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Self-Optimizing Low Energy Drive Circuit for Emitter-Switched Transistor

IP.com Disclosure Number: IPCOM000117205D
Original Publication Date: 1996-Jan-01
Included in the Prior Art Database: 2005-Mar-31

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+3]

Abstract

Driving high voltage bipolar junction transistors in high power switching applications involves very careful design. Without such care, these devices are prone to early failure.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 14% of the total text.

Self-Optimizing Low Energy Drive Circuit for Emitter-Switched Transistor

      Driving high voltage bipolar junction transistors in high power
switching applications involves very careful design.  Without such
care, these devices are prone to early failure.

      Emitter switching, as opposed to base, makes it significantly
easier optimize the base drive circuit, especially when the task is
complicated by the requirement to drive the transistor over a large
switching frequency range.  As the voltage rating of the switching
transistor is increased, the critical nature of the base current
waveform becomes progressively more evident.  This is the case for
both base and emitter switching, but especially true for the
inherently compromised base drive approach.  In addition to
simplifying power management requirements, emitter switching provides
the advantage of near-ideal drive conditions at all frequencies, with
reduced power dissipation in the horizontal deflection transistor and
in the deflection circuit as a whole, thereby increasing product
reliability.

      It may be desirable in some CRT displays to change a lower
voltage (e.g., 1500V) rated bipolar switching transistor in the
horizontal deflection stage to a higher voltage (e.g., 1700V) device.
On making this change however it may no longer be possible to
compromise on the shape of the base current waveform used to drive
the line output transistor in order to simplify circuit topology and
reduce component count.  This is due to the fact that higher voltage
devices, by necessity, have thicker collector regions such that they
can withstand the higher voltages applied during the flyback period
when the transistor is switched off.  As the collector region is made
thicker, it becomes progressively more difficult to saturate its
collector-base junction during the conduction period.

      Conventionally, a simple DC current was bled into the base of
the transistor to provide enough charge - stored in the
base-collector junction - (Qs) to ensure that it was driven into
saturation.  In addition, a base current proportional to the
collector current was provided by way of a current transformer, the
primary winding of which conducts the collector current and whose
secondary drives the base-emitter junction.  This scheme provided
near-optimum drive conditions for the 1500V transistor.  However, it
failed to do so for the 1700V device because the DC current bled into
the base did not provide sufficient charge to saturate the
base-collector diode.  The increased thickness of this junction
requires significantly more injected charge to ensure adequate
saturation for low loss conduction during the scan period.

      To provide enough charge by way of a DC current involves
excessive loss power in associated power supply dropper resistors and
other power supply components.  Furthermore, optimizing this DC bias
current becomes extremely difficult over the transistor's spec...