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LNK520PN Datasheet(PDF) 10 Page - Power Integrations, Inc. |
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LNK520PN Datasheet(HTML) 10 Page - Power Integrations, Inc. |
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10 / 20 page  10 LNK520 E 2/05 Feedback Resistor – R4 The value of R4 is selected to give a feedback current into the CONTROL pin of approximately 2.15 mA at the peak output power point of the supply. The actual value depends on the bias voltage, typically in the range 20 V to 35 V, selected during design. Higher values for the bias voltage will increase no-load power consumption. Any 1%, 0.25 W resistor is suitable. Output Diode – D7 PN fast, PN ultra-fast or Schottky diodes can be used depending on the efficiency target for the supply, Schottky diodes giving higher efficiency than PN diodes. The diode voltage rating should be sufficient to withstand the output voltage plus the input voltage transformed through the turns ratio (a typical V OR of 50 V requires a diode PIV of 50 V). Slow recovery diodes are not recommended (1N400X types). Output Capacitor – C6 Capacitor C6 should be selected such that its voltage and ripple current specifications are not exceeded. Selecting a capacitor with low equivalent series resistance (ESR) will reduce peak-peak output ripple and improve overall supply operating efficiency. LinkSwitch Layout considerations Primary Side Connections The copper area connected to SOURCE should be maximized to minimize temperature rise of the LinkSwitch device. The CONTROL pin capacitor C5 should be located as close as possible to the SOURCE and CONTROL pins. To minimize EMI coupling from the switching DRAIN node on the primary to both the secondary andAC input, the LinkSwitch shouldbepositionedawayfromthesecondaryofthetransformer and AC input. The length and copper area of all PCB traces connecting to the switching DRAIN node should be kept to an absolute minimum to limit EMI radiation. Y capacitor If a Y capacitor is required, it should be connected close to the transformersecondaryoutputreturnpin(s)andtheprimarybulk capacitor positive terminal. Such placement will maximize the EMI benefit of theYcapacitor and avoid problems in common- mode surge testing. Quick Design Checklist As with any power supply design, all LinkSwitch designs should be verified on the bench to make sure that component specifications are not exceeded under worst-case conditions. Performing the following minimum set of tests is strongly recommended: 1. Maximum drain voltage – Verify that V DS does not exceed 675 V at highest input voltage and peak output power. 2. Maximumdraincurrent–Atmaximumambienttemperature, maximuminputvoltageandpeakoutputpower,verifydrain current waveforms at start-up for any signs of transformer saturation and excessive leading edge current spikes. LinkSwitch has a minimum leading edge blanking time of 200 ns to prevent premature termination of the on-cycle. Verify that the leading edge current spike event is below current limit at the end of the 200 ns blanking period. 3. Thermal check – At peak output power, minimum input voltage and maximum ambient temperature, verify that the temperature specifications are not exceeded for LinkSwitch, transformer, output diode and output capacitors. Enough thermalmarginshouldbeallowedforpart-to-partvariationof theR DS(ON) of LinkSwitch as specified in the data sheet. Under low line, peak power, a maximum LinkSwitch SOURCE pin temperature of 100 °C is recommended to allow for these variations. 4. Centered output characteristic – Using a transformer with nominalprimaryinductanceandataninputvoltagemidway between low and high line, verify that the peak power point occurs at ~4% above the desired nominal output current, with the correct output voltage. If this does not occur, then the design should be refined (increase L P) to ensure the overall tolerance limits are met. Selecting Between LNK500 and LNK520 The LNK500 and LNK520 differ in the circuit location of the LinkSwitch device. The LNK500 is designed for high-side operation and the LNK520 is designed for low-side operation. TheLNK520can,however,beusedinthehigh-sideconfiguration in certain applications. Refer to Figure 9 and supporting description.Table2summarizestheconsiderationsforselecting which device to use. Design Tools Up to date information on design tools can be found at the Power Integrations Web site: www.powerint.com. |
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