Abstract:
A system including a switch and a control circuit. The switch is configured to receive a first voltage. The control circuit is configured to, during a rising portion of a half cycle of the first voltage, (i) turn on the switch in response to the first voltage reaching a first value, and (ii) turn off the switch in response to the first voltage reaching a second value, where the second value is greater than the first value. The control circuit is further configured to, during a falling portion of the half cycle of the first voltage, (i) turn on the switch in response to the first voltage reaching the second value, and (ii) turn off the switch in response to the first voltage reaching the first value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This present disclosure is a continuation of U.S. patent application Ser. No. 13/449,407 (now U.S. Pat. No. 8,742,735), filed on Apr. 18, 2012, which claims the benefit of U.S. Provisional Application No. 61/486,488, filed on May 16, 2011. 
         [0002]    This application is related to U.S. application Ser. No. 13/467,648, filed on May 9, 2012 which claims the benefit of U.S. Provisional Application No. 61/494,619, filed on Jun. 8, 2011. 
         [0003]    The entire disclosures of the applications referenced above are incorporated herein by reference. 
     
    
     FIELD 
       [0004]    The present disclosure relates to a high-voltage startup circuit for systems that require DC power to operate when power is initially turned on. 
       BACKGROUND 
       [0005]    The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
         [0006]    Referring now to  FIG. 1 , a power supply  100  converts an alternating current (AC) line voltage  101  to one or more direct current (DC) voltages that are suitable for a load  102 . The AC line voltage  101  may be 110V, 60 Hz or 220V, 50 Hz. The DC voltages may include a fraction of 1V, 1.5V, ±5V, ±12V, 24V, or any other suitable value to drive the load  102 . The power supply  100  includes a step-down transformer  104  and a rectifier  106 . The step-down transformer  104  converts the AC line voltage  101  to an AC voltage having a smaller value than the AC line voltage  101  (e.g., 24V AC, 12V AC, and so on) depending on the value of the DC voltage to be generated. The rectifier  106  converts the AC voltage output by the step-down transformer  104  to the DC voltage and outputs the DC voltage to the load  102 . 
         [0007]    Referring now to  FIG. 2 , a power supply  150  converts the AC line voltage  101  to one or more DC voltages that are suitable for the load  102 . The power supply  150  includes a rectifier  152  and a DC-to-DC converter  154 . The rectifier  152  converts the AC line voltage  101  to a DC voltage. The DC-to-DC converter  154  converts the DC voltage output by the rectifier  152  to the one or more DC voltages that are suitable for operating the load  102 . 
         [0008]    The DC-to-DC converter  154  typically includes a switching controller (e.g., a pulse width modulation (PWM) controller). The switching controller requires a DC voltage for operation. The DC voltage required to operate the switching controller at startup (i.e., when power is turned on) is typically generated using a resistor. The resistor drops the AC line voltage  101  to a low value, which is used to power the switching controller at startup. Subsequently, when the DC voltages to operate the load  102  are generated, the switching controller is operated using one of the DC voltages. 
         [0009]    An efficiency of a power supply is given by a ratio of an output voltage of the power supply to an input voltage of the power supply. The efficiency of the power supply  150  is very low. For example, if the value of the DC voltage supplied by the power supply  150  to the load  102  is 5V, and the value of the AC line voltage  101 , is 120V (Le., approximately 170V RMS), then the efficiency of the power supply  150  is 5/170=approximately 3%. If the DC voltage supplied to the load  102  is 12V, and the AC line voltage  101  is 220V (i.e., approximately 311V RMS), then the efficiency of the power supply  150  is 12/311=approximately 4%. 
         [0010]    Additionally, the resistor used to power the switching controller at startup dissipates power. Further, in some applications, the power supply  150  continues to operate and therefore dissipates power although the load  102  may be switched from a normal operating mode to a power-save mode. 
       SUMMARY 
       [0011]    A system comprises a power transistor configured to receive an alternating current (AC) line voltage and a control circuit. During a rising portion of a half cycle of the AC line voltage, the control circuit is configured to turn on the power transistor when the AC line voltage reaches a first value and turn off the power transistor when the AC line voltage reaches a second value. The second value is greater than the first value. During a falling portion of the half cycle, the control circuit is configured to turn on the power transistor when the AC line voltage reaches the second value and turn off the power transistor when the AC line voltage reaches the first value. 
         [0012]    In other features, the system further comprises a capacitance, where the power transistor charges the capacitance when the power transistor is turned on, and where the capacitance outputs a voltage having a value less than the first value. 
         [0013]    In other features, the control circuit is configured to turn off the power transistor when the voltage output by the capacitance is greater than or equal to the first value. 
         [0014]    In other features, the system further comprises a power supply configured to generate a direct current (DC) voltage based on the AC line voltage and a controller configured to control the power supply. The voltage output by the capacitance powers the controller. 
         [0015]    In other features, the control circuit is configured to disable the power transistor. 
         [0016]    In still other features, a system comprises a power transistor configured to receive an alternating current (AC) line voltage and charge a capacitance to an output voltage based on when the power transistor is turned on during a half cycle of the AC line voltage. The system further comprises a control circuit configured to turn on the power transistor to charge the capacitance when the AC line voltage is between a first value and a second value during a half cycle of the AC line voltage, where the first value is greater than or equal to the output voltage, and where the second value is greater than the first value by a predetermined amount. The control circuit is further configured to turn off the power transistor when the AC line voltage is not between the first value and the second value during the half cycle of the AC line voltage or when the capacitance is charged to the output voltage. 
         [0017]    In other features, the system further comprises a controller configured to control a power supply, where the power supply generates a direct current (DC) voltage based on the AC line voltage, and where the capacitance outputs the output voltage to the controller. 
         [0018]    In other features, the control circuit is configured to turn off the power transistor and components of the control circuit. 
         [0019]    In other features, the control circuit comprises a voltage divider configured to divide the AC line voltage, a comparator configured to compare an output of the voltage divider to a reference voltage, and a switch configured to, based on the comparison, turn on the power transistor when the AC line voltage is between the first value and the second value, and to turn off the power transistor when the AC line voltage is not between the first value and the second value. 
         [0020]    In other features, the control circuit comprises a voltage divider configured to divide the output voltage, a comparator configured to compare an output of the voltage divider to a reference voltage, and a switch configured to, based on the comparison, turn on the power transistor when the AC line voltage is between the first value and the second value and when the capacitance is charged to less than the output voltage, and to turn off the power transistor when the capacitance is charged to greater than or equal to the output voltage. 
         [0021]    In still other features, an integrated circuit (IC) comprises a first resistance having a first end connected to an alternating current (AC) line voltage, and a second end; and a second resistance having a first end connected to the second end of the first resistance, and a second end. The system further comprises a first comparator having a first input connected to the second end of the first resistance, a second input connected to a reference voltage, and a first output. The system further comprises a first transistor having a gate connected to the first output of the first comparator, a source connected to the second end of the second resistance, and a drain; and a second transistor having a source connected to the second end of the second resistance, a drain connected to the drain of the first transistor, and a gate. The system further comprises a second comparator having a second output connected to the gate of the second transistor, a first input connected to the reference voltage, and a second input. The system further comprises a third resistance having a first end connected to the second end of the second resistance and a second end connected to the second input of the second comparator; and a fourth resistance having a first end connected to the second input of the second comparator and a second end. The system further comprises a fifth resistance having a first end connected to the second end of the fourth resistance and a second end connected to the drain of the first transistor. The system further comprises a diode having a cathode connected to the first end of the fifth resistance and an anode. The system further comprises a third transistor having a source connected to the anode of the diode, a drain connected to the first end of the first resistance, and a control terminal connected to the drain of the second transistor. The system further comprises a capacitance having a first end connected to the cathode of the diode and a second end connected to the second end of the second resistance. 
         [0022]    Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]    The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0024]      FIG. 1  is a functional block diagram of a power supply that rectifies a stepped-down alternating current (AC) line voltage according to the prior art; 
           [0025]      FIG. 2  is a functional block diagram of a power supply that rectifies the AC line voltage and generates one or more DC voltages according to the prior art; 
           [0026]      FIGS. 3A and 3B  are functional block diagrams of a power supply including a startup circuit according to the present disclosure; 
           [0027]      FIG. 4A  is a schematic of the startup circuit; 
           [0028]      FIG. 4B  is a graph depicting the AC line voltage, an output voltage of the startup circuit, and a drain current supplied by the startup circuit as a function of time; and 
           [0029]      FIG. 5  is a flowchart of a method for powering a controller of a power supply at startup (i.e., when power is turned on). 
       
    
    
     DESCRIPTION 
       [0030]    The present disclosure relates to a startup circuit that supplies power at startup (i.e., when power is turned on) to a system that draws power from AC line voltage (e.g., 120V AC) and that requires power (e.g., 5V DC) to operate at startup. For example, the startup circuit provides power to a switching controller of a power supply at startup. Based on the power provided by the startup circuit, the switching controller can control the operation of the power supply at startup so that the power supply can generate one or more DC voltages from the AC line voltage to operate a load. 
         [0031]    After the power supply generates the DC voltages, one of the DC voltages can be used to power the switching controller. Based on the DC voltage generated by the power supply, the switching controller continues operation and controls the power supply. The startup circuit can be disabled after the DC voltage generated by the power supply is used to power the switching controller. The principles of the present disclosure, while described using a power supply as an example, can be applied to any system that draws power from the AC line voltage and that requires power such as 5V DC to operate at startup. 
         [0032]    Referring now to  FIGS. 3A and 3B , a power supply  200  comprising a startup circuit  202  according to the present disclosure is shown. In  FIG. 3A , the startup circuit  202  is arranged between a rectifier  204  and a DC-to-DC converter  206 . In  FIG. 3B , the startup circuit  202  is arranged between the AC line voltage  101  and the rectifier  204 . In either arrangement, the startup circuit  202  draws power from the AC line voltage  101  during startup and supplies a DC voltage suitable for operating components (e.g., a switching controller) of the DC-to-DC converter  206 . The DC-to-DC converter  206  generates one or more DC voltages suitable for operating the load  102 . After the DC voltages are generated, the DC-to-DC converter  206  uses one of the DC voltages to operate components such as the switching controller of the DC-to-DC converter  206  and disables the startup circuit  202 . 
         [0033]    Referring now to  FIGS. 4A and 4B , the startup circuit  202  is shown in detail. In  FIG. 4A , the startup circuit  202  charges a capacitor C out  during each half cycle of the AC line voltage at startup. The startup circuit  202  charges the capacitor C out  to an output voltage V out . The capacitor C out  supplies the output voltage V out  to a component such as a switching controller (not shown) of the DC-to-DC converter  206  at startup. For example only, suppose that the switching controller requires 5V DC to operate. The startup circuit  202  charges the capacitor C out  to 5V DC and supplies 5V DC to the switching controller at startup. 
         [0034]    The startup circuit  202  charges the capacitor C out  when the value of the AC line voltage is between a first value and a second value during each half cycle of the AC line voltage. The first value is selected based on the value of the output voltage V out . The second value is greater than the first value. For example, if V out =5V, the first value may be any value greater than 5V. For example only, suppose that the first value is 5.1V. The second value may be 6V, 7V, 8V, or any value greater than the first value. For example only, suppose that the second value is 6V. 
         [0035]    In  FIG. 4B , the startup circuit  202  begins charging the capacitor C out  at time t1 during a half cycle of the AC line voltage when the AC line voltage increases from zero to a first value greater than 5V RMS (e.g., 5.1V RMS). The startup circuit  202  charges the capacitor C out  until time t2 when the AC line voltage increases to a second value greater than the first value (e.g., 6V RMS). The startup circuit  202  stops charging the capacitor C out  at time t2 when the AC line voltage is greater than or equal to the second value (e.g., 6V RMS). 
         [0036]    Subsequently, the AC line voltage increases to a peak value (e.g., 1.44*110V) and begins to decrease. The startup circuit  202  again begins charging the capacitor C out  at time t3 when the AC line voltage decreases from the peak value to the second value (e.g., 6V RMS). The startup circuit  202  charges the capacitor C out  until time t4 when the AC line voltage decreases from the second value to the first value (e.g., from 6V RMS to 5.1V RMS). The startup circuit  202  stops charging the capacitor C out  at time t4 when the AC line voltage is less than or equal to the first value (e.g., 5.1V RMS). The AC line voltage then returns to zero, and the cycle is repeated according to a line frequency of the AC line voltage (e.g., 50 Hz). The capacitor C out  outputs the output voltage V out =5V to the switching controller. 
         [0037]    Based on the output voltage V out  supplied by the startup circuit  202 , the switching controller of the DC-to-DC converter  206  operates during startup, and the DC-to-DC converter  206  generates one or more DC voltages to operate the load  102 . Subsequently, one of the DC voltages generated by the DC-to-DC converter  206  (e.g., 5V) is used to power the switching controller, and the startup circuit  202  can be disabled. 
         [0038]    In the above example, the capacitor C out  is charged when the input voltage to the startup circuit  202  is between 5V RMS and 6V RMS. Since the maximum input voltage to the startup circuit  202  is 6V RMS, and the output voltage of the startup circuit  202  is 5V, the worst-case efficiency of the startup circuit  202  is 5/6=approximately 83%. The startup circuit  202  is now described in detail. 
         [0039]    In  FIG. 4A , the startup circuit  202  can be manufactured as an integrated circuit (IC) having four pins: V AC , V out , disable (DIS), and ground (GND). The V AC  pin is connected to the AC line voltage  101 . The V out  pin is connected to the output capacitor C out  and supplies the output voltage V out  generated by the startup circuit  202  to the DC-to-DC converter  206  at startup. The GND pin is connected to ground. The DIS pin can be used to input a disable signal to turn off the startup circuit  202  after the startup (i.e., after the DC-to-DC converter  206  generates the one or more DC voltages) to save power. For example, the DC-to-DC converter  206  may send a control signal to the DIS pin after the DC-to-DC converter  206  generates the one or more DC voltages. The control signal turns off the startup circuit  202 . Alternatively, the DIS pin can be connected to ground when unused. 
         [0040]    The startup circuit  202  includes a super-high voltage, depletion-mode power transistor M 1  that is controlled by comparators CI and C 2 ; transistors M 2 , M 3 , and M 4 ; and resistors R 1  through RS. The comparators CI and C 2 ; transistors M 2 , M 3 , and M 4 ; and resistors R 1  through RS may be called a control circuit that controls the power transistor MI. The transistors M 2 , M 3 , and M 4  may be CMOSFETs. The resistors R 1  and R 2  are high-voltage resistors. 
         [0041]    A gate voltage of the power transistor M 1  is determined by the resistor RS and the transistors M 2 , M 3 , and M 4 . The transistors M 2 , M 3 , and M 4  are controlled by the AC line voltage V AC , the output voltage V out , and the disable input (DIS), respectively. The resistor R 5  is used to charge the gate voltage of the power transistor M 1  to V out . A diode D is a reverse blocking diode that prevents the output voltage V out  from discharging through a body diode of the power transistor MI. 
         [0042]    When power is turned on (i.e., at startup), V out  is initially low; the transistors M 2 , M 3 , and M 4  are turned off; and the gate voltage of the power transistor M 1  is equal to V out . Since the power transistor M 1  is a depletion mode MOSFET, the threshold voltage is negative, and the channel is already formed. Consequently, the power transistor M 1  is turned on when power is initially turned on. The capacitor C out  is charged by the AC line voltage close to the threshold voltage of the power transistor MI. A bandgap reference (BGR) generator (not shown) supplies a reference voltage V ref  to the comparators CI and C 2 . 
         [0043]    The comparator CI receives a signal V ac     —     sense  that provides an indication of the AC line voltage V AC . The signal V ac     —     sense  is generated using a resistor divider comprising the resistors  131  and R 2 . Specifically, V ac     —     sense =V AC *R 2 /(R 1 +R 2 ). When V AC  is greater than V ac     —     sense , the transistor M 2  turns on and pulls the gate voltage of the power transistor M 1  to ground to turn off the power transistor MI. In the above example, the comparator CI turns off the power transistor M 1  when V AC  is greater than or equal to 6V RMS. The value of V AC  at which to turn off the power transistor M 1  (e.g., 6V RMS) can be set to any value (e.g., 7V RMS, 8V RMS, 9V RMS, and so on) by selecting values of the resistors R 1  and R 2 . 
         [0044]    The comparator C 2  receives a signal V out     —     sense  that provides an indication of the output voltage V out . The signal V out     —     sense  is generated using a resistor divider comprising the resistors R 3  and R 4 . Specifically, V out     —     sense =V out *R 4 /(R 3 +R 4 ). When the output voltage V out  is greater than V out     —     sense , the transistor M 3  turns on and pulls the gate voltage of the power transistor M 1  to ground to turn off the power transistor MI. In the above example, the comparator C 2  turns off the power transistor M 1  and stops charging the capacitor C out  when the output voltage V out  reaches SV. The output voltage V out  is therefore limited to 5V and cannot exceed 5V. 
         [0045]    Accordingly, in this example, the comparator CI turns on the power transistor M 1  and allows charging of the capacitor C out  when V AC  is less than 6V RMS and V out  is less than 5V, and turns off the power transistor M 1  and stops charging the capacitor C out  when V AC  is greater than or equal to 6V RMS. The comparator C 2  allows the comparator CI to turn on the power transistor M 1  when V AC  is less than 6V RMS and allows charging of the capacitor C out  when V out  is less than 5V, and turns off the power transistor M 1  and stops charging the capacitor C out  when V out  is equal to 5V. 
         [0046]    The disable (DIS) input of the startup circuit  202  is an optional control that can be used by an independent application-specific controller to turn off the start-up circuit  202  to save power. For example, when the DIS pin is pulled up, the transistor M 4  turns on and pulls the gate voltage of the power transistor M 1  to ground to turn off the power transistor MI. The transistor M 4  turns off the power transistor M 1  regardless of the states of the transistors M 2  and M 3  determined by the comparators CI and C 2 . Alternatively, the power transistor M 1  can also be turned off by applying a voltage greater than V out  at the V out  pin. The voltage greater than V out  may be generated by a power supply (e.g., the DC-to-DC converter  206 ). 
         [0047]    In  FIG. 4B , when power is turned on, the AC line voltage V AC  (or the output voltage V rect  of the rectifier  204 ) increases from zero. At time t1, V AC  increases from zero to 5.1V RMS, for example. The power transistor M 1  is turned on at time t1. At time t2, V AC  increases from 5.1V RMS to 6V RMS, for example. The power transistor M 1  is turned on until time t2 and turned off at time t2. Subsequently, V AC  increases to a peak value of V AC  and starts to decrease. At time t3, V AC  decreases from the peak value to 6V, for example. The power transistor M 1  is turned on at time t3. At time t4, V AC  decreases from 6V to 5.1V, for example. The power transistor M 1  is turned on until time t4 and turned off at time t4. Subsequently, V AC  decreases to OV, and the cycle repeats at the line frequency of the AC line voltage V AC . 
         [0048]    A drain current I drain  flows through the power transistor M 1  and charges the capacitor C out  to the output voltage V out  from time t1 to t2 and from time t3 to t4. The output voltage V out  increases from time t1 to t2 and from time t3 to t4. The power transistor M 1  is turned off and does not charge the capacitor C out  at other times during the half cycle. The capacitor C out  discharges from time t2 to t3 and from time t4 to t1. The output voltage V out  therefore decreases from time t2 to t3 and from time t4 to t1. 
         [0049]    Referring now to  FIG. 5 , a method  250  for powering a controller of a power supply at startup (i.e., when power is turned on) is shown. At  252 , control determines if power to a power supply (e.g., AC line voltage) is turned on and waits until power is turned on. At  254 , when power is turned on, control turns on a power transistor and charges a capacitor when the AC line voltage is between a first value and a second value during rising and falling portions of each half cycle of the AC line voltage. Control turns off the power transistor at other times during each half cycle. Control also turns the power transistor on and off based on whether the output voltage of the capacitor is less than or equal to a desired voltage (e.g., 5V DC). At  256 , control uses the voltage output by the capacitor to power the controller of the power supply. Accordingly, the power supply can generate one or more DC voltages from the AC line voltage. At  258 , control determines if the output of the power supply is stable. Control returns to  254  if the output of the power supply is not yet stable. At  260 , if the output of the power supply is stable, control uses the output of the power supply to power the controller and turns of the startup circuit comprising the power transistor and the capacitor. 
         [0050]    The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure.