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LNK403LG Datasheet(PDF) 6 Page - Power Integrations, Inc. |
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LNK403LG Datasheet(HTML) 6 Page - Power Integrations, Inc. |
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6 / 20 page  Rev. D 08/11 6 LNK403-409/413-419 www.powerint.com Diode D3 and VR1 clamp the drain voltage to a safe level due to the effects of leakage inductance. Diode D4 is necessary to prevent reverse current from flowing through U1 for the period of the rectified AC input voltage that the voltage across C2 falls to below the reflected output voltage (V OR). Diode D6, C5, R7 and R8 create the primary bias supply from an auxiliary winding on the transformer. Capacitor C4 provides local decoupling for the BYPASS pin of U1 which is the supply pin for the internal controller. During start-up C4 is charged to ~6 V from an internal high-voltage current source tied to the device DRAIN pin. This allows the part to start switching at which point the operating supply current is provided from the bias supply via R5. Capacitor C4 also selects the output power mode (10 mF for reduced power was selected to reduce dissipation in U1 and increase efficiency). Feedback The bias winding voltage is proportional to the output voltage (set by the turns ratio between the bias and secondary windings). This allows the output voltage to be monitored without secondary side feedback components. Resistor R6 converts the bias voltage into a current which is fed into the FEEDBACK pin of U1. The internal engine within U1 combines the FEEDBACK pin current, VOLTAGE MONITOR pin current and drain current information to provide a constant output current over a 1.5:1 output voltage variation (LED string voltage variation of ±25%) at a fixed line input voltage. To limit the output voltage at no-load an output overvoltage protection circuit is set by D7, C12, R20, VR3, C13, Q3 and R19. Should the output load be disconnected then the bias voltage will increase until VR3 conducts, turning on Q3 and reducing the current into the FEEDBACK pin. When this current drops below 20 mA the part enters auto-restart and switching is disabled for 1500 ms allowing time for the output and bias voltages to fall. Output Rectification The transformer secondary winding is rectified by D8 and filtered by C8 and C10. A Schottky barrier diode was selected for efficiency and the combined value of C8 and C10 were selected to give peak-to-peak and LED ripple current equal to 40% of the mean value. For designs where lower ripple is desirable the output capacitance value can be increased. A small pre-load is provided by R15 which limits the output voltage under no-load conditions. TRIAC Phase Dimming Control Compatibility The requirement to provide output dimming with low cost, TRIAC-based, leading edge phase dimmers introduces a number of trade-offs in the design. Due to the much lower power consumed by LED based lighting the current drawn by the overall lamp is below the holding current of the TRIAC within the dimmer. This can cause undesirable behaviors such as limited dimming range and/or flickering as the TRIAC fires inconsistently. The relatively large impedance the LED lamp presents to the line allows significant ringing to occur due to the inrush current charging the input capacitance when the TRIAC turns on. This too can cause similar undesirable behavior as the ringing may cause the TRIAC current to fall to zero and turn off. To overcome these issues two circuits, the Active Damper and Passive Bleeder, are incorporated. The drawback of these circuits is increased dissipation and therefore reduced efficiency of the supply. For non-dimming applications these components can simply be omitted. The Active Damper consists of components R9, R10, R11, R12, D1, Q1, C6, VR2, Q2 in conjunction with R13. This circuit limits the inrush current that flows to charge C2 when the TRIAC turns on by placing R13 in series for the first 1 ms of the TRIAC conduction. After approximately 1 ms, Q2 turns on and shorts R13. This keeps the power dissipation on R13 low and allows a larger value during current limiting. Resistor R9, R10, R11 and C6 provide the 1 ms delay after the TRIAC conducts. Transistor Q1 discharges C6 when the TRIAC is not conducting and VR2 clamps the gate voltage of Q2 to 15 V. The Passive Bleeder circuit is comprised of C11 and R18. This helps to keep the input current above the TRIAC holding current while the input current corresponding to the effective driver resistance increases during each AC half-cycle. |
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