SCY99194 Datasheet, PDF (31/40 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller

SCY99194
Figure 59. Needs for Line Compensation For True Overpower Protection
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 V_FB(OPCE). However, because the HV pin can be
connected to an ac voltage, there is needed an additional
circuitry to read or at least closely estimate the actual voltage
on the bulk capacitor.
Figure 60. Overpower Protection Current Relation to Feedback Voltage
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