FairchildFL77308.4W dimming LED driver solution

Fairchild’s FL7730 is a highly integrated PWM controller that provides enhanced performance for single-stage flyback converters. Its proprietary TRUECURRENT™ topology simplifies the circuit design of LED lighting. Application voltage 80VAC ~ 308VAC, starting current 20uA, working The current is 5mA, and the frequency hopping has better EMI performance. It is mainly used in LED lighting systems. This article introduces the main characteristics of FL7730, block diagrams, typical application circuits and the use of FL7730 TRIAC dimming 180V-265VAC and 90-140VAC LED driver reference Design the circuit diagram.

FL7730MY: Single-Stage Primary-Side-Regulation PWM Controller for PFC and LED Dimmable Driving

This highly integrated PWM controller, FL7730MY, provides several features to enhance the performance of single-stage flyback converters. The proprietary topology, TRUECURRENT™, enables the simplified circuit design for LED lighting applications.

TRIAC dimming is smoothly managed by dimming brightness control without flicker. By using single-stage topology with primary-side regulation, an LED lighting board can be implemented with few external components and minimized cost. It does not require an input bulk capacitor or feedback circuitry . To implement good power factor and low total harmonic distortion, constant on-time control is utilized with an external capacitor connected to the COMI pin.

Precise constant-current control regulates accurate output current versus changes in input voltage and output voltage. The operating frequency is proportionally changed by the output voltage to guarantee Discontinuous Conduction Mode (DCM) operation with higher efficiency and simpler design. The FL7730MY provides protections such as open-LED, short-LED, and over-temperature protections. Current-limit level is automatically reduced to minimize output current and protect external components in a short-LED condition.

The FL7730MY frequency-hopping function in the oscillator improves EMI performance. The FL7730MY controller is available in an 8-pin SOP package.

Main features of FL7730:

 Compatible with Traditional TRIAC Control (No need to change existing lamp infrastructure: wall switch & wire)

 Compatible with Non-Dimming Lamp Designs

 Cost-Effective Solution without Input Bulk Capacitor and Feedback Circuitry

 Power Factor Correction (PFC)

 Accurate Constant-Current (CC) Control, Independent Online Voltage, Output Voltage, Magnetizing Inductance Variation

 Line Voltage Compensation for CC Control

 Linear Frequency Control for Better Efficiency and Simple Design

 Open-LED Protection

 Short-LED Protection

 Cycle-by-Cycle Current Limiting

 Over-Temperature Protection with Auto Restart

 Low Startup Current: 20μA

 Low Operating Current: 5mA

 Frequency Hopping for Better EMI Performance

 SOP-8 Package Available

 Application Voltage Range: 80VAC ~ 308VAC

FL7730 application:

 LED Lighting System
FairchildFL77308.4W dimming LED driver solution
Figure 1. FL7730 block diagram
FairchildFL77308.4W dimming LED driver solution
Figure 2. FL7730 typical application circuit diagram

TRIAC dimming 8.4W LED driver reference design using FL7730

A LED has become a promising light source for replacing conventional lighting systems, such as fluorescent and incandescent lights. Especially in the conventional TRIAC dimmer infrastructure, there has been much research into development of an LED bulb compatible with TRIAC dimmers. Because the incandescent light source consumes a hundred watt with short life time, an LED bulb can be the excellent substitute with considerably less power dissipation and longer life.

The biggest recent issue of TRIAC dimmable LED bulb is dimmer compatibility. The conventional TRIAC dimmer was originally designed to handle hundreds of watts induced by incandescent bulbs. An LED bulb consuming less than 20W should interact with those dimmers composed of high-power devices. If the interaction between dimmer and LED bulb is not stabilized, visible flicker is perceptible.

To manage the interaction without flicker, some requirements for dimmer operation need to be considered.

TRIAC dimmer needs latching current at firing and holding current during TRIAC turn-on after firing. If those two currents are not met, TRIAC dimmer misfires and LED light flickers. The TRIAC dimmer blocks input line in the beginning of line cycle, then connects input line and LED bulb after firing. The TRIAC dimmer turns off if latching or holding current flowing through the dimmer is inadequate. The latching and holding currents are different from dimmer models. The typical range of latching and holding currents is around 5 ~ 50mA. Those operating requirement do not cause problems using incandescent bulbs due to high power consumption. An LED bulb with less than 20W output power cannot maintain this amount of current over the whole line cycle.

This application note provides a practical guideline of TRIAC dimmable LED bulb board design. Passive and active bleeder design guides detail how to maintain latching and holding current without visible flicker. Active damper design improves efficiency by minimizing the count of external components. The input filter design section covers the effect of filter components on PF, THD, and EMI.

The following user guide supports the demonstration kit for the FL7730.

Main features of TRIAC dimming LED driver reference design:

Compatible with traditional TRIAC control

Cost effective solution without input bulk capacitor and feedback circuitry

Line voltage compensation

Linear frequency control for better efficiency and easy design

Constant current regulation vs output voltage change (10~28V): <+/- 3.1%

Constant current regulation vs line voltage change (180~265Vac): <+/- 1.9%

Output open & short circuit protection with auto restart

Flicker-free TRIAC dimming control

System efficiency up to 84.5%

PF and THD at nominal voltages (220 ~ 230Vac): PF(>0.92), THD(<18%)

All components’ temperature (at room temp.): less than 55ºC

TRIAC dimming LED driver reference design indicators:
FairchildFL77308.4W dimming LED driver solution
FairchildFL77308.4W dimming LED driver solution
Figure 3. TRIAC dimming LED driver reference design circuit diagram
FairchildFL77308.4W dimming LED driver solution
Figure 4. Transformer block diagram of TRIAC dimming LED driver reference design
FairchildFL77308.4W dimming LED driver solution
Figure 5. TRIAC dimming LED driver reference design circuit diagram (90-140VAC)

Figure 6. TRIAC dimming LED driver reference design circuit diagram (90-140VAC) transformer block diagram

For details, see:
http://www.fairchildsemi.com/ds/FL/FL7730.pdf
and
http://www.fairchildsemi.com/an/AN/AN-9745.pdf
as well as
http://www.fairchildsemi.com/support/referencedesign/docs/RD-L020-2.pdf

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