Multi-level LED driver design for efficient dimming

　　Although the basic LED driver designed for general lighting applications is relatively simple, the design becomes very complex when additional features such as phase cut dimming and power factor correction are required. Non-dimming LED drivers without power factor correction generally include an offline switching power supply that is regulated to achieve a constant current output. This is not much different from the standard offline switching power supply and the types commonly used in AC-DC adapters. This design can use standard SMPS (switching power supply) circuit topologies such as buck, boost or flyback converters.

　　On December 3, 2009, the U.S. Department of Energy (DOE) released the final version of the "Energy Star" specification for the integrated LED lamp project, stipulating that the power factor of LED drivers used in the United States must be better than 0.7, while industrial applications are expected to be better than 0.9. Many products on the market today cannot meet such requirements, so more advanced products will be needed to replace them in the future. There are two ways to achieve power factor correction (PFC), each of which requires adding some additional circuits to the front end of the power converter: simple low-cost passive PFC, and more complex active PFC.

　　Before delving deeper into these methods, it is important to emphasize that in order to obtain an Energy Star rating, the LED driver must be dimmable.

　　Generally, this means that the dimmability will come from an existing electronic wall dimmer based on the phase-cut principle, which was originally designed for pure impedance incandescent lamps. Although other dimming methods, such as linear 0-10V dimming or DALI, may also be suitable, they are likely to be limited to high-end industrial LED drivers. Phase-cut dimmers have been widely used so far, and it is clear that LED lamps that can be dimmed effectively will have great advantages. Since there are many low-cost triac-based dimmers on the market, it is unrealistic to guarantee that LED drivers are compatible with all categories, especially since many dimmers are of basic design and have very limited performance. For this reason, the Energy Star program only requires LED driver manufacturers to specify which dimmers are compatible with their products on a single web page.

　　Another requirement worth noting in the Energy Star specification is that the LED operating frequency must be greater than 150Hz to eliminate the possibility of visible flicker. This means that the output current powering the LED cannot have any significant ripple at twice the line frequency (50Hz or 60Hz).

　　LED lighting is increasingly being used in offline applications such as office lighting, public buildings, and street lighting, and this trend will continue in the coming years. In these applications, high-power LEDs replace linear or high-power CFL fluorescent lamps, HID lamps, and incandescent lamps. These applications require an LED driver with a typical power range of 25W to 150W. In many cases, the LED load consists of an array of high-brightness white light LEDs, usually in various forms of chip packaging. The DC current used to drive these loads is usually at least 1 ampere. There are also AC current-driven LED systems, but it is generally believed that DC systems can provide more ideal driving conditions for LEDs.

　　Current isolation is required in LED lighting to prevent the risk of electric shock in accessible areas, which is possible in most cases unless an insulating mechanical system is used. This is because unlike products such as fluorescent lighting that do not require insulation for safety, the LED chip needs to be connected to a metal heat sink. In order to achieve good thermal conductivity, a thermal barrier needs to be formed between the LED chip and the heat sink, which eliminates the need to meet the insulation requirements by adding insulating materials. Therefore, forming insulation inside the LED driver is the best choice, and it also proves that the power converter topology technology is feasible.

　　The two possible solutions are a flyback converter or a multi-stage converter including a PFC stage, followed by an isolation and buck stage, and finally a back-end current regulation stage. Of the two solutions, the flyback is more widely used due to its relative simplicity and low cost.

　　The flyback converter provides a good solution for many applications (Figure 1), however, it has the following limitations: limited power factor correction capability; limited efficiency over a wide input voltage range; output ripple at twice the line frequency (<150Hz) is difficult to eliminate; and additional circuitry is required for dimming.

　　Figure 1: LED dimming using a flyback converter. Although the additional cost of the multi-stage design (Figure 2) limits its application to high-end products, this design can overcome some of these problems. It can achieve high power factor and low total harmonic distortion (THD) over a wide AC output voltage range, allowing the same LED driver to be powered from a 110V, 120V, 220V, 240V or 277V mains supply.

　　Figure 2: LED dimming using a multi-level converter.

　　It is able to maintain high efficiency over a wide range, rather than having efficiency peak at a specific line load point but then drop off significantly under different conditions. It is also easier to reduce ripple output at 150Hz, and the multi-level system lends itself to more efficient use of different dimming methods.

　　The rest of this article will delve into the design principles of a wide voltage input range, isolated, dimmable, regulated DC output multi-level LED driver, primarily for applications in the 25W to 150W range. The multi-level LED driver in this example will be divided into three parts: the front end, the power factor correction (PFC) section; the isolation and step-down section; and the back end, the current modulation section.

　　The front end section consists of a boost converter configured with a power factor correction pre-regulator to provide a high voltage DC bus at the output which is regulated to a fixed voltage over a wide range of line or load variations. Since the voltage regulation control loop responds very slowly, many cycles of the AC line frequency are affected by load variations and it draws only a basic sinusoidal line input current. This circuit typically operates in critical conduction mode, otherwise known as transition mode. In this mode, the PWM off period and thus the switching frequency is varied so that a new switching period begins when all the energy stored in the boost inductor has been transferred to the output. This resonant mode of operation is widely used and achieves high efficiency as it minimizes switching losses. It is optimal to use this design over the specified power range.

　　The intermediate stage converts the high voltage DC bus voltage (typically around 475V) to a low voltage output suitable for driving the LED load. For safety reasons, the LED load is usually driven at a low voltage, so the drive circuit is usually a minimum of 1 amp. The recommended isolation and step-down stage configuration is a resonant half-bridge, consisting of a pair of switching MOSFETs driven with signals in anti-phase to each other. One end of the primary winding of the high-frequency step-down transformer is connected to the midpoint of the two switching transistors, and the other end is connected to a capacitor divider network from the DC bus to ground. In this way, the transformer primary sees a square wave with equal positive and negative voltage amplitudes. The secondary winding will be center-tapped so that two diode rectifiers can be used to convert the output current to DC. The output current is high enough that the rectifier diodes can be replaced by MOSFETs, thus operating as a synchronous rectification system. In a typical application with a current of 3 amps, the surface temperature of the synchronous MOSFET is lower than that of a Schottky diode in the same package at an ambient temperature of 30 degrees.

　　We can see that as the current requirement increases, the thermal advantage of synchronous rectification becomes more significant. Finally, a smoothing capacitor is required to produce an isolated, low-ripple DC voltage. The capacitance of this capacitor is in the order of tens of farads, so a ceramic capacitor is used.

　　To make the half-bridge stage more efficient, it should be designed to operate in resonant mode, where the MOSFETs switch under zero voltage (ZVS) conditions. To achieve this, there must be a short delay between the turn-off of one MOSFET and the turn-on of the other, and during this delay the voltage commutates from one rail to the middle of the other rail. This is because the energy in the inductor is released and conducted through the body diode in the MOSFET. In the primary design of the transformer, it is necessary to maintain sufficient leakage inductance so that more energy can be stored and energy exchange can be carried out. This makes the transformer design more complicated, and a simple way to avoid these problems is to use a standard high-frequency transformer design without adding additional leakage inductance to its design, and just add another inductor in parallel with the primary inductor to facilitate energy exchange. This additional inductance can also be used to help the dimming operation of the triac-based dimmer and provides additional cost and space for adjustment. We will discuss this further. Such an inductor can use a gapped core or open core to increase energy storage.

　　The back-end stage of the LED driver includes a current modulation circuit with short-circuit protection. This can be achieved with a linear modulation circuit, but this method alone is not sufficient, it is only suitable for low output currents and cannot be used in multi-stage systems. The alternative is a simple buck regulator circuit that uses current feedback to limit the output current of each LED that exceeds the target LED drive current. This can compensate for changes in the total LED forward voltage caused by temperature and device tolerances, while also limiting the current in the event of a short circuit or other fault, protecting the driver from damage.

　　The multi-channel approach can also be used when multiple output stages are connected to a separate isolated DC voltage supplied by the previous stage. In such a design, an output short circuit in one channel will not prevent the normal operation of other channels. It also allows the modulated current of several channels to be provided to different LED arrays and eliminates the need to connect parallel LED arrays. As we all know, if the LEDs do not have similar forward voltage drops under similar temperature conditions, then connecting LEDs in parallel will cause problems. At this time, the advantage of using a driver with multiple independent outputs is obvious. Disadvantages of 　　TRIAC dimmers

　　Most existing dimmers generally operate in a leading edge phase cutting fashion, using a very simple triac based circuit. These dimmers were originally designed to be used with incandescent lamps as a resistive load. A triac is a semiconductor switch that will conduct current in either direction between its two main terminals only after a pulse is applied to its third gate to trigger it. This pulse can be of either polarity and is therefore easily created using a basic RC timing circuit. The operating principle involves triggering the triac at a point in the AC line cycle such that it will conduct until the end of the cycle, when the line voltage drops to zero and the current through the triac will then also be zero, causing the triac to turn off again. The triac has a minimum rated holding current below which the switch will turn off. A potentiometer in the regulator circuit controls the point at which the triac in the regulator circuit turns on and changes the overall average AC current by achieving dimming.

　　However, even if they include a power factor correction front end, LED converters and other power supplies or electronic ballasts do not present a purely resistive load to the dimmer. When the dimming level is reduced, the triac in the dimmer may fire erratically or miss switching cycles. The factors that affect this performance are very complex, and since we have found a simple solution that can largely overcome this problem in multi-level systems, it is not necessary to analyze it in depth here.

　　Instead of returning the rectifier commutation inductor in the primary side of the step-down transformer to the midpoint of the capacitive divider, current can flow back to the line input through a DC blocking capacitor. This provides a small amount of additional current just before the AC line cycles through, which will turn the triac on and operate it within the required dimming range. This solution helps dimming through a triac-based dimmer by utilizing current that would otherwise be wasted. (Figure 3)

　　Figure 3: Front end and half-bridge with dimming charge pump.

　　Dimming in this way is feasible because as the dimming level is reduced, the output bus voltage of the front-end stage is also reduced. This causes the secondary voltage to drop as well, and since the LED load has a fixed total voltage drop, a small change in voltage will also cause a large change in current and light output. In this way, linear dimming of the LED is achieved, thereby meeting the requirements of more complex PWM dimming circuits and avoiding possible patent infringement. Although dimmer compatibility requires a certain loss of efficiency, multi-level configurations are still an excellent choice for higher performance LED driver designs.