Abstract:
A switched mode power converter includes a feedback mechanism by which a coded train of pulses with well defined integrity is generated on a secondary side of the power converter and transmitted to the primary side for decoding and application by a waveform analyzer to regulate the power converter output. The pulse train is modulated by a secondary side controller and transmitted across an isolation galvanic barrier. The main transformer is used as the signal transmitter from the secondary side to the primary side of the power converter. The coded pulse train is recognized by a controller on the primary side and translated into a regulating driving signal for a main switching element. The transmitted coded pulse train can be embedded with very high frequency modulation that allows the isolation galvanic barrier to act as a capacitive signal transmitter.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application claims priority under 35 U.S.C. 119 (e) of the co-pending U.S. Provisional Application Ser. No. 61/793,099, filed Mar. 15, 2013, and entitled “New Power Management Integrated Circuit Partitioning”. This application incorporates U.S. Provisional Application Ser. No. 61/793,099 in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is generally directed to the field of power converters. More specifically, the present invention is directed to controlling a power converter. 
       BACKGROUND OF THE INVENTION 
       [0003]    In many applications a power converter is required to provide a voltage within a predetermined range formed from a voltage source having a different voltage level. Some circuits are subject to uncertain and undesirable functioning and even irreparable damage if supplied power falls outside a certain range. More specifically, in some applications, a precise amount of power is required at known times. This is referred to as regulated power supply. 
         [0004]    In order to control a power converter to deliver a precise amount of power as conditions require, some form of control of the power converter is required. This control can occur on the primary side of an isolation transformer or the secondary side. A closed loop feedback control system is a system that monitors some element in the circuit, such as the circuit output voltage, and its tendency to change, and regulates that element at a substantially constant value. Control on the secondary side of a power converter can use a monitored output voltage as feedback control, but requires the use of some communication from the secondary to the primary side of the isolation transformer to control the primary side switching element. Control on the primary side can readily control the primary side switching element, but requires some feedback mechanism from the secondary side to the primary side to convey the status of the monitored element. In some applications, an optical coupler circuit, or opto coupler, is used to transmit feedback signals while maintaining electrical isolation between the primary and secondary sides. 
         [0005]      FIG. 1  illustrates a conventional regulated switch mode power converter including an optical coupler circuit. The power converter  2  is configured as a traditional flyback type converter. The power converter  2  includes an isolation transformer  4  having a primary winding P 1  and a secondary winding S 1 . The primary winding P 1  is electrically coupled to an input voltage Vin and a driving circuit including a transistor  8 , a resistor  12 , and a controller  10 . A capacitor  28  is coupled across the input Vin and coupled with the primary winding P 1 . Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. The transistor  8  is a fast-switching device, such as a MOSFET, the switching of which is controlled by the fast dynamic controller  10  to maintain a desired output voltage Vout. The controller  10  is coupled to the gate of the transistor  8 . As is well known, the DC/DC conversion from the primary winding P 1  to the secondary winding S 1  is determined by the duty cycle of the PWM switching signal provided to the transistor  8 . The secondary winding voltage is rectified and filtered using the diode  6  and the capacitor  22 . A sensing circuit and a load  14  are coupled in parallel to the secondary winding S 1  via the diode  6 . The sensing circuit includes a resistor  16 , a resistor  18 , and a secondary controller  20 . The secondary controller  20  senses the output voltage Vout across the load. 
         [0006]    In this configuration, the power converter is controlled by driving circuitry on the primary side, and the load coupled to the output is isolated from the control. As such, a monitored output voltage used for voltage regulation is required as feedback from the secondary side to the control on the primary side. The power converter  2  has a voltage regulating circuit that includes the secondary controller  20  and an optical coupler circuit. The optical coupled circuit includes two galvanically isolated components, an optical diode  24  coupled to the secondary controller  20  and an optical transistor  26  coupled to the controller  10 . The optical diode  24  provides optical communication with the optical transistor  26  across the isolation barrier formed by the transformer  4 . The optical coupler circuit in cooperation with the secondary controller  20  provides feedback to the controller  10 . The controller  10  accordingly adjusts the duty cycle of the transistor  8  to compensate for any variances in an output voltage Vout. 
         [0007]    However, the use of an optical coupler circuit in and of itself presents issues. Firstly, the optical coupler circuit adds extra cost. In some applications, the optical coupler circuit can add more cost to the power converter than the isolation transformer. The optical coupler circuit also adds to manufacturing and testing costs. Furthermore, the performance of the optical coupler circuit degrades over time and therefore introduces another potential point of failure in the overall power converter. Also, characteristics of the optical coupler circuit must be accounted for in the overall circuit design. For example, the optical diode component is non-linear and as such a correlation between the optical diode and the optical transistor must be established. The optical coupler circuit also has delays related to the operation of the optical diode and the optical transistor, and the operation of the optical diode requires a well defined DC level. As a result, it is generally desirable to avoid the use of an optical coupler circuit. 
         [0008]    A next generation of feedback control does not use optical control circuitry. Instead, the transformer is used to convey real-time feedback signaling from the secondary side to the primary side. In such an application, the transformer includes an auxiliary winding on the primary side that is magnetically coupled to the secondary winding.  FIG. 2  illustrates a conventional regulated power converter including a magnetically coupled feedback circuit. The power converter  32  is configured as a traditional flyback type converter. The power converter  32  includes an isolation transformer  34  having a primary winding P 1  and a secondary winding S 1 . The primary winding P 1  is electrically coupled to an input voltage Vin and a driving circuit including a transistor  44 , a resistor  46 , and a controller  42 . A capacitor  58  is coupled across the input Vin and coupled with the primary winding P 1 . Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. Similar to the power converter in  FIG. 1 , the transistor  44  is a fast-switching device controlled by the fast dynamic controller  42  to maintain a desired output voltage Vout. The secondary winding voltage is rectified and filtered using the diode  36  and the capacitor  38 , with the output voltage Vout delivered to the load  40 . 
         [0009]    The power converter  32  has a feedback loop that includes a magnetically coupled feedback circuit coupled to the secondary winding S 1  of the transformer  34  and the controller  42 . The magnetically coupled feedback circuit includes a diode  48 , a capacitor  50 , resistors  52  and  54  and an auxiliary winding  56 . The auxiliary winding  56  is coupled in parallel to the series of resistors  52  and  54 . 
         [0010]    The voltage VA is proportional to the voltage across the auxiliary winding  56 . The voltage VA is provided as a feedback voltage VFB to the controller  42 . The current through the transistor  44  is also provided as feedback current IFB to the controller  42 . The controller  42  includes a real-time waveform analyzer that analyzes input feedback signals, such as the feedback voltage VFB and the feedback current IFB. 
         [0011]    The auxiliary winding  56  is also magnetically coupled to the secondary winding S 1 . When the current through the diode  36  is zero, the voltage across the secondary winding S 1  is equal to the voltage across the auxiliary winding  56 . This relationship provides means for communicating the output voltage Vout as feedback to the primary side of the circuit. The voltage across the auxiliary winding  56  is measured when it is determined that the current through the diode  36  is zero, which provides a measure of the voltage across the secondary winding S 1  and therefore the output voltage Vout. 
         [0012]    The feedback voltage VFB when the diode  36  current is zero is determined and is referred to as the “voltage sense”, and the feedback current IFB when the diode  36  current is zero is determined and is referred to as the “current sense”. The voltage sense and the current sense are input to the real-time waveform analyzer within the controller  42 .  FIG. 3  illustrates a functional block diagram of a conventional real-time waveform analyzer  60 . Error amplifiers  62  and  64  are acceptors of the regulating means, which in this configuration are the sensed voltage and the sensed current. The error amplifier compares the input sensed voltage to a reference voltage and outputs a first difference value. The first difference value is amplified by the gain of the error amplifier  62 . The error amplifier  64  compares the amplified first difference value to the sensed current and outputs a second difference value that is either High or Low. A pulse width modulation (PWM) block  66  is configured as a Flip-Flop digital device. The output of the PWM block  66  is set according to the switching frequency of the clock  68  and is Reset by the High or Low value input from the error amplifier  64 . The variable signal applied to the Reset pin generates an output signal that is a pulse train modulated by the pulse width. A multiple input OR gate  70  inputs the clock signal, the pulse train signal, a shutdown signal, and a OVP/UVP/OTP signal, where OVP stands for “over voltage protection”, UVP stands for “undervoltage protection” and OTP stands for “over temperature protection”. The waveform analyzer is configured to output a high voltage value when one of the inputs to the OR gate is high or to output a low voltage value when all of the inputs to the OR gate are low. The high voltage value output from the waveform analyzer corresponds to turning on the transistor  44  in  FIG. 2 . The low voltage value corresponds to turning off the transistor  44 . The OR gate also enables a high voltage signal output from the PWM block  66  to propagate to the output by monitoring abnormal conditions such as under voltage, over voltage, over temperature, etc. In this manner, the pulse width of each pulse can be modified which adjusts the output voltage into regulation. 
         [0013]    In general, control intricacies of the waveform analyzer are aligned with control argument sampling to achieve overall system functional performance. Sampling argument is in the form of current, voltage and impedance. System functional performance is in the form of pulse width modulation (PWM), pulse frequency modulation (PFM) and pulse amplitude modulation (PAM). The waveform analyzer of  FIG. 3  is limited to signal processing in DC or real-time switching waveforms. In either case, the feedback signal received by the waveform analyzer requires some status integrity, such as no noise on the DC level, no disturbance on the switching waveform and to some degree represent a combination of analog and digital representations. The voltage across the auxiliary winding typically forms a pulse train with frequency corresponding to the switching frequency of the driving transistor. The voltage across the auxiliary winding when the secondary winding current is zero, which corresponds to the diode  36  current equaling zero, corresponds to the falling edge of the pulse. As such, measuring an accurate voltage value requires that the pulse is well defined with sufficient pulse integrity particularly at the falling edge. Further, the voltage value immediately following the rising edge includes ringing due to the leakage impedance of the transformer. As such, pulse integrity also requires sufficient time for the voltage value to stabilize following the rising edge. Higher switching frequencies minimize the pulse width and therefore provide less time for voltage stabilization. For at least these reasons, providing a pulse with sufficient pulse integrity is often difficult to achieve. 
       SUMMARY OF THE INVENTION 
       [0014]    Embodiments of a switched mode power converter are directed to a feedback mechanism by which a coded train of pulses with well defined integrity is generated on a secondary side of the power converter and transmitted to the primary side for decoding and application by a waveform analyzer to regulate the power converter output. The pulse train is modulated by a secondary side controller. Such a technique allows the transmission of regulation data over an isolation galvanic barrier without using an optical coupling circuit. Coded information is included in the coded pulse train by modulating pulses of the pulse train including, but no limited to, the pulse width, the pulse amplitude, the pulse frequency, or any combination thereof. For example, the pulse train can be modulated by the number of pulses over a predetermined time period, or the number of pulses with different amplitudes over the time period. In some embodiments, the main transformer is used as the signal transmitter from the secondary side to the primary side of the power converter. In some embodiments, the coded train of pulses is recognized by a controller on the primary side and translated into a Set or Reset signal input to a Flip-Flop circuit within a waveform analyzer. The waveform analyzer generates a train of pulses that energize a main power switch. In some embodiments, the transmitted coded pulse train is embedded with very high frequency modulation that allows the isolation galvanic barrier to act as a capacitive signal transmitter. 
         [0015]    In an aspect, a method of controlling a switching mode power converter is disclosed. The method includes configuring a switching mode power converter that includes a transformer, an output circuit coupled to a secondary winding of the transformer, a first switch coupled to a primary winding of the transformer and a first controller coupled to the first switch. The output circuit includes a second switch coupled to the secondary winding of the transformer and a second controller coupled to the second switch. The method also includes measuring an output characteristic of the output circuit. The method also includes driving the second switch by the second controller to generate a coded signal. The coded signal includes coded control information based on the output characteristic. The method also includes transmitting the coded signal from the output circuit to the first controller, and decoding the coded signal by the first controller to identify the control information. The method also includes generating a driving signal according to the control information, and driving the first switch using the driving signal to regulate the output characteristic. 
         [0016]    The coded signal can be a pulse train signal having a modulated plurality of voltage pulses. The modulated plurality of pulses can be modulated by modulating one or more of a pulse amplitude, a pulse frequency, a pulse width, a pulse delay, and a number of pulse cycles. The coded signal can be transmitted during an OFF period of the first switch. The output characteristic can be one or more of an output voltage, an output current, and an output power of the power converter. The control information can be the output characteristic. The control information can be a control instruction. Generating the coded signal can include generating and applying a driving signal to the second switch to turn the second switch on and off according to a determined pattern resulting in a modulated plurality of voltage pulses across the second winding of the transformer. Turning the second switch on can enable a negative secondary current through the secondary winding. Enabling the negative secondary current can include enabling an alternative current path from an output capacitor in the output circuit to the secondary winding. The secondary winding can be magnetically coupled to an auxiliary winding, and the auxiliary winding can be coupled to the first controller, further wherein transmitting the coded signal can include using the magnetic coupling between the secondary winding and the auxiliary winding to transmit the coded signal from the output circuit to the first controller. Transmitting the coded signal can include using a parasitic capacitance between a primary side and a secondary side of the power converter. The parasitic capacitance can be a parasitic capacitance of the transformer. The parasitic capacitance can be an inherent capacitance of a printed circuit board onto which the power converter is assembled, wherein the inherent capacitance is a result of a component layout on either side of an isolation galvanic barrier. The first switch can be a first transistor and the second switch can be a second transistor. 
         [0017]    In another aspect, a switching mode power converter is disclosed. The power converter includes a transformer, a first switch, a first controller, a second switch, a second controller, and a sensing circuit. The transformer has a primary winding coupled to an input supply voltage and a secondary winding. The first switch is coupled in series to the primary winding. The first controller is coupled to the switch, wherein the first controller is configured to turn the first switch ON and OFF. The second switch is coupled in series to the secondary winding. The second controller is coupled to the second switch, wherein the second controller is configured to turn the second switch ON and OFF. The sensing circuit is coupled to the secondary winding and the second controller, wherein the sensing circuit is configured to sense an output characteristic of the power converter. The second controller is configured to drive the second switch to generate a coded signal that includes coded control information based on the output characteristic. The transformer is configured as a signal transmitter to transmit the coded signal from a secondary side of the transformer to a primary side of the transformer. The first controller is configured to decode the coded signal to identify the control information, generate a driving signal according to the control information, and drive the first switch using the driving signal to regulate the output characteristic. 
         [0018]    The coded signal can be a pulse train signal having a modulated plurality of voltage pulses. The modulated plurality of pulses can be modulated by modulating one or more of a pulse amplitude, a pulse frequency, a pulse width, a pulse delay, and a number of pulse cycles. The coded signal can be transmitted during an OFF period of the first switch. The output characteristic can be one or more of an output voltage, an output current, and an output power of the power converter. The control information can include the output characteristic. The control information can include a control instruction. The power converter can also include an auxiliary winding coupled to the first controller, wherein the auxiliary winding is magnetically coupled to the secondary winding. The transformer can be configured to transmit the coded signal from the secondary winding to the auxiliary winding using the magnetic coupling between the secondary winding and the auxiliary winding. The transformer can be configured to transmit the coded signal from the secondary winding to the auxiliary winding using a parasitic capacitance between a primary side and a secondary side of the power converter. The parasitic capacitance can be a parasitic capacitance of the transformer. The parasitic capacitance can be an inherent capacitance of a printed circuit board onto which the power converter is assembled, wherein the inherent capacitance is a result of a component layout on either side of an isolation galvanic barrier. The first switch can be a first transistor and the second switch can be a second transistor. The sensing circuit can be a voltage divider circuit. The power converter can also include a diode coupled in parallel to the second switch and an output capacitor coupled in series to the diode, wherein the diode is configured to enable current flow from the secondary winding to the output capacitor when forward-biased. When the second switch is ON, an alternative current path can be formed between the output capacitor and the secondary winding of the transformer, further wherein a negative secondary current flows from the output capacitor to the secondary winding via the alternative current path. The negative secondary current can be generated as discharge from the output capacitor when the alternative current path is formed. The output circuit can be configured as a bi-directional current flow circuit, wherein a first current flow direction corresponds to positive secondary current from the secondary winding to the output capacitor and to an output of the output circuit to deliver power, and a second current flow direction corresponds to negative secondary current from the output capacitor to the secondary winding to provide control for switching the first switch coupled to the primary winding. The power converter can be configured as one of a flyback-type power converter circuit, a forward-type power converter circuit, a push-pull-type power converter circuit, a half-bridge-type power converter circuit, and a full-bridge-type power converter circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Several example embodiments are described with reference to the drawings, wherein like components are provided with like reference numerals. The example embodiments are intended to illustrate, but not to limit, the invention. The drawings include the following figures: 
           [0020]      FIG. 1  illustrates a conventional regulated switch mode power converter including an optical coupler circuit. 
           [0021]      FIG. 2  illustrates a conventional regulated power converter including a magnetically coupled feedback circuit. 
           [0022]      FIG. 3  illustrates a functional block diagram of a conventional real-time waveform analyzer. 
           [0023]      FIG. 4  illustrates a power converter according to an embodiment. 
           [0024]      FIG. 5  illustrates a functional block diagram of a portion of the controller for processing the coded voltage pulse train according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    Embodiments of the present application are directed to a power converter. Those of ordinary skill in the art will realize that the following detailed description of the power converter is illustrative only and is not intended to be in any way limiting. Other embodiments of the power converter will readily suggest themselves to such skilled persons having the benefit of this disclosure. 
         [0026]    Reference will now be made in detail to implementations of the power converter as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts. In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application and business related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure. 
         [0027]      FIG. 4  illustrates a power converter according to an embodiment. The power converter  100  is configured to receive an unregulated DC voltage signal at an input node Vin and to provide a regulated output voltage Vout. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification. The input voltage is typically filtered, such as via capacitor  102 . 
         [0028]    The power converter  100  is configured as a flyback converter. It is understood that the concepts described herein can be applied to alternatively configured switched mode converters including, but not limed to, a forward converter, a push-pull converter, a half-bridge converter, and a full-bridge converter. The power converter  100  includes an isolation transformer  104  having a primary winding P 1  and a secondary winding S 1 . The primary winding P 1  is electrically coupled to the input voltage Vin and a driving circuit including a switch  106 , a sense resistor  112 , and a controller  110 . The switch  106  is coupled in series with the primary winding P 1  of the transformer  104  and the sense resistor  112 . The controller  110  is coupled to the switch  106  to turn the switch ON and OFF. 
         [0029]    The power converter  100  further includes output circuitry coupled to the secondary winding S 1  of the transformer  104 . The output circuitry includes a freewheeling rectifier diode  116 , a switch  118 , a controller  120 , and an output capacitor  126 . The switch  118  is coupled in parallel to the diode  116 . An anode of the diode  116  is coupled to a first terminal of the secondary winding. A cathode of the diode  116  is coupled to a first terminal of the output capacitor  126  and coupled to the output node Vout. The output capacitor  126  is coupled to the Vout node across an output load, represented by a resistor  128 . The controller  120  is coupled to the switch  118  to turn the switch ON and OFF. The output circuitry also includes a sensing circuit configured to measure a circuit characteristic to be regulated such as an output voltage, an output current, and/or an output power. In this exemplary configuration and succeeding description, the power circuit is described as sensing and regulating the output voltage Vout. In the exemplary configuration of  FIG. 4 , the sensing circuit includes a resistive voltage divider including the resistors  122  and  124  coupled in parallel to the capacitor  126  to measure a voltage across the capacitor  126 . It is understood that an alternative sensing circuit can be used to measure the output voltage Vout. In general, the sensing circuit can be configured to use any conventional technique for determining the value of the regulated circuit characteristic. 
         [0030]    The switch  106  and the switch  118  are each a suitable switching device. In an exemplary embodiment, the switch  106  and the auxiliary switch  118  are each a n-type metal-oxide-semiconductor field-effect transistor (MOSFET) device. Alternatively, any other semiconductor switching device known to a person of skill in the art can be substituted for the switch  106  and/or the switch  118 . Subsequent description is based on an n-channel MOSFET. 
         [0031]    The power converter  100  has a feedback loop that includes a magnetically coupled feedback circuit coupled to the secondary winding S 1  of the transformer  104  and the controller  110 . The magnetically coupled feedback circuit includes a diode  108 , a capacitor  130 , resistors  132  and  134  and an auxiliary winding  114 . The auxiliary winding  114  is coupled in parallel to the series of resistors  132  and  134 . The auxiliary winding  114  is also magnetically coupled to the secondary winding S 1 . When the current through the diode  116  is zero, the voltage across the secondary winding S 1  is equal to the voltage across the auxiliary winding  114  if the turns ratio is 1:1, or otherwise proportional depending on the turns ratio. This relationship provides means for communicating the voltage across the secondary winding S 1  as feedback to the primary side of the circuit. The value of the voltage across the secondary winding S 1  is a function of the secondary current through the secondary winding S 1 . With the current through the diode  116  equal to zero, the transistor  118  is selectively turned ON and OFF by the controller  120 . When the transistor  118  is ON, an alternative current path is formed from the charged capacitor  126  to the secondary winding S 1 . The alternative current path enables negative current flow through the secondary winding S 1 . In this manner, the controller  120  generates a driving signal that selectively turns the transistor  118  ON and OFF, thereby generating a coded train of voltage pulses across the secondary winding S 1 . The driving signal is configured such that the voltage pulses are modulated with coded information. In this manner, a coded voltage pulse train is transmitted during a delay period that corresponds to the switch  106  OFF and the positive secondary current through the diode  116  having dropped to zero. 
         [0032]    In some embodiments, the coded information is the measured output circuit characteristic that is to be regulated, such as the output voltage Vout. In this case, the controller  120  receives the sensed output voltage Vout, and generates a driving signal resulting in a modulated train of voltage pulses across the secondary winding S 1  that is coded to convey the sensed output voltage Vout. In this manner, a coded signal is generated in the form of a coded voltage pulse train, where the DC level of the measured output voltage Vout is coded into the coded signal. Coded information is included in the coded pulse train by modulating pulses of the pulse train including, but no limited to, the pulse width, the pulse amplitude, the pulse frequency, or any combination thereof. For example, the pulse train can be modulated by the number of pulses over a predetermined time period, or the number of pulses with different amplitudes over the time period. 
         [0033]    The auxiliary winding  114  is magnetically coupled to the secondary winding S 1 , and the voltage across the auxiliary winding  114  is equal to or proportional to the voltage across the secondary winding S 1  when the current through the diode  116  is zero. As such, the coded voltage pulse train is transmitted from across the isolation galvanic barrier via the magnetically coupled auxiliary winding  114  and secondary winding S 1 . 
         [0034]    The coded voltage pulse train across the auxiliary winding  114  is measured when the transistor  106  is OFF and the current through the diode  116  equals zero. The voltage VA is proportional to the voltage across the auxiliary winding  114  and therefore represents the coded voltage pulse train. The voltage VA is provided as a feedback voltage VFB to the controller  110 , wherein the feedback voltage VFB represents the coded voltage pulse train. In contrast to the conventional power converter of  FIG. 2  where the feedback voltage VFB is a single pulse per switching cycle of the main transistor  44 , the feedback voltage VFB input to the controller  110  is a train of pulses per switching cycle of the main transistor  106 . The train of pulses includes the coded information that identifies the measured output voltage Vout, again in contrast to the conventional power converter of  FIG. 2  where the single pulse represents the actual output voltage Vout. 
         [0035]    The controller  110  is configured to receive the feedback voltage FB. The current through the transistor  106  is also provided as feedback current IFB to the controller  110 . The controller  110  includes a real-time waveform analyzer that analyzes input feedback signals, such as the feedback voltage VFB and the feedback current IFB.  FIG. 5  illustrates a functional block diagram of a portion of the controller  110  for processing the coded voltage pulse train according to an embodiment. The feedback voltage VFB input to the controller  110  is received by a pulse train acceptor  140 . The pulse train acceptor is a digital filter element, such as a high pass filter, that filters the received coded voltage pulse train. The filtered signal output from the pulse train acceptor  140  is input to a signal integrity discriminator  142  where the signal is decoded and the measured output voltage Vout coded into the coded voltage pulse train is identified. The signal integrity discriminator  142  generates and outputs a driving signal that corresponds to the identified output voltage Vout. The driving signal is input to a digital to analog converter  144  and converted to a corresponding DC level. 
         [0036]    The DC level output from the converter  144  is input to the waveform analyzer  146  as the “voltage sense”. The feedback current IFB is input to the waveform analyzer as the “current sense”. The voltage sense is provided as a first input to an error amplifier  148 . The current sense is provided as a first input to the error amplifier  150 . Error amplifiers  148  and  150  are acceptors of the regulating means, which in this configuration are the voltage sense and the current sense. The error amplifier  148  compares the input voltage sense to a reference voltage and outputs a first difference value. The first difference value is amplified by the gain of the error amplifier  148 . The error amplifier  150  compares the amplified first difference value to the current sense and outputs a second difference value that is either High or Low. A pulse width modulation (PWM) block  152  is configured as a Flip-Flop digital device. The output of the PWM block  152  is set according to the switching frequency of the clock  154  and is Reset by the High or Low value input from the error amplifier  150 . The variable signal applied to the Reset pin generates an output signal that is a pulse train modulated by the pulse width. A multiple input OR gate  156  inputs the clock signal, the pulse train signal, a shutdown signal, and a OVP/UVP/OTP signal. The OR gate  156  outputs a high voltage value when one of the inputs to the OR gate is high or to output a low voltage value when all of the inputs to the OR gate are low. The output of the OR gate  156  is amplified by amplifier  158  and output to drive the gate of the transistor  106  ( FIG. 4 ). The high voltage value output from the OR gate  156  corresponds to turning ON the transistor  106  in  FIG. 4 . The low voltage value output from the OR gate  156  corresponds to turning OFF the transistor  106 . The OR gate  156  also enables a high voltage value to propagate to the output by monitoring abnormal conditions such as under voltage, over voltage, over temperature, etc. In this manner, the pulse width of each pulse output from the PWM block  152  can be modified to adjust the output voltage into regulation. 
         [0037]    In general, control intricacies of the waveform analyzer are aligned with control argument sampling to achieve overall system functional performance. Sampling argument is in the form of current, voltage and impedance. System functional performance is in the form of pulse width modulation (PWM), pulse frequency modulation (PFM) and pulse amplitude modulation (PAM). The waveform analyzer shown in  FIG. 5  is an exemplary implementation. It is understood that alternative circuits and methodologies can be used to process the DC level output from the converter  144  and to output a signal for appropriately driving the transistor  106 . It is also understood that the controller  110  can be alternatively configured to process the coded voltage pulse train and to generate a driving signal for controlling the transistor  106 . 
         [0038]    In operation, a circuit output characteristic is measured on the secondary side of a switching mode power converter. In an exemplary application, the circuit output characteristic is the output voltage Vout. The secondary side controller generates a driving signal for controlling the secondary side transistor while the primary side main transistor is OFF. The driving signal selectively turns ON and OFF the secondary side transistor resulting in a coded train of voltage pulses across the secondary winding. The measured output voltage Vout is coded into the coded voltage pulse train. In some embodiments, the coded voltage pulse train is transmitted from the secondary winding to the auxiliary winding by magnetic coupling between the two windings. In other embodiments, the coded voltage pulse train is transmitted form the secondary winding to the auxiliary winding using the parasitic capacitance of either the transformer or the inherent capacitance of the printed circuit board across the isolation galvanic barrier, where the printed circuit board capacitance is due to the formation of the transformer and corresponding circuitry layout of the power converter components. The coded voltage pulse train is received and decoded by the primary side controller. The primary side controller identifies the measured output voltage Vout according to the decoded information and generates a driving signal corresponding to the identified output voltage Vout. In some embodiments, the driving signal is converted to a DC level that is input as the voltage sense to a waveform analyzer. The waveform analyzer uses the input voltage sense to generate a driving signal for controlling the primary side main transistor and regulating the output voltage Vout. 
         [0039]    Although the coded information within the coded voltage pulse train is described above as including information that identifies the output voltage Vout, the coded voltage pulse train can be alternatively modulated to include additional or alternative information. Such information includes, but is not limited to, a simple instruction to turn ON or OFF the primary side main transistor, an indicator of a short circuit condition, or an indicator of a voltage out of regulation. Each type of information is represented by a separate coding. 
         [0040]    In an alternative configuration, a bi-directional switch is used in place of the diode  116  and the transistor  118 . A body diode of the bi-directional switch functions as the freewheeling diode  116 . Control of the bi-directional switch is the same as the transistor  118  to enable a negative secondary current Isec. 
         [0041]    The present application has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the power converter. Many of the components shown and described in the various figures can be interchanged to achieve the results necessary, and this description should be read to encompass such interchange as well. As such, references herein to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made to the embodiments chosen for illustration without departing from the spirit and scope of the application.