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TOP250RN-TL Datasheet(PDF) 29 Page - Power Integrations, Inc. |
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TOP250RN-TL Datasheet(HTML) 29 Page - Power Integrations, Inc. |
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29 / 52 page  TOP242-250 29 M 12/04 Input Capacitor Theinputcapacitormustbechosentoprovidetheminimum DC voltage required for the TOPSwitch-GX converter to maintain regulation at the lowest specified input voltage and maximum output power. Since TOPSwitch-GX has a higher DC MAX than TOPSwitch-II , it is possible to use a smaller input capacitor. ForTOPSwitch-GX,acapacitanceof2µFperwattispossiblefor universal input with an appropriately designed transformer. Primary Clamp and Output Reflected Voltage V OR A primary clamp is necessary to limit the peak TOPSwitch-GX drain to source voltage. A Zener clamp requires few parts and takes up little board space. For good efficiency, the clamp Zener should be selected to be at least 1.5 times the output reflectedvoltageV OR, as this keeps the leakage spike conduction time short. When using a Zener clamp in a universal input application, a V OR of less than 135 V is recommended to allow for the absolute tolerances and temperature variations of the Zener. This will ensure efficient operation of the clamp circuit and will also keep the maximum drain voltage below the rated breakdown voltage of the TOPSwitch-GX MOSFET. AhighV OR is required to take full advantage of the wider DCMAX of TOPSwitch-GX. An RCD clamp provides tighter clamp voltage tolerance than a Zener clamp and allows a V OR as high as150V. RCDclampdissipationcanbeminimizedbyreducing the external current limit as a function of input line voltage (see Figures 21 and 35). The RCD clamp is more cost effective than the Zener clamp but requires more careful design (see Quick Design Checklist). Output Diode The output diode is selected for peak inverse voltage, output current, and thermal conditions in the application (including heatsinking, air circulation, etc.). The higher DC MAX of TOPSwitch -GX, along with an appropriate transformer turns ratio, can allow the use of a 60 V Schottky diode for higher efficiency on output voltages as high as 15 V (see Figure 41: A 12 V, 30 W design using a 60 V Schottky for the output diode). Bias Winding Capacitor Due to the low frequency operation at no-load a 1 µF bias winding capacitor is recommended. Soft-Start Generally, a power supply experiences maximum stress at start-up before the feedback loop achieves regulation. For a periodof10ms,theon-chipsoft-startlinearlyincreasestheduty cycle from zero to the default DC MAX at turn on. In addition, the primary current limit increases from 85% to 100% over the same period. This causes the output voltage to rise in an orderly manner, allowing time for the feedback loop to take control of the duty cycle. This reduces the stress on the TOPSwitch-GX MOSFET, clamp circuit and output diode(s), and helps prevent transformersaturationduringstart-up. Also,soft-startlimitsthe amount of output voltage overshoot and, in many applications, eliminates the need for a soft-finish capacitor. EMI The frequency jitter feature modulates the switching frequency over a narrow band as a means to reduce conducted EMI peaks associated with the harmonics of the fundamental switching frequency. This is particularly beneficial for average detection mode. As can be seen in Figure 46, the benefits of jitter increase with the order of the switching harmonic due to an increase in frequency deviation. The FREQUENCY pin of TOPSwitch-GX offers a switching frequency option of 132 kHz or 66 kHz. In applications that require heavy snubbers on the drain node for reducing high -20 -10 0 -10 20 30 40 50 60 70 80 0.15 1 10 30 Frequency (MHz) EN55022B (QP) EN55022B (AV) TOPSwitch-II (no jitter) EN55022B (QP) EN55022B (AV) -20 -10 0 -10 20 30 40 50 60 70 80 0.15 1 10 30 Frequency (MHz) TOPSwitch-GX (with jitter) Figure 46a. TOPSwitch-II Full Range EMI Scan (100 kHz, No Jitter). Figure 46b. TOPSwitch-GX Full Range EMI Scan (132 kHz, With Jitter) with Identical Circuitry and Conditions. |
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Similar Description - TOP250RN-TL |
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