Patent Abstract:
A power supply including a regulation circuit that maintains an approximately constant load current with line voltage. An example circuit includes a switch including first, second and third terminals. The first terminal is coupled or decoupled to the second terminal in response to a control signal received at the third terminal. Voltage sense circuitry is included and is coupled to sense a voltage drop across the switch. The voltage drop is representative of a current in the switch during an on time of the switch. The voltage sense circuitry has a variable current limit threshold that increases between a first level and a second level during the on time of the switch. The control signal is responsive to the variable current limit threshold to regulate a power supply with an output characteristic having an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold.

Full Description:
REFERENCE TO PRIOR APPLICATION 
   This application is a continuation of U.S. application Ser. No. 10/892,300, filed Jul. 15, 2004, now pending, which is a continuation of U.S. application Ser. No. 10/253,307, filed Sep. 23, 2002, now U.S. Pat. No. 6,781,357 B2, which claims the benefit of and priority to U.S. provisional application serial No. 60/325,642, filed Sep. 27, 2001, entitled “Method And Apparatus For Maintaining A Constant Load Current With Line Voltage In A Switch Mode Power Supply.” 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to power supplies and, more specifically, the present invention relates to a switched mode power supply. 
   2. Background Information 
   All electronic devices use power to operate. A form of power supply that is highly efficient and at the same time provides acceptable output regulation to supply power to electronic devices or other loads is the switched-mode power supply. In many electronic device applications, especially the low power off-line adapter/charger market, during the normal operating load range of the power supply an approximately constant output voltage is required below an output current threshold. The current output is generally regulated below an output voltage in this region of approximately constant output voltage, hereafter referred to as the output voltage threshold. 
   In known switched mode power supplies without secondary current sensing circuitry, minimizing the variation of the output current at the output voltage threshold is performed with complex control schemes. Typically, these schemes include the measurement of input voltage, output diode conduction time and peak primary current limit. Some or all of this measured information is then used to control the regulator in order to reduce the variation of the output current at the output voltage threshold. 
   SUMMARY OF THE INVENTION 
   A power supply that maintains an approximately constant load current with line voltage below the output voltage threshold is disclosed. In one embodiment, a regulation circuit includes a semiconductor switch and current sense circuitry to sense the current in the semiconductor switch. The current sense circuitry has a current limit threshold. The regulation circuit current limit threshold is varied from a first level to a second level during the time when the semiconductor switch is on. In one embodiment, the regulation circuit is used in a power supply having an output characteristic having an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold. In another embodiment, a power supply is described, which includes a power supply input and a power supply output and that maintains an approximately constant load current with line voltage below the output voltage threshold. In one embodiment, the power supply has an output characteristic having an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold. A regulation circuit is coupled between the power supply input and the power supply output. The regulation circuit includes a semiconductor switch and current sense circuitry to sense the current in the semiconductor switch. The current sense circuitry has a current limit threshold. The regulation circuit current limit threshold is varied from a first level to a second level during the time when the semiconductor switch is on. In another aspect, the current limit threshold being reached coincides with the power supply output characteristic transitioning from providing an approximately constant output voltage to supplying an approximately constant output current. In yet another aspect, the semiconductor switch is a MOSFET. Additional features and benefits of the present invention will become apparent from the detailed description and figures set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention detailed illustrated by way of example and not limitation in the accompanying figures. 
       FIG. 1  is a schematic of one embodiment of a switched mode power supply regulator in accordance with the teachings of the present invention. 
       FIG. 2  is a diagram illustrating one embodiment of sawtooth, duty cycle and intrinsic current limit waveforms in accordance with the teachings of the present invention. 
       FIG. 3  shows one embodiment of a power supply that has an approximately constant voltage and constant current characteristic in accordance with the teachings of the present invention. 
       FIG. 4  shows one embodiment of a power supply that has an approximately constant voltage and constant current characteristic in accordance with the teachings of the present invention. 
       FIG. 5  is a diagram illustrating the typical relationship between the output current and output voltage of one embodiment of a power supply in accordance with the teachings of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of methods and apparatuses for maintaining a power supply output current substantially constant independent of input voltage at the point where the power supply output characteristic transitions from providing an approximately constant output voltage to supplying an approximately constant output current are disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. 
   Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
   In one embodiment here, a switched mode power supply is described in which the output current below the output voltage threshold, is regulated to be approximately constant. This provides an approximate constant voltage/constant current output characteristic. The output current level at the output voltage threshold in known power supplies sensed at the output of the power supply to provide feedback to a regulator circuit coupled to the primary winding of the power supply. If however, the approximate constant current functionality is achieved without feedback from the secondary winding side of the power supply, the output current at the output voltage threshold is a function of a peak current limit of the primary regulator. 
   Embodiments of the present invention reduce the variation of the output current at the output voltage threshold by reducing the peak current limit variation with changing input voltage. In general, the intrinsic peak current limit is set by internal circuitry in the regulator to be constant. In one embodiment, once the drain current reaches a current limit threshold, the switching cycle should, in theory, terminate immediately. However, a fixed delay is inherent from the time the threshold is reached until the power metal oxide semiconductor field effect transistor (MOSFET) is finally disabled. During this delay, the drain current continues to ramp up at a rate equal to the direct current (DC) input voltage divided by the primary inductance of the transformer (drain current ramp rate). Therefore, the actual current limit is the sum of the intrinsic current limit threshold and a ramp-rate dependent component (the overshoot), which is the drain current ramp rate multiplied by the fixed delay. Thus, at higher DC input voltages, the actual current limit ramps to a higher level above the intrinsic current limit level than at low DC input voltages. This can result in variations in the output current delivered to the load at the output voltage threshold over a range of input line voltages. 
   The actual current limit is the sum of the intrinsic current limit and the ramp-rate dependent component (the overshoot). The goal is to maintain a constant actual current limit over DC input voltage variations. Since the ramp-rate component (the overshoot) increases with respect to the DC input voltage, the only way to maintain a relatively constant current limit would be to reduce the intrinsic current limit threshold when the DC input voltage rises. 
   In discontinuous power supply designs, the point in time during the switching cycle in which the current limit is reached is dependent on the DC input voltage. In fact, the time it takes from the beginning of the cycle to the point where current limit is inversely proportional to the DC input voltage. Thus, the time elapsed from the beginning of the cycle can be used to gauge the DC input voltage. 
   Therefore, in order to create an intrinsic current limit which decreases relative to the DC input voltage, the time elapsed can be used. It is simply necessary to increase the intrinsic current limit as a function of the time elapsed during the cycle. A first approximation for increasing the intrinsic current limit with time can be obtained by using the Equation 1 below:
 
 I   LIM-INTRINSIC   =K   1   +K   2   * t   elapsed,   (Equation 1)
 
where is I LIM-INTRINSIC  the intrinsic current limit, K 1  and K 2  are constants and t elapsed  is the time elapsed.
 
   In one embodiment, the time elapsed can be detected by the internal oscillator output waveform. In one embodiment, this waveform is a triangular one. It starts at its minimum at the beginning of the cycle. It gradually ramps until it reaches the point of maximum duty cycle. 
   In one embodiment, he ramp is substantially linear with time. In another embodiment, the ramp can also be nonlinear depending on the requirements of the power supply in which the regulator is used. The intrinsic current limit threshold is basically proportional to the voltage seen at the input of the current limit comparator. This bias voltage is the product of the resistor value and the current delivered to this resistor. One way to increase the intrinsic current limit linearly as a function of the elapsed time would then be to derive a linearly increasing (with elapsed time) current source and deliver this current to the resistor. This linearly increasing (with elapsed time) current source can thus be derived from the oscillator. 
     FIG. 1  shows a schematic of one embodiment of a switched mode power supply in accordance with the teachings of the present invention. All of the circuitry shown in this schematic is used to control the switching of the power MOSFET  2 . The timing of the switching is controlled by oscillator  5 . Oscillator  5  generates three signals: Clock  10 , DMAX (Maximum duty cycle)  15 , and Sawtooth  20 . The rising edge of Clock signal  10  determines the beginning of the switching cycle. As shown in the illustrated embodiment, when Clock signal  10  is high, output latch  90  is set, which results in a control signal output from output latch  90  to enable power MOSFET  2  to begin conducting. The maximum conducting time is determined by DMAX  15  signal being high. When DMAX  15  signal goes low, latch  90  is reset, thus causing the control signal output from latch  90  to disable power MOSFET  2  from conducting. 
   The intrinsic current limit is, to the first order proportional to the voltage on node  22 . As stated earlier, the goal of the invention is to generate an intrinsic current limit proportional to the time elapsed in the switching cycle. The saw tooth waveform  20  can be used to perform this task. As the base voltage of NPN transistor  30  rises, the emitter voltage also rises at the same rate. Thus, the current through resistor  25  is linearly increasing with time elapsed during the switching cycle. After mirroring this current through current mirror  40 , the linearly increasing (with elapsed time) current source  27  is derived. The current limit threshold  22  is thus proportional to the product of the combination of linearly increasing current source  27  and constant current source  50  with the resistor  17 . The voltage on node  37  is proportional to the power MOSFET drain voltage because of the voltage divider network formed by resistors  55  and  60 . The drain current is proportional to the drain voltage. As the drain current  7  ramps up during the switching cycle, the voltage on node  37  rises proportionately. After the voltage on node  37  exceeds the voltage on current limit threshold node  22 , comparator  70  disables the power MOSFET by ultimately resetting latch  90 . 
   PWM Comparator  32  modulates the duty cycle based on the feedback signal coming from the output of the power supply. The higher the feedback voltage, the higher the duty cycle will be. 
     FIG. 2  shows an embodiment of three waveforms: sawtooth  20 , duty cycle max  15 , and intrinsic current limit  22 . The sawtooth waveform  20  and the duty cycle max waveform  15  are generated by the oscillator  5 . The duty cycle max  15  signal determines the maximum duration of a power MOSFET switching cycle, when it is high. The sawtooth waveform  20  starts increasing at the low point when the duty cycle max waveform  15  goes high. This signals the beginning of the power MOSFET switching cycle. The high point of the sawtooth  20  is reached at the end of the cycle, at the same time the duty cycle max signal  15  goes low. The intrinsic current limit  22  signal starts at the low point at the beginning of the cycle and then linearly increases with elapsed time throughout the cycle. At a time elapsed of zero, the intrinsic current limit is at K 1 . As time elapsed increases, the current limit increases by a factor of K 2 * t elapsed . As can be seen in  FIG. 2  therefore, the intrinsic current limit (I LIM-INTRINSIC ) is the sum of K 1  and K 2 *t elapsed.    
     FIG. 3  shows one embodiment of a power supply that has an approximately constant voltage and constant current characteristic in accordance with the teachings of the present invention. An energy transfer element  220  is coupled between DC output  200  and HV DC input  255 . In one embodiment, energy transfer element is a transformer including an input winding  225  and an output winding  215 . Regulation circuit  250  is coupled between HV DC input  255  and energy transfer element  220  to regulate DC output  200 . In the illustrated embodiment, feedback information responsive to DC output  200  is provided to the regulator  250  at its control pin. The current at the control pin is proportional to the voltage across resistor  235 , which in turn is related to the output voltage at DC output  200 . 
   In operation, the regulator circuit reduces the duty cycle of the power MOSFET when the voltage across resistor  235  increases above a threshold. In this section, the output is in approximately constant voltage mode. The regulator circuit reduces the current limit of the power MOSFET when the voltage across resistor  235  decreases below a threshold. The current limit is reduced as a function of the voltage across resistor  235  to keep the output load current constant. Thus, the load current is proportional to the current limit of the power MOSFET in regulator  250 . By keeping the current limit invariant to line voltage, the output load current would remain constant at all line voltages. 
     FIG. 4  shows one embodiment of a power supply that has an approximately constant voltage and constant current characteristic in accordance with the teachings of the present invention. The feedback information is provided to the regulator  350  at its control pin. The current at the control pin is proportional to the voltage across resistor  335 , which in turn is related to the output voltage. The regulator circuit reduces the duty cycle of the power MOSFET when the voltage across resistor  335  increases above a threshold. In this section, the output is in approximately constant voltage mode. The regulator circuit reduces the current limit of the power MOSFET when the voltage across resistor  335  decreases below a threshold. The current limit is reduced as a function of the voltage across resistor  335  to keep the output load current approximately constant. Thus, the load current is proportional to the current limit of the power MOSFET in regulator  350 . By keeping the current limit substantially constant with line voltage, the output load current would remain substantially constant at all line voltages. 
     FIG. 5  is a diagram illustrating the typical relationship between the output current and output voltage of one embodiment of a power supply in accordance with the teachings of the present invention. As can be seen in curve  400 , the power supply utilizing the invention exhibits an approximately constant output current and constant output voltage characteristic. That is, as output current increases, the output voltage remains approximately constant until the output current reaches an output current threshold. As the output current approaches the output current threshold, the output voltage decreases as the output current remains approximately constant over the drop in output voltage until a lower output voltage threshold is reached when the output current can reduce further as shown by the range of characteristics. It is appreciated that the constant output voltage and constant output current characteristics of the present invention are suitable for battery charger applications or the like. 
   In the foregoing detailed description, the method and apparatus of the present invention has been described with reference to specific exemplary embodiments thereof It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.

Technology Classification (CPC): 7