Forward voltage

Fig. 2. Representation of the band edges in a semiconductor p—n junction where shallow donor, acceptor energies, and the Fermi level are labeled Ejy E, and E respectively, (a) Without external bias is the built-in potential of the p—n junction (b) under an appHed forward voltage F. ...

Efwd is the forward voltage drop of the antieipated output reetifier. is the maximum expeeted duty cyele (0.95 is good). 

For those applications where high efficiency is important, synchronous rectification may be used on the higher current (power) outputs. Synchronous rectifier circuits are much more complicated than the passive 2-leaded rectifier circuits. These are power MOSFE B, which are utilized in the reverse conduction direction where the anti-parallel intrinsic diode conducts. The MOSFET is turned on whenever the rectifier is required to conduct, thus reducing the forward voltage drop to less than O.f V. Synchronous rectifiers can be used only when the diode current flows in the forward direction, that is in continuousmode forward converters. 

A small Schottky rectifier with a current rating of about 20 to 30 percent of the MOSFET current rating (/d) is placed in parallel with the MOSFET s intrinsic P-N diode. The parallel schottky diode is used to prevent the MOSFET s intrinsic P-N diode from conducting. If it were allowed to conduct, it would exhibit both a higher forward voltage drop and its reverse recovery characteristic. Both can degrade its efficiency of the supply by one to two percent. 

Cutc/j diode. The eateh diode needs to be a sehottky diode to minimize the eon-duetion loss and the switehing loss of the funetion. The diode that has a reasonable forward voltage drop at the 3 A peak eurrent is the MBRD330 with a 0.45 V drop at 3 A (at -i-25°C). 

Synchronous Diode. A Sehottky diode about 30 pereent of the rating of the eon-tinuous rating of the synehronous MOSFET needs to be plaeed in parallel with the MOSFET s intrinsie diode. This would be about 0.66 A at 30 V. I will use an MBRS130. This diode produees a 0.35 V forward voltage drop at 0.66 A. 

These heatsinks will help a marginal heat situation. The alternative is to use a reetifier in a power transistor paekage sueh as a TO-220, TO-218, ete., and plaee it on a heatsink or to investigate a different teehnology of diode that exhibits a lower forward voltage drop sueh as a Sehottky. 

Figure 2.13 The energy levels of a p-n junction (a) unbiased (b) with a forward voltage apphed.

Figure 8. Schematic representations of p-n junctions and corresponding energy band diagrams under various conditions (a) uniformly doped p-type and n-type semiconductors before junction is formed, (b) thermal equilibrium, (c) forward bias, and (d) reverse bias. Abbreviations are defined as follows Ec, electron energy at conduction band minimum E, , electron energy at valence band minimum IF, forward current Vf, forward voltage Vr, reverse voltage ...

Schottky contact — Alternative denomination of metal-semiconductor contact presenting a Schottky barrier. Depending on metal - work function, semiconductor electron affinity, doping of the semiconductor, conditions of the surface of the semiconductor before contact preparation, and preparation process, Schottky contacts with high rectification can be prepared. Devices encor-porating such contacts behave like a diode and for this reason, are also denominated Schottky diodes, whose main features are the capability of high frequency operations and low forward-voltage drop. 

Figure 31 Rigid-band model of the energy levels in a diode (a) zero voltage (b) flat band (voltage Vn, applied) (c) large forward voltage V.

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