Abstract:
A power supply has a rectifier for producing a supply voltage from an AC source. A transformer includes a primary winding, a secondary winding, and an auxiliary winding, wherein the supply voltage is applied to the primary winding by a first switch. A controller, powered by voltage at a node, pulses the first switch between conductive and non-conductive states. A second rectifier is coupled between the auxiliary winding and the node. A starting resistor applies voltage derived from the supply voltage to the node. A second switch, in series with the starting resistor, is rendered non-conductive by a delay circuit a defined time period after a given voltage occurs at the node. When the power supply initially activates, the starting resistor supplies voltage to the node, soon thereafter voltage is supplied from the secondary winding. When the defined time period elapses, the delay circuit operationally disconnects the starting resistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to a switchmode power supply that produces direct current from a higher voltage alternating current; and more particularly to such power supplies for driving dimmable light emitting diodes, such as used in lighting fixtures of a building. 
         [0005]    2. Description of the Related Art 
         [0006]    Light emitting diodes (LED&#39;s) are presently used in buildings as alternative light sources to incandescent and fluorescent light bulbs. LED&#39;s consume less power, emit less heat, and have a considerably longer life than other light sources. Nevertheless, the light emitting diodes require relatively low voltage direct current (VDC) and thus a power supply is required if the lighting assembly is to be powered by the standard electrical wiring in the building, The building electrical lighting circuits typically carry alternating current at either 120 volts or 240 volts, depending upon the country in which the building is located. Commercial and industrial buildings also use 277 volts for lighting devices. 
         [0007]    A wide variety of techniques can be employed to convert the relatively high voltage alternating current to the lower voltage direct current for the LED&#39;s. Many conversion techniques are not very efficient and are undesirable when maximum energy conservation is desired. One of the more efficient techniques employs a switchmode type power supply. 
         [0008]      FIG. 1  depicts a common type of switchmode power supply  10 , commonly known as a flyback power supply. This disclosure references the flyback power supply but all types of switching power supplies suffer from the same issue of how to start-up a low voltage load from a high voltage source. The alternating current from the building&#39;s electrical system  11  is rectified into a direct current by a diode bridge  12 . The output of the diode bridge is applied across a series connection of a primary winding  14  in transformer  15  and a semiconductor switch  16 . The transformer has a secondary winding  18  that is connected to a rectifier  20  to produce the low voltage direct current for powering a load such as light emitting diodes. 
         [0009]    The semiconductor switch  16  is operated by a controller  22  which pulse width modulates operation of the switch. That operation sends current pulses through the primary winding  14  thereby inducing a voltage pulses in the secondary winding  18 . Voltage pulses also are induced in an auxiliary winding  30  of the transformer, with via diode  32  provides power at node  34  for operating the controller  22 . 
         [0010]    When the power switch  24  is closed to activate the power supply, the semiconductor switch  16  is in a non-conductive state, i.e. is turned off, therefore a special start up circuit is required to power the controller  22  and initiate the pulse width modulation of the semiconductor switch. That start up circuit has a drop down resistor  26  that creates a low voltage at node  34  and across a storage capacitor  28  for powering the controller  22 . This enables the controller to operate during start up directly off the rectified AC line voltage at the output of the diode bridge. Once the controller  22  begins pulse width modulating the semiconductor switch  16  the resultant current pulses in the primary winding  14  induce voltage in the auxiliary winding  30  which provides sustained low voltage to node  34 . 
         [0011]    One drawback of this power supply is that energy is continuously dissipated through the start up resistor  26  even after the controller turns on and voltage also is supplied via the auxiliary winding  30 . This energy dissipation represents a continuing inefficiency while the power supply is operating. To minimize that energy dissipation, the start up resistor  26  has a relatively large resistance, (e.g. 200 kilohms). That large resistance affects the start up time which is determined by the RC time constant of the start up resistor  26  and capacitor  28  and the peak voltage at the output of the diode bridge  12 . If a conventional lighting dimmer is used in place of the power switch  24 , at low dimmer settings, it takes an appreciably long time (e.g., 30 seconds) for the LED&#39;s to turn on due to the large value of the start up resistor and the low output voltage from the diode bridge  12 . 
         [0012]      FIG. 2  depicts a flyback type switchmode power supply  40  that has a variation of the start up circuit in which a second semiconductor switch, in the form of a MOSFET  42 , is placed in series with the start up resistor  26 . The voltage produced by a voltage regulator  44  is slightly greater than the voltage of the Zener diode  41  minus the turn on threshold voltage of the MOSFET  42 . This effectively “back biases” the MOSFET  42  and turns it off, however that is not a sharp turnoff and the MOSFET continues to leak current. Additionally there are considerable tolerance issues with predicting the turn on voltage of the controller  22 , the voltage regulator  44 , and the turn on threshold voltage of MOSFET  42 . The result is that the second semiconductor switch  42  never fully turns off and along with the start up resistor  26  continues to dissipate power after start up. Since the start up resistor  26  should be disconnected after start up, it can have a relatively low resistance, (e.g. 2 kilohms) in comparison to the start up resistor in the  FIG. 1  circuit. This lower resistance shortens the turn on time of the power supply when connected to a dimmer. Some of power supplies of this type include a voltage regulator for the voltage produced by the auxiliary coil  30  and diode  32 . 
         [0013]      FIG. 3  depicts a third version of a previous switchmode power supply  50  in which a third semiconductor switch  52  has a conduction path connected between the gate of the second semiconductor switch  42  and ground. The gate of the third semiconductor switch  52  is connected to the output terminal of the transformer auxiliary winding  30 . When the controller turns on and voltage is induced across the auxiliary winding  30 , that voltage turns on the third semiconductor switch  52 , thereby clamping the gate of the second semiconductor switch  42  to ground and fully turning off the second semiconductor switch. As a consequence, once the controller  22  begins operating, the start up resistor  26  is fully disconnected. 
         [0014]    All these versions of prior power supplies were susceptible to failure should a short circuit occur at the load connected to the secondary winding  18 . If such a short circuit exists when the power switch  24  closes, the power supply will start normally. Although the short circuit load prevents a voltage from being produced across the auxiliary winding  30 , the start up circuit maintains the voltage level at node  34  and thus keeps the controller  22  operating. Eventually the short circuit induced current causes the first and second semiconductor switches  16  and  42  to fail catastrophically. 
         [0015]    Therefore, it is desirable to provide a power supply with at least partial immunity to effects from a short circuited load. It is further desirable to provide optimal energy efficiency while enabling the power supply to be used for dimmable lighting. 
       SUMMARY OF THE INVENTION 
       [0016]    A power supply is provided to derive a DC output voltage from an AC power source and has an input rectifier for producing a supply voltage from the AC power source. A transformer includes a primary winding, a secondary winding, and an auxiliary winding. The supply voltage is applied to the primary winding by a first switch that changes between conductive and non-conductive states in response to a control signal. 
         [0017]    A controller is powered by voltage at a circuit node and produces the control signal in the form of a series of pulses. A second rectifier is coupled between the auxiliary winding and the circuit node to apply a voltage to the circuit node. 
         [0018]    A starting resistor applies a voltage, that is derived from the supply voltage, to the circuit node. A second switch is operatively connected to the starting resistor to control application of the voltage derived from the supply voltage to the circuit node. A time delay circuit is activated by production of the supply voltage, wherein a given time period after being activated the time delay circuit causes the second switch to discontinue the application of the voltage derived from the supply voltage to the circuit node. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic block diagram of a first prior power supply; 
           [0020]      FIG. 2  is a schematic block diagram of a second prior power supply; 
           [0021]      FIG. 3  is a schematic block diagram of a third prior power supply; and 
           [0022]      FIG. 4  is a schematic block diagram of a first power supply according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    With reference to  FIG. 4 , a switchmode power supply  100  receives alternating current at 120 volts, 240 volts, or 277 volts from an external power source  102 , such a utility company power line. The external power source  102  is connected to the power supply  100  by a switch  103 , such as a standard wall switch of the electrical wiring system in a building. Alternatively, a standard light dimmer  105 , schematically depicted as a variable resistor, may be substituted for the wall switch  103 . Within the power supply  100 , an input rectifier, such as a diode bridge  104 , converts the alternating current into direct current at output terminals across which a first smoothing capacitor  106  is connected. 
         [0024]    The negative output terminal of the diode bridge  104  is attached to circuit ground and the positive terminal is connected to one end of a primary winding  108  of a transformer  110 . The opposite end of the primary winding  108  is connected to circuit ground through the conduction path of a first switch  112 . Preferably, the first switch is a semiconductor device, such as a MOSFET. The first switch  112  has a control terminal, such as the gate of the MOSFET, which receives a control signal from a controller  122 . The controller  122  pulse width modulates operation of the first switch  112  in a conventional manner. Preferably, the controller  122  provides power factor correction (PFC) to “smooth out” the pulsating AC current resulting from PWM operation of the first switch  112  and thereby improving the power factor of the power supply. Implementing power factor correction increases the power handling capability of the power supply. Power factor correction also is desirable in order for the solid state lighting system for qualify for the Energy Star program of the United States Department of Energy. For example, the controller  122  may be a transition-mode PFC controller, such as model L6562 produced by STMicroelectronics, 39, Chemin du Champ des Filles, C. P. 21, CH 1228 Plan-Les-Ouates. GENEVA, Switzerland. 
         [0025]    The transformer has a secondary winding  114 , with one end coupled by an output rectifier, such as an output diode  116 , to a first output terminal  118  of the power supply. The other end of the secondary winding  114  is connected directly to a second output terminal  119 . A second smoothing capacitor  120  is connected across the output terminals  118  and  119 . The load  121 , in this case an assembly of light emitting diodes, is connected to the output terminals  118  and  119 . 
         [0026]    The transformer  110  also has an auxiliary winding  109  in which current is induced by the current flowing through the primary winding  108 . One end of the auxiliary winding  109  is connected to the circuit ground and an opposite end is connected by an auxiliary rectifier, for example a first diode  124 , to a linear voltage regulator  126 . Any well known voltage regulator can be employed to utilize the rectified voltage from the first diode  124  to produce a relatively stable voltage level (Vcc) at a first circuit node  130 . That voltage is used to power the controller  122 . A storage capacitor  146  is connected between the first circuit node  130  and circuit ground. 
         [0027]    The exemplary voltage regulator  126  includes a transistor  128  that has a collector-emitter path connected between the first diode  124  and the first circuit node  130 . A first resistor  132  is connected between the collector and the base electrodes of the transistor  128 . A cathode of a first zener diode  134  is connected to the base of the transistor  128 . The anode of the first zener diode  134  is coupled to the first circuit node  130  by a second resistor  136  and a first capacitor  138  connected in parallel. 
         [0028]    The first circuit node  130  is connected to the positive terminal of the diode bridge  104  by a start up circuit  139  comprising a series connection of a second switch  140 , a starting resistor  142 , and a starting diode  144 . The starting diode  144  is poled so that current flows from the positive output terminal of the diode bridge  104  toward the first circuit node  130 . These second switch  140 , which also may be a semiconductor device such as a MOSFET, has the control input (e.g., a gate electrode) that is connected to a second circuit node  148 . 
         [0029]    The second circuit node  148  is coupled to the positive output terminal of the diode bridge  104  by a third resistor  150 . A second zener diode  152  is connected in a reverse biased fashion between the second circuit node  148  and circuit ground. A third switch  154 , which also may be a semiconductor device such as a MOSFET, has a conduction path connected between the second circuit node  148  and circuit ground. The control terminal (e.g., gate electrode) of the third switch  154  is connected to an output terminal of a time delay circuit  156 . Any conventional circuit that provides the requisite time delay, as will be described hereinafter, may be used as the time delay circuit  156 . A simple RC circuit may be employed. For example as shown in  FIG. 4 , a timing resistor  158  connects the first circuit node  130  to the control terminal of the third switch  154 , which terminal is coupled to circuit ground by a timing capacitor  160 . The values of the timing resistor  158  and the timing capacitor  160  define the RC time constant of the delay circuit  156 . A second diode  162  is coupled in a reverse biased fashion between the control terminal of the third switch and the first circuit node  130 . 
         [0030]    When a user desires to activate the load  121 , the wall switch  103  or dimmer  105  is operated to convey alternating electric current from the power source  102  to the diode bridge  104  of the power supply  100 . This produces a DC voltage across the positive and negative output terminals of the diode bridge. At the time that alternating current is initially applied to the diode bridge  104 , the first, second, and third switches  112 ,  140 , and  154  were in nonconductive states. The positive voltage at the output of the diode bridge  104 , applied through the third resistor  150 , causes the second zener diode  152  to turn on which in turn turns on the second switch  140 . Rendering the second switch  140  conductive begins charging the storage capacitor  146  thereby ramping up the supply voltage Vcc at the first circuit node  130 . Eventually the voltage Vcc at the first circuit node  130  reaches a level that enables the controller  122  to begin to operate. The positive voltage Vcc at the first circuit node  130  also causes the time delay circuit  156  to commence operation. 
         [0031]    Operation of the controller  122  provides a PWM control signal to the control terminal of the first switch  112  (e.g. to the gate of the MOSFET), thereby alternating the switch between conductive and non-conductive states. Alternating the conductive states of the states first switch  112  sends pulses of direct current from the diode bridge  104  through the primary winding  108  of the transformer  110 . Those current pulses induce current in the secondary winding  114  which is rectified by the output diode  116  to provide direct current to the load  121 . At the same time, the pulsating current flowing through the primary winding  108  also induces a current in the auxiliary winding  109 . The current from the auxiliary winding  109  is rectified by the first diode  124  and applied to the linear voltage regulator  126 . The resultant regulated voltage is applied to the first circuit node  130  to further charge the storage capacitor  146  and provide the supply voltage Vcc. 
         [0032]    At this point in time, the power supply is fully operational with the supply voltage produced from the auxiliary winding  109  being sufficient to continue maintain the operation. As a consequence, voltage is no longer required to be supplied to the first circuit node  130  via the start up circuit  139  and in particular via the second switch  140 , the starting resistor  142 , and the starting diode  144 . Nevertheless, that start up voltage continues to be furnished because the second switch  140  is still conductive at this time. 
         [0033]    After the predefined delay period provided by the time delay circuit  156 , that circuit applies a positive voltage potential to the control input of the third switch  154  that turns on that switch. This in turn pulls the control input of the second switch  140  to ground potential, thereby turning off that second switch. That latter action deactivates the start up circuit and disconnects the first circuit node from the positive terminal of the diode bridge  104 . 
         [0034]    The incorporation of the starting diode  144  in the start up circuit  139  is beneficial for a power factor corrected type switchmode power supply. In this such a power supply, the value of first smoothing capacitor  106  is minimized and the voltage on that capacitor is a half sine wave and not a flat line DC level. Without the starting diode  144 , the voltage on the storage capacitor  146  would discharge through the parasitic capacitance of the MOSFET second switch  140  and the controller  22  may never receive enough voltage to function. 
         [0035]    If upon activating the power supply by operation of either the wall switch  103  or the dimmer  105 , a short circuit condition exists across the load terminals  118  and  119 , the present circuit configuration prevents a catastrophic failure of the power supply. This is achieved by setting the delay period provided by the delay circuit  156  to be shorter than the interval that the diode bridge  104 , the primary winding  108 , and the first switch  112  can tolerate the short circuit condition current without failing. 
         [0036]    Initiating power supply operation under a short circuit condition, results in the power supply starting in the same manner as described above during a non-short circuit condition. That is, the second switch  140  initially turns on coupling the first circuit node to the positive output terminal of the diode bridge  104  to begin charging the storage capacitor  146 . When that capacitor&#39;s charge level reaches a point that the voltage (Vcc) at the first circuit node  130  is sufficient to operate the controller  122 , that latter component produces a control signal that turns on the first switch  112 . This results in a large short circuit condition current flowing through the primary winding  108  and the first switch  112 . Because of the effect that the short circuit load has on the secondary winding  114 , a voltage is not produced across the auxiliary winding  109 . As a consequence, the voltage that normally would be provided by the auxiliary winding and conveyed through the first diode  124  and the voltage regulator  126  to the first circuit node  130  does not occur. Therefore, when the time delay interval provided by delay circuit  156  expires and the third switch  154  turns on which in turn turns off second switch  140 , voltage no longer will be applied by either the start up circuit  139  or the voltage regulator  126  to the first circuit node  130 . Therefore, the charge across the storage capacitor  146  quickly dissipates and the controller  122  ceases operation turning off the first switch  112 . 
         [0037]    As the voltage at the first circuit node  130  decays, a level is reached at which the third switch  154  begins to turn off. This causes the voltage on the second Zener diode  152  to increase and eventually reach what is termed the “threshold voltage” of the MOSFET second switch  140 . At that time, the second switch  140  begins to partially conduct, maintaining the existing voltage level at the first circuit node  130 . The resultant voltage at the first circuit node  130  remains equal to the threshold voltage of the MOSFET third switch  154  (typically no more than 4 volts) and the power supply  100  is at stable equilibrium. The voltage level at the first circuit node  130  remains lower than the minimum voltage required by the controller  122  to operate (e.g. about 12 volts) and so the controller remains in the non-operational state. The only way to restart the power supply  100  is to remove the short circuit and reset the input power. 
         [0038]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.