Flicker-free MR16 LED lamp driver solution

Traditional halogen cup lamps use electronic transformers and AC input, so the LED-MR16 lamps currently on the market can directly replace traditional halogen cup lamps by installing a rectifier circuit inside. Although traditional electronic transformers are cheap, their quality varies. Since electronic transformers are actually controlled by half-bridge rectification and the output changes with the load impedance, the circuit stability is poor. When there is a problem with the circuit matching, the LED-MR16 lamps are prone to flickering or not lighting up.

The current reference design of 3-5W LED-MR16 lamps mostly uses three 1W LEDs in series for output, and the input voltage is AC 12V±10%. If the LED drive circuit architecture adopts DC buck mode, it can effectively output 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, causing the DC buck drive circuit to fail to work and the LED current to fail to conduct continuously. This is the main cause of the flickering of LED-MR16 lamps.

In order to solve the above flickering problem, the buck-boost converter is a better choice for the LED driver circuit architecture. The LED series circuit can be driven by an input voltage higher or lower than the output voltage. The buck-boost converter circuit can achieve an efficiency of more than 85%, and suppress the change of input voltage in the discontinuous working mode to provide excellent line voltage regulation.

Richtek Technology has launched a new RT8450, which is specially designed for MR16 LED lamp driving applications. Its internal circuit design architecture can easily realize buck-boost circuits, as shown in Figure 1.

Figure 1: LED-MR16 lamp driver circuit using RT8450 buck-boost architecture

In addition, the RT8450 uses a very small WDFN package and has a 4.5-40V input voltage range, which enables the RT8450 driver circuit to provide a wide driving voltage range and input protection. The operating temperature of the RT8450 is as high as 125°C, and it can work safely in the high temperature environment of the MR16 lamp. Its 500kHz switching frequency allows the driver circuit to use small-sized 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 lamp, as shown in Figure 2.

Figure 2: Driver board based on 5W MR16 LED lamp and RT8450

In fact, the above application circuit is combined with the common electronic transformer on the market. The RT8450 does not flicker under any load condition. As shown in Figure 3, even if the input voltage of the electronic transformer varies greatly due to poor load conditions, the LED-MR16 lamp using the RT8450 buck-boost circuit still maintains a constant current characteristic, and there is no flicker at all. The switching frequency of the RT8450 is about 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 reach 85%. The RT8450 uses peak current detection and average current control mode methods to ensure 5% LED current accuracy.

Figure 3: Channel 1 is the electronic transformer AC output voltage, Channel 2 is the input voltage after rectification and filtering, Channel 3 is the AC input voltage, and Channel 4 is the LED output current.

In addition, the buck-boost architecture of the RT8450 is not affected by input voltage fluctuations in the LED driver circuit, which can support the front-stage circuit to implement a three-terminal bidirectional thyristor (TRIAC) dimming solution. This is because this chip can use the TRIAC's chopped wave to convert it into a PWM signal and then control the RT8450 linear brightness adjustment pin, which can adjust the brightness of the overall lighting solution without flickering.