 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. The Field Of The Invention
The present invention relates to a switching high voltage power supply and
in particular to a fast acting amplifier for use in driving a capacitance
of a multi-colored cathode ray tube.
2. The Prior Art
Switchable high voltage power supplies are well known in and of themselves.
For example, U.S. Pat. No. 3,659,190 shows a switchable high voltage power
supply which uses two series connected transistor chains. However, this
device has a disadvantage in that it is relatively expensive due to the
large number of transistors involved and the numerous connections that are
required.
A further example of a switchable high voltage power supply may be found in
U.S. Pat. No. 3,892,977 which has a basic voltage source and a plurality
of switching voltage sources in series therewith. The sources are
switchable into and out of the circuit by logic circuitry which is not
electrically referenced to the sources. However, this device is still
relatively expensive due to the large number of components involved
therein.
SUMMARY OF THE INVENTION
The present invention relates to a high voltage power supply having the
capability of rapidly switching between voltage states thus making the
circuit suitable for use in driving a large capacitance of a multi-colored
cathode ray tube or the like. The subject circuit includes a pair of high
voltage/frequency planar triodes as a source and sink, respectively, for
the current to a highly capacitive load. The source tube receives a
control input from a low level source through a high voltage optical
isolator while the sink tube receives its control signal much in the same
manner, but without the use of an isolator.
It is therefore an object of the present invention to produce an improved
high speed, high voltage switching power supply which can amplify a low
level analog/digital signal to a very high voltage to drive a large
capacitance and remain relatively immune to high voltage arcing to ground
plane structures.
It is also an object of the present invention to produce an improved
switching power supply having rise and fall times which are much faster
than previously attained by known power supplies.
It is a further object of the present invention to produce an improved
switching high voltage amplifier which can be readily and economically
produced.
The means for accomplishing the foregoing objects and other advantages will
become apparent to those skilled in the art from the following detailed
description taken with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block level schematic of the subject switching high voltage
power supply; and
FIG. 2 is an electrical schematic detailing the novel portion of the
subject switching high voltage power supply.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The circuit involved with the subject power supply is shown in FIG. 1 and
includes a filament supply 10, a high voltage delay 12, a high voltage
supply 14, high voltage adjustment means 16, a plurality of analog
switches 18, a decoder 20, a source tube grid driver 22, a high voltage
isolator 24, a source error amplifier and shunt comparator 26, a source
triode 28, a sink triode 30, and an output voltage supply 32, sensing
elements 34, and a switched output to the load 36.
The above circuit functions as follows. The filament supply 10 provides the
necessary DC voltages and DC isolation to heat the cathodes of both
triodes 28, 30 to their operating temperatures. The voltage delay 12
retards application of high voltage to the triodes for from 60 seconds to
90 seconds to prevent stripping of the cathode before the cathode material
has been sufficiently warmed to allow emission. The ten kilovolt power
supply 14 provides an operating plate voltage for the planar triodes 28,
30. The voltage adjustment means 16 consists of four potentiometers which
each give an adjustable DC voltage to a respective analog switch 18. The
individual reference voltages are those voltages after which the output
potential levels are patterned. According to the digital control input to
the analog switches 18, selection is made for which of the reference
voltages will be applied to the source error amplifier 26. Thus, as the
digital control signal changes, the reference voltage also changes as well
as the output. The two line to four line decoder 20 changes the two-bit
input to a four-bit output to drive the four channel analog switches 18.
The source tube grid driver 22 converts the linear control inputs from the
source error amplifier 26, via the optical isolator 24, to a potential
level compatible to the triode grid voltage requirement. The high voltage
isolator 24 transfers the control signals to the source tube grid driver
22 while maintaining a large DC voltage isolation required by the source
triode 28. The source error amplifier and shunt comparator 26 is a dual
function block which distributes information to control both the source
and sink triodes 28, 30. The source error amplifier 26 compares the input
signal E.sub.s with an analogous divided down output voltage E.sub.o. The
amplifier (a differential configuration) forces the high voltage to a
level that causes the differential between E.sub.s and the analog to be
close to zero. This assures that E.sub.o, the output voltage, will always
follow the input E.sub.s.
The function of the comparator 26 can be understood by application of some
basic facts. The sink tube 30 only conducts during downward input
transitions, when the falling slope of the input E.sub.s is greater than
the slope represented by the R.sub.o C.sub.o discharge mechanism. The sink
tube 30 may also conduct momentarily should the output voltage overshoot
the intended potential during an upward transition. The source tube 28
conducts only when the sink tube 30 is in a non-conducting state. The sink
tube 30 receives information from the shunt comparator 26. When E.sub.s
undergoes a downward (negative going) transition, the source error
amplifier output is of a pulse nature. The amplitude of this pulse is
compared with a non-varying DC reference in the comparator 26. When the
pulse input exceeds the DC reference, the comparator 26 instructs the sink
tube 30 to conduct. When the output voltage slews to the new intended
level, the comparator 26 receives instructions to turn the sink tube 30
off. Thus the comparator 26 is a device that insures that the source tube
28 and the sink tube 30 never conduct coincidentally. The source tube 28
is responsible for rapidly charging the output capacitance to some desired
level while the sink tube 30 is responsible for rapidly discharging the
output capacitance to the new lower programmed voltage. The 10 KV supply
14 is a scaling voltage factor which raises the average DC output by a
factor of the difference between the voltage supplies. This allows a
higher circuit output level with a relatively lower plate voltage rating
on the planar triode since only the first power supply 14 is the maximum
plate voltage seen by either tube at any time. The output sensing elements
form a frequency compensated high voltage divider to provide a scaled down
analog of the output of the inverting terminal of the source error
amplifier and shunt comparator. The switched output load is represented
simply by the output capacitance and resistance.
With specific reference to the electrical schematic of the subject
invention as shown in FIG. 2, two conventional filament supply circuits
are shown to comprise the elements C.sub.7, CR.sub.5, CR.sub.6, CR.sub.7,
CR.sub.8, and C.sub.8, CR.sub.9, CR.sub.10, CR.sub.11, CR.sub.12,
respectively. One of the filament supply circuits is electrically
connected to the cathode of each of the triodes 28, 30 to heat the
cathodes to operating temperature.
The source tube grid driver comprises circuits elements Q.sub.3, Q.sub.4,
C.sub.4, C.sub.5, CR.sub.2, CR.sub.4, R.sub.12, R.sub.13, R.sub.14,
R.sub.16, R.sub.18, electrically connected in the manner indicated, and it
should be appreciated from FIG. 2 that transistor Q.sub.4 functions as the
optical isolator detector 24.sub.b intended to receive optical signals
from optical isolator source 24.sub.a. Further, the sensing elements
previously described are shown to include the components C.sub.3, R.sub.2,
C.sub.1, and R.sub.1 which are electrically connected across the load 36
represented by capacitance C.sub.o and resistance R.sub.o.
With continuing reference to FIG. 2, the shunt comparator comprises an
error amplification portion, represented by the circuit elements U.sub.1,
R.sub.3, R.sub.4, Q.sub.1, and a shunt comparator portion represented by
circuit components U.sub.2, R.sub.9, R.sub.10, R.sub.11, Q.sub.2,
CR.sub.13, CR.sub.3, C.sub.6, R.sub.15, and R.sub.17. The source error
amp/shunt comparator, as set forth above, functions to compare the input
signal E.sub.s with an analogous divided down voltage E.sub.o and
distributes control information to both triodes 28, 30 to ensure that both
triodes do not simultaneously conduct. As indicated, the high voltage D.C.
supply 32 is connected through the load 36 when triode 28 is on and
charging the output to the desired level. When the triode 28 is off, that
is, when the output voltage level is to be reduced or when the output
level has over-shot the intended level, the sink triode 30 activates and
the voltage across the load 36 slews to the desired level. The 10 KV
voltage supply, not shown in FIG. 2, is electrically connected to the
subject circuit so as to provide operating plate voltages to the triodes
28, 30.
The present invention may be subjected to many modifications and changes
without departing from the spirit or essential characteristics thereof.
The above described embodiment should therefore be considered in all
respects as illustrative and not restrictive of the scope of the invention
.
* * * * *
|
|
 |
|
 |
Description |
|
