Patent Publication Number: US-7583061-B2

Title: Charger circuit and PWM controller thereof

Description:
FIELD OF THE INVENTION 
   The present invention relates to pulse width modulation (PWM) and particularly to a lower power PWM control technique related to charger circuits and PWM controllers thereof. 
   BACKGROUND OF THE INVENTION 
   At present the chargers for handsets and digital cameras mostly adopt a switch power supply circuit. They are designed for a lower power supply and can be classified as follow: a first type in which the circuit on the primary side of a transformer adopts RCC separated elements to form self actuation control or a PWM controller to form external actuation control; a second type in which the secondary side of a transformer adopts a dual operational charge circuit to achieve constant current and a voltage base circuit to achieve constant voltage, or adopts a voltage base circuit to achieve constant voltage and a transistor to achieve constant current; and a third type which adopts an ASIC (application specific integrated circuit) design which has a dedicated integrated circuit on the primary side of a transformer to control output at constant voltage and constant current without an optical coupler and a constant voltage/current controller on the secondary side of the transformer. 
   The first type is simpler, but has notable disadvantages, such as greater element dispersion, lower efficiency, no short circuit function, lower production yield, thus the acceptance is dwindling now. The third type that adopts the ASIC design is simpler, but it requires a special technology available only to a small number of manufacturers. Moreover, it does not provide a vigorous voltage/current control circuit, hence system harmonic wave and noise performances are less desirable, and the cost also is higher, thus it still cannot fully meet customers&#39; requirements. The second type is most widely adopted. Its system performance can meet the constant voltage/current requirement of most high performance chargers. Environmental temperature variation also does not have significant impact on the system performance. However, it also has its share of problems, such as system cost is higher and circuit board area is greater. 
   To overcome the aforesaid problems, a higher performance solution is needed to provide a higher efficiency, improved short circuit characteristics, and lower system output harmonic wave and system cost. 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to provide a high performance charger circuit that has a higher efficiency, improved short circuit characteristics, lower system output harmonic wave and system cost. 
   The invention also provides a PWM controller which has a short circuit mode circuit in which input ends are simultaneously connected to a power supply input end and an output end of an output driving circuit. The PWM controller also has an oscillator which includes a temperature compensation circuit adopting a medium value multi-transistor resistor of a negative temperature coefficient and a well resistor of a positive temperature coefficient. 
   The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a conventional charger circuit in a PWM controller. 
       FIG. 2  is a chart showing a charging curve of a conventional charger circuit. 
       FIG. 3  is a circuit block diagram of an embodiment of the PWM controller of the invention. 
       FIG. 4  is a fragmentary circuit block diagram of an embodiment of a short circuit mode circuit in the PWM controller of the invention. 
       FIG. 5  is a fragmentary circuit diagram of an embodiment of a frequency temperature compensation circuit in an oscillator of the invention. 
       FIG. 6  is a circuit diagram of another embodiment of the charge circuit of the invention. 
       FIG. 7  is a chart showing a charging curve of another embodiment of the charge circuit of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Please refer to  FIG. 3  for a circuit block diagram of an embodiment of a PWM controller  100  of the invention. It includes: 
   a power on circuit  101  which is connected to a power supply input end  001  and determines the interim threshold duty voltage of the power supply input end  001  at the power on stage and the minimum duty voltage during normal operation; 
   an oscillator  108  which generates a square wave signal at a constant frequency and has an output end connecting to a PWM logic circuit  107  and a positive and negative temperature compensation circuit to generate the constant frequency used by the power supply; 
   a current limit comparator  106  which has an input end connecting to the power supply input end  001  in response to a current sampling signal of an output driving circuit  104  to carry out feedback of a current circuit. The current limit comparator  106  also responds to voltage variations of the power supply input end  001  to carry out feedback of a voltage circuit. Current feedback signals and voltage feedback signals are sent to the PWM logic circuit  107  in an error signal format through the current limit comparator  106 ; 
   the PWM logic circuit  107  which is connected to the oscillator  108  to respond to the square wave signal output therefrom and also is connected to the current limit comparator  106  to receive the error signal thereof to determine the duty cycle of output driving pulse. The PWM logic circuit  107  further responds to input signals of a short circuit mode circuit  103  and periodically stops output signals to protect the system; 
   the short circuit mode circuit  103  which has one input end connecting to the power supply input end  001  and another input end connecting to an output end of the output driving circuit  104 . During normal operation the voltage of the output end of the output driving circuit  104  is higher and the voltage of the power supply input end  001  is lower. In the event of short circuit or a light loading condition and the output end voltage of the output driving circuit  104  is lower, the voltage of the power supply input end  001  is higher, the short circuit mode circuit  103  makes the PWM controller  100  to enter a short circuit protection mode; and 
   the output driving circuit  104  which has an input end connecting to the PWM logic circuit  107  and output ends connecting to the short circuit mode circuit  103  and the current limit comparator  106  to output the PWM pulse signals. It is connected to and drives a power transistor  105  outside the PWM pulse controller  100  through power elements located inside the PWM controller  100 . 
   The power on circuit  101  may also be connected to an internal base circuit  102  to provide internal base signals. 
   As may be seen from the embodiment shown in  FIG. 3 , the PWM controller of the invention has features as follow: the input ends of the short circuit mode circuit  103  are connected simultaneously to the power supply input end  001  and the output end of the output driving circuit  104 . Refer to  FIG. 4  for a fragmentary circuit block diagram of an embodiment of the short circuit mode circuit  103 . The short circuit mode circuit  103  of the PWM controller  100  of the invention adopts a control method different from the conventional PWM controller. The conventional PWM controller receives input signals merely from the output driving end, equivalent to the output driving circuit  104  of the invention, but does not receive the input signals at the same time from the power supply input end  001 , hence response speed is slower and not accurate. By contrast, the short circuit mode circuit  103  of the invention is connected to the power supply input end  001  and the output end of the output driving circuit  104  at the same time, thus in normal operation condition the voltage at the output end of the output driving circuit  104  is higher, and the voltage at the power supply input end  001  is lower. In the event of short circuit or a light loading condition the voltage at the output end of the output driving circuit  104  is lower, and the voltage at the power supply input end  001  is higher. By means of such a design the system can enter the short circuit protection mode when short circuit occurs as previously discussed. It is to be noted that the PWM controller  100  does not enter the short circuit protection mode in the light loading condition. 
   The temperature compensation circuit in the oscillator  108  of the PWM controller  100  of the invention adopts a medium value multi-transistor resistor of a negative temperature coefficient and a well resistor of a positive temperature coefficient. Refer to  FIG. 5  for a fragmentary circuit diagram of the frequency temperature compensation circuit in the oscillator of an embodiment of the invention. The frequency of the oscillation circuit usually is determined by a capacitor C 0  and power supplies I 0  and I 1 . Current I flows into one power supply I 0 , and current  4 I flows into another power supply I 1 . They are obtained through transistors M 0  and MI and a mirror image bias current Jbias of other current mirror. In the general CMOS design the bias current Ibias is generated by applying a positive temperature coefficient ΔVbe (delta Vbe shown in the drawings) to resistors R 1  and R 2 . As there is no temperature coefficient of a resistor that can exactly match the temperature coefficient of ΔVbe, the temperature characteristics of the current is not desirable, and frequency stability is affected. To get a desired positive temperature coefficient of ΔVbe, the oscillation circuit of the invention employs a medium value multi-transistor R 1  of a negative temperature coefficient and a well resistor R 2  of a positive coefficient to reduce the temperature coefficient of the bias current. The formula is as follow:
 
 I bias=Δ Vbe /( R 2+ R 1).
 
   The temperature coefficient is determined by the following formula: 
   
     
       
         
           
             
               ∂ 
               Ibias 
             
             
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                   Vbe 
                 
                 
                   
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                   2 
                 
               
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   Based on the above two formulas, it can be seen that as the temperature coefficient of R 2  is greater, the sum of two previous items is negative. This results in a not desirable current temperature coefficient. By including the resistor R 1  of a negative temperature coefficient, the total is approaching zero. Hence a desired current temperature coefficient that can meet requirements can be obtained. And the oscillation frequency of a greater temperature stability can be obtained. 
   The power elements inside the PWM controller  100  set forth above may be built-in power transistors. And the power transistor  105  and PWM controller  100  can be packaged in a single TO-94 to save space. Moreover, the power on circuit  101  in the PWM controller  100  of the invention does not include a voltage stabilization diode. 
   By means of the design previously discussed, the invention provides an improved PWM controller  100 . A system adopting the invention does not need a voltage stabilization diode which now exists in many of the conventional techniques. It also provides improved short circuit protection characteristics over the conventional designs. 
   Refer to  FIG. 6  for the circuit diagram of another embodiment of the charger circuit of the invention. The charger circuit includes two portions, namely a PWM controller portion and a constant voltage/current control circuit portion that are coupled through a transformer T 1 . The PWM controller  401  shown in  FIG. 6  is same as the PWM controller  100  in  FIG. 3 . It has pins sequence numbers  1 - 4  corresponding to input/output end numbers  001 - 004  in  FIG. 3 . It has a constant voltage/current control circuit marked by  402  in  FIG. 6 . The remaining elements are auxiliary elements. The operating principle of the charger circuit  400  is as follow: The voltage after rectification is sent to the PWM controller  401  through the transformer T 1  to provide power on energy at the power supply input end of the PWM controller  401  to complete system power on process. In normal duty conditions, the auxiliary winding of the transformer T 1  provides power supply energy for the PWM controller  401 . A diode D 8  is connected to a resistor R 12  in series to provide required power for the constant voltage/current control circuit  402 . Thus constant current characteristics can be maintained when output voltage is in the range of 2.5V-5V, and short circuit power is less than 1 watt. The harmonic wave also is lower in the no loading condition. Refer to  FIG. 7  for the charging curve of the charger circuit  400 . In this embodiment the constant voltage/current control circuit  402  can also provide short circuit control. Hence the charger circuit of the invention can get a very low output voltage harmonic wave in the no loading condition. 
   Referring to  FIG. 1  for the circuit diagram of a charger circuit of a conventional PWM controller. It also has a PWM controller coupling with a constant voltage/current circuit through a transformer. However it differs from the invention as follow: The structure of the PWM controller is different and coupling of the auxiliary circuits also is different, and implementation of the constant voltage/current circuit also is different. The invention employs a core plate, while the conventional technique adopts a dual operational charge circuit. The embodiment shown in  FIG. 1  is a conventional three-end PWM controller including a standard processing amplifier to achieve constant voltage and current. 
     FIG. 2  depicts the charge curve of the charger circuit shown in  FIG. 1 . The charged circuit shown in  FIG. 6  has disadvantages, namely the constant current range is narrower (3V-5V), and the charging curve is also less desirable, and short circuit current is greater ( 2 A). Moreover, the system needs a great number of separated elements and the cost is higher. 
   In short, a system adopted the PWM control circuit and charger circuit  400  of the invention does not need a voltage stabilization diode that has to be used in many of the conventional designs. Short circuit protection characteristics are better than many of the existing designs. It adopts a novel approach to eliminate the low frequency harmonic wave. The power transistor and the controller can be packaged in a single TO-94, hence cluster density is enhanced. It overcomes the reliability problem occurred to the small package. The invention provides a high performance design at a lower cost that has a higher efficiency, improved short circuit characteristics and a lower system output harmonic wave. 
   While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.