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2022-08-09
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Introduction: how to build intelligent LED lamps

light emitting diodes have always been used as low-cost indicators in various electronic products. At present, they have become powerful light sources, which can be used in indoor lighting, signboards, display screens and decorative lighting applications. Compared with incandescent lamps and fluorescent lamps, the energy consumed to produce the same brightness is much smaller, and the status of LED is rising day by day. Energy is one of the hottest topics in this century, and will soon become one of the most important issues that designers around the world need to consider

for lamp manufacturers, using LEDs has many advantages. However, there are many manufacturers trying to catch up with the LED wave as soon as possible, so product differentiation is very important. In addition, when energy consumption and labor cost become the main issues to be considered in the design, large lighting equipment almost needs to be "intelligent". Let the lamps have the ability to communicate with the "mother" controller, monitor their own conditions, adjust the working mode according to the monitoring results, and enter the safe state of graphene after failure. We hope to strive to make better graphene materials, which are the expectations of the new generation of LED lamps. This paper will discuss the "intelligent" selection and implementation steps suitable for LED lamps

input low voltage lockout:

the input voltage of LED drive system is generally DC voltage. The power supply is provided by the off-line AC-DC converter or bus. In addition to supplying power to the LED driver, the power supply is also used to supply power to the controller in the system (it needs to be converted to 5V or 3.3V voltage suitable for the controller first). The controller power supply is generally designed to work when the input voltage is slightly higher than the required output voltage. For example, a 5V regulator will start working when the input voltage reaches v. However, the steady state of this power supply can be a 24V power supply with 1A current per LED string. Once the controller is powered on, the controller will assume that the power supply is available and turn on the LED drive system (assuming its configuration), and then the LED drive system will try to enter the normal working state. If the input voltage is only 10V at this time, the current required to be provided by the power supply will be much greater than the current under steady state, and the system will collapse due to this sudden increase in instantaneous current. This excess current demand will also exceed the rated bearing capacity of cables, connectors and other components of the input power supply, which may cause permanent damage to the system

in order to avoid this situation, the system should have "low voltage locking" function. The hardware used is a resistance voltage divider, which can gradually reduce the input voltage to the range that the input of the controller can bear. The input is internally connected to the comparator. The action of the controller (Firmware) should be designed to enable the power supply only when the input voltage exceeds the threshold of normal operation during long-term use. In addition, the voltage system does not start as soon as the comparator is turned on. The firmware shall poll the output of the comparator to check whether this state is consistent (because the comparator is part of the combinational logic circuit), and then start the power system. Figure 2 the hardware schematic diagram (schematic diagram) to realize this function is to maintain the relative humidity at 60 ± 5%

load (LED) monitoring:

the load here has a constant current that passes through the LED and is adjusted. Although the current regulation system is already monitoring the load, the purpose here is to ensure that the appropriate load current flows. LEDs are prone to damage, especially when an open or short circuit occurs. The causes of these types of faults include loose cables, loose connectors, PCB assembly problems, and so on. A short circuit in the channel will cause damage to the MOSFET (acting as a switch). In view of the power of these systems, in case of failure, it will lead to large current and release a lot of heat. In order to protect the system and its surroundings from the adverse effects of faults, the controller should have the ability to monitor the load status in real time

we may consider the open circuit situation, in which the path through which the power supply flow flows does not exist. If the current regulation system is allowed to handle by itself, it will keep the switch (MOSFET) open to try to let the current flow to the specified place. But this will not solve the problem. Similarly, in the case of short circuit with uncontrolled sudden increase of current. The feedback system will try to turn off the switch, but if the MOSFET is damaged, it will not respond to these control signals, and the problem cannot be solved

intelligent LED lamps should be able to detect these conditions and place the system in a condition that can safely avoid the adverse consequences caused by faults. One way to achieve this goal is to force the fuse to blow, thereby cutting off the power supply of the entire system. Another way is to send a signal to the "mother" controller, or stop sending a signal to the "mother" controller to indicate the fault condition. For this reason, the system must be able to monitor the load current or voltage value first. To measure the current, a current sensing resistor is introduced into the LED circuit, and the voltage at both ends (after amplification) is input into ADC. The digital output of ADC is monitored by the processor and takes corresponding actions according to the measured current value. For example, if the current through the LED is 500mA, but the ADC only measures 10mA, it can be regarded as a fault. The controller then sends a signal to the "female" controller to start the "fused fuse" circuit and forcibly blow the fuse

in circuits such as boost circuits, there is a large capacity capacitor, so it is very important to continuously monitor the voltage of the load. Under the condition of normal operation of the boost system, the high-capacity capacitor is charged during the period when the switch is disconnected and discharged during the period when the switch is closed. If the load has an open circuit, the capacitor will not discharge but the charging continues. If ignored, the voltage at both ends of the capacitor will quickly rise to a high level, which may cause damage to parts such as MOSFET. If the circuit is suddenly closed when the connection is loose, the overcharged capacitor will cause excessive current to pass through the load in a short time, which may permanently damage the LED

connect the resistance voltage divider at both ends of the large capacity capacitor, which can reduce the output voltage to the range that the microcontroller can handle. The signal is then input to the comparator, and the output of the comparator is connected to the current regulation system that can turn it off. When the voltage exceeds the preset limit, the comparator will turn on the switch and turn off the system

environmental condition monitoring:

led lamps will be placed in various environments. In the office environment, if the lamp can detect people entering and leaving the room, it will turn off or reduce the brightness after people leave the room, which is good for energy conservation. This can save electricity, thereby saving the company's electricity bill. In addition, LED can be effectively used to extend its service life

this function can be achieved by combining a simple ambient light sensor (photodiode or transistor) in the system. The output of such a sensor is generally a current (depending on the intensity of the light obtained), which can be converted into a voltage signal with a resistor. The voltage signal is then supplied to the controller through its pin, and then converted into a digital signal using ADC. The controller determines the appropriate action (reduce brightness, turn off or turn on) according to this value

led itself will also generate heat, but unlike incandescent lamp, the heat emitted by LED is transferred with its terminal, and the direction of heat transfer of incandescent lamp is consistent with the direction of illumination. In addition, LED lamps are generally installed in a compact space, with poor heat dissipation conditions. If there is an abnormal sudden increase in temperature, it may lead to various consequences, such as parts and l (2) "small and fine": accuracy is the permanent pursuit of various instruments and equipment. 1ed's life will decline or even be permanently damaged, and in extreme cases, it may even cause a fire

generally speaking, there are two methods to monitor the system temperature. The one with higher cost is to use the temperature sensor based on I2C to send the digital signal related to temperature to the controller. When the controller has a built-in I2C interface, such as the powerpsoc LED drive controller series, the processor overhead involved in this method is very small, because the temperature value is directly reported. Another low-cost method is to use a thermistor and an ordinary resistor to form a voltage divider. The partial voltage signal is input to the controller, which uses ADC to convert it into digital value, and takes corresponding actions according to the temperature. The processor needs to perform additional work to convert the digital value to the corresponding value of temperature. Then the voltage of the thermistor can be directly input to the comparator (similar to the jump function of load voltage monitoring). The comparator can turn on or off the LED drive system according to the preset threshold

real time load current control:

the previous article discussed how to monitor load current, voltage and environmental conditions. Intelligent lamps should also have the ability to change their behavior according to the monitored conditions. For example, intelligent lamps should be able to adjust the brightness by changing the driving current or the digital density (PWM) of the output. This can be done according to environmental conditions, load conditions or external inputs, such as buttons or communication interfaces. Actions can be made according to a variety of factors, and each factor can be assigned priority

for example, the controller should be able to respond to communication interfaces such as I2C or UART, and adjust the brightness according to the received data. However, when the input voltage drops below the locking threshold or the temperature rises above the safety value, the controller can gradually reduce the brightness (regardless of the communication interface) and turn off the lamp at the set value. There can be many actions set in this way

traditionally, LED drivers must have a drive current set by hardware (resistance value), so real-time changes are generally impossible. In a software configurable LED drive system, such as powerpsoc, changing the drive current of the LED only requires rewriting some DAC registers. In addition, the software configurable system can use a single hardware platform to design products with different functional items

fault recording and diagnosis:

after the lamps are installed in large buildings, parking lots, streets and other commercial occasions, repair and replacement will affect the cost and maintenance efficiency. If the controller is equipped with an interface to communicate with the "parent" controller to plan maintenance and replacement work, the controller can also be used to record and report fault conditions

for example, the lamp may be damaged (or the fuse is actively blown) due to any of the following reasons:

· open circuit of the load

· short circuit of the load

· the number of times that the system temperature exceeds the threshold is more than the preset number of times

· overvoltage at both ends of the load

· LED life ends or premature aging

· the time that the input voltage fails to reach the locking value exceeds the preset length of time

these conditions require monitoring the working hours of lamps. This function only requires a clock that is accurate enough to record the working hours (accurate to seconds). Most internally generated clocks in the controller have a certain degree of redundancy, which can determine the final accuracy of the timing system. More accurate clocks generally require external oscillators or clock sources

in order to record the cause of the fault, the system condition just before the system is turned off or the fuse blows must be stored in a non-volatile medium. Controllers with flash memory can store these conditions in flash memory. Another way is to use Serial EEPROM devices connected to the controller through I2C interface. Working condition of the system

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