Abstract:
This invention includes a thermally stable, low-cost charging circuit for rechargeable batteries. The circuit includes a thermal control circuit that employs a temperature dependent component such as a thermistor or positive temperature coefficient device. The temperature dependent device is thermally coupled to a charging pass element, which is typically a power transistor. When the transistor enters a danger zone, which is a region of operation characterized by elevated power dissipation in the pass element, the thermal control circuit is actuated to regulate the pass element in a constant power mode until the circuit exits the danger zone.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates generally to battery charging and protection circuits, and more specifically to a thermally-limited charging circuit with overcharge and undercharge protection. 
     2. Background Art 
     Electronic devices, including cellular phones, pagers, radios, compact disc players, MP3 players, portable computers, and the like are becoming ever more popular. These devices are gaining popularity due to their portability. The devices derive their portability from the use of rechargeable batteries as a power source. Rechargeable batteries, of course, require a battery charger to inject current or “charge”, thereby causing the battery to store energy for future use in the electronic device. 
     FIG. 1 illustrates a simple battery charger  100  that is well known in the art. The charger  100  consists of a power supply  101 , a linear regulator  102 , a pass element  103  and a battery cell  104 . The power supply  101  provides voltage and current to the battery cell  104 . The voltage and current must be regulated by the pass element  103  so as to avoid charging the battery cell  104  too, rapidly. The linear regulator  102  performs this regulation by dissipating as heat the difference between the power generated by the power supply  101  and the power stored by the battery cell  104 . 
     The problem with this prior art solution is that the pass element  103  can overheat. This is best explained by way of example. For a typical single-cell, lithium battery application, a fully charged battery cell  104  typically registers about 4.1 volts. Thus, to fully charge the battery cell  104 , and to give enough headroom for parasitic power losses in the pass element  103  and connecting circuitry, the power supply must be capable of supplying at least 5 volts. A typical battery cell  104  will charge optimally at a current of roughly 1 amp. 
     The problem arises with the battery cell  104  is fully discharged. A discharged battery cell  104  may register only 2 volts. As the power supply  101  would supply energy at a rate of 5 volts at 1 amp, or 5 watts, and the battery cell  104  stores energy at a rate of 2 volts at 1 amp, or 2 watts, the pass element  103  must dissipate energy at a rate of 3 watts. As typical pass elements  103  may come in an industry-common TO-220 package, 3 watts for extended periods of time may make the pass element  103  quite warm. Extended periods of heat my actually jeopardize reliability by approaching—or surpassing—the threshold junction temperature of the pass element  103 . 
     The problem is exacerbated when an incompatible power supply  101  is coupled to the circuit. For example, if someone accidentally couples a 12-volt supply to the charger, the pass element  103  may have to dissipate 10 watts! This can eventually lead to thermal destruction of the pass element  103 . 
     One solution to this problem is recited in U.S. Pat. No. 5,815,382, issued to Saint-Pierre et al. entitled “Tracking Circuit for Power Supply Output Control”. This solution provides a means of reducing the output voltage of a power supply when the battery is in a discharged state, thereby reducing the total output power of the power supply. This, in turn, reduces the amount of power a pass element would need to dissipate. 
     While this is a very effective solution to the problem, it requires a power supply that both includes a feedback input and is responsive to the input by changing the output voltage. The electronics associated with an adjustable power supply can be more expensive that those found is a simple linear transformer power supply. 
     There is thus a need for an improved means of regulating temperature in a power-dissipating element like those employed as pass elements in battery charging applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a prior art charging circuit. 
     FIG. 2 is an illustration of the characteristic output of a constant current, constant voltage power supply. 
     FIG. 3 illustrates a danger zone of operation in accordance with the invention. 
     FIG. 4 is an illustration of the characteristic output of a wall transformer power supply. 
     FIG. 5 illustrates a danger zone of operation in accordance with the invention. 
     FIG. 6 is a block diagram of a circuit in accordance with the invention. 
     FIG. 7 is one preferred embodiment of a circuit in accordance with the invention. 
     FIG. 8 is an alternate embodiment of a circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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.” 
     Prior to turning to the specifics of the invention, it is well to briefly examine the operating regions in which there is a risk of thermal damage to a pass element. This is best explained by looking at battery charging applications, although it will be obvious to those of ordinary skill in the art that the invention may be equally applied to other applications as well. 
     Referring now to FIG. 2, illustrated therein is the output characteristic  200  of a “constant-voltage-constant-current”, or “CCCV”, power supply. Such supplies are known in the art, as recited by U.S. Pat. No. 5,023,541, entitled “Power Supply Control Circuit Having Constant Voltage and Constant Current Modes”. Another such supply is taught in the application notes for the TL494 control IC manufactured by On-Semiconductor. Segment  201  illustrates a constant voltage of Vmax that is supplied for all load currents less than Imax. Once the load current attempts to exceed Imax, segment  202  represents the maximum current, Imax, that is delivered as the voltage tapers from Vmax to zero. 
     Referring now to FIG. 3, illustrated therein is a charging characteristic  203  of the circuit of FIG. 1 when a CCCV source is employed as the power supply. The characteristic  203  is represented as voltage versus percentage of charge. Presuming that an initially discharged battery cell is coupled to the supply, the charging curve begins at Vlow  204 , which essentially represents the voltage of the discharged battery cell. The power supply, by contrast, begins at Vmax  205 . Consequently, there is a difference Vmax  205  minus Vlow  204  that proportionally corresponds to the power that must be dissipated by the pass element. Experimental and theoretical results have shown that a threshold exists, Vok  206 , above which standard pass elements are capable of dissipating power for a given charge rate. However, when the battery cell voltage is below Vok  206 , the pass element is called upon to dissipate more power than it can withstand. Thus, the shaded region  207  represents the “danger zone” for the pass element. Note that the current is below Imax for the voltage to be Vmax. 
     Referring now to FIG. 4, illustrated therein is the output characteristic  300  for another common power supply, the linear transformer. It may be seen from segment  301  that voltage generally rolls off as current increases. A small peak at segment  302  may be caused by rectification circuitry that includes filter capacitors. In any event, the battery charges between the levels Vbatmin  303  and Vbatmax  304 . 
     Referring now to FIG. 5, illustrated therein is the power generated by the circuit of FIG. 1 when a linear transformer is employed as the power supply. When the battery cell voltage approaches its termination point, Vbatmax  304  of FIG. 4, the voltage of the transformer continues to increase while the battery voltage stays relatively constant. This means that the pass element must be able to dissipate the extra power that results from this increasing voltage differential. As a result of the extra power, a pass element danger zone for linear transformers exists in the shaded region  306 . 
     To summarize the preceding discussion, there are regions of operation in which a battery charger having a pass element works well with no temperature compensation. There are other danger zones, however, where pass element reliability may be compromised due to the high power dissipation. It is one object of this invention to provide a circuit that prevents pass elements or other power dissipating elements from entering danger zones. The invention regulates the power dissipation of the pass element by limiting the power dissipation to a predetermined level. 
     Referring again to FIG. 1, the power dissipated in the pass element  103  may be expressed as the voltage of the power supply  101 , minus the voltage of the battery cell  104 , multiplied by the charge current. If the pass element  103  comprises a PNP bipolar junction transistor, as is common in the art, the voltage of the power supply  101 , minus the voltage of the battery cell  104  may simply be represented as Vce, the voltage difference between the emitter  106  voltage and the collector  107  voltage. Thus, the power is given as: 
     
       
           P=Vce*Ichg   (EQ. 1) 
       
     
     The threshold junction temperature, Tj, of the pass element  103  transistor is the temperature above which the transistor integrity begins to degrade. In other words, if the pass element  103  gets hotter than its threshold junction temperature, it will probably stop working properly. The threshold junction temperature may be represented as: 
     
       
           Tj=P*k+Tamb   (EQ. 2) 
       
     
     where P is the power dissipated in the pass element, k is a constant dependent upon the physical characteristics of the pass element, and Tamb is the ambient temperature about the pass element. Thus, if the ambient temperature is 35 degrees C., and the threshold junction temperature is 150 degrees C., a power dissipation temperature of 115 degrees may be tolerated while still ensuring proper pass element operation. 
     Solving for P in EQ. 2 yields: 
     
       
           P =( Tj−Tamb )/ k   (EQ. 3) 
       
     
     From EQ. 3, two things may be inferred: First, for a given ambient temperature, power dissipation is roughly proportional to junction temperature. Second, for a given maximum junction temperature, there is a predetermined power dissipation level above which a pass element will fail. 
     This invention takes advantage of these two pieces of information to create a low cost, linear charger with a maximum pass element power dissipation limit. The charger is thus capable of operation in the danger zones without fear of failure. The invention keeps the power dissipation of the pass element below a maximum level by reducing Ichg prior to the pass element temperature exceeding the maximum junction temperature. In so doing, the invention provides a safeguard against component failure in battery charging applications. 
     Referring now to FIG. 6, illustrated therein one preferred embodiment of a power regulation and thermal management circuit in block diagram form in accordance with the invention. The circuit includes a traditional pass element  501 , as well as power supply terminals  502  and cell connection terminals  503 . The circuit includes a maximum current limit circuit  504  that keeps the charging current, Ichg, below a predetermined maximum threshold. A voltage termination circuit  505  causes the pass element  501  to open when the cell is fully charged. A protection circuit  507  is provided to ensure safe operation of the cell while charging and discharging. 
     A trickle/charge control circuit  505  controls the pass element  501 . Such a circuit is recited in commonly assigned, copending application Ser. No. 10/155790, entitled Battery Trickle Charging Circuit, Filed May 26, 2002, which is incorporated herein by reference for all purposes. 
     The circuit includes a thermal control  508  for regulating the maximum power dissipation in the pass element  501 . The thermal control  508  is thermally coupled to the pass element  501  by way of a thermal link  509 . The thermal link is preferably created by a close physical proximity between the pass element  501  and the thermal control circuit  508 . 
     Referring to FIG. 7, illustrated therein is a preferred circuit embodiment for the block diagram of FIG.  6 . Each block of FIG. 6, including the maximum current limit  504 , the pass element  501 , the thermal control  508 , the trickle control  505  and the voltage termination circuit  506 , are shown in FIG. 7 with dashed lines. 
     The current control  504  circuit comprises a resistor  601  coupled serially with the pass element  501  and a pair of diodes  602  coupled to the base  603  of the pass element  501 . The value of the resistor  601 , in combination with the forward bias voltage of the diodes  602  as they source current to the base  603 , establish a maximum current that will flow through the pass element. 
     The charge control  505  utilizes a pair of diodes in conjunction with a transistor to establish a current from the base  603  of the pass element  506 . This is recited in application Ser. No. 10/155790, as mentioned above. For the present discussion, it is sufficient to say that the diodes  604  establish a base to emitter voltage, and thus a current, in the transistor  606 . This current in transistor  606  actuates the pass element  501 . 
     The voltage termination circuit  506  utilizes a voltage regulator  607 , like the TL431 manufactured by Motorola for example, to sense the voltage difference across a blocking diode  608 . When the voltage across the cell terminals  503  reaches a predetermined threshold set by resistors  609  and  610 , the voltage regulator  607  actuates transistor  611 , thereby sourcing current into the charge control  505 . This current causes the voltage across resistor  612  to increase, thereby reducing the base to emitter voltage of transistor  606 . The reduction of the base to emitter voltage causes transistor  606  to reduce the current flowing through it, thereby reducing the current flowing through the pass element  501 . Note that the three terminals labeled  616  are preferably a common node, and may be used to actuate enabling transistors  617  and  618  when a power supply is coupled to the circuit. 
     A protection circuit  507  is provided as well. This may be any of a number of off the shelf protection circuits, like the NCP802 integrated circuit manufactured by Ricoh for example. Other protection circuits known in the art would substitute equally as well. 
     It is the thermal control circuit  508  that serves as the power limiting control for the pass element  501 . The cornerstone of the thermal control circuit is a positive temperature coefficient (PTC) device  613 . A PTC has a thermal characteristic such that its resistance increases with temperature. The PTC  613  includes a thermal link  509  that is created by designing the circuit such that the PTC  613  is in close physical proximity to the pass element  501 . Preferably, the PTC  613  is physically coupled to the pass element  501  for the most efficient thermal linkage. 
     When the pass element  501  operates in a danger zone, power dissipation in the pass element  501  increases. The increased power dissipation takes the form of heat, which is translated via the thermal link  509  to the PTC  613 . When the PTC  613  heats, the impedance changes, thereby decreasing the current sourced to the base of transistor  614 . The decreased base current (and corresponding decreased voltage) causes current to flow through transistor  614  to the charge control circuit  505 . As stated above, this current causes the voltage across resistor  612  to increase, thereby reducing the base to emitter voltage of transistor  606 . The reduction of the base to emitter voltage causes transistor  606  to reduce the current flowing through it, thereby reducing the current flowing through the pass element  501 . 
     By selecting the proper value for resistor  615 , the thermal characteristics of the thermal control circuit  508 , i.e. exactly where transistor  614  turns on, may be tailored to match the thermal characteristic (defined by the junction temperature) of pass element  501 . Thus, when the power dissipation of the pass element  501  increases to a predetermined threshold, the thermal control circuit  508  will regulate the pass element  501  at a constant power level. This regulation continues until the circuit is out of the danger zone and the pass element  501  begins to cool. 
     Note that the circuit of FIG. 7 is preferably suited for applications in which the circuit is either being used in a charging state (i.e. injecting current into the cell), or a discharging state (i.e. where current flows from the cell to a load). For example, the typical digital camera is either coupled to the wall and being charged, or is detached from the wall and in use. Rarely is it being simultaneously charged and discharged at the same time. 
     Cellular phones, by contrast, are sometimes being charged and put to use at the same time. A situation may arise regarding the circuit of FIG. 7 during the charge/discharge application. If the cell is being charged and the circuit is in a danger zone, the thermal control circuit  508  will reduce the current in the pass element  501 . The temperature of the PTC  613  drives this decrease in current. There is a finite amount of time necessary for the PTC  613  to cool. If a load is coupled to the circuit before the PTC  613  cools, the pass element  501  may prevent the necessary current from being delivered to the load. Consequently, the load may not operate properly. 
     One solution to this issue contemplated with the invention is to add a timer and voltage sense circuit. The timer periodically overrides the thermal control circuit and measures the voltage across the pass element  501 . If the pass element  501  is no longer in a danger zone, the timer circuit allows the pass element  501  to return to saturation by keeping the thermal control circuit override active until the PTC  613  has cooled. 
     Turning now to FIG. 8, illustrated therein is another solution to the simultaneous charge-discharge requirement. Illustrated in FIG. 8 is a circuit that is similar in many ways to the circuit of FIG.  7 . The circuit of FIG. 8 includes the pass element  501 , power supply terminals  502  and cell connection terminals  503 . Additionally, the maximum current limit circuit  504 , voltage termination circuit  505 , and protection circuit  507  are identical to those of FIG.  7 . The trickle/charge control circuit  505  is roughly the same, including the enabling transistor  618 . 
     However, in the circuit of FIG. 8, the thermal control circuit  508  is changed to accommodate dynamic charge-discharge capabilities. The thermal control circuit includes a thermally sensitive component  701 , which is preferably a thermistor, that is in close physical proximity to the pass element  501 . Note that a thermistor&#39;s impedance changes linearly with temperature. The changing impedance of the thermistor, coupled with resistor  708 , create a thermally proportional voltage  709  that is coupled to a first comparator  702  and a second comparator  703 . The first comparator  702  and second comparator  703  each have corresponding reference voltages, which are voltage  704  and  705 , respectively. The reference voltages  704 , 705  correspond to different, predetermined temperature levels. Note that the references may change with power supply voltage. 
     The operation of the thermal control circuit  508  is as follows: Presume for the purposes of this example that voltage  705  is less than  704 . In a danger zone, when the temperature of the pass element and thus the corresponding thermistor  701  increase above voltage  705 , node  707  is actuated. The actuation of node  707  deactuates transistor  712 . The deactuation of transistor  712  causes resistor  714  to be decoupled in parallel with resistor  612 , thereby decreasing the current in the pass element  501 . If the temperature, and thus voltage  709 , increases above voltage  704 , node  706  is actuated, thereby deactuating transistor  713 . This causes resistor  715  to be decoupled in parallel with resistors  714  and  612 , again reducing the current in the pass element  501 . Once the thermistor  701  cools, transistors  713  and  712  are eventually actuated, thereby allowing the pass element to return to a full-conduction state  501 . 
     In one preferred embodiment, each comparator  702 , 703  includes positive feedback in the form of high-impedance resistors  710  and  711 . This positive feedback turns the temperatures set by voltage  704  and  705  into bands of temperatures by way of hysteresis. In other words, if voltage  705  originally corresponded to 75° C., with hysteresis node  707  may actuate at 80° C. and deactuate at 70° C. By tailoring the values of hysteresis resistors  710  and  711 , four temperature thresholds may be designed into the system. 
     These thresholds expand the protection of the circuit by altering the current at four different pass element  501  power dissipation levels, thereby finding a maximum charging current that keeps the pass element  501  below the maximum power dissipation level with greater resolution. In a preferred embodiment, for a typical pass element in a TO-220 package, the four levels correspond to 75° C. and 100° C. for comparator  703  and 85° C. and 110° C. for comparator  702 . The circuit operates effectively so long as the first level is between 50° C. and 85° C., the second level is between 85° C. and 115° C., the third level is between 75° C. and 100° C. and the fourth level is between 85° C. and 130° C., depending upon the type of pass element being used. 
     Thus, if the pass element  501  exceeds predetermined temperature limits, the thermal control circuit  508  alters the current in the pass element  501  by way of the control circuit  505 . For example, using the preferred temperatures above, if the temperature exceeds 100° C., transistor  712  is deactuated to reduce the current in the pass element  501 . Transistor  712  will not actuate until the temperature drops below 75° C. Likewise, if the temperature the temperature exceeds 110° C., transistor  713  is deactuated, thereby reducing the current in the pass element  501 . Transistor  713  will not actuate until the temperature drops below 85° C. The maximum pass element charge current will not resume until the pass element temperature falls below 75° C. 
     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.