Abstract:
Disclosed is an SMPS protection circuit, which can prevent unnecessary electric power consumption that may be caused due to a light load as well as damage of the SMPS protection circuit caused by a heavy load. According to the SMPS protection circuit, a voltage lowered by the load is fed back. If the fed-back voltage corresponds to the light load, the switching frequency of an SMPS transformer is downed. If the fed-back voltage corresponds to the heavy load, the SMPS protection circuit ceases to operate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an SMPS protection circuit in an electronic appliance. 
     2. Description of the Related Art 
     The following is a description of an SMPS protection circuit in an electronic appliance according to the conventional technology made with reference to the accompanying drawings. 
     FIG. 1 is a block diagram illustrating a construction of the SMPS protection circuit in an electronic appliance according to the conventional technology. 
     As shown in FIG. 1, the SMPS protection circuit in an electronic appliance according to the conventional technology comprises an AC generating Section  10  for generating an AC voltage, an AC rectifying section  11  for receiving the AC voltage generated from the AC Generating Section  10  and rectifying the AC voltage into a DC voltage, an SMPS transformer  12  for receiving the DC voltage rectified by the AC rectifying section  11  at a primary winding and inducing a voltage proportional to the number of turns of the primary winding with respect to the corresponding secondary winding, a power supply section  16  for receiving the AC voltage generated from the AC Generating Section  10  and converting the same to DC voltage to supply as a power source, a pulse generating section  17  operated by the voltage supplied from the power supply section  16  to generate pulses of a predetermined frequency, a frequency oscillating section  15  for providing a time constant for setting the frequency of the pulse generating section  17 , a switching section for switching an input power source at the primary winding of the SMPS transformer  12 , an output rectifying section  13  for rectifying the voltage induced to the secondary winding of the SMPS transformer  12  into a DC voltage in accordance with an operation of the switching section  18 , and a load  14 . 
     The following is a detailed description of an operation of the conventional SMPS protection circuit in an electronic appliance constructed as above. 
     As shown in FIG. 1, the AC rectifying section  11  receives the DC voltage generated from the AC Generating Section  10 , and rectifies the same into a DC voltage so as to be transmitted to the SMPS transformer  12 . The power supply section  16  converts the AC voltage generated from the AC Generating Section  10  to a DC voltage so as to be applied as a driving voltage of the pulse generating section  17 . Subsequently, the pulse generating section  17  generates pulses of a predetermined frequency, and transmits the same to the switching section  18 . The switching section  18  switches the DC power source applied to the primary winding of the SMPS transformer  12  in accordance with the pulses provided by the pulse generating section  17 . Thereafter, an output rectifying section  13  rectifies an output from the secondary winding of the SMPS transformer  12  into the DC voltage. The rectified DC voltage is applied to the load  14 . Here, the current generated through the load  14 , i.e., the load current, is variable depending on a level of the load  14 . 
     However, the conventional SMPS protection circuit operates irrespective of variation of the load current. As a consequence, electric power is unnecessarily consumed when the load  14  is light, while the SMPS protection circuit is damaged when the load  14  is excessive. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an SMPS protection circuit that can prevent unnecessary electric power consumption and damage of itself by controlling the power source supplied to the SMPS transformer in accordance with the load on the SMPS protection circuit. 
     To achieve the above object, there is provided an SMPS protection circuit comprising: a power supply section for receiving and converting an AC voltage to a driving voltage, and outputting the same; a pulse generating section for receiving the driving voltage outputted by the power supply section to generate pulses of a predetermined frequency; a frequency oscillating section for providing a time constant to set the frequency of the pulses of the pulse generating section; an SMPS transformer having a primary winding for applying an input DC power source thereto, and a secondary winding for applying an AC power source thereto; a switching section for switching the DC voltage inputted to the primary winding in accordance with the pulses generated from the pulse generating section; an output rectifying section for rectifying the output DC power source of the SMPS transformer, and supplying the same to a load; an output current detecting section for outputting a predetermined signal corresponding to a difference between the DC power source outputted by the output rectifying section and the voltage descended due to the load; an output current feedback section for outputting a voltage corresponding to the predetermined signal outputted by the output current detecting section; a frequency down section for reducing the output power source of the SMPS transformer by downing the frequency of the output pulses from the pulse generating section through variation of the time constant of the frequency oscillating section, if the voltage outputted by the output current feedback section is within a first set voltage range representing that the voltage outputted by the output current feedback section is a light load region; and a power intercepting section connected between an output terminal of the driving voltage of the power supply section and the pulse generating section for ceasing operation of the SMPS transformer by ceasing operation of the pulse generating section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram illustrating a construction of an SMPS protection circuit according to the conventional technology; 
     FIG. 2 is a block diagram illustrating a construction of an SMPS protection circuit in an electronic appliance according to a preferred embodiment of the present invention; 
     FIGS. 3A and 3B are circuit diagrams illustrating detailed parts of FIG. 2; and 
     FIGS. 4A to  4 E are wave diagrams illustrating outputs from each part in FIGS.  3 A and  3 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention with unnecessary detail. 
     A construction of the SMPS protection circuit in an electronic appliance according to a preferred embodiment of the present invention will now be described with reference to FIG.  2 . The SMPS protection circuit in an electronic appliance including a pulse generating section  210  for generating pulses of predetermined frequency, an SMPS transformer  120 , an output rectifying section  130 , and a load  140  comprises a switching section  220  for receiving pulse generated from the pulse generating section and switching the SMPS transformer  120 , a power supply section for supplying power source to the pulse generating section  210 , an output current detecting section  150  for converting the current corresponding to a difference between the voltage generated depending on a level of the load  140  and the DC voltage outputted by the output rectifying section  130  to an optical signal, an output current feedback section  160  for outputting voltage proportional to the optical signal transmitted by the output current detecting section  150 , a frequency oscillating section  180  for providing a time constant for setting frequency of the pulse generating section  210 , a frequency down section  170  for varying the time constant of the frequency oscillating section  180  in accordance with the voltage level outputted from the output current feedback section  160 , and a power intercepting section  190  for bypassing the power source applied to the pulse generating section  210  in accordance with the voltage level outputted from the output current feedback section  160 . Here, the SMPS protection circuit further comprises an AC Generating Section  100 , and an AC rectifying section  110  for rectifying the voltage generated from the AC Generating Section  100  to apply the same to a primary winding of the SMPS transformer  120 . 
     The output rectifying section  130  comprises a first diode D 110  having one end terminal connected to a secondary winding of the SMPS transformer  120  for rectifying the signal generated from the secondary winding of the SMPS transformer  120  to a DC voltage of a predetermined level, and a first capacity C 110  having one end terminal connected to a cathode of the first diode D 110  and the other end terminal grounded. 
     The output current detecting section  150  comprises a first resistor R 110  and a second resistor R 120  connected in parallel, a third resistor R 130  and a fourth resistor R 140  for dividing an input voltage into voltages of a predetermined level, an amplifier  150   a  for receiving the voltage passed through the second resistor R 120  at an inverting input terminal −, and receiving the voltage divided by the third resistor R 130  and the fourth resistor R 140  at a non-inverting input terminal + to output a voltage of a predetermined level, a fifth resistor R 150  having one end terminal connected to an output terminal of the amplifier  150   a , a second diode D 120  having a cathode connected to the other end terminal of the fifth resistor R 150 , a sixth resistor R 160  having one end terminal connected to the cathode of the first diode D 110  of the output rectifying section  130 , a third diode D 130  having one end terminal connected to the other end terminal of the sixth resistor R 160  and the other end terminal connected to an anode of the second diode D 120 , and a seventh resistor R 170 . 
     The output current feedback section  160  comprises a first transistor Q 110  operated by receiving a signal outputted from the third diode D 130  of the output sending section  150  at a base to output a voltage of a predetermined level. 
     The frequency down section  170  comprises a ninth resistor R 190  and a tenth resistor R 200  serially connected to an emitter of the first transistor Q 110  of the output current feedback section  160 , a first Zener diode ZD 110  having a cathode connected between the ninth resistor R 190  and the tenth resistor R 200 , which are serially connected to each other, and a breakdown voltage of a predetermined level, an eleventh resistor R 210  having one end terminal connected to the anode of the first Zener diode ZD 110  and the other end terminal grounded on earth, a second transistor Q 120  turned on or off by receiving a signal transmitted through the first Zener diode ZD 110  at the base, and a second capacitor C 120  having one end terminal connected to the emitter of the second transistor Q 120 . 
     The power intercepting section  190  comprises a second Zener diode ZD 120  having a cathode connected to a collector of the first transistor Q 110  of the output current feedback section  160  and a breakdown voltage of a predetermined level, a third transistor Q 130  turned on or off by receiving the signal transmitted through the second Zener diode ZD 120  at the base and having an emitter connected to a ground terminal, a fourteenth resistor  240  and a fourth capacitor C 140  having one end terminal connected to the collector of the third transistor Q 130  in parallel and the other end terminal connected to a Vcc terminal of the pulse generating section  210 , and a fourth transistor Q 140  having a base connected to a part, which connects the 14 th  resistor R 240  and the fourth capacitor C 140  to the collector of the third transistor Q 130  in parallel, an emitter connected to a Vcc terminal of the pulse generating section  210  and a collector connected between the second Zener diode ZD 120  and the base of the third transistor Q 130 . 
     Here, the third diode D 130  of the output current detecting section  150  and the first transistor Q 110  of the output current feedback section  160  are a couple of photo couplers. 
     The following is a detailed description of an operation of the SMPS protection circuit in an electronic appliance according to the present invention. As shown in FIG. 2, the AC rectifying section  110  receives an AC voltage generated from the AC Generating Section, and rectifies the same into a DC voltage. Subsequently, the SMPS transformer  120  receives the DC voltage rectified by the AC rectifying section  110  at a primary winding, and induces the same to the secondary winding. Also, the power supply section  200  converts the AC voltage generated from the AC Generating Section  100  to a DC voltage, and applies the same to the driving power source. 
     The pulse generating section  210  generates pulses of a predetermined frequency, and transmits the same to the switching section  220 . The switching section  220  switches the power source inputted to the primary winding of the SMPS transformer  120  in accordance with the pulse transmitted from the pulse generating section  210 . The secondary output of the SMPS transformer  120  due to the switching operation is transmitted to the output rectifying section  130 . The output rectifying section  130  rectifies the output of the secondary winding of the SMPS transformer  120  as a DC voltage, and outputs the same. 
     As shown in FIG. 3A, the first diode D 110  connected to the secondary winding of the SMPS transformer  120  rectifies the pulses transmitted from the SMPS transformer  120  into a DC voltage of a predetermined level. The rectified voltage is accumulated in the first capacity C 110  by means of a subsequent current I 110 . The current I 120  passes the load  140  and the first resistor R 110  of the output current detecting section  150 . The voltage laid on the first resistor R 110  according to the stream of the current I 120  is as shown in FIG.  4 A. In other words, if the load  140  is light, a small amount of current is applied to the corresponding load  140 . As a consequence, a small amount of current is applied to the first resistor R 110 , and a low voltage is subsequently generated in the first resistor R 110 . On the other hand, if the load  140  is heavy, a great amount of current is applied to the load  140 . As a consequence, a great amount of current is applied to the load  140 , and a high voltage is subsequently applied to the first resistor R 110 . 
     The voltage applied to the first resistor R 110  of the output current detecting section  150  is applied to the inverting input terminal (−) of the amplifier  150   a  through the second resistor R 120 , while the voltage divided by the third resistor R 130  and the fourth resistor R 140  is applied to the non-inverting input terminal (+) of the amplifier  150   a . The output voltage laid on the output terminal of the amplifier  150   a  is as shown in FIG.  4 B. This means that the voltage applied to the non-inverting input terminal (+) of the amplifier  150   a  is a constant voltage. With respect to the voltage applied to the non-inverting input terminal (−), a voltage applied to the first resistor R 110  passes the second resistor R 120 . Therefore, the voltage within the light load region applied to the non-inverting input terminal of the amplifier  150   a  is lower than the reference voltage by a predetermined level applied to the corresponding non-inverting input terminal, and a voltage higher than a predetermined level is outputted to the output terminal of the amplifier  150   a  as shown in FIG.  4 B. By contrast, a voltage applied to the non-inverting input terminal of the amplifier  150   a  within a heavy load region is higher than the reference voltage by a predetermined level applied to the corresponding non-inverting input terminal. Therefore, a voltage lower than the reference voltage by a predetermined level is outputted from the output terminal of the amplifier  150   a  as shown in FIG.  4 B. 
     The third diode D 130  of the output current detecting section  150  receives a DC voltage of a predetermined level, which passes the first diode D 110  of the output rectifying section  130 , through the sixth resistor R 160 . As a consequence, the current difference between the third diode D 130  and the output voltage of the amplifier  150   a  flows in the third diode D 130 , and a subsequent optical signal is generated. The optical signal is inputted to the base of the first transistor Q 110  of the output current feedback section  160 . 
     Here, the DC voltage of a predetermined level rectified by the first diode D 110  of the output rectifying section  130  is applied to the anode of the third diode D 130  on a steady basis, and the amount of current flowing in the third diode D 130  is variable according to the output voltage of the amplifier  150   a . To be specific, if the output voltage of the amplifier  150   a  is higher than a predetermined level, the voltage difference between the DC voltage applied to the third diode D 130  and the output voltage of the amplifier  150   a  becomes small. Thus, as shown in FIG. 4C, a small amount of current flows in the third diode D 130 . If the output voltage of the amplifier  150   a  is lower in a heavy load region by a predetermined level, the voltage difference between the DC voltage applied to the third diode D 130  and the output voltage of the amplifier  150   a  becomes great. Therefore, as shown in FIG. 4C, a great amount of current flows in the third diode D 130 . 
     The first transistor Q 110  of the output current feedback section  160  receives the optical signal transmitted from the third diode D 130  of the output current detecting section  150  at the base, and outputs the corresponding current. To be specific, as shown in FIG. 3B, the first transistor Q 110  of the output current feedback section  160  receives the voltage outputted from the Vcc terminal of the pulse generating section  210  at the collector through the eighth resistor R 180 , and outputs a collector current  130  to the emitter as the predetermined signal transmitted by the third diode D 130  of the output current detecting section is inputted to the base of the first transistor Q 110 . 
     Subsequently, the voltage of the emitter A of the first transistor Q 110  is calculated as defined in the following Equation 1. 
     
       
         Emitter Voltage  A  of  Q   110   =I   130 ( R   190   +R   200 ) [ V]    Equation 1 
       
     
     Here, the emitter voltage A of the first transistor Q 110  is proportional to the current I 130  flowing in the corresponding collector, as defined by the Equation 1. 
     When the current within the light load region flows in the base of the first transistor Q 110  as shown in FIG. 4C, a small amount of current  130  from the collector is outputted to the emitter of the first transistor Q 110  as shown in FIG.  4 D. 
     If the current within the heavy load region is applied to the base of the first transistor Q 110  as shown in FIG. 4C, a great amount of current I 130  of the collector is outputted to the emitter of the first transistor Q 110  as shown in FIG.  4 D. 
     The frequency down section  170  receives the emitter voltage A of the first transistor Q 110  of the output current feedback section  160  that has been divided by the ninth resistor R 190  and the tenth resistor R 200 , which are connected in parallel. If the voltage divided by the ninth resistor R 190  and the tenth resistor R 200  is lower than the breakdown voltage of the first Zener diode ZD 110 , i.e., if the emitter voltage A of the first transistor Q 110  becomes lower than the breakdown voltage of the first Zener diode ZD 110  due to an output of the current I 130  within the light load region, the first Zener diode ZD 110  is turned off, and no signal is applied to the base of the second transistor Q 120 . As a consequence, the corresponding second transistor Q 120  is turned on. The time constant representing the frequent setting section  180  is subsequently varied, and the pulse generating section  210  generates pulses having a frequency according to the varied time constant. The frequency at that time can be expressed by the following Equation 2. 
     
       
           F   1 =1.8÷ [R   220 ×( C   120   +C   130 )]  Equation 2 
       
     
     Here, the frequency generation section  210  generates a frequency according to the time constant inputted to the R T /C T  terminal. In other words, the pulse generating section  210  generates pulses having the frequency F 1  in the Equation 2, and transmits the pulses to the switching section  220 . 
     On the other hand, the emitter voltage A of the first transistor Q 110  within the normal operation region is higher than the breakdown voltage of the first Zener diode ZD 110  of the frequency down section  170  and within the region lower than the breakdown voltage of the second Zener diode ZD 120 , as shown in FIG.  4 E. 
     Since the emitter voltage A of the first transistor Q 110  is higher than the breakdown voltage of the first Zener diode ZD 110 , the first Zener diode ZD 110  is turned on so as to apply a predetermined signal to the base of the second transistor Q 120 . 
     At this stage, the second transistor Q 120  is turned off, and the capacitor C 120  is not connected to the capacitor C 130  of the frequency oscillating section  180  in parallel. Subsequently, the time constant representing the frequency oscillating section is varied, and the pulse generating section  210  generates pulses having a frequency according to the varied time constant. The frequency at this stage can be expressed by the following Equation 3. 
     
       
           F   2 =1.38÷[ R   220   ×C   130 ]  Equation 3 
       
     
     The pulse generating section  210  supplies the pulses having the frequency F 2  in the Equation 3 to the switching section  220 . 
     Here, the frequency F 1 , applied to the pulse generating section  210  from the light load region is lower than the frequency F 2  by a predetermined level. Therefore, consumption of electric power generated in accordance with operation of the switching section  220  can be reduced by reducing operation of the switching section  220  through reduction of the frequency generated from the pulse generating section  210 . 
     If the third diode D 130  of the output current detecting section  150  outputs the current in the heavy load region as shown in FIG. 4C, the first transistor Q 110  of the output current feedback section  160  is turned on by receiving the signal transmitted by the third diode D 130  at the base, and outputs a great amount of current I 130  to the emitter of the corresponding first transistor Q 110  as shown in FIG.  4 D. Since the emitter voltage A of the first transistor Q 110  is proportional to the current I 130  as defined by the Equation 1, the emitter voltage A of the first transistor Q 110  is increased as shown in FIG. 4E if the current I 130  is increased. 
     If the emitter voltage A of the first transistor Q 110  is higher than the breakdown voltage of the second Zener diode ZD 120  of the power intercepting section  190 , the second Zener diode ZD 120  is turned on to output a predetermined signal to the base of the third transistor Q 130 . The third transistor Q 130  is subsequently turned on, and a signal of 0V is applied to the fourth transistor Q 140  having a base connected to the collector of the third transistor Q 130 . 
     The fourth transistor Q 140  is turned on, and the voltage applied to the Vcc terminal of the pulse generating section  210  from the power supply section  200  is by-passed to a collector terminal of the fourth transistor Q 140 . To be specific, if the current transmitted by the output current detecting section  150  is sensed to be heavy by the output current feedback section  160 , i.e., overloaded, the voltage applied to the pulse generating section  210  is by-passed to cease the corresponding pulse generating section  210 . The system connected to the first winding and the second winding of the SMPS transformer  120 , i.e., the entire system, ceases to operate by ceasing operation of the switching section  220 . 
     In short, damage of the SMPS circuit causable by an overload can be prevented by ceasing operation of the entire system after sensing of the heavy current generated from the secondary winding of the SMPS transformer  120  by the output current feedback section  160  of the primary winding. 
     As described above, the SMPS protection circuit in an electronic appliance according to the present invention has an effect of enhancing reliability of the product by including a single output current feedback section, i.e., a single photo coupler, in the primary winding of the SMPS to sense variation of the load at the secondary winding of the SMPS and to prevent damage of the SMPS circuit causable by an overload. 
     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.