Heat dissipation management of LED lights

Thermal management is the most difficult, demanding, and costly design aspect of a new LED lamp. Failure to adequately manage thermal performance can result in catastrophic consequences such as lighting failure or fire. However, thermal management of LED lamps is the most complex, demanding, and costly aspect of the entire design solution. This article will explore how to implement negative temperature coefficient (NTC) thermal management to maximize the safety of LED designs and significantly reduce power consumption.

In traditional incandescent bulbs, the only heat source is the filament, which is not in direct contact with anything. For LED lamps, the LED is the light source, and the heat dissipation of the LED is in direct contact with the LED bulb. This direct contact is caused by the connection between the LED and the driver circuit. In order to achieve heat dissipation, the heat must be released from the LED and the driver circuit or effectively managed. This is also the basic premise for keeping LED lamps working for a long time.

To understand the importance of thermal management, let's imagine an application where an LED lamp is installed in a common lighting socket such as a wall sconce or ceiling lamp, and the LED lamp is controlled by a wall switch. Since most standard lamps such as wall sconces or ceiling lamps rely mainly on heat convection or airflow to dissipate heat, the heat dissipation effect of this application is not ideal for LED lamps.

Without effective heat management, it will lead to frequent replacement of failed LED lights or catastrophic consequences such as building fires. Using intelligent LED light control to monitor the temperature of LED lights is a relatively simple heat management method. At the same time, since LED lights can reduce power when the temperature rises, safety will be greatly improved.

NTC Thermal Management

The basic principle of the NTC circuit is to improve the safety of LED lamps and reduce the design complexity by monitoring the temperature of the LED lamp. When the temperature rises, the controller reduces the lumens and thereby keeps the LED within a safe level. In other words, when the temperature rises, the lumens are reduced, and conversely, when the temperature drops, the lumens are increased.

We can detect the temperature change of the LED lamp by detecting the voltage on the NTC. The detected voltage is directly related to the temperature of the NTC, and the resistance of the NTC will decrease as the temperature of the NTC and its surrounding circuits increases. There are two basic methods to determine the temperature using the NTC.

Method 1: Use an NTC in a voltage divider circuit where a known voltage is forced into the system and then measure the voltage at the NTC node. As the temperature of the NTC increases, the resistance decreases. The decrease in resistance will result in a change in the voltage divider ratio. The voltage at the NTC node will also decrease as the temperature increases.

Method 2: Force a known current through the NTC and measure the voltage across the NTC. When the temperature of the NTC increases, the resistance decreases. According to Ohm's law, the decrease in resistance will change the voltage across the NTC node. If the resistance decreases and the current remains the same, the voltage across the NTC node will also decrease.

Both methods of monitoring LED lamp temperature are straightforward to implement in terms of improving operations and increasing safety. Figure 1 is a schematic diagram of the two methods using LEDs as the source of temperature rise.

Figure 1: Two basic methods of determining temperature using an NTC.

Too high temperature or LED failure?

When the lumen output of an LED lamp drops, it is important to know whether the drop is due to an excessively high temperature environment or a faulty LED. An indicator showing a lumen drop can be used to determine the cause of the drop.

The lumen drop in the system shown in Figure 2 is indicated by a low-power red LED. When the system is at maximum lumen output, the red LED is off. As the temperature of the LED lamp increases, the lumen output drops, and when the lumen output drops, the red LED turns on. As the lumen output continues to drop, the intensity of the red LED increases accordingly. When the lumen output drops to its minimum intensity, the red LED turns on fully.

Figure 2
When the lumen output is at minimum intensity and the temperature of the LED lamp is still high, the red LED indicator can also serve as an alarm to warn of serious problems. In alarm mode, the red LED will flash continuously while all white LEDs are turned off.

The block diagram in Figure 3 shows a generic LED driver and LED controller with an NTC and an alarm indicator. A generic LED lamp contains an LED driver that is configured to provide a set current through the LED. The driver cannot reduce lumens based on temperature. The temperature monitoring feature provided by the driver is only used for its own protection and shuts down completely in the event of extremely high temperatures.

LED controllers have all the control functions of ordinary LED drivers and can enhance the intelligence level of other functions such as temperature monitoring, communication and dimming control. The blue part of the block diagram is the basic module and components of the LED controller. The components shown in red are not required for basic operation, but are shown for the NTC and alarm functions described in this article.

By adding an NTC to a regular LED, the LED light can be turned off in a controlled sequence when the temperature reaches a preset limit. The two red components (resistor and NTC) on the right side of the LED controller are configured according to method one described in the NTC operation section. The controller provides a precise voltage to the resistor element. The voltage at the NTC node is measured by the controller for conversion to the corresponding system temperature.

The alarm mechanism allows the LED light to indicate that the temperature has risen to the point where it must shut down for safety. The two red components (resistor and LED) on the left side of the LED controller are the basic indicator LED configuration. The brightness of the LED is controlled by a PWM (pulse width modulation) signal. The LED will increase in brightness as the PWM duty cycle increases.

The smart LED light above displays the alarm information as another LED indicator. LED alarm is just one of many communication interfaces that smart LEDs can use. Other interfaces include PLC (power line communication), DMX (digital multiplexing) and DALI (digital addressable lighting interface).

Lumen adjustment

The flowchart in Figure 4 shows a simple algorithm that monitors the temperature of an LED lamp and adjusts the lumen level if the temperature reaches certain safe limits. The "Power-On Startup - System Initialization" block at the top of the flowchart is the microcontroller initialization block. When the wall switch is turned on, the LED lamp is powered up and this block configures the LED lamp for basic operations such as lumen output and temperature sensing.

Figure 4: LED Light Monitoring and Adjustment Flowchart The "Is the light on?" block detects if the light has turned off due to overtemperature. This simple bitwise test will determine if the light is on. If the light is set to the light on bit, the light is on, if not set to the light on bit, the light is not on. When power is first applied, the light is on by default and the light on bit is set.

The "Alarm" control block controls the on/off sequence after the temperature is too high and the LED light is turned off by the controller. The following "Is the light on?" block will start the test sequence again. The only way to exit the alarm condition is to remove and then reapply power using the wall switch.

The next "Sense Temperature" block will sense the voltage at the NTC node. NTCs typically change nonlinearly with temperature, so the sensed voltage can be compared to the relevant temperature based on a lookup table. This temperature will be used in the next two control blocks.

The "Safety Temperature" block is used to determine if the temperature of the LED lamp is within the safe range. When the temperature reaches the configured maximum value, the system will turn off the lamp. If the temperature is below the maximum allowed value, the system will continue with the temperature stability test.

The "Light Off" block turns off the light when the LED temperature is in an unsafe range. Next comes the "Light On?" block, which restarts the detection sequence again.

The "Temperature Change" block determines if the temperature change since the last lumen adjustment cycle requires an increase or decrease in light output. The "Temperature Increase" block determines if the temperature is increasing or decreasing. Since the previous control block has already determined that the temperature change since the last lumen adjustment cycle is large enough, there are only two choices here.

The "Max Lumens" block is used to determine if the LED light is set to its maximum lumen output. If the lumen output reaches the maximum value, the "Is the light on?" block is re-entered to restart the test sequence.

When the previous control block detects that the lumen output is not at maximum, the Lumens Up, Light Dim block is triggered. This control block increases the output by one step as configured during the Initialization block, and also dims the light LED by one step to match the lumen increase with the light dimming, before restarting the test sequence.

When the "Temperature Rise" block detects a temperature rise, it triggers the "Minimum Lumens" block. If the lumens have not reached the preset minimum value, the flow is directed to the "Reduce Lumens, Brighten Indicator Light" block. If the lumen output reaches the preset minimum value, the "Is the light on?" block is re-entered to restart the detection sequence.

The "Reduce Lumens, Increase Indicator" block will decrease the output by one step based on the configuration during the Initialization block, and will also increase the indicator LED by one step to match the decrease in lumens with the increase in indicator, before restarting the detection sequence.

The above flow chart shows the LED light remaining off during the input power cycle. A slight change in the flow chart can provide a sequence where the light is off, the temperature is monitored, and the LED light is turned back on when the temperature drops to safe limits.