Abstract:
An isolated integrated differential current comparator for comparatively measuring the current passing through one or two resistors using a thermal difference sensor employing the Seebeck effect in an integrated circuit that is coupled to the resistors. The thermal difference sensor detects the temperature difference between the resistors, which is proportional to the square of the current passing through them. The output of the current comparator is electrically isolated from the inputs. The output is scalable and in circuit topologies requiring full signal isolation. The integrated differential current comparator is applicable to hot swap applications and applications where isolation of a number of signals is needed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     N/A 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to electrical isolation between circuit functions in an integrated silicon device. More specifically, the invention relates to the transmission of DC and AC information across a dielectric barrier using a temperature transducer. 
     Thermocouples are commonly used as temperature sensors. A thermocouple is composed of two touching dissimilar metals forming a sensing junction. When the sensing junction is held at a temperature different from the open ends of the metal, an open-circuit voltage, which is a function of the temperature difference, is created. This thermo-electric voltage is known as the Seebeck voltage. By measuring a thermocouple&#39;s voltage, the temperature can be calculated. When two sensing junctions are connected in series, a differential thermocouple is formed and the open-circuit voltage, V s  is proportional to the temperature difference between the two junctions, V s =kV(ΔT), where kV is a millivolts per degree change constant for the particular set of metals used. Thermocouples are traditionally formed as cable assemblies that are strung to temperature generating points remote from the measuring station. 
     Many applications require electrical isolation between circuit functions. While many methods of performing this electrical isolation exist in discrete circuitry, there are relatively few methods to obtain isolation in an integrated silicon device. For example, differential capacitive and inductive devices encased within the inter-metal dielectric of the die or package have been utilized. These approaches require significant circuitry to process the coupled information. In addition, because these components require time varying signals, direct coupling of DC signals across the isolation barrier is not possible without additional circuit complexity. 
     A means to isolate circuit functions in an integrated circuit that responds to both AC and DC signals without complex circuitry is desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     A differential isolated current comparator achieves isolation using integrated thermo-electric devices and generated thermal gradients. This comparator forms a fundamental cell, which can be scaled for a multitude of new applications. This cell allows for isolation of a measured current, measured voltage or an output voltage from the remainder of the circuitry. 
     An integrated differential current comparator is formed on a silicon die for providing input to output electrical isolation. The comparator includes a first resistor disposed between a first contact point and a second contact point, the first resistor proximate to the silicon, die. The first resistor generates a first temperature when a first current passes between the first contact point and the second contact point. A second resistor is disposed between a third contact point and a fourth contact point, the second resistor spaced apart from the first resistor and proximate to the silicon die. The second resistor generates a second temperature when a second current passes between the third contact point and the fourth contact point. A thermal difference sensor is disposed on the silicon die. The thermal difference sensor has a first temperature junction thermally coupled to the first resistor and a second temperature junction thermally coupled to the second resistor. The thermal difference sensor provides an output signal that is a function of the temperature difference between the first resistor and the second resistor. Dielectric barriers are interposed between the first temperature junction and the first resistor, and the second temperature junction and the second resistor respectively. 
     Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be understood from the following detailed description in conjunction with the drawings, of which: 
     FIG. 1 is a cross section of a thermo-electric sensor formed on a silicon base according to the invention; 
     FIG. 2 is a symbolic top view of a thermo-electric device formed on a silicon base according to the invention; 
     FIG. 3 illustrates the relationship between time and temperature within a silicon die; 
     FIG. 4 is a cross section of an embodiment of an isolated differential thermo-electric device according to the invention; 
     FIG. 5 is a symbolic top view of a layout of multiple thermo-electric devices, such as those of FIG. 4, arrayed to reduce thermal interference; 
     FIG. 6 illustrates a symbol for the isolated differential thermo-electric device of FIG. 5; 
     FIG. 7 is a circuit diagram of an input signal isolator using the of the thermo-electric device of FIG. 6; 
     FIG. 8 is a circuit diagram of an isolated voltage driver using the thermo-electric device of FIG. 6; 
     FIG. 9 is a circuit diagram of a current comparator using the thermo-electric device of FIG. 6; 
     FIG. 10 is a block diagram of an integrated circuit to provide a power switch control capability implemented using an embodiment of the invention; 
     FIG. 11 illustrates a high voltage application of the circuit of FIG. 10; and 
     FIG. 12 is a block diagram of an integrated circuit to provide multiple isolators using an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A Seebeck effect device, a temperature transducer  18 , may be implemented in an integrated semiconductor device utilizing the structure illustrated in cross section in FIG.  1 . In the figures herein, a silicon die is illustrated and discussed although the techniques taught are applicable to semiconductor devices in general. In FIG. 1, a dielectric layer  26  separates a base silicon layer  28  from a pair of Seebeck junctions  20  and  30 . First junction  20  is formed of a first conductor  22  contacting a second, different material, conductor  24  at first junction  20 . Second junction  30  is formed of a third conductor  32  contacting the second conductor  24  at a second junction  30  spaced apart from the first junction  20 . If the junctions  20  and  30  are held at different temperatures, a voltage E out  is developed across the ends  34 ,  36  of the first and third conductors  22 ,  32 . The pair of junctions  20 ,  30  are referred to as a Seebeck pair. The temperature transducer  18  generates an output voltage without the requirement for biasing. The voltage output of this transducer  18  is directly proportional to the temperature difference between the junctions  20 ,  30  and does not have any DC offset. 
     The materials chosen for the distinct conductors determines the proportionality variable. In an embodiment where the first and third conductors  22 ,  32  are implemented with aluminum and the second conductor  24  is implemented with polysilicon, the thermal-EMF (electro-motive force) of the polysilicon-aluminum transducer  18  is about 0.7 mV/C. The embodiment utilizing aluminum and polysilicon is advantageous because the sensor can be integrated above field oxide, without any silicon diode junctions. This is particularly important as silicon diode junctions represent either an isolation voltage standoff limitation or a conductive path when the junctions become forward biased. 
     The gain of the temperature transducer  18  of FIG. 1 can be increased by connecting a number of Seebeck pairs, experiencing the same temperatures, in series. FIG. 2 illustrates a symbolic top view of a thermopile—a layout of a series of connections that increases the gain by a factor N, where N is the number of junction pairs in series that experience the same temperature difference. In FIG. 2, vertical lines  42  represent first conductors and angled lines  40  represent second conductors. Structure  44  (B) maintains a temperature T 44  that is “hot” relative to the temperature T 46  maintained by structure  46  (A). There are (N=21) pairs of junctions in series in FIG. 2 so, for an aluminum/polysilicon embodiment, the output voltage is [(21*0.7 mv)/C]*(T 44 −T 46 ). 
     FIG. 3 illustrates the thermal profile of a silicon die  50  as power is applied and in particular as one resistor  52  is heated by carrying current while another resistor  54  does not. Trace  56  illustrates the temperature profile taken 10 μsec after power turn-on showing that area B near resistor  52  rises to a high temperature (˜40° C.) almost immediately. The temperature in area A near resistor  54  exhibits some rise in temperature (˜20° C.) due to its proximity to region B, while the temperature at the regions near the edges of the die remain at approximately 0° C. At time=100 μsec, as shown by trace  58 , the whole die has heated up to 20° C., while regions A and B have maintained the same relative differential to each other and to the die as a whole. Within one second, as shown on trace  59 , the die has stabilized at a temperature of 120° C. with region B reaching 160° C. and region B 140° C. The heat-generating resistor generates a temperature differential that is identifiable regardless of the die temperature. 
     A structure that utilizes the temperature gradient of a resistor carrying current and a Seebeck pair to form a current comparator  60  is shown in cross section in FIG.  4 . In FIG. 4, the basic structure of FIG. 1 is augmented by end resistors positioned near the Seebeck junctions A and B, where the current through the resistors is understood to flow orthogonal to the page. The resistors are positioned in close proximity to the Seebeck junctions A and B, but isolated from the junctions by a dielectric material. The resistors in one embodiment are implemented as semiconductor structures such as polysilicon resistors  52 ′ and  54 ′ embedded in the dielectric layer  28  near the junctions A and B. Alternately, the end resistors are implemented as metal resistors  62  and  64  in an inter-metal dielectric layer above the highest layer of the temperature transducer  18 . A temperature gradient between junctions A and B is created by energizing one resistor in close proximity to one junction. Alternately, a temperature gradient between junctions A and B is established by energizing both resistors, but with different currents. Polysilicon and Metal  2  resistors are shown in the embodiment illustrated as these materials keep the resistors above the field oxide layer, although similar materials may be substituted to suit alternate semiconductor processes as is known in the industry. Current in the left resistor  52 ′ or  62  of FIG. 4 creates a positive output voltage across the terminals  34 ,  36  of the comparator  60 . Current in the right side resistor  54 ′ or  64  reverses the output polarity. The net effect is a current comparator or current controlled voltage source. This current comparator is a power measurement device as small differences of low level signals do not cause sufficient temperature difference to generate a significant voltage. When the resistors and associated sensing junctions are spaced further apart, the reduced thermal leakage path allows larger temperature differences to be maintained between the measurement points. 
     The structure of FIG. 4 forms a fundamental comparator cell  60  whose gain is increased by connecting multiple cells in series as previously discussed. FIG. 5 illustrates a symbolic top view of a layout that connects multiple cells while reducing or substantially eliminating the sensitivity of a resultant comparator  75  to external temperature gradients. The orientation of the resistors  52   a / 52   b  and  54   a / 54   b  creates a set of cold and hot junctions ABBA (FIG. 3) that cancel out a signal generated by die temperature gradients while measuring the temperature difference between the resistors. Input current I 1  passes from input  72  to output  80  heating resistor  54   a / 54   b  whose resistance is determined by the resisitivity per unit length R times the length of N 1 . Similarly, input current I 2  passes from input  82  to output  88  heating resistor  52   a / 52   b  whose resistance is determined by the resisitivity per unit length R times the length of N 2 . Serially connected comparator cells  60  develop a voltage V det  across the terminals  84 ,  86  expressed as: 
     
       
           V   DET   =kN   3   R ( N   1   I   1   2   −N   2   I   2   2 ) 
       
     
     where k is the thermal-EMF of the materials selected for the conductors and N 3  is the number of comparator cells  60  in the comparator  75 . 
     The comparator  75  can be represented schematically as shown in FIG.  6 . The output voltage across pins  84  and  86  V DET =kN 3 R(N 1 I 1   2 −N 2 I 2   2 ) responds to power dissipation, therefore the output is a function of the square of the input currents I 1  and I 2 . The polarity of the currents is irrelevant. The power of each input  72 ,  82  is scaled directly with the sizing of the end resistors RN 1 , RN 2  and the overall gain is controlled by N 3 , the number of thermopiles. Response characteristics of this device are generally in the μsecond range. Local thermal gradients on the die establish themselves in this timeframe, as shown in FIG.  3 . The response time can be improved by pulsing the power into the resistors RN 1  and RN 2  at a high level for a short duration, in the order of microseconds, and then returning the power to a nominal steady state level. The integrated differential isolated current comparator  75  transmits DC or AC information across a dielectric barrier. It does not require biasing to generate its output and as such can be used to process isolated input information or to provide an isolated output drive signal. 
     The comparator  75  may be used as a control signal isolator as shown in FIG.  7 . The two connections to resistor RN 1  are brought out of the integrated circuit at pins g and i in this application. When a control signal  90  causes current to flow in resistor RN 1 , the temperature T 1  generated thereby is compared against a reference temperature T 2  set by a logic threshold bias voltage  98  through resistor RN 2 . When T 1  exceeds T 2  by sufficient margin, isolated control signal  94  is brought high. Note that all of comparator  75  except the control inputs g,i are referenced to a integrated circuit ground  100 . The isolation provided by the dielectric layer is sufficient to allow voltages as large as 1500V between the control inputs and integrated circuit ground. To prevent oxide breakdown of the isolated circuits during electrostatic discharge (ESD) or other high voltage events, robust spark gaps  96  are utilized between the isolated pins and the circuit/substrate ground  100 . The spark gaps  96  are in one embodiment made of copper. 
     The comparator  75  may also be used as an isolated bipolar voltage generator as shown in FIG.  8 . Depending on the state of the inputs  112 ,  114 , that are typically driven from a controller, comparator  75  will generate either a positive or a negative voltage between the outputs  116 ,  117 . The outputs  116 ,  117  are protected by spark gaps  96 . 
     The comparator  75  may also be used for isolated high current sensing as illustrated in FIG.  9 . In FIG. 9, resistor RN 1  has a small value relative to resistor RN 2 . Therefore, only a high current on the connection  120  through RN 1  will be detected and cause the outputs  122  to change state. This function is useful in  10  detecting overcurrent and shutting down a system, such as in hot swapping or in other applications that control components of power systems. This application could be extended to perform isolated feedback for voltage or current linear control by adding a linear control loop. 
     The comparator  75  may be incorporated into an integrated circuit providing a simple and flexible design for totally isolated power switch control applications, such as hot swapping, as shown in FIG.  10 . Equipment that allows hot swapping is configured to allow devices to be connected while the equipment is powered on. This design provides complete electrical isolation among a control input  144  included in a bipolar gate drive interface region  154 , a current sense interface  152  and a switched bus  132 ,  134  in an isolated gate drive region  140 . Because of the fast response, the hot-swap can detect overcurrent situations to protect many circuits. An external N or P MOSFET  156  can be used by the proper connection of GATEPOS  132  and GATENEG  134  pins to control the application of voltage to the device. An internal undervoltage lockout (UVLO)  142  ensures that an adequate VDD supply is present before enables  144  are allowed. In the absence of VDD power  130 , a short is maintained across the external MOSFET&#39;s gate  157  and source  148 . The presence of VDD power  130  charges the MOSFET gate  157  to a reverse potential. A logic high on the enable pin  144  sets the enable latch  146  and starts charging th e MOSFET gate  157  to a positive potential. The magnitude of current flowing through the SENSE pins  136 ,  138  is monitored by the comparator  151 . The trip current is set by the value of resistor RSET  150  according to the formula: 
     
       
           I  trip=1000 *VDD/RSET   
       
     
     Once the over-current threshold is reached, the latch  146  is turned off and the gate  157  is quickly discharged to a reverse potential. Bus current sense  152  is completely isolated from controller  154  and gate drive  140 . As the current sense  152  responds to absolute value only, pin polarity is irrelevant. Sensing can be in the drain or source path of the switching power FET  156  or in some other remote section of the circuit. DC or AC currents can be sensed. Gate drive  158  is bipolar and completely isolated from all other pins of the device. Dual pins  132 ,  134  allow direct connection of the gate driver source to the external MOSFET  156 . This feature enables direct use of N or P MOSFETs, in low or high side configurations. This design is applicable to: hot swapping of positive or negative high voltage systems, applications requiring bus isolation from the control, very high noise environments, and similar power switching applications. 
     FIG. 11 illustrates a high-voltage use of the device illustrated in FIG.  10 . By setting the value of RSET  150 ′ to 4.9 KΩ in FIG. 11, the trip current through the isolated sense  152 ′ is set to 1 AMP in a high voltage hot swap controller. Similar configurations are applicable to hot swapping in a negative high voltage system or in an alternating current (AC) system. 
     The comparator  75  may be incorporated into an integrated circuit multi-channel signal isolation product, for example as a replacement for discrete opto-couplers. The invention is less expensive than opto-electronics because it uses conventional processes and the chip area can be a small portion of the integrated circuit. An implementation of a quad bi-directional isolated interface to obtain complete dielectric isolation between functional blocks is shown in FIG.  12 . The interface consists of four (4) single ended input to differential isolated, floating output circuits  170  and four (4) differential floating input to single ended open drain output circuits  172 . The single ended inputs  174  and outputs  176  are powered from the VDD supply. The differential I/O  172  has channel to channel and power ground isolation. In the embodiment illustrated, all differential floating inputs  180  have an input resistance of 2 K ohms while the floating differential outputs  182  have a +/−1V swing with an output impedance of 50 k ohms. A microsecond response time is achieved using the differential isolated transducer. The multichannel isolated interface product is applicable to isolation of hot swap control inputs and outputs, to controlling power over the Ethernet, and to system communications link isolation and to areas where a high level of noise rejection is required. 
     Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.