Abstract:
A low loss DC/DC converter uses the reset technique to reset the magnetizing current from the forward transformer during the OFF period, reducing voltage stress and extend the maximum work duty. The third winding reset circuit is an improved version of a conventional third winding reset circuit for a forward converter.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a forward converter, and more particularly to a third winding reset circuit on a transformer core. 
     BACKGROUND OF THE INVENTION 
     The forward converter is the most idealist topology in the power conversion field, it can be widely used in several applications such as AC-to-DC, DC-to-DC and other power electronic equipments. Conventional forward converters have some problems such as the voltage stress, the work duty which can&#39;t be over 50% under some conditions. Therefore, many new reset circuit technologies have been developed. The third winding reset circuit proposed before is not approved entirely by the public. Applicant believes that it could be improved and the following problems can be solved. FIGS. 1 a  and  1   b , show a convention forward converter and timing diagram, FIGS. 2 a  and  2   b , are the conventional third winding reset for a forward converter and timing diagram. 
     As shown in FIG. 1 a  a conventional forward converter, power switch Sm is coupled in series with the primary winding P 1  of a transformer. Each time, the power switch Sm is turned on and off is controlled by the gate driving signals of the pulse-width-modulated (PWM) controller. The secondary side of the converter has a forward rectifier D 1  coupled to the secondary winding S 1  of the transformer, a free-wheeling rectifier D 2  and an output filter consisting of an output choke Lout and an output capacitor Cout. The output filter transfers DC energy to RL load from the primary side DC source V1. 
     When power switch Sm is turned on, the input voltage V1 is applied across the primary winding P 1  of the transformer T 1 , and the voltage is coupled to secondary winding S 1 . The positive end of the secondary winding S 1  is turned positive, and the forward rectifier D 1  is turned on, the free-wheeling rectifier D 2  is turned off, the forward power current flows to output choke Lout, output capacitor Cout and load RL. 
     When power switch Sm is turned off, the positive end of the secondary winding S 1  is turned negative. The forward rectifier D 1  is turned off and the free-wheeling rectifier D 2  must be turned on. Because the power current of output choke Lout must be forwarded continually to the output load RL by the free-wheeling rectifier D 2 . 
     FIG. 1 b  shows the timing diagram of the circuit of FIG. 1 a . The conventional forward converter has some problems, such as, the voltage stress and the work duty. If the work duty design is over 50%, the voltage stress will be dangerous. From t=t0 to t2, the power switch Sm is turned off, during t0 to t1 time diagram, across Drain-Source pin of the power switch is twice more than V1 voltage stress., so that Drain-Source pin of the power switch must endure twice more than V1 voltage. In this case, the material cost will be high. If the VT product of transformer between t0 and t1 can be kept constantly during the time period of t0 to t2, the voltage stress will be reduced to a minimum value. 
     As shown in FIG. 2 a  a conventional third winding reset of the forward converter, the power switch Sm is coupled in series with the primary winding P 1  of a transformer. Each time, the power switch Sm is turned on and off is controlled by the gate driving signals of the pulse-width-modulated (PWM) controller. The DC source V1 of the primary side is coupled in parallel with the DL network, and comprises a diode D 3  and a third winding S 2 . The DL network is used to reset the magnetizing current of the primary winding of transformer T 1  and the current will be recycled to DC source. The secondary side of the converter has a forward rectifier D 1  coupled to the secondary winding S 1  of the transformer, a free-wheeling rectifier D 2  and an output filter consisting of an output choke Lout and an output capacitor Cout. The output filter transfers DC energy to RL load from the primary side DC source. 
     When the power switch Sm is turned on, the input voltage V1, is applied across the primary winding P 1  of the transformer T 1 , and the voltage is coupled to secondary winding S 1 . The positive end of the secondary winding S 1  is turned positive, now, the forward rectifier D 1  is turned on, the free-wheeling rectifier D 2  is turned off, the forward power current flows to output choke Lout, output capacitor Cout and load, RL. The positive end of the third winding S 2  is turned positive voltage, the diode D 3  is turned off. 
     When power switch Sm is turned off, the positive end of the secondary winding S 1  is turned into negative. The forward rectifier D 1  is turned off and the free-wheeling rectifier D 2  must be turned on. Because the current of output choke Lout must be forwarded continually to the output load RL by the free-wheeling rectifier D 2 . The negative end of the third winding S 2  is positive voltage, the diode D 3  will be turned on, the magnetic flux from the transformer will be reset, the magnetizing current flows back to DC source by D 3  and the clamp voltage will be twice to V1 in the power switch Drain-Source. 
     FIG. 2 b  shows the timing diagram of the circuit of FIG. 2 a . The conventional third winding of forward converter still has some problems such as, the Vds of power switch voltage is still high, and the work duty designs only 50%. If the design of work duty is over 50%, the voltage will be clamped, according to voltage-second balance, the transformer will be saturated when the drain current Id of power switch has moved to high peak and destroys power switch Sm. 
     From t=t0 to t2, the power switch Sm is turned off; during t0 to t1 time diagram, Drain-Source of the power switch is across twice to V1 voltage stress. So that, the component must also meet twice to V1 for power switch Vds. In this case, the material cost is still high. 
     SUMMARY OF THE INVENTION 
     The aforementioned issue has two problems, voltage stress and work duty, which will be improved effectively by this third winding reset circuit present invention. The present invention consists of a rectifier diode Dr, a storage capacitor Cs, an auxiliary switch Sa, a storage inductor La and a free-wheeling diode Df. When power switch Sm is turned off, the rectifier diode Dr provides a forward conduction path to transfer magnetizing energy from a transformer and this energy flows to storage capacitor Cs. In the same time, the auxiliary switch Sa is turned on by the negative end of the third winding S 2 , must be high level voltage, to provide a path to discharge the energy through the capacitor Cs and the storage inductor La to the DC source. The free-wheeling diode Df provides a path to discharge the current of storage inductor La when the power switch Sm is turned on and the auxiliary switch is turned off. 
     The third winding provides a new reset circuit technique to reset magnetized energy easier from the transformer, effectively to reduce the voltage stress, and to surpass work duty 50% maximum limited of the conventional third winding forward converter. So the new third winding reset circuit structure is simple and manufactured at low cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 a  is a conventional forward converter circuit; 
     FIG. 1 b  is a timing diagram of the circuit of FIG. 1 a;    
     FIG. 2 a  is a conventional third winding reset of the conventional forward converter; 
     FIG. 2 b  is a timing diagram of the circuit of FIG. 2 a;    
     FIG. 3 a  is a third winding reset of the conventional forward converter; 
     FIG. 3 b  is a timing diagram of the circuit of FIG. 3 a , the work duty is over 50%; 
     FIG. 4 a  is the simplified equivalent circuit of a third winding reset circuit when the power switch Sm is turned on; 
     FIG. 4 b  is the simplified equivalent circuit of a third winding reset circuit when the power switch Sm is turned off; 
     FIG. 5 is a third winding reset circuit for synchronous rectifier forward converter; 
     FIG. 6 is a third winding reset circuit with shift voltage level circuit for low voltage output; 
     FIG. 7 is a third winding reset circuit for primary side reset of the conventional forward converter; 
     FIG. 8 is a third winding reset circuit for primary side reset of the synchronous rectifier forward converter; 
     FIG. 9 is a third winding reset circuit using P-Mosfet component for auxiliary switch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 a  shows the third winding reset forward converter of the present invention, it comprises a conventional forward converter (including input DC source V1, the PWM control circuit, the primary side of the winding P 1 , the power switch Sm, the secondary side of the winding S 1 , the power rectifier D 1  and D 2 , the output choke Lout, the output capacitor Cout and resistor load RL. The power switch Sm is coupled in series with the primary winding P 1  of a transformer. The secondary side of the converter has a forward rectifier D 1  coupled to the secondary winding S 1  of the transformer, a free-wheeling rectifier D 2  and an output filter consisting of an output choke Lout and an output capacitor Cout. The output filter transfers DC energy to RL load from the primary side DC source and the third winding reset circuit (including the third winding S 2 , the rectifier Dr, the storage capacitor Cs, the auxiliary switch Sa, the storage inductor La and the free-wheeling diode Df). When power switch Sm is turned off, the rectifier diode Dr provides a forward conduction path to transfer magnetizing energy from transformer flows to storage capacitor Cs. At the same time, the auxiliary switch Sa is turned on by the voltage polarity change of the third winding S 2  to provide a path to discharge the energy through the storage capacitor Cs and the storage inductor La to the DC source. The free-wheeling diode Df provides a path to discharge the current of storage inductor La when the power switch Sm is turned on. FIG. 3 b  shows the timing diagram of the circuit of FIG. 3 a . The voltage stress and work duty are improved by the present invention of the third winding reset circuit. 
     FIG. 4 a  and FIG. 4 b  are the simplified equivalent circuit of a third winding reset circuit when the power switch Sm is turned on and off. The rectifier Dr, Df and the auxiliary switch Sa are switch components respectively, the storage capacitor is the voltage source, and the storage inductor La is the current source. Referring to FIG. 4 a , when power switch Sm is turned on, the switch of Dr and Sa are turned off, the current of magnetizing inductance Lm is linearly increased from zero by the input source V1 energy charging to the transformer T 1 . At the same time, the magnetic current by the Df switch is turned on and discharges the DC source. Referring to FIG. 4 b , when power switch Sm is turned off, the Dr Switch and the Sa switch are turned on, the Df switch is turned off. The magnetic energy coupled to the third winding through Dr and Sa switch charging to storage Cs and inductance La until the magnetic current linearly decreases zero from magnetic inductance Lm. The power switch Sm is turned on again to start next switching cycle. 
     FIG. 5 shows the self-driven synchronous rectifier of a forward converter circuit applied to the third winding reset circuit of the present invention. 
     When the power switch Sm is turned on, the power energy of the primary side from the DC source V1 coupling to the secondary side, the positive end of secondary side winding S 1  is turned positive, the synchronous rectifier Sfy is turned on and the synchronous rectifier Sfd is turned off, the forward power current of the secondary side winding S 1  flows to output choke Lout, output capacitor Cout and load RL. The auxiliary switch Sa is turned off and the free-wheeling diode Df provides a path to discharge the current of storage inductor La to the load RL when the positive end of the third winding is turn positive; at the same time, the current of magnetic inductance Lm is linearly increased from zero by the input source V1 energy charging to the transformer T 1 . 
     When the power switch is turned off, the negative end of secondary winding S 1  is turned positive. So the synchronous rectifier Sfy is turned off and the synchronous rectifier Sfd is turned on. The power current stored in the output choke Lout and output capacitor Cout will be flowed to load RL by the synchronous Sfd turned on. At the same time, the negative end of the third winding is turned positive, the auxiliary switch Sa is turned on and the rectifier diode Dr provides a forward conduction path to discharge the energy through the capacitor Cs and the storage inductor La to the load RL until the current of magnetic inductance Lm is linearly decreased to zero by coupling to the third winding S 2 . 
     FIG. 6 shows the self-driven synchronous rectifier of a forward converter circuit applied to the third winding reset circuit and gat drive network of the present invention. The gat drive network comprises a diode Dg, a capacitor Cg and resistor Rg, it proves the average level of drive signal to quickly turn on the auxiliary switch Sa when the third winding reset circuit of the present invention used in the low voltage output condition. 
     When the power switch Sm is turned on, the power energy of the primary side from the DC source V1 couples to the secondary side, the positive end of secondary side winding S 1  is turned positive, the gate drive network gets average level of drive signal by the positive end of the third winding S 2  is turned positive, the drive signal through the diode Dg storage the capacitor Cg and resistor Rg. The synchronous rectifier Sfy is turned on and the synchronous rectifier Sfd is turned off, the forward power current of the secondary side winding S 1  flows to output choke Lout, output capacitor Cout and load RL. The auxiliary switch Sa is turned off and the free-wheeling diode Df provides a path to discharge the current of storage inductor La to the load RL when the positive end of the third winding is turned into positive; at the same time, the current of magnetic inductance Lm is linearly increased from zero by the input source V1 energy charging to the transformer T 1 . 
     When the power switch is turned off and the negative end of secondary winding S 1  is turned positive. The gat drive network having average level of drive signal can be turned the auxiliary switch Sa on quickly, so that the synchronous rectifier Sfy is turned off and the synchronous rectifier Sfd is turned on. The power current stored in the output choke Lout and output capacitor Cout will be flowed to load RL by the synchronous Sfd being turned on. At the same time, the negative end of the third winding is turned into positive, the auxiliary switch Sa is turned on and the rectifier diode Dr provides a forward conduction path to discharge the energy through the capacitor Cs and the storage inductor La to the load RL until the current of magnetic inductance Lm is linearly decreased to zero by coupling to the third winding S 2 . 
     FIG. 7 shows the conventional forward converter circuit applied the third winding reset circuit of the primary side with voltage level shift circuit of the present invention. The voltage level shift circuit comprises a diode Dsv and a capacitor Csv, this circuit provides DC voltage level to the third winding reset circuit, because the DC source V1 voltage of the primary side is higher than the voltage of the third winding S 2  by the primary winding P 1  coupled to the third winding S 2 . The third winding S 2  can&#39;t provide reset energy to the DC source V1 of the primary side. So that the voltage level shift circuit must be used for primary side reset circuit. The operation of turning on and off for the power switch Sm and the auxiliary switch Sa is controlled by the two-channel gate driving signals of the pulse-width-modulated (PWM) controller. 
     When the power switch Sm is turned on, the power energy of the primary side from the DC source V1 coupling to the secondary side, the positive end of secondary side winding S 1  is turned into positive, the rectifier D 1  is turned on and the rectifier D 2  is turned off, the forward power current of the secondary side winding S 1  flows to output choke Lout, output capacitor Cout and load RL. The capacitor Csv can&#39;t discharge DC voltage level by the diode Dsv turned off, the auxiliary switch Sa is turned off and the free-wheeling diode Df provides a path to discharge the current of storage inductor La to the DC souce V1 when the positive end of the third winding turns into positive. At the same time, the current of magnetic inductance Lm is linearly increased from zero by the input source V1 energy charging to the transformer T 1 . 
     When the power switch is turned off and the negative end of secondary winding S 1  is turned positive, the rectifier D 1  is turned off and the rectifier D 2  is turned on. The power current stored in the output choke Lout and output capacitor Cout will be flowed to load RL when the rectifier D 2  is turned on. At the same time, the negative end of the third winding is turned into positive, the diode Dsv is turned on, the capacitor Csv can charge DC voltage level from the third winding S 2  through a diode Dsv, the auxiliary switch Sa is turned on and the rectifier diode Dr provides a forward conduction path to discharge the energy through the capacitor Cs and the storage inductor La to the DC source V1 until the current of magnetic inductance Lm is linearly decreased to zero by coupling to the third winding S 2 . 
     FIG. 8 shows the self-driven synchronous rectifier of a forward converter circuit applied the third winding reset circuit of the primary side with voltage level shift circuit embodying of the present invention. The voltage level shift circuit comprises a diode Dsv and a capacitor Csv, it provides DC voltage level to the third winding reset circuit, because the DC source V1 voltage of the primary side is higher than voltage of the third winding S 2  by the primary winding P 1  coupled to the third winding S 2 . The third winding S 2  can&#39;t provide reset energy to the DC source V1 of the primary side. So that the voltage level shift circuit must be used for primary side reset circuit. The turning on and off of the power switch Sm and the auxiliary switch Sa is controlled by the two-channel gate driving signals of the pulse-width-modulated (PWM) controller. When the power switch Sm is turned on, the power energy of the primary side from the DC source V1 coupling to the secondary side, the positive end of secondary side winding S 1  is turned into positive, the synchronous rectifier Sfy is turned on and the synchronous rectifier Sfd is turned off, the forward power current of the secondary side winding S 1  flows to output choke Lout, output capacitor Cout and load RL. The capacitor Csv can&#39;t discharge DC voltage level by the diode Dsv turned off, the auxiliary switch Sa is turned off and the free-wheeling diode Df provides a path to discharge the current of storage inductor La to the DC souce V1 when the positive end of the third winding is turn positive. At the same time, the current of magnetic inductance Lm is linearly increased from zero by the input source V1 energy charging to the transformer T 1 . 
     When the power switch is turned off and the negative end of secondary winding S 1  is turned into positive. The synchronous rectifier Sfy is turned off and the synchronous rectifier Sfd is turned on. The power current stored in the output choke Lout and output capacitor Cout will be flowed to load RL by the synchronous rectifier Sfd which is turned on. At the same time, the negative end of the third winding is turned positive, the diode Dsv is turned on, the capacitor Csv can charge DC voltage level from the third winding S 2  by through a diode Dsv, the auxiliary switch Sa is turned on and the rectifier diode Dr provides a forward conduction path to discharge the energy through the capacitor Cs and the storage inductor La to the DC source V1 until the current of magnetic inductance Lm linearly is decreased to zero by coupling to the third winding S 2 . 
     FIG. 9 shows the self-driven synchronous rectifier of a forward converter circuit applied the third winding reset circuit of the secondary side with using a P-Mosfet component for auxiliary switch of the present invention. 
     When the power switch Sm is turned on, the power energy of the primary side from the DC source V1 coupling to the secondary side, the positive end of secondary side winding S 1  is turned positive, the synchronous rectifier Sfy is turned on and the synchronous rectifier Sfd is turned off, the forward power current of the secondary side winding S 1  flows to output choke Lout, output capacitor Cout and load RL. The P-mosfet auxiliary switch Sa is turned off by the positive end of the third winding is turned to be positive, the free-wheeling diode Df provides a path to discharge the current of storage inductor La to the load RL when the positive end of the third winding is turn positive. At the same time, the current of magnetic inductance Lm is linearly increased from zero by the input source V1 energy charging to the transformer T 1 . 
     When the power switch is turned off, the positive end of secondary winding S 1  is turned negative. So the synchronous rectifier Sfy is turned off and the synchronous rectifier Sfd is turned on. The power current stored in the output choke Lout and output capacitor Cout will be flowed to load RL by the synchronous Sfd turned on. At the same time, the P-mosfet auxiliary switch Sa is turned on by the positive end of the third winding is turned negative and the rectifier diode Dr provides a forward conduction path to discharge the energy through the capacitor Cs and the storage inductor La to the load RL until the current of magnetic inductance Lm linearly is decreased to zero by coupling to the third winding S 2 . 
     While the invention has been described in terms of simple and practical preferred embodiments, the invention will be easy to reorganize and consolidate modification by other skilled in the art within the spirit and scope of the appended claims.