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NCP12401EBAAA1 Datasheet(PDF) 36 Page - ON Semiconductor |
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NCP12401EBAAA1 Datasheet(HTML) 36 Page - ON Semiconductor |
36 / 44 page NCP12401 www.onsemi.com 36 Internal Overpower Protection The power delivered by a flyback power supply is proportional to the square of the peak current in discontinuous conduction mode: POUT + 1 2 @ h @ LP @ FSW @ IP 2 (eq. 3) Unfortunately, due to the inherent propagation delay of the logic, the actual peak current is higher at high input voltage than at low input voltage, leading to a significant difference in the maximum output power delivered by the power supply. Figure 67. Needs for Line Compensation For True Overpower Protection time Ipeak High Line Low Line ILIMIT tdelay tdelay DIpeak to be compensated To compensate this and have an accurate overpower protection, an offset proportional to the input voltage is added on the CS signal by turning on an internal current source: by adding an external resistor in series between the sense resistor and the CS pin, a voltage offset is created across it by the current. The compensation can be adjusted by changing the value of the resistor. But this offset is unwanted to appear when the current sense signal is small, i.e. in light load conditions, where it would be in the same order of magnitude. Therefore the compensation current is only added when the FB voltage is higher than VFB(OPCE). However, because the HV pin is being connected to ac voltage, there is needed an additional circuitry to read or at least closely estimate the actual voltage on the bulk capacitor. Figure 68. Overpower Protection Current Relation to Feedback Voltage VFB IOPC VFB(OPCE) VFB(OPCF) VHV Figure 69. Overpower Protection Current Relation to Peak of Rectified Input Line AC voltage VHV IOPC 365 V 125 V IOPC(125) IOPC(365) |
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