Abstract:
In addition to other aspects disclosed, a method comprises providing a signal to a transformer for supplying power during a reduced activity state. The signal comprises a series of pulses that comprises a positive pulse and a negative pulse. The signal is a segmented version of another signal provided to the transformer during the active state.

Description:
BACKGROUND 
       [0001]    This disclosure relates to providing power during a reduced activity state. 
         [0002]    By connecting a load to the secondary side of a transformer, power may be provided to the load from a power source connected to the primary side of the transformer. The transformer may increase (e.g., step-up) or decrease (e.g., step-down) the voltage present on the primary side depending upon the supply needs of the load. Along with the load, the transformer itself may dissipate power being provided to the secondary side of the transformer. 
       SUMMARY 
       [0003]    In some aspects of the invention, a method is disclosed that comprises providing a signal to a transformer for supplying power during a reduced activity state. The signal includes a series of pulses that includes a positive pulse and a negative pulse. The signal is a segmented version of another signal provided to the transformer during an active state. The width of the positive and negative pulses may be adjustable. The series of pulses may include one or more pulse patterns such as the positive pulse being provided prior to the negative pulse, alternating positive pulses and negative pulses, alternating pairs of positive and negative pulses, etc. A predefined time delay may be introduced between the positive pulse and the negative pulse. A trigger may initiate a transition from the inactive state to the active state or a transition from the active state to the inactive state. The second signal may include an alternating current (AC) voltage signal. The series of pulses may include a series of half cycle pulses such as a series of half cycles of an AC voltage signal. 
         [0004]    In some aspects of the invention, an apparatus is disclosed that comprises a driver circuit to provide a signal to a transformer for supplying power during a reduced activity state. The signal includes a series of pulses that includes a positive pulse and a negative pulse. The signal is a segmented version of another signal provided to the transformer during an active state. The series of pulses may include one or more pulse patterns such as the positive pulse is provided prior to the negative pulse, alternating positive pulses and negative pulses, alternating pairs of positive and negative pulses, etc. A time delay may separate the positive pulse and the negative pulse. A counter may define the time delay, for example, based upon counting an odd number of cycles. The apparatus may include an audio equipment device. The driver circuit may receive a trigger signal to initiate a transition from the reduced-activity state to the active state. The signal provided during the active state may include an AC voltage signal. The driver circuit may include one or more bidirectional semiconductor devices such as a triac. The series of pulses may include a series of half cycle pulses such as a series of half cycles of an AC voltage signal. 
         [0005]    In some aspects of the invention, a system is disclosed that comprises a transformer to provide power to a load. The system also includes a driver circuit to provide a signal to the transformer for supplying power during a reduced activity state. The signal includes a series of pulses that includes a positive pulse and a negative pulse. The signal is a segmented version of another signal provided to the transformer during an active state. The system may also include a standby circuit to initiate a transition from the reduced-activity state to the active state. The load may include an audio equipment device. The signal provided to the transformer during the active state may be a signal that includes an AC voltage signal. A trigger signal received by the standby circuit may initiate the transition from the reduced-activity state to the active state. The transformer may provide power to the load during the active state. The series of pulses may include one or more pulse patterns such as the positive pulse is provided prior to the negative pulse, alternating positive pulses and negative pulses, alternating pairs of positive and negative pulses, etc. Various types of transformers may be implemented such as an iron core transformer. 
         [0006]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
     
       DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a diagram of a system that includes power supply, a load and a standby circuit. 
           [0008]      FIG. 2  is a schematic diagram of a transformer driver circuit. 
           [0009]      FIG. 3  is a diagram of waveforms for producing a positive half cycle pulse. 
           [0010]      FIG. 4  is a diagram of waveforms for producing a negative half cycle pulse. 
           [0011]      FIG. 5  is a diagram of waveforms for providing power during a reduced activity state and an active state. 
       
    
    
       [0012]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a system  100  provides power to a load  102  from a power source  104  such as a wall outlet, a generator, or other type of source that provides a voltage signal that is consistent with a regional power standard (e.g., 120 alternating current (AC) volts 60 Hertz in the United States, 230 volts AC 50 Hertz in the United Kingdom, etc.). To provide power to the load  102 , a power supply  106  conditions the voltage signal from the power source  104  prior to delivery to the load. In this example, power supply  106  includes a transformer  108  that may increase or decrease (e.g., step-up, step-down) the voltage signal dependent upon the power needs of the load  102 . Power supply  106  may also perform one or more operations such as rectification, filtering, isolation, etc. 
         [0014]    While operating, the load  102  typically draws more current from the power supply  106  than during periods of inactivity. For example, if the load  102  includes a powered speaker, a tuner, a compact disc (CD) player and an amplifier, or other type of audio equipment device, a significant amount of current may be drawn from the power supply  106  during operation of the equipment. Alternatively, when not in operation (e.g., in an inactive state), less current may be drawn by the load  102 . 
         [0015]    To initiate power delivery to the load  102 , a standby circuit  110  sends a signal to a transformer driver  112  (included in the power supply  106 ) to commence the delivery of a voltage signal from the power source  104  to the transformer  108 . To initiate signal transmission to the transformer driver  112 , the standby circuit  110  may receive a trigger signal from an external source. For example, a user may press a button to power on the load  102  and, thereby, send a trigger signal to the standby circuit  110  that correspondingly sends a signal to the transformer driver  112 . Upon receiving the signal, the transformer driver  112  directs a voltage signal from the power source  104  to the primary side of the transformer  108 . Alternatively, a trigger signal may be generated by a sensor or a watchdog circuit configured to send a trigger signal when a certain condition is met. For example, a sensor circuit may sense audio input signal lines. If the device is in an inactive state, the sensor circuit produces a trigger signal upon sensing the presence of an audio signal on the input lines. 
         [0016]    Along with providing power from the power source  104 , the transformer driver  112  may halt power delivery to the transformer  108 . For example, if the button is depressed (or pressed a second time), the standby circuit  110  may signal the transformer driver  112  to halt power delivery to the transformer  108 . However, even though the load  102  may not be operating, portions of the system  100  may still need to be powered during period of load inactivity. For example, in preparation of receiving a trigger signal, the standby circuit  110  may continuously need power. In some conventional systems, a second transformer may be implemented to provide power to the standby circuit. 
         [0017]    However, along with needing additional circuit board space, additional costs may be incurred. In this exemplary design, standby circuit  110  is provided power from transformer  108 , which also provides power to the load  102 . By providing power to the standby circuit  110 , a secondary transformer or separate power supply is not needed for the standby circuit, thereby conserving circuit board real estate and cost. 
         [0018]    In general, the power needed by the standby circuit  110  is significantly less than the power needed by the load  102  during an active state. The transformer  108  itself along with the standby circuit  110  may dissipate power while the load  102  is inactive. For example, certain types of transformers may draw current even if no load is present on the secondary side of the transformer. Iron core transformers may draw current levels such that power dissipation may range, e.g., from four to six watts when a load is absent. Thus, along with driving the transformer  108  to provide power to the standby circuit  110  for continuous operation, the power needs to be provided in a manner to reduce power loss through the transformer. Thus, the transformer driver  112  may direct the transformer  108  to produce less power when the  102  is inactive and relatively more power when the standby circuit  110  has been trigger to place the load in an active state. 
         [0019]    To provide appropriate power levels when the load is inactive, the transformer driver  112  provides a series of pulses to the primary side of the transformer  108 . By providing the pulses, power is provided to the standby circuit  110  such that it continuously operates and is ready to receive trigger signals. Additionally, by providing pulses to the transformer  108 , power dissipation by the iron core of the transformer  108  is reduced, thereby conserving power while providing power to the standby circuit  110  for operation while the load  102  is inactive. 
         [0020]    Referring to  FIG. 2 , an exemplary transformer driver  112  receives on a pair of conductors  200  a voltage signal (e.g., 120 volts AC) that is used to produce a series of pulses that are provided to the transformer  108  on a conductor pair  202 . To produce the pulses, the transformer driver  112  may selectively gate half cycle portions of the voltage signal provided by the power source  104 . For example, a pulse series may include alternating positive and negative half cycles of the voltage signal from power source  104 . To initiate the production of the pulse series, another pair of conductors  204  receives, e.g., trigger signals from the standby circuit  110 . 
         [0021]    If a trigger signal has not been received and the load  102  is inactive, the transformer driver  112  provides a series of pulses on the conductor pair  202  so that the transformer  108  continuously powers the standby circuit  110 . For example, a series of pulses may include a number of half cycles of an AC voltage signal provided by the power source  104 . The half cycle pulses may alternate in polarity so that the transformer may not become biased for one polarity. One or more time delays may be introduced between the half cycle pulses since less power is typically needed for the standby circuit  110  to operate (compared to the combined operating of the load  102  and the standby circuit  110 ). By adding time delays between the pulses, power is conserved while still providing voltage to the transformer  108  to supply power to the standby circuit  110 . Alternatively, if the load  102  is in an active state (e.g., operating), the entire voltage signal (e.g., 120 volt AC signal) from the power source  104  may be provided on the conductor pair  202  to the primary side of the transformer  108 . Although, in some arrangements the voltage signal may be processed (e.g., filtered, etc.) prior to being provided to the transformer  108 . Along with providing power to the load  102 , the transformer uses the voltage signal to provide power for operation of the standby circuit  110 . 
         [0022]    As described below, the transformer driver  112  includes timing circuitry such that portions (e.g., half cycle pulses) of the power source  104  voltage signal are provided to the transformer  108  so that the standby circuit  110  is provided power to compile with one or more national or international energy programs (e.g., the Energy Star program, California Energy Commission (CEC) 400-2005-012 standard, etc.). For example, the timing circuitry may segment the voltage signal from the voltage source  104  to provide a series of half cycle pulses to the transformer  108 . Furthermore, the transformer driver  112  may adjust parameters (e.g., pulse width, etc.) of the pulses provided to the transformer  108 . By implementing timing circuitry, cost is reduced along with circuit board real estate compared to using a second transformer to provide power to the standby circuit  110  when the load  102  is inactive. Additionally, by providing a series of pulse to the transformer  108 , magnetic biasing of the transformer may be reduced. For example, by alternating the polarity of the pulses, the transformer  108  may not be biased for one polarity, which may reduce transformer impedance and increase power drain. 
         [0023]    Power source  104  connects to transformer driver  112  via a fuse  206  to protect against one or more operational faults (e.g., a short circuit) that may occur. Additionally, transformer driver  112  may include a varistor  208  that protects the driver from voltage and current transients that may be present in power source signals. In some arrangements, a varistor may not be implemented. 
         [0024]    The voltage signal from the power source  104  is provided to three resistors  210   a ,  210   b ,  210   c  that may have equivalent resistances (e.g., 68.1 K-Ohm) or different resistance values. Various types of resistors may be used to implement the resistors  210   a - c , for example, the resistors may be surface mounted with packages approximately 0.12 inches in length and 0.06 inches in width (e.g., a 1206 two terminal package). Along with resistor size dimensions, resistor spacing may be selected to comply with one or more industry standards. For example, each resistor layout may provide approximately 1.8 mm of spacing, thus, two resistors may have a combined spacing of 3.6 mm and thereby may meet an Underwriters Laboratory (UL) requirement for providing at least 3 mm of clearance between components that may experience phases of a power supply signal. Also, by using three resistors, the safety margin is increased for scenarios such as if one of the resistors  210   a - c  experiences a short circuit. 
         [0025]    Transformer driver  112  also includes a Zener diode  212  that produces an approximate square wave by limiting the voltage of the power source signal to a relatively small positive value (e.g., +five volts) and small negative value (e.g., −0.7 volt). Typically the positive value of the square wave is less than a nominal Zener voltage due to loading and low bias currents. The square wave produced by the Zener diode  212  is provided to a resistor  214  and a capacitor  216  connected as a low pass filter to remove noise components on the power source  104  signal and to incorporate a time delay into the waveform. For example, a time delay of 1.5 milli-seconds may be introduced into the square wave. The square wave is also half wave rectified by a diode  218  and is filtered by a capacitor  220 . 
         [0026]    A logical exclusive OR (XOR) gate  222  has a capacitor  224  connected to both gate inputs to provide a comparator operation upon the square wave provided by the low pass filter (i.e., resistor  214  and capacitor  216 ) via a resistor  226 . By connecting a feedback resistor  228  between the output of the XOR gate  222  and one of its gate inputs, a Schmitt trigger is formed that provides hysteresis (e.g., exceed two thresholds to indicate a state change) for filtering the square wave. The XOR gate  222  may be implemented by various types of gates such as a 74VHC86 XOR gate, produced by ST Microelectronics of Geneva, Switzerland, which are CMOS based and exhibit low power, high speed, and are tolerant of input voltages that exceed the supply voltage values. Resistors (e.g., a resistor  226 ) connected to the inputs or the output of the XOR gate  222  also assist in tolerating excessive input voltage values. 
         [0027]    The resistor  230  and a capacitor  232  connect to form a low pass filter that introduces another time delay (e.g., 1.5 milli-second) into the waveform. Thus, a square wave output from XOR gate  222  (e.g., that is time delayed by 1.5 milli-second due to resistor  214  and capacitor  216 ) is provided to one input of an XOR gate  234  and a delayed version of the square wave (provided by the resistor  230  and the capacitor  232 ) is provided to the other input of the XOR gate  234 . Due to the time delay between the input signals, XOR gate  234  outputs a pulse for each transition (e.g., logic low to logic high or logic high to logic low) of the square wave output by XOR gate  222 . Thus a pulse is produced for each half cycle (e.g., positive half cycle, negative half cycle) of the sinusoidal voltage signal provided by the power source  104 . 
         [0028]    To control the periodicity of the pulses produced by the transformer driver  112 , the pulses output by XOR gate  234  are used as a clock signal for a counter  236  (after being filtered by a resistor  238  and a capacitor  240  pair connected as a low pass filter). In one implementation, the counter  236  is an eight-bit counter such as an 74HC590 made by Phillips Electronics of the Netherlands. By providing a clock signal (to the CLK port of the counter  236 ), the pulses control the incrementing of the counter. 
         [0029]    An XOR gate  242  is connected as an inverter such that the output of the XOR gate  234  signals the count data to be stored in a register included in the counter  236 . In this implementation the counting and register storing may be rising edge triggered (such that counting and register updating alternate), however other triggering techniques (e.g., falling edge triggering) may be implemented. 
         [0030]    A logic one is output from the counter  236  to indicate that a particular number of voltage signal half cycles from the power source  104  have been counted. For example, after a particular number of cycles are counted, the output “G” of the counter  236  outputs a logic one that is provided to an input to an XOR gate  244 . Similar to XOR gate  242 , XOR gate  244  is connected as an inverter, however, the gate  244  is used to clear the counter. Once cleared the counter may resume counting until the “G” output again outputs a logic one to indicate a particular number of half cycles have been counted, and then clears to count again. 
         [0031]    The “G” output is a binary output that corresponds to a decimal 2 6  (i.e., 64). Each time the count reaches decimal 64, a logic one is output from the “G” output. In this implementation, the “G” output is also used to drive the clock of a flip-flop unit  246 . The flip-flop unit  246  may be, for example, an 74HC174 hex D flip flop package made by Phillips Electronics that includes six positive-edge triggered flip-flops. Each of six data input ports (D 0 -D 5 ) are provided data from the output of XOR gate  222 . Since the clock of the flip-flop unit  246  is provided by the “G” output of the counter  236 , a clock pulse appears at the counter every 2 6  pulses. Additionally, an output is provided by each of the six output ports (Q 0 -Q 5 ) of the flip-flop unit  246  based on the clock pulse. 
         [0032]    For this design a square wave is provided by the flip-flop unit  246  that changes states every 2 n +1 half cycles of the voltage signal from the power source  104 . In this implementation, by using the “G” output of the counter  236  the square wave changes states every 2 6 +1=65 half cycles. However, in other implementations, the number of half cycles between states changes may be more or less than 65 half cycles, and may or may not conform to the 2 n +1 half cycle constraint. For example, the clock of the flip-flop unit  246  may be connected to the “H” output (e.g., 2 7  output) or the “F” output (e.g., 2 5  output) of the counter  236 , thereby providing a square wave from the six flip-flop unit  246  that changes states every 129 or 33 half cycles of the voltage signal from as the power source  104 . 
         [0033]    By counting 2 n +1 half cycles, an odd number of half cycles is counted (e.g., 33, 65, 129) prior to the output from the flip-flop unit  246  changing state. Due to the odd numbered count, the output of the flip-flop unit  246  (e.g., the six parallel connected flip-flops outputs) provides a series of pulses that correspond to half cycles of the power source voltage signal with alternating polarities (e.g., negative half cycle, positive half cycle, negative half cycle, etc.). 
         [0034]    By connecting each of the six output ports (Q 0 -Q 5 ) of the flip-flop unit  246  in parallel, sufficient drive current may be provided with the produced pulse series for a resistor  248  with a resistance of 33 Ohms, for example. A capacitor  250  is connected in series with the resistor  248  to provide AC coupling for the series of pulses with alternating polarity. The resistor  248  and the capacitor  250  are typically selected to produce a time constant so that triggering based on the pulses does not extend into the following half cycle. Thus, the selected half cycles are provided to the transformer  108  and two consecutive half-cycles (e.g., a positive half cycle directly followed by a selected negative half cycle) are not provided to the transformer. 
         [0035]    As mentioned above, during reduced activity periods transformer driver  112  provides a series of alternating polarity half cycles to the transformer  108 . Accordingly, a semiconductor device that bi-directionally conducts is incorporated into the transformer driver  112 . In this implementation a triac  252  is connected between the capacitor  250  and the one of the conductors in the conductor pair  202 . Other types of bidirectional semiconductor devices such as a thyristor, two silicon-controlled rectifiers (SCRs) configured in an inverse parallel arrangement, or other similar semiconductor devices may be implemented individually or in conjunction with the triac  252 . Combinations of two or more transistors (e.g., bipolar junction transistors, field effect transistors, etc.) may also be implemented to provide a bidirectional semiconductor device. 
         [0036]    The triac  252  is a three-quadrant triac and operates in two or the quadrants based on the polarities of the pulses provided by the AC coupling of the capacitor  250 . In particular, the triac  252  operates in the first quadrant (e.g., positive voltage present on main terminal one of the triac and a positive gate current) and the third quadrant (negative voltage present on main terminal one and a negative gate current). While in the first or third quadrant, the triac  252  provides a corresponding half cycle (e.g., positive half cycle or negative half cycle) present on the power source voltage signal to the transformer  108  via conductor pair  202 . When the triac  252  is not operating in either of the two quadrants, no signal is provided to the transformer  108 . A pair of diodes  254  are connected in parallel with the output ports of the flip-flop unit  246  to provide protection for voltage transients present on the power source  104  voltage signal that may be provided to the XOR circuitry (e.g., XOR  242 ) via the triac  252 . 
         [0037]    To assure that one half cycle is provided, a relatively narrow trigger pulse is provided to the triac  252 . Additionally, by delaying the trigger pulse, the provided half cycle begins slightly past the zero-crossing point so that an appropriate gate current may be provided to the triac  252 . This time delay is provided by the two approximately 1.5 milli-second time delays introduced by the low pass filters (e.g., resistor  230 , capacitor  232 ). By introducing the approximately 3.0 milli-second time delay along with a relatively narrow trigger pulse, the triac  252  is triggered to operate during one half cycle. In some implementations, a positive current pulse is provided to the gate of the triac  252  to trigger delivery of a positive half cycle to the transformer  108  and a negative pulse is provided to the gate to trigger delivery of a negative half cycle. A waveform  256  illustrates a positive half cycle and a negative half cycle that are produced for delivery to the transformer  108 . 
         [0038]    The pulse repetition frequency of the half cycles may be selected based upon one or more parameters such as efficiency. For example, for a 60 Hertz power source voltage signal and a count of 65 (i.e., 2 6 +1), alternating half cycles may be provided approximately every 0.54 seconds. Provided such a series of half cycles with alternating polarity, the transformer  108  may provide sufficient voltage from its secondary side to operate the standby circuit  110  during periods of inactivity while significantly reducing the power consumption. Some operations of the standby circuit  110  may include receiving and decoding trigger signals from a remote control, a button, or other type of user interface device. 
         [0039]    Upon receiving a signal to provide power to the load  102 , standby circuit  110  sends a signal to the transformer driver  112  to change from a standby state to an active state such that the load  102  may be provided appropriate operating power. For example, when placed in an active state, the transformer  108  may provide the load  102  (e.g., a powered speaker, a CD-player, amplifier, etc.) with power for performing typical operations (e.g., retrieving audio content from a CD, amplifying the content, playing the content over the powered speaker). 
         [0040]    In this implementation, an opto-coupler  258  is configured to receive a signal from the standby circuit  110  via the conductor pair  204 . For example, a signal may be provided to the opto-coupler  258  that requests that the power supply  106  provide operating power to the load  102 . Alternatively, a signal may be provided to the opto-coupler that represents that the load  102  is to be powered down and only the standby circuit  110  is to receive power. 
         [0041]    Using light emissions for a trigger, the opto-coupler  258  also provides electrical isolation between the standby circuit  110  and the transformer driver  112 . Thus, the opto-coupler  258  provides a safety layer while allowing signals to be sent from the secondary side of the transformer  108  to the primary side of the transformer. While the opto-coupler  258  is incorporated to pass signals while providing a level of safety, other light sensitive devices such as opto-isolators and photodiodes may also be implemented individually or in combination. 
         [0042]    When the opto-coupler  258  receives a signal from the standby circuit  110 , a signal is provided to the triac  252  for continuous operation. Once the triac  252  is biased by the opto-coupler signal, the voltage signal from the power source  104  is provided on the conductor pair  202  to the primary side of the transformer  108 . In this implementation, the sinusoidal voltage signal is stepped-down by the transformer  108  and then rectified to produce a direct current (DC) voltage signal. However, in other implementations, the transformer  108  may step-up the voltage signal being provided to the primary winding. 
         [0043]    Similar to triggering the transformer driver  112  to place the power supply  106  in an active state, the standby circuit  110  may also trigger the transformer driver  112  to return the power supply to a reduced activity state. In such a scenario, the opto-coupler  258  may send a signal to the triac  252  such that the triac does not continuously operate. Triggering control of the triac  252  is returned to the counter and flip-flop circuitry included in the transformer driver  112 . 
         [0044]    Referring to  FIG. 3 , a series of waveforms that represent signals present at particular locations in the transformer driver  112  is shown. Each waveform is represented as a function of time by a horizontal axis  300 . Voltage is represented by a vertical axis  302  and each waveform is offset for ease of visual comparison. 
         [0045]    Waveform  304  represents a typical sinusoidal voltage signal (e.g., 60 Hertz 120 Volts AC) that is provided by the power source  104  to the transformer driver  112 . The Zener diode  212  rectifies the signal to produce a square wave  306 . Waveform  308  represents the signal across the capacitor  216 . Due to the low pass filter produced by the resistor  214  and the capacitor  216 , the high frequency content of the square is removed and waveform  308  is smoothed compared to the waveform  306 . Waveform  310  represents the logic levels present at the output from the XOR gate  222 . As shown in  FIG. 2 , the waveform  310  is provided to an input of the XOR gate  234  and is also provided to the low pass filter formed by the resistor  230  and the capacitor  232 . The low pass filter introduces a time delay into the waveform  310  and provides a filtered waveform  312  to another input of the XOR gate  234 . Due to the time delay introduced into the waveform  312 , the output of the XOR gate  234  provides a logic “1” to represent the transitions of waveform  310  from a low to high level or from a high to low level. A waveform  314  represents the output of XOR gate  234 . The waveform  314  is inverted by the XOR gate  242  as represented by a waveform  316  and is provided to the counter  236  to control incrementing of the counter. 
         [0046]    As counter  236  increments, data is placed in a register and a pulse is output to represent attaining a particular count. For example, a waveform  318  represents a logic “1” provided from the “G” output of counter  236  to indicate that a count of 2 6 =64 has been achieved. The waveform  318  is also inverted by XOR gate  244  (connected as an inverter) to provide a counter clearing signal to the counter  236  as represented by a waveform  320 . The waveform  318  also provides a clock signal to the flip-flop unit  246 . To produce a positive pulse for the gate of triac  252 , a waveform  322  represents a low to high level output transition of the flip-flop unit  246 . The waveform  322  is provided to the resistor  248  and the AC coupling capacitor  250  for delivery to the gate of the triac  252 . 
         [0047]    A waveform  324  represents an exponentially decaying pulse that is provided to the gate of the triac  252 . By receiving the waveform  324 , the triac  252  is triggered to provide a corresponding positive half cycle from the power source voltage signal, represented by a waveform  326 , to the primary side of the transformer  108 . In this example, a positive half cycle is provided to the transformer  108  by the operations of the transformer driver  112 , as represented by the waveforms  304 - 326 . However, to reduce magnetic biasing of the transformer  108 , half cycle pulses with negative polarity may also be provided to the transformer. 
         [0048]    Referring to  FIG. 4 , a series of waveforms representative of operations performed by the transformer driver  112  for producing a negative half cycle pulse is shown. Equivalent to the waveforms  304 - 320 , waveforms  404 - 420  represent operations of components included in the transformer driver  112 . Also similar to the waveforms shown in  FIG. 3 , horizontal axis  400  and vertical axis  402  respectively represent time and offset voltage levels. 
         [0049]    To initiate the production of a negative half cycle pulse for delivery to transformer  108 , a high to low level transition is provided by the output of flip-flop unit  246  as represented by waveform  422 . This voltage transition signal is provided to the resistor  248  and the AC coupling capacitor  250  that provides the gate of the triac  252  with a signal represented by a waveform  424 . This exponentially increasing signal triggers the triac  252  and a negative half cycle pulse is produced, as represented by a waveform  426 , and provided to the transformer  108 . 
         [0050]    Referring to  FIG. 5 , two waveforms  500 ,  502  respectively represent voltage signals present on the primary and secondary side of the transformer  108  during a reduced activity state (e.g., when the power supply is in standby). In particular, waveform  500  represents a voltage signal on the primary winding of the transformer. Due to the reduced activity state, the transformer driver  112  provides only a portion of the voltage signal from the power source  104 . Half cycle pulses of alternating polarity are selected by the transformer driver  112  so that the primary side of the transformer  108  is not substantially magnetically biased. In this example, a time delay between the alternating half cycles is introduced by the counter  236  in the transformer driver  112 . The time delay may be increased or decreased depending upon the application of the power supply  106 . 
         [0051]    The voltage level of the half cycle pulses typically depends upon the voltage signal provided by the power source  104 . For example, for a 120 volts AC power source signal, the voltage level of each half cycle pulse is approximately +170 volts root mean square (RMS) for positive half cycles and −170 volts RMS for negative half cycles. 
         [0052]    The waveform  502  represents the processed (e.g., rectified, filtered) voltage signal present on the secondary side of the transformer  112  when the waveform  500  is present on the primary side. The waveform  502  is a sawtooth voltage signal that may provide a positive voltage for the time periods between the half cycle pulses. To provide power to the standby circuit  110  during the reduced activity state, the voltage signal, for example, may have peak values of approximately 16 volts and minimum values of 6 volts. While voltage levels in this range may be large enough to power the standby circuit, other voltage ranges may also be provided dependent upon the transformer  108  and the processing (e.g., rectification, filtering). 
         [0053]    A pair of waveforms  504  and  506  that represent voltage signals as the power supply  106  transitions from a standby state (in which power is only provided to the standby circuit) to an active state (in which power is provided to the load and the standby circuit). The waveform  504  represents the voltage signal present on the primary side of the transformer  108  and the waveform  506  represents the processed (e.g., rectified, filtered) signal from secondary side of the secondary side of the transformer  108 . During the time period between T 0  and T 1 , power supply  106  is operating in standby and the transformer driver  112  provides alternating half cycle pulses  508  and  510  to the primary winding of the transformer. Similar to the waveform  500 , the half cycle pulses alternate in polarity and provide power to the standby circuit  110 . At T 1 , a signal (e.g., button being pushed, remote control signal, etc.) is provided to the standby circuit  110  and the power supply  106  transitions to an active state in which power is provided to the load  102  and the standby circuit  110 . While in this state, the triac  252  is continuously on by a signal from the standby circuit  110  (via the opto-coupler  258 ). Accordingly, the voltage signal provided by the power supply  104  is provided to the primary side of the transformer  108 . In this example, a 120 volt AC signal is provided to the transformer  108  from time T 1  onward. 
         [0054]    The waveform  506  includes a sawtooth portion from T 0  to T 1  that is similar to the sawtooth waveform shown in waveform  502 . The voltage level of the waveform  506  ranges between a peak value of approximately 16 volts to a minimum value of approximately 6 volts for providing power to the standby circuit  110  during the reduced activity state. From T 1  onward, the voltage signal remains substantially constant at 16 volts for providing power to the load  102  and the standby circuit  110 . 
         [0055]    In the previous examples, the transformer driver  112  provided a series of half cycle pulses while in a reduced activity state. In particular, the series included half cycle pulses that alternated in polarity (e.g., positive pulse, negative pulse, positive pulse, etc.). However, other pulse arrangements, patterns and combinations may also be implemented. For example, rather than alternating polarity on a pulse by pulse basis, polarity may be alternated for each pair of pulses (e.g., positive pulse, positive pulse, negative pulse, negative pulse, etc.) or another similar pattern. 
         [0056]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, any voltage level or range of voltage levels may be provided by the power supply while in a reduce activity state (e.g., a standby state) or in a active state. Various time delays may be incorporated into the series of pulses provided to the transformer  108 . For example, time delays larger or smaller than the period of the power source  104  voltage signal may be implemented. Accordingly, other embodiments are within the scope of the following claims.