поискавой системы для электроныых деталей |
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FAN9611 датащи(PDF) 10 Page - Fairchild Semiconductor |
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FAN9611 датащи(HTML) 10 Page - Fairchild Semiconductor |
10 / 36 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.4 10 Figure 10. Input-Voltage Feedforward When the PWM ramp is made proportional to the input voltage squared, the system offers other noteworthy benefits. The first is the input voltage-independent small signal gain of the closed loop power supply, which makes compensation of the voltage regulation loop much easier. The second side benefit is that the output of the error amplifier becomes directly proportional to the input power of the converter. This phenomenon is very significant and it is re-visited in Section 9 describing light-load operation. 5. Starting a PWM Cycle The principle of boundary conduction mode calls for a pulse width modulator able to operate with variable frequency and initiate a switching period whenever the current in the boost inductor reaches zero. Therefore, BCM controllers cannot utilize a fixed frequency oscillator circuit to control the operating frequency. Instead, a zero current detector is used to sense the inductor current and turn on the power switch once the current in the boost inductor reaches zero. This process is facilitated by an auxiliary winding on the boost inductor. The voltage waveform of the auxiliary winding can be used for indirect detection of the zero inductor current condition of the boost inductor. Therefore it should be connected to the zero current detect input, as shown in Figure 11. Figure 11. Simple Zero-Current Detection Method The auxiliary winding can also be used to generate bias for the PFC controller when an independent bias power supply is not present in the system. At startup condition and in the unlikely case of missing zero current detection, the lack of an oscillator would mean that the converter stops operating. To overcome these situations, a restart timer is employed to kick start the controller and provide the first turn-on command, as shown in Figure 12. Figure 12. PWM Cycle Start 6. Terminating the Conduction Interval Terminating the conduction period of the boost transistor in boundary conduction mode controllers is similar to any other pulse width modulator. During normal operation, the PWM comparator turns off the power transistor when the ramp waveform exceeds the control voltage provided by the error amplifier. In the FAN9611/12 and in similar voltage-mode PWMs, the ramp is a linearly rising waveform at one input of the comparator circuit. Figure 13. Conduction Interval Termination In addition to the PWM comparator, the current limit circuit and a timer circuit limiting the maximum on-time of the boost transistor can also terminate the gate drive pulse of the controller. These functions provide protection for the power switch against excessive current stress. 7. Protecting the Power Components In general, power converters are designed with adequate margin for reliable operation under all operating conditions. However, it might be difficult to predict dangerous conditions under transient or certain fault situations. Therefore, the FAN9611/12 contains dedicated protection circuits to monitor the individual peak currents in the boost inductors and in the power transistors. Furthermore, the boost output voltage is sensed by two independent mechanisms to provide over-voltage protection for the power transistors, rectifier diodes, and the output energy storage capacitor of the converter. |
Аналогичный номер детали - FAN9611 |
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Аналогичное описание - FAN9611 |
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