Abstract:
A startup circuit for two switch forward converters with controller power supply connected at output side. With this startup circuit, all the controlling supporting circuitries are connected at the converter output low voltage side. This provides an opportunity for two switch forward converters to be easily designed with high input DC voltage without violating safety regulations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present description generally relates to power conversion.  
           [0003]    2. Description of the Related Art  
           [0004]    Distributed power systems (“DPS”) are employed in a large number of power generation applications. In particular, the use of small to medium size DPS in a variety of applications has risen in recent years. A DPS requires a low-voltage power supply (“LVPS”), typically in the range of 12 VDC to 24 VDC, for supplying power to a controller, gate drive, display control unit, customer interface unit, and other supporting units. The power rating of an LVPS typically ranges from a few hundred watts to one kilowatt. The input of the LVPS is usually from the output of a DPS, which is typically in the range from 400-600 VRMS line-to-line.  
           [0005]    The input voltage range of commercially available AC/DC converters is from 85 VRMS to 265 VRMS. Converters with input voltage range beyond 85-265 volts RMS, if even available, are very costly. An AC/DC converter of a few hundred watts having a 480 VRMS input costs between approximately $500 to $800. To make use of commercially available AC/DC converters with input voltage range of 85-265 VRMS, a step-down power transformer is required. The introduction of additional transformer adds extra costs, weight, size and many other negative factors to a DPS. Therefore, there is a need to design an AC/DC converter with wide input voltage range to cover all possible output voltages of a DPS.  
           [0006]    The main concern in designing such an AC/DC converter is the high input DC voltage of the LVPS. For example, when the nominal output voltage of a DPS is 600 VRMS line-to-line, the line-to-neutral voltage is 347 VRMS. Considering that the output voltage of a DPS has a tolerance of −12% to +6%, the maximum line-to-neutral voltage will be 367 VRMS. After the input rectifier, the input DC voltage will be 519 VDC. The maximum voltage that the switching device in an AC/DC converter, such as a one switch flyback or forward converter, may be subjected to is up to 2.5 times the DC input voltage, i.e., the required voltage rating of a switching device may be as high as 1300 VDC. Most power MOSFETs that are commercially available are rated at 1200 VDC. With the consideration of the power rating and the input DC voltage of an LVPS, a two switch forward converter is desirable for this application. The maximum voltage that the switching device in a two switch forward converter may be subjected to is the same as the maximum input DC voltage. Thus, for example, the converter may employ 600 V power MOSFETs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    In one aspect, an electrical power converter includes a high voltage node, a low voltage node, a high frequency power transformer having a high voltage side and a low voltage side, the high frequency power transformer coupled between the high voltage node and the low voltage node, a controller operatively coupled to provide control signals to the high frequency power transformer, a controller power supply electrically coupled between the controller and the low voltage side of the high frequency power transformer to provide power to the controller from the low voltage side of the high frequency power transformer, and a startup circuit electrically coupled between the high voltage node and the high voltage side of the high frequency power transformer to provide control signals to the high frequency power transformer in response to power being applied to the high voltage node.  
           [0008]    In another aspect, a circuit for an electrical power converter having a high voltage input and a low voltage output includes a high voltage bus having at least a first and a second high voltage rail, a low voltage bus having at least a first and a second low voltage rail, a transformer having a primary side and a secondary side, the primary side electrically coupled to respective ones of the first and second high voltage rails of the high voltage bus, the secondary side of the transformer electrically coupled to respective ones of the low voltage rails of the low voltage bus, the primary side having a number of power transistors, a startup circuit coupled to provide control signals in a first frequency range to the power transistors of the transformer in response to a voltage across the high voltage rails of the high voltage bus, a controller coupled to provide control signals in a second frequency range to the power transistors of the transformer, a controller power supply electrically coupled between the controller and the low voltage bus to provide a low voltage power to the controller during operation of the transformer, and a disable circuit electrically coupled to disable the control signals at the first frequency range while allowing the control signals at the second frequency range.  
           [0009]    In another aspect, a converter having a high voltage node and a low voltage node includes transformer means for transforming a high voltage to a low voltage, startup circuit means for providing a first set of control signals at a first frequency to the transformer means in response to a high voltage at a high voltage node, and control means electrically coupled to a low voltage side of the transformer means for providing a second set of control signals at a second frequency, different from the first frequency, to the transformer means in response to a low voltage produced by the transformer means.  
           [0010]    In a further aspect, a method of operating a converter having a startup circuit, a controller, and a high frequency power transformer having a high voltage side and a low voltage side includes providing a first set of control signals at a first frequency from the start up circuit to the high frequency power transformer in response to a high voltage supplied to the start up circuit, and providing a second set of control signals at a second frequency, different from the first frequency, from the control circuit to the high frequency power transformer in response to a low voltage supplied to the control circuit from the low voltage side of a high frequency power transformer. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0011]    In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative position of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and/or positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have solely been selected for ease of recognition in the drawings.  
         [0012]    [0012]FIG. 1 is an electrical schematic diagram of one illustrated embodiment of a converter having a transformer, a controller electrically powered from a low voltage side of the transformer, and a starting circuit.  
         [0013]    [0013]FIG. 2 is an electrical schematic diagram of one illustrated embodiment of a startup circuit for use in the starting circuit.  
         [0014]    [0014]FIG. 3 is a graph illustrating diac oscillation voltage and the resulting startup switching pulses during startup of the converter.  
         [0015]    [0015]FIG. 4 is a graph illustrating diac oscillation voltage and switching pulse waveforms resulting from modulation between the startup switching pulses and normal switching pulses, where the startup switching pulses have not been disabled.  
         [0016]    [0016]FIG. 5 is an electrical schematic illustrating a disable circuit for use in the starting circuit.  
         [0017]    [0017]FIG. 6 is a graph illustrating diac oscillation voltage and switching pulses waveforms during a transition from startup to normal switching, where the startup switching pulses are being disabled.  
         [0018]    [0018]FIG. 7 is a graph illustrating normal switching pulse waveform after startup is complete.  
         [0019]    [0019]FIG. 8 shows a flow diagram of one illustrated exemplary method of operating the converter. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with power converters and electrifiers have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.  
         [0021]    Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
         [0022]    The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention.  
         [0023]    In typical power converters, a controller power supply, a controller, and many other supporting circuitries are powered from the high voltage input side of the converter. Since the input DC voltage can be as high as 500 VDC, it is more desirable to connect the controller power supply and other supporting circuitries to the low voltage output side of the converter.  
         [0024]    While connecting the controller power supply to the low voltage output side avoids operating the control circuitries at high voltage, such a design introduces a starting problem. Normally, where the controller power supply is connected at the high voltage input side of the converter, the converter starts converting power immediately after the application of input power. If however, the controller power supply is connected the low voltage output side, the converter is unable to start itself upon the application of power at the input. This description describes a self startup circuit for a converter, such as a two switch forward converter, having a controller power supply connected at the low voltage output side of the converter.  
         [0025]    Converter  
         [0026]    [0026]FIG. 1 shows an exemplary power converter in the form of a two switch forward converter  10 . The converter  10  includes a high voltage node or input  12  couplable to a high voltage power source (not shown), and a low voltage node or output  14  couplable to a low voltage load (not shown). As used herein and in the claims, the terms high voltage and low voltage are used in a relative sense, and are not intended to be associated with any absolute voltage values. Also as used herein and in the claims, the term couplable means selectively coupled or permanently coupled. The high voltage power source can take any of a variety of forms, for example but not limited to, AC power sources such as a generator or turbine, or DC power sources such as a fuel cell stack, battery or ultra-capacitor. The load can also take a variety of forms, for example but not limited to, an electrical motor.  
         [0027]    The converter  10  includes a high frequency power transformer  16  coupled between the high voltage input  12  and the low voltage output  14 , via a high voltage bus  18  and a low voltage bus  20  respectively. The high frequency power transformer  16  has a high voltage or primary side  22  including first and second poles  24   a ,  24   b  electrically coupled to the high voltage input  12 , and a low voltage or secondary side  25  including first and second poles  26   a ,  26   b  electrically coupled to the low voltage output  14 . The high frequency power transformer  16  includes a pair of selectively controllable switches  28   a ,  28   b , for example, MOSFET or IGBT transistors.  
         [0028]    The converter  10  also includes a controller  30  for supplying control signals to the gates of the switches  28   a ,  28   b  of the high frequency power transformer  16 . The controller  30  supplies control signals to the switches  28   a ,  28   b  via a gate drive pulse transformer  32 . A pair of input diodes  34   a ,  34   b  are electrically coupled in series on respective ones of the rails of the high voltage bus  18  between the high voltage input  12  and the high voltage side  22  of the high frequency power transformer  16 .  
         [0029]    The converter  10  includes a controller power supply  36  for supplying low voltage power to the controller  30 . The controller power supply  36  is electrically coupled to receive power via the low voltage bus  20  from the low voltage side  25  of the high frequency power transformer  16 .  
         [0030]    The converter  10  may also include an input capacitor  38  electrically coupled across the high voltage rails of the high voltage bus  18  and the high voltage side  22  of the high frequency power transformer  16 .  
         [0031]    In the illustrated embodiment, the converter  10  includes a rectifier  40  for rectifying an AC current received at the high voltage input  12  from the power source. The rectifier  40  may be omitted where the converter  10  takes the form of a DC/DC converter and the input  12  to the converter  10  is a DC supply.  
         [0032]    The converter  10  may also include a coil or choke  42  in the low voltage bus  20 , between the low voltage output  14  and the low voltage side  25  of the high frequency power transformer  16 . The converter  10  may further include a first output diode  44   a  electrically coupled in series between the choke  42  and one of the poles  26   a  of the low voltage side  25  of the high frequency power transformer  16 . The converter  10  may also further include a second output diode  44   b  electrically coupled across the low voltage bus  20  between the choke  42  and the low voltage side  25  of the high frequency power transformer  16 . The converter  10  may even further include an output capacitor  46  electrically coupled across the low voltage bus  20  between the choke  42  and the low voltage output  14 .  
         [0033]    Starting Circuit  
         [0034]    The converter  10  includes a starting circuit  48  to start operation of the high frequency power transformer  16  when power is applied. Ideally, the starting circuit  48  generates gate drive pulses immediately after the application of power to the high voltage input  12 . The startup switching pulses are supplied to the switches  28   a ,  28   b  to cause the converter  10  to begin converting power from the high voltage input  12  to the low voltage output  14 . The length and time during which the pulses are generated should be such that the voltage level of the controller power supply  36  is sufficiently high to activate the controller  30 . Once active, the controller  30  generates normal switching pulses, and the starting circuit  48  should be disabled or the starting pulses suppressed.  
         [0035]    To implement the self starting functionality, the starting circuit  48  includes a startup circuit  50  and a disable circuit  52 . The startup circuit  50  senses the input DC voltage and generates startup switching pulses (i.e., gate drive pulses at a low frequency, e.g., 1 kHz). The startup switching pulses operate the switches  28   a ,  28   b  of the high frequency power transformer  16  to convert power from high voltage input  12  to the low voltage output  14 . Once the voltage of the controller power supply  36  reaches the desired operating level, the controller  30  generates normal switching pulses (i.e., gate drive pulses at a high frequency, e.g., 100 kHz). As used herein and in the claims, high and low frequency are used in a relative sense and are not intended to be associated with any absolute frequency values.  
         [0036]    The disable circuit  52  may incorporate a high pass filter at its front end. The high pass filter monitors the gate drive pulses, ignoring the low frequency startup switching pulses, and generating a trigger signal to disable the startup circuit  50  when the disable circuit senses the high frequency normal switching pulses. A detailed description of the startup circuit  50  and disable circuit  52  follows.  
         [0037]    [0037]FIG. 2 shows one illustrated example of the startup circuit  50 , including a diac DB 3 , an input resistor R 1 , charging-discharging capacitance or capacitor C 1 , and output resistor R 2 . As used herein and in the claims, the term capacitor refers to a discrete capacitor and/or an inherent or parasitic capacitance.  
         [0038]    The diac DB 3  is in a blocking state when the voltage across the terminals of the diac DB 3  is below a defined level. For example, the diac DB 3  may have a trigger voltage of approximately 32V. Thus, the diac DB 3  is in a blocking state until the terminal voltage of the diac DB 3  reaches 32 volts, at which point the diac DB 3  enters a conducting state, like an ordinary diode.  
         [0039]    When the input DC voltage is applied at the high voltage input  12 , the charging-discharging capacitor C 1  starts charging up from VDC through the input resistor R 1 . The voltage across the diac DB 3  is the same as the voltage across the charging-discharging capacitor C 1 . Thus, the diac DB 3  is in a blocking state. Once the voltage across the capacitor C 1  reaches 32 V, the diac DB 3  starts conducting with very small forward voltage drop. The energy stored in the charging-discharging capacitor C 1  is discharged through the output resistor R 2 . The voltage across the output resistor R 2  serves as the gate drive pulse. The required length of this pulse is determined by the duty ratio of the switch  28   a ,  28   b , for example, power MOSFET or IGBT transistors. The length of the pulse is controlled by the energy stored in the charging-discharging capacitor C 1 . After the energy in the charging-discharging capacitor C 1  is discharged through the output resistor R 2 , a new cycle starts with the charging-discharging capacitor C 1  charging again. The frequency of this charging-discharging cycle is determined by the time constant of the RC circuit formed by the input resistor R 1  and charging-discharging capacitor C 1 , and by the input voltage of VDC. A suitable range may, for example, be from 1 kHz to 5 kHz for the illustrated embodiment.  
         [0040]    Each charging-discharging cycle pumps a small amount of energy from the high voltage side  22  to the low voltage side  25  of the high frequency power transformer  16 , and the energy is stored in the capacitor  46  across the controller power supply  36 .  
         [0041]    [0041]FIG. 3 shows a waveform  56   a  of the diac oscillation voltage during the startup mode, and a waveform  58   a  of the resulting startup switching pulses.  
         [0042]    The charging-discharging cycle is repeated until the voltage of the controller power supply  36  reaches a minimum level that the controller  30  needs for normal operation. When the controller  30  has the required power, the controller  30  begins generating normal switching pulses. These normal switching pulses are modulated with the existing startup switching pulses. Modulated pulses control the switching of the switches  28   a ,  28   b.    
         [0043]    [0043]FIG. 4 shows a waveform  56   b  of the diac oscillation voltage, and a waveform  58   b  of the switching pulses resulting from the modulation between the startup switching pulses and the normal switching pulses. As a consequence of the modulation, the controller  30  loses control of the duty ratio of the switching pulse. Additionally, the amplitude of certain modulated pulses is not high enough to ensure that the switches  28   a ,  28   b  are fully saturated. As a result, the switches  28   a ,  28   b  may be operating in a linear mode, which may result in overheating of the switches  28   a ,  28   b . As discussed above, one approach to preventing the overheating of the switches  28   a ,  28   b  is to disable the startup circuit  50  after the controller  30  begins generating normal switching pulses.  
         [0044]    [0044]FIG. 5 shows one illustrated example of the disable circuit  52 , including a high pass filter  64 , an energy storage device such as a storage capacitor C 2 , electronic switch (e.g., transistor) T 1  and associated gate resistor R 3 . The high pass filter  64  may, for example, have a corner frequency of 50 kHz, passing pulses with a frequency above 50 kHz and blocking pulses with frequency below 50 kHz. Thus, the high pass filter  64  will pass the normal switching pulses having a frequency of 100 kHz, but will block the startup switching pulses having frequencies between 1 kHz and 5 kHz.  
         [0045]    The normal switching pulses pass through the filter  64  and charge up the storage capacitor C 2 . After a few pulses, the voltage across the storage capacitor C 2  will be high enough to drive the transistor T 1  to saturation. The saturation voltage V dis  of the transistor T 1 , is usually below 1 V. With passing reference to FIG. 2, the saturation voltage V dis  is connected to the charging-discharging capacitor C 1  of the startup circuit  50 , which serves as the source of the startup switching pulses. If the saturation voltage V dis  is below 1 volt, as in the case when the normal switching pulses are present, the voltage across the charging-discharging capacitor C 1  is locked at the saturation voltage V dis , and the startup switching pulses will not be generated.  
         [0046]    [0046]FIG. 6 shows a waveform  56   c  of the diac oscillation and a waveform  58   c  of the switching pulses during the transition between startup switching pulse operation and normal switching pulse operation employing the disable circuit  52 . As illustrated, there is only one modulated pulse immediately after the transition. This may be the result of residual flux in the high frequency power transformer  16 .  
         [0047]    [0047]FIG. 7 shows a waveform  58   d  of the switching pulses after the last modulation, indicated above with reference to FIG. 6. Only normal switching pulses exist in the waveform  58   d.    
         [0048]    [0048]FIG. 8 shows a flow diagram of one illustrated exemplary method  70  of operating the converter  10 . In act  72 , the startup circuit provides a first set of control signals to the high frequency power transformer  16  in response to a high voltage supplied to the startup circuit from the high voltage input  12 . The startup circuit  50  may provide the first set of control signals as starting switching pulses at a first frequency. In act  74 , the controller  30  provides a second set of control signals to the high frequency power transformer  16  in response to a low voltage supplied to the controller  30  via the controller power supply  36 . The controller  30  may provide the second set of control signals as normal switching pulses at a second frequency, higher than the first frequency. In act  76 , the disable circuit  52  disables the first set of control signals in response to the provision of the second set of control signals by the controller  30 . The method  70  may be implemented at each application of power to the high voltage input  12 .  
         [0049]    Although specific embodiments of, and examples for, the converter are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. For example, the transformer may take a form other than a high frequency power transformer having a pair of switches. In some embodiments, the functionality can be moved from one subsystem to another. The teachings provided herein can be readily applied to other converters, not necessarily the exemplary two switch forward converter generally described above. The various embodiments as described above can be combined to provide further embodiments. Aspects of the invention can be modified, if necessary, to employ other various systems, circuits and concepts as understood by those skilled in the art.  
         [0050]    In general, in the following claims, the terms used should not be construed to limit the invention to specific embodiments disclosed in the specification and claims, but should be construed to include all power converters that operate in accordance with the claims. Accordingly, the invention is not limited to the disclosure, but instead its scope is to be determined entirely by the following claims.