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FFP08S60S Scheda tecnica(PDF) 4 Page - Fairchild Semiconductor |
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FFP08S60S Scheda tecnica(HTML) 4 Page - Fairchild Semiconductor |
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4 / 17 page  AN-8027 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 8/26/09 4 RT/ CT OPFC OPWM OPWM (FAN4800C, FAN4802/2L) PFC dead time Figure 9. FAN480X Timing Diagram PWM Stage The PWM stage is capable of current-mode or voltage- mode operation. In current-mode applications, the PWM ramp (RAMP) is usually derived directly from a current sensing resistor or current transformer in the primary of the output stage and is thereby representative of the current flowing in the converter’s output stage. ILIMIT, which provides cycle-by-cycle current limiting, is typically connected to RAMP in such applications. For voltage-mode operation, RAMP can be connected to a separate RC timing network to generate a voltage ramp against which FBPWM voltage is compared. Under these conditions, the use of voltage feed-forward from the PFC bus can be used for better line transient response. No voltage error amplifier is included in the PWM stage, as this function is generally performed by a programmable shunt regulator, such as KA431, in the secondary-side. To facilitate the design of opto-coupler feedback circuitry, an offset voltage is built into the inverting input of PWM comparator that allows FBPWM to command a zero percent duty cycle when its pin voltage is below 1.5V. V BOUT RAMP R RAMP C RAMP REF + - PWM FBPWM 1.5V Figure 10. PWM Ramp Generation Circuit PWM Current Limit The ILIMIT pin is a direct input to the cycle-by-cycle current limiter for the PWM section. If the input voltage at this pin exceeds 1V, the output of the PWM is disabled until the start of the next PWM clock cycle. VIN OK Comparator The VIN OK comparator monitors the output of the PFC stage and inhibits the PWM stage if this voltage is less than 2.4V (96% of its nominal value). Once this voltage goes above 2.4V, the PWM stage begins to soft-start. PWM Soft-Start (SS) PWM startup is controlled by the soft-start capacitor. A 10µA current source supplies the charging current for the soft-start capacitor. Startup of the PWM is prohibited until the soft-start capacitor voltage reaches 1.5V. |
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