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FAN7530 데이터시트(PDF) 15 Page - Fairchild Semiconductor |
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FAN7530 데이터시트(HTML) 15 Page - Fairchild Semiconductor |
15 / 37 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.3 15 9. Input Voltage Sensing (VIN) The input AC voltage is sensed at the VIN pin. The input voltage is used in three functions: input under-voltage lockout (brownout protection), input over-voltage protection, and input voltage feedforward in the PWM control circuit. All the functions require the RMS value of the input voltage waveform. Since the RMS value of the AC input voltage is directly proportional to its peak, it is sufficient to find the peak instead of the more complicated and slower method of integrating the input voltage over a half line cycle. The internal circuit of the VIN pin works with peak detection of the input AC waveforms. One of the important benefits of this approach is that the peak indicates the correct RMS value even at no load when the HF filter capacitor at the input side of the boost converter is not discharged around the zero crossing of the line waveform. Another notable benefit is that during line transients, when the peak exceeds the previously measured value, the input- voltage feedforward circuit can react immediately, without waiting for a valid integral value at the end of the half line period. Furthermore, lack of zero crossing detection could fool the integrator while the peak detector works properly during light-load operation. The valid range for the peak of the AC input is between approximately 0.925V and 3.7V. This range is optimized for universal input voltage range of operation. If the peak of the sense voltage remains below the 0.925V threshold, input under-voltage or brownout condition is declared and the FAN9611/12 stops operating. When the VIN voltage exceeds 3.7V, the FAN9611/12 input voltage sense circuit saturates and the feedforward circuit is not able to follow the input any higher. Consequently, the slope of the PWM ramp remains constant corresponding to the VIN = 3.7V level amplitude for any VIN voltage above 3.7V. The input voltage is measured by a tracking analog-to- digital converter, which keeps the highest value (peak voltage) of the input voltage waveform. Once a measurement is taken, the converter tracks the input for at least 12ms before a new value is taken. This delay ensures at least one new peak value is captured before the new value is used. Figure 23. Input Voltage Sensing Circuit The measured peak value is then used in the following half-line cycle while a new measurement is executed to be used in the next half line cycle. This operation is synchronized to the zero crossing of the line waveform. Since the input voltage measurement is held steady during the line half periods, this technique does not feed any AC ripple into the control loop. If line zero crossing detection is missing, the FAN9611/12 measures the input voltage in every 32ms; it can operate from a DC input as well. The following figures provide detail about the input voltage sensing method of the controller. As shown in the waveforms, input voltage feedforward is instantaneous when the line voltage increases and has a half line cycle delay when the input voltage decreases. Any increase in input voltage would cause output over voltage due to the slow nature of the voltage regulation loop. This is successfully mitigated by the immediate action of the input-voltage feedforward circuit. |
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