LED lighting is quickly welcomed by users due to its high luminous efficiency, long service life, simple brightness control and environmental protection. As a new energy-saving light source, LED lamps will gradually replace traditional incandescent bulbs. The increasing popularity of LED lighting has placed increasing demands on dimming and control technology. The main concern of current users is that LED lamps must be safe to use, light in weight, long in life, and do not affect the health of users, and can be applied to existing dimming equipment and affordable prices.
To meet the user's wishes, the drive power conversion efficiency is high, the output current ripple is low, no optocoupler design is required, and the safety performance of the luminaire must be ensured when any dimmer is connected, whether it is a supported or unsupported model. . This poses a great challenge to the driving power of the LED. More and more LED luminaire manufacturers realize that the traditional driving method is difficult to meet all the requirements at the same time, and it is impossible to promote LED lights in large quantities. Digital power technology breaks through the limitations of traditional solutions, can integrate and optimize user requirements, and provides a complete solution for LED driver and dimming control. This article discusses the advantages of digital technology and the method of solving the problem for the specific design of LED lights.
LED drive technology
The high efficiency, no optocoupler-converted LED drive circuit converts energy from the AC grid to the DC form required for its own illumination. Energy is lost during the conversion process. The higher the conversion efficiency, the smaller the loss and the lower the heat dissipation requirement for the drive part. Most LED lamps use glue and aluminum heat sinks to solve the heat problem. For the user, a highly efficient driving scheme can reduce the heat dissipation cost of the driving circuit and reduce the weight of the LED lamp. Reducing the temperature rise of the circuit also helps to increase the service life of the LED lamp. The traditional isolated drive scheme uses an optocoupler to pass the secondary side current signal to the primary side controller to maintain a stable output current. The secondary side detection circuit increases the complexity, cost, and loss of the drive circuit. The use of optocouplers also reduces reliability. Therefore, mainstream LED lamp manufacturers have begun to use the primary side feedback technology without optocoupler. Currently, digital primary feedback technology has matured and is widely used. Digital control enables precise control of the output current without optocoupler feedback. Using transformer feedback waveforms, digital technology can also achieve valley turn-on to improve conversion efficiency.
a No optocoupler precise current control
Figure 1(a) shows a flyback converter with primary feedback. The current waveforms of the primary side and the secondary side are shown in Fig. 1(b). The average output current Iout=1/2XXXX, where Isp is the peak output current of the transformer secondary winding; Trst is the transformer magnetic recovery time; Tprd is the switching period. In the ideal case, the primary peak current Ipp = XXXX, where Np and Ns are the primary and secondary winding turns. Therefore, the output current Iout = XXXXXX. Now suppose that Iset is the design output current, and the digital controller can obtain the required output current by controlling the primary peak current Ipp=XXXXX.
An energy storage component must be present inside a power supply with an input resistance. When the input voltage is low, energy can be supplied to the load. If the energy is converted in a single pass and the input is required to be resistive, it requires a very large output capacitance to reduce the current ripple of the load. This problem can be solved if the energy is converted twice. The usual form of secondary conversion is to combine the Boost input stage with the flyback output stage. The input stage primarily controls the input impedance of the drive supply. The flyback power supply provides a low ripple output current. The complexity of the secondary conversion control is high. In particular, it is necessary to coordinate the energy balance between the input stage and the output stage when accessing the switch. Figure 3 is a diagram of a conventional secondary conversion system structure. The traditional secondary conversion control scheme needs to obtain the input voltage Vin, the Boost current IL, the voltage Vbulk on the intermediate capacitor, the flyback primary current Ip, and the voltage feedback Vout. The control cost is high, so it is difficult to be widely used. The digital control technology provides a simple primary-side feedback method that also predicts the intermediate capacitor voltage, so only the input voltage Vin is detected and the transformer feedback signal is resolved to achieve complete secondary conversion control. The control cost of the system is greatly simplified.
Comprehensive drive protection During the design, production and use of LED luminaires, the drive power supply may face short-circuit, open circuit of the LED load, short circuit of the drive power board, solder joint, wrong connection of the connector, reverse connection, etc. . Comprehensive drive protection simplifies the design and production of LED luminaires, extending service life and reducing production costs. Real-time monitoring of system status and making accurate judgments is a strength of digital control. The digital control can quickly realize the open circuit protection of the LED load, the short circuit protection of the LED load, the overheat protection of the LED load, the limited power control of the LED lamp, and the open and short circuit protection of each pin of the controller.
Dimming technology
Dynamic dimmer impedance in conjunction with conventional dimmers is primarily used to drive purely resistive loads, including leading edge phase-cut dimmers, trailing edge phase-cut dimmers, and intelligent dimmers. Since the load is incandescent, the traditional dimmer power is between 200W and 600W. The characteristics of LED drive power are just the opposite - low power, capacitive load. In order to be compatible with these dimmers, the LED driver must provide a resistive or resistive load to stabilize the dimmer. Directly providing resistive loads with power resistors is a traditional solution. The dimming effect of this method is good, but its main problem is low efficiency. This runs counter to the high efficiency of LED lights. Another common solution is to use power factor rectification techniques to cause the input current to follow the input voltage variation, thus providing a resistive load. This solution is often used in high power LED driver applications. For the popular low-power home and commercial LED drivers, the problem is that the input impedance is often too high, especially the interaction between the dimmer and the driving part of the EMI suppression component often makes it impossible to guarantee a sufficient input current to maintain the thyristor. Stable work. If the dimming signal is not handled well, the LED will flash.
Digital control technology can flexibly combine power factor rectification technology and dynamic impedance matching methods. When the controller detects the presence of a dimmer, the controller provides a matched impedance to maintain conduction of the thyristor based on the phase angle of the dimmer output. After the control phase angle judgment is completed, the controller can use high impedance to turn off the thyristor while maintaining the input waveform by power factor rectification. The rear cut and front cut dimmer waveforms shown in Figure 4. OUTPUT (TR) is the Boost drive control. For example, when a post-cut waveform is detected, the Boost driver is fully turned on to quickly bleed the input charge; conversely, after the current thyristor is turned off, the Boost driver slowly bleeds the input charge. In both cases, the phase of the input can be completely recovered. Many controllers on the market currently require the thyristor to conduct a complete AC cycle, which is very unfavorable for improving the efficiency of dimming. The use of digital technology can greatly reduce the loss of dimming, in line with the purpose of green lighting.
The perfect user dimming experience users have become accustomed to the dimming of incandescent lamps, so it is often expected that the dimming performance of LEDs will approach or exceed the previous experience. Therefore, the dimming performance is very important for the majority of users to accept LED lights. The quality of the dimming performance depends entirely on the control of the drive power. Some dimmable LED lights currently on the market cannot meet the needs of users in many ways. For example, if multiple LED lights are connected to the same switch, the brightness of each lamp will be significantly different, which is the consistency of dimming. Also, when the user dims, I hope to see the dimming effect right away, but I don't want to see a sudden brightness jump or even go out, which is the dynamic response of dimming. The illuminance of some LEDs varies with the input voltage, which affects the user's use in areas where grid voltage fluctuations are relatively large. More importantly, if the LED light does not stabilize the illumination but blinks continuously, the user is unacceptable.
Many LED lamps use an average input voltage or an approximately rms input voltage to control the output current. If each LED light has a difference in the detection and judgment of the input voltage, it will cause inconsistency in the output illumination. If the input voltage is reduced, the average voltage detected will decrease and the LED light output will decrease. The use of digital technology enables the detection of the phase of the input signal. Since the phase is an amount of time, the effect of changes in the input voltage on the phase is limited. Therefore, if combined with the detection of input voltage and phase, stable and consistent output illumination can be achieved. The digital algorithm can also detect the speed of the user's dimming to predict the position of the possible dimming so that the output current changes quickly following the user's instructions. This balances the dynamic response and accuracy of dimming, preventing over-dimming or over-lighting. The experience of user dimming is close to traditional incandescent lamps.
Dimming safety When a user purchases an LED light, the manufacturer cannot fully understand the environment in which it is used. The frequency of the AC input can be 50Hz or 60Hz; the regulator can be either supported or unsupported; the grid voltage will fluctuate and distortion will occur; Many factors can affect the brightness of LED lights and even safety. The design of the drive circuit must take into account these possible environmental changes and have corresponding countermeasures. Current digital control technologies are implemented:
Automatic dimming mode recognition. The controller automatically recognizes the front-cut phase dimmer and the rear-cut phase-level regulator, allowing the switching of the front and rear cut dimmers even during operation.
Automatically detect unsupported dimmers. If a dimmer is not supported by the LEDs produced, digital technology can force the LEDs into protection mode based on their output waveforms, ensuring user safety.
Automatically prevent multiple quick starts. Since the LED lamp requires fast start-up, when the LED lamp fails or the input voltage is severely distorted, the drive power source may be repeatedly restarted, causing overheating of the drive circuit. Digital control can easily determine the existence of roadblocks and prevent frequent restarts.
(0,0,0); WORD-BREAK: break-all; WORD-SPACING: 0px; PADDING-TOP: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px" >Typical digital LED control system
Digital Control LED System Structure Figure 5 is a schematic diagram of the iWatt iW3610 series digital dimming control system. The iW3610 controller is packaged in an 8-pin package and provides the following functions: dimmer impedance matching, input power factor control, Boost voltage prediction and control, flyback converter primary side constant current control, dimmer Type detection and dimming control, complete input, output and internal protection.
Dimmer identification and control process
Figure 6 is an internal block diagram of iWatt's iW3610 series digital controller. The output voltage waveform of the VIN sampling dimmer. The dimmer signal is passed through analog to digital conversion into the dimming control and phase detection digital blocks. The constant current control module calculates the required output current control amount based on the percentage of the front or back phase. The control is provided to the primary current control comparator (Ipeak) by digital to analog conversion. Isense detects the primary current signal and obtains a stable LED output current through the constant current control principle shown in Figure 1. Vsense provides a voltage signal for transformer flyback. By analyzing the flyback signal, the controller can obtain the output voltage, current, and valley time points to achieve various protection functions.
Figure 7 shows the startup detection of the dimmer. After the dimmer is turned on, the drive circuit begins to charge. When the VCC supply voltage reaches the startup level, the controller begins to operate. The Boost control signal OUTPUT (TR) is turned on for 3-4 AC half cycles, providing a low impedance loop of the dimmer to complete the initialization. During this period, the controller determines the input voltage range, frequency, and the type and phase angle of the dimmer based on the characteristic waveform of the dimmer output. If it is judged to be a supported dimmer, the drive circuit is activated to output the corresponding LED current.
iW3610 Series Application Plan Figure 8(a) shows a specific application of the iW3610 series controller. Figures 8(b) and (c) show the measured waveforms of the back-cut dimmer and the front-cut dimmer, respectively.
to sum up
Digital power control technology has the advantages of flexible control, good dimming performance and comprehensive protection in the field of LED lighting. In response to increasing control and protection requirements, iWatt's iW3610 series of digital controllers are gradually becoming the mainstream drive controller for LED general illumination. The iW3610 series of digital controllers are suitable for the built-in drive of the luminaire. A small number of components are used to achieve high-performance dimming, higher power factor, isolated drive and accurate constant current output design without optocoupler, optimizing the overall LED lamp. Thermal performance. The entire design can be as small as a bulb or PAR lamp built into the E27/E26 base. 5W design efficiency is greater than 80%, 10W design efficiency is greater than 85%, power factor meets Energy Star requirements, dimming range is 1%-100%, and supports mainstream dimmers in European and American markets and Asian markets.
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