Solutions for designing LED lamps using digital power technology

LED lighting is rapidly gaining popularity among 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 put forward higher and higher requirements for dimming and control technology. Currently, users are mainly concerned that LED lamps must be safe to use, light in weight, long in service life, not affect the health of users, and be applicable to existing dimming equipment and affordable.

To meet the wishes of users, the driver power supply must have high conversion efficiency, low output current ripple, and no optocoupler design. In addition, the safety performance of the lamp must be guaranteed when connected to any dimmer, whether it is a supported or unsupported model. This poses a great challenge to the LED driver power supply. More and more LED lamp manufacturers have realized that it is difficult for traditional driving methods to take all requirements into account at the same time, and it is impossible to promote LED lamps on a large scale. Digital power supply technology has broken through the limitations of traditional solutions, and can integrate and optimize user requirements to provide a complete solution for LED driving and dimming control. This article discusses the advantages of digital technology and solutions to specific design problems of LED lamps.

LED drive technology

High-efficiency non-optocoupler conversion The LED drive circuit converts energy from the AC grid into the DC form required for its own light emission. Energy will be lost during the conversion process. The higher the conversion efficiency, the smaller the loss, and the lower the requirement for heat dissipation of the drive part. Most LED lamps use glue filling and aluminum heat sinks to solve the heat dissipation problem. For users, high-efficiency drive solutions can reduce the heat dissipation cost of the drive circuit and reduce the weight of the LED lamp. Reducing the circuit temperature rise is also beneficial to increase the service life of the LED lamp. The traditional isolation drive solution uses an optocoupler to transmit the current signal on the secondary side 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 adopt primary feedback technology without optocouplers. At present, digital primary feedback technology has matured and has been widely used. Digital control can achieve precise control of the output current without optocoupler feedback. Using transformer feedback waveforms, digital technology can also achieve valley opening to improve conversion efficiency.

a Precise current control without optocoupler

Figure 1(a) shows a primary-side feedback flyback converter. The current waveforms of the primary and secondary sides are shown in Figure 1(b). The average output current Iout=1/2XXXX, where Isp is the peak output current of the secondary winding of the transformer; Trst is the transformer magnetic recovery time; and Tprd is the switching period. Ideally, the primary peak current Ipp=XXXX, where Np and Ns are the number of turns of the primary and secondary windings. Therefore, the output current Iout=XXXXXX. Now assuming that Iset is the designed output current, the digital controller can obtain the desired output current by controlling the primary peak current Ipp=XXXXX.

A power supply system with a resistive input must have an energy storage element inside to provide energy to the load when the input voltage is low. If the energy is converted once and the input is required to be resistive, it requires a very large output capacitor 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 a combination of a boost input stage and a flyback output stage. The input stage mainly controls the input impedance of the driver power supply. The flyback power supply provides a low ripple output current. The complexity of secondary conversion control is very high. In particular, when a regulator is connected, it is necessary to coordinate the energy balance between the input stage and the output stage. Figure 3 is a commonly used secondary conversion system structure. The traditional secondary conversion control scheme requires 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 to be obtained at the same time. The control cost is very high, so it is difficult to be widely used. Digital control technology provides a simple primary side feedback method and can also predict the intermediate capacitor voltage. Therefore, only the input voltage Vin and the transformer feedback signal need to be detected to achieve complete secondary conversion control. The control cost of the system is greatly simplified.

Comprehensive driver protection During the design, production and use of LED lamps, the driver power supply may face problems such as short circuit, open circuit of LED load, short circuit, cold solder joint of driver power supply board, wrong connection and reverse connection of connectors, etc. Comprehensive driver protection can simplify the design and production of LED lamps, extend service life and reduce production costs. Real-time monitoring of system status and making accurate judgments are one of the advantages of digital control. Digital control can quickly realize open circuit protection of LED load, short circuit protection of LED load, overheat protection of LED load, power limit control of LED lamp, open circuit and short circuit protection of each pin of the controller.

Dimming Technology

Dynamic dimmer impedance is mainly used with traditional dimmers to drive pure resistive loads, including leading edge phase dimmers, trailing edge phase dimmers and smart dimmers. Since the load is an incandescent lamp, the power of traditional dimmers is between 200W and 600W. The characteristics of LED driver power are just the opposite - low power, capacitive load. In order to be compatible with these dimmers, LED driver power must provide resistive or quasi-resistive loads to make the dimmer work stably. Using power resistors to directly provide resistive loads is a traditional solution. This method has a good dimming effect, but its main problem is low efficiency. This runs counter to the advantage of high efficiency of LED lamps. Another common solution is to use power factor rectification technology to make the input current follow the input voltage, thereby providing a quasi-resistive load. This solution is often suitable for high-power LED driver applications. For popular low-power household and commercial LED drivers, the problem is that the input impedance is often too high, especially the interaction between the dimmer and the EMI suppression components of the driver often makes it impossible to ensure that there is enough input current to maintain the stable operation of the thyristor. If the dimming signal is not processed properly, it will cause the LED to flicker.

Digital control technology can flexibly combine power factor rectification technology and dynamic impedance matching methods. When the controller detects the presence of a dimmer, it provides matching impedance to maintain the conduction of the thyristor according to the phase angle of the dimmer output. After the phase angle is determined, the controller can use high impedance to turn off the thyristor, while maintaining the input waveform through power factor rectification technology. Figure 4 shows the waveforms of the trailing and leading dimmers. OUTPUT (TR) is the Boost drive control. For example, when the trailing waveform is detected, the Boost drive is fully turned on and quickly discharges the input charge; on the contrary, when the leading dimmer thyristor is turned off, the Boost drive slowly discharges the input charge. In both cases, the input phase can be fully restored. Many controllers on the market currently require the thyristor to conduct a complete AC cycle, which is very unfavorable to improving the efficiency of dimming. The use of digital technology can greatly reduce the loss of dimming, which is in line with the purpose of green lighting.

Perfect user dimming experience Users are already accustomed to dimming of incandescent lamps, so they often expect LED dimming performance to be close to or even better than the previous experience. Therefore, dimming performance is very important for the majority of users to accept LED lamps. The quality of dimming performance depends entirely on the control of the driver power supply. Some dimmable LED lamps on the market currently cannot meet the needs of users in many aspects. For example, if multiple LED lamps are connected to the same dimmer, the brightness of each lamp will be significantly different, which is the consistency of dimming. In addition, when users dim, they want to see the dimming effect immediately, but they don’t want to see sudden brightness jumps or even extinguishing, which is the dynamic response of dimming. The illuminance of some LED lamps changes with the input voltage, which will affect the use of users in some areas where the voltage fluctuation of the power grid is relatively large. More importantly, if the LED lamp cannot illuminate stably but keeps flashing, users will not accept it.

Many LED lamps use average input voltage or approximate root mean square input voltage to control output current. If each LED lamp detects and judges the input voltage differently, the output light will be inconsistent. If the input voltage decreases, the average voltage detected will decrease, and the output light of the LED lamp will decrease. Digital technology can detect the phase of the input signal. Since the phase is a time quantity, the change of input voltage has limited impact on the phase. Therefore, if the input voltage and phase detection are combined, stable and consistent output light can be achieved. The digital algorithm can also detect the user's dimming speed to predict the possible dimming position, so that the output current changes quickly following the user's instructions. This balances the dynamic response and accuracy of dimming, preventing dimming too slow or over-adjustment of light. Make the user's dimming experience close to that of traditional incandescent lamps.

Dimming safety After users purchase LED lights, manufacturers cannot fully understand their usage environment. The frequency of the AC input can be 50Hz or 60Hz; the regulator can be supported or not; the grid voltage can fluctuate and be distorted; and so on. Many factors can affect the brightness and even safety of LED lights. The design of the drive circuit must take these possible environmental changes into account and have corresponding countermeasures. Current digital control technology has achieved:

Automatic dimming mode recognition. The controller can automatically identify leading and trailing phase dimmers, and even allow switching between leading and trailing phase dimmers during operation.

Automatically detect unsupported dimmers. If a certain dimmer is not supported by the LED lamp produced, digital technology can force the LED lamp to enter protection mode based on its output waveform, ensuring the safety of users.

Automatically prevent multiple rapid starts. Since LED lamps require fast start-up, when LED lamps fail or input voltage distortion is serious, the drive power supply may restart repeatedly, causing overheating of the drive circuit. Digital control can easily determine the existence of obstacles and prevent frequent repeated starts.

Typical digital LED control system

Figure 5 is a schematic diagram of the structure of iWatt's iW3610 series digital dimming control system. The iW3610 controller uses an 8-pin package to implement the following functions: dimmer impedance matching, input power factor control, boost voltage prediction and control, primary side constant current control of the flyback converter, dimmer type detection and dimming control, and complete input, output and internal protection.

Dimmer Identification and Control Process

Figure 6 is the internal structure diagram of iWatt's iW3610 series digital controller. VIN samples the output voltage waveform of the dimmer. The dimmer signal enters the dimming control and phase detection digital module through analog-to-digital conversion. According to the percentage of the leading or trailing phase, the constant current control module calculates the required output current control amount. The control amount is provided to the primary current control comparator (Ipeak) through 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 the voltage signal of the transformer flyback. By analyzing the flyback signal, the controller can obtain the output voltage, current and valley time point to implement various protection functions.

Figure 7 shows the start-up detection of the dimmer. After the dimmer is turned on, the driver circuit starts charging. When the VCC supply voltage reaches the start-up level, the controller starts working. The Boost control signal OUTPUT (TR) conducts for 3-4 AC half cycles, providing the dimmer with a low impedance loop to complete initialization. During this period, the controller determines the input voltage range, frequency, and dimmer type and phase angle based on the characteristic waveform output by the dimmer. If it is determined to be a supported dimmer, the driver circuit is started and the corresponding LED current is output.

iW3610 Series Product Application Scheme Figure 8(a) shows a specific application scheme of the iW3610 series controller. Figures 8(b) and (c) show the measured waveforms of the trailing dimmer and leading dimmer, respectively.

Summarize

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 more and more control and protection requirements, iWatt's iW3610 series digital controller is gradually becoming the mainstream driver controller for LED general lighting. The iW3610 series digital controller is suitable for the requirements of built-in driving of lamps. It uses a small number of components to achieve high-performance dimming, high power factor, isolated drive and precise constant current output design without optocoupler, optimizing the heat dissipation performance of the overall LED lamp. The volume of the entire design can be as small as a bulb or PAR lamp built into the E27/E26 lamp head. The 5W design efficiency is greater than 80%, and the 10W design efficiency is greater than 85%. The power factor meets the requirements of Energy Star, and the dimming range reaches 1%-100%. At the same time, it supports mainstream dimmers in the European, American and Asian markets.