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CM6903IS Datasheet(PDF) 8 Page - Champion Microelectronic Corp. |
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CM6903IS Datasheet(HTML) 8 Page - Champion Microelectronic Corp. |
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8 / 14 page  CM6903/4 Low Pin Count PFC/PWM CONTROLLER COMBO 2002/12/16 Preliminary Rev. 0.4 Champion Microelectronic Corporation Page 8 Equation 2 means: average boost inductor current equals to input current. d out l in I V I V × ≈ × ∴ (3) Therefore, input instantaneous power is about to equal to the output instantaneous power. For steady state and for the each phase angle, boost converter DC equation at continuous conduction mode is: ) 1 ( 1 d V V in out − = (4) Rearrange above equations, (1), (2),(3), and (4) in term of Vout and d, boost converter duty cycle and we can get average boost diode current equation (5): e out d R V d I × − = 2 ) 1 ( (5) Also, the average diode current can be expressed as: dt t I T I off T d sw d ⋅ = ∫ ) ( 1 0 (6) If the value of the boost inductor is large enough, we can assume d d I t I ~ ) ( . It means during each cycle or we can say during the sampling, the diode current is a constant. Therefore, equation (6) becomes: ) 1 ( ' d I d I T t I I d d sw off d d − × = × = × = (7) Combine equation (7) and equation (5), and we get: sw off e out d e out d e out d T t R V I R V d I R V d d I × = ∴ × = ∴ × = × ' 2 ' ' ) ( (8) From this simple equation (8), we implement the PFC control section of the CM6903. Leading/Trailing Modulation Conventional Pulse Width Modulation (PWM) techniques employ trailing edge modulation in which the switch will turn ON right after the trailing edge of the system clock. The error amplifier output is then compared with the modulating ramp. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned OFF. When the switch is ON, the inductor current will ramp up. The effective duty cycle of the trailing edge modulation is determined during the ON time of the switch. Figure 2 shows a typical trailing edge control scheme. In case of leading edge modulation, the switch is turned OFF right at the leading edge of the system clock. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned ON. The effective duty-cycle of the leading edge modulation is determined during OFF time of the switch. Figure 3 shows a leading edge control scheme. |
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