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FAN3121C データシート(PDF) 4 Page - Fairchild Semiconductor |
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FAN3121C データシート(HTML) 4 Page - Fairchild Semiconductor |
4 / 12 page AN-6069 APPLICATION NOTE © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.3 • 1/6/10 4 Synchronous Rectifier Operation A MOSFET operated as a synchronous rectifier (SR) experiences a switching interval significantly different from the case of a clamped inductive load. Figure 6 shows a simplified forward converter power stage with a synchronous rectifier QSR in place of the freewheel diode. Figure 6. Simplified Forward Converter In this example, an SR signal generated by the control circuit crosses the isolation boundary to keep the synchronous rectifier QSR on while Q1 is off. However, the SR signal should command QSR to turn off before Q1 turns on to apply positive voltage to the transformer. Figure 7 shows four intervals used to illustrate the turn-off sequence of the synchronous rectifier. (a) (b) (c) (d) CGD CGS IL RG VSEC VDC DBD RLOW - + IG RDS S D CGD CGS CDS IL RG VDC DBD RLOW VSEC - + D S VDC CGD CGS CDS IL RG VDC DBD RLOW CGD CGS CDS IL RG DBD RLOW IG VSEC - + VSEC -+ DD S S Figure 7. SR MOSFET Turn Off Prior to turn off, the MOSFET conducts load current IL through the resistive channel RDS and the drain-to-source voltage is negative. In Figure 7(a) the output of the driver is low and the combination of CGD and CGS are discharged in parallel in a time interval given by: G SR , Q off I Q t = (11) where QQSR is defined in reference [3] to be: DD SR , GD GS SR , Q V ) C C ( Q ⋅ + = (12) Also in reference [3], CGS,SR is estimated as: DD SPEC , DS SPEC , RSS SR , GD V 5 . 0 V C 2 C ⋅ ⋅ ⋅ = (13) From standard MOSFET nomenclature: RSS ISS GS C C C − = (14) In Figure 7(b), the MOSFET is fully off, IL flows through the body diode, and the VSEC polarity has not changed. When VSEC changes polarity, as shown in Figure 7(c), current flows from VSEC to recover the body diode stored charge and the diode commutates. In Figure 7(d), the body diode has been fully recovered and VDS rises quickly. The high dV/dT on the MOSFET drain can cause a capacitive current to flow through the CDS/CGS voltage divider, so a driver with strong current sink capability is essential to hold the gate voltage below the threshold voltage. In the synchronous rectifier application, IG does not affect switching losses as it did in the clamped inductive load application. However, the paralleled MOSFETS used in SR applications require high-current pulses to switch effectively, and high current drivers are often located in close proximity. Transformer Drive Applications In power converters such as a half-bridge, full-bridge, two- switch forward converters; and active clamp forward converters there are high-side switches or a combination of high/low switches that must be controlled. If galvanic isolation is not needed between the control and the power switches, the MOSFETs may be driven with a semiconductor half-bridge gate driver, but the inherent propagation delay must be considered in the design. For circuits that need isolation or can benefit from short propagation delays, the gate drive transformer should be considered as a potential solution. In a related application, it is often necessary to provide high- speed communication between the primary and secondary sides of an isolated converter. This can be accomplished using technologies such as opto-isolators with digital outputs or magnetic pulse transformers. These pulse transformers are similar to the gate drive transformer, but they are only required to transmit logic signals instead of delivering the high-current pulses to turn a power MOSFET on and off. The simplified circuit of Figure 8 is used to illustrate the basic operation of a low-side driver and pulse transformer used in a communication circuit. The transformer is shown as ideal transformer with turns ratio NP:NS = 1:1 in parallel |
同様の部品番号 - FAN3121C |
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同様の説明 - FAN3121C |
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