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NCP1608 Datasheet(PDF) 16 Page - ON Semiconductor |
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NCP1608 Datasheet(HTML) 16 Page - ON Semiconductor |
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16 / 24 page  NCP1608 http://onsemi.com 16 During the delay caused by RZCD and the ZCD pin capacitance, the equivalent drain capacitance (CEQ(drain)) discharges through the path shown in Figure 35. Figure 35. Equivalent Drain Capacitance Discharge Path + AC Line EMI Filter + D L Iin Cin IL Cbulk Vout CEQ(drain) CEQ(drain) is the combined parasitic capacitances of the MOSFET, the diode, and the inductor. Cin is charged by the energy discharged by CEQ(drain). The charging of Cin reverse biases the bridge rectifier and causes the input current (Iin) to decrease to zero. The zero input current causes THD to increase. To reduce THD, the ratio (tz / TSW) is minimized, where tZ is the period from when IL = 0 A to when the drive turns on. The ratio (tz / TSW) is inversely proportional to the square root of L. During startup, there is no energy in the ZCD winding and no voltage signal to activate the ZCD comparators. This means that the drive never turns on. To enable the PFC stage to start under these conditions, an internal watchdog timer (tstart) is integrated into the controller. This timer turns the drive on if the drive has been off for more than 165 ms (typical value). This feature is deactivated during a fault mode (OVP or UVP), and reactivated when the fault is removed. Wide Control Range The Ct charging threshold (VCt(off)) decreases as the output power is decreased from the maximum output power to the minimum output power in the application. In high power applications (>150 W), VControl is reduced to a low voltage at a large output power and Ct(offset) remains constant. The result is that VCt(off) is reduced to a low voltage at a large output power. The low VControl and VCt(off) voltages are susceptible to noise. The large output power combined with the low VControl and VCt(off) increase the probability of noise interfering with the control signals and on time duration (Figures 36 and 37). The noise induces voltage spikes on the Control pin and Ct pin that reduces the drive on time from the on time determined by the feedback loop (ton(loop)). The reduced on time causes the energy stored in the inductor (L) to be reduced. The result is that VZCD does not exceed VZCD(ARM) and the drive remains off until tstart expires. This sequence results in pulse skipping and reduced power factor. Figure 36. Control Pin Noise Induced On Time Reduction and Pulse Skipping DRV Remains Off DRV is Not Exceeded 0 V Noise Induced Voltage Spike tstart ton ton(loop) VZCD(ARM) Low VControl Voltage Low VCt(off) Voltage VControl − Ct(offset) VControl Ct(offset) VCt VCt(off) VZCD VZCD(ARM) VZCD(TRIG) VCL(NEG) |
Similar Part No. - NCP1608_10 |
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Similar Description - NCP1608_10 |
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