By Wonseok Kang, Principal Technical Marketing Engineer at Fairchild
A flyback converter is one of the best topologies for a switching power supply that is also cost effective. Additionally, LED lighting applications from AC input also requires high power factor and high system efficiency. This article reviews the challenges of designing for high performance LED lighting products and then demonstrates how these requirements can be met with a new generation highly integrated PWM controller.
Primary-side flyback controller
Single stage topology with primary-side regulation (PSR) is the topology of choice for LED lighting applications because it enable designs with the fewest external components by eliminating the input bulk capacitor and feedback circuitry. Besides costs, the additional benefit to eliminating the input capacitor is eliminating a component with lower operating life than other components. Furthermore, some energy efficiency standards require a LED lighting board to meet high power factor (PF) greater than 0.9 and low total harmonic distortion (THD) less than 20%. Therefore, a highly integrated PWM controller with constant on-time, fixed frequency control should be utilized to achieve the most simplified circuit design and meet excellent PF/THD performance at the same time. Fig. 1 shows the typical application circuit of a highly integrated PSR PWM controller.
Tight LED current regulation is another important requirement for LED lighting. A highly integrated PWM controller should implement precise constant-current control function to maintain accurate output current versus changes in input voltage and output voltage. The output current can be estimated by using the peak drain current of MOSFET and discharging time of inductor current, since output current is the average of the output diode current in steady state. This output current information is compared to internal precise reference to generate error voltage which determines the duty cycle. The constant output current can be precisely controlled as below.
Generally, DCM operation is preferred for PSR because it allows better output regulation. The PWM controller will need to change its operating frequency linearly in relation to the output voltage to guarantee DCM operation. One way to obtain output voltage information in PSR topology is by sensing the auxiliary winding voltage via the resistive divider connected to the VS pin. When output voltage decreases, secondary diode conduction time is increased and the linear frequency control feature of the PWM controller makes switching period longer. The frequency control also lowers primary rms current that results in better power efficiency.
For robust operation, the PWM controller should also provide the protection functions such as open LED, shorted LED and over temperature protection. One important requirement is that the current limit level is automatically reduced to minimize the output current and protect external components in the shorted LED condition.
Board level evaluation
The highly integrated PWM controller, FL7733A can meet all the aforementioned requirements and enable the most simplified design for LED lighting application. A 20W rated LED lighting power board is selected to evaluate the FL7733A together with Fairchild SuperFET®2 MOSFET. SuperFET®2 MOSFET is the latest generation super-junction technology. In addition to low on-resistance, the SuperFET®2 MOSFET also achieves less stored energy in output capacitance (Eoss). The Eoss is important for low power switching LED lighting solutions, because the energy dissipation occurs every switching cycle.
Figure 2 shows PF and THD results using rated LED load at 10 minutes after startup. The measured solution exceeds the standards with PF greater than 0.98 and THD performance less than 10%. Fig. 3 is efficiency test results with various AC inputs. The SuperFET®2 technology shows best efficiency over entire input range. The better result at high input voltage is good example of how stored energy in output capacitance affects system efficiency. Because the competitor MOSFET has same on-resistance to the SuperFET®2 MOSFET, the gap in efficiency can be considered as coming from switching losses. As shown in Fig. 4, the competitor MOSFET holds more energy in output capacitance as drain-source voltage increases. This means it dissipates more power during switching-on at higher input voltage. In Fig. 3, device level characteristics are well matched to board level test results.
The LED lighting power supply requires high power factor, high efficiency, isolated secondary-side to meet safety standard, and less components due to limited space. The FL7733A together with SuperFET®2 MOSFET provide complete solution to these requirements.
Fig. 1 Typical application circuit of PSR PWM controller FL7733A
Fig. 2 Power factor & total harmonic distortion for a 20W LED converter based on FL7733A
Fig. 3 System efficiency by MOSFETs
Fig. 4 Stored energy in output capacitance
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