Power regulation during start up and shut down

A switched mode power supply and horizontal deflection system comprises a first oscillator circuit for generating horizontal rate synchronizing trigger pulses, having a voltage supply input terminal; a horizontal output stage; and, a second oscillator circuit for driving the output stage, operable at a horizontal rate responsive to the trigger pulses and free running at a different rate absent the trigger pulses. An overcurrent protection circuit for the horizontal output stage responds to an overcurrent condition which can occur during free running of the second oscillator circuit. A flyback transformer is coupled to the horizontal output stage and has a secondary side voltage supply coupled to the voltage supply input terminal for energizing the first oscillator circuit during operation of the output stage. An energy storage device, for example a large value capacitor, is coupled to the voltage supply input terminal for energizing the first oscillator circuit for a period of time after the horizontal deflection system is deactivated. The capacitor and a resistor form a timing network for the first oscillator circuit. The first oscillator circuit continues generating synchronizing trigger pulses and prevents operation of the second oscillator circuit at the free running frequency. A quick charging path for the energy storage device, for example a Zener diode in parallel with the resistor, minimizes operating time of the second oscillator circuit at the free running rate prior to the initiation of the synchronizing trigger pulses when the power supply and horizontal deflection system is activated.

This invention relates to the field of switched mode power supplies for 
television apparatus, and in particular, to a control circuit for 
preventing overcurrent operation by the horizontal output stage in a 
horizontal deflection system when the television apparatus is turned on or 
off. 
Televisions with microprocessor control typically have certain circuits 
which are continuously active in a standby mode of operation, even when 
the television has been switched off. Other circuits are energized only 
after the television set has been switched on, in a run mode of operation. 
Problems can be encountered coordinating the interaction of systems which 
are always active and those which are active only during the run mode of 
operation. 
The horizontal output stage in a horizontal deflection system may comprise 
a horizontal output transistor driven by a sawtooth waveform oscillator. A 
configuration for one such output circuit known as a Wessel circuit is 
shown in accompanying drawings. A sawtooth oscillator generates the basic 
driving waveform, and is typically free running at a lower frequency than 
the horizontal scanning frequency. For an NTSC interlaced signal, the 
horizontal scanning frequency is approximately 15,750 Hz. The free running 
frequency might be between 13,000 Hz and 14,000 Hz. 
A horizontal oscillator is provided for generating a synchronizing timing 
signal precisely at the horizontal scanning rate, synchronized with the 
video input signal. Such a horizontal oscillator may be incorporated as 
one the circuits in a one-chip. Such a one-chip may be part No. M51408 
available from Mitsubishi. The horizontal oscillator circuit provides 
trigger pulses to the otherwise free running oscillator, to assure that 
the sawtooth waveform is precisely equal to the horizontal scanning 
frequency rather than the free running frequency. The sawtooth signal may 
be coupled through buffer and driver stages, to the horizontal output 
stage, which may be a horizontal output transistor. The horizontal output 
transistor is coupled to a flyback transformer, from which a number 
secondary voltage sources may be derived from energy in the flyback 
pulses. Rectifying circuits may be coupled to secondary windings of the 
flyback transformer for developing these voltage sources at different 
voltage levels which may be required by various load circuits in the 
television. 
Typically, neither the sawtooth waveform oscillator nor the one-chip are 
energized during the standby operation. In fact, the one-chip is typically 
energized by one or more secondary voltage sources generated by the 
switched mode operation of the power supply. Moreover, the switched mode 
power supply, which relies upon switching of a horizontal output 
transistor to develop the secondary derived voltage sources, cannot 
operate until the sawtooth waveform has been generated by the sawtooth 
oscillator. 
It can be a characteristic of such switched mode power supplies that 
sufficiently prolonged operation at the free running frequency results in 
the horizontal output transistor being conductive for too long a period of 
time, at each turn-on. This results in an overcurrent condition, which can 
damage the horizontal output transistor and other components in the 
switched mode power supply. Accordingly, a safety circuit is often 
provided for sensing the overcurrent condition and disabling the power 
supply. The safety circuit can be responsive to overcurrent or overvoltage 
conditions having other causes as well. 
Televisions with microprocessor control are programmed to undergo a certain 
sequence of operations when the television is switched off, in order to 
prevent undesirable or harmful transient conditions. In a switched mode 
power supply for a horizontal deflection system as described above, such 
an undesirable transient condition can occur when the television set is 
switched off. The horizontal oscillator in the one-chip providing the 
horizontal rate trigger pulses for the sawtooth oscillator can stop 
functioning before the sawtooth oscillator stops functioning. This sudden 
change of horizontal frequency as the sawtooth oscillator begins free 
running causes a large current spike to be conducted by the horizontal 
output transistor, which in turn causes operation of the safety circuit, 
disrupting the orderly, soft switch off of the television. 
The horizontal oscillator in the one-chip has a separate Vcc input 
terminal, which is coupled to derived secondary voltage source of the 
flyback transformer. The Vcc input pin of the chip requires a series 
resistor for frequency control and a capacitor for filtering out ripple. 
In accordance with an inventive arrangement, soft switch off can be 
assured by substantially increasing the capacitance value of the filtering 
capacitor, for example to 1,000 microfarads. This ensures that the 
horizontal synchronizing trigger pulses will continue to be generated long 
enough to maintain the horizontal frequency oscillation of the sawtooth 
oscillator until the soft switch off has been completed. 
Although the introduction of large capacitance filter capacitor solves the 
soft switch off problem, a further problem can remain. The value of the 
filter capacitor increases the R-C time constant at the Vcc input pin of 
the one-chip. Whenever the television is switched on, the filter capacitor 
can require so much time to charge that the sawtooth oscillator free runs 
long enough at the lower frequency to cause the overcurrent condition, 
which causes activation of the safety sense circuit, which interrupts 
operation of the switched mode power supply. In effect, the safety circuit 
can prevent the television from ever being successfully turned on. It is 
necessary to significantly decrease the start up time for the horizontal 
oscillator in the one-chip, without the sacrificing the filtering function 
of the R-C network and without sacrificing reliable soft switch off. In 
accordance with another inventive arrangement, a Zener diode can be 
coupled in parallel with the resistor and bypass the resistor during 
startup of the television, providing a quick charging path for the 
capacitor. The Zener diode stops conducting as soon as the operating 
voltage is reached, enabling the R-C network to provide ripple filtering 
as before. The quick charging path reduces the time during which the 
sawtooth oscillator operates at the free running frequency, and prevents 
the overcurrent condition of the horizontal output transistor. 
A switched mode power supply and horizontal deflection system according to 
inventive arrangements taught herein assures reliable and soft turn on and 
off. A power supply and deflection system in accordance with these 
inventive arrangements can comprise a first oscillator circuit for 
generating horizontal rate synchronizing trigger pulses, having a voltage 
supply input terminal; a horizontal output stage; and, a second oscillator 
circuit for driving the output stage, operable at a horizontal rate 
responsive to the trigger pulses and free running at a different rate 
absent the trigger pulses. An overcurrent protection circuit for the 
horizontal output stage responds to an overcurrent condition which can 
occur during free running of the second oscillator circuit. A flyback 
transformer is coupled to the horizontal output stage and has a secondary 
side voltage supply coupled to the voltage supply input terminal for 
energizing the first oscillator circuit during operation of the output 
stage. An energy storage device, for example a large value capacitor, is 
coupled to the voltage supply input terminal for energizing the first 
oscillator circuit for a period of time after the horizontal deflection 
system is deactivated. The first oscillator continues generating 
synchronizing trigger pulses and prevents operation of the second 
oscillator at the free running frequency. A quick charging path is 
established for the energy storage device, for example by a Zener diode 
coupled in parallel with the resistor. The quick charging path minimizes 
operating time of the second oscillator at the free running rate prior to 
the initiation of the synchronizing trigger pulses when the power supply 
and horizontal deflection system is activated.

In the drawings, all capacitances are in farads and EC equals 16 volts 
unless otherwise noted. All resistances are in ohms, 1/4 watt, unless 
otherwise noted. 
In FIG. 1, an AC mains supply is coupled to a diode bridge comprising 
diodes DP26, DP27, DP28 and DP29. Half wave rectified voltage is available 
as VSTANDBY, which is the source for power during the standby mode of 
operation. The standby voltage is an input to a voltage regulator, for 
example a series pass regulator, which supplies standby voltage to a 
microprocessor, not shown. The microprocessor is responsive to on-off and 
other control commands. 
Transistor TP11 acts as a gate for the remaining half wave rectified pulses 
from the diode bridge. Transistor TP11 is responsive to operation of 
switch transistor TP12. The base of switch transistor TP12 is coupled to a 
STANDBY line (FIG. 2) from the microprocessor. The STANDBY line goes high, 
turning transistor TP12 on, whenever the microprocessor initiates the run 
mode of operation. Half wave rectified pulses gated by TP11 provide energy 
for charging capacitor CP01 up to 18 volts as determined by Zener diode 
DP02. The 18 volt voltage level provides a bias voltage at the junction of 
a voltage divider formed by resistors RP26 and RP05. The half wave 
rectified pulses are also an input to a sawtooth waveform oscillator, 
generally comprising transistors TP01, TP02 and capacitor CP03. 
Transistors TP01 and TP02 are normally biased off. When capacitor CP03 is 
sufficiently charged, transistor TP02 turns on. This provides base drive 
for transistor TP01 which also turns on. This provides a rapid discharge 
path for capacitor CP03. When capacitor CP03 is fully discharged, 
transistors TP02 and TP01 turn off, enabling capacitor TP03 to recharge. 
The sequence repeats cyclically. The resulting waveform is a sawtooth at 
the base of transistor TP04. In a free running mode, absent trigger or 
synchronizing pulses delivered to the base of transistor TP01 through 
diode DP05, the sawtooth oscillator free runs at a frequency less than a 
standard horizontal scanning frequency. For the component values shown, 
the free running frequency is between 13,000 Hz and 14,000 Hz. 
The sawtooth waveform is conducted through buffer transistor TP04 and AC 
coupled to the pulse width modulating (PWM) transistor TP05 (FIG. 2) 
through capacitor CP48. The signal is clamped by diode DP37 and adjusted 
in amplitude by the voltage divider formed by resistors RP14 and RP15. 
With further reference to FIG. 2, the conduction time of PWM transistor 
TP05 is related to slope of the leading edge of the sawtooth waveform. The 
on/off pulse width modulating signal at the collector of transistor TP05 
is coupled through the horizontal driver circuit to the horizontal output 
stage, shown at the upper left hand portion of FIG. 4. In the 
configuration of a Wessel circuit, the horizontal output stage is 
essentially horizontal output transistor TP10. Horizontal output 
transistor TP10 drives both the power supply transformer and the flyback 
transformer. Briefly, the output stage transistor in a Wessel circuit 
operates with an unstabilized supply voltage, and draws from the operating 
voltage source only as much power is required to maintain a constant 
deflection current. The conduction time of the horizontal output 
transistor is regulated to maintain constant deflection current 
independently of fluctuations of operating voltage and real loads. 
Transistor TP10 is coupled to the horizontal yoke BP04, the flyback 
transformer LP04 and the power supply transformer LP03, as shown in FIG. 
4. Raw B+ voltage originating at the diode bridge rectifier circuit in 
FIG. 1 is coupled to tap 12 of transformer LP03. The raw B+ voltage is 
applied across the primary winding of transformer LP03 by the switching 
transistor TP10. The deflection winding of transformer LP04, retrace 
capacitor CP18 and damper diode DP13 are coupled across the collector to 
emitter junction of the switching transistor TP10 by a first diode DP10, 
poled for conduction in the same direction as the collector to emitter 
junction. A secondary winding of transformer LP03 is coupled across the 
deflection winding by a second diode DP11 poled to conduct and transfer 
energy from the primary winding to the deflection winding during the 
retrace interval. The first half of the retrace interval is the time 
during which the retrace capacitor CP18 is charged by energy in the 
retrace pulse flowing from the horizontal yoke. The retrace capacitor is 
fully charged at the middle of retrace, when the deflection current is 
zero. Current flows from the retrace capacitor back through the horizontal 
yoke during the second half of retrace, charging the linearity capacitor 
CP40. Retrace ends when the voltage across the retrace capacitor CP18 
reaches zero, and the damper diode conducts. The damper diode conducts 
until the deflection current reaches zero. Thereafter, the damper diode 
turns off. Transistor TP10 will start conducting sometime before the 
deflection current reaches zero, but not after, depending upon the extent 
of load losses. As the deflection current exceeds zero, the diode DP10 
becomes forward biased by reason of the charge on the linearity capacitor. 
This is possible because transistor TP10 will already be conducting for 
the power supply function, and the cathode of diode DP10 will be only 
slightly above ground. The start of conduction by transistor TP10 will not 
effect the deflection current, whereby regulation of the power supply 
function is independent of deflection. Conduction of the deflection 
current through diode DP10 and transistor TP10 continues until transistor 
TP10 is turned off, which initiates retrace. 
There are two safety sense circuits associated with operation of the 
transistor TP10. Emitter current is directly sensed by sampling resistors 
RP30 and RP31. The voltage across the sampling resistors is an input to 
the base of transistor TP08 (FIG. 1), which forms part of the safety sense 
circuit described in detail below. Current in the secondary winding of 
transformer LP03 is sampled by resistor RP50. The voltage across resistor 
RP50 is an input to a network of diodes DP31, DP31 and DP33, resistor RP48 
and capacitor CP45. If the sampled voltage is of sufficient magnitude, the 
DC level of the sawtooth signal on the base of PWM transistor TP05 will be 
pulled down. This will reduce the conduction time of the PWM transistor, 
which will reduce the conduction time of transistor TP10. This can protect 
against overcurrent conditions which might be reflected back through the 
transformer LP03 and otherwise cause damage before being sensed as emitter 
current of transistor TP10 by resistors RP30 and RP31. 
Switched mode operation of the horizontal output transistor enables a 
number of secondary voltage sources coupled to secondary windings of the 
flyback transformer to be developed. One of these voltages is the B+ 
voltage of 104 volts, which is fed back to resistor RP08 in FIG. 1 as the 
principle feedback signal for regulation of the switched mode power 
supply. Another secondary supply shown in FIG. 3 is a 13 volt supply 
developed by capacitor CP20 and diode DP18 coupled to pin 10 of 
transformer LP04. This 13 volt secondary supply is the voltage source for 
the horizontal oscillator circuit of the one-chip shown in FIG. 5. When 
the 13 volt supply is running, the horizontal oscillator circuit provides 
output pulses on pin 20 of the one-chip precisely at a standard horizontal 
scanning rate, synchronized with the video signal input. These pulses 
trigger the sawtooth oscillator circuit at the base of transistor TP01, 
assuring that the sawtooth oscillator operates synchronously, at the 
standard horizontal scanning frequency. Yet another secondary supply is a 
22 volt supply developed by capacitor CP23 and diode DP20 for energizing 
the vertical deflection driver integrated circuit shown in FIG. 6. 
Operation of the sawtooth oscillator in the free running mode is 
independent of operation of the horizontal oscillator in the one-chip, 
energized by the 13 volt supply. In fact, the sawtooth oscillator will 
operate in the free running mode before the 13 volt secondary supply 
becomes available, whenever the television is switched on. Moreover, the 
free running oscillator is apt to continue free running even when the 
switched mode power supply has ceased operation, when the television is 
switched off. The on-time of the horizontal output transistor is increased 
during the free running operation of the sawtooth oscillator. The 
transition from synchronized to free running is an abrupt transition 
occurring as soon as the synchronizing trigger pulses stop. The sawtooth 
oscillator can continue operating in the free running mode for long a 
enough period of time after the horizontal oscillator in the one-chip 
stops generating synchronizing pulses, for the horizontal output 
transistor to operate in a overcurrent condition. 
Overcurrent conditions are detected by sense resistors RP30 and RP31, 
connected to the emitter of transistor TP10. When the sense voltage is of 
sufficient magnitude, transistor TP08 in FIG. 1 will be turned on. 
Conduction of transistor TP08 turns on transistor TP07. Together, 
transistor TP07 and TP08 function in the manner of a silicon controlled 
rectifier. When transistor TP08 begins conducting, its collector pulls 
down the STANDBY control line through diode DP45, which turns off 
transistor TP12. This in turn turns off gate transistor TP11, which 
prevents further charging pulses for capacitor CP03. At the same time, a 
rapid discharge path for capacitor CP01 is provided through diode DP01. 
This quickly prevents further operation of the horizontal output 
transistor TP10 by turning off the sawtooth oscillator and effectively 
grounding the input to PWM transistor TP05. When all of the relevant 
capacitors have discharged, both the base and emitter of transistor TP07 
will be at a voltage level of approximately 2 diode drops below ground. 
This will turn off transistor TP07, and thereafter, will turn off 
transistor TP08. This will enable the STANDBY control line to go high 
again and will initiate operation of the sawtooth oscillator, and 
thereafter, the horizontal output transistor. Operation of this 
overcurrent protection circuit is of course desirable responsive to 
genuine overcurrent conditions. However, overcurrent conditions should not 
be generated merely because the television set is turned on or off. 
The safety sense circuit can also be activated responsive to other fault 
conditions. The x-ray protection (XRP) circuit shown in FIG. 2 is 
responsive to overvoltage conditions in the high voltage supply for the 
cathode ray tube through diode DX03. The output of the x-ray protection 
circuit is another input to the base of transistor TP08 and the collector 
of transistor TP07, through diode DX01. The x-ray protection circuit 
formed integrally with the one-chip is permanently disabled by grounding 
pin 15. Overcurrent conditions in the vertical yoke (FIG. 6), for example 
those resulting from a short circuit of S-shaping capacitor CF01, will 
generate a threshold voltage across resistor RF11. This signal is also 
coupled to the base of transistor TP08, through diode DF01. The vertical 
yoke overcurrent signal is tapped from the AC component of the vertical 
feedback (VFB) signal at potentiometer PF01. The DC component of the 
vertical feedback signal is developed by the resistive divider comprising 
resistors RF03, RF04 and RF05. The inventive arrangements taught herein do 
not interfere with the normal operation of the safety circuit. 
Accordingly, an inventive arrangement assures that the horizontal 
oscillator in the one-chip will continue operating long enough to maintain 
operation of the sawtooth oscillator at the horizontal scanning rate until 
the soft shut down of the television has been accomplished. This is 
achieved by substantially increasing the size of capacitor C121 as shown 
in FIG. 5 to 1,000 microfarads. This provides a continuing energy source 
for the horizontal oscillator. However, capacitor C121 requires a long 
charging time. When the television is turned on, this charging time is 
sufficiently long that the sawtooth oscillator operates in at the free 
running frequency long enough to cause an overcurrent condition in the 
horizontal output transistor, which trips the safety sense circuit. It may 
be impossible to turn on the television, as the safety sense circuit keeps 
turning the power supply off. This problem is solved in accordance with an 
inventive arrangement by providing a quick charging path for capacitor 
C121. The quick charging path is advantageously provided by Zener diode 
DP47, coupled in parallel to resistor RP06. Resistor RP06 and capacitor 
C121 provide filtering against ripple for the horizontal oscillator 
circuit in the one-chip. The Zener diode provides a short-circuit path 
around resistor RP06 when the television is turned on. This enables the 
horizontal oscillator circuit in the one-chip to begin operating soon 
enough to synchronize the sawtooth oscillator before the horizontal output 
transistor reaches an overcurrent condition. 
Other inventive arrangements may be appreciated by an analysis of the 
remaining parts of the circuit schematic, which have not been described in 
detail.