LED lamps and bulbs are now rapidly replacing incandescent lamps, halogen lamps and CFL (miniature fluorescent lamps) light sources in many general lighting applications. Most of the LED / DC converters are able to achieve the isolation between the DC power supply and the DC power supply.

Almost all direct change led bulbs have a large aluminum heat sink, the shape should be consistent with the design, there are many fins to expand the surface area. High brightness LED has high heat, so it must be dispersed into the surrounding air to prevent overheating and prolong its service life.

Although the LEDs themselves are inaccessible, they usually remain electrically connected to the heat sink because any isolation between the two acts as a thermal barrier. Designs with isolators require thinner fins to reduce this barrier, but do not provide reliable electrical isolation. Therefore, engineers usually prefer isolated flyback drive circuit instead of simple but non isolated buck structure. Flyback LED driver also has the ability of simple, low cost and high power factor, and can be compatible with triac (three terminal alternating current) dimmer by adding some circuits.

Design of LED driver circuit for flyback DC / DC converter

Figure 1, the core component of flyback LED driver circuit is a coupling inductor

The core component of flyback LED driver circuit is a coupling inductor (Fig. 1). High voltage MOSFET is used to switch the inductor primary on DC bus. When the switch is on, the current in the inductor rises and the energy is stored in the form of magnetic field. Therefore, an air gap is needed for the inductor core. Switching of the mosft interrupts the primary current; therefore, the current must flow into the secondary winding rather than through the diode and into the output capacitance and load. During this period, the energy in the inductor is transferred to the output. Because the current does not flow to the output when the MOSFET is on, the output needs a storage capacitor to provide continuous current for the LED.

The turn ratio of the inductor makes the transformer neither buck nor boost, but must consider the reflected voltage on the primary winding when the MOSFET is turned off. The voltage at the drain of the MOSFET shall not exceed its maximum rated drain source voltage at peak line voltage and the maximum LED output voltage. This voltage is equal to the DC bus voltage plus the LED output voltage, and then multiplied by the turn ratio, which is the reflected voltage. For a 120V AC circuit, the MOSFET should have 400V voltage; for 277v AC or wide input range circuit, MOSFET should have 650V voltage. Under these voltages, practical inductors with less secondary turns can be designed.

Flyback converter continuously stores and transmits energy through inductors. Therefore, the inductor only works in one quadrant on the curve of flux density and magnetic field intensity. Therefore, the core must be large to transmit the power provided by other more complex power structures, which use the core more efficiently. The flyback scheme is more suitable for power levels less than 50W, which covers all screw direct switching LED bulb products, as well as many spotlights and floodlights (Figure 2). Flyback designs can also operate at higher power levels; however, these designs are more complex, usually using multiple inductors and MOSFET interleaving circuits.

Design of LED driver circuit for flyback DC / DC converter

Figure 2, flyback scheme is most suitable for applications with power consumption less than 50W, covering all screw port direct conversion LED bulb products, as well as many spotlights and floodlights

With the performance standards gradually covering LED lighting products, environmental issues such as high power factor have become a requirement. Flyback LED driver can provide a power factor of about 0.9. It adopts passive circuit technology, without any pre adjustment stage which will significantly increase the cost and volume.

In order to provide high power factor, flyback circuit can be run from a full wave rectified DC bus with only a few capacitors for high frequency coupling, or a simple passive valley filling circuit consisting of two capacitors and three diodes can be added (Fig. 3). The first method is relatively cheap, but a large holding capacitor is required at the output to prevent the LED current from falling to the zero crossing point close to the AC line. Therefore, this method is only feasible when the LED is 350mA or less. The second method is more commonly used, it increases some costs, but overcomes the limitations of the first method.

Design of LED driver circuit for flyback DC / DC converter

Figure 3 shows that in order to provide high power factor, the flyback circuit can be operated from a full wave rectifier DC bus with only one small capacitor as high frequency coupling, or a simple passive valley filling circuit composed of two capacitors and three diodes can be added

The next problem to consider is how to adjust the LED current. Using a secondary voltage and current detection circuit, using an optocoupler to send the feedback signal back to the control IC at the primary end, this adjustment can be achieved. Another way is to adjust the peak current at the primary end in MOSFET, instead of directly detecting the voltage or current of the LED. Another option is to use a primary detection method, which provides some current regulation and over-voltage protection, but does not require optocouplers.

Using secondary voltage and current detection circuit is the most accurate method, but it requires the use of optocoupler and an output detection and voltage stabilizing circuit, all of which will affect the space and cost. Adjusting the peak current of primary terminal in MOSFET saves a lot of components, but the control precision is low. Only under certain line input and LED output voltage can the correct output current be provided. Although this solution may be acceptable for some low-end applications, it does not provide protection against open circuit conditions. If the load is open, the output of a flyback converter may generate a high voltage, for example, when one of the LEDs in a string fails to open, because the voltage will continue to rise until the inductor can release its stored current.

Now, manufacturers have adopted primary detection method in intelligent flyback control IC, which can detect the current and voltage at the primary end of the circuit, and determine the output current with an algorithm without directly detecting it. The LED Driver with this controller can provide a stable output current in a range of input voltage variation, but its output still needs to be set for certain number of LEDs because it can not adjust the voltage change. Such controllers may also contain circuits to detect open circuit conditions, thereby limiting the output voltage. This method is more accurate than the method of adjusting the peak current at the primary end of MOSFET, because the controller has higher complexity, but it is still weaker than the secondary voltage and current detection circuit using optocoupler.

The flyback driver in the LED bulb can use any PFC technology. However, the current trend is for users to use triac dimmers installed. This scheme adds more complexity to the design of LED driver. Triac dimmers generally work poorly on capacitive loads such as solid-state power converter circuits, because when triac is ignited, only when the current remains above a predetermined threshold, can the current continue to turn on. In the LED driver, some additional circuits are generally needed to ensure this activity. Without these additional circuits, the ignition of the triac would be irregular, causing flicker.

After solving this problem, the LED driver must be able to adjust the output current of LED according to the position of dimmer. The most basic circuit depends on the bus voltage drop when the dimmer level drops to reduce the output current. However, the performance provided by this scheme is limited and can only be used for a part of the adjustment range of the dimmer. Perhaps, it is more significant to design a better dimmer that can work with LED driver than to design more complex LED driver and make it adapt to the dimmer originally used in fluorescent lamp. Although the scheme seems to have a technical logic, the market is not adopting this direction now.

Now many designs provide good dimming control. The way is to add triac ignition angle detection circuit, convert it into a DC control voltage, and then adjust the output current accordingly. However, such schemes now require many components because they adopt the method of adjusting the primary terminal peak current in MOSFET, which usually requires many optocouplers. As a result, the price of these products is at least $30. The next generation of dimmable flyback design is likely to adopt primary detection methods, provided that new and more intelligent control ICs enter the market.

In addition to floodlights and spotlights, flyback LEDs can also be used as alternatives to fluorescent lamps. They look similar, but LEDs have higher lumens per watt and longer life (Figure 4). For example, you can concatenate LEDs into long chains to act as a continuous light source. In the figure, the product with 24W led replaces 32W T8 fluorescent lamp. At this level, flyback design provides the best option for low-cost drivers, meeting both security and performance requirements.

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