Problems and solutions of LED driver "application"

 LEDs are widely used in E27, GU10, PAR lamps, MR16 and other fields. However, there are some problems in the application of LED-MR16 driven by electronic transformers. This is mainly caused by 4 reasons:

　　First, the power of the MR16 lamp cup of LED is usually only 5W, and the magnetizing current generated in the half-bridge circuit cannot saturate the internal transformer, causing the electronic transformer to work abnormally.

　　Second, MR16 has a large electrolytic capacitor after the rectifier bridge, which is equivalent to the load changing from the original pure group load to a large capacitive load for the electronic transformer.

　　Third, in 5W applications, a capacitor greater than 487 (uF) is required to make the Buck circuit work properly. It is impossible to put such a large-capacity capacitor in the MR16 lamp cup, which has very strict volume requirements.

　　Fourth, the voltage output by the electronic transformer is not a sine wave, but a quasi-triangular wave with a minimum of 5.6V. If the operating voltage cannot reach 5.6V or lower, a larger filter capacitor is required. Otherwise, the LED output current will drop to zero when VCE is lower than the chip operating voltage, and low-frequency flicker of 100Hz or regular flickering at time intervals determined by the peripheral components of the COMP pin of the IC may occur.

　　In view of the current status of the above industries and the analysis of the causes of flicker, we recommend the use of RICHTEK8479 solution, which is a Two-Stage Topology (Boost+Buck) architecture and has a wide operating voltage of 4.5V to 36V, which can easily solve the flickering phenomenon and achieve stable output current and illumination.

　　This power supply solution adopts a two-stage architecture, that is, the voltage input from the electronic transformer is first boosted after fast recovery rectification to a constant voltage of about 20V, and then passes through the second stage circuit and is converted into a step-down constant current mode to keep the output current constant within a certain output voltage range.

　　Since this IC has two built-in MOS tubes, the peripheral circuit becomes compact and simple. And since this solution adopts a boost architecture, the input end does not need to use a large-capacity electrolytic capacitor to stabilize the input voltage. It is very suitable for use in small spaces such as MR16 bases.

　　This power supply can achieve a good constant current effect in a wide input voltage range of 8V to 19V, whether in AC or DC input state. It will be a general trend for LED-MR16 spotlights to replace halogen spotlights. The power supply driven by the electronic transformer has explored a method for implementing a driving power supply with good compatibility for the M7-MR16 spotlight.