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L6563STR Datasheet(PDF) 30 Page - STMicroelectronics |
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L6563STR Datasheet(HTML) 30 Page - STMicroelectronics |
30 / 42 page ![]() Application information L6563S 30/42 Doc ID 16116 Rev 3 Figure 41. Tracking output voltage vs Input voltage characteristic with TBO 6.6 Inductor saturation detection Boost inductor's hard saturation may be a fatal event for a PFC pre-regulator: the current up- slope becomes so large (50-100 times steeper, see Figure 42) that during the current sense propagation delay the current may reach abnormally high values. The voltage drop caused by this abnormal current on the sense resistor reduces the gate-to-source voltage, so that the MOSFET may work in the active region and dissipate a huge amount of power, which leads to a catastrophic failure after few switching cycles. However, in some applications such as ac-dc adapters, where the PFC pre-regulator is turned off at light load for energy saving reasons, even a well-designed boost inductor may occasionally slightly saturate when the PFC stage is restarted because of a larger load demand. This happens when the restart occurs at an unfavorable line voltage phase, i.e. when the output voltage is significantly below the rectified peak voltage. As a result, in the boost inductor the inrush current coming from the bridge rectifier adds up to the switched current and, furthermore, there is little or no voltage available for demagnetization. To cope with a saturated inductor, the L6563S is provided with a second comparator on the current sense pin (CS, pin 4) that stops the IC if the voltage, normally limited within 1.1 V, exceeds 1.7 V. After that, the IC will be attempted to restart by the internal starter circuitry; the starter repetition time is twice the nominal value to guarantee lower stress for the inductor and boost diode. Hence, the system safety will be considerably increased. Figure 42. Effect of boost inductor saturation on the MOSFET current and detection method |
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