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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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ML4804CS Fiches technique(PDF) 9 Page - Fairchild Semiconductor |
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ML4804CS Fiches technique(HTML) 9 Page - Fairchild Semiconductor |
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9 / 14 page  ML4804 REV. 1.0.2 3/9/2001 9 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 2.50V. The VFB should be set at a level where the active and passive external power components and the ML4804 are within their safe operating voltages, but not so low as to interfere with the boost voltage regulation loop. VCCOVP The VCCOVP feature of the ML4804 works along with the TriFaultTM Detect as a redundant PFC buss voltage limiter, to prevent a damaged and broken connection or component from causing an unsafe fault condition. VCCOVP assumes that VCC is generated from a bootstrap winding on the PFC boost inductor, or by some other means whereby VCC is proportional to VBUSS. If the proportionality is exact, then a nominal VBUSS of 385V at VCC = 15.0V will cause the VCCOVP comparator to shut the PFC down when VBUSS = [(16.4/15.0) x 385V] = 444V. The PFC will then remain in the shutdown state until VCC declines to 13.0V, at which time the PFC will restart. If the PFC VCC again encounters an over voltage condition, the protection cycle will repeat. Note that the PWM stage of the ML4804 remains operational even when the PFC goes into VCCOVP shutdown. For a real-world example, assume that the bootstrap supply is derived from a conventional boost inductor winding and rectified using Shottky diodes. Then it follows that the voltage from the bootstrap winding must equal 15.8V during regular circuit operation, and will increase to 17.2V at the point of VCCOVP shutdown. Then the output voltage from the PFC will have increased from a noninal VBUSS of 385VDC to (17.2/15.8) x 385V = 419VDC. When VBUSS reaches 419V, the PFC will shut off, thereby protecting the output (BUSS) capacitor and the semiconductors in both the PFC and PWM stages. To assure reasonable headroom in which to operate this device, VCCOVP tracks with UVLO. The VCCOVP threshold is always at least 2V above that of the UVLO. To assure reliable operation of the ML4804, VCC must be operated from a bootstrap winding on the PFC’s inductor, or from an external power supply whose output is regulated to 15.0V (nominal). In the case of a regulated power supply powering the ML4804, the VCCOVP function will be rendered non-operational. 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 PFC's 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 Figure 2. Compensation Network Connections for the Voltage and Current Error Amplifiers Figure 3. External Component Connections to VCC FUNCTIONAL DESCRIPTION (Continued) 15 VEAO IEAO VFB IAC VRMS ISENSE 2.5V – + 16 2 4 3 VEA + – IEA + – VREF 1 PFC OUTPUT GAIN MODULATOR ML4804 VCC GND VBIAS 0.22µF CERAMIC 15V ZENER RBIAS |
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Description similaire - ML4804CS |
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