Abstract:
A short-circuit protection circuit, particularly for power transistors, contains a first circuit for mirroring the output current of a power transistor which is parallel-connected to the power transistor, and a second mirroring circuit which is series-connected to the first mirroring means and is adapted to emit a current which is correlated to the current mirrored by the first mirroring circuit, for comparison with a reference current. The result of the comparison determines the need to intervene or not on the power transistor. The short-circuit protection circuit may also contain a circuit for sensing the voltage drop across the power transistor which is parallel-connected to the power transistor and to the first mirroring circuit, in order to adjust minimum and maximum values of the current in output from the power transistor, as a function of the voltage that is present across the transistor.

Description:
TECHNICAL FIELD 
     The present invention relates to a short-circuit protection circuit, particularly for power transistors. 
     BACKGROUND OF THE INVENTION 
     It is known that many integrated circuits use protections against short-circuits to control the maximum current that flows through the power transistors. In some applications it is important that the power transistor be able to supply a small current also when high voltages are applied to its terminals. 
     In single power-supply audio applications, during power-on transients the power transistor must charge the decoupling capacitor, which is connected between the loudspeaker and the output of the power stage, while the entire power supply voltage is applied to the terminals of the power transistor. 
     In such a situation, it is important to ensure that the power transistor is capable of supplying the current required to charge said capacitor, so that the amplifier can power-on correctly. 
     FIG. 1 illustrates a conventional protection circuit, in which the reference numeral  1  designates the power transistor and the reference numeral  2  designates a so-called sense resistor, which is suitable to sense the output current Io and is arranged in series to the power transistor  1 , a current mirror Q 1 , Q 2  with corresponding current sources Io and Iref which are series-connected, and finally a circuit branch which is connected between the ground and the bases of the transistors Q 1  and Q 2  and is constituted by a resistor  3  which is series-connected to a Zener diode  4 . 
     At a point that is intermediate between the transistor Q 2  and the current source Iref, a pin is provided for acquiring the signal for indicating intervention or lack thereof on the power transistor  1  in order to limit the maximum voltage across it. 
     The resistor  3  senses the voltage drop on the power transistor  1 . When he voltage across the transistor exceeds the value        Vds_Max   =       R                   I   ref          A2   A1       +     V   Z                              
     the protection circuit disconnects the power transistor  1  and no current can be supplied anymore: this limits the maximum supply voltage. 
     In the above relation, the terms A 2  and A 1  are, respectively, the areas of the transistors Q 2  and Q 1 , while Vds_Max is the maximum voltage between the drain and the source of the power transistor  1  and V z  is the voltage across the Zener diode  4 . 
     SUMMARY OF THE INVENTION 
     The aim of embodiments of the present invention is therefore to provide a short-circuit protection circuit, particularly for power transistors, in which means are provided for determining the maximum and minimum values of the output current of the power transistor as a function of the voltage applied to its drain and source terminals. 
     Within the scope of this aim, an advantage of the present invention is that it provides a short-circuit protection circuit, particularly for power transistors, which occupies a reduced area. 
     Another advantage of the present invention is that it provides a short-circuit protection circuit, particularly for power transistors, which is highly reliable, relatively easy to manufacture and at competitive costs. 
     This aim, these advantages and others which will become apparent hereinafter are achieved by a short-circuit protection circuit, particularly for power transistors, having first means for mirroring the output current of a power transistor which are parallel-connected to said power transistor, and second mirroring means which are series-connected to said first mirroring means and are suitable to emit a current which is correlated to the current mirrored by said first mirroring means, for comparison with a reference current. The result of said comparison determines the need to intervene or not on said power transistor. The short-circuit protection circuit also includes means for sensing the voltage drop across said power transistor which are parallel-connected to said power transistor and to said first mirroring means, in order to adjust minimum and maximum values of the current in output from said power transistor, as a function of the voltage that is present across said transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the circuit according to the invention, illustrated only by way of non-limitative example in the accompanying drawings. 
     FIG. 1 is a circuit diagram of a conventional protection circuit. 
     FIG. 2 is a conceptual circuit diagram of the short-circuit protection circuit according to the present invention. 
     FIG. 3 is a chart which plots the protection curve that can be obtained with the circuit according to the present invention. 
     FIG. 4 is a circuit diagram of a practical implementation of the circuit shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the above figures, the protection circuit according to the present invention, generally designated by the reference numeral  10 , comprises first means  11  for mirroring the output current of a power transistor  12 . 
     The mirroring means are conveniently constituted by a MOS transistor which is parallel-connected to the power transistor  12  so that its gate terminals are common-connected, although other acceptable mirroring circuits could be used. 
     Second mirroring means, conveniently constituted by bipolar transistors  13  and  14  with common-connected base terminals, are series-connected to the first mirroring means  11 . In particular, the collector terminal of the bipolar transistor  13  is connected to the drain terminal of the transistor  11 , while its emitter terminal is connected to the drain terminal of the power transistor  12 . 
     In the bipolar transistor  14 , instead, the emitter terminal is connected to the drain terminal of the power transistor  12  and the collector terminal is connected to a reference current source  15 , which generates a reference current Iref. 
     The bipolar transistor  13  is diode-connected. 
     FIG. 2 illustrates the case of a protection circuit applied to one of the two power transistors usually used in a final power stage. In particular, the case of FIG. 2 is the one in which the protection circuit is applied to the power transistor whose source terminal is connected to the ground and whose drain terminal constitutes the output of the final power stage. 
     The above-described circuit can likewise be used also in the case of a power transistor whose drain terminal is connected to the supply voltage and whose source terminal constitutes the output of the final power stage. 
     The circuit according to an embodiment of the invention further comprises means for sensing the voltage drop across the power transistor  12 ; said means are parallel-connected to the power transistor  12  and to the first mirroring means 
     The means for sensing the voltage drop across the power transistor  12  comprises a second MOS transistor  16 , whose gate terminal is common-connected to the gate terminal of the power transistor  12  and whose source terminal is connected to the source terminal of the power transistor. A resistor  17  is series-connected to the drain terminal of the second MOS transistor  16 , and a Zener diode  18  is in turn series-connected to said resistor  17 ; the anode terminal of said Zener diode  18  is connected to the collector terminal of the bipolar transistor  13 . 
     The area of the first MOS transistor  11  is much smaller than the area of the second MOS transistor  16 . 
     A signal, designated by the reference numeral  20 , is acquired in an intermediate point between the collector terminal of the bipolar transistor  14  and the current source  15 ; said signal provides an indication as to the need to intervene or not on the power transistor  12  to reduce its supplied current. 
     With reference now to FIG. 2, the operation of the circuit according to the invention is as follows. 
     The output current of the power transistor  12  is sensed by mirroring the power transistor  12  by means of the MOS transistor  11  and then by means of the bipolar transistors  13  and  14  on the output, i.e., in the comparison mode wherefrom the signal  20  is output, in order to compare said current with the reference current set by the reference current source  15 . 
     The effect of the resistor  17  is to decrease the current supplied by the power transistor  12  until the voltage across the power transistor  12  is high enough to cause the current flowing through the resistor R to be equal to the current mirrored by the MOS transistor  16 . 
     The transistor  16  behaves like a switch when low voltages are applied across the power transistor  12  and instead acts as a current source when high voltages are applied across the same transistor. 
     In the second case, the resistor  17  has no effect and the maximum current is set by the area ratios of the transistors and by the reference current Iref. 
     Explicitly with reference now to the circuit of FIG. 2, when a low voltage (Vds) is applied between the drain terminal and the source terminal of the power transistor  12 , no current flows through the MOS transistor  16 . The protection current can be calculated by making the current supplied by the bipolar transistor  14  equal to the reference current Iref. Therefore:                         I   out            A   11       A   12         =     I   ref                              
     where A 11  is the area of the MOS transistor  11  and likewise A 12  is the area of the power transistor  12 , while I out  is the current in output from the power transistor  12 . 
     Solving the above equation as a function of I out  provides the expression of the protection current at low voltage:                       I   outLV     =       I   ref            A   12       A   11                                  
     when the voltage between the drain and the source of the power transistor  12  is higher than the voltage across the Zener diode  18 , a certain current flows through the resistor  17 , lowering the protection current, in a manner which is linear with respect to the increase in voltage across the terminals of the power transistor. Ignoring the voltage drop between the base and the emitter of the bipolar transistor  13 , it is possible to obtain:              I   out            A   11       A   12         +         V   DS     -     V   Z       R       =     I   ref                            
     wherefrom, by solving as a function of the output current I out , the expression of the protection current is obtained as a function of the voltage drop across the power transistor  12 . 
     Accordingly, the following relation applies:          I   out     =       (       I   ref     -         V   DS     -     V   Z       R       )            A   12       A   11                                
     where I out  is the current of the safe operating area of the power transistor  12 . 
     When the voltage across the power transistor  12  increases, the second MOS transistor  16  becomes a current source and no further dependence of the protection current from the voltage across the drain and source terminals of the power transistor  12  is observed. This occurs when the current flowing through the MOS transistor  16  equals the current flowing through the resistor  17 :          V   DS     =       R                   I   ref            A   16         A   16     +     A   11           +     V   Z                              
     In this condition, the protection current can be calculated by rendering the sum of the current mirrored by the transistors  16  and  11  equal to the reference current Iref:            I   out              A   16     +     A   11         A   12         =     I   ref                            
     Solving as a function of the output current I out  produces the expression of he high-voltage protection current applied to the drain and source terminals of the power transistor  12 :          I   outHV     =       I   ref            A   12         A   16     +     A   11                                  
     FIG. 3 plots the resulting protection curve, wherein the axis of the ordinates represents the output current I out  of the power transistor and the axis of the abscissae represents the voltage across the drain and source terminals of the power transistor  12 . 
     It is evident that for voltage values Vds below the voltage across the Zener diode  18 , Vz, the protection curve is flat and parallel to the axis of the abscissae, while for voltages applied to the power transistor  12  which are higher than the Zener voltage Vz the curve descends in a linear manner as the voltage Vds increases, until it settles again so that it is parallel to the axis of the abscissae for voltage values exceeding a certain value thereof, equal to:            A   16         A   16     +     A   11         =     R                   I   ref                              
     FIG. 4 shows a particular implementation of the circuit according to another embodiment of the invention. The circuit, similar to the one shown in FIG. 2, differs in that the output of the circuit, taken between the collector terminal of the bipolar transistor  14  and the source  15 , is connected to the base terminal of a third bipolar transistor  21 , whose emitter terminal is common-connected to the gate terminal of the power transistor  12  and whose collector terminal is common-connected to the source terminal of the power transistor  12 . 
     The transistor  21  of the PNP type limits the gate voltage of the power transistor  12 , thus limiting the output current of said transistor. 
     A fourth PNP transistor  22  is connected so that its base terminal is common-connected to the gate terminal of the power transistor  12  and its collector terminal is common-connected to the source terminal of the power transistor  12 . 
     The emitter terminal of the transistor  22  is biased by a biasing current source  23 , which is connected to the supply voltage V dd . 
     The transistor  22  is necessary because in the circuit layout of FIG. 4 the second mirroring means  15  are connected to the ground, while the branch with the transistor  16 , the resistor  17  and the Zener diode  18  is connected to the output of the final power stage. 
     In practice it has been observed that the circuit according to embodiments of the present invention fully achieves the intended aim. 
     The circuit thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may also be replaced with other technically equivalent elements. 
     In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to requirements and to the state of the art.