How to Overcome High Efficiency LED Lighting with a Flyback Converter

This document addresses the use of high power factor flyback converters to overcome the challenges of high efficiency LED lighting solutions. It will include the rationale and reasons why designers prefer this topology over other types. The requirements to produce valid output will also be briefly discussed.

This document addresses the use of high power factor flyback converters to overcome the challenges of high efficiency LED lighting solutions. It will include the rationale and reasons why designers prefer this topology over other types. The requirements to produce valid output will also be briefly discussed.

This article discusses the challenges of using high power factor flyback converters to overcome high efficiency LED lighting solutions. It will include the rationale and reasons why designers prefer this topology over other types. The requirements to produce valid output will also be briefly discussed.

A flyback converter is one of the best topologies for switching power supplies and is also very cost-effective. In addition, LED lighting applications with AC input also require high power factor and high system efficiency. This article reviews the design challenges of high-performance LED lighting products, and then demonstrates how these requirements can be met using a new generation of highly integrated PWM controllers.

Primary Side Flyback Controller

A single-stage topology with primary-side regulation (PSR) is the preferred topology for LED lighting applications because it eliminates input bulk capacitors and feedback circuits, allowing designs with minimal external components. Aside from cost, another benefit of eliminating input capacitors is the elimination of components that have a lower lifespan than others. In addition, some energy efficiency standards require LED lighting panels to meet a high power factor (PF) greater than 0.9 and a low total harmonic distortion ( PF ) of less than 20%. Therefore, a highly integrated >span class=”caps”>PWM controller with constant on-time, fixed frequency control should be used to achieve the most simplified circuit design and at the same time satisfy excellent PF/THD performance. As shown.

How to Overcome High Efficiency LED Lighting with a Flyback Converter
Typical application circuit of PSR PWM controller FL7733A

Tight LED current regulation is another important requirement for LED lighting. A highly integrated PWM controller should implement precise constant current control to maintain accurate output current versus changes in input and output voltages. Since the output current is the average value of the output diode current in steady state, the output current can be estimated using the peak drain current of the MOSFET and the discharge time of the Inductor current. This output current information is compared to an internal precision reference to generate an error voltage that determines the duty cycle.

Typically, for PSR, DCM operation is preferred as it provides better output regulation. The PWM controller will need to vary its operating frequency linearly with respect to the output voltage to ensure DCM operation. One way to obtain output voltage information in a PSR topology is to sense the auxiliary winding voltage through a resistor divider connected to the VS pin. When the output voltage decreases, the conduction time of the secondary diode increases, and the linear frequency control function of the PWM controller prolongs the switching period. Frequency control also reduces primary rms current, improving power supply efficiency.

For stable operation, the PWM controller should also provide protection functions such as LED open circuit, LED short circuit and overheat protection. An important requirement is that the current limit level is automatically reduced to minimize output current and protect external components during shorted LED conditions.

Board Evaluation

The highly integrated PWM controller FL7733A can meet all the above requirements and can provide the most simplified design for LED lighting applications. A 20W rated LED lighting power strip was chosen to evaluate the FL7733A with the Fairchild SuperFET® 2MOSFET. SuperFET® 2MOSFETs are the latest generation of superjunction technology. In addition to low on-resistance, the SuperFET® 2MOSFET also achieves less stored energy in the output capacitor (Eoss). Eoss is important for low power switching LED lighting solutions because energy dissipation occurs during each switching cycle.

How to Overcome High Efficiency LED Lighting with a Flyback Converter
Power Factor and Total Harmonic Distortion of 20W LED Converter Based on FL7733A

Figure 2 shows the PF and THD results when using the rated LED load 10 minutes after startup. The measured solution exceeded the standard with PF greater than 0.98 and THD performance less than 10%.

How to Overcome High Efficiency LED Lighting with a Flyback Converter
System Efficiency of MOSFETs

Figure 3 shows the efficiency test results for various AC inputs. SuperFET®2 technology shows the best efficiency over the entire input range. Better results at high input voltages are a good example of how energy stored in the output capacitor affects system efficiency. Since competitor MOSFETs have the same on-resistance as SuperFET® 2 MOSFETs, it can be assumed that the efficiency gap is caused by switching losses.

How to Overcome High Efficiency LED Lighting with a Flyback Converter
energy stored in the output capacitor

As shown in Figure 4, as the drain-source voltage increases, the competitor MOSFET holds more energy in the output capacitance. This means that more power is dissipated during turn-on at higher input voltages. In Figure 3, the device-level characterization exactly matches the board-level test results.

in conclusion

LED lighting power supplies require high power factor, high efficiency, an isolated secondary side to meet safety standards, and fewer components due to space constraints. Together with the SuperFET® 2MOSFET, the FL7733A provides a complete solution to meet these requirements.

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