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Transistor Fast Turnoff Insensitive to Drive Transition

IP.com Disclosure Number: IPCOM000081985D
Original Publication Date: 1974-Sep-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Hoffman, HS: AUTHOR [+2]

Abstract

This circuit utilizes storage in an antisaturation diode to provide continued on-drive to a transistor, until the main drive is fully reversed. The sharp snap-off of the antisaturation diode then provides sudden initiation of substantial off-drive at the transistor base.

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Transistor Fast Turnoff Insensitive to Drive Transition

This circuit utilizes storage in an antisaturation diode to provide continued on-drive to a transistor, until the main drive is fully reversed. The sharp snap-off of the antisaturation diode then provides sudden initiation of substantial off-drive at the transistor base.

Power transistor Q is driven with two junction diodes 10, 12 in series with its base, and one antisaturation diode 14 between the drive point 16 and the collector of Q. Diodes 10, 12 and the base-collector junction of Q will conduct much less current than diode 14. Diodes 10, 12 are chosen to have larger values of minority carrier storage and long recombination times relative to Q. Diode 14 is driven into deep saturation by the antisaturation current it carries. Therefore, there will be minimum storage in transistor Q, but considerable storage in diode 14 which can continue to provide transistor drive after the original drive to node 16 cuts off.

Since the drive at node 16 is through a transformer X having low magnetizing inductance, cutoff of drive at the primary 18 will cause a sizeable magnetizing current flow in secondary 20 opposite to the original drive current. This will begin to remove stored charge from 14. Transistor Q will still be on full, because of excess charge in Q. Under the continued flow of magnetizing current, diode 14 will deplete and the base of transistor Q will be pulled negative through diodes 10, 12 (which are still...