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The Multi-Stage Offline LED Driver

Driving LEDs gets more complicated when you start to add features to the final product. The multi-stage configuration is a good option for LED driver designs where high performance is required.

Although it is relatively straightforward to design a basic LED driver for a general lighting application, it becomes far more complex when additional functionality, such as phase-cut dimming and power factor correction, is also required.

A non-dimming LED driver with no power factor correction (PFC) generally consists of an offline switching power supply configured to regulate the output at a constant current. This is not much different from a standard offline switching power supply, such as the types commonly used in AC-DC adapters. Such designs can be based on standard switched-mode power supply (SMPS) circuit topologies like buck, boost, or flyback converters.

On December 3, 2009, the US Department of Energy (DOE) released the final version of "ENERGY STAR Program Requirements for Integral LED lamps," which mandates that the power factor must be better than 0.7 for LED drivers for domestic applications. The requirement for industrial applications is expected to be better than 0.9. Many products currently on the market fail to meet these requirements and therefore more advanced designs are needed to replace them in the future. There are two basic approaches to power factor correction, each of which requires some additional circuitry at the front end of the converter: the simple low-cost passive PFC and the more complex active PFC. Before exploring these methods in greater depth, it should be mentioned that in order to gain ENERGY STAR rating, the LED driver must also be dimmable.

This generally means dimmable using existing wall dimmers, based on the phase-cut principle of operation originally designed to work with purely resistive incandescent lamps. Although other dimming methods such as linear 0 to 10 V dimming or Digital Addressable Lighting Interface (DALI) would presumably also qualify, they are likely to be limited to high-end industrial type LED drivers. Phase-cut dimmers are by far the most widely used, and it is clear that there would be a significant advantage for LED lamps to be able to be dimmed effectively by them. Since many low-cost, triac-based dimmers exist in the market, it is not practical for LED drivers to guarantee compatibility with all types, especially since many dimmers are of very basic design and limited performance. For this reason, the ENERGY STAR program requires only that the LED driver manufacturers specify on a web page which dimmers are compatible with their product.

Another ENERGY STAR requirement worth mentioning is that the LED operating frequency has to be greater than 150 Hz in order to eliminate the possibility of visible flicker. This means that the output current supplying the LEDs may not include any significant amount of ripple at twice the line frequency of 50 or 60 Hz.

The adoption of LED lighting in offline applications, such as office lighting, public buildings, and street lighting is increasing, and that increase is predicted to continue for the next few years. In these applications, high-power LEDs replace linear or high-power CFL fluorescent lamps, high-intensity discharge (HID) lamps such as metal halide and high-pressure sodium, as well as incandescent lamps. These applications require an LED driver, which typically ranges from 25 W to 150 W. In many cases the LED load comprises an array of high-brightness white LEDs, often packaged in multiple die form. The DC current required to drive these loads is often at least one ampere. AC current-driven LED systems also exist, but DC systems are generally considered to provide more optimal driving conditions for LEDs.

In LED light fixtures, galvanic isolation is required to prevent electric shock risk where LEDs are accessible which is the case in most situations unless a mechanical system of isolation is employed. This is because unlike fluorescent light fixtures that do not need to be isolated for safety, the LED die need to be connected to a metal heatsink. For good thermal conductivity, a thermal barrier between the LED die and the heatsink is necessary, which precludes the possibility of adding insulating material in between that would be thick enough to satisfy isolation requirements. Therefore, it is the best option to provide isolation within the LED driver itself, and this dictates the power converter topologies that are suitable. The two possibilities are the flyback converter or a multi-stage converter that includes a PFC stage, followed by an isolation and step down stage, and finally a back-end current regulation stage. Of the two, the flyback is the more popular due to its relative simplicity and low cost (See Figure 1). The flyback converter offers a good solution for many applications; however, it has the following limitations:
  1. Limited power factor correction ability.
  2. Limited efficiency over a wide input voltage range.
  3. Output ripple at twice the line frequency (<150 Hz), which cannot be easily eliminated.
  4. Additional circuitry is required for dimming.

Figure 1: Simple diagram of a flyback converter.

The multi-stage design can overcome some of these problems, although its additional cost limits its adoption in higher-end products (See Figure 2). High-power factor and low total harmonic distortion (THD) can be achieved over a wide AC input voltage range, allowing the same LED driver to operate from a 110 V, 120 V, 220 V, 240 V, or 277 V mains supply. Efficiency can be maintained over this range, rather than peaking at a specific line load point and dropping off significantly under different conditions. It is also much easier to minimize output ripple under 150 Hz, and the multi-stage system lends itself more effectively to the different methods of dimming.

Figure 2: Simple diagram of a multi-stage converter.

The remainder of this article will discuss, in detail, the design of a wide input voltage range, isolated, dimmable, regulated DC output, multi-stage, LED driver design concept intended for applications in the 25 W to 150 W range. The multi-stage LED driver in this example will be broken down into three sections:
  1. The front end, PFC section.
  2. The isolation and step down section.
  3. The back end, current regulation section.
The front end section consists of a boost converter. It is configured as a power factor correcting pre-regulator that delivers a high-voltage DC bus at the output regulated to a fixed voltage over variations in line or load. Since the regulating control loop response is slow, and takes many cycles of AC line frequency to react to line load changes, it draws an essentially sinusoidal line input current. This circuit typically operates in critical conduction mode, otherwise known as transition mode. In this mode the Pulse Width Modulation (PWM) off period, and therefore the switching frequency, is variable such that the new switching cycle begins at the point when all of the energy stored in the boost inductor has been transferred to the output. This resonant mode of operation is widely used and offers high efficiency due to minimal switching losses. It is the best approach to use in the power range required.

The middle stage converts the high-voltage DC bus voltage (typically around 475 V) to a low-voltage output suitable for driving LED loads. For safety reasons, LED loads are normally driven from low voltage and therefore drive current is often at least one ampere. The configuration of the isolation and step down stage recommended here is a resonant half bridge consisting of a pair of switching MOSFETs driven in anti-phase with each other. The midpoint of these switches supplies one end of the primary winding of a high frequency step down transformer, and the other end is connected to a capacitive divider network from the DC bus to the zero volt return. In this way, the transformer primary sees a square wave voltage of equal positive and negative amplitude. The secondary winding will be center-tapped in order that a two-diode rectifier can be used to convert the output back to DC.

Where the output current is sufficiently high, the rectifying diodes can be replaced with MOSFETs operating as a synchronous rectification system. In a typical application running at three amperes, the surface temperature of synchronous MOSFETs was measured at 30°C lower than Schottky diodes with the same package. It can be seen that as the current requirement increases, the thermal benefits of synchronous rectification become very significant. Finally, a smoothing capacitor is required to produce an isolated DC voltage with low ripple. This can be on the order of tens of microfarads, and therefore, ceramic capacitors can be used.

In order for the half bridge stage to operate efficiently, it should be designed to operate in resonant mode where the MOSFETs switch at zero voltage. This is accomplished by ensuring that there is a short delay between the time when one MOSFET switches off and its counterpart switches on, and that during this time the voltage at the midpoint commutates from one rail to the other. This happens due to the release of energy stored in the inductor conducting through the integral body diodes of the MOSFETs. The primary of the transformer must possess sufficient leakage inductance for sufficient energy to be stored, allowing commutation to take place. This makes the transformer design rather complicated. One easy way to get around this difficulty is to use a standard high-frequency transformer design without additional leakage inductance added into its design, and to simply add another inductor in parallel with the primary solely to facilitate commutation. This extra inductor can also be used to aid dimming operation from triac-based dimmers, therefore adding justification for the extra cost and space. This will be discussed later. Such an inductor can be built around a gapped or open core to facilitate energy storage.

The back end stage of the LED driver consists of a current regulating circuit with short circuit protection. This can be realized with a linear regulating circuit; such an approach is inherently inefficient and therefore only suitable for low output currents, which will not generally apply in a multi-stage system. The alternative is a simple buck regulator circuit with a current feedback to limit the output current from exceeding the intended LED drive current. This compensates for variations in total LED forward voltage over temperature and device tolerance, and limits the current in the event of a short circuit or other fault condition, thereby protecting the driver against damage.

A multi-channel approach is also possible, where several output stages are connected to a single isolated DC voltage supplied by the previous stage. This is advantageous because with such an arrangement, a short circuit at the output of one of the channels would not prevent the other channels from working normally. Furthermore, it allows several channels of regulated current to supply different LED arrays and avoids the need for connecting LED arrays in parallel. It is well known that connecting LEDs in parallel is problematical unless the LEDs are of similar forward voltage drop and operating at similar temperature. Therefore, the advantage of a driver with multiple independent outputs is apparent.

The drawbacks of using triac-based dimmers

Most commonly available dimmers operate by means of leading-edge phase cutting using a very simple circuit based on a triac. These dimmers were originally designed to work with incandescent light bulbs, which are purely resistive loads. The triac device is a semiconductor switch that conducts current in either direction between its two main terminals only after it has been fired by a pulse applied to the third gate terminal. This pulse can be of either polarity and is therefore simple to create with a basic RC timing circuit. The principle of operation consists of firing the triac at a point in the AC line cycle so that it conducts until the end of the cycle, at which point the line voltage drops to zero and consequently, so does the current flowing through the triac, causing it to switch off again. Triac devices have a minimum rated holding current below which they will switch off. Adjusting a potentiometer in the circuit controls the firing point of the triac in the dimmer circuit and changes the overall average AC current passed through, enabling dimming.

LED converters and other power supplies or electronic ballasts, however, do not represent a purely resistive load to the dimmer even when they include a power factor correcting front end. The triac in the dimmer therefore tends to fire erratically and miss cycles when the dimming level is lowered. The factors that influence this behavior are quite complicated and it is not necessary to go into a deep analysis, since a simple solution has been found that can overcome the problem to a large extent in the multi-stage system.

Instead of returning the commutating inductor from the primary side of the step down transformer to the midpoint of the capacitive divider, the current can be fed through a DC blocking capacitor back to the line input. This provides a small amount of additional current which will keep the triac from switching off before the end of the AC line cycle, and allow it to operate as required over the range of dimming. This solution uses current that would otherwise be wasted to facilitate dimming using triac-based dimmers (See Figure 3).

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

Dimming in this way works because as the dimmer level is reduced the output bus voltage from the front end stage also falls. This results in the secondary voltage dropping. Since LED loads have a fixed total voltage drop, a small variation in voltage causes a large variation in current and therefore light output. In this way, linear dimming of LEDs is realized, circumventing the need for more complicated PWM dimming circuitry, as well as avoiding possible patent infringement.

Although dimmer compatibility necessitates some loss in efficiency, the multi-stage configuration remains a good option for LED driver designs where high performance is required.