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FAN4800IM Fiches technique(PDF) 9 Page - Fairchild Semiconductor |
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FAN4800IM Fiches technique(HTML) 9 Page - Fairchild Semiconductor |
9 / 20 page © 2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN4800 Rev. 1.0.5 9 Functional Description The FAN4800 consists of an average current controlled, continuous boost Power Factor Correction (PFC) front end and a synchronized Pulse Width Modulator (PWM) back end. The PWM can be used in either current or voltage mode. In voltage mode, feed forward from the PFC output bus can be used to improve the PWM’s line regulation. In either mode, the PWM stage uses conven- tional trailing-edge, duty-cycle modulation. This patented leading/trailing edge modulation results in a higher usable PFC error amplifier bandwidth and can signifi- cantly reduce the size of the PFC DC bus capacitor. The synchronization of the PWM with the PFC simplifies the PWM compensation due to the controlled ripple on the PFC output capacitor (the PWM input capacitor). The PWM section of the FAN4800 runs at the same fre- quency as the PFC. In addition to power factor correction, a number of pro- tection features are built into the FAN4800. These include soft-start, PFC over-voltage protection, peak cur- rent limiting, brownout protection, duty cycle limiting, and under-voltage lockout (UVLO). Power Factor Correction Power Factor Correction treats a nonlinear load like a resistive load to the AC line. For a resistor, the current drawn from the line is in phase with and proportional to the line voltage, so the power factor is unity (one). A common class of nonlinear load is the input of most power supplies, which use a bridge rectifier and capaci- tive input filter fed from the line. The peak charging effect, which occurs on the input filter capacitor in these supplies, causes brief, high-amplitude pulses of current to flow from the power line, rather than a sinusoidal current in phase with the line voltage. Such supplies present a power factor to the line of less than one (i.e., they cause significant current harmonics of the power line frequency to appear at the input). If the input current drawn by such a supply (or any nonlinear load) can be made to follow the input voltage in instantaneous amplitude, it appears resistive to the supply. To hold the input current draw of a device drawing power from the AC line in phase with and proportional to the input voltage, that device must be prevented from load- ing the line except in proportion to the instantaneous line voltage. To accomplish this, the PFC section of the FAN4800 uses a boost mode DC-DC converter. The input to the converter is the full-wave, rectified, AC line voltage. No bulk filtering is applied following the bridge rectifier, so the input voltage to the boost converter ranges (at twice line the frequency) from zero volts to a peak value of the AC input and back to zero. By forcing the boost converter to meet two simultaneous conditions, it is possible to ensure that the current drawn from the power line is proportional to the input line voltage. One of these conditions is that the output voltage of the boost converter must be set higher than the peak value of the line voltage. A commonly used value is 385VDC, to allow for a high line of 270VAC rms. The second condition is that the current drawn from the line at any given instant must be proportional to the line voltage. Estab- lishing a suitable voltage control loop for the converter, which in turn drives a current error amplifier and switch- ing output driver, satisfies the first of these requirements. The second requirement is met by using the rectified AC line voltage to modulate the output of the voltage control loop. Such modulation causes the current error amplifier to command a power stage current that varies directly with the input voltage. To prevent ripple, which necessar- ily appears at the output of boost circuit (typically about 10VAC on a 385VDC level), from introducing distortion back through the voltage error amplifier, the bandwidth of the voltage loop is deliberately kept low. A final refine- ment is to adjust the overall gain of the PFC section to be proportional to 1/VIN 2, which linearizes the transfer func- tion of the system as the AC input voltage. Since the boost converter in the FAN4800 PFC is current averaging, no slope compensation is required. 1. PFC Section 1.1 Gain Modulator Figure 1 shows a block diagram of the PFC section of the FAN4800. The gain modulator is the heart of the PFC, as the circuit block controls the response of the current loop to line voltage waveform and frequency, RMS line voltage, and PFC output voltages. There are three inputs to the gain modulator: 1. A current representing the instantaneous input voltage (amplitude and wave shape) to the PFC. The rectified AC input sine wave is converted to a proportional cur- rent via a resistor and is then fed into the gain modula- tor at IAC. Sampling current in this way minimizes ground noise, required in high-power, switching-power conversion environments. The gain modulator responds linearly to this current. 2. A voltage proportional to the long-term RMS AC line voltage, derived from the rectified line voltage after scaling and filtering. This signal is presented to the gain modulator at VRMS. The output of the gain modu- lator is inversely proportional to VRMS 2 (except at unusually low values of VRMS, where special gain con- touring takes over to limit power dissipation of the cir- cuit components under heavy brownout conditions). The relationship between VRMS and gain is called K and is illustrated in Figure 5. |
Numéro de pièce similaire - FAN4800IM |
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Description similaire - FAN4800IM |
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