Abstract:
When a signal having a large shifted level is needed in a level shift circuit, a false operation is generated by the increase of noise level. And, when the signal level is very low, the circuit does not operate and generates distortion. The present circuit improves such problems. To do this, a capacitor or diode for coupling a signal generated from a signal source and level shifter for shifting the DC-coupled signal by a predetermined shift level is utilized. Accordingly, a variation of a small signal is transmitted without distortion to exactly and precisely control a device and noise level is reduced in a source signal transmission process, thereby preventing false operation of the device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a level shift circuit and more particularly to a small signal level shift circuit for shifting the level of a small signal without distortion. 
     Generally, in circuits or devices for processing a signal generated from a signal source such as a sensor, a monitoring circuit, a signal generating circuit and a level sensing circuit, etc., the levels of the generated signals required in the above circuits are different. A signal level is shifted by a level required in a level shift circuit shown in FIG. 1. In FIG. 1, the collector terminal of transistor 2 is coupled to a power source voltage Vcc, its base terminal is coupled to a signal Vs generated from a signal source, and its emitter terminal is grounded through serially connected resistors 4 and 6, thereby obtaining an output signal Vo at a node between the resistors 4 and 6. If a signal Vs such as FIG. 2A is supplied to the base terminal of the transistor 2, the transistor 2 amplifies the signal Vs. At this time, at the node between resistors 4 and 6, a level shifted signal Vo such as FIG. 2B is generated by adding the amplified signal to a signal level obtained by dividing the power source voltage Vcc. Accordingly, the signal Vs is shifted by a predetermined voltage level V SET . 
     As described above, the conventional level shift circuit generates the following problems according to the use of transistor. First, when a signal level is shifted largely, a transistor having a large small-signal current amplification rate h fe  should be used. But, in this case, noise level is also increased, thereby generating a false operation, so that an extra circuit for preventing the false operation is needed. Secondly, when the signal level is very low, the transistor is in a cut-off region, so that the transistor does not operate. Thirdly, distortion caused by several operation characteristics of transistor is generated. 
     Meanwhile, in a switching mode power supply (hereinafter referred to as SMPS) widely used as power supply of several electric or electronic devices, signal transfer is needed, which is described in detail as follows. With reference to FIG. 3 showing an example of multi-output SMPS, a schematic operation useful for understanding of the present invention is described. An input rectifying circuit 12 rectifies an AC (alternating current) power source through an AC input power source 10 and supplies the rectified power source to a transformer 14 and a switching control unit 16. Generally, as the switching control unit 16, one-chip IC such as UC1842 which is a PWM controller of UNITRODE company in U.S.A. is used. The switching control, unit 16 is operated by the DC (direct current) power source supplied from the input rectifying circuit 12, thereby generating a pulse width modulation (hereinafter referred to as PWM) signal having a predetermined frequency. A switching circuit 18 coupled between a primary side of the transformer 14 and the switching control unit 16 induces power source at a secondary side of the transformer 14, by switching the DC power source supplied to the primary side of the transformer 14 in response to the PWM signal. The induced power source is rectified and smoothed in first and second output rectifying circuits 22 and 24, and then is supplied to a load through first and second DC output power sources 26 and 28, respectively. An insulating circuit 30 and a feedback circuit 32 feed back the output power source voltage of the second DC power source 28 to the switching control portion 16. According, the switching control unit 16 varies duty of PWM signal according, to the state of feedback voltage, thereby stabilizing the output power source. 
     And, a current sensing circuit 20 senses the state of the primary side current flowing through the switching unit 18 and supplies the sensed current state to the switching control unit 16. At this time, if current over a regulated value is sensed due to abnormality of input power source or the abnormality of load or SMPS, the switching control unit 16 is shut down, thereby stopping the generation of PWM signal to protect load or SMPS from overcurrent. 
     FIG. 4 is a diagram of a conventional current sensing circuit for sensing a current state of primary side as described above, where a switching control unit 16, a switching unit 18, a current sensing circuit 20, and lines 101 to 104 correspond to the corresponding circuits of the FIG. 3, respectively. The switching control unit 16 is constituted by a PWM controller as described above. A field effect transistor (hereinafter referred to as FET) 36 of the switching unit 18 switches a primary-side power source in response to a PWM signal, such as that of FIG. 5A, outputted from an output terminal OUTPUT of the switching control unit 16. In the line 101 which is a primary side of transformer 14, a voltage waveform such as FIG. 5B is shown by the FET 36. Resistors 32 and 34 are coupled between the output terminal OUTPUT of the switching control unit 16 and a gate terminal of the FET 36, to properly set on/off time of FET 36. A resistor 42 of the current sensing circuit 20 is a current sensing resistor, which limits current flowing through the FET 36 and at the same time, generates a voltage corresponding to the amount of current in the line 103. The generated voltage shows a waveform such as FIG. 5C and is supplied to a current sensing terminal I SENSE  of the switching control unit 16 through a resistor 40 as a current sensing voltage having the waveform such as FIG. 5D. 
     Generally, a shut-down voltage where the switching control unit 16 senses overcurrent and is shut down is set as 1 V, and accordingly, a current sensing resistance Rs according to a maximum current Ismax for sensing overcurrent is determined by the following equation (1): 
     
         Ismax˜1.0V/Rs                                        (1) 
    
     Thus, if a current sensing voltage supplied to the current sensing terminal I SENSE  of the switching control unit 16 through the resistor 40 reaches 1 V as the current passing through the FET 36 increases, the switching control unit 16 is shut down. At this time, since power proportional to the current flowing through the FET 36 is consumed on the resistor 42, heat loss is generated. For instance, when a maximum current Ismax where the current sensing voltage becomes 1 V, is 15A, a duty D of PWM signal is 0.8, and a resistance R 42  of resistor 42 is 65 mΩ, the power consumption Pt is given by the following equation (2): ##EQU1## 
     That is, to obtain the current sensing voltage of 1V, loss of 11.7 W is generated. Accordingly, when large current such as 15A is sensed, excessive heat loss is generated, so that additional radiating processing is required. Also, there is a problem in that the efficiency of SMPS is deteriorated by the generation of heat loss. Also, the resistor 42 should be a resistor having rated dissipation which is sufficiently large with respect to the power of 11.7 W, so that there are problems of occupying large space and raising the cost. 
     Accordingly, another current sensing circuit constituted by a transformer (troidal core transformer) 44 for sensing current by a magnetic element instead of the resistor 42, as shown in FIG. 6 is used. In FIG. 6, the switching control unit 16, the switching unit 18, the current sensing circuit 20, and the lines 101 to 104 correspond to the corresponding circuits of FIGS. 3 and 4, respectively. And, the switching unit 18 switches the primary-side power source in response to a PWM signal, such as FIG. 7A, outputted from the switching control unit 16 as described above. Then, the voltage waveform such as FIG. 7B is shown in the line 101. A resistor 46 converts magnetic current induced in the secondary side of the transformer 44 into a voltage, which is generated with the waveform such as FIG. 7C in the line 105. The voltage of the line 105 appears in the line 106 without negative voltage, as shown in FIG. 7D, through a diode 48, and is supplied to the current sensing terminal I SENSE  of the switching control unit 16 through the resistor 52 as a current sensing voltage having the waveform such as FIG. 7E. A resistor 50 stabilizes the voltage of the line 106. 
     In this case, the power proportional to the current flowing through the switching unit 18 is consumed also in the resistor 46, so that heat loss is generated. For instance, when a maximum magnetic current Imax where the current sensing voltage becomes 1 V, is 150 mA and a resistance R 46  of resistor 46 is 15Ω, the maximum voltage V 105  in the line 105 is given by the following equation (3): 
     
         i V.sub.105 =150×10.sup.-3 ×15=2.25V           (3) 
    
     Thus, when the duty D of PWM signal is 0.8, the power consumption Pt dissipated in the resistor 46 is given by the following equation (4): ##EQU2## 
     That is, the loss of 270 mW which is greatly reduced compared with the circuit of FIG. 4 is generated to obtain the current sensing voltage of 1 V. However, according to the use of magnetic element, the following problems are generated. First, the number of steps in manufacturing of products is increased and the automatic insert machine cannot be used. Secondly, in the design of the magnetic element, saturation of magnetic core should be considered, so that its realization is difficult. 
     As described above, in SMPS, since the conventional current sensing circuit uses a resistor element or a magnetic element to sense current state, the above-mentioned problems are generated and also there is another problem of causing a false operation since noise level with respect to large current is transferred as a current sensing voltage, as it is, during the transfer of current sensing voltage. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a small signal level shift circuit which can solve the aforementioned problems. 
     It is another object of the present invention to provide a small signal level shift circuit which can exactly and precisely control a device by shifting a small signal by a predetermined level without distortion. 
     It is still another object of the present invention to provide a small signal level shift circuit which can prevent a false operation of a device by reducing noise level generated during transmitting signal. 
     It is yet another object of the present invention to provide a small signal level shift circuit which can minimize heat loss by controlling the sensing of large current only by a small signal, in a current sensing circuit of SMPS. 
     It is yet still another object of the present invention to provide a small signal level shift circuit which can sense current without using a resistor element or a magnetic element having large rated dissipation. 
     (*To achieve the objects, the present invention comprises signal coupling means for DC-coupling a signal generated from a signal source and level shift means for shifting the DC-coupled signal by a predetermined shift level.*) 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: 
     FIG. 1 is a conventional level shift circuit; 
     FIGS. 2A and 2B are waveforms according to operation of FIG. 1; 
     FIG. 3 is a block diagram of a general switching mode power supply; 
     FIG. 4 is a current sensing circuit according to a conventional embodiment; 
     FIGS. 5A to 5D are waveforms at the respective circuits of FIG. 4; 
     FIG. 6 is a current sensing circuit according to another conventional embodiment; 
     FIGS. 7A to 7E are waveforms at the respective circuits of FIG. 6; 
     FIG. 8 is a level shift circuit of an embodiment according to the present invention; 
     FIGS. 9A and 9B are waveforms of FIG. 8 according to the present invention; 
     FIG. 10 is a level shift circuit of another embodiment according to the present invention; 
     FIG. 11 is a circuit diagram as an example where the level shift circuit according to the present invention is applied to that of FIG. 3; and 
     FIGS. 12A to 12D are waveforms at the respective circuit of FIG. 11 according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 8, resistors 62 and 64 are serially coupled between a power source voltage Vcc and a ground, and constantly divide the power source voltage Vcc. A capacitor 60 which is a capacitive element is coupled between a signal source Vs and a connection point of the resistors 62 and 64, and DC-couples the signal Vs. An output signal Vo is obtained at the connection point of the resistors 62 and 64. 
     An operational example of FIG. 8 is described with reference to the waveform view of FIG. 9 as follows. Assuming that the signal Vs generated from a signal source is such as that of FIG. 9A, the signal Vs appears as a level-shifted signal Vo such as FIG. 9B by being shifted by a shift level set by resistors 62 and 64 through the capacitor 60. That is, the signal Vs is shifted to a set voltage level V SET . The shift level V SFT  is given by the following equation (5) according to resistances R 62  and R 64  of resistors 62 and 64. That is, 
     
         V.sub.SFT ={R.sub.64 /(R.sub.62 +R.sub.64)}×Vcc      (5) 
    
     And, the level shifted output signal Vo is given by the following equation (6): 
     
         Vo=Vs+{R.sub.64 /(R.sub.62 +R.sub.64)}×Vcc           (6) 
    
     Accordingly, when the output signal Vo is equal to or smaller than the power source voltage Vcc, the resistances of resistors 62 and 64 can be adjusted, thereby shifting the level of signal Vs to a desired level. As described above, the circuit of FIG. 8 does not amplify the level of signal Vs generated from a signal source and shifts only the level, so that the distortion of signal or the increase of noise level is prevented, thereby preventing a false operation of device. Also, the signal Vs of a very small level can be shifted. Here, the capacitor 60 performs the DC cut-off between the signal source and the resistors 62 and 64, thereby preventing the shift level voltage from being supplied to the signal source. 
     FIG. 10 is a level shift circuit of another embodiment according to the present invention, which is an example of replacing the capacitor 60 of FIG. 8 by a diode 66 which is a DC cut-off element. Accordingly, the resistors 62 and 64 are the same as those of FIG. 8, and the diode 66 is forwardly coupled to a connection point between resistors 62 and 64 from the signal source Vs. Accordingly, the diode 66 performs the DC-cut off of the signal source side from the resistors 62 and 64, thereby preventing the shift level voltage from being supplied to the signal source side. 
     Meanwhile, FIG. 11 shows an example where the level shift circuit of FIG. 8 according to the present invention is applied in the current sensing circuit 20 of SMPS shown in FIG. 3. In FIG. 11, a resistor 70 is coupled between the switching unit 18 and a ground. The resistor 70 is a current sensing resistor, which converts the current flowing through the switching unit 18 into a voltage of level corresponding to the current. A capacitor 72 is coupled between a connection point of switching unit 18 and resistor 70, and a connection point of resistors 74 and 76, and DC-couples the converted voltage level. The resistors 74 and 76 are serially coupled between a reference voltage terminal Vref of the switching control unit 16 and a ground, and constantly divide the reference voltage to set a shift level. Also, the connection point of resistors 74 and 76 are coupled between the capacitor 72 and a current sensing terminal I SENSE  of the switching control unit 16. Here, the level shift circuit 78 having the capacitor 72 and the resistors 74 and 76 corresponds to that of FIG. 8. The switching control unit 16, the switching unit 18, the current sensing circuit 80 and the lines 101 to 104 correspond to the corresponding circuits of FIGS. 3 and 4 described above, respectively. 
     In FIG. 11, contrarily to the conventional one, the resistance of resistor 70 is set to a very small value, thereby generating in the line 103 a small voltage level, which is shifted by the shift level through the capacitor 72, and then is supplied to the current sensing terminal I SENSE  of the switching control unit 16 as a current sensing voltage. 
     Meanwhile, a reference voltage Vref of generally 5.1 V (±1%) is generated in a reference voltage terminal Vref of the switching control unit 16. Accordingly, assuming that the shut-down voltage of the switching control unit 16 is set to 1.0 V as described above, for example, the shift level is set to 0.95 V. In this state, if the current sensing voltage of line 103 generated in the resistor 70 reaches 0.05 V, it becomes 1.0 V by being shifted by the shift level 0.95 V through the capacitor 72, and is supplied to the current sensing terminal I SENSE  of the switching control unit 16. Accordingly, the switching control unit 16 senses overcurrent state and is shut down, thereby stopping the generation of PWM signal to protect the load or SMPS from overcurrent. Here, when the PWM signal outputted in the switching control unit 16 is identical to that of FIG. 12A, voltage waveform such as FIG. 12B appears in the line 101, and voltage waveform such as FIG. 12C appears by the resistor 70 in the line 103. The voltage of the line 103 is shifted by the shift level, and is generated in line 104 as a current sensing voltage such as FIG. 12D. At this time, if it is assumed that the maximum current Ismax where the current sensing voltage becomes 1 V is set to be 15A as shown in FIG. 4, the resistance R 70  of the resistor 70 is determined as 50 mV/15 A=3.3 mΩ. In this state, when the duty D of PWM signal is 0.8 as in the above case, the power consumption Pt of the resistor 70 is given by the following equation (7): ##EQU3## 
     That is, to obtain the current sensing voltage of 1 V, the loss of 0.6 W is generated, so that the loss is greatly reduced, compared with the aforementioned circuit of FIG. 4. Accordingly, even if large current is sensed, the heat loss is minimized, thereby improving the efficiency of SMPS, and the current can be sensed without using a resistor element or a magnetic element of rated dissipation. Moreover, noise level is reduced, and also, current can be exactly and precisely limited without distortion. 
     While the aforementioned description of the present invention describes a preferred embodiment, several variations can be made without departing from the spirit of the invention. Particularly, FIG. 8 illustrates that the shift level is set using only two resistors 62 and 64, but the shift level can be set by dividing a power source voltage Vcc by a plurality of resistors, and can be differently set using a variable resistor as shown in FIG. 8, if necessary. Similarly, only one capacitor 60 of FIG. 8 or one diode of FIG. 10 is used in the present invention, but a plurality of capacitors or diodes can be used if necessary, and other capacitive element or one-directional DC cut off means can be used. **Also, FIG. 11 illustrates that a shift level is set from a reference voltage of switching control unit 16. But, the shift level can be set by dividing a power source voltage as shown in FIG. 8 and instead of using the level shift circuit of FIG. 8, that of FIG. 10 can be used to obtain the same effect. 
     As described above, according to the present invention, a variation of small signal is transmitted without distortion and fine control can be made, thereby exactly and precisely controlling a device. Also, there is an advantage of preventing a false operation of device by reducing noise level in signal transmission process. Also, in SMPS, even if large current is sensed, heat loss is minimized, thereby improving the efficiency of SMPS, and a resistor element or a magnetic element of rated dissipation is not used, thereby realizing the miniaturization of device and the cost reduction. Also, current can be exactly and precisely limited by reducing noise level and being transmitted without distortion. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that modifications in detail may be made without departing from the spirit and scope of the invention.