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NCP1653A Datasheet(PDF) 12 Page - ON Semiconductor |
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NCP1653A Datasheet(HTML) 12 Page - ON Semiconductor |
12 / 19 page NCP1653, NCP1653A http://onsemi.com 12 Vin even though Vout is reduced in follower boost operation. As a result, the on time t1 is reduced. Reduction of on time makes the loss of the inductor and power MOSFET smaller. Hence, it allows cheaper cost in the inductor and power MOSFET or allows the circuit components to operate at a lower stress condition in most of the time. Output Feedback The output voltage Vout of the PFC circuit is sensed as a feedback current IFB flowing into the FB pin (Pin 1) of the device. Since the FB pin voltage VFB1 is much smaller than Vout, it is usually neglected. (eq.14) IFB + Vout * VFB1 RFB [ Vout RFB where RFB is the feedback resistor across the FB pin (Pin 1) and the output voltage referring to Figure 2. Then, the feedback current IFB represents the output voltage Vout and will be used in the output voltage regulation, undervoltage protection (UVP), and overvoltage protection (OVP). Output Voltage Regulation Feedback current IFB which represents the output voltage Vout is processed in a function with a reference current (Iref = 200 mA typical) as shown in regulation block function in Figure 32. The output of the voltage regulation block, low−pass filter on Vcontrol pin and the Icontrol = Vcontrol / R1 block is in Figure 30 is control current Icontrol. And the input is feedback current IFB. It means that Icontrol is the output of IFB and it can be described as in Figure 32. There are three linear regions including: (1) IFB < 96% × Iref, (2) 96% × Iref <IFB < Iref, and (3) IFB > Iref. They are discussed separately as follows: Figure 32. Regulation Block Icontrol Iref Iref 96% IFB Icontrol(max) Region (1): IFB < 96% × Iref When IFB is less than 96% of Iref (i.e., Vout < 96% RFB × Iref), the NCP1653 operates in follower boost mode. The regulation block output Vreg is at its maximum value. Icontrol becomes its maximum value (i.e., Icontrol = Icontrol(max) = Iref/2 = 100 mA) which is a constant. (eq.13) becomes (eq.15). Vout + h 2RS R vac Icontrol(max) Vref Vac RM RCS Pout (eq.15) T Vac Pout The output voltage Vout is regulated at a particular level with a particular value of RMS input voltage Vac and output power Pout. However, this output level is not constant and depending on different values of Vac and Pout. The follower boost operating area is illustrated in Figure 33. Figure 33. Follower Boost Region V V V Vout ac(max) ac(min) ac Pout(min) Pout(max) 12 V in 96% Iref RFB 1. Pout increases, Vout decreases 2. Vac decreases, Vout decreases Region (2): 96% × Iref < IFB < Iref When IFB is between 96% and 100% of Iref (i.e., 96% RFB × Iref < Vout < RFB × Iref), the NCP1653 operates in constant output voltage mode which is similar to the follower boost mode characteristic but with narrow output voltage range. The regulation block output Vreg decreases linearly with IFB in the range from 96% of Iref to Iref. It gives a linear function of Icontrol in (eq.16). (eq.16) Icontrol + Icontrol(max) 0.04 1 * Vout RFB Iref Resolving (eq.16) and (eq.13), Vout + Vac RM RCS 2RS R vac Vref 0.04 Icontrol(max) Pout h ) Vac RFB Iref (eq.17) According to (eq.17), output voltage Vout becomes RFB × Iref when power is low (Pout ≈ 0). It is the maximum value of Vout in this operating region. Hence, it can be concluded that output voltage increases when power decreases. It is similar to the follower boost characteristic in (eq.15). On the other hand in (eq.17), output voltage Vout becomes RFB × Iref when RMS input voltage Vac is very high. It is the maximum value of Vout in this operating region. Hence, it can also be concluded that output voltage increases when RMS input voltage increases. It is similar to another follower boost characteristic in (eq.15). This characteristic is illustrated in Figure 34. Figure 34. Constant Output Voltage Region V V V Vout ac(max) ac(min) ac Pout(min) Pout(max) 12 96% Iref RFB 1. Pout increases, Vout decreases 2. Vac decreases, Vout decreases Iref RFB Region (3): IFB > Iref When IFB is greater than Iref (i.e., Vout > RFB × Iref), the NCP1653 provides no output or zero duty ratio. The regulation block output Vreg becomes 0 V. Icontrol also |
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