Low voltage supply cutoff circuit for an electronic appliance

A low voltage supply cutoff process and circuit for terminating supply of electric power less than a required minimum voltage to a monitor or a computer system in order to prevent malfunction of the appliance. The circuit may be constructed with a power supply unit is used for rectifying an input alternating current voltage (AC voltage) to a direct current voltage (DC voltage); a switching unit for supplying the DC voltage provided by the power supply unit to the electronic appliance on the basis of a switching signal; a voltage detecting unit for detecting a voltage level applied by the switching unit to the electronic appliance; an overvoltage detecting unit for detecting whether the switching unit supplies an overvoltage to the electronic appliance; a low voltage detecting unit for comparing the output voltages of the power supply unit and the voltage detecting unit in order to determine whether the input AC voltage of the power supply unit is a low voltage with an amplitude less than a predetermined voltage level; a control unit for generating a switching control signal according to the voltage level detected by the voltage detecting unit and generating a switching disenable signal when the overvoltage detecting unit detects the overvoltage or the low voltage detecting unit detects the low voltage; and an oscillating unit for generating the switching signal to operate the switching unit by being oscillated according to the switching control signal and for terminating the switching signal by stopping oscillation when the switching disenable signal is generated.

CLAIM FOR PRIORITY 
This application makes reference to, incorporates the same herein, and 
claims all benefits flow accruing under 35 U.S.C. .sctn.119 from 
applications for LOW VOLTAGE SUPPLY CUT OFF CIRCUIT FOR AN ELECTRONIC 
APPLIANCE earlier filed in the Korean Industrial Property Office on 29 May 
1996, and there duly assigned Ser. No. 1996/26069, a copy of which is 
annexed hereto. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electronic appliance such as a monitor 
or a computer system, and, more particularly, to a low voltage supply 
cutoff process and circuit for an electronic appliance for interrupting 
the supply of electric power to the appliance when the supply exhibits 
less than a required minimum voltage to the appliance. 
2. Description of the Related Art 
In general, delicate electronic appliance such as a monitor or a 
microprocessor driven computer system will malfunction when an excessively 
high voltage is applied to the appliance. For example, application of an 
overvoltage will cause a monitor to display an unstable image and will 
cause a computer system to incorrectly recognizes data. Therefore, an 
electric appliance such as a monitor or a computer system is equipped with 
a SMPS for supplying stable electric power to the appliance on the basis 
of the electric power consumption of the appliance. Typical conventional 
power supply systems seek to protect electronic appliances by stopping the 
operation of the oscillator when an excessive current flows, that is, when 
an abnormal voltage occurs in the second power supply. Generally, a low 
alternating current voltage may be supplied for household use during a low 
alternating current voltage is supplied; I have noticed that problems will 
occur however with the operation of an electronic appliance supplied with 
an abnormal voltage because an electronic appliance will perform abnormal 
operations when the voltage provided by the power supply is less than the 
voltage required by each of the circuit elements of the appliance. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved process and circuit for protecting electrical appliances 
attributable to deleterious variations in the voltage supplied to the 
appliance. 
It is another object to provide a low voltage supply cutoff process and 
circuit for an electronic appliance using a low voltage detecting unit for 
detecting an input of a voltage that has an amplitude that is less than a 
certain voltage, to cut off application of electric power to the appliance 
and maintain stability of the appliance. 
In order to achieve these and other objects, a low voltage supply cutoff 
circuit for an electronic appliance may be constructed according to the 
principles of the present invention by comparing the output voltage of the 
electric power supplied to the appliance with an output voltage of a 
voltage detecting unit, determining whether an AC voltage of the electric 
power is less than a predetermined voltage level, and stopping operation 
of an oscillating unit by applying a control signal to a control unit when 
the AC voltage is a low voltage that is less than the predetermined 
voltage level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a view for schematically showing a conventional SMPS circuit. 
Alternating current power source AC is applied to a bridge diode rectifier 
BD. An output terminal of the bridge diode rectifier BD is connected to a 
grounded capacitor C1 and one terminal of a primary winding 100 of a 
transformer T of switching unit 13 through bridge diode rectifier BD. The 
output terminal of the bridge diode rectifier BD is connected to first 
input terminal of an oscillating unit 10 through series connected 
resistors R1 and R2. A first power supply unit 5 is constituted as shown 
in FIG. 1. 
An output terminal of the oscillating unit 10 is connected to the base 
electrode of a transistor TR1 of switching unit 13 through a resistor R3. 
Switching unit 13 is constructed with transistor TR1, resistor R3 coupled 
between oscillating unit 10 and the base electrode of transistor TR1, 
resistor R4 coupled between the emitter electrode of transistor TR1 and a 
local reference potential such as a circuit ground, and transformer T. The 
other terminal of the primary winding 100 of the transformer T is 
connected to the collector electrode of transistor TR1, and the emitter 
electrode of transistor TR1 is connected to grounded resistor R4 and a 
second input terminal of the oscillating unit 10. 
A secondary winding 200 of transformer T is connected to a second power 
supply unit 15 for supplying operational electric power to an appliance. 
A voltage detecting unit 20 may be constructed with a tertiary winding 300 
of the transformer T being connected through a diode D2 to one terminal of 
a resistor R9, a grounded capacitor C5, and the first input terminal of 
oscillating unit 10. 
An overvoltage detecting unit 25 may be constructed with tertiary winding 
300 being connected to a grounded resistor R6 and the cathode electrode of 
a zener diode ZD through a diode D1, a grounded capacitor C2, and a 
resistor R5 in turn, and the anode electrode of the zener diode ZD is 
connected to a grounded capacitor C3. 
A control unit 30 may be constructed with the juncture of the anode 
electrode of the zener diode ZD and the grounded capacitor C3, which is 
the output terminal of overvoltage detecting unit 25, being connected to 
grounded resistor R7, the base electrode of transistor TR2, and the 
collector electrode of transistor TR3. The other terminal of the resistor 
R9, which is the output terminal of the voltage detecting unit 20, is 
connected to the emitter electrode of the transistor TR3 as well as being 
connected to the collector electrode of transistor TR2 and the base 
electrode of transistor TR3 through resistor R8 and capacitor C4, which 
are connected in parallel to each other. The juncture of resistor R8, 
capacitor C4, the collector electrode of transistor TR2, and the base 
electrode of transistor TR3 is connected to an third input terminal of 
oscillating unit 10 through reverse-biased diode D3. 
In operation, a conventional power supply unit constructed as set forth in 
FIG. 1, bridge diode rectifier BD rectifies an alternating current voltage 
(AC voltage) applied across its input terminals, the rectified AC voltage 
is smoothed to a direct current voltage (DC voltage) through capacitor C1, 
and the DC voltage is applied to oscillating unit 10 as an initial 
operating voltage through serially coupled resistors R1 and R2. 
Oscillating unit 10 generates a pulse signal through its output terminal 
in accordance with the operating voltage input. The pulse signal from 
oscillating unit 10 turns transistor TR1 connected to primary winding 100 
of transformer T on and off; consequently, application of the DC voltage 
from the capacitor C1 to the primary winding of the transformer T is 
turned on and off. Therefore, voltages are induced in secondary winding 
200 of transformer T, and the induced voltages are rectified and smoothed 
through the second power supply unit 15 so as to be supplied to an 
electrically powered appliance (not shown) as an operating voltage. 
Voltages are induced across tertiary winding 300 in proportion to the 
levels of the induced voltages across the secondary winding 200. The 
voltages induced across the tertiary winding 300 are rectified through 
diode D2 of voltage detecting unit 20, smoothed through capacitor C5, and 
supplied to oscillating unit 10 as an operating voltage, and are applied 
to control unit 30 through resistor R9. The voltage applied from voltage 
detecting unit 20 to control unit 30 is applied to the emitter electrode 
of transistor TR3 as well as to the base electrode of transistor TR3 
through resistor R8 and capacitor C4. Accordingly, transistor TR3 is 
turned on so that a bias voltage that is established in accordance with 
the level of the output voltage generated by voltage detecting unit 20 is 
applied to the base electrode of transistor TR2 at node b since the output 
voltage of voltage detecting unit 20 drops in amplitude to ground 
potential through transistor TR3 and resistor R7. Therefore, the turn-on 
(i.e., the electrical conduction of the semiconducting channel between the 
collector-emitter electrodes) of transistor TR2 allows an output voltage 
from voltage detecting unit 20 to flow through the semiconducting channel 
between the collector-emitter electrodes of transistor TR2 through 
resistor R8. That is, since a current having an amplitude established in 
accordance with the bias voltage applied to the base electrode of 
transistor TR2 flows through the collector electrode of transistor TR2, 
the voltage measured at the collector electrode of transistor TR2 varies 
in accordance with an output voltage level of voltage detecting unit 20. 
The voltage occurring at the collector electrode of transistor TR2 is 
applied to oscillating unit 10 through diode D3 so that oscillating unit 
10 outputs a pulse signal having a pulse width that is established in 
accordance with an output voltage level of control unit 30. Since the 
level of a voltage induced across secondary winding 200 is established in 
accordance with the level of the voltage applied across primary winding 
100, and varies when the output pulse signal varies the duration of 
turn-on and turn-off time periods of transistor TR1, a rated voltage is 
normally continuously supplied to the electronic appliance. 
A voltage induced across tertiary winding 300 of the transformer T is 
rectified through diode D1 of overvoltage detecting unit 25, smoothed 
through capacitor C2, divided by resistors R5 and R6, and applied to the 
cathode electrode of the zener diode ZD. Resistance values of the 
resistors R5 and R6 are set in the event that an overvoltage is applied to 
the electronic appliance, in order for the voltage of the cathode 
electrode of zener diode ZD to exceed over a control voltage. With this 
setting, when the overvoltage is applied to the appliance, zener diode ZD 
is turned on and a high voltage is applied to the base electrode of 
transistor TR2 to operate transistor TR2 in the saturation state, a 
voltage at the collector electrode of transistor TR2 turns to a low level 
and a low level voltage is applied to oscillating unit 10. Accordingly, 
oscillating unit 10 determines that an overvoltage has been applied to the 
electronic appliance, and does not output a switching signal to transistor 
TR1 to keep the transistor TR1 in a turned-off state, so that any voltage 
is not applied to the appliance. 
As described above, the conventional power supply system protects an 
electronic appliance by stopping the operation of the oscillating unit 
when an overcurrent flows, that is, when an abnormal voltage occurs in 
second power supply unit 15. Generally, a low alternating current voltage 
may be supplied to a household appliance when a low alternating current 
voltage is supplied. 
Hereinafter, a low voltage supply cutoff circuit for an electronic 
appliance according to an embodiment of the present invention will be 
described in detail with reference to the accompanying drawing of FIG. 2, 
a view for showing a low voltage supply cutoff circuit for an electronic 
appliance according to an embodiment of the present invention. Description 
and explanation as to the same parts shown in the conventional power 
supply system of FIG. 1 will be omitted. As shown in FIG. 2, the low 
voltage supply cutoff circuit for an electronic appliance constructed as 
an embodiment of the present invention includes a low voltage detecting 
unit 40. The low voltage detecting unit 40 is connected to the juncture 
node b positioned at the base electrode of transistor TR2 of control unit 
30 for comparing a voltage at the juncture node a and a voltage at the 
juncture node c and stopping the driving of oscillating unit 10 when it is 
determined as a result of the comparison that a low voltage has been 
inputted. Juncture node a is a connection between resistors R1 and R2 of 
the first power supply unit 5 as shown in FIG. 1, and juncture node c is 
positioned at the cathode electrode of diode D2 of voltage detecting unit 
20 for inputting a certain voltage to drive oscillating unit 10. Resistors 
R10 and R11 of low voltage detecting unit 40 are connected to the juncture 
node a between resistors R1 and R1, and one terminal of resistor R11 is 
grounded. The cathode electrode of diode D4 is connected to the junction 
between the resistors R10 and R11, the anode electrode of diode D4 is 
connected to the base electrode of a PNP-type transistor TR4. The emitter 
electrode of transistor TR4 is connected to the juncture node c of voltage 
detecting unit 20. Further, the collector electrode of transistor TR4 is 
connected to the base electrode of transistor TR2 of control unit 30 
through resistor R12. 
Operations of the embodiment of the present invention will be described in 
detail hereinafter, under the assumption that a low voltage is a voltage 
less than 80 volts in amplitude. First, operations of the embodiment of 
the present invention will be described when a voltage less than 80 volts 
is applied. When a voltage of about 50 volts is applied, the inputted 
voltage is rectified through the bridge diode rectifier BD and filtered 
through capacitor C1. The voltage filtered by capacitor C1 is applied 
through resistors R1 and R2, which are called "beginning resistors", to 
oscillating unit 10 for generating a pulse signal. At this time, the 
voltage at the junction between resistors R1 and R2 is in proportion to 
the input voltage of 50 volts and divided again by resistors R10 and R11. 
A voltage divided by resistors R10 and R11 becomes about 13 volts. A 
voltage at the juncture node c for supplying a voltage to drive 
oscillating unit 10 is a certain voltage of 15 volts. Accordingly, diode 
D4 is turned on and transistor TR4 is also turned on since transistor TR4 
is a PNP type, so that a collector electrode voltage of transistor TR4 is 
applied to the base electrode of transistor TR2 of control unit 30. At 
this time, the collector electrode of transistor TR2 goes to a low level 
of voltage and the low level of voltage is applied to oscillating unit 10 
in order for the operation of the oscillating unit 10 to stop. Therefore, 
a switching signal is not applied to transistor TR1 so as to keep the 
transistor TR1 in a turn-off state, so that electric power is not applied 
to an appliance. 
Next, operations of the embodiment of the present invention will be 
described when a voltage more than 80 volts in amplitude is applied. Since 
the diode D4 is turned off when a voltage between the resistors R10 and 
R11 is over 15 volts, transistor TR4 is also turned off. Therefore, since 
a bias voltage according to an output voltage level of voltage detecting 
unit 20 is applied to the base electrode of transistor TR2, transistor TR2 
is turned on so that an output voltage of the voltage detecting unit 20 is 
applied to transistor TR2 through resistor R8. That is, since a current 
according to a bias voltage applied to the base electrode of transistor 
TR2 is supplied to the collector electrode of transistor TR2, the 
collector electrode voltage of transistor TR2 varies in accordance with an 
output voltage level of voltage detecting unit 20. When the collector 
electrode voltage of transistor TR2 is applied to oscillating unit 10 
through diode D3, oscillating unit 10 outputs a pulse signal having a 
pulse width that is established in accordance with an output voltage level 
of control unit 30 so as to change the turn-on and turn-off time period of 
transistor TR1. The changed time period varies a voltage level which is 
induced from the primary winding 100 to secondary winding 200 of the 
transformer T, thereby supplying a rated voltage to an appliance. 
As described above, the embodiments detailed in the foregoing paragraphs 
describe a low voltage supply cutoff process and circuit for an electronic 
appliance, that endeavors to prevent malfunction of the appliance by 
automatically interrupting the supply of electric power to the appliance 
when the supply exhibits less than a required minimum voltage to the 
appliance. In accordance with the principles of the invention, the low 
voltage supply cutoff circuit is provided with a low voltage detecting 
unit for detecting a low voltage less than a certain voltage to operate a 
monitor and for automatically terminating application of electric power to 
the appliance so as to prevent malfunction of the monitor, so that the 
stability of the monitor operation is assured despite the low voltage 
condition of the power supply. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended thereto be limited to the descriptions 
set forth herein, but rather that the claims be constructed as 
encompassing all the features of the patentable novelty that reside in the 
present invention, including all the features that would be treated as 
equivalents thereof by those skilled in the art to which this pertains.