and adjustable trigger temperature for the start of thermal protection to overcome the limitations of conventional LED system designs. The new technology was implemented in the recently announced ILD6150 and ILD6070 DC/DC hysteretic buck LED driver with high side current sensing.
The implementation of the specific smart temperature protection provides the following key benefits to LED system integrators which will be elaborated in more detail:
1) Saving system cost by using the driver IC as a thermal sensor for thermal protectionThe driver IC can be used as a reference temperature to determine the LED temperature in systems where the driver is thermally coupled to the LEDs. This saves cost of NTC, additional wiring and connector pins. In addition the design for thermal protection is simplified.
2) Protection of LED lifetime by slope dimming mechanismReducing the LED average current with an integrated slope dimming mechanism protects the lifetime of the LEDs by reducing their junction temperature. The protection circuit should ideally result in LED power deliver that has a smooth protection characteristic as a hard switching may lead to flickering. 
If a light source driven by the new LED drivers is exposed to a higher temperature condition than foreseen in the thermal design the driver ICs will reduce the average current of the LED continuously in a slope dimming characteristic, not abruptly.
3) Optimisation between lifetime and cost for thermal system by adjusting the triggering point for thermal protectionLuminary manufacturers can adjust the temperature that the protection engages by selecting the value of a low cost external resistor according to the end customer and application needs.
4) Maintaining light colour during thermal protection due to PWM dimming methodDimming by generating an internal PWM signal during thermal protection ensures that the light colour doesn’t change. Since the light colour remains practically unchanged it might not be visible to end users that the LED system is in thermal protection mode.The integrated temperature protection circuit will start to reduce the LED current by internal PWM modulation once the certain temperature trigger point of the slope characteristic of the IC is exceeded.
New LED driver generationThe new driver generation implementing the described smart thermal protection technology consists of hysteretic buck LED driver ICs for driving high power LEDs in general lighting applications. With a wide range of supply voltages, they are suitable for various LED applications. A multifunctional PWM input signal allows dimming of the LEDs with an analog DC voltage or an external PWM signal.
SummaryA key challenge in implementing high-quality, durable LED lighting systems is managing the heat emitted from the LED components and electronics. Luminary designers are advised to consider other ways than only the heat sink design in order to manage the high thermal load at the PCB. With the patent pending smart thermal management technology described here and used in the latest LED driver generation, the lifetime and cost of LED designs can be significantly improved.
Authored by—Bernd Pflaum – Principal Engineer (for concept and development of linear LED driver ICs and switching regulator ICs) Infineon Technologies AG
Hakan Yilmazer – Senior Manager (and global head of Application Marketing for Lighting) Infineon Technologies AG.

Click to comment

You must be logged in to post a comment Login

Leave a Reply

Thermal management for LEDs

LED driver ICs with smart over temperature protection help in increasing lifetime and optimizing system cost of LED lighting systems
 LEDs offer an enormous potential to save energy and enhance the quality of light in indoor and outdoor lighting applications. In addition, LEDs enable a service free, long lifetime of lighting fixtures which allow a significant advantage for total cost of ownership calculations. In order to benefit from the long lifetime of LED lighting fixtures, it has to be considered that the LED lifetime is a function of the LED junction temperature. The fact that LEDs enable new compact lighting designs increases even more the necessity of putting high emphasis on thermal management of LED lighting solutions.  System manufacturers are addressing thermal challenges by designing systems with appropriate heat sinks, high-thermal conductivity enclosures and other advanced thermal design techniques. Generally, these manufacturers do not consider the LED driver IC as a control component in the thermal system. The use of an LED driver IC with intelligent over temperature protection provides an additional control mechanism that can increase the lifetime of LED light sources significantly, ensuring the rated lifetime and reducing the incidence of defective products. Depending on the lighting manufacturer and application, useful lifetime for LED lighting products range from about 20,000 hours to more than 50,000 hours compared to less than 2,000 hours for incandescent bulbs. The intelligent thermal protection helps also reduce system cost by enabling system integrators to design the heat sink with lower safety margin.
Despite the very high efficiency of LEDs compared to other light sources, a large portion of the energy used to drive LEDs is converted to heat. Unlike other light sources like incandescent bulbs, LEDs generate little or no infrared radiation. That’s why the generated heat must be conducted from the LED die to the underlying circuit board and heat sinks, housings or luminaries frame elements. If this heat transfer system has design or manufacturing flaws, or the system experiences extremely high temperature conditions the LEDs, the driver IC or other heat sensitive components like electrolytic capacitors can be damaged.
The patent pending smart thermal management technology included in the latest LED driver ICs can help protect the system from thermal failure resulting in improved LED system designs.
LED lifetime depends on temperatureHigh-quality LEDs are in general robust devices that can operate in excess of 100,000 hours, when properly operated. The lifetime of LEDs is directly related to the junction temperature to which they are exposed, increasing temperatures can reduce the operating lifetime significantly.
Different approaches for thermal protectionThe thermal design is often focused on the heat sink and PCB design, while the opportunities for thermal management by the LED driver IC and driving circuit are not considered. Intelligent over temperature protection by the LED driver IC can increase the lifetime of LED light sources significantly. 
Temperature protection with LED driver ICs can be implemented in a variety of ways. Some LED driver devices include a sense pin to which an external temperature sensor may be attached. Different temperature sensing devices, including diodes, on-chip sensors, positive temperature coefficient (PTC) or negative temperature coefficient (NTC) thermistors can be used in LED lighting applications to protect the LEDs from overheating. NTC thermistors are often the preferred choice for temperature sensing and control in many of these applications, primarily because of their small package sizes and attractive price/performance ratio. But accuracy, response time and thermal gradient depend on the way the NTC thermistor is mounted. Once the temperature is accurately sensed, the response to over temperature must be considered.  The typical response is to trigger an abrupt turn off of the current to the LEDs when a critical temperature has been detected.  These devices then can “restart” when the temperature is reduced or wait until a power cycle to restart.
Smart over temperature protection implemented at the LED driver ICInfineon Technologies has a granted patent which describes a circuit arrangement of a power supply unit for LED current control including a temperature sensor and a trigger circuit to provide a temperature dependent LED output current to protect the LEDs from overheating.
Leveraging the key idea of the above mentioned patent to use the LED driver IC as a thermal sensor Infineon Technologies has developed pending patent technology to protect the LEDs with a slope dimming characteristic and adjustable trigger temperature for the start of thermal protection to overcome the limitations of conventional LED system designs. The new technology was implemented in the recently announced ILD6150 and ILD6070 DC/DC hysteretic buck LED driver with high side current sensing.
The implementation of the specific smart temperature protection provides the following key benefits to LED system integrators which will be elaborated in more detail:
1) Saving system cost by using the driver IC as a thermal sensor for thermal protectionThe driver IC can be used as a reference temperature to determine the LED temperature in systems where the driver is thermally coupled to the LEDs. This saves cost of NTC, additional wiring and connector pins. In addition the design for thermal protection is simplified.
2) Protection of LED lifetime by slope dimming mechanismReducing the LED average current with an integrated slope dimming mechanism protects the lifetime of the LEDs by reducing their junction temperature. The protection circuit should ideally result in LED power deliver that has a smooth protection characteristic as a hard switching may lead to flickering. 
If a light source driven by the new LED drivers is exposed to a higher temperature condition than foreseen in the thermal design the driver ICs will reduce the average current of the LED continuously in a slope dimming characteristic, not abruptly.
3) Optimisation between lifetime and cost for thermal system by adjusting the triggering point for thermal protectionLuminary manufacturers can adjust the temperature that the protection engages by selecting the value of a low cost external resistor according to the end customer and application needs.
4) Maintaining light colour during thermal protection due to PWM dimming methodDimming by generating an internal PWM signal during thermal protection ensures that the light colour doesn’t change. Since the light colour remains practically unchanged it might not be visible to end users that the LED system is in thermal protection mode.The integrated temperature protection circuit will start to reduce the LED current by internal PWM modulation once the certain temperature trigger point of the slope characteristic of the IC is exceeded.
New LED driver generationThe new driver generation implementing the described smart thermal protection technology consists of hysteretic buck LED driver ICs for driving high power LEDs in general lighting applications. With a wide range of supply voltages, they are suitable for various LED applications. A multifunctional PWM input signal allows dimming of the LEDs with an analog DC voltage or an external PWM signal.
SummaryA key challenge in implementing high-quality, durable LED lighting systems is managing the heat emitted from the LED components and electronics. Luminary designers are advised to consider other ways than only the heat sink design in order to manage the high thermal load at the PCB. With the patent pending smart thermal management technology described here and used in the latest LED driver generation, the lifetime and cost of LED designs can be significantly improved.
Authored by—Bernd Pflaum – Principal Engineer (for concept and development of linear LED driver ICs and switching regulator ICs) Infineon Technologies AG
Hakan Yilmazer – Senior Manager (and global head of Application Marketing for Lighting) Infineon Technologies AG.

Click to comment

You must be logged in to post a comment Login

Leave a Reply

Most Popular

To Top