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NCP1608 Datasheet(PDF) 14 Page - ON Semiconductor |
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NCP1608 Datasheet(HTML) 14 Page - ON Semiconductor |
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14 / 24 page  NCP1608 http://onsemi.com 14 Figure 31. On Time Generation Control Ct + − PWM + DRV Icharge ton VControl − Ct(offset) ton VCt VCt(off) VDD DRV VControl Ct(offset) VControl varies with the rms input voltage and output load, which naturally satisfies Equation 1. The on time is constant during the ac line cycle if the values of compensation components are sufficient to filter out the Vout ripple. The maximum on time of the controller occurs when VControl is at the maximum. The Ct capacitor is sized to ensure that the required on time is reached at maximum output power and the minimum input voltage condition. The on time is calculated using Equation 7: ton + Ct @ VCt(MAX) Icharge (eq. 7) Combining Equation 7 with Equation 1, results in Equation 8: Ct w 2 @ Pout @ LMAX @ Icharge h @ VacLL 2 @ VCt(MAX) (eq. 8) To calculate the minimum Ct value: VCt(MAX) = 4.775 V (minimum value), Icharge = 297 mA (maximum value), VacLL is the minimum rms input voltage, and LMAX is the maximum inductor value. Off Time Sequence In CrM operation, the on time is constant during the ac line cycle and the off time varies with the instantaneous input voltage. When the inductor current reaches zero, the drain voltage (Vdrain in Figure 27) resonates towards Vin. Measuring Vdrain is a way to determine when the inductor current reaches zero. To measure the high voltage Vdrain directly is generally not economical or practical. Instead, a winding is added to the boost inductor. This winding, called the Zero Current Detection (ZCD) winding, provides a scaled representation of the inductor voltage that is sensed by the controller. Figure 32 shows waveforms of ideal CrM operation using a ZCD winding. Figure 32. Ideal CrM Waveforms Using a ZCD Winding DRV 0 A 0 V 0 V 0 V 0 V Diode Conduction MOSFET Conduction TSW ton tdiode toff VCL(NEG) VZCD(TRIG) VZCD(ARM) VCL(POS) VZCD(WIND),on VZCD VZCD(WIND),off VZCD(WIND) Vout Vdrain IL(peak) IL The voltage induced on the ZCD winding during the switch on time (VZCD(WIND),on) is calculated using Equation 9: VZCD(WIND),on + −Vin NB :NZCD (eq. 9) Where Vin is the instantaneous input voltage and NB:NZCD is the turns ratio of the boost winding to the ZCD winding. The voltage induced on the ZCD winding during the switch off time (VZCD(WIND),off) is calculated using Equation 10: VZCD(WIND),off + Vout * Vin NB :NZCD (eq. 10) When the inductor current reaches zero, the ZCD pin voltage (VZCD) follows the ZCD winding voltage (VZCD(WIND)) and begins to decrease and ring towards zero volts. The NCP1608 detects the falling edge of VZCD and turns the driver on. To ensure that a ZCD event is not inadvertently detected, the NCP1608 logic verifies that VZCD exceeds VZCD(ARM) and then senses that VZCD decreases to less than VZCD(TRIG) (Figure 33). |
Similar Part No. - NCP1608_10 |
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Similar Description - NCP1608_10 |
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