Abstract:
A power supply includes an energy transfer element, a switch, and a controller. The controller includes a modulator, a drive signal generator, a comparator, and a variable current limit generator. The modulator generates an enable signal having logic states responsive to a feedback signal. The drive signal generator either enables or skips enabling a switch of the power supply during a switching period in response to the logic state of the enable signal. The comparator asserts an over current signal to disable the switch if the switch current exceeds a variable current limit. The variable current limit generator sets the variable current limit to a first current limit in response to one logic state of the enable signal and sets the variable current limit to a second current limit if the enable signal transitions logic states and the over current signal is asserted during the switching period.

Description:
REFERENCE TO PRIOR APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/180,227, filed Jul. 11, 2011, which issued as U.S. Pat. No. 8,143,875 on Mar. 27, 2012, which is a continuation of U.S. application Ser. No. 12/488,427, filed Jun. 19, 2009, which is issued as U.S. Pat. No. 7,990,125 on Aug. 2, 2011, which is a continuation of U.S. application Ser. No. 12/052,609, filed Mar. 20, 2008, which is issued as U.S. Pat. No. 7,567,070 on Jul. 28, 2009, which is a continuation of U.S. application Ser. No. 11/732,209, filed Apr. 2, 2007, which issued as U.S. Pat. No. 7,359,225 on Apr. 15, 2008, which is a continuation of U.S. application Ser. No. 11/179,144, filed Jul. 11, 2005, which issued as U.S. Pat. No. 7,215,107 on May 8, 2007. U.S. patent application Ser. No. 13/180,227 and U.S. Pat. Nos. 7,990,125, 7,567,070, 7,359,225, 7,215,107 are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates generally to electronic circuits, and more specifically, the invention relates to switched mode power supplies. 
     2. Background Information 
     A typical requirement for power supplies of electronic equipment is that they limit their output power. One reason to limit output power is to meet the requirements of safety agencies for prevention of personal injury. Another reason to limit output power is to avoid damage to electronic components from an overload. 
     Power supplies typically have self-protection circuits that respond when an output becomes unregulated for a specified time. However, if output power is not limited, a fault at a load can consume enough power to cause damage or to exceed regulatory requirements while the outputs remain regulated. Thus, the self-protection feature can be ineffective if the power supply can deliver too much power. 
     A common way to limit output power of a switching power supply is to limit the current in a power switch at the input of the power supply. The maximum output power is related to the peak current in the switch. Inherent delays in the responses of electrical circuits create an error between the desired limit for peak current in the switch and the actual maximum peak current in the switch. The error is greater at higher input voltages, causing the maximum output power to be greater at higher input voltages than it is at lower input voltages. 
    
    
     
       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 functional block diagram of one embodiment of a switching power supply that may limit output power in accordance with the teaching of the present invention. 
         FIG. 2  is a graph of power capability for one embodiment of a switching power supply with respect to the peak current of the switch. 
         FIG. 3  shows waveforms of the current in the switch for one embodiment of a switching power supply in accordance with the teaching of the present invention. 
         FIG. 4  shows parameters of timing signals with parameters of the current in a switch of a power supply that may limit output power in accordance with the teaching of the present invention. 
         FIG. 5  is a flow diagram that illustrates a method to limit output power of a switching power supply in accordance with the teaching of the present invention. 
         FIG. 6  shows timing signals with waveforms of the current in a switch of a switching power supply to illustrate operation of one embodiment of the present invention. 
         FIG. 7  is a functional block diagram of one embodiment of the present invention that includes the power switch in an integrated circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a power supply regulator that may be utilized in a power supply 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. Well-known methods related to the implementation 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 “for 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. 
     As will be discussed, the power from a switching power supply may be limited according to embodiments of the present invention by limiting the current in a switch of the power supply. For one embodiment, a switch is coupled to an energy transfer element of a power supply with a controller generating a drive signal to control switching of the switch to regulate the output of the power supply. The controller includes a current limiter, which will adjust the drive signal to limit a current though the switch to a variable current limit value. For one embodiment, the current limiter based on the input line voltage of the power supply sets the variable current limit value. For example, the variable current limit may be to a nominal current limit value for nominal or a low input line voltage. If, however, the input line voltage is relatively high, then the variable current limit is set to a reduced current limit value in accordance with the teachings of the present invention. For one embodiment, the controller deduces the magnitude of the input voltage by measuring how long the current takes to go between two values and the variable current limit is then adjusted accordingly. 
     The variable current limit value for the switch is adjusted according to the input voltage of the power supply to compensate for a delay between the time when the current reaches the current limit and the time when the switch turns off. A lower value of current limit for higher input voltages prevents excess output power at high input voltage. For one embodiment, input voltage may be determined indirectly from a measurement of time to reach current limit from an initial value of zero current when the power supply operates in discontinuous conduction mode. In general, a design can deliver a required output power and also limit the maximum output power over the operating range of input voltage by compensating for the error between desired maximum peak current in the switch and the actual maximum peak current in the switch. 
     As will be discussed, a measurement of time is used to determine an appropriate adjustment of the desired maximum peak current in the switch to meet the requirements of the design. A current limit threshold for a switch is adjusted in response to a measurement of time during the conduction of the switch to compensate for the undesirable influence of input line voltage on the actual peak current in the switch. For instance, a relatively high input line voltage is indicated for an embodiment of the present invention if an over current condition is identified during a first switching cycle after a skipped switching cycle of the switch. 
     As will be discussed, it is likely that the power supply will operate in a discontinuous conduction mode of operation in the first switching cycle after a skipped switching cycle. In this situation, the energy in the energy transfer element typically goes to zero before the switch turns on in the next switching cycle. Therefore, if an over current condition occurs during this first switching cycle with the energy in the energy transfer element initially at zero at the beginning of the switching cycle, a high input line condition is indicated, and the variable current limit is set accordingly to the reduced value in accordance with the teachings of the present invention. If, on the other hand, an over current condition is not identified in the first switching cycle after a skipped switching cycle of the switch, then it is assumed that the input line voltage of the power supply is nominal or relatively low, and the variable current limit is set accordingly to the nominal value in accordance with the teachings of the present invention. 
     To illustrate,  FIG. 1  shows a functional block diagram of a power supply that may include an embodiment of a method that limits peak switch current in accordance with the teachings of the present invention. The topology of the power supply illustrated in  FIG. 1  is known as a flyback regulator. It is appreciated that there are many topologies and configurations of switching regulators, and that the flyback topology shown in  FIG. 1  is provided to illustrate the principles of an embodiment of the present invention that may apply also to other types of topologies in accordance with the teachings of the present invention. 
     As illustrated in the power supply example of  FIG. 1 , an energy transfer element T 1   125  is coupled between an unregulated input voltage V IN    105  and a load  165  at an output of the power supply. A switch S 1   120  is coupled to the primary winding  175  at an input of energy transfer element  125  to regulate the transfer of energy from the unregulated input voltage V IN    105  to the load  165  at the output of the power supply. A controller  145  is coupled to generate a drive signal  157  that is coupled to be received by the switch S 1   120  to control switching of switch S 1   120 . In the example of  FIG. 1 , the energy transfer element T 1   125  is illustrated as a transformer with two windings. A primary winding  175  has N P  turns with an inductance L P . A secondary winding has N S  turns. In general, the transformer can have more than two windings, with additional windings to provide power to additional loads, to provide bias voltages, or to sense the voltage at a load. 
     A clamp circuit  110  is coupled to the primary winding  175  of the energy transfer element T 1   125  to control the maximum voltage on the switch S 1   120 . In one embodiment, switch S 1   120  is a transistor such as for example a power metal oxide semiconductor field effect transistor (MOSFET). In one embodiment, controller  145  includes integrated circuits and discrete electrical components. The operation of switch S 1   120  produces pulsating current in the rectifier D 1   130  that is filtered by capacitor C 1   135  to produce a substantially constant output voltage V O  or a substantially constant output current I O  at the load  165 . 
     The output quantity to be regulated is U O    150 , that in general could be an output voltage V O , an output current I O , or a combination of the two. A feedback circuit  160  is coupled to the output quantity U O    150  to produce a feedback signal U FB    155  that is an input to the controller  145 . Controller  145  also includes a current sensor coupled to receive current sense  140  that senses a current I D    115  in switch S 1   120 . Any of the many known ways to measure a switched current, such as for example a current transformer, or for example the voltage across a discrete resistor, or for example the voltage across a transistor when the transistor is conducting, may be used to measure current I D    115 . The controller may use current sense signal  140  to regulate the output U O    150  or to prevent damage to the switch S 1   120 . 
       FIG. 1  also shows an example waveform for current I D    115 . During any switching period T S    190 , switch S 1   120  may conduct in response to drive signal  157  from controller  145  to regulate the output U O    150 . When current I D    115  reaches a current limit value I PEAK    195  after a time t ON    180  from the beginning of the switching period T S    190 , switch S 1   120  turns off and stays off for a time t OFF    185 , which is the remainder of the switching period T S    190 . The current waveform shows two fundamental modes of operation. The trapezoidal shape  170  is characteristic of continuous conduction mode (CCM) whereas the triangular shape  175  is characteristic of discontinuous conduction mode (DCM). 
       FIG. 2  shows how the peak current I PEAK    190  is related to the maximum output power of the power supply in  FIG. 1 . In DCM, the output power increases as the square of I PEAK . In CCM, the output power increases linearly with I PEAK . The current limit value I PEAK  is used to help limit the output power of the power supply. A difficulty in limiting the current limit value I PEAK  is that there is always a delay between the time when the current reaches the limit and the time the switch turns off. 
     To illustrate,  FIG. 3  shows how a delay influences peak current in the switch. In the example illustrated in  FIG. 3 , I PMAX  is the maximum desired value for I D . A controller having a current limit threshold I LIMIT1  that is the same value of I PMAX  takes action to turn off the switch when I D  exceeds I LIMIT1 . The unavoidable time delay t d  allows I D  to exceed I LIMIT1  by an amount ΔI DELAY  that depends on the delay t d  and on how fast I D  is changing after it passes I LIMIT1 . A current limit I LIMIT1  produces a peak current I PEAK1  that is greater than the desired I PMAX . If the delay t d  and the rate of change of I D  are known, the current limit can be compensated to a lower value I LIMIT2  such that addition of ΔI DELAY  will give a peak current I PEAK2  that is less than I PMAX . 
     A complication in the use of a lower current limit value to compensate for the delay is that in general ΔI DELAY  will be larger at higher input voltages than at lower input voltages because I D  increases at a greater rate when the input voltage is high. Therefore, a power supply that uses a single compensated current limit I LIMIT2  to limit maximum output power to the desired value at a high input voltage would have less than the desired maximum output power at low input voltage. Indeed, if the circuit to limit the power has only one desired limit for peak current such as I PEAK2 , a design that meets the requirement for maximum power at high input voltage may be unable to deliver the required power at low input voltage. 
     For one embodiment, a power supply may use a first compensated current limit I LIMIT1  at a low input voltage and a second compensated current limit I LIMIT2  at high input voltage to limit the maximum output power to a desired value over a wide range of input voltages in accordance with the teachings of the present invention. 
     To illustrate,  FIG. 4  shows example timing signals that are used with the current I D  for one embodiment of the invention to determine whether the current limit will be I LIMIT1  or I LIMIT2 . In particular,  FIG. 4  shows two full switching periods, T 1  and T 2  of switch current I D  with timing signals I LIM , I LIMMAX , and D MAX . In  FIG. 4 , current limit signal I LIM  is high whenever I D  is greater than the current limit. Signal I LIMMAX  is a timing reference that is compared to current signal I LIM  to determine whether the current limit will be I LIMIT1  or I LIMIT2 . Signal D MAX  sets the maximum on time of the switch. The switch is forced off when D MAX  is high. 
       FIG. 5  is a flow diagram for one embodiment of a power supply controller that uses the timing signals of  FIG. 4  in accordance with the teachings of the present invention. The flow starts at Block  505  when the switch is off. Block  510  sets a nominal current limit, which for one embodiment corresponds to I LIMIT1  in  FIG. 4 , and is more suitable for a nominal or low input voltage. Block  515  interprets feedback signal U FB  to determine whether the switch should turn on or remain off in the next switching period. If the switch is enabled, then Block  520  directs the switch to turn on in Block  525 . If the switch is not enabled, then Block  520  directs the switch to be off in Block  545 . 
     Once the switch is turned on, the state of the current limit signal I LIM  is evaluated in Block  535 . The on time of the switch is compared to the maximum permissible on time in Block  540 . Block  545  turns off the switch immediately if I LIM  is high or if the on time exceeds the maximum on time t DMAX . After the switch turns off, Block  550  directs the flow depending on whether the mode of operation was CCM or DCM when the switch turned on. The mode is DCM if the energy in the energy transfer element goes to zero before the switch turns on. In one embodiment, a single switching period with the switch disabled is sufficient to reduce the energy to zero. Therefore, in one embodiment, Block  550  has a memory of whether or not the switch was enabled during a previous switching period to determine the mode of operation at the start of the present switching period. 
     If the mode of operation was not DCM, the controller continues with the interpretation of the feedback signal in Block  515 . If the mode of operation was DCM, the flow is diverted to Block  555 . Block  555  compares the time to reach current limit against the reference time t LIMMAX . Although delays in practical circuits prevent exact measurement of the time t LIM  to reach the current limit, it is sufficient to measure a signal that includes the delays for an approximate measurement of t LIM . For one embodiment, the sum of t LIM  and delay t d , which is the on time t ON  in  FIG. 3 , is measured in Block  555  as an approximation to t LIM  for comparison against the reference time t LIMMAX . When the operation is in DCM, the current can reach current limit in less time than t LIMMAX  only if the input voltage is high in accordance with the teachings of the present invention. 
     If the time to reach current limit is less than t LIMMAX , the controller sets a reduced current limit in Block  530 . The reduced current limit for a high input voltage corresponds to I LIMIT2  in  FIG. 4 . If the time to reach current limit is not less than the reference time t LIMMAX , then the controller sets the nominal current limit in Block  510 . The latter condition is also true when the switch turns off before the current reaches current limit, causing the controller to set the nominal current limit in Block  510 . 
       FIG. 6  shows several switching periods that illustrate operation according to the flow diagram of  FIG. 5 . In Period  1 , the switch operates at a high input voltage when the current limit has been set at the nominal value I LIMIT1  that is appropriate for a nominal or low input voltage. The surplus energy from the high peak current at the high input voltage causes the controller to disable the switch in Period  2 . The controller detects a high input voltage condition from the short time to reach current limit in Period  3 , and sets the reduced current limit I LIMIT2  in Period  4 . The operation continues with the reduced current limit until the controller detects a period of DCM operation where the time to reach current limit is not less than the reference time t LIMMAX . In Period n, the switch is disabled and the input voltage is low. The controller has determined that the time to reach current limit in a period of DCM was not less than the reference time t LIMMAX . Consequently, the controller sets the current limit to the nominal value I LIMMAX  in Period (n+1). The current does not reach current limit in Period (n+1) so the switch is turned off by maximum on time and the current limit remains at I LIMIT1 . The power supply operates in CCM at low input voltage and current limit I LIMIT1  in Period (n+2) and Period (n+3). 
       FIG. 7  shows one embodiment that includes a power switch  736  in an integrated circuit  700 . Power switch  736  is a MOSFET that conducts current between a drain terminal  702  and a source terminal  758 . Circuits internal to the integrated circuit are powered from an internal voltage V CC    705  that is referenced to source terminal  758 . For one embodiment, drain terminal  702  provides internal voltage V CC    705 . Internal voltage V CC  may be provided from drain terminal  702  or from a different terminal of the integrated circuit by several techniques that are known to one skilled in the art. 
     A feedback terminal  754  receives a feedback signal U FB . A modulator  752  interprets the feedback signal U FB  to set an enable signal  744  high or low. An oscillator  756  provides a clock signal  748  and a D MAX  signal  746  to determine respectively the length of a switching period and the maximum on time of the switch  736 . Switch  736  may be on while D MAX    746  is low. Switch  736  is off while D MAX    746  is high. AND gate  740  sets latch  738  to turn on switch  736  with drive signal  757  at the beginning of a switching period if the enable signal  744  is high. OR gate  742  resets latch  738  to turn off switch  736  with drive signal  757  if switch current I D    706  exceeds the current limit or if signal D MAX    746  goes high. 
     Switch current I D    706  is sensed as a voltage V D  that is compared to a current limit voltage V LIMIT  by a comparator  704 . Resistor  732  with current sources  728  and  730  generates current limit voltage V LIMIT . Current source  730  is switched on and off by p-channel transistor  724 . In one embodiment, current source  730  is one-tenth the value of current source  728 . Thus, the current limit voltage V LIMIT  increases by 10 per cent to make a nominal current limit 10 per cent higher than a reduced current limit when current source  730  is switched on. 
     The drive signal  757  that is output by latch  738  is delayed by leading edge blanking time t LEB  delay  734  before being received by AND gate  708 . AND gate  708  receives the output of current limit comparator  704  and the output of leading edge blanking time delay  734  to provide an over current signal  760 . Leading edge blanking time t LEB  delay  734  is long enough to allow switch  736  to discharge stray capacitance on drain terminal  702 . Discharge of stray capacitance at drain terminal  702  can produce a high drain current I D    706  that temporarily exceeds the current limit, but is unrelated to the output of the power supply. The leading edge blanking time t LEB  delay  734  prevents the over current signal  760  from going high during a time t LEB  after switch  736  turns on. Over current signal  760  in  FIG. 7  corresponds to signal I LIM  in  FIG. 4  or  FIG. 6 . 
     Flip-flop  750  remembers the state of enable signal  744  at the beginning of the switching period. Flip-flop  750  is clocked at the start of every switching period by the complement of D MAX  signal  746  from inverter  720 . A change in the state of the clocked enable signal  745  from one switching period to the next switching period is detected by XOR gate  716 . 
     XOR gate  716  with delay  718  at one input receives the clocked enable signal  745  to set latch  714  whenever there is a change in the clocked enable signal  745 . Delay  718  is long enough to produce an output that sets latch  714 . In one embodiment, delay  718  is ten nanoseconds. Latch  714  is set at the beginning of a switching period whenever there has been a change in the state of the clocked enable signal  745  from the previous switching period. 
     Latch  726  is allowed to set if enable signal  744  is high at the beginning of the current switching period. Inverter  722  resets latch  726  if enable signal  744  is low at the beginning of the current switching period. 
     Latch  714  is set to indicate DCM operation in the present switching period. DCM is indicated when the output of latch  714  is high. Latch  726  is set to reduce the current limit. 
     In the embodiment of  FIG. 7 , the maximum on time signal D MAX    746  is also the timing reference that is compared to over current signal L LIM    760  to determine whether the current limit will be I LIMIT1  or I LIMIT2 . In the embodiment of  FIG. 7 , t LIMMAX =t DMAX , representing an embodiment where the signals I LIMMAX  and D MAX  of  FIG. 4  are identical. For another embodiment, however, it is appreciated that t LIMMAX  does not necessarily have to equal t DMAX  in accordance with the teachings of the present invention, such as the example illustrated in  FIG. 4 . Latch  726  will not be set if there is no over current condition detected or the current limit is not reached during the time when D MAX    746  is low. Thus, current source  730  remains switched on by transistor  724  if the over current condition is not detected in accordance with the teachings of the present invention. 
     It is appreciated that although  FIG. 7  illustrates an integrated circuit  700  for an example of the present invention that employs a switching regulator that may skip switching cycles of power switch  736  in response to enable signal  744 , other examples of integrated circuits may also be covered in accordance with the teachings of the present invention. For example, a pulse width modulated (PWM) regulator circuit may also be covered in accordance with the teachings of the present invention. For instance, an example PWM controller deduces the magnitude of the input voltage by measuring how long the current takes to go between two values and then adjusts the variable current limit in accordance with the teachings of the present invention. 
     In the foregoing detailed description, the methods and apparatuses of the present invention have been described with reference to a specific exemplary embodiment 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.