Building LED drivers should achieve flicker-free dimming control

1 Challenges of LED lighting design

The digital revolution of the global lighting industry is coming, and energy-efficient LED lamps will replace incandescent, M16 halogen and CFL bulbs. However, LED lighting designers are facing new challenges in recent times, which is to meet the new requirements of dimming control functions that can be achieved with LED drivers for incandescent and M16 halogen lamps, and dimming control with high power factor and no flicker, especially to be compatible with the existing infrastructure, including corner dimming and electronic transformer dimming to support flicker-free dimming design.

It should be said that dimming is a very common function of lighting systems. Compared with incandescent lamps or M16 halogen lamps, it can be easily achieved at a low cost, but it is difficult to dim LED lamps, especially to achieve dimming control without any flicker. Usually, the most fearful thing for buildings and users to convert incandescent lamps or M16 to LED lighting is the fear of losing the advantages of dimming control applications, that is, there should be no flicker. Of course, power factor is also a very important factor, because high power factor can reduce the loss and waste of the distribution network. Therefore, regulatory agencies at home and abroad are further tightening their power factor specifications. For example, the Energy Star solid-state lighting energy efficiency specification stipulates that the power factor of residential lighting products should be greater than 0.7, and the power factor of commercial lighting products should be greater than 0.9. Therefore, LED bulb and lamp manufacturers are responding to these requirements, requiring LED driver circuits to be suitable for various dimming units, with high efficiency and power factor (PF)>0.9, so that their products have high versatility.

Therefore, how to deal with the challenges brought by LED lighting design and meet the new requirements has become an urgent concern for manufacturers, designers and consumers. Based on this, this article will discuss the dimming control technology and construction of high power factor performance without any flicker in LED lighting applications, and focus on the analysis of two high power factor without any flicker dimming control, LED drivers built with multiple controllers and highly integrated MOSFET LED drivers.

2 The LED driver constructed by applying multiple controllers is a flyback power supply dedicated to constant current LED load

To this end, let's start with the basic concept of the dimming controller.

2.1 Problems with thyristor dimmers in LED lighting applications

* Silicon controlled dimmer (see Figure 1(a)). It uses the phase shear principle to reduce VRMS to reduce the power of ordinary load (resistance load), with high working efficiency and stable performance.

Therefore, it is a commonly used method for LED lighting, as shown in Figure 1(b) which shows the architecture of the thyristor dimmer and LED driver.

The thyristor dimming in Figure 1(b) is to extract the conduction angle information when the power supply is cut in the chip or application circuit, and control the LED drive current according to the signal control to achieve the dimming effect. However, the thyristor dimmer has flickering, uneven light emission, audio noise and flickering in LED lighting applications. There are two reasons for this: First, the 100Hz frequency flash of the thyristor dimming exists. Because the power frequency of the power grid is 50/60Hz, after the phase-cut waveform is rectified, a 100/120Hz pulse signal can be obtained. This can be used directly to generate a dimming PWM signal. In actual applications, any slight voltage fluctuation or current change will affect the duty cycle of the pulse, which is equivalent to a 100/120Hz flicker. But there is no such problem for incandescent lamps? It is because the physical inertia of the tungsten filament of the bulb makes it difficult for people to feel it; secondly, the dimming control of the thyristor dimmer is achieved by changing the phase angle of each half cycle of the thyristor conduction. In order to maintain the stable operation of the thyristor, one of its important parameters, the holding current (IH), cannot be zero (its typical value is between 8mA and 40mA). When it drives incandescent lamps, the IH holding current is not a problem. However, when driving high-efficiency LEDs, it is impossible to maintain the holding current when the lamp is off. Especially in the case of oscillation, it is more likely to cause flickering, uneven light emission, audio noise and flickering. If the oscillation causes the current to drop below IH (8mA~40mA), the thyristor will be turned off, resulting in multiple restarts of the thyristor in the same input line cycle, and the LED lamp will flicker frequently.

* Improving the LED driver load capacity is the key to suppressing flicker

100Hz flicker of thyristor dimming - Solution: You cannot dim directly with the phase-cut waveform, you need to shift the modulation signal to a higher frequency band so that people cannot see the flicker. The driver controller chip adjusts the dimming signal to a higher frequency band of several kHz.

Regarding the load problem of thyristor dimming, for the thyristor dimmer to work stably, a stable IH is required to ensure the operation of the dimmer. In order to solve the overheating of the thyristor dimming load, it is necessary to apply signal filtering technology to shape and filter the intercepted conduction angle signal to improve the availability of the dimming signal; in order to solve the vibration when the load is insufficient, it is necessary to add signal smoothing and phase locking technology to smoothly lock the two adjacent conduction angle signals. This can greatly reduce the occurrence of flickering.

To avoid the problems associated with thyristor dimming, LED drivers must meet the various requirements of LED loads while being compatible with dimming circuits designed for incandescent lamps. LED lamps used to replace standard incandescent lamps usually contain multiple LEDs to ensure uniform lighting. These LEDs are connected together in series. The brightness of each LED is determined by the current it receives. The forward voltage drop of an LED is approximately 3.4V, but is typically between 2.8V and 4.2V (±20%). Despite the large load variations, the LED string must still be driven by a constant current power supply, so the current must be tightly controlled to ensure high matching between adjacent LED lamps. For an LED lamp to be dimmable, its power supply must detect the variable phase angle output of the thyristor controller and use this information to change the constant current drive of the LED.

2.2 Construction of flyback power supply for constant current LED load

It should be said that there are many types of controllers that can be used to build a constant current LED driver power supply - a flyback power supply for LED loads. Here we only take the LED driver built with the integrated multi-controller LinkSwitch.-PH as an example for analysis.

2.2.1 The LED driver constructed with multiple controllers is a flyback power supply dedicated to constant current LED loads

Figure 2 shows a flyback power supply architecture designed using the latest LinkSwitch.-PH controller, which is a flyback power supply specifically designed for constant current LED loads. The constant current source for LED driving helps ensure that the brightness of the LED is consistent and does not flicker during the working period of the light.

The LinkSwitch-PH controller in Figure 2 integrates several new features specifically designed for driving LEDs. Unlike the standard flyback topology used in LED drivers, this circuit uses primary-side regulation. This eliminates the need for an optocoupler and secondary-side control circuitry. The secondary-side winding (bias winding) on ​​the transformer has two functions: it supplies power to the LinkSwitch-PH through the BP pin and provides current feedback through the FB pin. The two secondary-side windings are tightly coupled so that the voltage on the bias winding is proportional to the current flowing through the LED load. After receiving current feedback at the FB pin, the controller adjusts the duty cycle of the integrated high-voltage power MOSFET to maintain current regulation. The circuit can operate under rectified and unsmoothed AC mains input, and its controller continuously adjusts the duty cycle of the high-voltage power MOSFET as the mains input rises and falls in each half cycle, and controls the average charge in each half cycle to maintain the output current regulation. It can be seen that the flyback power supply for constant current LED load built by LinkSwitch-PH controller can be configured to provide constant current output and realize thyristor phase angle detection and power (power factor) control.

2.2.2 Implementation of dimming control without any flicker

*The LinkSwitch-PH controller has features that guarantee flicker-free dimming control. Two of the features of the LinkSwitch-PH controller are continuous conduction mode and frequency jitter. In addition to helping simplify input filtering, these two features are: continuous conduction mode has two major advantages, namely reduced conduction losses (thereby improving efficiency) and reduced EMI characteristics, which helps to meet the requirements of EMI standards with a low-cost, small input EMI filter, eliminating a large capacitor and eliminating or reducing the size of the common-mode choke; the controller in LinkSwitch-PH can also apply jitter to the switching of the high-voltage power MOSFET, which can extend the range of switching frequencies and further reduce filtering requirements. Adding active attenuation circuits and bleeder circuits ensures that the LED lamp operates stably over an extremely wide 1000:1 dimming range without any flicker.

In particular, LinkSwitch-PH can be set to dimming mode or non-dimming mode by connecting a programming resistor. In non-dimming mode, the circuit can provide a constant current output over the full AC input range with a power factor close to 1. In dimming mode, the zero crossing point and phase angle of the rectified input are used to set the output current level, thereby providing dimming functionality. LinkSwitch-PH can be used to design high-performance LED drivers that can operate over the full input voltage range and enable the dimming range of low-cost thyristor dimmers to reach 1000:1 without any flicker.

*The application of LinkSwitch-PH controller is determined by its unique features. So what are the application features? It should be said that it is a LED driver IC with single-stage power factor correction (PFC), primary-side constant current output and thyristor dimming.

LinkSwitch-PH (such as PH LNK403EG) can greatly simplify the design of high-efficiency LED drivers with power factor greater than 0.9 and thyristor dimming. This highly integrated controller device uses a new control technology that not only provides very high power factor and precise constant current output control, but also eliminates the passive circuits required for power factor correction, as well as optocouplers and secondary current control circuits. Each device integrates a 725 V power MOSFET, a continuous mode PWM controller, a self-biased high-voltage switch current source, frequency jitter, cycle-by-cycle current limit and hysteresis thermal shutdown circuits on a single IC. The application features of LinkSwitch-PH are as follows: First, it is a greatly simplified offline LED driver. It has single-stage power factor correction (PFC) and precise constant current (CC) output and flicker-free phase-controlled thyristor dimming, which can eliminate optocouplers, all secondary current control circuits, all control loop compensation circuits and simplify the primary side PWM dimming interface; second, precise and stable performance. It can quickly start up time, compensate for transformer inductance tolerance and input voltage variation and frequency modulation technology, thus greatly reducing the size and cost of EMI filter; third, advanced protection and safety features. It is an open-circuit fault detection mode, which can provide short-circuit protection through automatic restart, automatic thermal shutdown or hysteresis automatic restart, whether on the PCB board or on the package, to ensure that the high-voltage drain pin and all other reference pins meet the high-voltage creepage requirements; fourth, EcoSmart? - high-efficiency energy-saving technology. Low standby power remote ON/OFF control function (< 50 mW at 230 VAC input), no current detection resistor is required, which can improve efficiency. LinkSwitch-PH is a green package, that is, a halogen-free and RoHS-compliant package. It is a typical example to use PI's PH LNK403EG controller to build a 7W dimmable LED driver.

3 New LED construction solution that can directly replace MR16 halogen lamps

Overview The application of highly integrated MOSFET LED drivers can build a new type of offline and MRl6 LED lighting solution. It uses unique technology to cooperate with dimmers and electronic transformers to achieve flicker-free dimming. The scheme is shown in Figure 3.

This LED solution, which directly replaces MR16 halogen lamps, ensures compatibility with the electronic infrastructure of existing lighting solutions and helps manufacturers calmly deal with the barriers of mass production. It is also called a pin-compatible alternative to offline/MRl6 lamps. It can effectively extend the service life and smoothly adjust the brightness using the installed phase-cut dimmer, and can be as low as 0%; it can work with most electronic transformers to achieve flicker-free dimming (MRl6); it can support global lighting architectures (offline) with the same design under 90-265VAC universal input; no electrolytic capacitors are required, so it effectively extends the life of the lamp. Here, the implementation of the solution is analyzed from the application perspective.

3.1 Unique construction scheme

* Questions raised

Halogen lamp is a pure resistance load, which can work regardless of whether it is powered by AC or DC. Usually, when driving halogen lamp, the voltage is reduced, the brightness of halogen lamp is reduced, and there is no other change. When driving LED, the voltage is reduced, and LED will flicker to a certain extent. Why? Halogen lamp is a pure resistance load. Generally, halogen lamps use electronic transformers to reduce 220V AC high voltage to 12V high frequency (50kHz) low voltage, so halogen lamp electronic transformers can work well with it. LED driver is nonlinear, not a pure resistance load, but a load of capacitive impedance + inductive impedance. When halogen lamp electronic transformer accepts LED lamp load, its output capacity will be greatly reduced, which is manifested as the output voltage drops from 12V to 7V. If the driver IC is started with 8V, it will enter the undervoltage protection state. At this time, the output voltage of halogen lamp electronic transformer rises to 12V again. Therefore, when using the existing circuit architecture, it will repeat this cycle, so that you can see LED lamp flickering, unable to dim, and even unable to turn on in some cases. For ease of use, LED replacement lamps must be compatible with electronic transformers and angle dimmers. The use of the MAX16840 LED driver is an effective choice.

*Construction scheme It uses the new MAX16840 LED driver. The architecture of this chip uses electronic transformers and angle-cut dimmers to achieve flicker-free dimming, forming a unique LED driver solution, which can realize LED lamp design compatible with most electronic transformers and angle-cut dimmers MR16. It technically ensures that MR16 halogen lamps can be directly replaced with LED replacement lamps without any modification to the existing circuit architecture. This solution removes a major obstacle for market application, allowing end users to obtain all the advantages of LED lighting at a very low configuration cost, namely: it is an LED replacement solution that ensures flicker-free and dimmable operation, is compatible with most electronic transformers and angle-cut dimmers, and can directly replace MR16; there is no need to change the existing electrical architecture, and the board size is reduced and the BOM cost is reduced. So how is the construction design of LED replacement lamps achieved?

3.2 Implementation of LED replacement lamp construction technology

The application features of the MAX16840 are the guarantee of the construction technology of LED replacement lamps. The MAX16840 integrates a MOSFET LED driver and can be used for MR16 and other 12V AC input lamps. It uses a unique technology to control the input current of the lamp tube and shape the input current. It can support flicker-free operation with most electronic transformers and dimmers, which well solves the trouble of the above "problem".

The MAX16840 integrated MOSFET LED driver IC includes all the functions required for 12V AC and 24V AC input (such as MR16) LED lamp drivers with minimal external components. Its proprietary input current control mechanism makes the constructed LED lamp compatible with electronic transformer designs and supports trailing edge dimming (with electronic transformers); it can be used to build buck, boost, and buck-boost topologies, with a built-in 0.2Ω (maximum), 48V switching MOSFET; and uses a fixed frequency and average current control mode. The built-in switching MOSFET can further save space, reduce component count, and reduce solution cost.

The MAX16840 chip has three important pin functions: the FB pin voltage can detect the input current and adjust its average value; the input pin (REFI) is used to set the input current value. When the pin voltage is lower than a specific threshold, the input current is proportional to the voltage. When the voltage exceeds a specific threshold, the input current will be set to a fixed preset level. Based on this, by using the nonlinear characteristics of REFI and connecting an NTC resistor, thermal foldback protection can be obtained; the IN pin has internal overvoltage protection to prevent the internal switch MOSFET from being damaged when the LED string is open or the LED string voltage is too high; it has an independent EXT pin to ensure that a trigger startup current is provided when the low input voltage is turned on to cooperate with the normal operation of the electronic transformer. EXT drives the external npn transistor. Once IN drops below the 5.5V UVLO threshold, EXT is pulled to the ground level and the external npn transistor is turned off. Its IC uses a 3mm x 3mm, 10-pin, TDFN package and has an operating temperature range of -40°C to +125°C. Figure 4 is a typical application diagram of the MAX16840. It is widely used in MR16 and other 12V AC or DC input LED lighting.

Due to the application of MAX16840, the service life of the lamp tube is extended and the size requirements of MR16 lamps are met. In addition, MAX16840 can adopt an electrolytic capacitor-free design, which effectively extends the service life of LED lamps, because electrolytic capacitors are usually the first components to fail in the driver circuit. The electrolytic capacitor-free design can also reduce the cost and size of the driver, and also meet the small size requirements of MR16 lamps. It is worth noting that its use allows LED lamp designers to directly replace MR16 halogen lamps, eliminating the expensive circuit upgrade costs required by similar competitive solutions. This makes it clearer where the advantages of the unique MR16 LED lighting solution are, that is, the LED replacement lamp (or retrofit lamp) is constructed. Compared with incandescent lamps, halogen lamps and fluorescent lamps, the replacement lamp can provide ultra-high efficiency and longer service life with LED light sources, especially compatible with many electronic transformer dimming and support flicker-free dimming design. However, it should be reminded that in order to obtain a more valuable alternative, the LED replacement lamp must be compatible with the existing infrastructure, including angle dimming and electronic transformer dimming. In terms of application, for indoor lighting replacement solutions, high energy efficiency and long service life make LED an economical and practical solution to replace traditional lighting. Here is an example of an application, as shown in Figure 5, which is a schematic diagram of a low-voltage LED bulb.

This LED driver solution is used for MR16 and similar replacement lamps with 12VAC input. It can work with electronic transformers and supports the trailing edge dimming of electronic transformers. No electrolytic capacitors are required in the circuit, which effectively extends the working life of the LED lamp. In addition to the electronic transformer, the system uses a trailing edge dimmer, which reduces the brightness of the lamp by cutting off the last part of each half cycle of the AC power.

4. Afterword

From the above application cases, it can be seen that if an LED driver built with an integrated multi-controller or a highly integrated MOSFET LED driver is used, which can perform power factor correction, constant current drive and phase angle detection, stable operation with flicker-free dimming can be achieved. In addition, the circuit can meet the efficiency, power factor, harmonic and EMI requirements of all international standards. In the past, incandescent bulbs or MR16 had to be manufactured for a specific power supply voltage. Now there is no need to be subject to this restriction, and the dimmable LED lamps manufactured can be used at home and abroad without any modification.