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NCP1608 Datasheet(PDF) 15 Page - ON Semiconductor |
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NCP1608 Datasheet(HTML) 15 Page - ON Semiconductor |
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15 / 24 page  NCP1608 http://onsemi.com 15 Figure 33. Implementation of the ZCD Block ZCD + − Demag + − Reset Dominant Latch R Q S DRIVE ZCD Clamp RZCD Vin NZCD NB Rsense VZCD(ARM) VZCD(TRIG) Q This sequence achieves CrM operation. The maximum VZCD(ARM) sets the maximum turns ratio and is calculated using Equation 11: NB :NZCD v Vout * 2 @ VacHL VZCD(ARM) (eq. 11) Where VacHL is the maximum rms input voltage and VZCD(ARM) = 1.55 V (maximum value). The NCP1608 prevents excessive voltages on the ZCD pin by clamping VZCD. When the ZCD winding is negative, the ZCD pin is internally clamped to VCL(NEG). Similarly, when the ZCD winding is positive, the ZCD pin is internally clamped to VCL(POS). A resistor (RZCD in Figure 33) is necessary to limit the current into the ZCD pin. The maximum ZCD pin current (IZCD(MAX)) is limited to less than 10 mA. RZCD is calculated using Equation 12: RZCD w 2 @ VacHL IZCD(MAX) @ (NB :NZCD) (eq. 12) The value of RZCD and the parasitic capacitance of the ZCD pin determine when the ZCD winding signal is detected and the drive turn on begins. A large RZCD value creates a long delay before detecting the ZCD event. In this case, the controller operates in DCM and the power factor is reduced. If the RZCD value is too small, the drive turns on when the drain voltage is high and efficiency is reduced. A popular strategy for selecting RZCD is to use the RZCD value that achieves minimum drain voltage turn on. This value is found experimentally. Figure 34 shows the realistic waveforms for CrM operation due to RZCD and the ZCD pin capacitance. Figure 34. Realistic CrM Waveforms Using a ZCD Winding with RZCD and the ZCD Pin Capacitance DRV 0 A 0 V 0 V 0 V 0 V Diode Conduction MOSFET Conduction tz RZCD Delay Minimum Voltage Turn on ton toff TSW tdiode VCL(NEG) VZCD(TRIG) VZCD(ARM) VCL(POS) VZCD(WIND),on VZCD VZCD(WIND),off VZCD(WIND) Vout Vdrain IL(peak) IL IL(NEG) |
Similar Part No. - NCP1608_10 |
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Similar Description - NCP1608_10 |
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