The traditional halogen cup lamp uses an electronic transformer and uses an AC input. Therefore, the current LED- MR16 lamp product on the market has a rectifier circuit directly substituted for the conventional halogen cup lamp. Although the traditional electronic transformer is cheap, the relative quality is not good. Because the electronic transformer is actually controlled by half-bridge rectification, the output varies with the load impedance, so the circuit stability is poor. When there is a problem with the circuit matching, the LED - The MR16 luminaire is prone to flicker or no light.
In the current reference design of the 3-5W LED-MR16 lamp, three 1W LEDs are used in series, and the input voltage is AC 12V±10%. If the LED drive circuit architecture adopts DC buck mode, it can output effectively under normal power supply conditions, but due to the stability of the electronic transformer, the input voltage of the LED drive circuit may be higher or lower than the output voltage, resulting in DC buck. The drive circuit does not work, and the LED current cannot be continuously turned on, which is the main cause of the LED-MR16 lamp flickering.
In order to solve the above flicker problem, the LED driver circuit architecture uses a buck-boost converter as a better choice. The LED series circuit can be driven with an input voltage higher or lower than the output voltage. The buck-boost conversion circuit can achieve an efficiency of more than 85%, and suppress the change of the input voltage in the discontinuous operation mode to provide excellent line voltage regulation.
Lixin Technology is currently introducing the new RT8450, which can be specifically designed for driving applications of MR16 LED lamps. Its internal circuit design architecture makes it easy to implement buck-boost circuits, as shown in Figure 1.
Figure 1: Driving circuit for LED-MR16 lamp using RT8450 buck-boost architecture
In addition, the RT8450 is available in a very small WDFN package with an input voltage range of 4.5-40V, which allows the RT8450 driver circuit to provide a wide drive voltage range and input protection. With an operating temperature of up to 125 ° C, the RT8450 can operate safely in high temperature environments within MR16 luminaires. Its 500kHz switching frequency allows the driver circuit to use small inductors and capacitors. In addition, the buck-boost circuit architecture can reduce the input filter capacitor capacitance, so that the driver circuit can be placed in the MR16 luminaire, as shown in Figure 2.
Figure 2: Driver Board Based on 5W MR16 LED Light and RT8450
Actually, the above application circuit is combined with the common electronic transformer on the market. The RT8450 does not flicker under any load conditions. As shown in Figure 3, the input voltage varies greatly due to poor load conditions of the electronic transformer. The LED-MR16 luminaire with RT8450 buck-boost circuit maintains a constant current characteristic of the LED, and there is no flicker at all. The RT8450 switching frequency is approximately 500kHz, and the buck-boost circuit can use an input 68uF capacitor. The total converter efficiency (LED power divided by the power from VBAT) in this example can be as high as 85%. The RT8450 uses peak current sensing and average current control mode to ensure 5% LED current accuracy.
Figure 3: Channel 1 is the AC output voltage of the electronic transformer, channel 2 is the rectified and filtered input voltage, channel 3 is the AC input voltage, and channel 4 is the LED output current.
In addition, the RT8450 buck-boost architecture is characterized by the fact that the LED driver circuit is not affected by input voltage variations, and can support the pre-stage circuit to implement a triac TRIAC dimming scheme because this chip can utilize the TRIAC cutoff. After switching to the PWM signal, the RT8450 linear brightness adjustment pin can be controlled to control the brightness and brightness of the overall lighting scheme without flickering.
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