Abstract:
A thermal protection device is for an integrated power MOSFET transistor including an interdigitated array of source regions and drain regions defined in a well region of the monocrystalline silicon substrate, and gate structures overhanging channel regions defined between adjacent source and drain regions. The thermal protection device may include a temperature sensor and a comparator for generating an over temperature flag signal usable for turning off the overheated power transistor. The thermal protection device may sense, in a very accurate manner, the temperature of the power MOS and may include a circuit for forcing a fixed current through a small number of source regions of the interdigitated array separately connected from the other source regions electrically connected in common of the power transistor; and a comparator, integrated on the substrate outside the well region, comparing the source voltage present on the small number of separately connected source regions with a threshold voltage for producing on an output the over temperature flag signal.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the protection of power transistors, and, more precisely, to a thermal protection device for an integrated power MOSFET that generates an over temperature flag signal for turning off the overheated power transistor.  
       BACKGROUND OF THE INVENTION  
       [0002]     During operation, power MOS transistors warm up and accidentally may reach temperatures high enough to cause their failure. For this reason, it is important to know the so-called Safe Operating Area (SOA) of MOS transistors, for ensuring that they function in safe operating conditions.  
         [0003]      FIG. 1  illustrates a SOA. A Safe Operating Area of a transistor corresponds to a set of the working points of the transistor bordered by certain curves. These curves are calculated for a certain drain-source voltage and a certain working temperature of the transistor. Typically, they indicate limit functioning conditions for safe operations of a power transistor at a certain working temperature when a square drain-source voltage pulse is applied.  
         [0004]     The typical approach to prevent failures of power transistors includes integrating together with the power MOS a protection device that monitors the current flowing in the transistor and the voltage across it (Vds). If the working point identified by these two values approaches a border of the SOA, the protection device intervenes to keep the working point inside the SOA.  
         [0005]     An important parameter to be considered for determining the SOA of a power MOSFET is its working temperature. It is a well known fact that the SOA of a transistor shrinks when the working temperature increases. Therefore, a certain driving voltage appropriate for driving a power transistor at a certain temperature, may damage it if the working temperature of the power transistor is higher.  
         [0006]     Indeed, a protection device capable of considering all variables that may influence the SOA of a transistor is practically impossible to implement. For this reason, certain protection devices overprotect the power MOS transistor, thus strongly limiting it functioning, while other protection devices though allowing a full exploitation of the capabilities of the transistor, may be unable to prevent failure by overheating under any condition.  
         [0007]     To prevent power transistors from heating up to a temperature potentially dangerous for its integrity, a temperature sensor may be realized near the power MOS or inside it, for sensing its working temperature. The protection device of the power MOS may thus limit power dissipation when the working temperature exceeds a pre-established threshold.  
         [0008]     Commonly, a suitable temperature sensor is realized in the form of a bipolar transistor, as disclosed in U.S. Pat. No. 5,396,119 assigned to the assignee of the present invention.  
         [0009]     A drawback of this approach may be that the sensor is generally integrated on the chip at a certain distance from the power MOS, and it may not sense exactly the real working temperature of the MOS transistor. Moreover, parasitic activations of this sensor, caused by below ground voltages of the drain of the power MOS (in case of an N-channel MOS) are likely to occur.  
       SUMMARY OF THE INVENTION  
       [0010]     It has been found a thermal protection device for an integrated power MOS transistor that overcomes the above mentioned drawbacks.  
         [0011]     Basically, the temperature in the well diffusion containing the interdigitated power MOS structure is sensed by forcing a certain current through a small number of the interdigitated source regions of the power MOS, purposely connected separately from the others.  
         [0012]     Of course, the voltage of these separately connected source regions through which a certain current is forced will depend on their temperature, thus an over temperature flag may be generated by comparing the voltage of these separately connected source regions with a threshold voltage.  
         [0013]     The thermal protection device of this invention benefits from the outstandingly precise manner in which the temperature of the power MOS is monitored because the temperature sensor is essentially a portion of the integrated power MOS itself.  
         [0014]     More precisely, this invention provides a thermal protection device for an integrated power MOSFET transistor including interdigitated array of source regions and drain regions defined in a well region of the monocrystalline silicon substrate, and gate structures overhanging channel regions defined between adjacent source and drain regions, having a temperature sensor and a comparator for generating an over temperature flag signal usable for turning off the overheated power transistor.  
         [0015]     The thermal protection device senses in a very accurate manner the temperature of the power MOS because it includes: means or a circuit for forcing a fixed current through a small number of source regions of the interdigitated array separately connected from the other source region selectrically connected in common of the power transistor; and a comparator, integrated on the substrate outside the well region, comparing the source voltage present on the small number of separately connected source regions with a threshold voltage for producing on an output the over temperature flag signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The features and advantages of this invention will become even more evident through the following detailed description of an embodiment and by referring to the attached drawings, wherein:  
         [0017]      FIG. 1  shows as already discussed a typical Safe Operating Area of a power MOS as in the prior art;  
         [0018]      FIG. 2  shows a first embodiment of the thermal protection device of this invention;  
         [0019]      FIG. 3  shows an alternative embodiment of the thermal protection device of this invention;  
         [0020]      FIG. 4  shows a sample layout positioning of the components of the device of  FIG. 3 ;  
         [0021]      FIG. 5  shows a preferred circuit embodiment of the comparator of the device of  FIG. 3 ;  
         [0022]      FIG. 6  shows the circuit of a generator of the current If(t, Rv) for the comparator of  FIG. 5 ;  
         [0023]      FIG. 7  is a diagram showing the loci of pair of temperatures of the well region (Thot) and of the substrate outside the well region (Tcold) for which the comparator of  FIG. 5  generates an over temperature flag; and  
         [0024]      FIG. 8  shows a third embodiment of the thermal protection device of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     In the ensuing description, the invention will be described for the case of a N-channel power MOS, but any skilled person would immediately recognize that the protection device that will be illustrated may be easily adapted to the case of a P-channel MOS by reversing polarities and type of conductivity of transistors.  
         [0026]     A first embodiment of the thermal protection device of this invention is depicted in  FIG. 2 , together with the power transistor Dw to be protected.  
         [0027]     The protection device includes a transistor Dw/n for sensing the temperature that is provided by a small portion of the integrated structure of the power transistor, and a comparator of the gate-source voltage on the temperature sensing transistor Dw/n with a certain threshold, corresponding to a maximum allowed temperature.  
         [0028]     Commonly, an integrated power transistor is includes an interdigitated array of source regions and drain regions, all defined within a diffused well region of the semiconductor chip, and of gate structures overhanging the channel regions defined between adjacent source and drain fingers.  
         [0029]     An aspect of the device is that the temperature sensor, besides being intimately located in the well region of the power transistor structure, has the same identical characteristics of the power transistor structure because it is provided by a small number n of separately connected source regions of the same power transistor to be controlled. Because of the separate connection of the n source regions, a pre-established source current Is 1  may be forced through the temperature sensor Dw/n.  
         [0030]     Being known the characteristic curve of the relationship between the gate-source voltage and the functioning temperature, it is possible to determine the threshold value of the gate-source voltage, VgsRef, that corresponds to the maximum allowed temperature for the integrated structure of the power MOS. The comparator Comp compares the gate-source voltage of the temperature sensor with the threshold VgsRef for generating an over temperature flag signal when it is exceeded.  
         [0031]     The temperature sensor is located in the same well of the power transistor, therefore the distance between the sensor and the power MOSFET is minimized (few microns). Moreover, the sensor usually has the same structure as the host power transistor. Therefore, the protection device has an enhanced precision compared to similar devices of the prior art and it is substantially not influenced by below ground voltages or parasitic activations.  
         [0032]     The thermal protection device of  FIG. 2  may still be affected by variability of parameters of the sensor with temperature, such as the threshold voltage of the transistor Dw/n, carrier mobility and the like, that may limit its precision. To obviate these residual causes of imprecision because of possible drifts of the gate-source voltage of the transistor Dw/n due to other causes, the thermal device may be optionally provided with a second temperature sensor Dw/m, as shown in  FIG. 3 .  
         [0033]     This optional second sensor is a MOS transistor having the same structure of the integrated power transistor, in practice including a second number m of source finger regions like those of the integrated power transistor, but purposely integrated outside the well region containing the power transistor structure and the first temperature sensing transistor Dw/n, and in a location as close as possible to the comparator, where a lower temperature than that in the well region normally exists.  
         [0034]     Moreover, the protection device of this alternative embodiment depicted in  FIG. 3 , includes means or a circuit for generating a voltage Vcomp that in general depends on the temperatures T“hot” and T“cold” as sensed by the first sensor Dw/n and second sensor Dw/m, respectively. Preferably, the voltage Vcomp is generated so as to decrease when the temperature T“cold” increases.  
         [0035]     The gate of the MOSFET Dw/m, defining the second integrated temperature sensor, is in common with the gate of the power transistor, while its drain may be connected either to the drain of the power MOSFET or even to a voltage Vd(cold), close to but not identical to the drain voltage of the power MOSFET.  
         [0036]     A current Is 1 *m/n, a scaled replica of the current Is 1  by the ratio between the integer numbers m and n, is forced through the sensor Dw/m so that when the two sensors Dw/n and Dw/m are at the same temperature, their gate-source voltages are perfectly equal to each other.  
         [0037]     The protection device of the embodiment of  FIG. 3  may be insensitive to any cause (different from temperature) that could modify the threshold voltages of the sensors Dw/n and Dw/m or the mobility of carriers, because in such an event it would affect both sensors in the same way. Moreover, the protection device is not affected by temperature variations of the second sensor Dw/m because the voltage drop Vcomp compensates eventual variations of its source voltage Vc.  
         [0038]     In the circuit of  FIG. 3 , when the output voltage is close to the source voltage of the power MOSFET, the sensors tend to enter the ohmic region of their functioning characteristic. Tests carried out by the applicants have shown that when functioning in the ohmic region whereat the laws that tie the Vgs to the temperature are no longer those on which the protection is designed and this phenomenon may jeopardize reliability.  
         [0039]     Even when the drain-source voltage of the MOSFET is relatively low, the power dissipated in the MOSFET may still increase dangerously the temperature thereof. A further embodiment shown in  FIG. 8  overcomes the above mentioned possible effect. According to this embodiment, the gate electrodes of the temperature sensors and of the power MOSFET are not shorted to each other, as in the embodiment of  FIG. 3 , but biased to a certain minimum voltage V GateSens .  
         [0040]     Being Vgs Max  the maximum gate-source voltage of the sensors when the current Is 1  flows through them, and being V Is1min  the minimum voltage drop on the current generators Is 1 , the minimum bias voltage V GateSens  applied to the gates of the temperature sensors will be: 
 
 V   GateSens   =Vgs   Max   +V   Is1min  
 
         [0041]     By applying such a minimum gate-source voltage, the sensor is prevented from entering into its ohmic region even if the drain-source voltage of the MOSFET becomes relatively low. Of course, the embodiment of  FIG. 8  requires two distinct gate electrode corrections instead of a single one.  
         [0042]      FIG. 4  is a sample illustration of the layout positioning of the sensors of the device of  FIG. 3 .  FIG. 5  shows a possible circuital embodiment of the comparator of  FIG. 3 .  
         [0043]     Supposing the comparator of  FIG. 5  to be fully balanced (M 1 =M 2 , M 3 =M 4  etc.), the threshold voltage of the comparator is the voltage drop on the resistor R, when the current flowing in the two transistors of the differential pair M 1  and M 2  equal each other.  
         [0044]     When the currents If(Rf)i and If(Rf)o equal each other 
 
If( Rf ) i= If( Rf ) o =If( Rf ) 
 
 the voltage drop Vcomp is given by the following equation: 
 
 Vcomp=R *If( t, Rv )/2 −R *If( Rf ) 
 
         [0045]     Therefore, the threshold voltage Vcomp depends linearly on the currents If(t, Rv) and If(Rf).  
         [0046]     By adjusting these currents, it is possible to establish the temperatures T“hot” and T“cold” on the basis of which the comparator eventually generates the over temperature signal.  
         [0047]     To have a voltage drop Vcomp that decreases when the temperature T“cold” increases, it is desired that the current If(t, Rv) decreases when T“cold” increases. A suitable circuit for generating the currents If(t, Rv) and If(Rf)i and If(Rf)o is shown in  FIG. 6 .  
         [0048]     The operational amplifier imposes a certain constant voltage Vref on the source node of the MOSFET Mfb, thus the current flowing through it is inversely proportional to the resistance Rfb. Both MOSFETs Mfa and Mfb are kept in a conduction state by the operational amplifier, thus a current inversely proportional to the source degeneration resistance Rfa circulates through the transistor Mfa.  
         [0049]     In practice, the MOSFETs Mfa and Mfb are the two current generators that generate the currents If(Rf)o and If(Rf)i, respectively, for the comparator of  FIG. 5 , as a function of the resistances Rfa and Rfb.  
         [0050]     The transistor B 1  is turned on by forcing through it a bias current delivered by the current generator I. Therefore, the resistor Rv takes the base-emitter voltage of the transistor B 1 . Given that B 1  is in a conduction state, the MOSFET M 1  is activated and thus the current If(t, Rv) is the current flowing through the resistor Rv, that is 
 
If( t, Rv )= Vbe   B1   /Rv  
 
 wherein Vbe B1  is the base-emitter voltage of the transistor B 1 . 
 
         [0051]     It is worth noticing that the base-emitter voltage Vbe B1  of the transistor B 1  decreases with the working temperature of B 1 . Therefore, the current If(t, Rv) varies with temperature and may be adjusted by varying the resistance Rv.  
         [0052]     By properly dimensioning the transistors Msa-Msb of the current mirror that generates If(Rf)i, the transistors Mfa and Mfb and the source degeneration resistors Rfa and Rfb, it is possible to make 
 
If( Rf ) i= If( Rf ) o =If( Rf ) 
 
         [0053]     A possible dimensioning for obtaining this condition is the following: 
 
 Msa=Msb; Mfb=Mfa; Rfa=Rfb.  
 
         [0054]     Therefore, with the circuit of  FIG. 6  it is possible to adjust the currents If(Rf) and If(t, Rv) by varying the resistances Rv, Rfa and Rfb, and thus to adjust the threshold voltage Vcomp of the comparator. This feature allows choosing the pair of temperatures T“cold” and T“hot” for which the comparator generates the over temperature flag.  
         [0055]      FIG. 7  shows a sample diagram of possible loci, that are substantially straight lines, of pairs T“hot”, T“cold” values for which the over temperature flag is generated. Hereinbelow, these straight lines will be referred as the “lines of intervention” of the comparator.  
         [0056]     In the example shown, the vertical lines indicate that the flag is generated when the temperature T“hot” reaches 220° C. or 180° C. whichever the temperature T“cold” is, while the inclined lines indicate that the temperature T“hot”, at which the over temperature flag is generated, depends on the temperature T“cold”.  
         [0057]     As suggested by the arrows, by adjusting the ratio R/Rf it is possible to translate horizontally the line of intervention, while by adjusting the ratio R/Rv it is possible to modify the slope of the line of intervention of the comparator.  
         [0058]     By summarizing, the protection device according to the first embodiment of  FIG. 2  senses only the temperature of the power DMOS T“hot”, while the device according to the second embodiment of  FIG. 3  generates the over temperature flag by considering also the temperature T“cold” of the substrate outside the well region of the power MOS. Of course, in both cases, the maximum allowable temperature T“hot” may be fixed as desired.  
         [0059]     With the device of  FIG. 3  and the circuit of  FIG. 7 , it is also possible to vary the maximum allowable temperature T“hot”, as a function of the temperature T“cold”, for obtaining a certain line of intervention of the comparator. Moreover, the functioning of the device made according to the embodiment depicted in  FIG. 3  is substantially insensitive to eventual variations of parameters of the device, such as carrier mobility, threshold voltage of the temperature sensing transistor Dw/n that could be induced by causes other than temperature.