Part Name Includedstart withendmatch Description
FAN4800IN

 Low Start-Up Current PFC/PWM Controller Combos ( 20 Page)

FAIRCHILD
 100%
 10 page © 2005 Fairchild Semiconductor Corporationwww.fairchildsemi.comFAN4800 Rev. 1.0.5103. The output of the voltage error amplifier, VEAO. Thegain modulator responds linearly to variations in thisvoltage.The output of the gain modulator is a current signal, inthe form of a full wave rectified sinusoid at twice the linefrequency. This current is applied to the virtual ground(negative) input of the current error amplifier. In this way,the gain modulator forms the reference for the currenterror loop and ultimately controls the instantaneous cur-rent draw of the PFC from the power line. The generalform of the output of the gain modulator is:More precisely, the output current of the gain modulatoris given by:where K is in units of V-1.Note that the output current of the gain modulator is lim-ited around 228.57µA and the maximum output voltageof the gain modulator is limited to 228.57µA x 3.5K =0.8V.This 0.8V also determines the maximum input power.However, IGAINMOD cannot be measured directly fromISENSE. ISENSE = IGAINMOD – IOFFSET and IOFFSET canonly be measured when VEAO is less than 0.5V andIGAINMOD is 0A. Typical IOFFSET is around 60µA.1.2 Selecting RAC for IAC pinIAC pin is the input of the gain modulator. IAC is also acurrent mirror input and requires current input. Selectinga proper resistor RAC provides a good sine wave currentderived from the line voltage and helps program themaximum input power and minimum input line voltage.RAC = VIN peak x 7.9K. For example, if the minimum linevoltage is 80VAC, the RAC = 80 x 1.414 x 7.9K = 894kΩ.1.3 Current Error Amplifier, IEAOThe current error amplifier’s output controls the PFC dutycycle to keep the average current through the boostinductor a linear function of the line voltage. At the invert-ing input to the current error amplifier, the output currentof the gain modulator is summed with a current, whichresults from a negative voltage being impressed uponthe ISENSE pin.The negative voltage on ISENSE represents the sum of allcurrents flowing in the PFC circuit and is typically derivedfrom a current sense resistor in series with the negativeterminal of the input bridge rectifier.The inverting input of the current error amplifier is a vir-tual ground. Given this fact, and the arrangement of theduty cycle modulator polarities internal to the PFC, anincrease in positive current from the gain modulatorcauses the output stage to increase its duty cycle untilthe voltage on ISENSE is adequately negative to cancelthis increased current. Similarly, if the gain modulator’soutput decreases, the output duty cycle decreases toachieve a less negative voltage on the ISENSE pin.1.4 Cycle-By-Cycle Current Limiter and Selecting RSAs well as being a part of the current feedback loop, theISENSE pin is a direct input to the cycle-by-cycle currentlimiter for the PFC section. If the input voltage at this pinis ever less than -1V, the output of the PFC is disableduntil the protection flip-flop is reset by the clock pulse atthe start of the next PFC power cycle.RS is the sensing resistor of the PFC boost converter.During the steady state, line input current x RS equalsIGAINMOD x 3.5K.Since the maximum output voltage of the gain modulatoris IGAINMOD maximum x 3.5k = 0.8V during the steadystate, RS x line input current is limited to below 0.8V aswell. Therefore, to choose RS, use the following equation:For example, if the minimum input voltage is 80VAC andthe maximum input RMS power is 200Watt,RS = (0.8V x 80V x 1.414) / (2 x 200) = 0.226Ω.1.5 PFC OVPIn the FAN4800, the PFC OVP comparator serves to pro-tect the power circuit from being subjected to excessivevoltages if the load changes suddenly. A resistor dividerfrom the high-voltage DC output of the PFC is fed to VFB.When the voltage on VFB exceeds 2.78V, the PFC outputdriver is shut down. The PWM section continues to oper-ate. The OVP comparator has 280mV of hysteresis andthe PFC does not restart until the voltage at VFB dropsbelow 2.50V. VCC OVP can also serve as a redundantPFC OVP protection. VCC OVP threshold is 17.9V with1.5V hysteresis.21ACEAOGAINMODRMSIVIVV×=×(3)(0.625)GAINMODEAOACIKVI=×−×(4)(5)0.82INPEAKSVVRLineInput Power×=×

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