Abstract:
A high voltage DC to low voltage converter having a plurality of switches, connected in series, paired to form half bridges, inputs connected in series across a high voltage DC source, with outputs summed together using one or more primaries of one or more transformers, with one or more secondaries rectified and filtered to form an isolated DC output at a lower voltage. Each half bridge has an input voltage that is less than the overall input voltage.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to power supplies and more particularly the conversion of a high voltage direct current (DC) to a lower voltage DC.  
       BACKGROUND OF THE INVENTION  
       [0002]     Conversion of high voltage DC to a lower voltage has become a problem with the advancement of a number of technologies. One is the continuing development of the Electrohydrodynamic or Electrokinetic Generator, where modern versions produce a high voltage DC output in the order of a few 10s of kV. Such a high voltage has few useful direct applications, and for that reason must be converted to a lower voltage which is more usable by current systems and devices.  
         [0003]     Advancement in solid stage microwave amplifiers has necessitated the development of replacement modules for high voltage vacuum tube based microwave devices. The commercial requirement is for a drop-in replacement for the vacuum tube, which requires an added converter to change the high voltage previously used by the vacuum tube amplifier to a lower voltage required by the solid state replacement.  
         [0004]     Another emerging market pertains to advances in energy storage in high voltage capacitors which may involve the need to efficiently convert a high voltage to a more usable lower voltage DC.  
         [0005]      FIG. 1 , taken from U.S. Pat. No. 3,022,430, Feb. 20, 1962, “Electrokinetic Generator” uses a rotating switch arrangement with a capacitor divider to convert high voltage DC to a lower voltage DC. A brief explanation follows; a rotating switch alternately connects capacitors  68  and  69  across capacitors,  52 ,  53 ,  54 ,  55 ,  56  and  57 . The charge is transferred from these capacitors and stored on  68 ,  69 , then applied to output capacitor  52 . The output is thereby reduced in voltage from the original value applied across Vdc+ 70  and Vdc− 58 . The use of mechanical switches requires frequent maintenance and requires large capacitance values for the capacitors.  
         [0006]      FIG. 2  shows part of another typical technology employed by the power industry for transmitting high voltage high power DC across large distances. The technical reference Dennis A. Woodford “HVDC Transmission” Manitoba HVDC Research Centre, 18 Mar. 1998 (27 pages), provides much more detail. Typical voltages are 500 kV, using 200 or more high voltage solid-state switches in series. These solid-state switches are slow and designed for very high levels of power, and are not suitable for use at the relatively lower power levels addressed by the current invention. In  FIG. 2  which also describes the prior art, switches  101 ,  102 ,  103  are in series and pull the end of capacitor  107  to Vdc+ 150  when the SWITCH DRIVE  155  is in the first state shown by the table called SWITCH DRIVE  154 . Alternately, as the process progresses along, the clock table switches  101 ,  102 ,  103  open and then  104 ,  105 ,  106  are closed and connect the end of capacitor  107  to Vdc− 152 . The resulting action of alternating the connections of capacitor  107  between Vdc+ 150  and Vdc− 151  creates a square wave on the primary of transformer  108 , which is then reduced in voltage and then rectified into a lower voltage DC V out+ 152  and V out− 153 . Alternately, (again referring to  FIG. 2 ) the output of transformer  108  is filtered to make a clean AC waveform by removing rectifiers  109 ,  110  and replacing them with a suitable filter. The disadvantage of this technology is that for lower power operation the switch losses are large when the frequency of operation is increased. The very high losses encountered when operating at high frequency are undesirable from a cost of operation standpoint. Further, the potential benefits of operating at high frequency and smaller component size for transformer  108  and capacitors  107 , 111  are not possible with current methods. As well, the large number of switches stacked in series in the prior art requires special protection circuits (not shown in  FIG. 2 ), to ensure that all switches share the voltage equally, increasing the cost of manufacture, and adding to device complexity.  
         [0007]     The following patents are relevant styles of power converters but not all are designed specifically for high voltage DC-to-DC operation: U.S. Pat. No. 5,199,285, Jun. 2, 1992; “Solid State Power Transformer Circuit”; U.S. Pat. No. 5,666,278, Sep. 9, 1997, “High Voltage Inverter Utilizing Low Voltage Power Switches”; U.S. Pat. No. 5,943,229, Aug. 24, 1999, “Solid State Transformer”.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to obviate or mitigate at least one disadvantage of previous power converters.  
         [0009]     In one aspect, the invention provides an improved method of converting a high voltage DC into low voltage DC. A plurality of (N) switches are connected in series to a high voltage DC source and operated as pairs to form a plurality of half bridges. The SWITCH DRIVE operates the switches using a predefined, controlled switching sequence. The SWITCH DRIVE operates using 100% duty such that only one switch belonging to a switch pair is ON for half the time (with the other being ON for the other half), and with the pattern alternating sequentially between the two switches in a pair. The SWITCH DRIVE circuit may be powered by a separate power source or alternately a special start-up run control circuit that operates from the high voltage input. The outputs of the switches are then connected to either a single or plural number of isolation transformers with a single or multiple primaries.  
         [0010]     In one embodiment, each primary of the isolation transformer(s) will have one or more capacitor in series to block the flow of DC voltage. This preferred embodiment has at least one or a plurality of isolated secondaries that have the output rectified and filtered to provide the intended low voltage DC output.  
         [0011]     Another preferred embodiment provides a well-regulated low voltage DC output. It consists of a plurality of (N) switches connected in series to a high voltage DC source and operated as pairs to form a plurality of half bridges. The switches are operated using a predefined, controlled switching sequence by a SWITCH DRIVE. The SWITCH DRIVE uses a variable switch ON time or duty, but only one switch belonging to a switch half bridge is ON at any time. For a portion of a cycle both switches are OFF and the pattern alternates sequentially between the two switches in a half bridge. The switch drive circuit may be powered by a separate power source or alternately a special start-up run control circuit that operates from the high voltage input. The outputs of the switches are then connected to either a single or plural number of isolation transformers with a single or multiple primaries. In an embodiment of this variant, each primary of the isolation transformer(s) will have one or more capacitor in series to block the flow of DC voltage. This embodiment has at least one isolated secondary that has the output rectified by diodes with the output of each diode feeding the input of one or more inductor(s). The output of this inductor is then connected to a capacitor to filter out any undesired ripple current. The resulting DC output may be then changed or regulated using feedback and a control circuit that alters the duty of the drive signals applied to the switches (and thus the ON time).  
         [0012]     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying Figures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:  
         [0014]      FIG. 1  depicts a known method using mechanical rotary switch to perform the DC-to-DC conversion;  
         [0015]      FIG. 2  depicts a known method showing current method of converting high voltage DC to lower voltage AC or DC;  
         [0016]      FIG. 3  is a generalized schematic representation of a converter of the present invention;  
         [0017]      FIG. 4  is a schematic representation showing another embodiment of the converter of the present invention;  
         [0018]      FIG. 5  is a schematic representation showing another embodiment of the converter of the present invention; and  
         [0019]      FIG. 6  is a schematic representation showing a variation of the convert of the present invention, providing a regulated output. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Generally, the present invention provides a system and use of that system for converting high voltage DC into low voltage AC or low voltage DC, for a wide variety of applications.  
         [0021]     Referring generally to  FIG. 3 , a simple generic schematic of the converter  10 , in accordance with the present invention is shown. Switches  201 ,  202  form a half bridge with switches  203 ,  204  forming another half bridge and switches  205   206  forming a third. All three half bridges are operated in the same manner, with switches  201 ,  203 ,  205  having the same waveforms shown in a SWITCH DRIVE  254  and the switches  202 ,  204   206  having similar switching waveforms. The depiction of three half bridges is merely an example of the number, N, of half bridges, and it is obvious to those skilled in the art that the number of half bridges may be increased or decreased as part of the overall design of the converter  10 . The switches are shown as a generic representation of a switching device, which may be typically a solid-state device, preferably with a reverse diode across it. The capacitors  207 ,  208 ,  209  may reduce the ripple voltage and current that appears across the groups of half bridges. The Capacitors  210 ,  211 ,  212  are used to couple the AC waveform output of the switches to the primary of a transformer  213 . The capacitors block the DC component present on the half bridge outputs from the primary of the transformer  213 . The primary of the transformer  213  is shown connected to Vdc− 251  by example, but it may alternatively be connected to any lead, of any of the appropriately sized capacitors  207 ,  208  or  209 . The waveform outputs of all three half-bridge sets comprising switches  201 ,  202 ;  203 ,  204 ;  205 ,  206  share equally in the load and the voltages across capacitors  207 ,  208  and  209  during operation are nearly identical. When high voltage is applied across Vdc+ 250  and Vdc− 251  then the output of these three half bridges is typically square wave and is reduced by the transformer  213  in amplitude as well as isolated from the primary high voltage as required. The secondary of the transformer  213  is typically rectified by diodes  214 ,  215  and filtered as required by capacitor  216 , component types and values of output filtering characteristics selected to provide the desired degree of filtering of the isolated DC output.  
         [0022]     An advantage of this circuit is that is able to use low power high speed solid state switches, making possible the design of compact low power, efficient converters, not possible using previous methods. The use of high frequency solid-state switches reduces considerably the size of the converter when appropriate parts are selected, principally the size of capacitors  207 ,  208 ,  209 ,  210 ,  211 ,  212 ,  216  as well as the transformer  213 . It will be obvious to those skilled in the art to recognize that the secondary of transformer  213  may be left as AC and not converted into DC if AC is needed as an output.  
         [0023]     Referring to  FIG. 4 , a variation of the converter  10 , similar to that of  FIG. 3  in design and function is shown. In this variation, the output transformer  313  is connected to three half bridges, in this case comprising switches  301 ,  302 ;  303 ,  304 ;  305 ,  306 . Capacitors  307 ,  308 ,  309  filter the switching noise appearing as an AC ripple or transient across the three half bridges. Capacitor  310  connects one primary of the transformer  313  to the half bridge made up of switches  301 ,  302 . Similarly, capacitor  311  connects one primary of the transformer  313  to the half bridge made up of switches  303 ,  304  and finally capacitor  312  connects one primary of the transformer  313  to the half bridge made up of switches  305 ,  306 . This arrangement uses the same clocking sequence for the switches as the converter in  FIG. 3  and the arrangement is shown in the table called TYPICAL SWITCH DRIVE  354 . This configuration has advantages as the physical layout of high power converters, as well as the reverse phasing of every other half-bridge group may under some circumstances reduce an AC ripple that appears across Vdc+ 350  and Vdc− 351  as well as reducing any radiated noise (EMI) from the converter. The secondary of the transformer  313  may be rectified by diodes  314 ,  315  and filtered as required by capacitor  316  into a filtered isolated DC output. It will be obvious to those skilled in the art to recognize that the secondary of Transformer  313  may be left as AC and not converted into DC if the AC is desired.  
         [0024]     Referring to  FIG. 5 , another variation of the converter  10 , similar to that of  FIGS. 3 and 4  in design and function is shown. In this variation, the output transformer  415  is connected to four half bridges comprising switches  400 ,  401 ;  402 ,  403 ;  404 ,  405 ;  406 ,  407 . Capacitor  408 ,  410 ,  411 ,  414  filter the DC across the four half bridges. Capacitor  409  connects one primary of transformer  415  to the half bridge made up of switch  400 ,  401  to a reverse phased half bridge made up of switches  402 ,  403 . Similarly, capacitor  412  connects another primary of transformer  415  to the half bridge made up of switches  404 ,  405  to a reversed phased half bridge made up of switches  406 ,  407 . This arrangement has a different clocking sequence for the switches than in FIGS.  3  or  4  and the new arrangement is shown in the table called SWITCH DRIVE  454 .  
         [0025]     This configuration has advantages for the design of the physical layout of high power converters as the half bridges are configured as full bridges. The use of this configuration and different phased switch drive signals group can be used to reduce an AC ripple that appears across Vdc+ 450  and Vdc− 451  as well as reduce any radiated noise created by the converter. The secondary of transformer  415  may be rectified by diodes  416 ,  417  and filtered as required by capacitor  413  into a filtered isolated DC output. It will be obvious to those skilled in the art to recognize that the secondary of Transformer  415  may be left as AC and not converted into DC if the AC is needed for another purpose.  
         [0026]     To those skilled in the art it is obvious that other combinations and permutations of switch arrangement than the examples in  FIG. 3 ,  FIG. 4  and  FIG. 5  are possible.  
         [0027]     Referring generally to  FIG. 6 , a variation of the converter  10  of the present invention is shown having a regulated output achieved by a feedback system. A PWM (Pulse Width Modulation) SWITCH DRIVE  554  may be PWM controlled in a similar manner as used by commercial AC to DC switching power supplies. Switches  500 ,  501 ;  502 ,  503 ,  504 ,  505  form three half bridges that are connected in series in a similar manner to  FIG. 3 ,  FIG. 4  and  FIG. 5 . Capacitors  506 ,  507 ,  511 ,  512  filter the switch current pulses reducing the AC that is generated by the half bridges across the high voltage DC input Vdc+ 550  and Vdc− 551 . The addition of resistors  514 ,  515  and  516  are used to force the voltages to be equal across capacitors  506 ,  507  and  511  during the start-up time where the half bridges are off. Capacitor  512  is used to provide start-up power for the START MODULE  531  which has various components that store sufficient charge to run the half bridges for a specific time after which an auxiliary winding  560  from transformer  518  supplies the necessary power to run the control electronics. Alternately, an external DC or AC power source, not shown, may provide power to operate the converter, and may be either common to or close to either Vdc+ 550  or Vdc− 551 .  
         [0028]     The FEEDBACK  530  supplies an error signal used by the PWM MODULE  532  to generate appropriate width clock signals that are supplied to the SWITCH DRIVER  533 , which then drives the switches  500 ,  501 ,  502 ,  503 ,  504 ,  505 . The additional circuits function as follows. When high voltage power is first applied to Vdc+ 550  and Vdc− 551 , the resistors  514 ,  515  and  516  charge capacitor  512 . The START MODULE  531  determines when it has enough charge to operate the PWM MODULE  532  and SWITCH DRIVER  533  for a predetermined time. Alternately, the START MODULE  531  may be powered by an external low voltage DC or AC source. After initially powering the converter electronics, the START MODULE  531  receives a low voltage AC from transformer  518  through secondary  560 . The power from this secondary  560  then provides the low voltage power to sustain operation of the PWM MODULE  532  and SWITCH DRIVER  533 .  
         [0029]     After the START MODULE  531  has started the converter the FEEDBACK  530  provides to the PWM MODULE  532 , a signal, which is representative of the output voltage (for example being proportional in some manner to the output voltage).  
         [0030]     The FEEDBACK  530  may use optical isolation, an isolation transformer etc., not shown, to provide an isolated feedback signal to the PWM MODULE  532 . This feedback mechanism will be obvious known to one skilled in the art, and is similar to that used in traditional power supplies except that the isolation voltage rating is substantially greater. When the SWITCH DRIVE  554  is decreased from full duty (50% of full duty is shown as an example) then the waveform that appears on the secondary of transformer  518  is not a full duty square wave but has positive and negative phases which are proportional in width to the SWITCH DRIVE  554  wave form. The Diodes  519 ,  520  rectify the secondary AC into a pulsating DC, which is then filtered by inductor  521  and capacitor  510 . The output inductor  521  and capacitor  510  filters the pulsating DC into an average value equal to the duty of the waveform times its amplitude. This portion of the circuit will be obvious to one skilled in the art, and may be used, for example in a switching power supply commonly called a FORWARD CONVERTER, except that in the present invention, it provides a regulated low DC voltage output from a very High voltage input.  
         [0031]     The switches,  500 ,  501 ,  502 ,  503 ,  504 ,  505  are typically semi-conductor devices that have a reverse diode across them to clamp any reverse voltage that may be generated by transformer  518  during the time the SWITCH DRIVE  554  changes state. The combination of the switches  500 ,  501 ,  502 ,  503 ,  504 ,  500  capacitor  508 ,  509 ,  513  and primary of transformer  518  may be combined in any way shown in  FIG. 3 ,  FIG. 4  or  FIG. 5  or combination of  FIG. 2 ,  FIG. 3 ,  FIG. 4  or  FIG. 5 , implied thereby.  
         [0032]     As used herein, the term high voltage DC refers generally to voltages greater than the intended high range tolerance voltage of a single semi-conductor switch used in the intended application. For medium power applications, an exemplary lower limit of a range of high voltages might be 800 V DC.  
         [0033]     The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.