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FAN7930BMX Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN7930BMX Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 22 page  © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930B • Rev. 1.0.2 11 Applications Information 1. Startup: Normally, supply voltage (VCC) of a PFC block is fed from the additional power supply, which can be called standby power. Without this standby power, auxiliary winding to detect zero current detection can be used as a supply source. Once the supply voltage of the PFC block exceeds 12V, internal operation is enabled until the voltage drops to 8.5V. If VCC exceeds VZ, 20mA current is sinking from VCC. Figure 22. Startup Circuit 2. INV Block: Scaled-down voltage from the output is the input for the INV pin. Many functions are embedded based on the INV pin: transconductance amplifier, output OVP comparator and disable comparator. For the output voltage control, a transconductance amplifier is used instead of the conventional voltage amplifier. The transconductance amplifier (voltage- controlled current source) aids the implementation of OVP and disables function. The output current of the amplifier changes according to the voltage difference of the inverting and non-inverting input of the amplifier. To cancel down the line input voltage effect on power factor correction, effective control response of PFC block should be slower than the line frequency and these conflicts with the transient response of controller. Two- pole one-zero type compensation may be used to meet both requirements. The OVP comparator shuts down the output drive block when the voltage of the INV pin is higher than 2.675V and there is 0.175V hysteresis. The disable comparator disables the operation when the voltage of the inverting input is lower than 0.35V and there is 100mV hysteresis. An external small-signal MOSFET can be used to disable the IC. The IC operating current decreases to reduce power consumption if the IC is disabled. 0 is the timing chart of the internal circuit near the INV pin when rated PFC output voltage is assumed at 390VDC and VCC supply voltage is 15V. Figure 23. Circuit Around INV Pin Figure 24. Timing Chart for INV Block 3. OVP Pin: Over-Voltage Protection (OVP) is embedded by the information at the INV pin. That information comes from the output through the voltage dividing resistors. To scale down from high voltage to low one, high resistance normally used with low resistance. In cases the resistor of high resistance get a damage and resistance is changed to high, though INV pin information is normal output voltage exceeds its rated output. Once this happen, output electrolytic capacitor may be exploded. To prevent such a catastrophe additional OVP pin is assigned to double check output voltage. Thus additional OVP may be called 2 nd OVP while INV pin OVP can be called 1st OVP. |
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