This engineering report describes a TRIAC dimmable, isolated flyback LED driver designed to drive a nominal LED voltage string of 27 V at 350 mA from an input voltage range of 90 VAC to 132 VAC. The LED driver utilizes the LYT3315D from the LYTSwitch-3 family of devices.
The LYTSwitch-3 LYT3315D combines a high-voltage powr MOSFET switch with a power supply controller in a single package. The LYTSwitch-3 controller provides a single-stage power factor correction, LED current control and dimming control.
Fusible resistor RF1 provides safety protection against component failures. Varistor RV1 provides clamping during differential line surge events to limit the maximum voltage spike across the primary.
The AC input is rectified by BR1 to provide the pulsating DC input to the pi filter consisting of C2, C3 and L1. Values of C2, C3 and L1 were chosen to provide the best balance between high power factor, EMI performance and dimming compatibility. Inductor L3 is a common mode inductor to provide filtering against common mode noise.
LYTSwitch-3 Primary Control Circuit
The topology is an isolated flyback converter with primary side regulation. During turn-on of the LYT3315D internal MOSFET, current is fed to the un-dotted end of the primary winding and flows out of the dotted end through the internal MOSFET and back to the input. During this period energy is stored in the magnetizing inductance of the primary winding. This energy is then transferred to the secondary winding and is discharge to the load through the output diode D3 during internal MOSFET turn-off. Output capacitor C11 provides filtering to minimize LED ripple current. R18 serves as pre-load.
Bias supply to the LYT3315D is provided by the bias circuit consisting of R17, D2 and C10 which provides the bias voltage from the auxiliary winding. Resistor R14 limits the current flowing into the BYPASS (BP) pin of LYT3315D with C8 providing both decoupling and energy storage for the device. The value of R14 is chosen such that device dissipation is minimized without compromising device supply cutoff which could occur during dimming conditions where input conduction angle is at a minimum.
Output current is controlled through the FEEDBACK (FB) pin. A voltage across the parallel combination of R12 and R21 is induced by the drain current. This voltage is compared to the voltage across R13 internally to a reference voltage which varies linearly with the conduction angle when in dimming mode. The reference is 300 mA during full conduction (no dimmer detected). Capacitor C7 filters the voltage across the sense resistors. C14 filters noise from the line sense pin.
TRIAC Phase Dimming Control with LYTSwitch-3 Smart Bleeder Drive
LYTSwitch-3 uses an active bleeder control to provide the highest dimmer compatibility possible for LED drivers without sacrificing driver efficiency, power factor and %THD.
Resistor R2 dampens the driver input current ringing when the TRIAC in a dimmer turns on. The fusible resistor RF1 also provides significant damping to the ringing.
Diode D1 serves as a blocking diode to prevent the active bleeder from drawing current from the input capacitors once it turns on. Resistor R1 serves as a discharge path for the input capacitors. This is needed to increase dimmer compatibility of the driver.
The active bleeder and control circuit consists of R4, R5, R7, R8, R9, R25, C5, C6, Q1 and Q2. This network is directly driven by the LYT3315D through the BLEEDER CONTROL (BL) pin. Given that the input current is the sum between the bleeder current when dimming and the converter current, this current flows through R7 and induces a voltage which is then compared through R10 to a 120 mV reference on the BLEEDER SENSE (BS) pin. Based on the input current level, the BL pin drives the bleeder circuit to maintain the input current as programmed by R7.
Resistor R4, R5, and R27 are bleeder resistors with Q1 and Q2 forming a Darlington pair to act as switch for the bleeder resistor. Resistor R8, R9, C5 and C6 form a stabilization network.