Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a power system with temperature compensation control. 
     2. Description of Related Art 
     Referring to  FIG. 1 , an electronic device often requires two power supply paths. When it is operated under an external power source, the external power source supplies power to a load  2  and charges a battery BATT; when the external power source does not exist, the battery BATT supplies power to the load  2 . In the prior art shown in the drawing, the external power supply path is controlled by a linear voltage converter circuit  10 . This linear voltage converter circuit  10  for example is a simple switch or an LDO (Low Drop-Out) circuit, including a power transistor switch P 0  controlled by an LDO control circuit or a switch control circuit  11 . The path through which the external power source charges the battery BATT and the battery BATT supplies power to the load  2  is controlled by a power transistor switch P 1 , wherein the switch P 1  is controlled by a constant current or constant voltage (CC/CV) dynamic control circuit  20 . The switch P 1  is CC/CV dynamically controlled when the battery BATT is being charged, but operates as a simple switch when the battery BATT supplies power to the load  2 . 
     In the foregoing prior art, the LDO  10  and the CC/CV dynamic control circuit  20  are well-know to a person skilled in this art, and their details are thus omitted here. 
     The foregoing prior art in  FIG. 1  has the following drawback. Because the circuit uses the linear voltage converter circuit, the voltage difference between the external power source and the battery causes a heat dissipation issue. This issue occurs in either the power switch P 0  or the power switch P 1 . Thus, it is desired to provide a solution to it. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing drawback, the present invention provides a power system with temperature compensation control. 
     To achieve the foregoing objective, in one perspective of the present invention, it provides a power system with temperature compensation control, for selectively supplying power from an external power source or a battery to a load, or charging the battery from the external power source, the power system comprising: a buck converter electrically connected between the external power source and the load, and a temperature compensation control circuit for adjusting an output voltage of the buck converter according to a sensed temperature. 
     In a preferred embodiment of the foregoing circuit, the output voltage is preferably set within a range between an upper limit and a lower limit. The output voltage is allowed to reach the upper limit when the sensed temperature is lower than a predetermined temperature. A maximum level of the output voltage decreases as a temperature difference between the sensed temperature and the predetermined temperature increases, but the output voltage is still higher than or equal to the lower limit, when the sensed temperature is higher than the predetermined temperature. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram showing a prior art power system with two power supply paths. 
         FIG. 2  is a schematic circuit diagram showing an embodiment of the present invention. 
         FIG. 3  illustrates the relationship between temperature and output voltage of the present invention. 
         FIG. 4  is a schematic circuit diagram illustrating embodiments of a temperature sensor circuit and an output voltage upper and lower limit setting circuit. 
         FIG. 5  shows how to control a buck converter according to an output of a temperature compensation control circuit and an over current protection circuit. 
         FIGS. 6-7  illustrate two embodiments of the over current protection circuit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Please refer to  FIG. 2 , which shows an embodiment of the present invention by a schematic circuit diagram. The present invention employs a buck converter  30  to replace the linear voltage converter  10  in the prior art. The buck converter  30  has better power utilization efficiency and is less likely to cause heat dissipation issue. In addition, the buck converter  30  is operated under temperature compensation control so that the circuit temperature is even better controlled. 
     More specifically, the power system of the present invention includes two power supply paths. The first power supply path is connected between an external power source and an output voltage node Vout which supplies an output to a load  2 , the first power supply path being controlled by the buck converter  30 . The second power supply path is connected between the output voltage node Vout and a battery BATT. The buck converter  30  is controlled by a temperature compensation control circuit  40 , wherein the temperature compensation control circuit  40  includes a temperature sensor circuit  41  and an output voltage upper and lower limit setting circuit  42 . The temperature sensor circuit  41  senses the circuit temperature. When the circuit temperature is too high, the output voltage Vout is adjusted to reduce the circuit temperature. The output voltage upper and lower limit setting circuit  42  sets an upper limit VH and a lower limit VL of the output voltage Vout. Referring to  FIG. 3 , the function achieved by the temperature sensor circuit  41  and the output voltage upper and lower limit setting circuit  42  is thus. When a sensed temperature is lower than a predetermined temperature T, the output voltage is allowed to reach the upper limit VH, such that the load  2  and the battery BATT can obtain a maximum level of power. When the sensed temperature is equal to or higher than the predetermined temperature, the maximum level of the output voltage Vout decreases gradually to reduce the circuit temperature. Yet, when the sensed temperature is much higher than the predetermined temperature T, the output voltage Vout is still not lower than the lower limit VL, such that the load  2  can obtain the basic power that it requires. 
     In addition, in this embodiment, an additional over current protection circuit  50  may be optionally provided. The over current protection circuit  50  is used for controlling current through the first power supply path, such that the current does not exceed a predetermined safety range. 
       FIG. 4  shows a specific embodiment of the temperature compensation control circuit  40 . The temperature sensor circuit  41  includes an operational amplifier  411  and a transistor  412 , wherein the operational amplifier  411  has an output controlling a gate of the transistor  412 . The operational amplifier  411  compares a signal representing a sensed temperature with a signal representing a predetermined temperature T, and controls the conduction of the transistor  412  to determine a current I 1  according to the comparison result. The output voltage upper and lower limit setting circuit  42  includes a current source  12 , a resistor R 1  and a comparator  421 , wherein the product of the current I 2  and the resistance R 1  is equal to the upper limit VH of the output voltage. When the sensed temperature is lower than the predetermined temperature T, the operational amplifier  411  controls the transistor  412  and turns it off; hence, I 1  is zero. Therefore, the voltage at the node V 1  is equal to I 2 ×R 1  (i.e., VH is equal to I 2 ×R 1 ). When the sensed temperature is higher than the predetermined temperature T, the conduction of the transistor  412  increases in accordance with the increase of the temperature difference. In this case, the voltage at the node V 1  is equal to (I 2 −I 1 )×R 1 . The comparator  421  selects a highest one from its three positive inputs, i.e., the battery voltage VBATT, the lower limit VL, and the node voltage V 1 , and compares it with the output voltage Vout. When the comparison result shows that the negative input (output voltage Vout) is lower, a high level signal Pon is generated. 
     When the over current protection circuit  50  is provided and the temperature compensation control circuit  40  is embodied by the one shown in  FIG. 4 , the circuit can supply power to the load in the following way, for example: 
     First, when an over current protection is not triggered, and when the sensed temperature is much lower than the predetermined temperature T, the output voltage Vout can be set to the upper limit VH. When the sensed temperature exceeds but is still close to the predetermined temperature T, the maximum level of the output voltage Vout decreases, such that the voltage difference between the output voltage Vout and the battery BATT decreases. Hence, power dissipation by the power transistor P 1  decreases, so that the circuit temperature decreases and eventually balances at the predetermined temperature T. When the sensed temperature is far higher than the predetermined temperature T, if the battery voltage VBATT is higher than the lower limit VL, the maximum level of the output voltage Vout is VBATT, such that the voltage difference of the output voltage Vout and the battery voltage VBATT is zero; hence, the power dissipation by the power transistor P 1  is zero. Yet, if the battery voltage VBATT is lower than the lower limit VL, the output voltage Vout is maintained at the lower limit VL, such that the load  2  can obtain basic power that it requires. 
     Second, when the total current flowing to the load  2  and for charging the battery BATT is too large that the over current protection circuit  42  is triggered, because the product of the input current and input voltage of the buck converter  30  is almost equal to the product of its output current and output voltage, when the input current is limited and the output current increases, the output voltage Vout naturally decreases, such that the power transistor P 1  enters its saturation region, and the current charging the battery BATT decreases accordingly. If the current required by the load  2  is more than the over current protection setting, the circuit will stop charging the battery BATT; instead, the external power source and the battery BATT will both supply current to the load  2 . 
     When the over current protection circuit  50  is not provided, the output signal Pon of the temperature compensation control circuit  40  can solely determine the switching time of a power switch in the buck converter  30 . If the over current protection circuit  50  is provided, as an example, the circuit may be embodied as shown in  FIG. 5 . The buck converter  30  includes upper and lower gate power switches  301  and  302 , and an inductor  303 . By operation of the upper and lower gate power switches  301  and  302 , an input voltage Vin at the left side is converted to an output voltage Vout at the right side. Each of the upper and lower gate power switches  301  and  302  can be a PMOS transistor or an NMOS transistor. Depending on the type of the transistor, the gate control signal thereof may need to be inverted. A logic circuit  304  performs a logic operation on the output signal Pon from the temperature compensation control circuit  40  and the output signal OC from the over current protection circuit  50 ; the result is used to drive the power switch  301  via a driver gate  305 . Assuming that the upper gate power switch  301  is a PMOS transistor, when the output signal OC is low, indicating that the over current status does not occur, the signal Pon determines the on-time of the power switch  301  (since the power switch  301  is a PMOS transistor, the logic circuit  304  outputs the signal Pon in inverted form). When the output signal OC is high, indicating that the over current status occurs, the logic circuit  304  outputs a high level signal, and the power switch  301  is turned off. 
     The over current protection circuit  50  can be embodied in many forms.  FIG. 6  shows one example thereof, wherein a voltage difference across the resistor R 2  is used to indicate a current flowing through the resistor R 2 . By comparing the voltage difference with a predetermined reference voltage Vref, it can be determined whether an over current status has occurred.  FIG. 7  shows another example of the over current protection circuit  50 , wherein it senses a current flowing through the first power supply path, and causes the current to flow through a resistor R 3 . Similarly, whether an over current status has occurred can be determined by comparing the voltage across the resistor R 3  with the reference voltage Vref. 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, the power switch  302  can be replaced by a diode. As another example, an additional circuit device which does not substantially affect the primary function of the circuit can be interposed between two devices shown to be in direct connection in the embodiments of the present invention. As yet another example, in the embodiment shown in  FIG. 4 , it is not necessarily required to compare all signals in one comparator  421 ; instead, the signals can be compared two by two, and the results are consolidated by logic operation, or the like. In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.

Technology Category: 5