Abstract:
Power converters, e.g., AC/DC and DC/DC, typically have unique circuitry for a proper graceful start-up and to develop correct operating voltage biases. Typically this unique circuitry is incorporated in a primary-side controller. This primary-side controller could also be the primary means of control of the power converter once started. However, a secondary-side controller is typically needed for more exact output voltage regulation, duplicating circuitry already present in the primary-side controller. Complication is typically added by linear communication between the two controllers across an isolation barrier. A simplified primary-side start-up controller is envisioned providing minimal circuitry to power up a converter until a secondary-side controller activates and takes control by sending discrete PWM commands across the isolation barrier instead of a linear signal. The start-up controller can provide voltage and current protection if the secondary-side controller fails. The secondary-side controller can be an analog and/or digital design for sophisticated converter control.

Description:
RELATED PATENT APPLICATION 
     This application claims priority to commonly owned U.S. Provisional Patent Application No. 62/082,317; filed Nov. 20, 2014; which is hereby incorporated by reference herein for all purposes. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to power converters, and, in particular, to start-up controller methods and apparatus for DC-to-DC and AC-to-DC power converters. 
     BACKGROUND 
     Power converters, e.g., DC-to-DC and AC-to-DC, typically have unique circuitry for a proper graceful start-up (soft start) and to develop correct operating voltage biases. This unique circuitry may require custom integrated circuits and/or proprietary designs which may increase the cost and delivery schedule of such power converters.  FIG. 3  illustrates a schematic diagram of a prior art flyback converter. Transformer T 1  is shown having a primary-side bias winding  302 . This is used to bias the Primary-Side Controller device  301  via its VDD port. The voltage at VDD is cross-regulated to the output voltage Vo via transformer coupling. Therefore it is possible to regulate the voltage at Vo by a controller  301  monitoring the voltage at its VDD port. Typically regulating Vo using transforming coupling to the controller  301  is not accurate enough for most applications, therefore an additional feedback path is needed from the secondary-side of the flyback converter  300  to its primary side. Voltage reference  304  (U 3 ) is a device that provides a precision reference (Vo is compared to that precision reference), a voltage error amplifier (with its compensation components for stability) and a driver for driving an optical isolation coupler (optocoupler)  303 . The controller  301  also contains a precision reference and voltage amplifier, but these circuits are not utilized when the additional feedback path is included. The optocoupler  303  is driven linearly. Therefore the current transfer ratio (CTR) of the optocoupler  303  is an issue. CTR adds gain to the additional feedback path. This gain can vary from device to device, and a device&#39;s CTR can change as it ages. 
     The controller  301  is located on the primary-side of the flyback converter  300 . The secondary-side of the flyback converter  300  is where the load (application) is coupled. Typically the application device (not shown) contains a microprocessor with its capability of programmability. The controller  301  is isolated from the benefits that programming can provide for more sophisticated flyback converter control techniques. The power MOSFET switch Q 1  is an external device, resistor R 6  is an external resistor that scales the voltage analogous to current through the MOSFET switch Q 1  and is used by the controller  301  for current sensing. 
     SUMMARY 
     Therefore there is a need for a low cost solution to start-up power converters using a conventional, low cost integrated circuit (IC) solution on the primary side that does not duplicate the resources of a secondary side controller and minimizes discrete components on the primary side electronic devices. 
     According to an embodiment, a method for starting up a power converter may comprise the steps of: applying a first DC voltage to a start-up controller; turning on and off a power switch with the start-up controller, wherein the first DC voltage and the power switch may be coupled to a primary winding of a transformer, whereby an AC voltage may be produced on a secondary winding of the transformer; rectifying the AC voltage from the secondary winding of the transformer with a second rectifier to provide a second DC voltage for powering a secondary-side controller and a load; and transferring control of the power switch from the start-up controller to the secondary-side controller when the second DC voltage may be at a desired voltage value. 
     According to a further embodiment of the method, the start-up controller may be initially powered directly from the first DC voltage and then from a tertiary winding of the transformer. According to a further embodiment of the method, the step of turning on and off the power switch with the start-up controller may comprise the steps of: turning on the power switch until a maximum current through the primary winding of the transformer may be reached; and thereafter turning off the power switch for a fixed time period. According to a further embodiment of the method, the fixed time period may be determined by a capacitance value of a capacitor coupled to the start-up controller. 
     According to a further embodiment of the method, decoupling the load from the second DC voltage until requested to couple the load to the second DC voltage. According to a further embodiment of the method, the load may be coupled to the second DC voltage after the secondary-side controller starts controlling the power switch. According to a further embodiment of the method, preventing an overvoltage of the second DC voltage may be provided by coupling a voltage shunt thereacross. According to a further embodiment of the method, the voltage shunt may be a Zener diode having a breakdown voltage higher than a desired value for the second DC voltage. 
     According to a further embodiment of the method, the step of transferring control of the power switch from the start-up controller to the secondary-side controller may comprise the steps of: sending PWM signals from the secondary-side controller to the start-up controller when the second DC voltage may be at the desired voltage value; detecting the PWM signals from the secondary-side controller with the start-up controller; and turning on and off the power switch with the detected PWM signals from the secondary-side controller. 
     According to a further embodiment of the method, the second DC voltage may be regulated by the secondary-side controller after the start-up controller detects the PWM signals from the secondary-side controller. According to a further embodiment of the method, the step of controlling the power switch further comprises the steps of: turning on and off the power switch at a low frequency with the start-up controller to conserve power; and turning on and off the power switch at a higher frequency with the secondary-side controller. 
     According to a further embodiment of the method, the step of sending PWM signals from the secondary-side controller to the start-up controller further comprises the step of sending PWM signals through a voltage isolation circuit. According to a further embodiment of the method, the voltage isolation circuit may be an optical-coupler. According to a further embodiment of the method, the voltage isolation circuit may be a pulse transformer. According to a further embodiment of the method, the AC-to-DC power converter may comprise an AC-to-DC flyback power converter. According to a further embodiment of the method, the AC-to-DC power converter may comprise an AC-to-DC forward power converter. 
     According to a further embodiment of the method, the start-up controller may protect a power switch driver from under and over voltages. According to a further embodiment of the method, the step of limiting a maximum allowable transformer primary winding current may be proved with the start-up controller. According to a further embodiment of the method, the step of preventing the flyback power converter from going into too deep a continuous conduction mode may be provided with a current-sense comparator, whereby the flyback power converter may be protected from an over-current fault. 
     According to a further embodiment of the method, may comprise the steps of: providing bias voltage to the start-up controller from a primary-side tertiary winding of the transformer, wherein the bias voltage may be coupled to the second DC voltage and provides voltage feedback thereof; detecting an overvoltage condition from the bias voltage when the secondary side controller fails to properly operate; and locking out the start-up controller when the overvoltage condition may be detected. 
     According to a further embodiment of the method, providing a linear regulator between an output of a primary-side tertiary winding of the transformer and a bias input of the start-up controller. According to a further embodiment of the method, clamping a secondary side reset winding of the transformer to provide a transformer reset. According to a further embodiment of the method, providing initial bias for the secondary-side controller from an active clamp circuit until a bias from a tertiary winding of an output filter inductor may be established. According to a further embodiment of the method, applying AC power to a first rectifier for providing the first DC voltage. 
     According to another embodiment, a power converter may comprise: a start-up controller coupled to a first DC voltage; a transformer having primary and secondary windings, wherein the transformer primary winding may be coupled to the first DC voltage; a current measurement circuit for measuring current through the primary winding of the transformer and providing the measured primary winding current to the start-up controller; a power switch coupled to the transformer primary, and coupled to and controlled by the start-up controller; a secondary-side rectifier coupled to the transformer secondary winding for providing a second DC voltage; a secondary-side controller coupled to the start-up controller and the secondary-side rectifier; wherein when the start-up controller receives the first DC voltage it start to control the power switch on and off whereby a current flows through the transformer primary, an AC voltage develops across the transformer secondary winding, a DC voltage from the secondary side rectifier powers up the secondary-side controller, and the secondary-side controller takes over control of the power switch from the start-up controller when the second DC voltage reaches a desired voltage level. 
     According to a further embodiment, the power converter may comprise a flyback power converter. According to a further embodiment, the power converter may comprise a forward power converter. According to a further embodiment, a switching post regulator may be coupled between the secondary side rectifier and a load, wherein the switching post regulator may be controlled by the secondary-side controller. According to a further embodiment, the power switch may be a power metal oxide semiconductor field effect transistor (MOSFET). 
     According to a further embodiment, the secondary-side controller may be coupled to and control the start-up controller through an isolation circuit. According to a further embodiment, the isolation circuit may be an optocoupler. According to a further embodiment, the isolation circuit may be a pulse transformer. 
     According to a further embodiment, a fixed off time circuit may be provided for keeping the power switch off for a certain time period after the start-up controller has turned off the power switch. According to a further embodiment, the certain time period may be determined by a capacitance value of a capacitor coupled to the fixed off time circuit. According to a further embodiment, an AC-to-DC rectifier and filter adapted for coupling to an AC power source and used to provide the first DC voltage. According to a further embodiment, a microcontroller integrated circuit may comprise the power converter. 
     According to yet another embodiment, a start-up controller may comprise: a high voltage regulator having an input and an output; internal bias voltage circuits coupled to the high voltage regulator output; under and over voltage lockout circuits coupled to the high voltage regulator output; a current regulator; logic circuits for generating pulse width modulation (PWM) control signals; a fixed off time circuit coupled to the logic circuits; a power driver coupled to the logic circuits and providing PWM control signals for control of an external power switch; an external gate command detection circuit coupled to the logic circuits and adapted to receive an external PWM control signal, wherein when the external PWM control signal may be detected the external gate command detection circuit causes control of the external power switch to change from the logic circuits to the external PWM control signal; and first and second voltage comparators having outputs coupled to the internal current regulator and inputs coupled to a current sense input. 
     According to a further embodiment, a blanking circuit may be coupled between the current sense input and the first and second voltage comparator inputs. According to a further embodiment, the fixed off time circuit time period may be determined by a capacitance value of a capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a schematic block diagram of a flyback power converter comprising a primary-side start-up technique, according to a specific example embodiment of this disclosure; 
         FIG. 2  illustrates a schematic block diagram of a start-up controller, according to specific example embodiments of this disclosure; 
         FIG. 3  illustrates a schematic diagram of a prior art flyback converter; and 
         FIG. 4  illustrates a schematic block diagram of a forward power converter comprising a primary-side start-up technique, according to another specific example embodiment of this disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein. 
     DETAILED DESCRIPTION 
     Power supplies, in particular DC-to-DC and AC-to-DC power converters, typically have unique circuitry to start them up. According to various embodiments of this disclosure, a power converter may comprise a start-up controller and a secondary-side controller, wherein the start-up controller is utilized to send power to the secondary-side controller when power (voltage) is first applied to the primary side of the power converter. This provides a low cost integrated circuit (IC) solution for start-up of a power converter using conventional devices on the primary side that does not duplicate the resources of a secondary-side controller and minimizes discrete components on the primary side. 
     The start-up controller is specifically designed for starting up a power converter, wherein the start-up controller is located on the primary-side of the power converter and a secondary-side controller is located on an electrically isolated, secondary-side of the power converter (transformer). The start-up controller may have two modes of operation: 1) start-up controller operates as an open loop current regulator, and 2) the start-up controller receiving external PWM commands from the secondary-side controller for control of the power switch. In the open loop current regulator mode the start-up controller is initially powered directly from a DC source voltage, e.g., battery or rectified AC line. During an ON time of the power switch that couples the DC or rectified AC line voltage to the transformer, current in the primary winding of the transformer is allowed to rise to a maximum current level monitored by the start-up controller. The OFF time of the power switch is set by an external capacitor such that the power converter outputs only a small fraction of its rated power capacity. This small fraction of the rated output power charges an output capacitor of the power converter and powers up the secondary-side controller. The load on the power converter may be disconnected during this time. 
     When the power converter&#39;s output charges to a sufficient voltage level the secondary-side controller will activate and take control of the power switch from the start-up controller. As the power converter powers up the start-up controller may receive bias from a primary-side tertiary winding of the transformer. Because the output power is only a small fraction of the power converter&#39;s rated power, wherein the output voltage may easily be protected against over-voltage by simple voltage shunt techniques, such as a power Zener diode, if the secondary-side controller fails to operate. 
     When the start-up controller receives external PWM commands (signals) from the secondary-side controller, the start-up controller switches to an external PWM commands mode when the external PWM commands from the secondary-side controller are detected. Wherein the ON and OFF times of the power switch are determined by the secondary-side controller such that the power converter can deliver its rated power or the power necessary to regulate the output voltage to the load. In normal operation the secondary-side controller regulates the output voltage to the load from the power converter. The secondary-side controller may connect the load to the power converter (either via a switch, or via a switching post regulator). 
     PWM commands from the secondary-side controller are sent to the start-up controller via an isolation circuit, e.g., an optocoupler or a pulse transformer. The isolation circuit is not required to operate linearly thereby alleviating problems caused by the optocoupler&#39;s current transfer ratio (CTR) issues if linear control was used. The secondary-side controller may make use of microprocessor resources located in the load (application) that the power converter is powering such that sophisticated power converter control techniques may be employed. 
     If the start-up controller ceases receiving external PWM commands it will revert back to its open loop current regulator mode. In either mode the start-up controller protects the power switch driver from under and over voltages. The start-up controller limits the maximum allowable transformer primary current. The start-up controller may be used to start-up either a flyback power converter or a forward power converter. When used in a flyback power converter application the start-up controller has some additional features such as, for example but not limited to, an additional current-sense comparator that prevents the flyback power converter from entering too deeply into a continuous conduction mode of operation, thereby protecting the output of the flyback power converter from an over-current fault condition. 
     The voltage from the transformer&#39;s primary-side tertiary winding, used to bias the start-up controller, may be coupled to the output voltage of the flyback converter. Therefore the voltage on the tertiary winding can be used as an output voltage feedback mechanism that can be used by the start-up controller&#39;s over-voltage lockout (OVLO) circuit as an additional level of over-voltage protection if the secondary-side controller fails to operate properly. 
     When used in a forward converter application the forward converter design may require the following: A linear regulator may be required between the output of the transformer&#39;s primary-side tertiary winding and the bias input to the start-up controller. This is due to the fact that the tertiary winding is coupled to the rectified AC voltage and not the converter&#39;s output voltage. The forward converter&#39;s transformer&#39;s reset winding is located on the power converter&#39;s secondary side, and is actively clamped to provide a transformer reset. In addition, the active clamp may be designed to provide the initial bias for the secondary-side controller until the main source of bias for the secondary-side controller is established from the tertiary winding of the forward converter&#39;s output filter inductor. 
     Referring now to the drawings, the details of example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     Referring now to  FIG. 1 , depicted is a schematic block diagram of a flyback power converter comprising a primary-side start-up technique, according to a specific example embodiment of this disclosure. A flyback power converter, generally represented by the numeral  100 , may comprise a primary side power rectifier and filters  104  coupled to an AC line power source  102 , a start-up controller  106 , a capacitor  107 , a transformer  122 , MOSFET switch  116 , a current sensing resistor  124 , a bias voltage rectifier  114 , a power rectifier  135 , a Zener diode  130 , a secondary-side controller  118 , a switching post regulator  120 , and an isolation circuit  108 . The flyback power converter  100  provides regulated voltage to an application load  128  after start-up. The AC line power source  102  may be in a universal range of from about 85 to 265 volts alternating current (AC) at a frequency of from about 47 Hz to about 63 Hz. It is contemplated and within the scope of this disclosure that the embodiments disclosed herein may be adapted for other voltages and frequencies. A DC source may be used instead of using the primary side power rectifier and filters  104  coupled to an AC source. 
     When AC line power  102  is applied to the primary side power rectifier and filters  104  a DC voltage, V_Link, results. This DC voltage, V_Link, is coupled to the primary of transformer  122  and the V IN  input of the start-up controller  106 . The start-up controller  106  becomes active when the voltage, V_Link, reaches a sufficient voltage for proper operation thereof. Once activated the start-up controller  106  starts driving the MOSFET switch  116  from its gate node (output pin). The start-up controller  106  controls the switching of the MOSFET switch  116  in an open-loop manner based upon regulation of the peak current through the MOSFET switch  116 . A voltage is developed across resistor  124  in series with the MOSFET switch  116  and primary of the transformer  122  that is proportional to the peak current therethrough. This voltage is coupled to the C/S (current sense) input of the start-up controller  106  which senses it and adjusts the on time of the MOSFET switch  116  to limit the peak current to a certain design value. An internal linear regulator (see  FIG. 2 , regulator  230 ) in the start-up controller  106 , whose input is the DC voltage, V_Link, regulates a voltage, V DD , usable by the internal circuits of the start-up converter  106 . V DD  is the peak voltage at the gate node of the start-up controller  106 . Initially, the internal linear regulator supplies V DD  for operation of the start-up controller  106 , but once a DC voltage is provided from a primary-side tertiary winding of the transformer  122  through the power diode  114  this internal linear regulator stops supplying current to the internal circuits of the start-up controller  106 . This allows internal thermal dissipation in the start-up controller  106  to be reduced. 
     Driving the MOSFET switch  116  on and off will cause the transformer  122  through rectifier  135  to charge a capacitor  126  to a voltage, V_Bulk. The switching post regulator  120  is off, therefore no output voltage, V_Out, is present therefrom. Thus the application load  128  is isolated from the output of the transformer  122 . As the voltage, V_Bulk, increases the secondary-side controller  118  becomes active. When the voltage, V_Bulk, at the V/S input of the secondary-side controller  118  reaches a desired value the secondary-side controller  118  will start controlling the gate output from the start-up controller  106  by sending pulse width modulation (PWM) commands via an isolation circuit  108  to the PWM input of the start-up controller  106 . Now the secondary-side controller  118  controls the MOSFET switch  116 . 
     The transformer  122  also provides bias voltage, V-Bias, via diode  114 . V-Bias may be cross-regulated to the start-up controller  106  by transformer coupling. The winding turns ratio of the transformer  122  is such that V_Bias is higher than the output voltage set point of the internal linear voltage regulator  230  ( FIG. 2 ) of the start-up controller  106 , thereby effectively shutting off this internal linear voltage regulator  230  and reducing the internal thermal dissipation of thereof. Once V_Bulk has risen to its design voltage the secondary-side controller  118  will control the switching post regulator  120  to provide V_Out to the application load  128 , thereby power loading the flyback converter  100 . 
     Referring now to  FIG. 2 , depicted is a schematic block diagram of a start-up controller, according to specific example embodiments of this disclosure. The start-up controller  106  may comprise a high voltage regulator  230 , internal bias voltage circuits  232 , a first voltage comparator  234 , a second voltage comparator  238 , a fixed blanking time circuit  240 , internal current regulator and logic circuits  236 , an external gate command detection circuit  242 , a signal buffer  244 , a switch  246  controlled by the logic circuits  236 , a MOSFET driver  248 , a fixed off-time timer  250 , and over and under voltage lockout circuits  252 . 
     V IN  input is coupled to a voltage provided from the bridge rectifier and filters  104  ( FIG. 1 ) and is used as an input voltage to the high voltage regulator  230  dependent upon the AC line voltage  102 . The high voltage regulator  230  may be a linear regulator that provides a lower voltage V DD  for powering the MOSFET driver  248  and other internal bias voltages (bias circuits  232 ). V DD  may also be provided from an external source (e.g., V_Bias from the transformer  122  ( FIG. 1 ) such that the internal high voltage regulator  230  may turn off, thereby saving internal power dissipation within the start-up controller  106 . The voltage V DD  may be monitored by the over and under voltage lockout circuits  252  to protect the circuits within the start-up controller  106  from out of design specification voltages. Internal biases and voltage references may be provided by the internal bias voltage circuits  232  which may receive its input operating voltage from the high voltage regulator  230  or an external source for V DD , e.g., transformer  122 . 
     Gate drive commands to the gate driver  248  may be switched between two sources using switch  246  that may be controlled by the logic circuits  236 . The first source may be the internal current regulator and logic circuits  236 , and the second may be from an external source coupled to the PWM input and internally buffered by the signal buffer  244 . 
     Current flowing through the MOSFET switch  116  may be monitored by an analogous voltage developed across resistor  124  that may be coupled to the current sense (C/S) input of the start-up controller  106 . The MOSFET current is the same as the primary current of transformer. When the gate driver  248  begins driving the MOSFET switch, the logic circuits  236  start the fixed blanking time circuit  240  that then momentarily blanks the signal at the current sense (C/S) node from reaching the internal current regulator and logic circuits  236  so that the internal current regulator therein may ignore the initial turn-on current spike through the MOSFET switch  116 . The first comparator  234  and the second comparator  238  monitor the voltage at the current sense (C/S) input. The first comparator  234  monitors the voltage at the C/S node for a brief time interval after the blanking time period of the fixed blanking time circuit  240  has finished. If the voltage at the C/S node exceeds a first voltage reference (V REF1 ) during this brief time interval then the gate drive is terminated. The second comparator  238  sets the maximum voltage allowed (current through the MOSFET switch  116 ) at the current sense (C/S) input. If the voltage at the current sense (C/S) input is greater than a second voltage reference (V REF2 ) then the gate drive is also terminated. When the gate drive is terminated it remains off for a time period determined by the fixed off-time circuit  250 . This off time period may be externally selected by the capacitance value of a capacitor  107  at the T OFF  node of the start-up controller  106 . 
     When an external signal is applied to the pulse width modulation (PWM) input node it may be detected by the external gate command detection circuit  242 . When an external PWM signal is so detected, logic within the logic circuits  236  cause the switch  246  to couple this external PWM signal to drive the MOSFET driver  248 , thereby controlling the power MOSFET switch  116  from a PWM source external to the start-up controller  106 . The PWM signal frequency may be, for example but is not limited to, from about 20 kHz to about 65 kHz. If the PWM signal at the PWM input node ceases switching (e.g., remains in either a high or low state) for more than a certain number of switching periods, e.g., five switching periods at 20 kHz (250 microseconds) then the logic within the logic circuits  236  causes the switch  246  to switch back to the PWM output of the logic circuits  236 , whereby the MOSFET driver  248  is then driven from the PWM output of the logic circuits  236 . The ground node (Gnd) is the circuit ground or common point for the circuits in the start-up controller  106 . This ground node may provide a return point for both the PWM drive current to the external MOSFET switch  116  and for bias return currents of the voltages at the V IN  and V DD  nodes. 
     Referring back to  FIG. 1 , the start-up controller  106  is not a primary-side power supply controller that can linearly regulate the output of a flyback power converter via transformer coupling. It does not duplicate the precision reference and voltage error amplifier of a secondary-side controller  118 . The start-up controller  106  basically has two modes of operation: In the first mode, during start-up of the flyback power converter  100 , it performs as an open-loop current regulator that drives the MOSFET switch  116  until the secondary-side controller  118  takes control (command) of the PWM signals that drive the MOSFET switch  116 . In the second mode, once the secondary-side controller  118  is fully operational, it starts sending PWM signal commands to the start-up controller  106  through the isolation circuit  108 . Once external PWM signal commands from secondary-side controller  118  (via isolation circuit  108 ) are received by the start-up controller  106 , its internal gate driver  248  may be coupled to the external PWM signal, whereby the secondary-side controller  118  now controls the MOSFET switch  116 . 
     The secondary-side controller  118  may be either an analog controller or a digital controller (or an analog/digital hybrid). Very sophisticated control methods may be used by the secondary-side controller  118 , as long as the output of these control methods provides a PWM signal (which is typical). The secondary-side controller  118  may communicate with the application load  128  (which loads the flyback power converter  100  via switching post regulator  120 ) for additional control sophistication. 
     Since the PWM signal commands (PWM pulses) from the secondary-side controller  118  drive the isolation circuit  108  (e.g., optocoupler, pulse transformer) on or off, and does not require any circuit linearity, optocoupler CTR concerns are not an issue, according to the teachings of this disclosure. The open-loop current regulator, comprising the start-up controller  106 , is designed to operate the flyback power converter  100  in a highly discontinuous mode of operation that provides a small amount of start-up power to a secondary winding of the transformer  122  whereby an output capacitor  126  is charged up and supplies operating voltage to the secondary-side controller  118 . 
     The ON time (driving the external MOSFET switch  116  on) is typically determined by the amount of time it takes the PWM signal at the C/S node of the start-up controller  106  to ramp from zero volts to the V REF2  voltage of the second comparator  238 . The OFF time (driving the external MOSFET switch  116  off) may be determined by the fixed time-off timer  250 . The time duration of the fixed time-off timer  250  may be determined by the value of a capacitor  107  coupled to the T OFF  node of the start-up controller  106 . For example, a flyback converter rated for 20 watts of power can be made to deliver approximately one (1) watt of output power using the open-loop current regulator techniques and a long enough OFF time set by the capacitor  107  coupled to the T OFF  node. 
     When an external PWM signal is applied to the PWM node of the start-up controller  106  and detected by the external gate command detection circuit  242 , switch  246  changes the input to the gate driver  248  from the internal current regulator and logic circuits  236  to the external source (from the PWM node via signal buffer  244 ). This allows the secondary-side controller  118  to drive the flyback converter  112  at the proper frequency and PWM duty cycle to achieve rated output power and output voltage regulation. In this mode the start-up controller  106  is simply a primary-side biased gate driver. However, the start-up controller  106  still provides the current protection afforded by the first and second voltage comparators  234  and  238 . If either of the first or second voltage comparators  234  or  238  trips (changes output state) then switch  246  will change back to the position where it gets its commands from the internal current regulator and logic circuits  236  where the OFF time is set by the fixed time-off timer  250 . Wherein the switch  246  cannot change position back to receiving commands via the signal buffer  244  until the end of the time period set by the fixed off-time timer  250 . When the external PWM signal from the secondary-side controller  118  via the isolation circuit  108  ceases (remaining in either a high state or a low state) (no longer being detected by the external gate command detection circuit  242 ) for a time period exceeding 250 μs, switch  246  will change back to the position where it gets its commands from the internal current regulator and logic circuits  236 . 
     The over and under voltage lockout circuits  252  ensure that the peak voltage at the gate node is within the proper range for the external power MOSFET switch  116  of the flyback converter  112 . The under-voltage lockout (UVLO) circuit ensures enough voltage is available to properly enhance the gate of the MOSFET  116 . The over-voltage lockout (OVLO) circuit ensures that the voltage does not exceed the typical gate voltage ratings of the power MOSFET  116 . The OVLO circuit  252  also provides another important function: It must protect from a failure of the secondary-side controller  118  to start up and regulate. If the secondary-side controller  118  does not take command, the start-up controller  106  will continue to charge the output capacitor  126  until it reaches the over-voltage threshold. This voltage on the output capacitor  126  is reflected back to the V DD  node of the start-up controller  106  via the transformer  122  winding coupling and will trip the OVLO circuit in the start-up controller  106 . When the high voltage limit of the OVLO part of circuit  252  is exceeded the MOSFET driver  248  output will be inhibited. The OVLO circuit  252  may have, for example but is not limited to, a two (2) volt hysteresis band. Therefore gating of the MOSFET switch  116  is halted until the voltage at the V DD  node of the start-up controller  106  decays below the lower limit of OVLO circuit&#39;s  252  hysteresis band. For an additional layer of over-voltage protection (in case the secondary-side controller  118  fails) a power Zener diode  130  (or some other form of active shunt regulator) may be placed across the output of the transformer  122  (e.g., across capacitor  126 ). Since the output power of the flyback power converter  100  can be set low by choosing a long OFF time with the capacitor  107  on the T OFF  node of the start-up controller  106 , wherein the output of the transformer  122  via rectifier  135  can be reasonably protected against over-voltage by using a power Zener diode  130  shunted across the DC output therefrom. 
     Referring now to  FIG. 4 , depicted is a schematic block diagram of a forward power converter comprising a primary-side start-up technique, according to another specific example embodiment of this disclosure. A forward power converter, generally represented by the numeral  400 , may comprise a primary side power rectifier and filters  404  coupled to an AC line power source  402 , a start-up controller  106 , a capacitor  107 , a regulator  430 , a MOSFET switch  416 , a resistor  424  for current sensing, a bias voltage rectifier  414 , a transformer  422 , a secondary-side controller  418 , power rectifiers  435  and  436 , an active clamp circuit  440 , a current sense transformer  445 , an inductor  450 , a diode  455 , a clamp Zener diode  465 , a switch  460 , an isolation circuit  408 , and an application load  428 . A DC source may be used instead of using the primary side power rectifier and filters  404  coupled to an AC source. 
     The transformer  422  may comprise four (4) windings: 1) a primary winding coupled to V_Link, 2) a secondary winding coupled to power rectifiers  435  and  436 , 3) a reset winding coupled to the active clamp circuit  440 , and 4) a tertiary winding coupled to rectifier  414 . The AC line  402  may be in a universal range of from about 85 to 265 volts alternating current (AC) at a frequency of from about 47 Hz to about 63 Hz. It is contemplated and within the scope of this disclosure that the embodiments disclosed herein may be adapted for other voltages and frequencies. When AC line power source  102  is applied to the primary side power rectifier and filters  404 , a DC voltage, V_Link, results. This DC voltage, V_Link, is coupled to a primary winding of transformer  422  and the V IN  input of the start-up controller  106 . The start-up controller  106  is initially biased by V_Link (via its V IN  node) upon application of the AC Line power source  402 . The start-up controller  106  becomes active when the voltage, V_Link, reaches a sufficient voltage for proper operation thereof. Once so biased, the start-up controller  106  gates MOSFET switch  416  on and off. The start-up controller  106  provides open-loop regulation of the current through the primary winding of transformer  422  by monitoring the voltage developed across the current sense resistor  424  coupled to its C/S node. 
     When the MOSFET switch  416  is gated on, the dot sides (phasing) of the Transformer  422  windings are positive allowing current to flow through the primary winding, secondary winding, and tertiary winding. Current flows through rectifier  414  and through the voltage regulator  430  to provide bias to the V DD  port of the start-up controller  106 . Current also flows through rectifier  435 , current sense transformer  445 , the main winding of inductor  450 , and charges capacitor  426 . At this time the application load  428  is isolated because switch  460  is open. When the MOSFET  416  switch is gated off, current flows through the reset winding to the active clamp circuit  440 . The active clamp circuit  440  clamps the reset winding voltage by the Zener diode on the gate of the PNP transistor thereof. The Zener diode on the collector of the PNP transistor clamps the voltage V CCS . V CCS  is the bias voltage for the secondary-side controller  418 . Magnetization energy from the reset of transformer  422  may be used to help bias the secondary-side controller  418 . When MOSFET switch  416  is gated off current flows through the tertiary winding of inductor  450  coupled to diode  455 . This also allows energy to flow to provide voltage V CCS . Once the forward power converter  400  is operational current flowing via diode  455  to the voltage V CCS  will be the main source of operating power for the secondary-side controller  418 . 
     When V CCS  reaches a sufficient voltage, the secondary-side controller  418  can send gating commands to the start-up controller  106  via the isolation circuit  408 . Now the gating of the MOSFET switch  416  is controlled by the secondary-side controller  418 . The secondary-side controller  418  may then regulate the voltage V_OUT, close the switch  460 , and apply power to the application load  428 . 
     There are some key differences when using a start-up controller  106  to start-up a flyback power converter  100  or a forward power converter  400 . For example, the voltage on the tertiary winding of transformer  422  is not coupled to the output voltage of the forward converter  400 . Instead it is coupled to V_Link. Therefore no secondary voltage information is available via transformer coupling. That is why voltage regulator  430  is needed to regulate the voltage on the V DD  port of the start-up controller  106 . Also, because of the lack of voltage information via the transformer  422  tertiary winding, the over-voltage protection strategy is different in the event of the failure of the secondary-side controller  418 . During start-up the power delivered to the output is set to be low with a selected value capacitor  107  coupled to the T OFF  node (port) of the start-up controller  106  (see  FIG. 2 ). The Zener diode on the collector of the PNP transistor of the active clamp circuit  440  clamps the voltage on V CCS  and protects the secondary-side controller  418  from over-voltage. Components across the output of the forward converter  400  may be protected by Zener diode  465 . Both of these Zener diodes act as protective shunt regulators. Comparator  234  shown in  FIG. 2  is not needed in the forward power converter  400  design. Its purpose is to keep the flyback power converter  100  from entering into a continuous conduction mode of operation. However, the main winding of inductor  450  of a forward power converter  400  is typically kept in a continuous conduction mode. 
     A power Zener diode  130 / 465  may be placed in parallel with capacitor  126 / 426 , wherein the cathode of the Zener diode  130 / 465  is coupled to the positive side of capacitor  126 / 426  and the anode of the Zener diode  130 / 465  may be coupled to the negative side of capacitor  126 / 426 . In this configuration the Zener diode  130 / 465  is shunted across the output of the flyback power converter  100  or forward power converter  400 . The Zener diode  130 / 465  breakdown voltage is higher than a normal voltage output on the capacitor  126 / 426 . If a secondary-side controller  118  failure occurs and an over-voltage results, the output voltage will rise until the Zener diode  130 / 465  breaks over and clamps the overvoltage. The Zener diode  130 / 465  will dissipate the output power of the flyback or forward power converters  100  or  400 , respectively, determined by the capacitance value of the capacitor  107  at the T OFF  pin of the start-up controller  106 . The Zener diode  130 / 465  should be rated for at least that power dissipation. It is contemplated and within the scope of this disclosure, that the function of the Zener diode  130 / 465  may be replaced by active circuitry that performs this shunt clamp function. This is typically done if a more accurate breakdown voltage is required. 
     Basically, a purpose of the start-up controller  106  is to start-up a power converter  100 / 400  by having an open loop-current regulator with a short ON time (the MOSFET switch  116 / 416  is gated ON) and a very long OFF time (the OFF time is determine by the capacitor value placed on the T-off node of the start-up controller  106  in  FIG. 2 ). In this way a power converter  100 / 400  rated for powers ranging from about 20 watts to 60 watts may have a start-up power of about one (1) watt. So, in an open-loop manner, one (1) watt of power may be delivered to the secondary to charge up the converter&#39;s output capacitor  126 / 426  and start up the secondary-side controller  118 / 418 . Normally the secondary-side controller  118 / 418  would start up in time to prevent the output capacitor  126 / 426  from over-charging (over-voltage). However, if the secondary-side controller  118 / 418  fails to start then the open loop start-up controller  106  will continue to charge the output capacitor  126 / 426  (its open loop, meaning it gets no voltage feedback). So it&#39;s necessary to clamp the voltage across the output capacitor  126 / 426  to a voltage somewhere around 125% of the normal rated output voltage for protection. This can be done simply using a Zener diode  130 / 465  with the appropriate breakdown voltage. This Zener diode  130 / 465  needs to be rated to handle the start-up power. For example, a Zener diode rated for two (2) watts will easily handle the (1) watt start-up power. A power converter  100 / 400  with a failed secondary-side controller  118 / 418  will remain in this Zener-clamped state until the AC line power source  102 / 402  is removed. For a forward converter  400 , this is the only way to protect against overvoltage if the secondary-side controller  418  fails to start. For a flyback converter  100  the start-up controller&#39;s  106  OVLO lockout circuits  252  may also be employed to prevent over-voltage in the event of a failure of the secondary-side controller  118 . In this case, the Zener  130  clamp provides an additional level of protection.