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TOP250RN-TL Datasheet(PDF) 30 Page - Power Integrations, Inc. |
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TOP250RN-TL Datasheet(HTML) 30 Page - Power Integrations, Inc. |
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30 / 52 page  TOP242-250 M 12/04 30 frequency radiated noise (for example, video noise sensitive applications such as VCR, DVD, monitor, TV, etc.), operating at 66 kHz will reduce snubber loss resulting in better efficiency. Also, in applications where transformer size is not a concern, use of the 66 kHz option will provide lower EMI and higher efficiency. Note that the second harmonic of 66 kHz is still below 150 kHz, above which the conducted EMI specifications get much tighter. For 10 W or below, it is possible to use a simple inductor in place of a more costly AC input common mode choke to meet worldwide conducted EMI limits. Transformer Design It is recommended that the transformer be designed for maximum operating flux density of 3000 Gauss and a peak flux densityof4200Gaussatmaximumcurrentlimit. Theturnsratio should be chosen for a reflected voltage (V OR) no greater than 135 V when using a Zener clamp, or 150 V (max) when using an RCD clamp with current limit reduction with line voltage (overload protection). For designs where operating current is significantly lower than the default current limit, it is recommended to use an externally setcurrentlimitclosetotheoperatingpeakcurrenttoreducepeak flux density and peak power (see Figures 20 and 34). In most applications,thetightercurrentlimittolerance,higherswitching frequency and soft-start features of TOPSwitch-GX contribute to a smaller transformer when compared to TOPSwitch-II. Standby Consumption Frequency reduction can significantly reduce power loss at light or no load, especially when a Zener clamp is used. For very low secondary power consumption, use a TL431 regulator for feedback control. Alternately, switching losses can be significantly reduced by changing from 132 kHz in normal operation to 66 kHz under light load conditions. TOPSwitch-GX Layout Considerations As TOPSwitch-GX has additional pins and operates at much higherpowerlevels compared to previous TOPSwitch families, the following guidelines should be carefully followed. Primary Side Connections Use a single point (Kelvin) connection at the negative terminal of the input filter capacitor for the TOPSwitch-GX SOURCE pin and bias winding return. This improves surge capabilities by returning surge currents from the bias winding directly to the input filter capacitor. The CONTROL pin bypass capacitor should be located as close as possible to the SOURCE and CONTROL pins and its SOURCE connection trace should not be shared by the main MOSFET switching currents. All SOURCE pin referenced components connected to the MULTI-FUNCTION, LINE- SENSE or EXTERNAL CURRENT LIMIT pins should also be located closely between their respective pin and SOURCE. Once again, the SOURCE connection trace of these components should not be shared by the main MOSFET switching currents. It is very critical that SOURCE pin switching currents are returned to the input capacitor negative terminal through a seperate trace that is not shared by the components connected to CONTROL, MULTI-FUNCTION, LINE-SENSE or EXTERNAL CURRENT LIMIT pins. This is because the SOURCE pin is also the controller ground reference pin. Any traces to the M, L or X pins should be kept as short as possible and away from the DRAIN trace to prevent noise coupling. LINE-SENSE resistor (R1 in Figures 47-49) should be located close to the M or L pin to minimize the trace length on the M or L pin side. In addition to the 47 µF CONTROL pin capacitor, a high frequency bypass capacitor in parallel may be used for better noise immunity. The feedback optocoupler output should also be located close to the CONTROL and SOURCE pins of TOPSwitch-GX. Y-Capacitor The Y-capacitor should be connected close to the secondary output return pin(s) and the positive primary DC input pin of the transformer. Heat Sinking The tab of the Y package (TO-220) or F package (TO-262) is internally electrically tied to the SOURCE pin. To avoid circulating currents, a heat sink attached to the tab should not be electrically tied to any primary ground/source nodes on the PC board. When using a P (DIP-8), G (SMD-8) or R (TO-263) package, a copper area underneath the package connected to the SOURCE pins will act as an effective heat sink. On double sided boards (Figure 49), top side and bottom side areas connected with vias can be used to increase the effective heat sinking area. In addition, sufficient copper area should be provided at the anode and cathode leads of the output diode(s) for heat sinking. In Figures 47, 48 and 49, a narrow trace is shown between the output rectifier and output filter capacitor. This trace acts as a thermal relief between the rectifier and filter capacitor to prevent excessive heating of the capacitor. |
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Similar Description - TOP250RN-TL |
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