Abstract:
A power converter with a self synchronized synchronous rectifier includes one or two drive windings to provide positive drive to the control electrodes of the controlled switches (FETs) of a self synchronized synchronous rectifier. The polarities of these windings are selected so that the switched devices are driven appropriately to rectify the periodic signal output of the secondary winding of the power transformer of the converter. In some arrangements one or two drive windings are included as extra windings in the power transformer and connected to provide the proper polarity drive signals. The turn ratios of the drive windings to the other windings are selected to provide the proper gate drive signal levels. In an alternative arrangement a separate drive transformer may be provided to supply the gate drive signals.

Description:
FIELD OF THE INVENTION 
     This invention relates to synchronous rectification and to a power converter employing synchronous rectification. It is particularly concerned with a self synchronized rectifier combined with a converter. 
     BACKGROUND OF THE INVENTION 
     A converter is a power processing circuit, that may have an input-output transformer isolation, that operates to convert an input voltage waveform with a DC component into an output DC voltage waveform. The presence of an isolation transformer requires the use of a rectifier circuit in the converter output circuit to perform the waveform conversion. The traditional rectifier uses rectifying diodes that conduct the load current only when forward biased in response to the input waveform. In some rectifiers (i.e. synchronous rectifiers) the diodes are replaced by controlled switches that are periodically biased into conduction and nonconduction in synchronism with the periodic waveform to be rectified. In self-synchronized synchronous rectifiers the biasing of the synchronous switches is supplied directly from a secondary winding of a transformer without requiring a separate drive to activate the synchronous switches. 
     Self-synchronized synchronous rectifiers come in many forms, all designed to meet specified operating constraints. The challenge, in each instance, is to devise synchronous rectifier circuitry that is efficient (i.e. has low power dissipation) in performing the rectification process. The specific circuit topology of the synchronous rectifier is dependent in large part on the converter type being used and its operating characteristics (i.e. hard switched rs. soft switched). Application of self synchronized synchronous rectifiers to hard switched buck derived converter topologies, for example, is limited by a variable transformer reset voltage that often causes the voltage across the transformer windings to be essentially zero during a portion of each switching cycle. During this time, the synchronous rectifier switch that should be conducting is operating in a dissipative or cut-off mode causing a serious shortfall in efficiency. An example of a circuit that eliminates the problem of zero voltage across the transformer is provided in the U.S. Pat. No. 5,303,138 which discloses an improved forward converter combined with a self synchronized synchronous rectifier. In this circuit the reset voltage is clamped and maintained over the non conducting interval of the main power switch and hence causes the rectifier to operate over the entire non conducting interval. In this arrangement the gate drive signal is directly dependent upon the voltage of the secondary winding which in turn is dependent upon the input voltage and load. In practice the voltages of the secondary winding may vary over a substantial range and there is the possibility of insufficient drive voltage for a rectifier that is conducting, causing it to operate in either a dissipative mode or a cut-off mode. This deficiency is quite likely for converters that deliver low output voltages. 
     In a circuit disclosed by L. Hubler et al (APEC 94 page 645, entitled &#34;Design of a High Efficiency Power Converter for a Satellite Solid-State Power Amplifier&#34;), the problem of insufficient drive voltage is overcome by including separate windings on the power transformer to drive the synchronous rectifier switches. However, when the turns of the drive windings are set high enough to ensure adequate drive voltage for all operating conditions of input voltage and load, excessive drive voltage is typically generated at some operating condition. This causes excessive power dissipation or failure of the synchronous rectifier switch. 
     In another U.S. Pat. No. 5,274,543 voltage limiting (gate drive) switches are disclosed as a means for limiting dissipation in the drive circuit for the synchronous rectifiers. 
     SUMMARY OF THE INVENTION 
     A power converter with a self-synchronized rectifier that includes one or two drive windings that do not carry load current but instead drive the control electrode(s) of one or both controlled rectifier switches (FETs). The drive winding(s) are connected in such a way that the switched devices rectify the periodic voltage waveform present at the secondary winding of the power transformer of the converter, with the turns of the drive winding(s) selected to provide sufficient drive signal levels under all operating conditions of input voltage and load. Additional switches may be connected in series with the control electrodes of the rectifier switches to limit the applied voltage. This drive circuit ensures that the drive voltage is always large enough to bias the proper synchronous-rectifier switch conducting, but not so large that it damages the switch or dissipates excessive power. 
     In one arrangement of the drive circuit, an extra winding is included in the power transformer, and each of its leads is connected to the control electrode of one synchronous-rectifier switch. In another arrangement, a separate drive transformer is provided to supply the gate drive signals. In either of these arrangements, there may be voltage-limiting switches connected between the drive winding and the control electrode of each synchronous-rectifier switch. 
     In a third arrangement, one or two extra windings are included in the power transformer and for each one, one lead is connected to the secondary winding and the other is connected to a voltage-limiting switch, of a series connection of two voltage limiting switches which is connected to the control electrode of a synchronous-rectifier switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic of a clamped forward converter with a drive for a self synchronized synchronous rectifier; 
     FIG. 2 is a graph of waveforms to assist in describing the operation of the converter of FIG. 1; 
     FIG. 3 through 14 are schematics of other versions of a self synchronized synchronous rectifier, embodying the principles of the invention; 
     FIG. 15 is a clamped forward converter with an alternate drive for a self-synchronized synchronous rectifier; and 
     FIG. 16 is a generalized circuit diagram embodying the principles introduced in FIG. 15. 
    
    
     DETAILED DESCRIPTION 
     A power converter, such as shown in FIG. 1, includes a main FET power switch 101 connected to and periodically switched to enable an input DC voltage to be applied to the primary winding 112 of the power transformer 102. An auxiliary switch 103 and a capacitor 107 connected in series and placed in shunt connection with the power switch 101 operates to clamp the voltage level across the primary and secondary windings during non conducting intervals of the power switch 101. This assures that a voltage exists at the transformer windings over the entire time of non-conduction of the main power switch to assure drive for synchronous rectifier devices connected in the secondary circuit. The secondary winding 111 is connected to two rectifier switches 105 and 106 which are controllably switched to rectify the periodic waveform supplied to the rectifier by the secondary winding 111. A low pass filter including inductor 104 acts on the rectified waveform to supply the output voltage V out . A regulation control 121 senses the output voltage V out , via lead 127, and controls the duty cycle of the main power switch 101 and auxiliary switch 103, via the drive circuit 122. 
     The power transformer 102 includes a third or auxiliary winding 110 having a winding polarity so that its voltage is utilized to appropriately drive the FET synchronous rectifier switches 105 and 106. Drive to the FET synchronous rectifier switches 105 and 106 is applied through the drain-source path of the gate drive FET devices 109 and 108 respectively. The drive level is determined by the turn ratio of the auxiliary winding 110 with respect to the primary winding 112, selected to assure that there is sufficient drive for the gates of synchronous rectifier switches 105 and 106 over the entire operating cycle and permitted range of input voltage V in . The FET devices 108 and 109 limit the voltage applied to the gates of the synchronous rectifier switches 105 and 106 to reduce dissipative losses and to reduce the possibility of voltage overstress of the switches 105 and 106. A voltage source V b , is used to supply a bias voltage to the FET devices 108 and 109. 
     The operation of the converter may be readily understood through the following description and by reference to the voltage waveforms shown in the FIG. 2. At the initial t=t 0  start time the power switch 101 is non-conducting and the auxiliary switch 103 is conducting with substantially zero impedance in its main conductive path. Switch 103 is then turned off, and before time t=t 1  switch 101 is turned on, causing the input voltage V in  to be fully impressed across the primary winding 112 of transformer 102. The voltages ν 111  across winding 111 and ν 110  across winding 110 (dotted ends are positive with respect to undotted ends) are determined by the respective turn ratios. Typically more turns are included in winding 110 to boost the synchronous-rectifier drive voltage above that available at winding 111. As the voltage ν 110  increases, current flows out of the dotted terminal of the winding 110 through the switch 108 to the gate capacitance of the rectifying switch 106 causing the gate-source voltage ν gs106  of switch 106 to increase. This voltage continues to increase until the gate-source voltage of the switch 108 falls below the sustaining threshold and the switch 108 becomes non-conducting at time t=t 1 . Non-conductance of switch 108 clamps the voltage at the gate of switch 106 to a value determined by the difference between the DC bias voltage V b  connected to the gate of switch 108 and the threshold voltage of the switch 108. 
     Current now has stopped flowing into the gate of switch 106, supplied by the secondary winding 110. The same current flowed out of the gate of switch 105 and impressed a negative voltage v gs105  across the gate to source junction of switch 105, causing it to turn off to a non-conducting state. The load current now flows through switch 106, secondary winding 111 and output inductor 104. 
     Between times t=t 2  and t=t 3 , the switches 101 and 103 change state, after which switch 101 is non-conducting and switch 103 is conducting. For a short period during this interval, the drain-source channel of neither switch 105 or 106 is conducting and the output inductor current is conducted through the body diodes comprising a part of the switch of these devices. A negative voltage V in  minus V c107  is impressed across the primary winding 112 of transformer 102 causing the voltage across the secondary winding 111 and auxiliary winding 110 to reverse. 
     Initially, at the start of this interval, switch 108 is non-conducting and switch 109 is conducting. The negative voltage V 110  across winding 110 causes the gate to source capacitance of switch 106 to discharge through the body diode of switch 108. The gate to source capacitance of switch 105 is charged and the enabled switch 105 and output inductor 104 carry the load current. At time t=t 4 , the steady-state operating cycle that began at t=t 0 , repeats itself. 
     Circuit resonances are produced by the switching in the circuit due to the interaction between parasitic capacitances and inductances of the circuit. These resonances cause tinging in the drain-to-source voltage waveforms of switches 105 and 106 and as shown by the waveforms of FIG. 2 as they change conductive states. Coupling of these tinging voltages through the parasitic drain-to-gate capacitances of switches 105 and 106 could cause them to inappropriately conduct, however the bias circuit arrangement described herein causes negating voltages to be clamped across the gate to source terminals of switches 105 and 106 when they are normally non-conducting. Hence temporary voltage increases can not inadvertently turn on these devices and cause turn on losses that impair circuit efficiency. 
     Many variations of the circuit of FIG. 1 operate in the same manner without departing from the spirit and scope of the invention. The circuit of FIG. 3 discloses a flyback converter having a drive winding on the power transformer. In the circuit of FIG. 4 the output filter is connected to a tap in the power secondary winding to achieve both forward and flyback mode operation. 
     In the circuit variations of FIGS. 5 through 10, the drive winding for the synchronous rectifiers is contained in a separate drive transformer rather than the power transformer 102. In FIGS. 5 through 7, the primary winding of the drive transformer 110, and a series connected capacitor (to block DC current) are connected across the power switch 101. This drive circuit is shown in a forward converter in FIG. 5, a flyback converter in FIG. 6, and a converter with a tapped secondary winding in FIG. 7. In FIGS. 8 through 10, the drive transformer and capacitor are connected instead across a winding of the power transformer 102. This drive configuration is shown for the same three basic converters, a forward converter in FIG. 8, a flyback converter in FIG. 9, and a converter with a tapped secondary winding in FIG. 10. 
     FIG. 11 is a schematic diagram that represents all nine of the aforementioned circuit variations; i.e., all combinations of three circuit topologies and three possible locations of the synchronous-rectifier drive winding. FIGS. 12 through 14 show part of the circuit of FIG. 11, to illustrate some further circuit variations that embody the principles of the invention and can be applied to any of the nine aforementioned circuits. 
     The drive circuit in FIG. 12 accomplishes the same action as that in FIG. 11, but for a different kind of synchronous rectifier switch. This switch is biased conducting when the gate-to-source voltage is zero or positive, and it is biased nonconducting by a negative gate-to-source voltage. Diodes 121 and 122, which may be intrinsic to synchronous-rectifier switches 105 and 106, limit the positive voltage applied to the gate electrodes of the switches. Switches 117 and 118, along with the voltage source V p , limit the negative voltage applied to the gate electrodes of 105 and 106. In the embodiment shown in FIG. 12, FETs 117 and 118 are p-channel enhancement-mode FETs, which turn on when the gate-to-source voltage is less than the negative threshold voltage of the device. These switches also contain intrinsic body diodes that conduct to permit turning on the corresponding synchronous-rectifier switch. 
     During turn-on of synchronous-rectifier switch 106, in FIG. 12, current flows out of the dotted terminal of winding 110, through the body diode of switch 118, and charges the gate capacitance of 106. The gate-to-source voltage of 106 rises to a positive value, clamped by the forward drop of diode 122, turning switch 106 on. Simultaneously, switch 105 is turned off by current flowing out of the gate capacitance of 105 through switch 117. When the source voltage of switch 117 falls to V p  minus the negative threshold voltage of 117, switch 117 turns off, clamping the gate-to-source voltage of switch 105 to a negative value within the limits of the device. During the other switching transition, when synchronous-rectifier switch 106 turns off, the roles of 106, 118, and 122 are exchanged with those of 105, 117, and 121. 
     FIGS. 13 and 14 show other variations on the circuit of FIG. 11, with either synchronous-rectifier switch replaced by a diode. Specifically, synchronous-rectifier switch 105 in HG. 11 can be replaced by diode 113 in FIG. 13, with capacitor 115 substituted for the gate-to-source capacitance of switch 105. Alternatively, synchronous-rectifier switch 106 in FIG. 11 can be replaced by diode 114 in FIG. 14, with capacitor 116 substituted for the gate-to-source capacitance of switch 106. These circuits operate in essentially the same manner as described previously for FIG. 1, except that a diode is conducting during one portion of the switching cycle instead of a synchronous-rectifier switch. 
     Another means of boosting the drive voltage for self-driven synchronous rectifiers is illustrated in FIG. 15. In this example, the voltage available at the secondary power winding 211 is sufficient to drive one of the two synchronous-rectifier switches, 206, but not to drive the other, switch 205. The solution is to add drive winding 210 to the main power transformer 202, with one lead connected to the secondary power winding 211 and the other lead connected in series with one or two voltage-limiting switches and the control electrode of synchronous-rectifier switch 205. Gate-drive switches 209 and 208 are included as in FIG. 1 to limit the positive voltage applied to the gates of the synchronous rectifier switches. New gate-drive switch 217 and new bias voltage V p  are included to limit the &#34;off&#34; voltage applied to the gate of switch 205 during the time synchronous rectifier switch 206 is conducting. During turn-on of switch 205, gate-drive current can flow through the body diode of p-channel FET 217, but during turn-off, gate-drive current flows through the channel of switch 217 until its source voltage (between switches 209 and 217) falls to less than -V p  minus the negative threshold voltage of switch 217. This action protects the gate of synchronous-rectifier switch 205 from excessive negative voltage. 
     FIG. 16 is a generalized diagram of the invention illustrated in FIG. 15, and a circuit suitable for the alternate type of synchronous-rectifier switch introduced in FIG. 12. Both portions 211 and 219 of the power secondary winding may be present, yielding an isolated buck converter with a tapped secondary winding, or the turns of winding 219 may be set to zero, yielding a forward converter as in FIG. 15, or the turns of winding 211 may be set to zero, giving a flyback converter. Independently, the turns of either drive winding 210 or 218 may be set to zero if sufficient drive voltage is present for a particular design and range of operating conditions. Either of switches 208 and 216 may be eliminated, and either of switches 209 and 217 may be eliminated, as long as the associated gate voltage is not excessive. Finally, either synchronous-rectifier switch 205 or 206 may be replaced by a diode, if desired. Any of these circuit variations embodies the principles of the invention.