This engineering report describes a TRIAC tunable optically isolated flyback LED driver designed to drive a nominal 27 V voltage of 27 V at 350 mA over an input voltage range of 90 VAC to 132 VAC. The LED driver utilizes the LYT3315D from the LYTSwitch-3 family of devices. LYTSwitch-3 is a family of devices specifically designed for TRIAC dimmable LED drivers with single-stage PFC capability and precise LED current control. The design is a 9.5 W TRIAC dimmable LED driver with constant current output. The main design goals are high efficiency, compact PCB and excellent dimming compatibility. This design is intended for downlight LED applications. Circuit description The LYTSwitch-3 LYT3315D integrates a high voltage MOSFET power switch and a power controller in one package. The LYTSwitch-3 controller provides single stage power factor correction, LED current control and dimming control. Input stage The fuse resistor RF1 provides safety protection against component failure. Varistor RV1 provides a clamp function during differential line surge events to limit the maximum voltage spike on the main line. The AC input is rectified by BR1, which supplies the pulsating DC input to a pi filter consisting of C2, C3 and L1. The values ​​of C2, C3 and L1 are chosen to provide the best balance between high power factor, EMI performance and dimming compatibility. Inductor L3 is a common mode inductor that provides filtering for common mode noise. LYTSwitch-3 main control circuit This topology is an isolated flyback converter with primary side regulation. During the turn-on of the LYT3315D internal MOSFET, current is fed to the non-dot end of the primary winding and flows out of the dummy through the internal MOSFET and back to the input. During this time, energy is stored in the magnetizing inductance of the primary winding. This energy is then transferred to the secondary winding and discharged to the load through output diode D3 during the internal MOSFET turn-off. Output capacitor C11 provides filtering to minimize LED ripple current. R18 is preloaded. Bias Circuit The bias circuit provided to the LYT3315D consists of R17, D2 and C10, which provides the bias voltage of the auxiliary winding. Resistor R14 limits the current flowing into the LYT3315D bypass (BP) pin, which provides decoupling and energy storage for the device. Selecting the value of R14 minimizes device dissipation without affecting device power cuts that may occur at dimming conditions where the input conduction angle is minimal. The output current is controlled by the feedback (FB) pin. The drain current causes a voltage on the combination of R12 and R21 in parallel. This voltage is compared to the voltage across internal R13 to obtain a reference voltage that varies linearly with the conduction angle in dimming mode. The reference voltage during full conduction is 300 mA (no dimmer detected). Capacitor C7 filters the voltage across the sense resistor. C14 filters noise from the line sense pins. TRIAC phase dimming control with LYTSwitch-3 Smart Bleeder driver LYTSwitch-3 uses an active bleeder controller to provide the highest dimmer compatibility for LED drivers without sacrificing drive efficiency, power factor and %THD. When the TRIAC in the dimmer is turned on, the resistor R2 suppresses the input current ringing of the driver. The fusible resistor RF1 also provides significant damping for ringing. Diode D1 acts as a blocking diode to prevent the active bleeder from drawing current from the input capacitor once it is turned "on". Resistor R1 acts as a discharge path for the input capacitor. This is the dimmer compatibility that needs to be added to the drive. The active bleed and control circuit consists of R4, R5, R7, R8, R9, R25, C5, C6, Q1 and Q2. The network is directly driven by the LYT3315D via the BLEEDER CONTROL (BL) pin. Assuming the input current is the sum of the bleeder current and the converter current at dimming, the current flows through R7 and senses a voltage, which is then compared to the 120 mV reference on the BLEEDER SENSE (BS) pin via R10. Based on the input current level, the BL pin drives the bleeder circuit to maintain the R7 programmed input current. Resistors R4, R5 and R27 are bleeder resistors, and Q1 and Q2 form a Darlington pair that acts as a bleeder resistor. Resistors R8, R9, C5 and C6 form a stable network. headset Guangzhou Lufeng Electronic Technology Co. , Ltd. , https://www.lufengelectronics.com Figure 1 fills the board, top view.
Figure 2 fills the board, bottom view
Figure 3 schematic