Note: Descriptions are shown in the official language in which they were submitted.
Related Patent
U.S. Patent 3,838,332 issued September 24, 1974, and
assigned to the present assignee, discloses a regulated power supply
using an SCR as a combined rectifier and control device. The
present invention is useful in such a power supply, particularly
where protection against load device arcing is desired.
Background of the Inven~on
; Arcing in a cathode ray tube CCRT) is a well-known
phenomenon although the detailed cause is not well enough under-
stood to produce a complete cure. Monochrome CRT's arc on
occasion, but color CRT's, due to their higher operating potent-
ials, tend to arc more frequently. Accordingly, color TV receivers
commonly employ an extensive array of spark gaps to act as pro-
tective elements for the circuit devices associated with the CRT.
In the manufacture of a CRT, stringent quality control
measures are taken in the various processes to avoid the conditions
- that lead to arc production. In addition a process known as spot
knocking is employed. In this technique a much higher than normal
voltage is applied to the finished CRT and this will enhance
arcing. Each arc tends to destroy the arc-producing mechanism so
that, after a short period of intense arcing at a higher-than
;~ normal voltage, the tube will be substant~ally arc-free at normal
';
.
.
- , - ~ . ,.
104~)706
voltage. While the measures taken during manufacture have reduced
arcing to a very lo~ incidence it still occurs, particularly after
the CRT has been handled extensively as it is in shipment and/or
mounting in a TV receiver.
In conventional TV receivers, the startin~;of an arc will
quickly reduce the high voltage, due to poor supply regulation,
and the arc will tend to be self-extinguishing. However in modern
solid state TV receivers much better high voltage regulation is
being achieved in order to produce more satisfactory picture
reproduction performance. For example in a conventional switching-
type solid state TV deflection circuit, the circuit is regulated
to provide deflection that is substantially independent of power
line voltage. In some circuits regulation is acoomplished by the
use of a regulated low voltage power supply. In either case the
high voltage is regulated usually by controlling the deflection
circuit. While these modern circuits provide much more satisfact-
ory picture performance, they tend to make arcing a more serious
problem. In effect, when an arc does occur, the energy discharged
is much greater. Under some conditions the regulation is good
enough that the arc will be sustained for an extended period. This
can be harmful to the CRT as well as to the horizontal deflection
and high voltage components. Fast acting circuit breakers can be
used to interrupt the applied voltage when the current drawn by
the circuit exceeds some predetermined value. However such
devices are expensive, difficult to set properly, and are subject
to false triggering. In addition they must be reset ma~ually,
thereby making them a nuisance.
Summary of the Invention
It is an object of the invention to provide a regulated
power supply that will sense a sudden large increase in current
drain and respond by shutting off momentarily.
It is a further object to provide a regulated power supply
~0407Q6
for a TV receiver in which a CRT arc will be sensed and the power
supply turned off long enough toreceive the arc and then be
turned back on again.
It is a feature of the invention that the above objects
are accomplished by adding a small number of inexpensive components
to a known regulated power supply circuit so that very little cost
is associated with the invention.
These and other ob~ects and features are achieved in a
momentary shutdown circuit for a regulated power supply, aaid
power supply including at least one SCR connected as a rectifier
between an a-c input source and a smoothing filter, a pulse gener-
ator synchronized by said a-c input source and connected to fire
said SCR, a reference supply source connected for comparison with
said power supply output, a variable pulse delay circuit connected
into said pulse generator so as to vary the timing of firing said
SCR, said pulse delay connected to sense the difference between
the potentials of said power supply and said reference supply and
to reduce said delay as said difference decreases, said shutdown
circuit comprising:
a storage capacitor having t~o terminals, one terminal of
said capacitor being connected to the output of said power supply
and the second terminal being connected through a series resistor
to a shutdown source of potential independent from said power
supply and having a potential value substantially lower than the
nominal potential of said power supply, and
a unidirectional switch connected between said second ter-
minal of said capacitor and a control element of said pulse gener-
ator, said switch being connected in a p~larity such that said
shutdown source biases said switch to be normally nonconductive
but conductive when a sudden drop in power supply voltage is
coupled by way of said capacitor to said switch, said switch
when conductive acting upon said control element to render said
1~4~1706
pulse generator inactive thereby shutting down the operation of
said power supply for a period of time determined by the time
constant of said resistor-capacitor combination.
Brief Description of the Drawing~
Figure 1 is a schematic diagram of a prior art circuit;
Figure 2 is a graph showing the operation of the circuit
of Figure l;
Figure 3 is a schematic diagram of a circuit employing the
invention; and
Figure 4 is a schematic diagram showing in greater detail
a power supply employing the invention.
Description of the Prior Art
Referring to Figure 1, SCR 1 is connected as a half wave
rectifier to the a-c input line. This input can be the secondary
of a transformer, but in the interests of economy, and if no
voltage transformation is re~uired, it is convenient to connect
the SCR directly to the 120-volt a-c line. The half-wave rectified
output is applied to a conventional filter 2 which smooths the
rectifier output to provide a steady d-c output voltage. If the
circuit used a simple-diod~e, or if the SCR gate electrode were
returned through a suit~ble resistor to the anode, a conventional
half-wave rectifier would result. In such a circuit the rectifier
would turn on during the rise of the positive excursion of the a-c
input voltage and would charge filter capacitor 4 to the peak
a-c line voltage. The rectifier would turn off when the a~c
input voltage drops to the cathode voltage level. The filter
capacitor would discharge until the next positive a-c input ex-
cursion whereupon the cycle repeats. Clearly, the d-c output
voltage would be a function of the peak input voltage.
By adding the pulse generator 3 to the circuit, the SCR
can be fired when desired by a controlled gate pulse. As shown
in Figure 2, the SCR is fired somewhere near the a-c input peak,
1~40706
the firing angle being designated ~. ~ will be located in the
range extending from shortly ahead of the 90 point to shortly
ahead of the 180 point on the input wave form. This is because
the maximum power supply energy capability will occur when the
SCR is fired sli~htly ahead of the 90 point on the input wave,
and to have any output at all the SCR must be fired ahead of the
180 point. ~he dashed line shows the voltage waveform ~
across the input filter capacitor 4. At ~, when the SCR is fired
by a pulse from generator 3, the capacitor will quickly charge
along an exponential rise to whatever voltage is present at the
SCR anode. Then the capacitor will discharge between positive
input alternations until the next firing interval. It can be
seen that the capac~tor charge is no longer related to peak line
voltage. In addition, the magnitude of charge can easily be
controlled or varied by changing the firing angle ~.
As 8hown in Figure 1, the pulse generator is synchronized
by connecting it to the a-c input. The generator supplies a pulse,
delayed with respect to the a-c input, that will turn the SCR on n
in the range of less than ~0 to less than 180. The delay is
made variable in response to the difference between the d-c output
voltage and a separate fixed d-c reference voltage applied between
the indicated terminals. If the d-c output voltage is too low,
the delay is reduced so that the SCR fires sooner; this will
locate the firing point closer to the a-c input peak. If the d-c
output voltage is too high, the delay is increased so that the SCR
; fires later and, therefore, lower down on the a-c input waveform.
By making the firing angle 0 a strong function of d-c output,
;~ close control of the output voltage is obtained. Clearly, the
circuit will compensate for input or power line variations as well
as load variations.
General Description of the Invention
Figure 3 shows the components that are added to the circuit
_5_
1040706
of Figure 1 to practice the invention and how the power supply is
connected to a CRT display.
Capacitor 7 charges through resistor 8 to the difference
between the d-c output voltage and the d-c reference voltage.
Thus the cathode of diode 9 will be at the positive potential of
the d-c reference voltage. Because the anode of diode 9 is at a
substantially lower potential (as will be apparent from the more
detailed description of the circuit of Fig. 4), diode 9 will be
reverse biased and in its open switch state. For these conditions
the circuit will function normally as a regulated power supply.
The power supply operatesi the CRT display circuits 30 which
include a high voltage supply 31. The display circuits and high
voltage output are connected to CRT 32.
If for some reason, such as an arc in CRT 32, the d-c
voltage output drops suddenly so that capacitor 7 will not have
time to discharge, the cat:Hod?eof diode 9 can be driven more
negative than its anode and it will be driven into its on switch
state. This action causes pulse generator 3 to stop supplying
pulses to SCR 1 which will then remain off. The d-c output voltage
will then continue to fall thereby holding diode 9 conductive until
the energy stored in filter 2 is discharged. This means that when
a CRT arc occurs the influx of energy to the system is halted and
the arc will only trigger the dissipation of the energy stored in
the power supply filter, the CRT horizontal deflection circuit and
the high voltage supply. Once that energy is dissipated, the arc
will be relieved and the circuit restored to normal operation. If
- this arc termination were not accomplished, the regulated power
supply could drive a considerable quantity of energy into the
system possibly damaging the CRT or its associated components.
Once the d-c output voltage has dropped to some low value
and capacitor 7 has discharged through resistor 8 to a value that
will no longer hold diode 9 in its on switch state, the diode will
1040706
cease to conduct and the pulse generator will resume its
functioning. This will again turn SCR 1 on periodically and
filter 2 will again become charged and the system will resume
normal ~unctioning. The time constant of capacitor 7 and
resistor 8 is established to give the circuit an off interval of
about 2 seoonds. This period will be long enough to make sure
that any arc will be completely interrupted but is short enough
to have only a small effect on TV viewing. Clearly this interval
can be selected within wide limits to achieve the desired action.
Many times a single arc occurance will, as described above
in ~ackground-of the Invention, destroy the arc-producing mechanism
and the system will operate normally after a single interruption
cycle. However if the arc-producing mechanism still exists, an
arc will develop again and the shutdown sequence repeated. Only
in rare cases will the arc-producing mechanism survive several
arc ~equences with the interposed shut-down intervals. In the
event that the arc-producing mechanism remains present, the shut-
down seq~e~ce~ wil} continue to operate until the display is
turned off manually.
; 20 Figure 4 is a complete power supply schematic showing how
the pulse generator of figure 3 can be implemented using a
combined pulse generator-variable delay circuit.
SCR 1 is connected directly to the a-c input and provides
rectified d-c output by way of filter 2 which is composed of
input filter capacitor 4, output filter capacitor 5, and series
filter inductor 6. Under typical conditions, the d-c output
voltage may be of the order of 100 volts. Compl-ementary tran-
~ sistors 10 and 11 are connected into a well known bistable latch-
;~ ing complementary pair configuration. ~ransistor 10, in addition
to being part of a latching pair, also acts as a Miller effect
delay device. When transistor 10 is turned on from its off state,
it will come on slowly due to the Miller effect. After a delay
1040706
period, its output will become sufficient to turn transistor 11
on and the complementary pair will then rapidly switch on to
saturation. This latter turn on action provides a pulse that is
coupled by way of coupling capacitor 25, to the SCR so as to fire
it. Since the turn on of transistor 10 is synchronized to the
a-c input by way of the network consisting of resistors 13 and 18
and diode 12, the SCR firing angle is then controlled by the
durationi^~of the Miller effect delay. The delay is in turn cont-
rolled by applying tbY way of resistance 17, 1~, 20, 21, 22 and
23) a fraction of the d-c output voltage to the base of transistor
11 to act as a bias or threshold control. As the d-c output
voltage varies, the threshold at which the complementary pair
switches on will be varied. This action varies the duration of
the Miller effejct delay. The sense of this feed-back loop is such
that the d-c output voltage will tend to remain nearly constant
as will be discussed hereinafter.
A 20-volt reference supply is shown operatingithe delay
and switching circuits. In applications where a suitable regulated
voltage is available it would be used. If no such regulated
voltage is available, the reference can be obtained from any con-
venient unregulated source that supplies more than the required
value at the lowest a-c line voltage condition. A dropping resis-
tor and zener diode having the required voltage rating are con-
nected in series across the unregulated source. The voltage
present across the zener diode is then used as the reference. If
no such unregulated source is available an additional rectifier
- diode and filter capacitor combination can be connected to the
a-c input line and the above mentioned resistor-zener diode
combination connected across the filter capacitor. These latter
components, associated with the 20-volt reference supply, are
conventional and are not shown in the drawing~.
In the quiescent state when the a-c input is near zero,
--8--
1~4~)706
diode 12 is forward biased by eIectron flow due to the 20-volt
reference supply. Electrons will ~low from ~round through
resistor 13, from the cathode of diode 12 to its anode, and then
up through resistor 14 to the positive supply terminal. Resistor
13 is made quite small, about 0.1% of the value of resistor 14,
so that the drop across it is negligible. Diode 12 being forward
biased will develop about 0.6 volt at the base of transistor 10.
Resistors 15 and 16 are proportioned to apply a positive voltage
of about 2 volts to the emitter of transistor 10. Thus, trans-
istor 10 will not conduct until its base is more than about 2.7volts positive. For these quiescent conditions transistor 10
will be cut off and its collector will rest at almost 20 volts.
This will apply a high positive potential to the base of tran-
sistor 11 through resistor 17 and transistor 11 will be held in
its off state. This is a stable state for both transistors~
Resistors 13 and 18 form a voltage divider across the a-c
input and are proportioned to apply about 2~ of the input to the
cathode of diode 12. This will be about 3.4 peak volts for a 120-
~- volt line condition. As the a-c inputs swings from zero toward
- 20 its positive peak, at some point along the excursion, diode ~2
will be cut off or reverse biased. When this occurs, the base
voltage on transistor 10 will rise toward the positive 20-volt
supply because of resistor 14. When this rise exceeds about 2.7
volts, transistor 10 will start to conduct and its collector will
.
start to fall toward ground potential. However, as soon as the
collector starts to fall, capacitor 29 will couple the fiall back
into the base and oppose the change. This circuit configuration
is called a Miller-effect circuit. As far as the base of tran-
sistor 10 is concerned, the circuit behaves as if a large capacitor
~ere connected between base and ground. This effective capacitor
has a value approximately equal to the value of c~pacitor 29
multiplied by the circuit gain of transistor 10. Thus, a
_g_
104~706
relatively small capacitor 29 will act in conjunction with
resistor 14 to ~orm a relatively long time constant integrator.
In effect, a .02 microfarad capacitor can be made to act as if
it were a 0.4 microfarad capacitor if the transistor circuit gain
is only 20, an easily achieved value.
It will be noted that the emitter of transistor 11 is
returned to the 20-volt reference. Thus, when its base drops
to about 19.3 volts it will begin to conduct. Resistors 17, 19
and 20 are selected, along with the values of voltage divider
resistors 21, 22 and 23 so that transistor 11 will be well below
cutoff when transistor 10 is cut off. As the collector voltage
of transistor 10 falls, at some point the base of transistor 11
will reach the 19.3-volt level. When this occurs and transistor
11 starts to conduct, collector conduction will tend to drive the
base of transistor 10 positive. This in turn lowers the ~ollect-
or voltage of transistor 10 so as to turn transistor 11 on even
harder. This regenerative action terminates the Miller-effect
ramp and both transistors are quickly driven into saturation. At
this time the collector of transistor 10 will have dropped to about
5 volts. Ramp termination can occur rapidly because resistor 24
is made much smaller than resistor 14 which, in conjunction with
the Miller-effect capacitance, established the ramp rate. In
practice, the value of resistor 24 is made as about equal to the
value of resistor 15. Thus, if résistors 15 and 16 produce a
quiescent off state voltage of 2 volts at the emitter of transistor
10, the value will increase to about 5 volts in the saturation or
on state.
Thus, when the complementary transistor pair switches on,
a positive pulse will appear at the emitter of transistor 10. This
pulseL--will~-~ou~ d--^to-~-he`gaté of S~R L~ap~àci"to~''~5,-"the^~eby
causing it to fire. As the a-c input continues to fall, a time is
reached at which point diode 12 becomes again forward bia~ed and
--10--
104~)'706 `
cuts off transistor 10. This, in turn, cuts off transistor 11
because the collector of transistor 10 quickly returns to is
20-volt level where it will remain until the above sequence is
repeated on the next positive half of the a-c input cycle.
If, for some reason, such as an increased load, the d-c
output voltage were to decrease, it can be seen that a less
positive quiescent bias will appear at the base of transistor 11.
This means that less ramp action is needed at the collector of
transistor 10 to produce the level at which the complementary
transistor pair switches on. This means that an early switching
pulse occurs at the emitter of transistor 10. Since firing SCR 1
earlier will produce more d-c output voltage, the output voltage
will be increased by the reduced firing angle to compensate for
the postulated d-c output voltage reduction.
Conversely, if the d-c output voltage were to rise, for
example as a result of reduced loading, a more positive bias will
be applied to tran&istor 111 This means that the ramp on the
collector of transistor 10 will have to run longer to cause switch-
ing. Since the SCR is now fired later and lower on the a-c
input waveform, the d-c output voltage will be reduced to compens-
ate for the postulated rise.
For a given set of component values the circuit will act to
maintain nearly a constant d-c output voltaqe ~alue. It will, in
fact, act to maintain a nearly fixed difference between the d-c
output voltage and the reference voltage. Thus, it is responsive
to variations of a-c input as well as load variations.
As a matter of convenience, such regulators usually include
~; an adjustable element because otherwise each part would have to be
held to excessively strict tolerance. In the circuit of Figure 4
resistor 23 is made variable. As this resistor is varied, a
variable fraction of the d~c output voltage is supplied to the base
of transistor 11. This will vary the length of the ramp needed to
--11--
1040706
drive transistor 11 into conduction and will, therefore, vary the
SCR 1 firing angle. For a given set of input conditions resistor
23 can vary the d-c output voltage over a substantial range. The
circuit will then act to maintain the d-c output voltage constant
at nearly the set value.
The circuit thus far described constitutes the prior art
and is disclosed in detail in above-mentioned Patent Number
3,838,322. That application also shows further circuit refinements
that enhance power supply performance. These refinements are not
shown here because they are not necessary to understand the pre-
sent invention which will now be detailed.
Detailed Description of the Invention
Components 7, 8 and 9 comprise the elements added to the
prior art power supply to practice the invention. Capacitor 7 will
charge through resistor 8 to the difference in potential between
the 20-volt reference supply and the d-c output voltage. Thus if
the power supply were to be set at 100 volts, capacitor 7 would
.,
` charge to 80 volts. This would place the cathode of diode 9 at a
` positive 20 volts. Since the anode of diode 12 is connected to
':~
the base of transistor 10 which operates at a positive potential
' of only a few volts, the diode will be biased in its off switching
state. Thus for normal operating conditions the circuit behaves
as if components 7, 8, and 9 were not present.
If a sudden increase in power supply loading occurs, such
as would be produced by an arc in a color CRT operated by the
~; power supply t capacitor 7 will not have time to dis~harge and the
voltage change is coupled directly to the cathode of diode 9. If
the drop in power supply voltage is sufficient to drive diode 9
into its on switch state the base of transistor 10 will suddenly
become connected to capacitor 7. This will drive the base of
transistor to negative with respect to its emitter thereby cutting
it off. This will halt the pulsing of SCR 1. The elements of
1040706
filter 2 will then start to discharge and the output voltage
will continue to ~all. This falling output will keep diode 9
turned on by way of the action of capacitor 7. Thus the filter
section 2 will discharge, the power supply output will be termin-
ated, and the causal arc extinguished. As soon as the d-c output
drops, capacitor 7 will start to discharge through resistor 8. At
some point along the discharge curve, the cathode of diode 9 will
be driven to a potential with respect to its anode that will turn
it off. The values of resistor 8 and capacitor 7 are selected
10 to have a time constant that will establish a suitable power
supply off interval. A Yalue found to be useful is 2 seconds.
Once diode 9 is driven to its off switch state, the pulse generator
; will resume functioning, SCR 1 will again receive pulses, and
filter 2 will be recharged to where the regulator action takes
s over circuit operation as described above.
A suitable set of component values for the circuit of
figure 4 is as follows:
SCR 1 --C107Dl tGeneral Electricl
Capacitor 4 --400 microfarads 175 volts
Capacitor 5 - 400 microfarads 150 volts
Inductor 6 - 0.2 henry
~; Capacitor 7 - 10 microfarads 150 volts
Resistor 8 - 100K Ohms
~ Diode 9 - lN456
Transistor 10 - 2N4424
Transistor 11 - 2N5366
. Diode~12 , - lN456
Resistor 13 - lK Ohms
Resistor 14 - 910K Ohms
:- .
; 30 Resistor 15 - 3.3K Ohms
Resistor 16 - 330 Ohms
Resistor 17 - 82K Ohms
. .~
-13-
1~)40706
Resistor 18 - 47K Ohms
Resistor 19 - 6.8K Ohms
Resistor 20 - 18K Ohms
Resistor 21 - 160K Ohms
Resistor 22 - 33K Ohms
Resistor 23 - 15K Ohms
Resistor 24 - 3.3K Ohms
- Capacitor 25 - .047 Microfarad
The circuit of figure 4 with the above component values
functioned with good regulation. With the inception of a ~udden
load increase such as would be occasioned by a CRT arc, the out-
put voltage was interrupted for a period o~ about 2 seconds.
The invention has been described in detail and a set of
component values has been shown to provide suitable performance.
Numerous m~di~i~ationswill occur to a person skilled in the art.
For examp}e, while the invention has been shown in connection with
a half-wave line-connected rectifier, transformer operation could
be employed and a full wave rectifier could be used. Further,
resistor 8 could-be returned to a source of potential other than
the 20-volt reference or even a fraction thereof. The lower the
potential to which resistor 8 is returned, the more sensitive the
.
circuit will be to a drop in d-c output potential. For the
; conditions given a drop in d-c output of slightly less than 20
volts will initiate momentary shutoff. If resistor 8 were return-
~- ed to a 10-volt supply (or 1/2 of the 20-volt reference supply)
shutoff would occur when a drop of slightly less than 10 volts
occurred. It is clear that still further modifications could be
`~ made within the scope and intent of the invention. It is intend-
~ ed that the invention be limited only by the following claims.
