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IRS2166DSPBF Datasheet(PDF) 14 Page - International Rectifier |
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IRS2166DSPBF Datasheet(HTML) 14 Page - International Rectifier |
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14 / 20 page  IRS2166D(S)PbF www.irf.com Page 14 Fig. 14: External over-current protection circuit Should high currents occur, the voltage across the current-sensing resistor will exceed the 4.3 V over- voltage protection threshold at the VBUS pin and the PFC MOSFET will turn off safely limiting the current. The watch-dog timer will then restart the PFC as normal (Fig. 15). The current sensing resistor value should be selected such that the over-current protection does not false trip during normal operation over the entire line voltage range and load range. A current-sensing resistor value, for example, of 1.0 W will set the over-current protection threshold to about 5 A peak. Fig. 15: PFC inductor current limited using over-current protection circuit (upper trace: DC Bus, 100 V/div; middle trace: AC line input voltage, 100 V/ div; lower trace: PFC inductor current 1 A/div). The effect that these line and load conditions have on the performance of the ballast depends on the saturation level of the PFC inductor, the selection of the PFC MOSFET, the DC bus capacitor value, the maximum on- time limit set by DZCOMP, and, how fast VCC decreases below UVLO- when the DC bus drops during ignition (the 3 V reset on the VBUS pin does not become active until RUN mode). For these reasons, the ballast designer should perform these mains interrupt and ignition tests carefully to determine the robustness of their final design and to decide if this additional over-current protection circuit is necessary. Ballast Design Equations Note: The results from the following design equations can differ slightly from experimental measurements due to IC tolerances, component tolerances, and oscillator over- and under-shoot due to internal comparator response time. Step 1: Program Deadtime The deadtime between the gate driver outputs HO and LO is programmed with timing capacitor CT and an internal deadtime resistor RDT. The deadtime is the discharge time of capacitor CT from 3/5 V CC to 1/3 VCC and is given as: 1475 ⋅ = T DT C t [s] (1) or 1475 DT T t C = [F] (2) Step 2: Program Run Frequency The final run frequency is programmed with timing resistor RT and timing capacitor CT. The charge time of capacitor CT from 1/3 V CC to 3/5 VCC determines the on- time of HO and LO gate driver outputs. The run frequency is therefore given as: ) 1475 51 . 0 ( 2 1 + ⋅ ⋅ = T T RUN R C f [Hz] (3) or 2892 02 . 1 1 − ⋅ ⋅ = RUN T T f C R [Ω] (4) Step 3: Program Preheat Frequency The preheat frequency is programmed with timing resistors RT and RPH, and timing capacitor CT. The timing resistors are connected in parallel internally for the duration of the preheat time. The preheat frequency is therefore given as: |
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