Digital power technology promotes the development of LED lighting

introduction

LED lighting has quickly gained popularity among users for its advantages of high luminous efficiency , long service life, simple brightness control and environmental protection. As a new type of 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. The main concern of current users is that LED lamps must be safe to use, light in weight, long in service life, not affect the health of users, and can be applied to existing dimming equipment and affordable.

Digital power technology breaks through the limitations of traditional solutions, can integrate and optimize user requirements, and provide a complete solution for LED drive and dimming control. This article discusses the advantages of digital technology and solutions to specific design problems of LED lamps .

1. LED driver technology

High-efficiency optocoupler-free conversion The LED drive circuit converts energy from the AC grid to 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 optocouplers to transmit the current signal of 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 waveform, digital technology can also achieve valley opening to improve conversion efficiency.

1.1 Precise current control without optocoupler

Figure 1(a) shows a flyback converter with primary-side feedback. The current waveforms on the primary and secondary sides are shown in Figure 1(b). 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.

1.2 Valley opening control

The main purpose of valley switching is to achieve high efficiency. Figure 2 shows the voltage waveform coupled to the auxiliary winding of the transformer after the MOSFET is turned off. As shown in Figure 2, the transformer completes magnetic recovery at time T1. Then the magnetizing inductance and the stray capacitance of the MOSFET drain begin to resonate. If the MOSFET is turned on at the bottom of the drain-source voltage resonance T3, the lowest switching loss can be achieved. At the same time, the reduction of electromagnetic interference is conducive to improving the efficiency of the input filter. By using digital technology to analyze the voltage waveform on the auxiliary winding, the function of valley switching can be very simple to achieve.

1.3 Low current ripple design

LED lighting requires not only precise and stable current, but also very low current ripple. Scientists have shown that flicker below 165Hz, whether from visible or invisible light, may cause migraines or visual discomfort. Flicker below 70Hz may even cause epilepsy in a small number of people. Therefore, the Institute of Electrical and Electronics Engineers ( IEEE ) is developing relevant standards to guide the design of LED lighting drivers that are harmless to human health.

A power supply with a resistive input must have an energy storage element inside the system to provide energy to the load when the input voltage is low. If the energy is converted once and the input is 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 board, wrong connection, 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 achieve

* Open circuit protection for LED load

* Short circuit protection for LED load

* Overheat protection for LED load

* LED light power limit control

* Open circuit and short circuit protection for each pin of the controller

2. Dimming technology

2.1 Dynamic dimmer impedance matching

Traditional dimmers are mainly used 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 supplies are just the opposite - low power, capacitive load. In order to be compatible with these dimmers, LED driver power supplies must provide resistive or quasi-resistive loads to enable the dimmer to 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 a large 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 rear-cut and front-cut dimmers. OUTPUT (TR) is the Boost drive control. For example, when the rear-cut waveform is detected, the Boost drive is fully turned on and quickly discharges the input charge; on the contrary, when the front-cut dimmer thyristor is turned off, the Boost drive slowly discharges the input charge. In both cases, the input phase can be fully restored. At present, many controllers on the market require the thyristor to conduct a complete AC cycle, which is very unfavorable to improve 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.

2.2 Perfect user dimming experience

Users are already accustomed to dimming incandescent lamps, so they often expect LED dimming performance to be close to or even better than their 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 driving 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 hope to see the dimming effect immediately, but they do not want to see sudden brightness jumps or even extinguishing, which is the dynamic response of dimming. The light illumination of some LED lamps changes with the input voltage, which will affect the use of users in some areas where the power grid voltage fluctuates greatly. 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 RMS 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 effect on the phase. Therefore, if the input voltage and phase detection are combined, stable and consistent output light can be achieved. Digital algorithms 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.

2.3 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 lights require fast start-up, when an LED light fails or the input voltage is severely distorted, the drive power supply may restart repeatedly, causing the drive circuit to overheat. Digital control can easily determine the existence of obstacles and prevent frequent repeated starts.

3. Typical digital LED control system

Digital Control LED System Structure

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 and implements the following functions:

* Dimmer impedance matching

* Input power factor control

* Boost voltage prediction and control

* Primary side constant current control of flyback converter

* Dimmer type detection and dimming control

* Complete input, output and internal protection

4. 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.

5. Conclusion

Digital control technology has the advantages of flexible control, good dimming performance and comprehensive protection in the field of LED lighting. In response to the increasing 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 lamp driving. 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.