Abstract:
This invention includes a circuit that approximates the thermal behavior of a fuse or other electronic device that is coupled in series with the circuit. In one preferred embodiment, the circuit protects a fuse coupled in series with a rechargeable battery from clearing during soft short conditions. Thus, when the instantaneous current is temporarily above the current rating of the fuse, yet the root mean squared current is below the current rating of the fuse, the circuit works to estimate the heating of the fuse element and limit the current to a root mean squared value that is less than the current rating of the fuse. One embodiment includes a programmable comparator that actuates a counter which, in turn, increments to estimate heating of the element when the current exceeds a predetermined threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation in part of U.S. Non-provisional application Ser. No. 09/545,135, filed Apr. 7, 2000, which claims priority to U.S. Provisional Application Ser. No. 60/172,273, filed Dec. 17, 1999, the disclosures of which, including all attached documents and appendices, are incorporated by reference in their entirety for all purposes.  
         [0002]    This application is also a continuation in part of U.S. Non-provisional application entitled “Silicon Equivalent PTC Circuit”, John Wendell Oglesbee, inventor, filed Oct.  17 ,  2000 , which claims priority to U.S. Provisional Application Ser. No. 60/161,191, filed Oct. 22, 1999, the disclosures of which, including all attached documents and appendices, are incorporated by reference in their entirety for all purposes.  
         [0003]    This application claims priority from U.S. Provisional Application Ser. No. 60/172,272, filed Dec. 17, 1999, the disclosures of which, including all attached documents and appendices, are incorporated by reference in their entirety for all purposes.  
         [0004]    This application further claims priority from U.S. Provisional Application Ser. No. 60/202,150, filed May 5, 2000, the disclosures of which, including all attached documents and appendices, are incorporated by reference in their entirety for all purposes.  
         [0005]    This application further claims priority from U.S. Provisional Application Ser. No. 60/203,795, filed May 9, 2000, the disclosures of which, including all attached documents and appendices, are incorporated by reference in their entirety for all purposes.  
     
    
     
       TECHNICAL FIELD  
         [0006]    This invention relates generally to electronic circuits incorporating fuses coupled in series, and the preferred embodiment relates more specifically to protection circuits in battery charging circuits incorporating series fuses for circuit protection.  
         BACKGROUND  
         [0007]    Electronic circuits often employ fuse elements for protection. When too much current runs through a fuse, i.e. a current in excess of the rating, the soft fuse material vaporizes, thereby opening the circuit. For example, a hair dryer may include a fuse that protects the hair dryer from over-current damage. If one accidentally drops a hair dryer in a sink, the water may create a path for excessive current to flow. This could cause the dryer to stop working. To avoid damage to the dryer, a fuse in series with the power cord will clear, thereby disconnecting the hair dryer from the wall outlet.  
           [0008]    Rechargeable batteries often use fuses to protect the cell. When rechargeable batteries, like those made with lithium, if they are overcharged they can release gasses at high temperatures. When such a situation occurs, reliability of the battery may be compromised.  
           [0009]    Consequently, rechargeable battery pack manufacturers often place fuses (in addition to battery protection circuits) in the battery packs. If a temporary problem occurs, the battery protection circuit, which includes transistor switches, will open the circuit to prevent an over-current situation. However, in the event that a catastrophic condition of battery protection circuit failure occurs, the fuse will clear thereby providing another layer of protection.  
           [0010]    The problem with such fuses is that they are often non-replaceable. In other words, manufacturers often solder them to a circuit board that is inaccessible to the end user. Thus, when the fuse clears, the battery is protected, but it is also rendered useless as the battery cell is now disconnected from the terminals of the battery pack. Occasionally, a short current spike may cause the fuse to inadvertently blow. This is known as “nuisance tripping”. If a nuisance trip occurs, the user must throw the battery away and buy another one, even though the cell and circuitry may be imperfect condition. With the cost of some batteries for cellular phones exceeding one-hundred dollars, there is thus a need for an improved protection circuit to prevent nuisance tripping in the series fuse. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic block diagram in accordance with the invention.  
         [0012]    [0012]FIG. 2 is a comparison graph of current versus time in accordance with the invention.  
         [0013]    [0013]FIG. 3 is an analogous thermal estimator circuit in accordance with the invention.  
         [0014]    [0014]FIG. 4 is one preferred embodiment of a thermal estimator circuit in accordance with the invention.  
         [0015]    [0015]FIG. 5 is a linear current regulator in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
         [0017]    Referring now to FIG. 1, illustrated therein is a battery protection circuit as disclosed in U.S. Provisional Application Ser. No. 60/202,150, filed May 5, 2000 and U.S. Provisional Application Ser. No. 60/203,795, filed May 9, 2000. The protection circuit  100  includes a shunt transistor  101  and a series transistor  102 , as well as a control circuit  103 . The control circuit  100  includes a shunt regulator with thermal crowbar as described in copending U.S. patent application Ser. No. 09/545,135, filed Apr. 7, 2000. The shunt regulator  104  essentially monitors the voltage across the cell  105 . When this voltage increases above a predetermined threshold, the shunt regulator  104  causes the shunt transistor  101  to begin regulating voltage in a linear fashion by bypassing current around the cell  105 . This current flowing through the shunt transistor  101  causes the shunt transistor  101  to heat due to I 2 R losses. When the temperature in the shunt transistor  101  reaches a predetermined threshold, the shunt regulator  104  causes the shunt transistor to go into a full conduction mode, thereby minimizing the impedance across the shunt transistor  101 . This action is called a “thermal crowbar” effect.  
         [0018]    One skilled in the art is quick to notice that when the shunt transistor  101  moves into saturation, a low-impedance path is coupled across the cell  105 . In other words, the conducting shunt transistor  101  becomes an effective “short” across the cell  105 . To prevent the cell  105  from discharging itself, the shunt regulator opens the series transistor  102  when the crowbar action commences. Thus, the cell  105  becomes electrically decoupled from the shunt transistor  101 .  
         [0019]    In addition from the opening action of the series transistor  102 , the series transistor  102  performs other functions as well. In addition to the shunt regulator  104 , the control circuit also comprises a silicon-equivalent PTC circuit  106  as described in copending U.S. Non-provisional Application entitled “Silicon Equivalent PTC Circuit”, John Wendell Oglesbee, inventor, filed Oct. 17, 2000. The silicon-equivalent PTC circuit  106  is best explained by way of example. Under certain conditions, the voltage across the cell  105  may be below the predetermined threshold (and thus the shunt transistor  101  is not conducting), but the current flowing through the cell  105  may be in an abnormal range. Abnormally high currents are undesirable because they may potentially compromise the cell  105  or a host device. Consequently, when the current flowing in the series transistor  102  exceeds a predetermined current threshold, the silicon-equivalent PTC circuit  106  causes the series transistor to begin conducting in its ohmic, or linear, region. It does this by causing the impedance of the series transistor  102  to increase. The increase in impedance causes the device to heat due to I 2 R losses. When the temperature reaches a predetermined threshold, the silicon-equivalent PTC circuit  106  causes the series transistor  102  to open.  
         [0020]    The control circuit  100  also contains a current limit circuit  107 . As explained above, the silicon-equivalent PTC circuit  106  causes the series transistor  102  to open when the temperature reaches a predetermined threshold. This heating takes a time. If a short circuit were connected across the device, a very high current could pass through the cell  105 . If the current were sufficiently high, damage to the cell  105  could occur before the silicon-equivalent PTC circuit  106  has had time to heat enough to trip. In such an event, the current limit circuit  107  limits impulse currents to a predetermined level, thereby protecting the cell  105 .  
         [0021]    While the above mentioned circuits all provide protection for the cell, there is another problem that can slip “under the radar” if only the elements above are included. This condition is known as a “soft short”. One object of this invention is to provide a protection mechanism against soft shorts.  
         [0022]    A soft short is a condition whereby the current flowing through the cell  105  is sufficiently high to potentially cause nuisance tripping of the fuse, but is not high enough for the current limit circuit  107  to trip, nor is it high enough to cause the silicon-equivalent PTC circuit  106  to open the series transistor in a short enough time to ensure that the fuse  109  does not clear. Thus, a fuse protection circuit  108  is included in the control circuit  103 .  
         [0023]    Referring now to FIG. 2, (circuit numbers from FIG. 1 will be carried throughout the discussion of FIG. 2 for reference convenience) a graph is shown explaining the necessity for the fuse protection circuit  108 . Plot  200  illustrates the action that occurs when a silicon-equivalent PTC  106  circuit and current limit circuit  107  are included in the control circuit  100 . Assuming that the voltage across the cell  105  remains below the predetermined threshold, a soft short connected to the circuit may cause a significant current pulse as illustrated by line  202 . Normally, this line  202  would continue to a self-determined maximum (dependent upon the impedance of the battery circuit and the impedance of the short). Here, however, it is arrested and limited by the current limit circuit at point  203 . The current stays limited as per point  203  until the series transistor  102  heats to a predetermined threshold. When this occurs, the silicon-equivalent PTC circuit  106  actuates at point  204 . The actuation causes the impedance of the series transistor  102  to rise (and the corresponding current to fall, line  205 ) until the power generated in the series transistor  102  reaches equilibrium with the power dissipation at line  206 .  
         [0024]    It is well to briefly discuss fuses here. Fuses clear when a sufficient current flows through the fuse for a sufficient amount of time. For example, a 2 A fuse will pass 2 A indefinitely. A 2.1 A current, will cause a 2 A fuse to clear after a certain amount of time t1. A 10 A current will cause a 2 A fuse to clear after a time t2, where t2 is less than t1. Thus, the product of current and time (which is proportional to the I 2 R heating of the fuse element) will cause a fuse to clear.  
         [0025]    Referring again to FIG. 2, if a soft short is placed across the terminals of a battery, the battery may discharge at a current that is sufficient to cause the fuse to clear before the silicon-equivalent PTC circuit  107  opens. By way of example, assume that the current limit circuit  107  is set to limit current at 10 A. Additionally, assume that the fuse is rated at 2 A. If the user puts the battery in his pocket and a key chain, with a specific impedance, shorts the terminals of the battery pack, the impedance of the key chain may cause a current such as 5 A to flow through the series transistor  102 . Were this scenario to present itself, line  209  would represent the current. As the series transistor  102  heated, the silicon-equivalent PTC circuit  106  would eventually trip at point  208 . This time, however, may not be correlated to the thermal characteristics of the series fuse  109 . Consequently, the fuse may have only been able to conduct current until point  209 . Under these conditions, the battery pack would be rendered useless by a nuisance trip, even though the battery cell  105  is still in working condition. Consequently, one object of this invention is to produce a circuit that emulates the thermal characteristics of a fuse by limiting current at lint  210 , for a time as point  211 , such that the area under the curve  212 , which represents current times time, is insufficient to clear the fuse. The key chain problem of nuisance tripping due to soft shorts would thereby be eliminated.  
         [0026]    Referring now to FIG. 3, illustrated therein is an electrical circuit  300  that approximates the thermal response of a fuse. A first resistor  301  is analogous to the parasitic impedance that causes power generation in a fuse. This first resistor  301  limits the current of an input voltage in proportion to the heating of the fuse. Thus, the first resistor  301 , in combination with the capacitor  303 , define a circuit that allows the capacitor  303  to charge at a rate which is proportional to the heating of a fuse, defined by the impedance and thermal mass. A second resistor  302 , correspondingly discharges the capacitor  303  at a rate that is proportional to fuse&#39;s loss of heat to the atmosphere. The discharge rate is often slower than the charging rate. In any event, if the output of this circuit is coupled to a comparator  305  having a reference voltage  304 , the reference voltage  304  may be set to a level so as to correspond to a predetermined time when the fuse would normally clear. Thus, when the voltage across the capacitor  303  exceeds the reference voltage  304 , the fuse protection circuit can open the series transistor, thereby preventing the fuse from a nuisance trip.  
         [0027]    While the circuit shown in FIG. 3 is an effective fuse heating approximation circuit, it is difficult to realize in a silicon integrated circuit (IC). This is because resistive elements are difficult to trim on silicon. In addition, the capacitor may be of such great size that it occupies a tremendous amount of the silicon real estate at a prohibitive cost.  
         [0028]    Referring now to FIG. 4, illustrated therein is a preferred embodiment of the invention. The circuit  400  is a thermal fuse estimator capable of being inexpensively realized on silicon. The circuit  400  includes a current input that is proportional to the level of current flowing in a pass element. This level can be measured using a variety of techniques known in the art, including sensing the voltage across a series resistor. A threshold voltage  402  is also provided.  
         [0029]    A comparator  403  compares the threshold voltage  402  to the current level  401 . When the current level  401  exceeds the threshold voltage  402 , indicative of a potential nuisance trip current flow, the output of a comparator  404  goes active high and actuates the enable pin of a first clock  405 . The first clock  405  is coupled directly to an up/down counter  407 . When the counter  407  is counting up, this is analogous to the series fuse in the circuit heating.  
         [0030]    When the current level  401  is less than the threshold voltage  402 , the comparator output  404  is active low. This turns the first clock  405  off, but causes an active high signal to enable a second counter  413  via a series inverter  414 . When the second clock  413  is active, the clock output  415  is coupled into a frequency divider  412  that divides the clock output  415  by a predetermined factor, which is in this exemplary embodiment 2. This divided output  416  causes the counter to count down, which is analogous to the series fuse element cooling due to the loss of heat to the environment. It will be clear to those skilled in the art that the frequency divider  412  is optional, in that the clocks could be designed to oscillate at different frequencies. The frequency counter  412  does have benefits in that it can be programmed to any arbitrary factor, which need not be 2 as in this exemplary embodiment.  
         [0031]    The division of frequency occurs because a series fuse loses heat to the environment at a rate slower that the current flowing through the fuse heats it. Thus, the counting down, or cooling, is at a slower rate (due to the frequency division) than is the counting up, or heating. It is well to note that the series fuse does lose some heat to the environment while current is flowing through it. For this reason, an optional cooling input  417  has been logically “OR&#39;ed” with the second clock  413  through the “OR”ing diodes  418  and  419 . Thus, while the series fuse is cooling both when the current level  401  exceeds and is less than the threshold voltage  402 , the temperature can never descend below ambient, which is represented by a count of zero in the counter  407  which may be calibrated to a predetermined temperature.  
         [0032]    There are advantages in having the second clock  413  stop counting when the counter  407  reaches zero. The primary incentive to have the second clock stop is to prevent the internal protection circuits from unnecessarily discharging the battery. Thus, a latch, like an S/R flip flop could be added such that when the counter  407  reaches a zero count, the second clock  413  would become disabled. Neither the first clock  405  nor the second clock  407  would be enabled, in this case, until the current exceeded the predetermined threshold once again.  
         [0033]    The output of the counter, which in this exemplary embodiment is represented by the three most significant bits  408 , is then coupled to a digital to analog (D/A) converter  409  that converts the digital inputs  408  to an analog voltage  410 . This analog voltage  410  is then coupled to a linear regulator  411  that limits the current in the series fuse such that the series fuse is prevented from nuisance tripping.  
         [0034]    To recap, when the current exceeds a predetermined threshold the comparator output  404  goes high, causing the first clock  405  to be enabled and the second clock  413  to be disabled. The clock output  406  is coupled to the up count (representing heating of the series fuse) of an up/down counter  407 , while a divided clock  416  is coupled to the down count (representing slower cooling of the series fuse) of the counter  407 . The most significant bits  408  are coupled to a D/A converter  411 . As the count increases, the output of the D/A  410  increases, causing current flowing in the series fuse to be limited by way of a linear current regulator  411 . Thus, as current exceeds the threshold across time, the circuit  400  limits current to prevent the series fuse from clearing.  
         [0035]    Correspondingly, when the current level  401  is below the threshold voltage  402 , the second clock  413  is enabled which represents cooling of the series fuse. This clock  413  represents the continued cooling when current in the series fuse has dropped below the threshold. The cooling cannot drop below ambient, which is represented by a zero count of the counter.  
         [0036]    It will be clear to those skilled in the art that the invention is not limited to a single comparator or a single clock rate. The invention could comprise any number of comparators and clocks of varying frequencies to approximate the thermal response of any component. In it&#39;s most general form, the invention would comprise at least one comparator coupled to digital select logic that selects at least one corresponding clock. The output would be coupled to the up or down input of the counter, depending upon whether the device was heating or cooling. The clocks could all be disabled when the counter reached a zero count to prevent current drain on the battery.  
         [0037]    It will further be clear that the invention need not include more than one clock. The clock select logic could be made so as to switch the clock input from increment to decrement depending upon whether the current was above or below the predetermined threshold. Additionally, as clocks are now programmable, when the switch from increment to decrement, or vice versa, were made, the clock could be programmed to any desired frequency. This could also be accomplished with a programmable frequency divider.  
         [0038]    Referring now to FIG. 5, illustrated therein is a preferred embodiment of a linear current regulator as represented by block  411  in FIG. 4. The analog voltage output  505  from the D/A  409  is fed into a linear amplifier  501 . Simultaneously, a voltage corresponding to the current flowing through the series fuse  506  is also coupled into the linear amplifier. A linear current regulator, comprising a current sense resistor  503  and a second amplifier  502  as is known in the art, generates the voltage corresponding to the current flowing through the series fuse  506 .  
         [0039]    When the voltage from the D/A  505  is below the voltage corresponding to the current flowing through the series fuse  506 , the linear amplifier operates the pass transistor  504  in its fully conducting mode. When the voltage from the D/A  505  exceeds the current flowing through the series fuse  506 , indicative of the fuse element heating, the output of the linear amplifier  507  increases, causing the pass transistor&#39;s impedance to increase, thereby reducing the current. The circuit  500  employs negative feedback to continue the cessation of current until an equilibrium state is reached.  
         [0040]    Thus, in summary, a problem in traditional battery circuits occurs if a soft short overload current should occur just under the protection circuit current limit threshold. For this situation, the protection circuit may sustain a current just below its maximum limit value, for example 4.5 amps. If 4.499 amps were to flow in the protection circuit, the heating of the protection circuit is 0.708 watts based on a minimum on resistance of 35 milliohms for the series pass transistor. Under unfavorable conditions, this 0.708 watts may not trip the protection circuit thermally, or it may take so long to trip thermally that the fuse opens during the time the protection circuit is reaching its trip temperature.  
         [0041]    To protect the fuse, the protection circuit must have a protection delay characteristic that approximates that of the time delay fuse. The protection circuit must sustain “short” current pulses to support normal load current pulses. But if the RMS discharge load current is sustained for a significant time above a predetermined limit, the protection circuit must begin to limit the current to protect the fuse. In the ideal world, the protection circuit would contain a thermal and electrical model of the fuse trip characteristics, and would shutoff just before any damage to the fuse occurred. Such circuits can be difficult and expensive to realize on silicon, however.  
         [0042]    This invention thus contemplates a protection circuit that includes a time delay characteristic that corresponds to the thermal characteristics of the fuse. For short pulses, the current limit value may be, for example 4.5 A maximum. For sustained overloads however, the invention reduces the sustainable current across time as the series fuse heats.  
         [0043]    This invention includes a comparator threshold for detecting an instantaneous discharge current in excess of a predetermined threshold. If the instantaneous current is above the predetermined threshold, clock a counter up, indicating an overload condition. If the instantaneous current is below the predetermined threshold, clock a counter down, indicating no overload. In simple terms, counting up indicates the fuse element is heating up past normal design temperature limits, counting down indicates that the fuse element is cooling down below normal design temperature limits.  
         [0044]    If the counter continues to accumulate counts up past a particular count, begin decreasing the value at which current limiting regulation action of the protection circuit occurs. For each additional count (in the up direction) continue decreasing the value of the current limiting regulating threshold. This is a poor man&#39;s approximation to an integrator and fuse thermal/electrical model. The counter approximates the fuse element temperature. If the temperature approximation in the counter becomes critical, the current limit regulator circuit begins progressively decreasing current through the fuse to protect the fuse. The counter must be protected from over-flow and under-flow.  
         [0045]    By way of a mathematical example: Although not perfect, the math is such that it guarantees that the fuse RMS current stays within design bounds. For this example assume the up-count rate is twice the down-count rate, and that the clock and counter scaling is such that the counter reaches it&#39;s critical value after one second of continuous counting in the up direction. Also assume that the peak current limit is 4.5 A, the fuse is rated for 3 A, and the predetermined threshold is 2 A. Since the up rate is twice the down rate, and since peak current is always less than 4.5 A, and since the current is always less than 2.0 A when counting down, then averaged over one period the mean squared value of the current cannot exceed:  
         Root Mean Squared= SQRT ([4.5  A *4.5  A* 1  Sec]+[ 2.0  A* 2.0  A* 2  Sec]/ 3 second)=3.1  A    
         [0046]    The integrating nature of the counter algorithm is such that an RMS value of 3. 0  amps is never exceeded over any period of operation. Therefore, the fuse is always protected.  
         [0047]    While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims.