Flyback LED drivers offer superior balance among operating tradeoffs

LED-based light fixtures and bulb replacements are now rapidly replacing incandescent, halogen and CFL light sources in many general-lighting applications. The flyback DC/DC converter is the power-supply topology of choice for large segment of the LED driver market. The primary reason for this is that the flyback design allows electrical isolation between the LEDs and the AC line, which is a safety requirement in most LED lamps.

Almost all LED-based light-bulb replacements include a large, aluminum heat sink shaped to fit the design, with many fins to maximize the surface area. High-brightness LEDs generate heat which must be radiated out into the ambient air to prevent overheating and reduced life.

Although the LEDs themselves are not accessible, they are often electrically connected to the heatsink, since any insulator between the two imposes a thermal barrier. If insulators are used, they need to be very thin in order to minimize this, and in so doing cannot offer reliable electrical isolation. This explains why the isolated flyback-driver circuit is frequently favored over the simpler but non-isolated buck topology.

Other benefits of the flyback LED driver are simplicity, low cost, the ability to achieve a high power factor and, with some additional circuitry, compatibility with common TRIAC-based dimmers.

Figure 1 shows the basic elements of a flyback LED driver. The core element of the circuit is a coupled inductor, sometimes incorrectly referred to as the transformer. A single high-voltage MOSFET switches the primary of the inductor across the DC bus.

Figure 1: Basic flyback LED driver

When the switch is on, current rises in the inductor and energy is stored in the magnetic field. In order for this to happen, the inductor cores require an air gap. When the MOSFET switches off, the primary current is interrupted and therefore current must flow in the secondary winding instead of through the diode and into the output capacitor and load.

During this phase, the energy stored in the inductor is transferred to the output. Since current does not flow to the output when the MOSFET is on, a storage capacitor at the output is necessary to provide continuous current in the LEDs.

The inductor turns ratio does not provide a step-down or step-up function as in a transformer; instead, the ratio must be derived by considering the reflected voltage that appears at the primary winding during the period when the MOSFET is off.

The voltage appearing at the drain of the MOSFET must not exceed its maximum VDS rating under conditions of peak line voltage and maximum LED output voltage. This voltage is equal to the DC bus voltage plus the LED output voltage, multiplied by the turns ratio, which is the reflected voltage. For a 120 VAC circuit, the MOSFET should be rated to 400V and for a 277VAC or wide input range circuit the MOSFET should be rated to 650V. This allows a practical inductor design that does not require too many turns on the secondary.

Since the flyback converter continually stores and transfers energy through the inductor, the inductor is used in only one quadrant of the B-H curve. This means that the core needs to be larger to transfer a given power than it is in some other more-complex power-supply topologies which use the cores more efficiently.

For this reason, the flyback approach is most suited to power levels below 50 W, which covers all screw-in, LED-based light-bulb replacement products as well as many downlights and luminaires. In fact, flyback designs can also be used at higher power levels; however, these become more complicated and often use multi-inductor and MOSFET-interleaved circuits.

As performance standards are introduced to cover LED lighting products, environmental considerations such as high power factor become requirements as well. The flyback LED driver is capable of providing a power factor around 0.9 using passive circuit techniques, without the need for any pre-regulating stage which would add significantly to the cost and size.

There are two main methods used to provide a high power factor. The first is to run the flyback circuit from a full-wave rectified DC bus, with only a small capacitor for high-frequency coupling. The second is to add a simple, passive valley-fill circuit consisting of two capacitors and three diodes, Figure 2.

Figure 2: Passive valley-fill circuit

The first method is cheaper but requires a larger hold-up capacitor at the outputs to prevent the LED current dropping out close to the AC-line zero crossings. For this reason this method is not feasible except when the LED current is low (350 mA or less). The second method adds some cost but overcomes this limitation and is widely used.

The next important issue to consider is how the LED current is to be regulated. This can also be done in several ways:

1. Using a secondary-voltage and current-sense circuit with an optoisolator, to transfer the feedback signal back to the primary-side control IC.
2. Regulating the primary-side peak current in the MOSFET only, and not directly sensing the LED voltage or current.
3. Using a smart primary-sensing method that provides some current regulation and overvoltage protection, but without the need for an optoisolator.

•The first method is the most accurate, but it requires the use of an optoisolator coupled with an output-sensing and regulation circuit, all of which impacts space and cost.

•The second method eliminates a significant number of components but offers a less-accurate form of control, which can only provide the correct output current to the LEDs at a specific line input and LED output voltage. Although this may be acceptable in some low-end applications, it offers no protection against an open-circuit condition. The output of a flyback converter can produce high voltages if the load becomes open circuit, for example, if one LED in the chain fails in an open-circuit state. This is because the energy stored in the inductor needs to discharge and so the voltage continues to rise until it can do so.

•The third method is now being introduced in the form of smart flyback-control ICs capable of sensing the current and voltage at the primary side of the circuit, and using an algorithm to determine the output current without needing to sense it directly. An LED-driver based on one of these controllers can provide a regulated output current over some range of input-voltage variation, although it still needs to be set to operate for a specific number of LEDs at the output since it cannot adjust for voltage variations.

Such controllers can also include circuitry for detection of an open-circuit condition and thereby limit the output voltage. This method is more accurate than the second method with all the added complexity integrated into the controller, but is still less accurate than the first method.

A flyback driver used in an LED-based lightbulb replacement could use any combination of the above PFC techniques and control methods. However, the trend in the market now is towards products that can be dimmed from existing TRIAC-based dimmers. This adds another degree of complexity to the LED driver design. The triac-based dimmer generally does not work well with capacitive loads such as solid-state power-converter circuits.

This is because the TRIAC device, once fired, continues to pass current only while the current remains above a defined threshold. In LED drivers, some additional circuitry is usually necessary in order to guarantee that this is the case. Without it, the TRIAC tends to fire erratically, which results in flickering.

After addressing this issue, it is then necessary to enable the LED driver to adjust the LED output current depending on the dimmer position. The most-basic circuit simple relies on the drop in bus voltage as the dimmer level is lowered to provide reduction in output current. However, this produces limited performance and operates only over a portion of the adjustment range of the dimmer.

It would probably make a lot more sense to design better dimmers intended to work with LED drivers, as an alternative to designing more-complicated LED drivers to work with dimmers that were originally intended for incandescent lamps. However, while it appears that this approach seems more logical technically, the market is not going that route at present.

Many designs now produce very good dimming control by adding some additional circuitry that detects the firing angle of the TRIAC and converts it to a DC control voltage, which is then used to adjust the output current accordingly. However, such implementations currently contain a high component count because they use the first control method described above, often requiring multiple optoisolators.

This explains why such products end up costing $30 or more. It is likely that the next generation of dimmable flyback-based designs will use the third control method as new and more intelligent control ICs come on to the market.

As noted earlier, flyback-based LED drivers are also used in many luminaires and downlights since the power levels required for these generally fall below 50 W, Figure 3.

Figure 3: A typical LED replacement lamp

LEDs are also used as fluorescent-tube replacements, which actually look very similar but deliver higher lumens per watt and longer life. Figure 4 shows an example where many small LEDs in series have been used arranged in a long chain, to give the appearance of a continuous light source. This 24 W LED-based product replaces a 32 W T8 fluorescent lamp. At this level, a flyback design provides the best option for a low-cost driver that complies with safety and performance requirements.

Figure 4: An LED tube replacement.

Compatibility with TRIAC dimmers is generally not necessary in this type of LED lighting, which often operates using a 0-to-10 V analog-dimming control or digital-control schemes such as DALI  (Digital Addressable Lighting Interface, see http://www.dali-ag.org/index.php?n=w ) in more advanced applications. This eliminates many of the problems associated with dimming and allows more precise control of the light output, since both PWM and/or linear dimming can be incorporated.

About the author

Peter B. Green is the LED Group Manager at International Rectifier Corp.(El Segundo, CA), responsible for LED driver-IC product-line definition and specifications, where he has worked for 10 years. He has a BSc (Electrical Engineering) from Queen Mary College, University of London.

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