Low power consumption design of heat dissipation management for LED lamps

Thermal management is the most difficult, demanding, and expensive design aspect of a new LED lamp. If thermal management is not adequately performed, it can lead to catastrophic consequences such as lighting failure or fire. However, thermal management of LED lamps is the most complex, demanding, and expensive 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. In LED lamps, however, 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 due to the way the LED is connected to the driver circuit. In order to achieve heat dissipation, the heat must be removed from the LED and driver circuit or effectively managed, which is also the basic premise for the long-term operation of the LED lamp.

To understand the importance of thermal management, let's imagine an application where an LED lamp is installed in a general 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 thermal management, the consequences can be disastrous, from frequent replacement of failed LED lamps to building fires. Using smart LED lamp control to monitor the temperature of the LED lamp is a simpler way to manage thermal management, and it also greatly improves safety because the LED lamp can reduce power when the temperature rises.

NTC Thermal Management

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

We can detect the temperature change of the LED lamp by sensing the voltage across the NTC. The sensed voltage is directly related to the temperature of the NTC, and the resistance of the NTC decreases as the temperature of the NTC and its surrounding circuits increases. There are two basic methods for determining temperature using the NTC.

Method 1: Use the NTC in a voltage divider circuit that forces a known voltage on the system and then measures the voltage at the NTC node. As the temperature of the NTC increases, the resistance decreases. The reduction in resistance will cause the voltage divider ratio to change. 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 at the NTC node. If the resistance decreases and the current remains constant, the voltage at the NTC node will also decrease.

Both methods of monitoring the temperature of LED lamps are simple and straightforward to implement in terms of improved operation and safety. Figure 1 is a schematic diagram of the two methods using the LED as the source of the temperature increase.



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

Too hot or LED failure?

When the lumen output of an LED lamp drops, it is important to know whether the LED output is dropping due to excessive temperature environment or due to LED failure. We can use an indicator that shows the lumen drop 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; when the temperature of the LED lamp increases, the lumen output decreases, and when the lumen output decreases, the red LED turns on. As the lumen output continues to decrease, the intensity of the red LED increases accordingly. When the lumen output drops to its minimum intensity, the red LED will turn on completely.



Figure 2

When 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 off.

The block diagram of Figure 3 shows a general LED driver and LED controller with NTC and alarm indicators. The general 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 function provided by the driver is only used for its own protection and completely shuts down when the temperature is extremely high.



The LED controller has all the control functions of the general LED driver and can enhance the intelligence 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.

The addition of NTC to the general LED can shut down the LED lamp in a controlled sequence when the temperature reaches the preset limit. The two red components on the right side of the LED controller (resistor and NTC) 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 a basic indicator LED configuration. The brightness of the LED is controlled by a PWM (pulse width modulation) signal. The LED increases in brightness as the PWM duty cycle increases.

The above smart LED light displays the alarm information in the form of 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 for monitoring the temperature of an LED light and adjusting the lumen level when the temperature reaches a certain safety limit. 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 light is powered up and this block will configure the LED light for basic operations such as lumen output and temperature detection.


Figure 4: LED light monitoring and adjustment flowchart

The "Is the light on?" block detects whether the light is off due to excessive temperature. This simple bitwise test will determine whether the light is on. If set to the Light On position, the light is on, if not set to the Light On position, the light is not on. When power is first applied, the light is on by default and the Light On position is set.

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

The next “Sense Temperature” block will sense the voltage at the NTC node. NTCs typically change non-linearly with temperature, so the sensed voltage can be compared to a lookup table for a relative temperature. This temperature is used in the next two control blocks.

The “Safe Temperature” block is used to determine if the temperature of the LED light is within a safe range. When the temperature reaches the configured maximum value, the system will turn the light off. If the temperature is below the maximum allowed, the system will continue the temperature stability test. The “

Light Off” block is used to turn the light off if the LED light temperature is within an unsafe range. Next is the “Is the light on?” block, which restarts the test sequence again.

The “Temperature Change” block is used to determine if the temperature change since the last lumen adjustment cycle requires an increase or decrease in light output. The "Temperature Increase" block is used to determine whether the temperature is rising or falling. Since the previous control block has already determined that the temperature has changed enough since the last lumen adjustment cycle, there are only two choices here.

The Max Lumens block determines if the LED light is set to maximum lumen output. If the lumen output is at the maximum, the Is Light On? block is re-entered to restart the test sequence.

When the previous control block detects that the lumen output is not at the maximum, the Lumens Increase, Indicator Dim block is triggered. This control block increases the output by one step as configured during the Initialization block, and also turns the indicator LED down by one step to match the increase in lumens with the dimming of the indicator, and then restarts the test sequence.

When the Temperature Increase block detects an increase in temperature, the Min Lumens block is triggered. If the lumens are not at the preset minimum value, the flow is directed to the Lumens Decrease, Indicator Brighten block. If the lumen output is at the preset minimum value, the Is Light On? block is re-entered to restart the test sequence. The Lumens Decrease,

Indicator Brighten block turns the output down by one step as configured during the Initialization block, and also turns the indicator LED up by one step to match the decrease in lumens with the increase in the indicator, and then restarts the test sequence.

The above flowchart shows the case where the LED light remains off during an input power cycle. With a slight change in the process, a sequence can be provided that monitors the temperature after the light is turned off and turns the LED light back on when the temperature drops to safe limits.