Abstract:
A switching voltage regulator adapted for providing a regulated voltage at an output terminal is described which comprises at least one MOS transistor having a non-drivable terminal coupled with said output terminal and a control circuit receiving a signal that is representative of the current signal flowing in said MOS transistor. The control circuit comprises a compensation device adapted for cancelling the thermal variation of said signal that is representative of the current signal flowing in said MOS transistor.

Description:
TECHNICAL FIELD  
         [0001]    The present invention refers to a switching voltage regulator.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is usually known the use of control circuits in switching static regulators employed in power suppliers wherein particular features on the precision of the regulation are requested, as in the power supply circuits of the high performance modern microprocessors. Particularly such power suppliers must provide higher and higher currents and lower and lower voltages.  
           [0003]    Switching regulators are used to perform said power suppliers. Each regulator comprises at least one MOS power transistor; particularly some regulators comprise a pair of MOS transistors or several pairs of MOS transistors which are arranged in parallel to each others and which are connected with a single output terminal by means of an inductance for each transistor pair (Multi-phase converters). The output currents of said pairs of transistors are automatically balanced by means of a control operation which detects each single current by detecting the voltage drop between the drain and source terminals of the MOS transistor. This voltage drop is also employed for implementing a well precise and programmable load regulation as a function of the current provided to the load, as it is required from particular loads such as the microprocessors.  
           [0004]    It is however known that the MOS transistors are provided internally of a resistance between the drain and source terminals in the conduction or firing phase, known as on or conduction resistance Rdson, which changes with the temperature and which, for this reason, can cause variations of the voltage signal between the drain and source terminals of the MOS transistor with currents of the same value. This occurs above all in the power MOS transistors because the current flowing through the source and drain terminals thereof is high and consequently the variation of the voltage drop at the terminals of the MOS transistor which is due to the thermal variation of the on resistance, is also high above all in the cases wherein, for size problems, thermal dissipators or fans are not used.  
           [0005]    Therefore the thermal variation of the on resistance (Rdson) of the MOS may cause very high variations of the voltage regulated by the power suppliers which employ the voltage detected between the terminals of the MOS transistor for regulating the output voltage on the load to be supplied. This may bring to non-respect of some specifications of the loads such as the microprocessors.  
           [0006]    A possibility to avoid the variations of the voltage detected between the terminals of the MOS transistor of the voltage regulators consists of adding and consequently using measurement elements that are substantially invariant in temperature, for example resistors known as “sense resistors”. The signal measured between the terminals of the sense resistors acts in the control operation of the MOS transistor for regulating suitably the output voltage of the regulator. Such resistors have thermal variations lower than the resistance Rdson, are very precise but the high precision thereof is compromised by the high contact resistance due to the welding thereof on the printed circuit of the regulator. Also the power dissipated by the sense resistors decreases the effectiveness of all the regulator and the use thereof causes a higher cost of all the appliance.  
           [0007]    In view of the art described, it is an object of the present invention to provide a switching voltage regulator that overcomes the aforementioned disadvantages.  
         SUMMARY OF THE INVENTION  
         [0008]    According to principles of the present invention, a switching voltage regulator adapted for providing a regulated voltage at an output terminal, comprising at least one MOS transistor.  
           [0009]    In one embodiment the MOS transistor includes a non-drivable terminal coupled with said output terminal and a control circuit receiving a signal that is representative of the current signal flowing in said MOS transistor, characterized in that said control circuit comprises a compensation device adapted for cancelling the thermal variation of said signal that is representative of the current signal flowing in said MOS transistor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The features and the advantages of the present invention will be made evident by the following detailed description of an embodiment thereof, illustrated as not limiting example in the annexed drawings, wherein:  
         [0011]    [0011]FIG. 1 is a simplified circuit scheme of a switching regulator according to prior art;  
         [0012]    [0012]FIG. 2 is a scheme of a circuit part of a switching regulator according to invention;  
         [0013]    [0013]FIG. 3 is a diagram of the output voltages of the circuit in FIG. 1 and of the regulator comprising the circuit part in FIG. 2 as functions of the temperature;  
         [0014]    [0014]FIG. 4 is a circuit scheme of a modular power supplier according to prior art;  
         [0015]    [0015]FIG. 5 is a more detailed scheme of a circuit part of a module of the power supplier in FIG. 4;  
         [0016]    [0016]FIG. 6 is a scheme of a circuit part of a module according to the present invention;  
         [0017]    [0017]FIG. 7 is a diagram of the average current of the power supplier in FIG. 5 and of a modular power supplier employing the circuit part in FIG. 6 in each module as functions of the temperature.  
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring to FIG. 1 a switching regulator according to the prior art is shown; said regulator comprises two MOS power transistors M 1  and M 2  where the source terminal of the transistor M 1  is in common with the drain terminal of the transistor M 2  and it is connected with an inductance L the other terminal of which is the output terminal of the regulator. The drain terminal of the transistor M 1  is connected with an input voltage Vin while the source terminal of the transistor M 2  is connected to ground. The gate terminals of the transistors M 1  and M 2  (which may constitute even the electric equivalent of more MOS transistors connected with each others in parallel) are driven by means of a control circuitry  1 . The transistors M 1  and M 2  may be discrete components or may be integrated in the same chip with the control circuitry  1 . The current flowing between the drain and source terminals of the transistor M 2  is detected by means of a detecting device  2  that preferably comprises an amplifier able to convert the voltage drop between the terminals of the transistor M 2  into a proportional current signal Ifb. The detected current Ifb is in input at the inverting terminal of an error operational amplifier  3  having the non-inverting terminal connected with a reference voltage Vprog (for example of about 1.5 V) and the output terminal connected with an impedance Z the other terminal of which is connected with the inverting terminal of the amplifier  3 . The detected current Ifb is brought to the output terminal of the regulator by means of a resistor Rfb arranged between the inverting terminal of the amplifier  3  and the output terminal of the regulator. In accordance with the variations of the current required from the load the current Ifb changes proportionally by causing a well precise and desired variation of the regulated voltage. The variation of the on resistance Rdson2 of the MOS transistor M 2  in a way depending on the temperature determines however an undesired variation of the provided current Ifb that in turn determines an undesired variation of the output voltage Vout of the regulator. In fact it occurs that Vout=Vprog−Rfb*Ifb wherein the current Ifb is given by Ifb=Iout*Rdson2/K where K is a proportionality constant that is function of the current detecting device  2  and lout is the current flowing between the drain and source terminals of the transistor M 2 . It occurs  
       Vout   =     Vprog   -     Rfb   *     Rdson2   K     *   Iout                             
 
         [0019]    [0019]FIG. 2 shows one embodiment of the present invention. This embodiment has the advantage of avoiding variations of the regulated voltage Vout. The temperature coefficient of the term Rfb*Rdson2/K is cancelled and this is possible by using a thermal compensation device  9 . The device  9 , shown in FIG. 2, comprises preferably an element  10  having a dependence on the temperature with a negative coefficient, as shown in FIG. 2; in such way the total temperature coefficient may be minimized or even cancelled. A device comprising for example a series of two resistors R 1  and R 2  may be introduced in the place of the resistor Rfb; said element  10  is set in parallel to the resistor R 2 . Said element  10  is preferably constituted by a resistor NTC but it may be constituted by a diode having a suitable interface circuitry.  
         [0020]    The thermal compensation device  9  may be formed in another way, for example using a MOS transistor and a suitable circuitry or even any bipolar transistor or JFET which is connected always with a suitable circuitry. Any component sensitive to the temperature may be used with a suitable interface circuitry for compensating the variation of the Rdson of the MOS transistor M 2 . The advantage of using MOS transistors or diodes as element  10  is due to integrability thereof directly on chip of the power transistor at the terminals of which the detection is effectuated.  
         [0021]    In FIG. 3 the waveforms of the output voltages Vout 1  (with a sketch line) and Vout 2  (with a continuous line) as functions of the temperature are shown which respectively regard the voltage regulator in FIG. 1 and the regulator employing the thermal compensation device according to invention. The voltages Vout 1  and Vout 2  are valued in the different cases wherein the value of the current lout (in FIG. 3 Iout=1) is 0 A, 10 A, 20 A, 30 A, 40 A, 50 A; using 1=0 A the voltages Vout 1  and Vout 2  are equal. The used element NTC is a PANASONIC ERTJ1VT102H. From the diagram it is evident that the voltage Vout is substantially constant changing the temperature.  
         [0022]    In FIG. 4 a modular power supply is shown according to prior art. Said power supply comprises various modules  100  arranged in parallel to each other and which have a same input voltage V 1 . Each module  100  comprises a supply  101 , a MOS transistor  102  (which may constitute even the electric equivalent of more MOS transistors connected with each other in parallel) connected in series with the power supply  101  and with the output terminal OUT of the modular power supply and a control circuit  103 . The last detects the current  1102  flowing through the transistor  102  and provides an input signal to the supply  101  and a signal Vbus which finds on the bus  200  (current-sharing BUS) that is common to all the modules  100  and on which the information relating to the average current brought by the modular power supply is formed wherein it is meant by average current the mean of the currents brought by each module  100 . Each module  100  compares the own current with the average current and amends its operation to cancel such difference.  
         [0023]    One embodiment of the control device  103 , shown in FIG. 5, comprises an amplifier  104  adapted for detecting the current  1102  flowing through the MOS transistor  102 , a buffer  105  having an input voltage signal at the inverting terminal which is given by the current signal Iout 1  in output from the amplifier  104  which is multiplied by a resistor Rcga and the output signal thereof is in input to a current-sharing BUS  200 . The control device  103  comprises an error amplifier  106  having in input the same voltage signal given by the current signal Iout 1  in output from the amplifier  104  which is multiplied by a resistor Rcga and a signal storing the information relating to the average current deriving from the current-sharing BUS  200 . The output signal of the amplifier  106  is in input to the power supply  101  and it is a correction signal; said signal allows to correct the operation of each power supply  101  in such a way to make equal the current  1102  of each module  100  with the average current brought totally by the modular power supply.  
         [0024]    The voltage Vbus for each single module  100  is given by:  
       Vbus   =     Rcga   *     Rdson   k1     *     Iout1   .                             
 
         [0025]    [0025]FIG. 6 shows the inventive circuit used to avoid variation of the voltage Vcbus with respect to the temperature. The temperature coefficient of the term Rcga*Rdson/K1 is cancelled; this is possible by using a thermal compensation device  9  already described. The device  9  comprises preferably an element  10  having a dependence on the temperature with a negative coefficient, as shown in FIG. 6; in such way the total temperature coefficient may be cancelled. A device comprising for example a series of two resistors R 1  and R 2  may be introduced in the place of the resistor Rcga; said element  10  is set in parallel to the resistor R 2 . Said element  10  is preferably constituted by a resistor NTC but it may be constituted by a diode having a suitable interface circuitry.  
         [0026]    The thermal compensation device  9  may be formed in another way, for example using a MOS transistor and a suitable circuitry or even any bipolar transistor or JFET which is connected always with a suitable circuitry. Any component sensitive to the temperature may be used with a suitable interface circuitry for compensating the variation of the Rdson of the MOS transistor  102 .  
         [0027]    In FIG. 7 the waveforms of the output voltages Vbus 1  (with a sketch line) and Vbus 2  (with a continuous line) on the bus  200  as function of the temperature are shown which respectively regard the power supply in FIG. 4 and the power supply employing the thermal compensation device according to invention. The voltages Vbus 1  and Vbus 2  are valued in the different cases wherein the value of the current I 102  (in FIG. 7 I 102 =1) is 10 A, 25 A, 40 A. The element NTC used is a PANASONIC ERTJ1VT102H. From the diagram it is evident that the voltage Vbus 2  is substantially constant changing the temperature.  
         [0028]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0029]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.