Abstract:
A power converter includes an input and an output. A regulation circuit is coupled between the power converter input and the power converter output. The regulation circuit is coupled to receive a feedback signal representative of the power converter output. The feedback signal has a first feedback state that represents a level at the power converter output that is above a threshold level and a second feedback state that represents a level at the power converter output that is below the threshold level. An oscillator is included in the regulation circuit that provides an oscillation signal that cycles between two states. The regulation circuit is coupled to be responsive to the oscillation signal and to a change between the first and second feedback states to enable or disable a flow of energy from the power converter input to the power converter output.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. application Ser. No. 13/117,982, filed May 27, 2011, now pending, which is a continuation of and claims priority to U.S. application Ser. No. 12/330,277, filed Dec. 8, 2008, now U.S. Pat. No. 7,974,112 B2, which is a continuation of U.S. application Ser. No. 11/824,425, filed Jun. 28, 2007, now U.S. Pat. No. 7,477,534, which is a continuation of U.S. application Ser. No. 11/389,184, filed Mar. 24, 2006, now U.S. Pat. No. 7,248,029 B2, which is a continuation of U.S. application Ser. No. 11/066,026, filed Feb. 25, 2005, now U.S. Pat. No. 7,038,439 B2, which is a continuation of U.S. application Ser. No. 10/805,661, filed Mar. 18, 2004, now U.S. Pat. No. 6,876,181 B1, which is a continuation of U.S. application Ser. No. 10/438,207, filed May 13, 2003, now U.S. Pat. No. 6,747,444 B2, which is a continuation of U.S. application Ser. No. 10/092,705, filed Mar. 6, 2002, now U.S. Pat. No. 6,608,471 B2, which is continuation of U.S. application Ser. No. 09/927,273, filed Aug. 10, 2001, now U.S. Pat. No. 6,414,471 B1, which is a continuation of U.S. application Ser. No. 09/630,477, filed Aug. 2, 2000, now U.S. Pat. No. 6,297,623 B1, which is a divisional of U.S. application Ser. No. 09/032,520, filed Feb. 27, 1998, now U.S. Pat. No. 6,226,190 B1. U.S. application Ser. No. 13/117,982 and U.S. Pat. Nos. 7,974,112 B2, 7,477,534, 7,248,029 B2, 7,038,439 B2, 6,876,181 B1, 6,747,444 B2, 6,608,471 B2, 6,414,471 B1, 6,297,623 B1 and 6,226,190 B1 are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present inventions pertain to the field of power supplies, and among other things to the regulation of power supplies. 
         [0004]    2. Background 
         [0005]    Accurate regulation of power supplies is important in many areas. For instance in sensitive electronic devices such as computers and televisions maintaining a constant power supply is important for the operation of the computer or television. Additionally, the advantages of accurate power supply regulation include reduced overall power consumption and reduced damage to equipment by preventing voltage spikes during start up and operation. 
         [0006]    Power supplies are regulated by keeping either a current or voltage delivered to a load within a specified range. A power supply is deemed to be in regulation if the load current or voltage is within the specified range and is deemed to be out of regulation if the load current or voltage is outside the specified range. 
         [0007]    Problems associated with out of regulation conditions include damage to the load, improper load functioning, and the consumption of power when no power is necessary to operate the load. Therefore, power supplies that regulate output power provided to the load are desired. 
         [0008]    A known regulated power supply is depicted in  FIG. 1 . The regulated power supply of  FIG. 1  includes an EMI filter  10  that receives an AC mains voltage. The output of the EMI filter  10  is coupled to rectifier  15  that rectifies the AC mains voltage and then provides the rectified voltage to capacitor  20 . Capacitor  20  provides a substantially DC voltage to a primary winding  25  of transformer  30 . 
         [0009]    A monolithic power supply control chip  40  includes a MOSFET  45  that is controlled by pulse width modulator  50 . When MOSFET  45  is conducting, primary winding  25  has current flowing through it allowing transformer  30  to store energy. When MOSFET  45  is not conducting, the energy stored in the transformer  30  induces a voltage across the secondary winding  55  which is transferred to a load  60  connected at output terminals  65 . A capacitor  70  is coupled to secondary winding  55  in order to maintain the voltage that is being supplied to the load  60  when MOSFET  45  is on. 
         [0010]    A feedback circuit  75  is coupled to the load  60 . The feedback circuit  75  includes a resistor  80 , zener diode  85  and an optocoupler  90 . A bias winding  95  is magnetically coupled to primary winding  25  and is used to supply power to the output of the optocoupler  90 . When the voltage at load  60  is above combination of the reverse bias voltage of zener diode  85  and the forward voltage drop of light emitting diode  100 , a current is generated in the phototransistor  105  by light emitting diode  100 . The phototransistor  105  current flows from the bias winding  95  to the control terminal  110  of monolithic power supply control chip  40 . The current provided to the control terminal  110  of monolithic power supply control chip  40  controls the duty cycle of MOSFET  45 . When the control terminal  110  current increases the duty cycle of MOSFET  45  decreases and the amount of current through primary winding  25  decreases. Therefore, the power provided to the load  60  decreases. As the power supplied to the load  60  decreases, the load voltage decreases which in turn reduces the optocoupler  90  current increasing the duty cycle of MOSFET  45 . Thus, the output voltage is regulated at a voltage equal to zener  85  reverse breakdown voltage plus the forward drop of LED  100  in an analog closed loop. Resistor  80  controls the gain of the analog loop. 
         [0011]    It should be noted that pulse width modulator  50  is switching at some duty cycle to provide power to the feedback circuit  75  even when there is no load connected to the output terminals  65 . This will cause power consumption from switching losses occurring at the operating frequency of the MOSFET  45 . 
         [0012]    The regulated power supply of  FIG. 1  is able to maintain the voltage at the load at a reasonably constant level, while reducing voltage transients due to load and line variations. 
         [0013]    However, the addition of a feedback winding and pulse width modulation controller makes application of the regulated power supply of  FIG. 1  expensive for many power suppliers operating at low powers, especially those below five (5) watts. Additionally, the use of analog pulse width modulation feedback control requires compensation circuitry to stabilize the circuit and to prevent oscillations. The compensation circuit limits the bandwidth of the control loop to one (1) or two (2) kilohertz. The Pulse Width Modulated feedback circuit while effective at regulating the voltage still has time periods when the voltage is above and below the desired level, because of the limited bandwidth of the feedback loop which is in the range of one (1) or two (2) kilohertz even though the switching frequency of the MOSFET  45  may be as high as one hundred (100) kilohertz. 
         [0014]    It is therefore desired to create a power supply that is cost effective for low power solutions. 
         [0015]    It is further desired to create a power supply that utilizes the minimum amount of components possible. 
         [0016]    It is additionally desired to create a power supply that can respond quickly to load transients without losing output regulation. 
       SUMMARY OF THE INVENTION 
       [0017]    A presently preferred DC to DC converter comprises an energy storage element that receives a first power level and that provides a second power level, a feedback circuit coupled to the energy storage element, and a regulator circuit coupled to the feedback circuit and to the energy storage element. When a feedback signal is above a threshold the regulator circuit is disabled and when the feedback signal is below said threshold level the regulator circuit is enabled. 
         [0018]    In another embodiment a power supply comprises a transforming element that transfers energy and is coupled to receive a first power level and a regulator circuit coupled to the transforming element. The regulator circuit controlling input of the first power level to the transforming element. When an output voltage or current of the transforming element is above a threshold level the regulator circuit is disabled and when output voltage or current of the transforming element is below a threshold level the regulator circuit operates. 
         [0019]    In yet another embodiment a regulator circuit comprises a feedback input, a switch operating when a control signal is received at its control terminal, an oscillator that provides a duty cycle signal comprising a high state and a low state. The control signal is provided when no feedback signal is provided and the duty cycle signal is in said high state. 
         [0020]    In a further embodiment a power supply comprises an energy storage element coupled to receive a first power level and a regulation circuit coupled between the energy storage element and a source of the first power level. The regulation circuit prevents the energy storage element from receiving the first power level when a current or voltage at the input of the energy storage element is at or above a predetermined threshold level. 
         [0021]    In an additional embodiment a power supply comprises a transforming element coupled to receive a first power level and a regulation circuit coupled between the transforming element and a source of the first power level. The regulation circuit prevents the transforming element from receiving the first power level when a current or voltage at the input of the transforming element is at or above a predetermined threshold level. 
         [0022]    It is an object of an aspect of the present inventions to create a power supply that is accurately regulated with a minimum amount of time spent out of regulation. 
         [0023]    It is another object of an aspect of the present inventions to create a power supply that is cost effective for low power solutions. 
         [0024]    It is a further object of the present inventions to create a power supply that utilizes the minimum amount of components possible. 
         [0025]    It is also an object of the present inventions to create a power supply that is low cost. 
         [0026]    This and other objects and aspects of the present inventions are taught, depicted and described in the drawings and the description of the invention contained herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  depicts a known regulated power supply. 
           [0028]      FIG. 2  is a presently preferred regulated DC to DC power supply according the present inventions. 
           [0029]      FIG. 3  is a presently preferred power supply according to the present inventions. 
           [0030]      FIG. 4  is an alternate presently preferred power supply according to the present inventions. 
           [0031]      FIG. 5  is a diagram of the presently preferred regulator circuit switch current presently preferred power supplies of  FIG. 2 ,  3  or  4  according to the present inventions. 
           [0032]      FIG. 6  is a functional block diagram of a presently preferred power supply regulation circuit according to the present inventions. 
           [0033]      FIG. 7  is a block diagram of a presently preferred bypass voltage regulation circuit according to the present inventions. 
           [0034]      FIG. 8  is a diagram of the presently preferred bypass terminal voltage and maximum duty cycle signal according to the present inventions. 
           [0035]      FIG. 9  is a block diagram of a presently preferred circuit allowing for increasing the clock frequency of the oscillator according to the present inventions. 
           [0036]      FIG. 10  is a block diagram of a presently preferred circuit allowing for stoppage of the oscillator according to the present inventions. 
           [0037]      FIG. 11  is a diagram of the presently preferred enable signal and the saw tooth waveform according to the present inventions. 
           [0038]      FIG. 12  is a diagram of the signals generated in a presently preferred mode of operation within the presently preferred power supply regulation circuit of  FIG. 7  according the present inventions. 
           [0039]      FIG. 13  is a diagram of the signals generated in a presently preferred mode of operation within the presently preferred power supply regulation circuit of  FIG. 9  according the present inventions. 
           [0040]      FIG. 14  is a diagram of the signals generated in an alternate preferred mode of operation within the presently preferred power supply regulation circuit of  FIG. 10  according the present inventions. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Referring to  FIG. 2 , a DC to DC converter  200  receives a first DC voltage  210  having a first magnitude. The first DC voltage  210  is converted to a second DC voltage  220  that has a second magnitude by energy storage element  205 . Although, the presently preferred DC to DC converter utilizes an energy storage element, other elements may be used by the present invention without departing from the scope and spirit of the present invention. For instance a transforming element, may be used as well. The second DC voltage  220  is provided to a load  230  to supply power to the load  230 . It is presently preferred that the second voltage level is below the first voltage level and that DC to DC converter  200  is a step down converter. In operation the presently preferred regulation circuit  240  operates at fixed frequency, allowing current to be provided into the energy storage element input  250  for a same time period in-each cycle of the operating frequency. The output of feedback circuit  260  is utilized to enable or disable operation of the regulation circuit  240 . The magnitude of second DC voltage  220  will vary depending on the ratio of the enable time to the disable time, i.e. the larger the ratio the greater the magnitude of second DC voltage  220 . 
         [0042]    To maintain second DC voltage  220  at a regulated level feedback circuit  260  is coupled to the positive terminal  270  of the load  230 . A presently preferred feedback circuit  260  includes an optocoupler  280  and a zener diode  290 . Feedback circuit  260  will trigger when the second DC voltage  220  is above a threshold level which is presently preferred to be a combination of the voltage drop across the light emitting diode  300  of optocoupler  280  (preferably one volt) and the reverse break down voltage of zener diode  290 . Upon triggering feedback circuit  260  will activate phototransistor  310  of the optocoupler  280 . The activation of phototransistor  310  causes a current to flow into the feedback terminal  320 . The current input into feedback terminal  320  is utilized to disable regulation circuit  240 . Disabling regulation circuit  240  prevents switching current at the operating frequency from flowing to energy storage element input  250  and prevents power from being supplied to the load  230 . When regulation circuit  240  is not conducting a current source  330  is triggered within the regulation circuit  240 . The current source  330  allows a small current to flow through a bypass terminal  340  of regulation circuit  240  to charge regulation circuit power supply bypass capacitor  350 . Regulation circuit power supply bypass capacitor  350  is used to supply power to operate regulation circuit  240  when it is conducting. In this way when the second DC voltage  220  is above the desired threshold level virtually no power is supplied to the load and a minimum amount of power is being consumed by the DC to DC converter  200 . 
         [0043]    It is presently preferred that at the moment when second DC voltage  220  reaches a level below the threshold level, phototransistor  310  will cease conducting. When the phototransistor  310  is not conducting, no current flows into feedback terminal  320  and regulation circuit  240  is enabled. When the regulation circuit  240  is enabled a switching current at the operating frequency is supplied to the energy storage element input  250 . 
         [0044]    It is presently preferred, that the threshold level is equal to the regulated value of the output voltage, e.g. the second DC voltage  220 . Alternatively, the output current can also be regulated by utilizing a current threshold. Referring to  FIG. 3 , a power supply  400  comprises a bridge rectifier  410  that rectifies an input AC mains voltage. Power supply capacitors  420  charge with the rectified AC mains voltage to maintain an input DC voltage  430 . A presently preferred range for input DC voltage  430  is approximately one hundred (100) to four hundred (400) volts to allow for operation based upon worldwide mains voltages which range between eighty five (85) and two hundred sixty five (265) volts. The presently preferred power supply  400  also includes harmonic filter components  440  which in combination with capacitors  420  reduce the harmonic current injected back into the power grid. Transformer  450  includes a primary winding  460  magnetically coupled to secondary winding  470 . The secondary winding  470  is coupled to a diode  480  that is designed to prevent current flow in the secondary winding  470  when the regulation circuit  240  is conducting (on-state). A capacitor  485  is coupled to the diode in order to maintain a continuous voltage on a load  490  which has a feedback circuit  260  coupled to it. A presently preferred feedback circuit  260  comprises an optocoupler  280  and zener diode  520 . The output of optocoupler  280  is coupled to the feedback terminal  320  of regulation circuit  240 . The presently preferred regulation circuit  240  switches on and off at a duty cycle that is constant at a given input DC voltage  430 . A regulation circuit power supply bypass capacitor  350  is coupled to and supplies power to regulation circuit  240  when the regulation circuit  240  is in the on-state. 
         [0045]    Operation of the power supply  400  will now be described. An AC mains voltage is input into bridge rectifier  410  which provides a rectified signal to power supply capacitors  420  that provide input DC voltage  430  to primary winding  460 . Regulation circuit  240 , which preferably operates at a constant frequency and about constant duty cycle at a given input DC voltage  430 , allows current to flow through primary winding  460  during its on state of each switching cycle and acts as open circuit when in its off state. When current flows through primary winding  460  transformer  450  is storing energy, when no current is flowing through primary winding  460  any energy stored in transformer  450  is delivered to secondary winding  470 . Secondary winding  470  provides then provides the energy to capacitor  485 . Capacitor  485  delivers power to the load  490 . The voltage across the load  490  will vary depending on the amount of energy stored in the transformer  450  in each switching cycle which is turn dependent on the length of time current is flowing through primary winding  460  in each switching cycle which is presently preferred to be constant at a given input DC voltage  430 . The presently preferred regulation circuit  240  allows the voltage delivered to the load to be maintained at a constant level. 
         [0046]    It is presently preferred that the sum of the voltage drop across optocoupler  280  and the reverse break down voltage of zener diode  520  is approximately equal to the desired threshold level. When the voltage across the load  490  reaches the threshold level, current begins to flow through the optocoupler  280  and zener diode  520  that in turn is used to disable the regulation circuit  240 . Whenever regulation circuit  240  is in the off-state the regulation circuit power supply bypass capacitor  350  is charged to the operating supply voltage, which is presently preferred to be five point seven (5.7) volts by allowing a small current to flow from bypass terminal  340  to the regulation circuit power supply bypass capacitor  350 . Regulation circuit power supply bypass capacitor  350  is used to supply power to operate regulation circuit  240  when it is in the on-state. 
         [0047]    When the regulation circuit  240  is disabled, an open circuit condition is created in primary winding  460  and transformer  450  does not store energy. The energy stored in the transformer  450  from the last cycle of regulation circuit  240  is then delivered to secondary winding  470  which in turn supplies power to the load  490 . Once the remaining energy in transformer  450  is delivered to the load  490  the voltage of the load  490  will decrease. When the voltage at the load  490  decreases below the threshold level, current ceases to flow through optocoupler  280  and regulation circuit  240  resumes operation either instantaneously or nearly instantaneously. 
         [0048]    The presently preferred regulation circuit  240  has a current limit feature. The current limit turns off the regulation circuit  240 , when the current flowing through the regulation circuit  240  rises above a current threshold level. In this way regulation circuit  240  can react quickly to changes such as AC ripple that occur in the rectified AC mains voltage, and prevents the propagation of the voltage changes to the load. The current limit increases the responsiveness of the regulation circuit to input voltage changes and delivers constant power output independent for the AC mains input voltage. 
         [0049]    Although the presently preferred power supplies of  FIGS. 2 &amp; 3  utilize current mode regulation and a feedback circuit that includes an optocoupler and zener diode, the present invention is not to be construed as to be limited to such a feedback method or circuit. Either current or voltage mode regulation may be utilized by the present invention without departing from the spirit and scope of the present invention so long as a signal indicative of the power supplied to the load is supplied to the feedback terminal  320  of the regulation circuit  240 . Additionally, although the presently preferred power supplies both utilize an optocoupler and zener diode as part of feedback circuits other feedback circuits may be utilized by the present invention without departing from the spirit and scope of the present invention. 
         [0050]    Advantages associated with the power supplies depicted in  FIGS. 2 and 3  include a “digital” on and off for the power supply making the regulation of the power supply extremely fast. Further, unlike known pulse width modulated regulated power supplies, no compensation of the regulation loop is required. Additionally, in known analog pulse width modulated control the bandwidth, which is usually one to two kilohertz, is less than its switching frequency. The bandwidth of the presently preferred regulation circuit  240  is capable of operating at its switching frequency. The presently preferred switching frequency is between forty (40) and fifty (50) kilohertz. Also, since there is no compensation loop or bias winding the cost of the power supply is reduced below the cost of known pulse width modulation regulated power supplies and 50/60 Hz transformers utilized in linear regulation solutions 
         [0051]    Referring to  FIG. 4 , a presently preferred low power supply  600  produces an output power preferably ranging between zero (0) and one (1) watt, but can also be used with higher power levels without departing from the scope and spirit of the present invention. Bridge rectifier  610  Us receives the AC mains voltage. Power supply capacitors  615  take the rectified voltage and then generate a DC voltage  620  that is supplied to primary winding  630  of transformer  640  and is then supplied to secondary winding  650 . The secondary winding  650  provides power to capacitor  660  that supplies power to load  670 . Load  670  has a zener diode  680  coupled in parallel with it. A regulation circuit  240  is coupled in series with primary winding  630 , so that when regulation circuit  240  is conducting, on-state, current flows through primary winding  630  and when regulation circuit  240  is not conducting, off-state, current does not flow through primary winding  630 . In the on-state power is supplied to regulation circuit  240  by regulation circuit power supply bypass capacitor  350 . 
         [0052]    Operation of the low power supply  600  of  FIG. 4  will now be described. The AC mains voltage input into bridge rectifier  610  is rectified and the rectified voltage is supplied to power supply capacitors  615  that provide DC voltage  620 . The DC voltage  620  is then provided to primary winding  630  that is in series with regulation circuit  240 . Regulation circuit  240  preferably operates at a peak current limited duty cycle at a constant frequency and delivers power to the primary winding  630 . At the beginning of each cycle when regulation circuit  240  is in the on-state the current through it ramps up at a rate determined by the inductance of primary winding  630  and the input DC voltage  620 . When the current reaches the current limit regulation circuit  240  goes into the off-state. When current flows through the primary winding  630  energy is stored by transformer  640  and when no current flows through primary winding  630  energy is delivered to load  670 . A constant power is delivered by the secondary winding  640  to the zener diode  680  and the load  670 . As long as the load  670  consumes less power than delivered by the secondary winding  640  at the zener diode  680  reverse break down voltage, part of the power is consumed by the zener diode  680  and the output voltage is regulated at the reverse break down voltage. 
         [0053]    Referring to  FIG. 5 , a current limit  710  (I) is designed into the regulation circuit  240  for faster response. The current flowing through primary winding  460  or  630  will rise to the level of current limit  710  and then cease to flow. A high input voltage current  720  rises at a first rate, while a low input voltage current  730  rises at a second rate. The second rate is lower than first rate, but both currents reach the current threshold limit  710  (I) although at different times. The rate of rise of the current is a function of the inductance of primary winding (L) and magnitude of the input voltage. The power supplied to the load is proportional to the area under the curves of the current multiplied by the input voltage, which is constant. Since the primary winding current is limited at the current limit  710  (I) the power supplied to the load, can be expressed as in Equation 1 below: 
         [0054]    Based upon Equation 1 the power supplied to the primary winding by high input voltage current  720  and low input voltage current  730  will be the same, assuming the same regulation circuit  240  is operating with the same current threshold limit  710  and at the same frequency (.function.). This is true regardless of the rate of rise of the primary winding current. This means that the power supplied to the load in the power supply of  FIG. 3  or  FIG. 4  will be constant and independent of the DC input voltage  430  or  620 . This means that the power supplied to the load is independent of the AC Mains voltage. Thus, a constant power is delivered utilizing the presently preferred ted regulation circuit  240   
         [0055]    The power supplied to the load is a function of the current limit  710  (I), frequency of operation (.function.) and the inductance of the primary winding (L). Since the inductance of the primary winding and current limit are determined by the circuit designer in designing the power supply, the designer can design in the power delivered to the load easily and effectively by utilizing the presently preferred regulation circuit  240 . 
         [0056]    It should be noted that the above discussion assumes, as is presently preferred, that the inductance of the primary winding is chosen such that the all of the energy input into the transformer is delivered in each cycle of operation. As a result, the presently preferred primary winding current begins at zero at the start of each cycle of operation. However, the present invention will still deliver power if the inductance of the primary winding is chosen such that not all of the energy input into the transformer is delivered in each cycle of operation and the primary winding current begins at a non-zero value at the start of each cycle of operation. 
         [0057]    Referring to  FIG. 6 , a presently preferred regulation circuit  240  comprises a MOSFET  800  that is coupled between a drain terminal  804  and a source terminal  806 . MOSFET  800  is switched on and off according to a drive signal  850  input into its gate by first and-gate  810 . The input of first and-gate  810  comprises an output of first latch  820 , a bypass terminal voltage indicator signal  845  provided by undervoltage comparator  860 , and a thermal status signal  870  from thermal shut down circuit  880 . Maximum duty cycle signal  830  determines the maximum time that MOSFET  800  can conduct in each cycle of operation. 
         [0058]    Thermal shut down circuit  880  monitors the temperature of the primary winding by monitoring the temperature of regulation circuit  240  and provides the thermal status signal  870  as long as the temperature is below a threshold temperature. It is presently preferred that the threshold temperature is 135 degrees Celsius. 
         [0059]    The inputs to latch  820  include an or-gate output signal  900  and and-gate output signal  910 . The and-gate output signal is provided when no phototransistor  310  current is provided to feedback input  320 . Feedback gate  920  provides output when enable signal  905  is received and clock signal  930  is provided by oscillator  840 . Additionally, first current source  940  will pull enable signal  905  to a logic high state when the current present in the phototransistor  310  is less than the current source  940  current. In operation when enable signal  905  is high, the clock signal  930  is transferred to latch  820  by the and-gate  920 , thereby setting the latch  820  and enabling that cycle to go through and turn on the MOSFET  800 . Conversely, when the enable signal  905  is low, it blocks the clock signal from setting the latch  820 , and keeps the MOSFET  800  off during that cycle. 
         [0060]    Or-gate output signal  900  is provided by or-gate  945  when the current threshold limit  710  is reached or during the time when maximum duty cycle signal  830  is in an off state. In operation or-gate output signal  900  will be provided when the maximum duty cycle signal is off or when the current limit  710  is reached in order to turn off the MOSFET  800 . 
         [0061]    Current threshold limit monitoring is performed by current threshold comparator  950  that compares the voltage level across the MOSFET  800  on-resistance, if that voltage is above the current threshold limit voltage  960  the current limit signal is triggered and the MOSFET  800  is turned off and then will not begin conducting until the beginning of the next on-time when no current limit signal is provided. 
         [0062]    In this way the presently preferred regulator circuit  240  turns off the MOSFET  800  after the current on cycle when the phototransistor  310  pulls the enable signal  905  low and creates a condition where there will be no additional power supplied to the load. When the phototransistor  310  current falls below the first current source  940  current, enable signal  905  is high due to the operation of current source  940  and MOSFET  800  will resume operation upon the beginning of the next on-period of the maximum duty cycle signal  830 . 
         [0063]    Bypass circuit  970 , which includes current source  330 , regulates the power level of regulation circuit power supply bypass capacitor  350  at a voltage level which is presently preferred to be five point seven (5.7) volts. This is done by charging the regulation circuit power supply bypass capacitor  350  when the MOSFET  800  is not conducting. Undervoltage circuit  860  prevent the MOSFET  800  from conducting again until the voltage at bypass terminal  340  reaches the desired voltage level. 
         [0064]    Referring to  FIG. 7 , maximum duty cycle signal  830  is provided to first inverter  1000  the output of which is provided to a first terminal  1005  of bypass latch  1010  and to bypass and-gate  1015 . The output of bypass latch  1010  is provided to bypass and-gate  1015 . The second input terminal  1020  of bypass latch  1010  receives the output of second bypass inverter  1025  that receives input from bypass comparator  1030 . Bypass comparator  1030  determines whether the voltage at bypass terminal  340  has reached the voltage level for terminating input to the regulation circuit power supply bypass capacitor  350 . A bypass MOSFET  1035  conducts or interdicts depending on the output of bypass and-gate  1015 . When the bypass MOSFET  1035  conducts current source  330  allows current to flow bypass terminal  340  and allows the regulation circuit power supply bypass capacitor  350  to charge. 
         [0065]    In operation bypass latch  1010  is turned on when the maximum duty cycle signal  830  is high and MOSFET  800  is conducting. However, the output of bypass latch  1010  is blocked by bypass and-gate  1015  from turning on the current source  330  during this time. When the maximum duty cycle signal  830  goes low MOSFET  800  turns off and the bypass and-gate  1015  will no longer block the output of bypass latch  1010  from turning on current source  330 . When the current source  330  is turned on, it charges the regulation circuit power supply bypass capacitor  350 . When the bypass terminal voltage  1037  at the bypass terminal  340  reaches the voltage threshold level, which is presently preferred to be five point seven (5.7) volts, the bypass latch  1010  is reset by the output of bypass comparator  1030  and current source  330  is turned off. In this way bypass MOSFET  1035  will conduct only when the maximum duty cycle signal  830  is low and regulation circuit power supply capacitor  350  will charge only when the MOSFET  800  is not conducting. 
         [0066]    Referring to  FIG. 8 , the bypass terminal voltage  1037  will decrease while the maximum duty cycle signal  830  is high. When the maximum duty cycle signal  830  goes low, the current source  330  is activated and the regulation circuit power supply bypass capacitor  350  is charged which in turn increases the bypass terminal voltage  1037 . It is presently preferred, that the bypass terminal voltage  1037  reaches the voltage threshold prior to when the maximum duty cycle signal  830  goes high. Once the voltage threshold is reached, the bypass latch  1010  is reset and the bypass MOSFET  1035  ceases to conduct the regulation circuit power supply bypass capacitor  350  will discharge until the next low period of maximum duty cycle signal  830 . 
         [0067]    Referring to  FIG. 9 , normal oscillator current source  1040  provides a current to oscillator  840  when the oscillator is outputting the maximum duty cycle signal  830 . A presently preferred speed-up current source  1045  provides a current that has a greater magnitude than the current provided by the oscillator current source  1040 . When both the speed-up current source  1045  and the oscillator current source  1040  provide current to drive oscillator  840  the clock frequency is increased. Speed-up latch  1050  is set at the beginning of each clock cycle. When speed-up latch  1050  is set, the speed-up switch  1055  is allowed to conduct allowing current from speed-up current source  1045  to increase the clock frequency of oscillator  840 . A speed-up or-gate  1060  will reset speed-up latch  1050  when the maximum duty cycle signal  830  is low or when the latch  820  is set. It should be noted that latch  820  is set when the enable signal  905  is high. Therefore, in those cycles when enable signal  905  is high and MOSFET  800  conducts the speed-up latch  1010  is reset immediately at the beginning of that cycle and the clock frequency of the oscillator is normal with only oscillator current source  1040  providing current. In those cycles when enable signal  905  is low and MOSFET  800  is not conducting, the speed-up latch  1010  is not reset until maximum duty cycle signal  830  is low and the oscillator  840  operates at the predetermined higher frequency by the addition of the speed-up current source  1045 . 
         [0068]    Referring to  FIG. 10 , oscillator  840  includes a saw tooth output  1070  that provides a saw tooth waveform  1075 . The saw tooth waveform  1075  is the voltage across saw tooth capacitor  1080  that charges and discharges within each cycle. A disable nor-gate  1085  is provided with the enable signal  905  and the maximum duty cycle signal  830 . Disable nor-gate  1085  will provide an output when the enable signal  905  is low and the duty cycle signal  830  is low. The output of disable nor-gate  1085  is provided to clamp switch  1090  allowing the clamp switch  1090  to conduct and clamp the saw tooth waveform.  1075  to a voltage level between its high and low peaks during its falling edge. The presently preferred clamp switch  1090  is a MOSFET. When the clamp switch  1090  conducts a current flows from the bypass terminal  340  through biasing transistor  1100  and clamp switch  1090  to clamp the voltage level of saw tooth capacitor  1080 . The voltage across saw tooth capacitor  1080  is then clamped to a fixed value and the oscillator  840  ceases to function. In this way, oscillator  840  ceases to function when the load voltage is above the threshold level and the regulation circuit  240  is disabled. 
         [0069]    Referring to  FIG. 11 , saw tooth waveform  1075  oscillates between a higher voltage level and a lower voltage level. The presently preferred higher voltage level is two (2) volts and the presently preferred lower voltage level is one (1) volt. Once the enable signal  905  is removed, and the saw tooth waveform  1075  reaches the clamp voltage of saw tooth capacitor  1080  the saw tooth waveform  1075  is maintained at that clamp voltage. Once the enable signal  905  is provided again, the clamp switch  1090  no longer conducts and the saw tooth waveform  1075  continues its cycle. Once the saw toothed waveform  1075  reaches the lower voltage level at resume time  1125  another clock cycle begins and oscillator  840  resumes operation. 
         [0070]    It is presently preferred that regulation circuit  240  comprises a monolithic device. 
         [0071]    Referring to  FIG. 12 , maximum duty cycle signal  830  has an on-time  1200  and off-time  1210 . Enable signal  905  is provided and then stops at time  1220  when feedback from phototransistor  310  is received. When the enable signal  905  is terminated, drive signal  850  is maintained on for the remainder of on-time  1200 . Once the on time  1200  is completed drive signal is disabled. At time  1230 , which is during on-time  1200  enable signal  905  is provided again because feedback from phototransistor  310  is no longer received. The drive signal  850  will not be provided again until the beginning of the next on-time  1200  of the maximum duty cycle signal  830 . 
         [0072]    Referring to  FIG. 13 , in the alternate approach depicted in  FIG. 9 , maximum duty cycle signal  830  includes on-time  1200  and off-time  1210  while the enable signal  905  is received. Also, during the on-time  1200  drive signal  850  is provided. When the enable signal  905  is terminated, drive signal  850  is maintained on for the remainder of on-time  1200 . Once the enable signal  905  is discontinued, it is presently preferred that the oscillator  840  speeds up to a higher frequency by having a shortened on-time  1240  while maintaining the same length off-time  1210 . When the enable signal  905  is provided again, MOSFET  800  resumes operation in approximately half the time compared to the embodiment of  FIG. 12 . The drive signal  850  will not be provided again until the beginning of the next on-time  1200  of the maximum duty cycle signal  830 . This approach may have some advantages in certain applications as it minimizes the response time of the regulation circuit  240 . The shorter response time decreases of the voltage ripple at the load. 
         [0073]    Referring to  FIG. 14 , in the approach depicted in  FIG. 10 , maximum duty cycle signal  830  includes on-time  1200  and off-time  1210  while the enable signal  905  is received. Also, during the on-time  1200  drive signal  850  is provided. When the enable signal  905  is terminated, drive signal  850  is maintained on for the remainder of on-time  1200 . Once the enable signal  905  is discontinued, oscillator  840  ceases functioning. The drive signal  850  will not be provided again until the beginning of the next on-time  1200  of the maximum duty cycle signal  830 , which will be immediately upon receiving the enable signal  905 . Like the embodiment of  FIG. 13  this approach has the advantage of minimizing the response time of the regulation circuit  240  The shorter response time decreases of the voltage ripple at the load. 
         [0074]    While the embodiments, applications and advantages of the present invention have been depicted and described, there are many more embodiments, applications and advantages possible without deviating from the spirit of the inventive concepts described herein. Thus, the inventions are not to be restricted to the preferred embodiments, specification or drawings. The protection to be afforded this patent should therefore only be restricted in accordance with the spirit and intended scope of the following claims.