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NCP1653A Datasheet(PDF) 14 Page - ON Semiconductor |
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NCP1653A Datasheet(HTML) 14 Page - ON Semiconductor |
14 / 19 page NCP1653, NCP1653A http://onsemi.com 14 Cvac is to maintain the In pin (Pin 3) voltage in the calculation to always be the peak of the sinusoidal voltage due to very little current consumption (i.e., Vin = √2 Vac and Ivac ≈ 0). This Ivac current represents the RMS input voltage Vac and will be used in overpower limitation (OPL) and the PFC duty modulation. Figure 37. Input Voltage Sensing In Current Mirror 3 12 k Cvac Rvac Ivac 4 V V in 9 V There is an internal 9 V ESD Zener Diode on the pin. Hence, the value of Rvac is recommended to be at least 938 k W for possibly up to 400 V instantaneous input voltage. Rvac 400 V * 9V u 12 k W 9V * 4V (eq.19) Rvac u 938 kW Overpower Limitation (OPL) Sense current IS represents the inductor current IL and hence represents the input current approximately. Input−voltage current Ivac represents the RMS input voltage Vac and hence represents the input voltage. Their product (IS × Ivac) represents an approximated input power (IL × Vac). Figure 38. Overpower Limitation Reduces Vcontrol FB ref ref reg 300 k Vcontrol V I I 96% I Limitation Overpower Regulation Block 2 0 1 When the product (IS × Ivac) is greater than a permissible level 3 nA2, the output Vreg of the regulation block is pulled to 0 V. It makes Vcontrol to be 0 V indirectly and VM is pulled to be its maximum. It generates the minimum duty ratio or no duty ratio eventually so that the input power is limited. The OPL is automatically deactivated when the product (IS × Ivac) becomes lower than the 3 nA2 level. This 3 nA2 level corresponds to the approximated input power (IL × Vac) to be smaller than the particular expression in (eq.20). IS Ivac t 3nA2 IL @ RCS RS Vac @ 2 Rvac ) 12 kW t 3nA2 IL @ Vac t RS RCS Rvac ) 12 kW 2 3nA2 (eq.20) Biasing the Controller It is recommended to add a typical 1 nF to 100 nF decoupling capacitor next to the VCC pin for proper operation. When the NCP1653 operates in follower boost mode, the PFC output voltage is not always regulated at a particular level under all application range of input voltage and load power. It is not recommended to make a low−voltage bias supply voltage by adding an auxiliary winding on the PFC boost inductor. Alternatively, it is recommended to get the VCC biasing supply from the second−stage power conversion stage as shown in Figure 39. Figure 39. Recommended Biasing Scheme in Follower Boost Mode AC Input Output EMI Filter NCP1653 V cc V bulk Second−stage Power Converter Voltage When the NCP1653 operates in constant output voltage mode, it is possible to make a low−voltage bias supply by adding an auxiliary winding on the PFC boost inductor in Figure 40. In PFC boost circuit, the input is the rectified AC voltage and it is non−constant versus time that makes the auxiliary winding voltage also non−constant. Hence, the configuration in Figure 40 charges the voltages in capacitors C1 and C2 to n ×(Vout − Vin) and n×Vin and n is the turn ratio. As a result, the stack of the voltages is n ×Vout that is constant and can be used as a biasing voltage. |
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