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NCP1337 Datasheet(PDF) 11 Page - ON Semiconductor |
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NCP1337 Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 15 page NCP1337 http://onsemi.com 11 Soxyless The “Valley point detection” is based on the observation of the Power MOSFET Drain voltage variations. When the transformer is fully demagnetized, the Drain voltage evolution from the plateau level down to the VIN asymptote is governed by the resonating energy transfer between the LP transformer inductor and the global capacitance present on the Drain. These voltage oscillations create current oscillation in the parasitic capacitor across the switching MOSFET (modelized by the Crss capacitance between Gate and Drain): a negative current (flowing out of DRV pin) takes place during the decreasing part of the Drain oscillation, and a positive current (entering into the DRV pin) during the increasing part. The Drain valley corresponds to the inversion of the current (i.e., the zero crossing): by detecting this point, we always ensure a true valley turn−on. Lprim Crss DRV Isoxy Vswitch TSWING t Figure 6. Soxyless Concept The current in the Power MOSFET gate is: Igate = Vringing/Zc (with Zc the capacitance impedance) so Igate = Vringing S (2 S p S Fres S Crss) The magnitude of this gate current depends on the MOSFET, the resonating frequency and the voltage swing present on the Drain at the end of the plateau voltage. The dead time TSWING is given by the equation: Tswing + 0.5 Fres + p *Lp * Cdrain (eq. 1) (where LP is the primary transformer inductance and CDRAIN the total capacitance present on the MOSFET Drain. This capacitance includes the snubber capacitor if any, the transformer windings stray capacitance plus the parasitic MOSFET capacitances COSS and CRSS). Internal Feedback Circuitry To simplify the implementation of a primary regulation, it is necessary to inject a current into the FB pin (instead of sourcing it out). But to have a precise primary regulation, the voltage present on FB pin must be regulated. Figure 8 gives the FB pin internal implementation: the circuitry combines the functions of a current to voltage converter and a voltage regulator. FB + - + 3 V Vdd Internal Setpoint 20 kHz Low−pass Filter Figure 7. Internal Implementation of FB Pin |
Similar Part No. - NCP1337_11 |
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Similar Description - NCP1337_11 |
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