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LYT4215 Datasheet(PDF) 7 Page - Power Integrations, Inc. |
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LYT4215 Datasheet(HTML) 7 Page - Power Integrations, Inc. |
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7 / 20 page  Rev. B 02/13 7 LYT4211-4218/4311-4318 www.powerint.com Diode D6, C5, C9, R19 and R20 create the primary bias supply from an auxiliary winding on the transformer. Capacitor C8 provides local decoupling for the BYPASS pin of U1 which is the supply pin for the internal controller. During start-up C8 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 R17. Capacitor C8 also selects the output power mode (47 µF for reduced power was selected to reduce dissipation in U1 and increase efficiency for this design). 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 R18 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, the 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 D8, C15, R22, VR4, R27, C14 and Q2. Should the output load be disconnected then the bias voltage will increase until VR4 conducts, turning on Q2 and reducing the current into the FEEDBACK pin. When this current drops below 10 µA the part enters auto-restart and switching is disabled for 300 ms allowing time for the output and bias voltages to fall. Output Rectification The transformer secondary winding is rectified by D7 and filtered by C11 and C12. An ultrafast TO-220 diode was selected for efficiency and the combined value of C11 and C12 were selected to give peak-to-peak LED ripple current equal to 30% 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 R23 which discharges residual charge in output capacitors when turned off. 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 simple two circuits, the SCR active damper and R-C 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 SCR active damper consists of components R6, C3, and Q1 in conjunction with R8. This circuit limits the inrush current that flows to charge C4 when the TRIAC turns on by placing R8 in series for the first ~1 ms of the TRIAC conduction. After approximately 1 ms, Q1 turns on and bypasses R8. This keeps the power dissipation on R8 low and allows a larger value during current limiting. Resistor R6 and C3 provide the delay on Q1 turn on after the TRIAC conducts. Diode D9 blocks the charge in capacitor C4 from flowing back after the TRIAC turns on which helps in dimming compatibility especially with high power dimmers. The passive bleeder circuit is comprised of R1 and C1. This helps 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. |
Similar Part No. - LYT4215 |
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Similar Description - LYT4215 |
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