The Circuit Designer's Companion - diagramas.diagram...

116 The Circuit Designer’s CompanionI F I F “fast” recoveryreverse recoveryV FV FI RV Rforward recoveryaggressivedi/dtV RFigure 4.6 Diode forward and reverse recoveryprocessing reduces the reverse recovery time to a minimum, though not to zero. Typicalrecovery times are 150−200ns and fast recovery diodes are used extensively in highvoltage,high-speed switching. When even these speeds are too slow “ultra-fast” diodesare also available which can have recovery times down to 20ns.Interference due to fast recoveryFast recovery brings its own problems, though. The characteristic tail on turn-off“snaps” back very quickly to I R, producing a very high transient rate of change ofcurrent (di/dt). Usually it is the highest di/dt within the circuit and so is responsible formost of the unwanted electromagnetic interference output. To save on the need for extracomponents to limit this current, yet another class of diodes has been developed, the“soft recovery” diodes, in which a compromise has been reached between speed ofrecovery and a comparatively gentle turn-off characteristic.However, bear in mind that all p-n junction diodes exhibit some form of reverserecovery and are therefore capable of generating interference at harmonics of theswitching frequency (even mains!) and of dissipating some power during this period.4.1.6 Schottky diodesThe p-n semiconductor junction is not the only arrangement to show rectifyingproperties. A metal-semiconductor junction also rectifies. Devices which use thisproperty are known as Schottky diodes. The important differences betweenconventional p-n silicon diodes and silicon Schottky diodes can be summarised asshown in Table 4.1.ConventionalSchottkyForward voltage typically 0.6V Forward voltage typically 0.4Vat medium currentsat medium currentsMinority carrier charge storageeffects limit speedNo minority carriers, no chargestorage, high speedHigh reverse breakdown voltageLow reverse breakdown voltage,achievable, in excess of 1kV ranging generally from 30–100VLow reverse leakage currentHigher reverse leakage currentTable 4.1 Schottky versus conventional diodes