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STRF6600 Datasheet(PDF) 8 Page - Allegro MicroSystems |
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STRF6600 Datasheet(HTML) 8 Page - Allegro MicroSystems |
8 / 16 page Series STR-F6600 OFF-LINE QUASI-RESONANT FLYBACK SWITCHING REGULATORS 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 ™ 8 TM Functional Description and Operation (cont’d) Figure 7 - Voltage Regulation Waveforms Normal Operation (Quasi-Resonant) Mode Refer to the Functional Block Diagram, Typical Appli- cation diagram (figure 6), and Quasi-Resonance Wave- forms (figure 8). Regulation is achieved as in fixed off-time mode but instead of having a fixed off-time, the demagnetization of the transformer is sensed by a second comparator. This comparator threshold, V th(2) is nominally 1.45 V. Quasi- resonance sensing makes use of the natural magnetizing and leakage inductances and self-capacitances of the power circuit. Figure 8 shows the drain voltage waveform, (V DS), on pin 3 of the STR-F66xx, as well as V P, the voltage on the primary of the transformer. Once the current in the output diode stops flowing, the primary stored energy ‘rings’ as shown by V P and VDS. The resonant frequency (fr) is determined by the magne- tizing inductance of the transformer and the capacitor C4. The addition of this capacitor sets the ringing frequency and reduces the harmonic content in the V DS waveform, lowering EMI. Also since V DS falls to a minimum during the first half-cycle of the ring this point can be sensed and used to turn on the MOSFET with minimum voltage across it. Thus the MOSFET is low voltage and zero current switched (LVS/ZCS). Figure 8 – Quasi-Resonance Waveforms Dwg. GK-021 VP VDS VD ID t = π√LpC4 f R = 1/2 π√LpC4 VDS (min) VP VIN VOCP ≈ 2.8 V Vth(2) ≈ 1.45 V Vth(1) ≈ 0.73 V VFDBK |
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