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ML4801 Folha de dados(PDF) 8 Page - Micro Linear Corporation |
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ML4801 Folha de dados(HTML) 8 Page - Micro Linear Corporation |
8 / 14 page ML4801 8 Overvoltage Protection The OVP comparator serves to protect the power circuit from being subjected to excessive voltages if the load should suddenly change. A resistor divider from the high voltage DC output of the PFC is fed to VFB. When the voltage on VFB exceeds 2.75V, the PFC output driver is shut down. The PWM section will continue to operate. The OVP comparator has 250mV of hysteresis, and the PFC will not restart until the voltage at VFB drops below 2.5V. The OVP trip level should be set at a level where the active and passive external power components and the ML4801 are within their safe operating voltages, but not so low as to interfere with the regulator operation of the boost voltage regulation loop. Error Amplifier Compensation The PWM loading of the PFC can be modeled as a negative resistor; an increase in input voltage to the PWM causes a decrease in the input current. This response dictates the proper compensation of the two transconductance error amplifiers. Figure 2 shows the types of compensation networks most commonly used for the voltage and current error amplifiers, along with their respective return points. The current loop compensation is returned to VREF to produce a soft-start characteristic on the PFC: as the reference voltage comes up from zero volts, it creates a differentiated voltage on IEAO which prevents the PFC from immediately demanding a full duty cycle on its boost converter. There are two major concerns when compensating the voltage loop error amplifier; stability and transient response. Optimizing interaction between transient response and stability requires that the error amplifier’s open-loop crossover frequency should be 1/2 that of the line frequency, or 23Hz for a 47Hz line (lowest anticipated international power frequency). Rapid perturbations in line or load conditions will cause the input to the voltage error amplifier (VFB) to deviate from its 2.5V (nominal) value. If this happens, the transconductance of the voltage error amplifier will increase significantly. This increases the gain-bandwidth product of the voltage loop, resulting in a much more rapid voltage loop response to such perturbations than would occur with a conventional linear gain characteristic. The current amplifier compensation is similar to that of the voltage error amplifier with the exception of the choice of crossover frequency. The crossover frequency of the current amplifier should be at least 10 times that of the voltage amplifier, to prevent interaction with the voltage loop. It should also be limited to less than 1/6th that of the switching frequency, e.g. 16.7kHz for a 100kHz switching frequency. There is a also a degree of gain contouring applied to the transfer characteristic of the current error amplifier, to increase its speed of response to current-loop perturbations. However, the boost inductor will usually be the dominant factor in overall current loop response. Therefore, this contouring is significantly less marked than that of the voltage error amplifier. For more information on compensating the current and voltage control loops, see Application Notes 33, 34, and 55. Application Note 16 also contains valuable information for the design of this class of PFC. Oscillator (RTCT) The oscillator frequency is set by the values of RT and CT, which determine the ramp and off-time of the ML4801's master oscillator: f tt OSC RAMP DEADTIME = + 1 (2) The deadtime of the oscillator is derived from the following equation: tC R V V RAMP T T REF REF =´ ´ - - F HG I KJ ln . . 125 375 (3) at VREF = 7.5V: tC R RAMP T T =´ ´ 051 . The ramp of the oscillator may be determined using: t V mA CC DEADTIME T T =´ = ´ 25 55 455 . . (4) The deadtime is so small (tRAMP >> tDEADTIME) that the FUNCTIONAL DESCRIPTION (Continued) 15 VEAO IEAO VFB IAC VRMS ISENSE 2.5V - + 16 2 4 3 VEA - + IEA + - GND VREF 1 PFC OUTPUT GAIN MODULATOR Figure 2. Compensation Network Connections for the Voltage and Current Error Amplifiers |
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Descrição semelhante - ML4801 |
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