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LNK408LG Datasheet(PDF) 9 Page - Power Integrations, Inc. |
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LNK408LG Datasheet(HTML) 9 Page - Power Integrations, Inc. |
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Rev. D 08/11 9 LNK403-409/413-419 www.powerint.com provides tighter clamp voltage tolerance than a Zener clamp. The RCD clamp is more cost effective than the Zener clamp but requires more careful design to ensure that the maximum drain voltage does not exceed the power FET breakdown voltage. These V OR limits are based on the BVDSS rating of the internal FET, a V OR of 60 V to 100 V is typical for most designs, giving the best PFC and regulation performance. Series Drain Diode An ultra-fast or Schottky diode in series with the drain is necessary to prevent reverse current flowing through the device. The voltage rating must exceed the output reflected voltage, V OR. The current rating should exceed two times the average primary current and have a peak rating equal to the maximum drain current of the selected LinkSwitch-PH device. Line Voltage Peak Detector Circuit LinkSwitch-PH devices use the peak line voltage to regulate the power delivery to the output. A capacitor value of 1 mF to 4.7 mF is recommended to minimize line ripple and give the highest power factor (>0.9), smaller values are acceptable but result in lower PF and higher line current distortion. Operation with Phase Controlled Dimmers Dimmer switches control incandescent lamp brightness by not conducting (blanking) for a portion of the AC voltage sine wave. This reduces the RMS voltage applied to the lamp thus reducing the brightness. This is called natural dimming and the LinkSwitch-PH LNK403-409 devices when configured for dimming utilize natural dimming by reducing the LED current as the RMS line voltage decreases. By this nature, line regulation performance is purposely decreased to increase the dimming range and more closely mimic the operation of an incandescent lamp. Using a 49.9 kW REFERENCE pin resistance selects natural dimming mode operation. Leading Edge Phase Controlled Dimmers The requirement to provide flicker-free 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 causes undesirable behaviors such as limited dimming range and/or flickering. 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 so for non-dimming applications these components can simply be omitted. Figure 9(a) shows the line voltage and current at the input of a leading edge TRIAC dimmer with Figure 9(b) showing the resultant rectified bus voltage. In this example, the TRIAC conducts at 90 degrees. Figure 10 shows undesired rectified bus voltage and current with the TRIAC turning off prematurely and restarting. If the TRIAC is turning off before the end of the half-cycle erratically or alternate half AC cycles have different conduction angles then flicker will be observed in the LED light due to variations in the output current. This can be solved by including a bleeder and damper circuit. Dimmers will behave differently based on manufacturer and power rating, for example a 300 W dimmer requires less dampening and requires less power loss in the bleeder than a 600 W or 1000 W dimmer due to different drive circuits and TRIAC holding current specifications. Line voltage also has a significant impact as at high-line for a given output power the input current and therefore TRIAC current is lower but the peak inrush current when the input capacitance charges is higher creating more ringing. Finally multiple lamps in parallel driven from the same dimmer can introduce more ringing due to the increased capacitance of parallel units. Therefore when testing dimmer operation verify on a number of models, different line voltages and with both a single driver and multiple drivers in parallel. 50 100 150 200 250 300 350 400 Conduction Angle (°) 350 250 150 50 -50 -150 -250 -350 0.35 0.25 0.15 0.05 -0.05 -0.15 -0.25 -0.35 PI-5983-060810 Voltage Current 0.5 0 50 100 150 200 250 400 350 300 Conduction Angle (°) 350 300 250 200 150 100 50 0 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 PI-5984-060810 Voltage Current Figure 9. (a) Ideal Input Voltage and Current Waveforms for a Leading Edge TRIAC Dimmer at 90° Conduction Angle. (b) Resultant Waveforms Following Rectification of TRIAC Dimmer Output. |
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