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LNK304GN Datasheet(PDF) 8 Page - Power Integrations, Inc. |
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LNK304GN Datasheet(HTML) 8 Page - Power Integrations, Inc. |
8 / 20 page 2-8 Rev. I 11/08 8 LNK302/304-306 Pre-load Resistor R4 In high-side, direct feedback designs where the minimum load is <3 mA, a pre-load resistor is required to maintain output regulation. This ensures sufficient inductor energy to pull the inductor side of the feedback capacitor C3 to input return via D2. The value of R4 should be selected to give a minimum output load of 3 mA. In designs with an optocoupler the Zener or reference bias current provides a 1 mA to 2 mA minimum load, preventing “pulse bunching” and increased output ripple at zero load. LinkSwitch-TN Layout Considerations In the buck or buck-boost converter configuration, since the SOURCE pins in LinkSwitch-TN are switching nodes, the copper area connected to SOURCE should be minimized to minimize EMI within the thermal constraints of the design. In the boost configuration, since the SOURCE pins are tied to DC return, the copper area connected to SOURCE can be maximized to improve heatsinking. The loop formed between the LinkSwitch-TN, inductor (L1), freewheeling diode (D1), and output capacitor (C2) should be kept as small as possible. The BYPASS pin capacitor C1 (Figure 6) should be located physically close to the SOURCE (S) and BYPASS (BP) pins. To minimize direct coupling from switching nodes, the LinkSwitch-TN should be placed away from AC input lines. It may be advantageous to place capacitors C4 and C5 in-between LinkSwitch-TN and the AC input. The second rectifier diode D4 is optional, but may be included for better EMI performance and higher line surge withstand capability. Quick Design Checklist As with any power supply design, all LinkSwitch-TN designs should be verified for proper functionality on the bench. The following minimum tests are recommended: 1) Adequate DC rail voltage – check that the minimum DC input voltage does not fall below 70 VDC at maximum load, minimum input voltage. 2) Correct Diode Selection – UF400x series diodes are recommended only for designs that operate in MDCM at an ambient of 70 °C or below. For designs operating in continuous conduction mode (CCM) and/or higher ambients, then a diode with a reverse recovery time of 35 ns or better, such as the BYV26C, is recommended. 3) Maximum drain current – verify that the peak drain current is below the data sheet peak drain specification under worst-case conditions of highest line voltage, maximum overload (just prior to auto-restart) and highest ambient temperature. 4) Thermal check – at maximum output power, minimum input voltage and maximum ambient temperature, verify that the LinkSwitch-TN SOURCE pin temperature is 100 °C or below. This figure ensures adequate margin due to variations in R DS(ON) from part to part. A battery powered thermocouple meter is recommended to make measurements when the SOURCE pins are a switching node.Alternatively, the ambient temperature may be raised to indicate margin to thermal shutdown. In a LinkSwitch-TN design using a buck or buck boost converter topology, the SOURCE pin is a switching node. Oscilloscope measurements should therefore be made with probe grounded to a DC voltage, such as primary return or DC input rail, and not to the SOURCE pins. The power supply input must always be supplied from an isolated source (e.g. via an isolation transformer). |
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