# Patent

## Title
Circuit and method for controlling the secondary FET of transformer coupled synchronous rectified flyback converter

## Abstract

A secondary FETsc control circuit is disclosed for controlling FETsc of transformer coupled synchronous rectified flyback converter (TCSC). The control circuit includes source-drain voltage VSD sense trigger with output VSD-trigger activated upon positive 0-crossing of VSD. Drain-source current IDS sense trigger with output IDS-trigger activated upon positive 0-crossing of IDS. Secondary coil voltage Vsec sense trigger with output Vsec-trigger activated upon sensing negative Vsec. A multi-trigger gate driver (MTGD) has trigger inputs coupled to VSD-trigger, IDS-trigger, Vsec-trigger and drive output driving the FETsc gate. The MTGD has logic states of state-I where FETsc is turned off and latched, state-II where FETsc is turned off but unlatched, state-III where FETsc is turned on but unlatched. The MTGD is configured to enter state-III upon VSD-trigger, enter state-I upon IDS-trigger and enter state-IT upon Vsec-trigger. The control circuit thus avoids false triggering of the FETsc by numerous undesirable Vsec ringings.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
1. Field of Invention
This invention relates generally to the field of power electronics. More specifically, the present invention is directed to precise control of a switching metal oxide semiconductor field effect transistor (MOSFET) on the secondary side of a flyback converter.
2. Background of the Invention
The usage of switched power converters has become ubiquitous in the electronics industry. A few examples are switching power supplies, DC-DC voltage converters and DC-AC voltage converters.
FIG. A1 through FIG. A4 illustrate a prior art transformer coupled conventional rectified flyback converter (TCCC) 1 with a primary circuit 10 and a secondary circuit 30 magnetically coupled through a coupling transformer 20 having a primary transformer coil (PTC) 11 and a secondary transformer coil (STC) 31. The primary circuit 10 has a primary switching network (PSN) 12 with internal cyclic active switching by a primary switching FET (FETpm) 13 in turn controlled by a primary gate drive signal VGpri. As a result, a primary coil current Ipri and a secondary coil current Isec respectively flows in the primary transformer coil (PTC) 11 and the secondary transformer coil (STC) 31 effecting a magnetic power transfer from the primary circuit 10 to the secondary circuit 30. correspondingly, an alternating current (AC) secondary coil voltage Vsec is developed across the secondary transformer coil (STC) 31. The secondary circuit 30 has a network of power diode 33 and an output capacitor (Cout) 32 for rectifying and filtering Vsec into a desired output voltage Vout. During operation, each switching cycle of the primary gate drive signal VGpri 14, the developed secondary coil voltage Vsec and associated primary coil current Ipri plus secondary coil current Isec are respectively illustrated in FIG. A2, FIG. A3 and FIG. A4. Each switching cycle is characterized by a sequence of time marks tPCR, tPSX and tSIC. At time mark tPCR the signal VGpri 14 is turned on marking the start of primary coil current (Ipri) ramp up accompanied by a negative zero crossing (0-crossing) of the secondary coil voltage Vsec. At time mark tPSX the signal VGpri 14 is turned off marking the instant of a forceful Ipri-to-Isec transfer due to transformer action accompanied by a positive 0-crossing of Vsec with ringings 40a. Notice that, to simplify illustration the Ipri and the Isec current amplitudes are shown to be equal following a normalization using the coil turn ratios between the PTC 11 and the STC 31. As an important remark, the Vsec ringings 40a are induced during the forceful Ipri-to-Isec transfer owing to the existence of numerous leakage and parasitic inductances inherent in the transformer coils and the overall TCCC 1 circuit. Finally, near time mark tSIC the tail end of Isec decay causes additional Vsec ringings 40b due to a sharp power diode 33 current turn-off (following an injected charge carrier-storage decay) in combination with the numerous leakage and parasitic inductances.
Observe that, due to its substantial forward voltage drop (about 0.7 Volt to 1 Volt) during high current conduction, the power diode 33 can cause substantial power loss from the TCCC 1 between time marks tPSX and tSIC. For this reason, the power diode 33 can be replaced with a secondary switching FET (FETsc) 52 in the secondary circuit 51 of a transformer coupled synchronous rectified flyback converter (TCSC) 50 illustrated together with its operating signal waveforms in FIG. B1 through FIG. B3. To those skilled in the art, the FETsc 52 comes with an inherent parasitic body diode BDsc 52a. The secondary gate drive and device current of the FETsc 52 are labeled as VGsec and IDS. The source-drain voltage of the FETsc 52, the same as the forward voltage of the power diode 33, is labeled as VSD. Thus, as soon as the BDsc 52a gets forward biased near the time mark tPSX the FETsc 52 should be turned on hard via VGsec, shunting the BDsc 52a, causing a substantially lower forward voltage drop VSD (about 0.1 to 0.2 Volt) hence vastly reducing the associated power loss from the TCSC 50. On the other hand, near time mark tSIC the FETsc 52 needs to be turned off and kept off via VGsec either upon an imminent Vsec fall or upon sensing a first quadrant current (a positive IDS) through the FETsc 52 to avoid shorting the Cout 32 through STC 31. This is signified by a dashed counter clockwise arc arrow.
While the above-stated requirements for turning on and off FETsc 52 are understood, the actual reliable implementation of VGsec control can be quite challenging due to the noisy presence of Vsec ringings 40a and 40b in the vicinity of time marks tPSX and tSIC. A prior art example solution by International Rectifier Company (El Segundo, Calif.) is their IR1167S controller illustrated in FIG. C1 and FIG. C2. Similar in circuit topology to the just described TCSC 50, the secondary circuit here is powered by a secondary coil of a transformer XFM with a coil current ID—SEC flowing through a secondary switching FET Q1 with its gate controlled by VGATE output of the IRI 167S. The drain-to-source voltage of FET Q1 is labelled as VDS—SEC. A regulated DC output Vout is developed across an output filter capacitor Co with a LOAD. The IR1167S itself is powered by an RC network made up of Rdc and Cdc. Thus, for correct operation the Gate Drive of FET Q1 must be turned ON and OFF at time marks referenced to time periods T1 and T2 where VDS—SEC respectively crosses two threshold voltages VTH2 and VTH1 where VTH2=−100 mV and VTH1=−10 mV. In the presence of the aforementioned VDS—SEC ringings near these threshold crossings, such low magnitude threshold voltages (VTH2 and VTH1) have necessitated additional measures such as time windows MOT (minimum ON-time) and tblank (blanking period) to be implemented as part of the IR1167S design. In essence, any additional threshold crossings of VDS—SEC within either time windows MOT or tblank are to be ignored to reduce the chance of falsely switching the Gate Drive in the presence of VDS—SEC ringings. Regardless of such additional measures, the IR1167S further recommends tight physical layout rules for the application circuit to reduce false Gate Drive switching.
Another prior art example solution by ON Semiconductor (Phoenix, Ariz.) is their NCP4302 controller and driver for controlling Gate Drive of the secondary switching FETSC as illustrated in FIG. D1 and FIG. D2. The NCP4302 senses voltage drop VDS—SEC across the FETSC with a turn on threshold of 0.5 Volt. For zero detection of the secondary current, the voltage drop developed across a 75 Ohm resistor by a 230 microAmp current source is used with a 30 mV offset. However, to reduce the chance of falsely switching Gate Drive in the presence of VDS—SEC ringing noises, minimum turn-ON and turn-OFF time intervals, both settable by a DLYADJ pin voltage, are still needed.
There are other examples of prior art which use discrete components. Here, besides using both voltage and current sensing to turn ON and OFF the secondary switching FETSC, an extra winding on the transformer is also required. Such solutions require a large number of extra components and are not very efficient in their performance.
Therefore, there remains a need to reliably switching the secondary switching FETSC of a transformer coupled synchronous rectified flyback converter with very few additional discrete components and without complex user adjustments.
SUMMARY OF THE INVENTION
A circuit is proposed for controlling the secondary FET of a transformer coupled synchronous rectified flyback converter (TCSC) having a primary circuit and a secondary circuit coupled with a transformer. The primary circuit has a primary transformer coil (PTC) coupled with a primary switching network (PSN) switched by its primary switching FET (FETpm). The secondary circuit has a serial connection of a secondary transformer coil (STC) with a secondary coil voltage (Vsec), an output capacitor (Cout) and a secondary switching FET (FETsc) having a built-in parasitic body diode BDsc with a forward voltage VSD. Each TCSC switching cycle is characterized by time marks tPCR, tPSX and tSIC where tPCR marks the start of primary coil current (Ipri) ramp up followed by a negative 0-crossing of Vsec, tPSX marks the instant of primary coil current (Ipri)-to-secondary coil current (Isec) transfer followed by a positive 0-crossing of VSD with Vsec ringing and tSIC marks the start of FETsc first-quadrant conduction followed by a positive 0-crossing of IDS also with Vsec ringing. The secondary FET control circuit includes:

    
    
        a) A VSD sense trigger with a trigger output VSD-trigger and its analog inputs coupled to the FETsc terminals for sensing the VSD and, upon sensing a positive 0-crossing of VSD, for activating the VSD-trigger.
        b) An IDS sense trigger with a trigger output IDS-trigger and its analog inputs coupled to the FETsc terminals for sensing a drain-to-source current IDS of the FETsc and, upon sensing a positive 0-crossing of IDS, for activating the IDS-trigger.
        c) A Vsec sense trigger with a trigger output Vsec-trigger and its analog inputs coupled to the STC terminals for sensing the Vsec and, upon sensing a negative Vsec, for activating the Vsec-trigger.
        d) A multi-trigger gate driver (MTGD) having trigger inputs VSD-input, IDS-input and Vsec-input respectively coupled to the VSD-trigger, IDS-trigger and Vsec-trigger, a drive output VGATE coupled to the gate of FETsc and a set of logic states of:
        
            State-I where the FETsc is turned off and latched thus it can not be triggered on.
            State-II where the FETsc is turned off but unlatched thus it can be triggered on.
            State-III where the FETsc is turned on but unlatched thus it can be triggered off.
            The MTGD is configured to enter state-III upon activation of the VSD-trigger, enter state-I upon activation of the IDS-trigger and enter state-II upon activation of the Vsec-trigger.
        
        
    
    


As a result, the secondary FET control circuit avoids false triggering of the FETsc by the numerous undesirable Vsec ringings and entry into incorrect states around the time marks tPSX and tSLC resulting in an otherwise increased secondary power loss of the TCSC.
As a more specific embodiment, the secondary FET control circuit includes a bias voltage input terminal (BVIT) for receiving a low power bias voltage Vcc powering the VSD sense trigger, the IDS sense trigger, the Vsec sense trigger and the MTGD. The Vcc can be generated by a low-power bias network (LPBN) in parallel connection with the STC. Alternatively, the Vcc can also be generated by a low-power bias network (LPBN) in parallel connection with the Cout.
In a more specific embodiment, the MTGD includes:

    
    
        A switchable gate driver (SGD) having a digital trigger input ON coupled to the VSD-trigger, a digital trigger input OFF and a drive output VGATE coupled to the gate of FETsc so as to turn on FETsc upon activation of the digital trigger input ON whereas turn off FETsc upon activation of the digital trigger input OFF.
        A turn off and latch (TOL) having a digital trigger input SET coupled to the IDS-trigger, a digital trigger input RESET coupled to the Vsec-trigger and a digital output QTOL coupled to the digital trigger input OFF such that activation of the digital trigger input SET activates the QTOL whereas activation of the digital trigger input RESET deactivates the QTOL.
    
    


In a more specific embodiment, the analog inputs of the IDS sense trigger are integrated into the FETsc with a four-terminal current sensing Kelvin connection producing a sampled IDS sense current that is a small fraction of the IDS.
In a more specific embodiment, the VSD sense trigger, the Vsec sense trigger and the MTGD are co-packaged with the FETsc so as to form a 4-terminal replacement device for the FETsc with further increased noise immunity against false triggering of the FETsc. The 4-terminal replacement device can be deployed to locate at either the high-side or the low-side of the STC.
A method is proposed for controlling the secondary FETsc of the transformer coupled synchronous rectified flyback converter (TCSC). The method includes:

    
    
        a) Sensing the forward voltage VSD across the body diode BDsc, a drain-to-source current IDS of the FETsc and the Vsec.
        b1) Upon sensing a positive 0-crossing of VSD, turning on the FETsc and maintaining it in a conductive but unlatched state so it may subsequently be turned off.
        b2) Upon sensing a positive 0-crossing of IDS, turning off the FETsc and maintaining it in a latched state so it may not be subsequently turned on.
        b3) Upon sensing a negative Vsec, turning off the latch of FETsc and maintaining it in an unlatched state so it may subsequently be turned on.
    
    


As a result, the method avoids false triggering of the FETsc by the numerous undesirable Vsec ringings and entry into incorrect states around the time marks tPSX and tSIC resulting in an otherwise increased secondary power loss of the TCSC.
As a refinement, sensing the drain-to-source current IDS can be done by integrating a four-terminal current sensing Kelvin connection into the FETsc for producing a sampled IDS sense current that is a small fraction of the IDS.
These aspects of the present invention and their numerous embodiments are further made apparent, in the remainder of the present description, to those of ordinary skill in the art.



BRIEF DESCRIPTION OF THE DRAWINGS
In order to more fully describe numerous embodiments of the present invention, reference is made to the accompanying drawings. However, these drawings are not to be considered limitations in the scope of the invention, but are merely illustrative.
FIG. A1 through FIG. A4 illustrate a prior art transformer coupled conventional rectified flyback converter;
FIG. B1 through FIG. B3 illustrate another prior art transformer coupled synchronous rectified flyback converter;
FIG. C1 and FIG. C2 illustrate a prior art control circuit for controlling the secondary switching FET in a transformer coupled synchronous rectified flyback converter;
FIG. D1 and FIG. D2 illustrate another prior art control circuit for controlling the secondary switching FET in a transformer coupled synchronous rectified flyback converter;
FIG. 1A and FIG. 1B illustrate the present invention control circuit for controlling the secondary switching FET in a transformer coupled synchronous rectified flyback converter;
FIG. 2 illustrates a more detailed circuit schematic of the present invention control circuit for controlling the secondary switching FET in a transformer coupled synchronous rectified flyback converter; and
FIG. 3A and FIG. 3B illustrate two embodiments of the present invention control circuit as deployed in a transformer coupled synchronous rectified flyback converter.



DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
The description above and below plus the drawings contained herein merely focus on one or more currently preferred embodiments of the present invention and also describe some exemplary optional features and/or alternative embodiments. The description and drawings are presented for the purpose of illustration and, as such, are not limitations of the present invention. Thus, those of ordinary skill in the art would readily recognize variations, modifications, and alternatives. Such variations, modifications and alternatives should be understood to be also within the scope of the present invention.
FIG. 1A and FIG. 1B illustrate the present invention FETsc control circuit 100 for controlling the secondary switching FETsc 52 in a TCSC (see FIG. B1). Recall that the secondary circuit 51 has a serial connection of a secondary transformer coil STC 31 with a secondary coil voltage Vsec, an output capacitor Cout 32 and the FETsc 52. The FETsc 52 has a built-in parasitic body diode BDsc 52a with a forward voltage VSD. As shown again in FIG. 1B, each TCSC switching cycle is characterized by time marks tPCR, tPSX and tSIC where tPCR marks the start of primary coil current (Ipri) ramp up followed by a negative 0-crossing of Vsec, tPSX marks the instant of primary coil current (Ipri)-to-secondary coil current (Isec) transfer followed by a positive 0-crossing of VSD with Vsec ringing and tSIC marks the start of FETsc first-quadrant conduction followed by a positive 0-crossing of IDS also with Vsec ringing (notice the opposite polarity of Isec and IDS). The FETsc control circuit 100 includes:

    
    
        A VSD sense trigger 102 with a digital trigger output signal VSD-trigger 106. The analog inputs 104a and 104b of the VSD sense trigger 102 are respectively coupled to the FETsc 52 source and drain terminals for sensing its VSD. The VSD sense trigger 102 is designed to activate its trigger output signal VSD-trigger 106 upon sensing a positive 0-crossing of VSD.
        An IDS sense trigger 110 with a digital trigger output signal IDS-trigger 114. The analog inputs 112 of the IDS sense trigger 110 are coupled to the FETsc 52 terminals for sensing its drain-to-source current IDS. The IDS sense trigger 110 is designed to activate its trigger output signal IDS-trigger 114 upon sensing a positive 0-crossing of IDS.
        A Vsec sense trigger 116 with a digital trigger output signal Vsec-trigger 120. The analog inputs 118 of the Vsec sense trigger 116 are coupled to the STC 31 terminals for sensing the secondary coil voltage Vsec. The Vsec sense trigger 116 is designed to activate its trigger output signal Vsec-trigger 120 upon sensing a negative Vsec.
        A serial connection of a gating circuit 130 and a buffer driver 132 for generating a drive output signal VGATE 136 to turn ON or OFF the FETsc 52. The gating circuit 130 can be turned ON via its VSD-input 106a that is the trigger output signal VSD-trigger 106 of the VSD sense trigger 102. The gating circuit 130 can be turned OFF via its OFF-signal 129.
        A latchable turn-off logic 128 has an output that is the OFF-signal 129. The LATCH and UNLATCH input signals of the latchable turn-off logic 128, respectively labeled as IDS-input 114a and Vsec-input 120a, function to turn OFF the gating circuit 130 via the OFF-signal 129 while respectively keeping the gating circuit 130 in an ON or OFF state. With the latchable turn-off logic 128 in its latched state, the gating circuit 130 can not be turned on by its VSD-input 106a. With the latchable turn-off logic 128 in its unlatched state, the gating circuit 130 can be turned on by its VSD-input 106a. The IDS-input 114a and Vsec-input 120a of the latchable turn-off logic 128 are respectively hooked up to the trigger output signal IDS-trigger 114 and the trigger output signal Vsec-trigger 120.
    
    


Thus, upon activation of the trigger output signal IDS-trigger 114 the FETsc 52 is turned off. However, upon activation of the trigger output signal VSD-trigger 106 the FETsc 52 can only be turned on if the VSD-trigger 106 activation is preceded by a Vsec-trigger 120 activation. Rephrased within the context of controlling the FETsc 52 of the secondary circuit 51, the following statement can be made:

    
    
        Upon sensing a positive 0-crossing of IDS or a negative Vsec the FETsc 52 is turned off. However, upon sensing a positive 0-crossing of VSD the FETsc 52 can only be turned on if this positive 0-crossing of VSD is preceded by a sensing of a negative Vsec.
    
    


Hence the present invention FETsc control circuit 100, rather than switching the FETsc 52 on and off simply according to the thresholded instantaneous Vsec and IDS signal levels, switches the FETsc 52 on and off according to the natural logical operating sequence of the TCSC 50 despite the presence of the Vsec ringings 40a and 40b. As a result, the FETsc control circuit 100 can substantially reduce the chance of false triggering of the FETsc 52 due to Vsec ringings 40a and 40b and entry into incorrect ON/OFF states around the time marks tPSX and tSIC that could otherwise increase secondary power loss of the TCSC 50.
To those skilled in the art, the interconnected latchable turn-off logic 128, gating circuit 130 and buffer driver 132 function as a multiple-trigger gate driver (MTGD) 126 as outlined in FIG. 1A. The MTGD 126 has a drive output signal VGATE 136 driving the gate of FETsc 52. The MTGD 126 has trigger inputs VSD-input 106a, IDS-input 114a and Vsec-input 120a respectively coupled to the VSD sense trigger 102, the IDS sense trigger 110 and the Vsec sense trigger 116. Additionally, the MTGD 126 has a set of logic states of:

    
    
        State-I where the FETsc 52 is turned off and latched thus it can not be triggered on by the VSD sense trigger 102.
        State-II where the FETsc 52 is turned off but unlatched thus it can be triggered on by the VSD sense trigger 102.
        State-III where the FETsc 52 is turned on but unlatched thus it can be triggered off by the IDS sense trigger 110.

The MTGD 126 is further configured to enter state-III upon activation of the VSD sense trigger 102, enter state-I upon activation of the IDS sense trigger 110 and enter state-II upon activation of the Vsec sense trigger 116. It should also become clear that numerous other logic building blocks, other than the latchable turn-off logic 128 and the gating circuit 130 just described, can be designed and configured to perform a function equivalent to the MTGD 126. Terminal-A and terminal-K of the FETsc control circuit 100 are provided, as will be presently illustrated, to hook it up along the conduction path of the secondary coil current Iscc.

    
    


FIG. 2 illustrates a more detailed circuit schematic of the present invention FETsc control circuit 200 for controlling the FETsc 52 in a TCSC 50. The VSD sense trigger 102 of the previous FETsc control circuit 100 is embodied in a voltage comparator 202. The IDS sense trigger 110 of the previous FETsc control circuit 100 is embodied in a Kelvin connection IDS sensor 210 plus a voltage comparator 204. More specifically, the Kelvin connection employs a four-terminal current sensing configuration that is integrated into the FETsc 52 producing a sampled IDS sense current that is a small fraction of the IDS. As a specific example of application, the ratio of IDS to IDS sense current can range from about 100:1 to about 100000:1. The Vsec sense trigger 116 of the previous FETsc control circuit 100 is embodied in a voltage comparator 206. Without showing unnecessary obscuring details, the threshold voltages for the various voltage comparators 202, 204 and 206 can be created by a band gap voltage reference 208. As additional examples of application, the threshold voltage for the voltage comparator 202 can be set from about 20 mV to about 200 mV. The threshold voltage for the voltage comparator 204 can be set to correspond to an IDS threshold of from about 0.1 mA to about 10 mA. The threshold voltage for the voltage comparator 206 can be set from about −0.1 V to about −1 V. The MTGD 126 is now embodied in a turn off and latch (TOL) 212 and a switchable gate driver (SGD) 214 with a digital output QTOL of the TOL 212 coupled to a digital input OFF of the SGD 214. The digital output DRV of SGD 214 is connected to the drive output signal VGATE 136 driving the FETsc 52. The digital inputs SET and RESET of the TOL 212 are respectively coupled to the outputs of the voltage comparators 204 and 206 while the digital input ON of the SGD 214 is coupled to the output of the voltage comparator 202. Thus, activation of the digital inputs SET and RESET would respectively activate and deactivate the QTOL. In a similar manner, activation of the digital inputs ON and OFF would respectively turn the FETsc 52 ON and OFF. Finally, all the just described signal processing building blocks of the FETsc control circuit 200 are powered by a bias voltage input terminal (BVIT) 201 at voltage Vcc.
FIG. 3A and FIG. 3B illustrate two embodiments of the present invention control circuit as deployed in a TCSC. In FIG. 3A a present invention FETsc plus control circuit 220 is deployed with its Terminal-A and terminal-K hooked up along the low-side conduction path of the secondary coil current Isec. Hence the FETsc plus control circuit 220 forms a 4-terminal replacement device for the FETsc 52. A low-power bias network (LPBN) 222, in parallel connection with Cout 32, is provided to generate the Vcc. In FIG. 3B a present invention FETsc plus control circuit 220 is deployed with its Terminal-A and terminal-K hooked up along the high-side conduction path of the secondary coil current Isec. Hence the FETsc plus control circuit 220 also forms a 4-terminal replacement device for the FETsc 52. A low-power bias network (LPBN) 222, in parallel connection with the STC 31, is provided to generate the Vcc.
Throughout the description and drawings, numerous exemplary embodiments were given with reference to specific configurations. It will be appreciated by those of ordinary skill in the art that the present invention can be embodied in numerous other specific forms and those of ordinary skill in the art would be able to practice such other embodiments without undue experimentation. The scope of the present invention, for the purpose of the present patent document, is hence not limited merely to the specific exemplary embodiments of the foregoing description, but rather is indicated by the following claims. Any and all modifications that come within the meaning and range of equivalents within the claims are intended to be considered as being embraced within the spirit and scope of the present invention.

## Claims

1. A circuit for controlling the secondary FET of a transformer coupled synchronous rectified flyback converter (TCSC) having a primary circuit and a secondary circuit with transformer coupling there between, the primary circuit having a primary transformer coil (PTC) coupled with a primary switching network (PSN) being switched by its built-in primary switching FET (FETpm), the secondary circuit having a serial connection of a secondary transformer coil (STC) with a secondary coil voltage (Vsec) there across, an output capacitor (Cout) and a secondary switching FET (FETsc) having a built-in parasitic body diode BDsc with a forward voltage VSD there across, each of the synchronous switching cycles of the TCSC is characterized by a sequence of time marks tPCR, tPSX and tSIC wherein tPCR marks the start of primary coil current (Ipri) ramp up followed by a negative 0-crossing of Vsec, tPSX marks the instant of primary coil current (Ipri)-to-secondary coil current (Isec) transfer followed by a positive 0-crossing of VSD with Vsec ringing and tSIC marks the start of FETsc first-quadrant conduction followed by a positive 0-crossing of IDS also with Vsec ringing, the secondary FET control circuit comprises:
a) a VSD sense trigger with a digital trigger output signal VSD-trigger and its analog inputs coupled to the FETsc terminals for sensing the VSD and, upon sensing a positive 0-crossing of VSD, for activating said VSD-trigger;
b) an IDS sense trigger with a digital trigger output signal IDS-trigger and its analog inputs coupled to the FETsc terminals for sensing a drain-to-source current IDS of the FETsc and, upon sensing a positive 0-crossing of IDS, for activating said IDS-trigger;
c) a Vsec sense trigger with a digital trigger output signal Vsec-trigger and its analog inputs coupled to the STC terminals for sensing the Vsec and, upon sensing a negative Vsec, for activating said Vsec-trigger; and
d) a multiple-trigger gate driver (MTGD) having digital trigger inputs VSD-input, IDS-input and Vsec-input respectively coupled to said VSD-trigger, IDS-trigger and Vsec-trigger, a drive output signal VGATE coupled to the gate of FETsc and a set of logic states of:
state-I where the FETsc is turned off and latched thus can not be triggered on;
state-II where the FETsc is turned off but unlatched thus can be triggered on; and
state-III where the FETsc is turned on but unlatched thus can be triggered off; and
said MTGD is configured to enter state-III upon activation of said VSD-trigger, enter state-I upon activation of said IDS-trigger and enter state-II upon activation of said Vsec-trigger


whereby avoid false triggering of the FETssc caused by the numerous undesirable Vsec ringings, into incorrect states around the time marks tPSX and tSIC that would otherwise result in an increased secondary power loss of the TCSC.

2. The secondary FET control circuit of claim 1 further comprises a bias voltage input terminal (BVIT) for receiving a low power bias voltage Vcc powering the VSD sense trigger, the IDS sense trigger, the Vsec sense trigger and the MTGD.

3. The secondary FET control circuit of claim 2 wherein the Vcc is generated by a low-power bias network (LPBN) in parallel connection with the STC.

4. The secondary FET control circuit of claim 2 wherein the Vcc is generated by a low-power bias network (LPBN) in parallel connection with the Cout.

5. The secondary FET control circuit of claim 1 wherein said MTGD further comprises:
a switchable gate driver (SGD) having a digital trigger input ON coupled to said VSD-trigger, a digital trigger input OFF and a drive output signal VGATE coupled to the gate of FETsc so as to turn on FETsc upon activation of the digital trigger input ON whereas turn off FETsc upon activation of the digital trigger input OFF; and
a turn off and latch (TOL) having a digital trigger input SET coupled to said IDS-trigger, a digital trigger input RESET coupled to said Vsec-trigger and a digital output QTOL coupled to the digital trigger input OFF such that activation of the digital trigger input SET activates the QTOL whereas activation of the digital trigger input RESET deactivates the QTOL.

6. The secondary FET control circuit of claim 1 wherein the analog inputs of said IDS sense trigger are integrated into the FETsc with a four-terminal current sensing Kelvin connection producing a sampled IDS sense current that is a small fraction of the IDS.

7. The secondary FET control circuit of claim 6 where the VSD sense trigger, the Vsec sense trigger and the MTGD are co-packaged with the FETsc so as to form a 4-terminal replacement device for the FETsc with further increased noise immunity against false triggering of the FETsc.

8. The secondary FET control circuit of claim 7 where the 4-terminal replacement device is located at the high-side of the STC.

9. The secondary FET control circuit of claim 7 where the 4-terminal replacement device is located at the low-side of the STC.

10. A method for controlling the secondary FET of a transformer coupled synchronous rectified flyback converter (TCSC) having a primary circuit and a secondary circuit with transformer coupling there between, the primary circuit having a primary transformer coil (PTC) coupled with a primary switching network (PSN) being switched by its built-in primary switching FET (FETpm), the secondary circuit having a serial connection of a secondary transformer coil (STC) with a secondary coil voltage (Vsec) there across, an output capacitor (Cout) and a secondary switching FET (FETsc) having a built-in parasitic body diode BDsc with a forward voltage VSD there across, each of the synchronous switching cycles of the TCSC is characterized by a sequence of time marks tPCR, tPSX and tSIC wherein tPCR marks the start of primary coil current (Ipri) ramp up followed by a negative 0-crossing of Vsec, tPSX marks the instant of primary coil current (Ipri)-to-secondary coil current (Isec) transfer followed by a positive 0-crossing of VSD with Vsec ringing and tSIC marks the start of FETsc first-quadrant conduction followed by a positive 0-crossing of IDS also with Vsec ringing, the method comprises:
a) sensing the forward voltage VSD across the body diode BDssc a drain-to-source current IDS of the FETsc and the Vsec; and
b1) upon sensing a positive 0-crossing of VSD, turning on the FETsc and maintaining it in a conductive but unlatched state so it may subsequently be turned off;
b2) upon sensing a positive 0-crossing of IDS, turning off the FETsc and maintaining it in a latched state so it may not be subsequently turned on; and
b3) upon sensing a negative Vsec, turning off the latch of FETsc and maintaining it in an unlatched state so it may subsequently be turned on

whereby avoid false triggering of the FETssc caused by the numerous undesirable Vsec ringings, into incorrect states around the time marks tPSX and tSIC that would otherwise result in an increased secondary power loss of the TCSC.

11. The method for controlling the secondary FET of claim 10 further comprises providing a low-power bias network (LPBN) in parallel connection with the STC for generating a low power bias voltage Vcc powering the sensing of VSD, IDS and Vsec as well as generating a gate drive signal VGATE for driving the FETSC.

12. The method for controlling the secondary FET of claim 10 further comprises providing a low-power bias network (LPBN) in parallel connection with the Cout for generating a low power bias voltage Vcc powering the sensing of VSD, IDS and Vsec as well as generating a gate drive signal VGATE for driving the FETSC.

13. The method for controlling the secondary FET of claim 10 wherein sensing the drain-to-source current IDS comprises integrating a four-terminal current sensing Kelvin connection into the FETsc for producing a sampled IDS sense current that is a small fraction of the IDS.