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NCP1280DR2G Datasheet(PDF) 11 Page - ON Semiconductor |
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NCP1280DR2G Datasheet(HTML) 11 Page - ON Semiconductor |
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11 / 18 page  NCP1280 http://onsemi.com 11 TYPICAL CHARACTERISTICS Figure 27. Outputs Rise Time versus Load Capacitance CL, LOAD CAPACITANCE (pF) 200 150 100 50 0 0 10 20 30 40 50 60 80 Figure 28. Outputs Fall Time versus Load Capacitance CL, LOAD CAPACITANCE (pF) 200 150 100 50 0 0 5 10 15 20 25 35 70 30 TJ = −40°C TJ = 25°C TJ = 125°C TJ = −40°C TJ = 25°C TJ = 125°C 175 125 75 25 175 125 75 25 Measured from 10% to 90% of VOH VAUX = 12 V Measured from 90% to 10% of VOH VAUX = 12 V DETAILED OPERATING DESCRIPTION Introduction An NCP1280 based system offers significant efficiency improvements and system cost savings over a converter using a traditional forward topology. The NCP1280 provides two control outputs. OUT1 controls the primary switch of a forward converter. OUT2 has an adjustable overlap delay, which can be used to control an active clamp/reset switch or any other complementary drive topology, such as an asymmetric half−bridge. In addition, OUT2 can be used to control a synchronous rectifier topology, eliminating the need of external control circuitry. Other distinctive features include: two mode overcurrent protection, line under/overvoltage detectors, fast line feedforward, soft−start and a maximum duty cycle limit. The Functional Block Diagram is shown in Figure 2. The features included in the NCP1280 provide some of the advantages of Current−Mode Control, such as fast line feedforward, and cycle by cycle current limit. It eliminates the disadvantages of low power jitter, slope compensation and noise susceptibility. Active Clamp Topology The transformer reset voltage in a traditional forward converter is set by the turns ratio and input voltage. Where as the reset voltage of an active clamp topology is constant over the converter off time and only depends on the input voltage and duty cycle. This translates into a lower voltage stress on the main switch, allowing the use of lower voltage MOSFETs. In general, lower voltage MOSFETs have lower cost and ON resistance. Therefore, lower system cost and higher efficiency can be achieved. In addition, the lower voltage stress allows the converter to operate at a higher duty cycle for a given primary switch voltage stress. This allows a reduction in primary peak current and secondary side voltage stress as well as smaller secondary inductor size. High Voltage Startup Regulator The NCP1280 contains an internal 700 V startup regulator that eliminates the need for external startup components. In addition, this regulator increases the efficiency of the supply as it uses no power when in the normal mode of operation, but instead uses power supplied by an auxiliary winding. The startup regulator consists of a constant current source that supplies current from the input line voltage (Vin) to the capacitor on the VAUX pin (CAUX). The startup current is typically 13.8 mA. Once VAUX reaches 11 V, the startup regulator turns OFF and the outputs are enabled. When VAUX reaches 7 V, the outputs are disabled and the startup regulator turns ON. This “7−11” mode of operation is known as Dynamic Self Supply (DSS). The VAUX pin can be biased externally above 7 V once the outputs are enabled to prevent the startup regulator from turning ON. It is recommended to bias the VAUX pin using an auxiliary supply generated by an auxiliary winding from the power transformer. An independent voltage supply can also be used. If using an independent voltage supply and VAUX is biased before the outputs are enabled or while a fault is present, the One Shot Pulse Generator (Figure 2) will not be enabled and the outputs will remain OFF. As the DSS sources current to the VAUX pin, a diode should be placed between CAUX and the auxiliary supply as shown in Figure 29. This will allow the NCP1280 to charge CAUX while preventing the startup regulator from sourcing current into the auxiliary supply. |
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