Abstract:
A power supply ( 300 ) includes a rectification means ( 303 ) for providing a voltage from an AC mains input ( 301 ). An inverter ( 307 ) is used for supplying a switched AC voltage at high frequency from the rectified voltage to a transformer ( 311 ) for modifying the amplitude and/or providing galvanic isolation of the switched AC voltage. Output rectification ( 313 ) is used to convert the switched AC voltage at the secondary of the transformer back to a rectified voltage. An inductor ( 309 ) is used in series with the primary of the transformer ( 311 ) for reducing the peak and ripple current in both the primary and secondary of the transformer while minimizing or eliminating the need for an inductive component in the output filter of the supply.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to power supplies and more particularly to switching power supplies for providing substantially high output voltages. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many different types of power supplies have been developed for use in applications requiring a high output voltage. These devices often supply either high direct current (DC) or alternating current (AC) voltages to one or more output loads. One application for this type of high voltage supply is for use with a vacuum tube oscillator. This type of oscillator is used for providing substantially high power radio frequency (RF) voltages at its output. 
         [0003]    Many factors commonly affect the design of these types of power supplies. These factors include the amount of power needed from the supply, the duration and stability of the voltage and current under various load conditions, and the acceptable range of input voltages for supply operation. Moreover, the load placed on the input power source of the supply and the efficiency at which the supply can convert power are also factors in its design and operation. 
         [0004]    Power supplies for electronic devices can be broadly divided into either linear or switching power supplies. A linear supply is usually a relatively simple design but becomes increasingly bulky and heavy for high voltage and high current equipment. This is due to the use of relatively large mains-frequency transformers operating at 50-60 Hz. The overall size of a linear supply can be very large and expensive to manufacture depending on its application. In contrast, a “switching” or switched-mode power supply that has the same voltage and current ratings as a linear supply will be smaller in size but will be more complex in construction. This type of switched-mode supply works on a different principle of operation so that either a DC input voltage or a rectified AC input voltage can be used as a power source. 
         [0005]    In operation, an input or supply voltage is switched on and off at a very high speed (typically 10 kHz to 1 MHz) by electronic switching circuitry, called an inverter. The high-frequency inverter then drives a smaller, lighter, and less expensive transformer to step-up or step-down the switched voltage to a specific amplitude. This amplitude is typically controlled by varying the “on” time, or duty cycle of the inverter. The high frequency output of the transformer is rectified and filtered to remove the switching frequency components and average the output waveform. In addition to transformer size, another advantage to this design is that much smaller filter elements, such as inductors and capacitors, are used when filtering the high frequency signal components. This is in contrast to the larger filter elements used in the design of a linear power supply operating at a 50-60 Hz mains frequency. 
         [0006]      FIG. 1  illustrates a prior art block diagram of a linear type supply known as a phase fired controller mains supply  100 . The supply  100  includes a mains input  101  that feeds a phase fired control  103 . The phase fired control  103  controls the conduction angle of the mains frequency that supplies a mains frequency transformer  105  used to step up the voltage supplied at its primary winding. Optionally, the mains frequency transformer  105  can include multiple taps for allowing operation from various nominal mains input voltages. The secondary winding or output of the mains frequency transformer  105  feeds an output rectifier  107 . The output rectifier  107  is used for providing a phase chopped full wave rectified AC waveform to a load  109 . The voltage at the load  109  is monitored by a phase controller  111  so that the phase angle of the phase fired controller  103  can regulate output voltage at the load  109 . 
         [0007]    In contrast to that shown in  FIG. 1 , a switched supply topology uses differing methods to control voltage at the load. One commonly used topology is referred to as a forward converter, which uses the turns ratio of the transformer to increase or decrease the output voltage. This technique has the advantage of providing galvanic isolation for the load. In the forward converter, an input voltage to the transformer is switched using a variable duty cycle. This technique is also called pulse width modulation (PWM). The transformer provides a PWM voltage at its secondary that is a scaled version of the PWM primary voltage. The PWM secondary voltage is filtered to provide an output voltage that has the average value of the PWM secondary voltage. The output voltage is subsequently controlled by varying the PWM duty cycle. 
         [0008]    Another switching supply topology is known as a flyback converter. In the flyback converter, the input voltage to the transformer is switched with a variable duty cycle. While applying a voltage to the transformer primary, the transformer stores the applied energy as magnetic flux rather than delivering it to the load. When the primary voltage is switched off, the energy stored in the transformer is delivered to the transformer secondary winding and a load at its output. This supply topology includes a capacitor at its output for energy storage, delivering power to the load during the “on” time of the transformer primary. Thus, the flyback converter technique uses the transformer as an energy storage device while also providing galvanic isolation between the transformer primary and secondary windings. 
         [0009]    An issue associated with switching power supplies using PWM for varying the output voltage involves parasitic oscillation or “ringing.” PWM power supplies can be plagued with ringing waveforms that can degrade performance, impact electromagnetic interference (EMI) measurements, and cause transformer failure in high power applications. Ideally, the forward converter should generate sawtooth shaped current waveforms in the output filter inductor. This provides a scaled version of the waveform shape at the transformer primary. However, the basic forward converter often includes undesirable parasitic oscillations also known as “ringing” due to parasitic inductances and capacitances in both the transformer and output filter inductor.  FIG. 2  shows a graphical representation of oscilloscope waveforms of the primary current  201  and voltage  203  appearing at the primary winding of a switching power supply transformer. The graph shows an undesirable amount of oscillation or “ringing” at the primary. 
         [0010]    In use, there are numerous parasitic elements that cause ringing in a power supply circuit. These factors include, but are not limited to, printed circuit board trace inductance, transformer leakage inductance, transformer magnetizing inductance, transformer primary capacitance, transformer primary-to-secondary capacitance and transformer secondary capacitance. Additional factors include, output filter inductor capacitance, output filter capacitor inductance, switching transistor output capacitance and diode junction capacitance. In many cases, these elements can be voltage and frequency dependent such as in semiconductor junction capacitances and transformer leakage inductance. Ringing waveforms are typically suppressed using snubbers and clamp circuits for suppressing a dominant parasitic; however, these techniques are not always effective for high voltage and high power applications. 
         [0011]    Thus, it is important to protect the power supply circuit in differing modes of operation under varying operating conditions. Since transient events can excite circuit resonances, circuit failure often can occur during such transients due to the additional stress placed on power supply components. In the case illustrated in  FIG. 2 , the power supply transfer function is not monotonic, which results in an unstable control loop and an undesirable power supply design. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0012]    The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
           [0013]      FIG. 1  is a prior art block diagram of a standard phase fired control type power supply. 
           [0014]      FIG. 2  is a graph illustrating the primary current and voltage of a transformer which shows the characteristics of oscillation or ringing at the transformer primary. 
           [0015]      FIG. 3  is a schematic diagram illustrating a forward-flyback power supply topology used in accordance with an embodiment of the invention. 
           [0016]      FIG. 4  is a schematic diagram of a full bridge inverter used in connection with the switching power supply in accordance with an embodiment of the invention. 
           [0017]      FIG. 5  is a graph illustrating the primary current and voltage of the transformer using a forward-flyback topology as shown in  FIG. 3 . 
           [0018]      FIG. 6  is a schematic diagram of an RF oscillator used in connection with the switching power supply shown in  FIG. 3 . 
       
    
    
       [0019]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to a forward-flyback power supply. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
         [0021]    In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
         [0022]    It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of a forward-flyback power supply as described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to supply power to an RF oscillator in an induction furnace. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
         [0023]      FIG. 3  is a schematic diagram illustrating a forward-flyback power supply  300  using an inductor  309  in series with a primary winding of a transformer  311 . The power supply  300  is used to power an RF oscillator in accordance with an embodiment of the invention. The power supply  300  includes an AC mains voltage input  301  that typically has an input line voltage between 85-265 VAC at 47-63 Hz. An input filter  302  can be used between the AC mains  301  and an input rectifier  303  for reducing harmonic distortion of the AC mains voltage or other voltage source. 
         [0024]    The input rectifier  303  includes one or more switching devices used to provide a rectified voltage to an inverter  307 . A capacitor  305  is used across an input of the inverter  307  for sustaining peak currents. The capacitor  305  also acts as a “snubber” for inverter current transients and prevents the switching currents of the inverter  307  from affecting the AC mains voltage input  301 . The inverter  307  uses a switching controller (not shown) for switching the input voltage at a substantially high frequency to drive an input circuit comprised of the series combination of the inductor  309  and the primary winding of the transformer  311 . The voltage at the secondary winding of the transformer  311  feeds an output rectifier  313 . An output capacitor  315  is used to smooth the voltage at the output  317  for supplying one or more load(s) (not shown). Thus, the inductor  309  is connected in series with the primary winding the transformer  311  for filtering an output voltage applied to a load coupled with the secondary winding of transformer  311 . For example, using a 175-275 VAC input and a 4 kVAC/0.5 A output at a 25 kHz switching frequency, the inductor  309  might have an optimized value in a range between 18-4701 when used with a transformer with a turns ratio between 1:12 and 1:10. Although a single transformer  311  is shown, it should be evident to those skilled in the art that alternative embodiments using a plurality of transformers having one or more primary and secondary windings may also be used. 
         [0025]      FIG. 4  is a schematic diagram of a switching inverter  400  comprised of a plurality of switching devices used in combination to form parallel connected half bridge networks. The inverter  400  uses two parallel connected half bridges. The first half bridge is comprised of switching devices  401 ,  403 ,  409 ,  411  and the second half bridge is comprised of switching devices  405 ,  407 ,  413 ,  415 . In this diagram, the switching devices are represented as insulated gate bipolar transistors (IGBTs)  401 ,  403 ,  405 ,  407  and diodes  409 ,  411 ,  413 ,  415 . Since IGBTs can only pass current from collector to emitter, anti-parallel diodes  409 ,  411 ,  413 ,  415  are included to allow current to flow in the opposite direction. 
         [0026]    The first half bridge is a switching network formed using first transistor pair  401 ,  403  connected in series between the positive (+) and negative (−) rails of a respective input bus  402 ,  404  with diodes  409 ,  411  connected in anti-parallel across each transistor. The series connection is formed from the emitter of transistor  401  to the collector of transistor  403  and the anti-parallel connections are formed with the diode  409  anode and cathode tied to transistor  401  emitter and collector, respectively, and diode  411  anode and cathode tied to transistor  403  emitter and collector, respectively. The second half bridge is identically connected and placed in parallel with the first half bridge in a manner such that the collectors of transistors  401 ,  405  and cathodes of diodes  409 ,  413  are connected by the positive (+) bus and the emitters of transistors  403 ,  407  and anodes of diodes  411 ,  415  are connected by the negative (−) bus. These positive and negative bus connections (+,−) provide the input voltage connections to the inverter  400 . The center points of each half bridge, that is the emitter-collector connection between first transistor pair  401 ,  403  and anode-cathode connection between first diode pair  409 ,  411  (U) and the emitter-collector connection between second transistor pair  405 ,  407  and anode-cathode connection between second diode pair  413 ,  415  (V), are used for providing the output voltage connections  406 ,  408  of the inverter. 
         [0027]    In use, the inverter  400  is operated as a phase controlled full bridge that includes a first half bridge and a second half bridge, as previously described. Unlike a conventional pulse width modulated inverter, each half bridge is continuously operated at a substantially fifty percent (50%) duty cycle. In doing so, the full bridge provides four switching states dependent on a switching voltage applied to the switching devices  401 ,  403 ,  405 ,  407 . 
         [0028]    In a first state, switching devices  401 ,  407  are switched to an “on” state and the inverter  400  is “on” providing a positive output voltage at output  406 ,  408 . In a second state, switching devices  401 ,  405  are in an “on” state and the inverter is “off” with a shorted output. In a third state, switching devices  403 ,  405  are in an “on” state and the inverter is “on” with a negative output voltage at output  406 ,  408 . Finally, in a fourth state, switching devices  403 ,  407  are in an “on” state and the inverter is “off” with a shorted output. 
         [0029]    When in operation, the inverter  400  delivers a switched output voltage to the output  406 ,  408 . The output voltage is based upon the voltage input at the bus  402 ,  404  and is controlled by varying the phase between each half of the full bridge inverter  400 . When each half of the bridge is switched in-phase, either transistors  401 ,  405  or transistors  403 ,  407  will be “on” at the same time, providing no output power. When each half of the bridge is switched out of phase, either transistors  401 ,  407  or transistors  403 ,  405  will be “on” at the same time. This provides full power at the output  406 ,  408 . The output power can be varied continuously between zero and full power by changing the phase delay between each half of the bridge. Although a single inverter output  406 ,  408  is shown, it should be evident to those skilled in the art that alternative embodiments using a plurality of half bridges having one or more inverter outputs may also be used. 
         [0030]      FIG. 5  illustrates various waveforms that occur at the inverter  307  output shown in  FIG. 3 . These waveforms include the output current  501 , the primary transformer voltage  503  (i.e., the voltage across the transformer  311  primary) and the inductor voltage  505  (i.e., the voltage across the inductor  309 ). These waveforms illustrate a transformer primary voltage and current that is free of oscillation and ringing. 
         [0031]    The forward-flyback topology, as described herein, applies an input voltage to the primary winding of the transformer  311  that is in series with the inductor  309 . The inverter  307  is switched as a phase controlled full bridge for providing duty cycle control. This topology is similar to a forward converter since during the “on” time, the transformer provides an output voltage that is a “scaled” version of its primary voltage (the inverter output voltage less the voltage on the inductor  309 ). The topology also provides characteristics of a flyback converter since during the “on” time, the inductor  309  stores a portion of the applied energy as magnetic flux. During the “off” time of the inverter, this stored energy is delivered to the output  317  through the transformer  311 . 
         [0032]    As described herein, the output voltage at the transformer secondary is controlled by varying the duty cycle of the inverter. Unlike supplies used in the prior art, such as U.S. Pat. No. 5,349,514 to Ushiki et al. entitled “Reduced-Resonant-Current Zero-Voltage-Switched Forward Converter Using Saturable Inductor,” which is incorporated herein by reference, the present invention does not require the use of an inductive component in an output filter network. Unlike the supply shown by Ushiki et al., the inductance provided by the inductor  309  is not used to “resonate” the switching waveforms from the switching network. Instead, it is used to store energy. 
         [0033]    The invention provides a substantially one hundred percent (100%) utilization of the transformer  311  over a wide operating voltage range, improving efficiency and reducing primary and secondary peak currents and ripple currents. Moreover, this operation simplifies filtering requirements and the value of any output filter inductor used in an output filter network can be greatly reduced or eliminated. Thus, in one embodiment, the inductor  309  acts as a filter element of a forward converter during its “on” time while acting as an energy storage element of a flyback converter during the “off” time. Neither a substantially high value output filter capacitance nor a filter inductor is required to provide a substantially low ripple output voltage. Finally, another advantage is that the load presented by the inverter  307  to an AC mains voltage input  301  will have a near unity power factor with low harmonic distortion. 
         [0034]      FIG. 6  is a schematic diagram of an RF oscillator that may be used in connection with the switching power supply shown in  FIG. 3 . The RF oscillator  600  includes a rectified AC input  601  supplied by the power supply shown in  FIG. 3 . An input filter consisting of a capacitor  603  and an inductor  605  allow the low frequency modulated DC voltage (47-63 Hz) to power the RF oscillator  600  while preventing any RF energy from returning to the power supply. The vacuum tube  607  includes a plate or anode that is connected to the power supply through the inductor  605 . The plate is connected by the capacitor  615  to a resonant network consisting of an induction coil  621  and the capacitors  617  and  619 . Although the vacuum tube  607  is depicted as a triode, other types of high power vacuum tube types can be used for supplying a substantially high amount of RF energy at a predetermined frequency. An input  609  depicts a cathode voltage input while an input  611  is a filament supply voltage input. A grid capacitor  613  works in combination with the resonant network for providing feedback to the grid of the vacuum tube  607  which induces an oscillation at a predetermined frequency. Thereafter, a substantially high RF voltage and current is supplied to the induction coil  621  in an analytical induction furnace. The induction furnace is used for combusting various materials to create vaporized gases for subsequent analysis. 
         [0035]    Thus, an embodiment of the invention is a switching power supply for use with an analytical induction furnace for providing power to a transformer coupled load containing large parasitic circuit elements between the primary and secondary load. The power supply includes an inverter operating at a high switching frequency and a transformer. An inductor is connected in series with a primary winding of the transformer for providing energy storage and filtering of the transformer secondary load circuit at the inverter switching frequency. 
         [0036]    In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.