Abstract:
A pulse-width modulated apparatus to convert an input voltage to an output voltage at advantaged duty-cycles utilizes more than two switches in a converter design. The switches implement current paths that allow a single primary winding to be formed of primary winding segments. Intermediate points in the transformer primary winding are connected to switches and diodes such that during the OFF phase of the duty-cycle, the transformer segments discharge more quickly, thereby allowing the ON Phase of the duty-cycle to be longer than the OFF phase. During the OFF phase, the switches isolate the segments from each other while the diodes provide a magnetizing current discharge path. An output stage connected across the secondary winding regulates the output voltage by implementing a forward converter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    N/A  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    N/A  
         BACKGROUND OF THE INVENTION  
         [0003]    Power converters are electrical devices that are used to convert one voltage to another voltage. In most instances, they take a variable voltage and convert that voltage to a fixed voltage. In the case of the topologies discussed herein, the converters do this buy varying the duty cycle of the power switches. If the input voltage decreases, the pulse width, which is controlled by the control circuitry (not shown) increases the ON time of the transistor switches so that the output remains at the desired voltage.  
           [0004]    One of the topologies used in converting power from one voltage to another is the two-switch forward converter. In this topology, a primary side voltage is applied across a winding of a transformer and, through that, a voltage is induced in the secondary of the transformer. The voltage across the primary is applied and removed by cycling switches. The alternating application and removal of the primary voltage causes an AC voltage to appear on the secondary of the transformer. This voltage is rectified, filtered, and applied to a load across the output.  
           [0005]    When the switches are on, the voltage across the primary of the transformer leads to a build up of magnetizing current within the transformer. This build up of the magnetizing current, if left unchecked, would saturate the transformer, resulting in a decrease of the primary inductance of the transformer and the eventual failure of the circuit due to excessive current. To prevent this failure, the switches are cycled with a duty-cycle that is limited to 50% or less. During the OFF portion of the cycle, the inductive current built up during the ON part of the cycle is dissipated, by returning the energy to the input source. This cycling creates an equal and opposite voltage-time integral across the primary, while limiting the voltage seen by the switches to the input voltage. When the voltage-time integral for each cycle is zero, there is no net increase of the magnetizing current and hence no saturation of the transformer and no reduction of the inductance of the transformer primary, therefore stable operation is possible.  
           [0006]    For the forward converter, the 50% duty-cycle limit imposes undesirable limitations on the output inductor, voltage range, transformers, and downstream converters. The output inductor must be larger than in circuits having higher duty-cycles to achieve the same minimum load continuous current in the inductor. The transformer must be able to handle higher RMS current and peak currents for the lower duty-cycle. Therefore, there is impetus to increase the duty-cycle.  
           [0007]    One way to increase the duty-cycle has been to use a single-switch converter. In some versions, this variant allows duty-cycles of greater than 75%. However, the single-switch converter requires a way to absorb the magnetizing current thereby resetting the transformer. A snubber/clamp circuit has been used for this, but generates both heat and electrical noise in the form of EMI. A separate reset winding in the transformer has been used also, but increases the cost due to the special transformer and, because of leakage inductance, may not work as well as desired. Because the single-switch converter with a duty cycle greater than 50% places voltages that are more than twice the input voltage across the semiconductors for the off time of the duty-cycles, the semiconductors must be rated for a higher voltage than those used in the two-switch converter. A circuit configuration that increases the duty-cycle above 50% while permitting the use of conventional lower voltage components is needed.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    A forward converter implemented with three or more switches allows a transformer to reset more quickly permitting duty-cycles greater than 50% for converters implemented with non-high voltage components. The multi-switch converter uses a transformer in which the primary is implemented in segments with the ends of the segments accessible. The switches bridge the segments and the connections between the primary and input power allowing current flow through the primary when all switches are ON. When all switches are OFF, each segment is separate. Diodes connecting the segments ends and the power rails permit resetting current flow when the switches are OFF. When the switches are ON, the voltage across the entire primary is approximately the input voltage. When the switches are OFF and the diodes are allowing the magnetizing current to flow, each segment of the primary has approximately the input voltage across it. The equivalent of multiple times the input voltage is present across the primary while no more than the input voltage appears across any component. The increased effective primary reset voltage allows a faster reset time during a shortened OFF cycle.  
           [0009]    The normal topology of power transformers, implementing the primary winding as two segments with the secondary sandwiched between them, makes the three-switch converter an economical way to implement duty-cycles up to 67%. When higher duty-cycles are required, additional segments, switches and diodes are incorporated in the converter. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]    The invention will be understood from the following detailed description in conjunction with the drawings, of which:  
         [0011]    [0011]FIG. 1 is a circuit diagram of a prior art two-switch forward converter;  
         [0012]    [0012]FIG. 1A is a circuit diagram of a prior art single-ended forward converter;  
         [0013]    [0013]FIG. 2A shows waveforms in the circuit of FIG. 1 when operating with a 33% duty-cycle;  
         [0014]    [0014]FIG. 2B shows waveforms in the circuit of FIG. 1 when operating with a 67% duty-cycle;  
         [0015]    [0015]FIG. 3 is a circuit diagram of an embodiment of a three switch forward converter according to the invention;  
         [0016]    [0016]FIG. 4A shows waveforms in the circuit of FIG. 3 when operating with a 50% duty-cycle;  
         [0017]    [0017]FIG. 4B shows waveforms in the circuit of FIG. 3 when operating with a 67% duty-cycle; and  
         [0018]    [0018]FIG. 5 is a circuit diagram of an embodiment of a four switch forward converter according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The present invention relates generally to pulse-width modulated transformer-coupled power supplies, termed converters, and, more specifically, to converters capable of operating at a duty-cycle of greater than 50%.  
         [0020]    A two-switch forward converter, as is known in the industry, is illustrated in FIG. 1. The input voltage VIN is applied across a primary winding  100  of transformer T 1  when two switches, implemented by transistors M 2  and M 3 , are closed. This applied voltage induces a voltage V L  across the secondary  102  of transformer T 1 . Transistors M 2  and M 3  are controlled in tandem by pulse generators V 3  and V 4  that are generated by any number of known methods. The alternating application and removal of primary voltage VIN causes an AC voltage V L  to appear on secondary  102  of transformer T 1 . Voltage V L  is rectified by diode D 13  and low-pass filtered through filter L 2  and C 3  resulting in output voltage VOUT. A load (not shown) is applied across the output VOUT. VOUT is fed to a control circuit (not shown) that controls the duty-cycle of switches M 2  and M 3 .  
         [0021]    Switches M 2  and M 3  are controlled such that they are both ON or both OFF. When both M 2  and M 3  are ON, the voltage on primary  100  of transformer T 1  is equal to input voltage VIN, and this voltage induces voltage V L  on the secondary  102 . Simultaneously, input voltage VIN applied across primary  100  leads to a build up of magnetizing current within the transformer. The increasing magnetizing current, if not reset, would saturate transformer T 1 , resulting in a decrease of the primary inductance of the transformer T 1  and eventually failure of the circuit due to excessive current.  
         [0022]    To build functional converters, the voltage-time integral on the transformer primary over each cycle (transistors ON then OFF) must equal zero. The ON time of transistors M 2 , M 3  is usually limited to less than 50% of the cycle (a 50% duty-cycle). During the OFF portion of the cycle, the inductive current built up during the ON part of the cycle is returned to the input source by diodes D 6  and D 15 . The reversed current causes an equal and opposite voltage across primary  100  until the voltage-time integral is zero for the cycle. When the duty-cycle is less than 50%, the current will flow until the integral of voltage over time equals zero and then will cease flowing. Because no magnetizing current remains, the inductance of primary  100  stays constant. The timing diagram of FIGS. 2A and 2B illustrates operation of the prior art in FIG. 1. FIG. 2A shows the operation of the converter operated at a duty-cycle less than 50%, in this case a 33% duty-cycle. Transistors M 2  and M 3  are ON for an ON-interval  202  lasting for one-third of the cycle and they are OFF for an OFF-interval  204  lasting two-thirds of the cycle. The voltage VT across the primary of transformer T 1 , is equal to input voltage VIN during the ON-interval  202 . For approximately an equal time period  208 , VT is equal to the negative of VIN. VT then returns to approximately zero, except for some ringing of the signal, for the remainder of the cycle  210 . Hence, the voltage-time integral for VT over one cycle is zero. The primary combined current ITC, consisting of the magnetization current and the load current in the transformer primary, rises during the ON-interval  212  and decreases for an equal time  214 . Once ITC has returned to zero, it remains there. The current IT LOAD  shows the load portion of the combined current and IT MAG  shows the magnetizing current of the ITC combined waveform. The voltage V L  on the secondary after the diode is equal to VIN (for a 1:1 transformer) during the ON-interval  218  and is equal to zero the remainder of the cycle  220 , producing an average VOUT  222  equal to V L *duty-cycle.  
         [0023]    [0023]FIG. 2B shows the destructive buildup of primary current that occurs when the converter is operated at greater than a 50% duty-cycle, in this case at a 67% duty-cycle. Transistors M 2  and M 3  are ON during an ON-interval  230  lasting two-thirds of the cycle, and are OFF for an OFF-interval  232  lasting for one-third of the cycle. The voltage VT across the transformer is equal to input voltage VIN during the ON-interval  234 . VT is equal to the negative of VIN for the entire OFF-interval  236 , but this is not sufficient to drive the voltage-time integral to zero. Consequently, the total current ITC and especially the magnetizing current component I MAG  rises during each ON-interval  238 ,  238 A,  242 ,  242 A and does not return to its starting point during the OFF-intervals  240 ,  240   a,    244 ,  244   a.  Within a short period, the magnetizing current ratchets up out of control as the transformer saturates  246 ,  246   a  resulting in the voltage across the winding starting to collapse  247  because of the high currents and a voltage build up across the switches. This situation prevents this circuit from operating at greater than 50% duty-cycle.  
         [0024]    The 50% duty-cycle limitation affects many of the parameters of the circuit. The output inductor L 2  must be larger than in a circuit that can operate at a higher duty-cycle to achieve the same minimum load continuous current through the inductor. The input voltage range is limited because of the need to maintain a volt second balance across the transformer. If an increased duty-cycle could have been used, the RMS and peak currents in the transformer could be reduced allowing for lower losses and potentially smaller design.  
         [0025]    A single-switch forward converter, as shown in FIG. 1A, can operate with a duty-cycle greater than 50% when a reset path is provided. The reset path may involve a resistor R 3 /capacitor C 3 /diode D 3  clamping circuit (RCD) to absorb the magnetizing current and allow the reverse voltage generated by the magnetizing current to reset transformer T 1  by bringing the volt-second integral to zero. However, energy is lost in the RCD circuit due to heat and conducted and radiated noise. The EMI goes up and the efficiency goes down with this circuit. In addition, because the voltage across the transformer primary  100 A during the OFF-interval increases in inverse proportion to the duty-cycle, the switching transistor M 1  must be a high voltage device to handle this voltage, including any ringing and spikes. In addition, transformer T 1  must be able to handle these higher voltages. Therefore, the cost of the entire single-switch forward converter circuit increases due to these factors.  
         [0026]    A forward converter using three or more switches according to the invention allows use of lower cost, lower resistance parts while allowing the duty-cycle to increase beyond 50%. FIG. 3 illustrates an embodiment of a three-switch forward converter implemented with FETs as the switches, although both bi-polar transistors and IGBTs can be used also. The input voltage VIN is applied across a primary winding  300  of transformer T 1 ′ when three switches, implemented by transistors M 2 , M 3  and M 4  are closed. In many cases, primary winding  300  is usually split to get better coupling with the secondary. Half  302  of primary winding  300  is placed next to the center of transformer T 1 ′ and half  304  of primary winding  300  is placed on the outer layer of transformer T 1 ′. Secondary  102  is physically sandwiched between the two halves of the primary winding  300  thereby improving coupling. The three-switch converter accesses the terminals of each of the halves  302 ,  304  to allow the switches M 2 , M 3 , and M 4  to be connected in series with the segments of the primary. The applied voltage induces a voltage V L  across the secondary  102  of transformer T 1 ′. Transistors M 2 , M 3  and M 4  are controlled in tandem by pulse generators V 3 , V 4  and V 5  that are controlled by any number of known control methods as previously discussed. The alternating application and removal of primary voltage VIN causes an AC voltage V L  to appear on secondary  102  of transformer T 1 ′. Voltage V L  is rectified by diode D 13  and low-pass filtered through filter L 2  and C 3  resulting in output voltage VOUT. A load (not shown) is applied across the output VOUT. In many cases, VOUT is fed to a control circuit (not shown) that controls the duty-cycle of switches M 2 , M 3  and M 4 .  
         [0027]    Switches M 2 , M 3  and M 4  are controlled such that they are all ON or all OFF. When all switches are ON, the voltage on primary  300  of transformer T 1 ′ is equal to input voltage VIN, and this voltage induces voltage V L  on the secondary  102 . Simultaneously, input voltage VIN applied across primary  300  causes a build up of magnetizing current within transformer T 1 ′. The increasing magnetizing current, if unchanged, would saturate transformer T 1 , resulting in a decrease of the primary inductance of the transformer T 1  and eventually failure of the circuit due to excessive current. The OFF-interval of the duty-cycle must prevent this failure.  
         [0028]    To build functional converters, the voltage-time integral over each cycle (transistors ON then OFF) must equal zero. During the OFF portion of the cycle, the segments  302  and  304  of the primary  300  are separated with no current flowing through the switches M 2 , M 3  and M 4 . The diodes D 21  &amp; D 6 , D 15  &amp; D 19  connecting the segment terminals to the power rails allow the inductive current to continue to flow during the OFF-interval, causing a reversed voltage across each segment. Since the inductive current portion of the primary current flowing in the segments has the same magnitude and polarity as when the primary segments were connected, the voltage across each separate segment equals the voltage that had been across the primary, VIN.  
         [0029]    When switches M 2 , M 3  and M 4  are all turned ON, no current flows in diodes D 6 , D 15 , D 19  and D 21 . When the switches are turned OFF, the magnetizing current still flows, resulting in the reversal of the voltage across the windings. This reverse voltage increases until the diodes conduct and the magnetizing current returns to the power source from the separate segments  302 ,  304  of the primary winding  300 . In the case of winding  302 , the current flows from ground, through D 15  into winding  302 , and out through D 19  to the positive side of VIN. In the case of winding  304 , the current flows from ground, through D 21  into winding  304 , out through D 6  to the positive side of VIN. Since the magnitude of the magnetizing current remains the same during the transition from ON to OFF-interval, the voltage across each winding  302 ,  304  equals VIN during the OFF-interval and the effective voltage across T 1 ′ is  2 VIN during the OFF-interval. Each winding however experiences only the input voltage VIN across it, because switch M 4  isolates the windings  302 ,  304 . With an effective doubling of the reverse voltage, the negative portion of the voltage-time integral is satisfied in half the time of the positive portion.  
         [0030]    The timing diagrams of FIGS. 4A and 4B illustrate the operation of this embodiment. FIG. 4A shows the operation of the converter operated at a duty-cycle of 50%. Switches M 2 , M 3  and M 4  are ON for an ON-interval  402  that is equal to the time when they are OFF, OFF-interval  404 . The voltage VT′ across the transformer T 1 ′ is equal to input voltage VIN during the ON-interval  406 . During the OFF-interval  404 , made up of time  408  and  410  each winding  302 ,  304  has a voltage equal to −VIN across it for the time  408 , making the effective voltage across transformer Tl′ equal to −2VIN  408 . During the time  410 , the magnetizing energy has mainly dissipated and the parasitic elements cause the windings to ring. The integral of VT′ over the ON and OFF-intervals ( 402  and  404 ) totals zero halfway through the OFF time, so VT′ settles to zero volts after some oscillation  410 .  
         [0031]    The voltage across the secondary of the transformer V L  when the switches are ON is VIN (for a 1:1 transformer) and, when the switches are OFF, is zero. For a 50% duty-cycle, this averages to ½VIN. V L  is smoothed out by the filter formed by L 2  and C 3 , so that VOUT also equals ½ VIN.  
         [0032]    In FIG. 4B, a 67% duty-cycle is represented. Switches M 2 , M 3  and M 4  are ON for an ON-interval  422  that is two-thirds of the cycle and are OFF for an OFF-interval  424  that is one-third of the cycle. The voltage VT′ across the transformer T 1 ′ during the ON-interval  422  equals VIN  426 . During the OFF-interval  424 , each winding  302 ,  304  has a voltage equal to −VIN across it, making the effective voltage across transformer T 1 ′ equal to −2VIN  428 . The voltage across the transformer secondary V L  when the switches are ON is VIN, which is 1.5VOUT. When the switches are OFF, V L  is zero. Over the cycle therefore, V L  averages to VOUT because the duty-cycle is 67%. The output voltage VOUT during both parts of the duty-cycle stays constant after filtering. If VIN decreases, the control system (not shown) would increase the duty cycle to try to maintain VOUT constant. The integral of VT′ over the ON and OFF-intervals ( 422  and  424 ) totals zero exactly as the cycle ends. If a higher duty-cycle were attempted, the current would build up, as described above, destroying the system.  
         [0033]    The three-switch forward converter allows the use of lower voltage transistors than the one-switch design, because the voltage experienced by any transistor is limited to the input voltage. In addition, since the magnetizing energy is returned to the source, efficiency is improved over the energy dissipating one-switch design. Further, the generated electro-magnetic-interference (EMI) is comparable to that of the two-switch converter.  
         [0034]    The principles behind the duty-cycle improvement from a two-switch converter to a three-switch converter can be extended if a higher duty-cycle is needed, but custom primary windings will be needed. A four-switch forward converter operates at up to a 75% duty-cycle, and requires a primary winding that has been explicitly formed as three segments as shown in FIG. 5. In FIG. 5 the primary of transformer T″ includes three segments  602 ,  604  and  606  with corresponding terminals. Switches M 4  and M 5  are each interposed between adjacent segments of the primary winding as shown. Diodes (D 15 , D 26 , D 21 ) are connected between the negative rail and the high end terminal of each segment ( 1   a,    3   a,  and S a ), and diodes (D 19 , D 27 , D 6 ) are connected between the low end terminal of each segment ( 2   a,    4   a,    6   a ) and the positive rail. The four-switch forward converter has an effective VT″ during the OFF time equal to three times VIN, yielding a maximum duty-cycle of 75%. It will be appreciated that a maximum duty-cycle of 80% can be obtained for a five-switch converter, 83% for a six-switch converter, etc.  
         [0035]    Having described preferred embodiments of the invention it will be apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.