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Motore di ricerca datesheet componenti elettronici |
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FAN4800 Scheda tecnica(PDF) 10 Page - Fairchild Semiconductor |
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FAN4800 Scheda tecnica(HTML) 10 Page - Fairchild Semiconductor |
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10 / 20 page  © 2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN4800 Rev. 1.0.5 10 3. The output of the voltage error amplifier, VEAO. The gain modulator responds linearly to variations in this voltage. The output of the gain modulator is a current signal, in the form of a full wave rectified sinusoid at twice the line frequency. 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 current error loop and ultimately controls the instantaneous cur- rent draw of the PFC from the power line. The general form of the output of the gain modulator is: More precisely, the output current of the gain modulator is 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 voltage of 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 from ISENSE. ISENSE = IGAINMOD – IOFFSET and IOFFSET can only be measured when VEAO is less than 0.5V and IGAINMOD is 0A. Typical IOFFSET is around 60µA. 1.2 Selecting RAC for IAC pin IAC pin is the input of the gain modulator. IAC is also a current mirror input and requires current input. Selecting a proper resistor RAC provides a good sine wave current derived from the line voltage and helps program the maximum input power and minimum input line voltage. RAC = VIN peak x 7.9K. For example, if the minimum line voltage is 80VAC, the RAC = 80 x 1.414 x 7.9K = 894kΩ. 1.3 Current Error Amplifier, IEAO The current error amplifier’s output controls the PFC duty cycle to keep the average current through the boost inductor a linear function of the line voltage. At the invert- ing input to the current error amplifier, the output current of the gain modulator is summed with a current, which results from a negative voltage being impressed upon the ISENSE pin. The negative voltage on ISENSE represents the sum of all currents flowing in the PFC circuit and is typically derived from a current sense resistor in series with the negative terminal 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 the duty cycle modulator polarities internal to the PFC, an increase in positive current from the gain modulator causes the output stage to increase its duty cycle until the voltage on ISENSE is adequately negative to cancel this increased current. Similarly, if the gain modulator’s output decreases, the output duty cycle decreases to achieve a less negative voltage on the ISENSE pin. 1.4 Cycle-By-Cycle Current Limiter and Selecting RS As well as being a part of the current feedback loop, the ISENSE pin is a direct input to the cycle-by-cycle current limiter for the PFC section. If the input voltage at this pin is ever less than -1V, the output of the PFC is disabled until the protection flip-flop is reset by the clock pulse at the 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 equals IGAINMOD x 3.5K. Since the maximum output voltage of the gain modulator is IGAINMOD maximum x 3.5k = 0.8V during the steady state, RS x line input current is limited to below 0.8V as well. Therefore, to choose RS, use the following equation: For example, if the minimum input voltage is 80VAC and the maximum input RMS power is 200Watt, RS = (0.8V x 80V x 1.414) / (2 x 200) = 0.226Ω. 1.5 PFC OVP In the FAN4800, the PFC OVP comparator serves to pro- tect the power circuit from being subjected to excessive voltages if the load changes suddenly. A resistor divider from the high-voltage DC output of the PFC is fed to VFB. When the voltage on VFB exceeds 2.78V, the PFC output driver is shut down. The PWM section continues to oper- ate. The OVP comparator has 280mV of hysteresis and the PFC does not restart until the voltage at VFB drops below 2.50V. VCC OVP can also serve as a redundant PFC OVP protection. VCC OVP threshold is 17.9V with 1.5V hysteresis. 2 1 AC EAO GAINMOD RMS IV IV V × =× (3) ( 0.625) GAINMOD EAO AC IK V I =× − × (4) (5) 0.8 2 INPEAK S VV R LineInput Power × = × |
Codice articolo simile - FAN4800_06 |
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Descrizione simile - FAN4800_06 |
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