Functional on/off switch for switched-mode power supply circuit with burst mode operation

A switched-mode power supply circuit having an operating mode and a stand-by mode, includes a functional ON/OFF switch. The switched-mode power supply circuit includes a transformer and a controllable switch connected to a primary winding of the transformer for switchably connecting the primary winding to a source of d.c. voltage. In the stand-by mode, based on a detected voltage level at an input of a controller IC, the switched-mode power supply circuit is arranged to switchable connect the primary winding to the d.c. voltage source in bursts which occur at a low frequency. By switchably connecting this input to a power source for the switched-mode power supply circuit, the controller IC detects the absence of this power source, and turns off the switched-mode power supply circuit allowing a minimum current consumption by the switched-mode power supply circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The subject invention relates to switched-mode power supply circuits for 
television receivers. 
2 . Description of the Related Art 
Switched-mode power supply circuits are used in television receivers to 
provide main operating power and stand-by power to the various circuit in 
the television receiver. A particular type of switched-mode power supply 
circuit provides stand-by power using a burst mode of operation. Although 
power consumption during this stand-by burst mode is low, there is still 
an appreciable amount of power being consumed. Thus, the television 
receiver also includes a main power switch for terminating all power to 
the television receiver when a user anticipates that the television 
receiver will not be used for a significant period of time. 
This main power switch is usually in the form of a mechanical switch which 
is bulky and expensive. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a functional ON/OFF 
switch for a switched-mode power supply circuit in a television receiver, 
which is low-cost and small in size, and which is capable of very low 
power consumption during an OFF state. 
The above object is achieved with a switched-mode power supply circuit 
having an operating mode and a stand-by mode, said switched-mode power 
supply circuit comprising means for generating a d.c. supply voltage 
having a first output terminal and a second output terminal; a transformer 
having a primary winding, an auxiliary primary winding, a first secondary 
winding and a second secondary winding, said primary winding having a 
first terminal coupled to the first output terminal of said generating 
means, and a second terminal; a controllable switch connected in a series 
with the second terminal of said primary winding and the second output 
terminal of said generating means; a main output capacitor coupled across 
output terminals of said first secondary winding for providing a first 
main output voltage in the operating state and a second main output 
voltage in the stand-by mode, said second main output voltage being lower 
than said first main output voltage; a control output capacitor coupled 
across output terminals of said second secondary winding for providing a 
control output voltage; means for selectively coupling one of the output 
terminals of said first secondary winding to said second secondary winding 
during said stand-by mode; an opto-coupler having light emitting means and 
light detecting means optically coupled to said light emitting means; 
means for selectively coupling the light emitting means of said 
opto-coupler across said control output capacitor during said stand-by 
mode; means for causing said light emitting means to emit light to said 
light detecting means when the control output voltage across said control 
output capacitor exceed a predetermined value during said stand-by mode; a 
controller having an output for supplying switching signals to said 
controllable switch, an auxiliary voltage sensing input coupled, via a 
shunting auxiliary capacitor, to said auxiliary primary winding of said 
transformer, and a stand-by mode detecting input coupled to an output of 
said light detecting means, said light detecting means having an input 
coupled to receive said auxiliary voltage, said controller comprising a 
start-up current source for charging said auxiliary capacitor during 
start-up of said switched-mode power supply, whereby, during said stand-by 
mode, said stand-by current source is used to intermittently charge the 
auxiliary capacitor when said controllable switch is not switching; and 
means for selectively coupling said stand-by mode detecting input of said 
controller to said first output terminal of said generating means, wherein 
said controller further comprises means for turning off said start-up 
current source and means for stopping said switching signals, thereby 
turning off said switched-mode power supply circuit, when said first 
output terminal is not coupled to said stand-by mode detecting input. 
In the above switched-mode power supply circuit, when the OFF mode is 
desired, the coupling of the first output terminal of the generating means 
to the stand-by mode detecting input of the controller is removed. The 
drop in voltage at the stand-by mode detecting input is detected by the 
turning off means which, in response, turns off the start-up current 
source and activates the means for stopping the switching signals. This 
then effectively turns off the switched-mode power supply circuit, in that 
it now only consumes approximately 300 .mu.A of current. 
Applicants have found that when replacing the main mechanical switch with 
the functional ON/OFF switch of the subject invention, it is necessary to 
ensure that the switched-mode power supply circuit consumes a minimum 
amount of power, e.g., 15 watts, even in the event of faults. 
To that end, the switched-mode power supply circuit as described above, is 
characterized in that the controller further comprises means for limiting 
power consumption when said coupling means selectively uncouples the first 
output terminal of said generating means from said stand-by detecting 
input of said controller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a schematic block diagram of a known switched-mode power 
supply using primary control. In particular, a diode rectifier bridge REC 
is connected to a line voltage source through a master switch SWM. An 
output from the rectifier bridge REC is connected to ground through a 
capacitor C11 and to one end of a primary winding L11 of a transformer TR. 
The other end of primary winding L11 is connected to one terminal of a 
controllable switch Tr11, the other terminal of which being connected to 
ground through a sense resistor R.sub.SENSE. A first secondary winding L12 
of the transformer TR has a first end and a second end connected to each 
other through a series arrangement of a diode D11 and a main output 
capacitor C12, the second end of the first secondary winding L12 also 
being connected to ground. A load (not shown) may be connected across the 
main output capacitor C12. 
The transformer TR also includes a second secondary winding L13 having a 
first end and a second end connected to each other through a series 
arrangement of a diode D12 and a control output capacitor C13, the second 
end of the second secondary winding L13 also being connected to ground. A 
microprocessor (not shown), for controlling, for example, a television 
receiver in which the switched-mode power supply circuit is installed, is 
connected across the control output capacitor C13 to receive operating 
power. 
The first end of the first secondary winding L12 is also connected, via a 
diode D13 and a controllable switch Sw1, to one end of the control output 
capacitor C13, while the control output capacitor C13 is shunted by a 
series arrangement of a light emitting diode D14 of an opto-coupler, a 
Zener diode Z1 and a controllable switch Sw2. The controllable switches 
Sw1 and Sw2 are controlled by a signal from the microprocessor to initiate 
the stand-by mode of the switched-mode power supply circuit. 
The transformer TR further includes an auxiliary primary winding L14 which 
has one end connected to a diode D15, and then to ground through a 
V.sub.AUX capacitor C14, to a V.sub.AUX input of a controller IC, and to 
one terminal of a light sensor Tr12 of the opto-coupler, the other 
terminal of the light sensor Tr12 being connected to a stand-by mode 
detecting input (OOB) of the controller IC. The other end of the auxiliary 
primary winding L14 is connected to ground. The controller IC also has a 
V.sub.IN input connected to the output of the rectifier bridge REC, a 
DEMAG input connected through a resistor R11 to the one end of the 
auxiliary primary winding L14, a driver output connected to the control 
input of controllable switch Tr11, and an I.sub.SENSE input connected to 
the resistor R.sub.SENSE. 
In order to turn off the switched-mode power supply circuit, one merely 
activated the switch SWM which cut off power to the rectifier bridge REC. 
However, this type of switch is costly and bulky due to the amount of 
power that it must handle. 
FIG. 2 shows the switched-mode power supply circuit of FIG. 1 in which the 
subject invention has been incorporated. In particular, the series 
arrangement of two resistors R12 and R13 and a Zener diode Z2 is connected 
between the first output terminal of the rectifier bridge REC and ground. 
The junction between the Zener diode Z2 and resistor R13 is connected to 
the OOB input of the controller IC via a switch Sw3, which is further 
connected to ground via a resistor R14. 
When it is desired to turn off the switched-mode power supply circuit, 
switch Sw3 is opened, removing the voltage across the Zener diode Z2 from 
the OOB input of the controller IC. The controller IC detects this drop in 
the voltage on the OOB input and stops the controllable switch Tr11 from 
switching. In this state, the controller IC draws less than 300 .mu.A 
thereby effecting the OFF state. 
The embodiment shown in FIG. 2 uses a separate opto-coupler in primary 
sensing to signal the controller IC that burst mode stand-by is desired. 
However, in most switched-mode power supplies, an opto-coupler is already 
being used to regulate the control voltage during normal operation. FIG. 3 
shows a second embodiment of the switched-mode power supply circuit of the 
subject invention in which the already existing opto-coupler is 
additionally used in secondary sensing to signal burst mode standby 
operation. In particular, the light emitting diode D14 of the opto-coupler 
is connected through resistors R15 and R16 to the anode of a Zener diode 
Z3, the cathode of which being connected to ground. A series combination 
of a resistor R17 and a capacitor C15 connects the anode of the Zener 
diode Z3 to a control terminal of the Zener diode Z3 and to junction point 
between resistors R18 and R19 connected between the output of the first 
secondary winding L12 and ground. Switch Sw2 connects the anode of Zener 
diode Z1 to the output of the second secondary winding L13, the cathode of 
Zener diode Z1 being connected to ground through the series arrangement of 
resistors R20 and R21. The junction between resistors R15 and R16 is 
connected to the collector of an NPN transistor Tr13, having an emitter 
connected to ground and a base connected to the junction between resistors 
R20 and R21. On the primary side, the junction between the light sensor 
Tr12 of the opto-coupler and the diode D16 is further connected through a 
resistor R22 to a V.sub.CNTL input of the controller IC which is also 
connected to ground through the parallel arrangement of a resistor R23 and 
a capacitor C16. 
During normal operation, transistor Tr13 is off and the intensity of the 
light being emitted by the light emitting diode D14 of the opto-coupler is 
controlled by the circuit R16-R19, C15 and Z3. This variable light 
intensity causes a corresponding response in the light sensor Tr12 which 
applies a portion of the V.sub.AUX voltage to the V.sub.CNTL input of the 
controller IC for regulating the duty cycle of the controllable switch 
TR11, which is beyond the scope of the present invention and will not be 
described further. However, when the microprocessor signals burst mode 
stand-by operation by closing switches Sw1 and Sw2, due to the increased 
control output voltage across the control output capacitor, transistor 
Tr13 turns on fully causing the light emitting diode D14 to emit a much 
increased light output, which, in turn, causes the light sensor Tr12 to 
apply the whole of the V.sub.AUX voltage to the OOB input of the 
controller IC. 
As shown in FIG. 4, the controller IC includes a start-up current source 30 
coupled to the V.sub.IN input and a Vcc management circuit 32 connected to 
the V.sub.AUX input. The OOB input is connected to a resistor R24 and then 
to a first comparator 34 for comparing the voltage thereon to +2.4V, and 
generating an "OFF/ON" signal. This OFF/ON signal is applied to an input 
of the Vcc management circuit 32. The resistor R24 is also connected to 
the collector of an NPN transistor Tr15. The base of transistor Tr15 is 
connected to a voltage source Vcca, and to ground via a Zener diode Z4. 
The emitter of the transistor Tr14 is connected to a second comparator 36 
for comparing the voltage thereon to +1.4V, for generating a "Burst Mode 
Stand-by" signal S6. This signal S6 is applied to the start-up current 
source 30 and to one input of an OR-gate 38. An output (S5) from the Vcc 
management circuit 32 is also applied to the start-up current source 30 
and to an inverting input of OR-gate 38. An output from OR-gate 38 is 
applied to the reset input of an RS flip-flop 40, the set input being 
connected to an output of an oscillator 42. The Q output from the RS 
flip-flop 40 is connected to one input of an AND-gate 44 which has an 
output connected to a driver 46 for driving the controllable switch Tr11. 
The operation of the switched-mode power supply circuit will now be 
described. When line voltage is applied to the rectifier bridge REC, with 
switch Sw3 open, the controller IC is in a "sleep mode" and the current 
consumption is less than 300 .mu.A. Once switch Sw3 is closed, the voltage 
on the OOB input is then equal to the Zener diode Z2 voltage (i.e., higher 
than +2.4V) causing the output from the OFF/ON comparator 34 to go "high". 
This commences a start-up sequence and Vcc management circuit 32 turns off 
the S5 signal causing the start-up current source 30 to generate a current 
I1 for charging the V.sub.AUX capacitor C14. Once the V.sub.AUX voltage 
rises above a predetermined level, the Vcc management circuit 32 turns on 
the signal S5 and the controller IC (at t.sub.2 &lt;t&lt;t.sub.4) now starts 
causing the controllable switch Tr11 to switch which then causes the 
transformer TR to start transferring energy from the primary winding. L11 
to the secondary windings L12 and L13, and also to the auxiliary primary 
winding L14 which then takes over supplying the V.sub.AUX capacitor C14. 
The switched-mode power supply circuit is now in normal operation. 
When burst mode stand-by operation is desired, the microprocessor closes 
switches Sw1 and Sw2 thereby coupling the first secondary winding L12 to 
the second secondary winding L13 thereby removing energy from the main 
output capacitor C12. In addition, switch Sw2 connects the light emitting 
diode D14 of the opto-coupler and the Zener diode Z1 across the second 
secondary winding L13. The coupling of the first and second secondary 
windings L12 and L13 now causes an increase in the control output voltage 
across the control output capacitor C13. When the control output voltage 
exceeds the Zener diode Z1 voltage, the light emitting diode D14 is 
energized. This causes the light sensor Tr12 to couple the V.sub.AUX 
voltage to the OOB input of the controller IC. Since the V.sub.AUX voltage 
is in excess of, for example, +5.6V, the comparator 36 generates the 
signal S6 resetting the flip-flop 40 which stops the controllable switch 
Tr11 from switching. 
Once the controllable switch Tr11 stops switching, transformer TR ceases 
transferring energy form the primary winding L11 to the first and second 
secondary windings L12 and L13 and to the auxiliary primary winding L14. 
As a result, the control output capacitor C13 and the V.sub.AUX capacitor 
C14 begin to drain causing the control voltage and the V.sub.AUX voltage 
to begin to drop. When the control voltage drops below the Zener diode Z1 
voltage, the light emitting diode D14 stops emitting light, the light 
sensor Tr12 removes the V.sub.AUX voltage from input OOB of the controller 
IC, and the comparator 36 stops generating the signal S6. The V.sub.AUX 
voltage has been dropping during this time and when the V.sub.AUX voltage 
has dropped to an under-voltage V.sub.UVLO level, the Vcc management 
circuit 32 stops generating the signal S5 thereby maintaining the reset 
condition of the flip-flop 40, and activating the start-up current source 
30 for charging up the V.sub.AUX capacitor C14 thereby raising the 
V.sub.AUX voltage. Once the V.sub.AUX voltage is at a start-up voltage 
V.sub.START level, the Vcc management circuit 32 generates the S5 signal 
which turns off the start-up current source 30 and allows the controllable 
switch Tr11 to commence switching. The switching of the controllable 
switch Tr11 allows the transformer TR to transfer energy from the primary 
winding L11 to the first and second secondary windings L12 and L13 and to 
the auxiliary primary winding L14. This cycle then repeats itself until 
the microprocessor opens switches Sw1 and Sw2 indicating an end to the 
burst mode stand-by operation and a return to normal operation. 
When using the functional switch Sw3, it is important that the power 
consumption of the switched-mode power supply circuit be kept to a minimum 
level, e.g., 15 watts, even in the event of a fault. The circuit of 
resistors R14, R24 and R25, and transistor Tr15 provide this protection. 
In particular, when switch Sw3 is open, resistor R14 ensures that the 
voltage at the OOB input is close to ground. The first comparator 34 then 
senses an OFF state and the controller IC goes into a "sleep" mode where 
it does not start the switched-mode power supply circuit. 
Burst mode is initiated by the light sensor Tr12 of the opto-coupler. When 
activated, the light sensor Tr12 forces a voltage greater than +5.6V at 
the OOB input and at the same time, provides a current large enough to 
force a voltage greater than the reference voltage, i.e., +1.4V, of the 
second comparator 36 on the resistor R25. The output of second comparator 
36 then goes high and causes the switched-mode power supply circuit to go 
into burst mode stand-by operation. 
Resistor R24 is chosen based on a fault condition where the input OOB is 
shorted to the V.sub.AUX input. In such a situation, the system should 
then go into burst mode operation thereby keeping the dissipation in the 
switched-mode power supply circuit below 15W. To this end, R24 acts as a 
current limiter. Transistor Tr13 is chosen to be a bipolar NPN transistor, 
instead of a PMOS type transistor, because of the high threshold voltage 
of a PMOS transistor (2.1V). Due to this higher threshold voltage, the 
burst mode trip level would be raised. The higher trip level would then 
make it impossible to put the switched-mode power supply into burst mode 
stand-by operation when a fault occurred. 
When there is a short circuit between the OOB input and the V.sub.AUX 
input, the voltage V.sub.AUX is applied to the OOB input causing the 
second comparator 36 to detect burst mode stand-by operation. The 
controllable switch Tr11 is turned off and the Vcc management circuit 32 
monitors the V.sub.AUX voltage. Due to the short circuit, the V.sub.AUX 
voltage gets clamped at a voltage level V.sub.CLAMP, where: 
EQU V.sub.CLAMP =V.sub.CCA +0.6 +(I.sub.start *R24) 
The voltage V.sub.CLAMP has a dependence on the value of R24, I.sub.START 
from the start-up current source 30, and temperature. Depending on the 
value of V.sub.CLAMP, the V.sub.AUX voltage can get clamped at a voltage 
which is lower than the start-up voltage for the controller IC. In such a 
state, the controller IC "hangs" and never initiates a start-up. If 
V.sub.CLAMP is higher than the start-up voltage, the controller IC starts 
up and immediately senses a burst mode condition. 
Numerous alterations and modifications of the structure herein disclosed 
will present themselves to those skilled in the art. However, it is to be 
understood that the above described embodiment is for purposes of 
illustration only and not to be construed as a limitation of the 
invention. All such modifications which do not depart from the spirit of 
the invention are intended to be included within the scope of the appended 
claims.