The role and topology of led drive circuit

LED is a kind of photon emission caused by excess energy. When a forward voltage is applied across the LED, minority carriers and majority carriers in the semiconductor react in a composite reaction. Different colored materials can be used to produce colored light of different colors, which is referred to as a light-emitting diode. With our further research on led, the performance of led light has been further improved, and the corresponding production costs have been gradually reduced.

1. LED performance advantages

Compared to incandescent lamps, LEDs have many advantages in performance. LED lamp can last up to 100,000 hours, and its service life can exceed the life of a person under normal use. In addition to its long life, LEDs also have the advantage of saving power. A typical LED headlamp can provide continuous illumination for tens of hours or even more than one hundred hours, which is like a long-lighted lamp compared to a few hours of illumination in an incandescent light. Super power-saving and shiny LED products are favored by many outdoor enthusiasts

2. LED drive circuit function

Unlike electronic ballasts for fluorescent lamps, the main function of the LED driver circuit is to convert the AC voltage to a constant current source and simultaneously match the voltage and current of the LED. The LED driver circuit preferably controls the magnitude of the LED current when factors such as input voltage and ambient temperature fluctuate. Otherwise, the light output of the LED will change with factors such as input voltage and temperature, and if the LED current is out of control, long-term operation of the LED at high current will affect the reliability and life of the LED, and may fail.

3. Analysis of topology structure of low power lighting LED driver circuit

Despite the many advantages of white LEDs, the design of LED driver circuits faces significant challenges. Space requirements and heat dissipation requirements are limited by design. In low-power (≤3W) lighting applications, designers use off-the-shelf, non-isolated, inductor-based buck and lift switch mode power supplies. The buck-boost converter has a major advantage over the buck converter in that the output diode is connected in series with the load. In a buck converter, if a MOSFET has a short-circuit fault, the input is tied directly to the output. In the case of such a buck-boost converter, the reverse bias output diode blocks the path between the input and output.

In both converters, the AC input is rectified and filtered by D1, D2, C1, C2, RF1 and RF2. Two diodes enhance input surge withstand capability and conducted EMI performance. Designers should use a fusible flame retardant resistor as RF1, but a flame retardant resistor can be used as the RF2. The on/off control in the Linkswitch-TN device is used to regulate the output current. Once the current into the feedback (FB) pin exceeds 49μA, the MOSFET switch will be disabled to enter the next switching cycle.

The buck-boost topology is slightly less efficient than the buck topology because power is not transmitted to the output every time the MOSFET switch is turned on. Therefore, it generates more heat than the buck topology. But the difference is not obvious.

To ensure that the circuit topology meets thermal regulation requirements, the designer mounts the power components into the lamp holder and then measures the temperature of the LNK306DN source pin. Ideally, the temperature of the source pin should not exceed 100 °C. Measurements at an ambient temperature of 25 ° C indicate that the source pin temperature will exceed 100 ° C when the Vin value rises to 265 VAC. In light of these results, the designer concluded that there may be thermal limits on some additional heat sinks, such as placing the LED heat sink on top of the U1 SO-8C package.

These two circuit topologies have similar advantages from a cost perspective. Importantly, a typical design requires the use of approximately 25 devices and allows the use of off-the-shelf, low-cost inductors instead of custom transformers.

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