Abstract:
A method and apparatus for providing welding type power is disclosed. The power source is capable of receiving any input voltage over a wide range of input voltages and includes an input rectifier that rectifies the ac input into a dc signal. A dc voltage stage converts the dc signal to a desired dc voltage and an inverter inverts the dc signal into a second ac signal. An output transformer receives the second ac signal and provides a third ac signal that has a current magnitude suitable for welding, cutting or induction heating. The welding type current may be rectified and smoothed by an output inductor and an output rectifier. A controller provides control signals to the inverter and a controller power supply can also receive a range of input voltages and provide a control power signal to the controller, and a voltage independent of the input voltage.

Description:
This is a continuation of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 09/969,535, filed Oct. 1, 2001, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding, Plasma Or Heating Power Source, which is a continuation of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 09/540,567, filed Mar. 31, 2000, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding, Plasma Or Heating Power Source, which issued on Dec. 11, 2001 as U.S. Pat. No. 6,329,636. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to power sources. More particularly, this invention relates to power sources employed in welding, cutting and heating applications. 
     Power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. In welding applications, power sources typically receive a high voltage alternating current (VAC) signal and provide a high current output welding signal. Around the world, utility power supplies (sinusoidal line voltages) may be 200/208V, 230/240V, 380/415V, 460/480V, 500V and 575V. These supplies may be either single-phase or three-phase and either 50 or 60 Hz. Other power supplies, such as that available in mines or subways, may be dc. Additionally, power may be provided from generators that attempt to provide power at such voltages and frequencies, or at other voltages and frequencies, or at dc. 
     Welding power sources receive such inputs and produce an approximately 10-40 volt dc high current welding output. Substantial power is delivered to a welding arc which generates heat sufficient to melt metal and to create a weld. Cutting power sources receive such inputs and produce an approximately 80 volt dc high current cutting output. Induction heating power sources receive such inputs and produce an approximately 200 volt ac high current heating output. Because welding, heating and cutting require similar high power outputs, welding type power source or supply, as used herein, includes welding, plasma and induction heating power sources and supplies. Welding type power, as used herein, refers to welding, plasma or heating power. 
     Given the various utility and generator power inputs it is desirable for a welding/plasma/heating power supply to be able to receive any of a wide range of power inputs. Several hurdles must be overcome to allow a power supply to receive multiple input voltages. First, the power circuit must be able to receive the expected voltage magnitudes and frequencies, yet still provide the desired output voltage. Second, the desired control voltage must be provided, regardless of the input voltage. Also, when aux power (for tools etc.) is being provided, the desired output voltage and frequency (110V ac at 60 Hz, e.g.) must be provided regardless of the input voltage and frequency. 
     Early power supplies overcame these hurdles by having taps on transformers correspond to each expected voltage. The taps were selected by the user manually “relinking” the power supply for each input voltage. This was time consuming, and required the user open the power supply. Operating an improperly linked power source could result in personal injury, power source failure or insufficient power. 
     A prior art welding source that improved upon manual linking provided an automatic linkage. For example, the Miller Electric AutoLink®, described in U.S. Pat. No. 5,319,533, incorporated herein by reference, tested the input voltage when they are first turned on, and automatically set the proper linkage for the input voltage sensed. The power supply included two inverters connected in parallel (for 230V, for example) or in series (e.g., for 460V). Such arrangements generally allow for two voltage connection possibilities. However, the higher voltage must be twice the lower voltage. Thus, such a power source cannot be connected to supplies ranging from 230V-460V to 380V-415V or 575V. 
     Another prior art power supply that was a significant advance in the ability of a power source to receive a wide range of power is described in U.S. Pat. No. 5,601,741, issued Feb. 11, 1997, on application Ser. No. 08/342,378, filed Nov. 18, 1994, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding Power Source, and is owned by the assignee of the present invention. The power supply described in U.S. Pat. No. 5,601,741 is implemented commercially in the Miller Omniline® power supply. 
     The welding/plasma/heating power supply of U.S. Pat. No. 5,601,741 (incorporated herein by reference), includes an input stage, a preregulator stage, and an output stage. Also, a controller (with a power source) controls the power supply to produce a desired output. The power supply for the controller is referred to as an auxiliary power supply therein. However, the present invention also includes a power output for tools etc that is often referred to as an auxiliary power output. Thus, to avoid confusion, the power output for tools will be referred to herein as aux power, and the power for the controller will be referred to herein as control power. 
     Generally, the Omniline® input stage receives ac utility or generator power, and rectifies that power to provide a first dc signal. The rectified dc signal is provided to the preregulator, which includes a boost converter. The boost converter boosts the rectified signal to create a dc bus. The output stage includes an inverter, transformer, and rectifier which create welding, cutting, or heating power (welding type power) from the bus. 
     Because a dc bus is created (by the boost converter) and then inverted to create the output power, the output power voltage and frequency is independent of the input voltage and frequency. This allows a wide range of input voltages and input frequencies to be used. 
     However, power for the controller is derived by transforming the input voltage. The control power circuit determines the magnitude of the incoming power, and configures taps on a transformer to obtain the desired control power. The control power transformer is relatively small since the amount of control power needed is relatively small. While a wide range of input power are thus acceptable, the input must be sufficient that, for a selected tap, the control voltage is acceptable. 
     Thus, the Omniline® provided the desired output voltage by inverting a dc bus having a magnitude independent of the input voltage. Also, the Omniline® created control power by selecting taps on a transformer. This allowed a wide range of input voltages to be used, but still required the input voltage to have an appropriate magnitude for being transformed into a control voltage. Additionally, this prior art did not provide an aux power (for tools), that had a voltage and frequency independent of the input voltage and frequency. 
     Accordingly, a welding power source that may receive any common input voltages or frequency is desirable. Preferably, this is accomplished without the need of any linkages for the welding power input and for the control power input. Additionally, it is desirable to have such a welding power source that produces aux power and weld power having a frequency and voltage independent of the input frequency and voltage. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention a welding type power source is capable of receiving a range of input voltages and frequencies. It includes an input circuit, a preregulator, an output circuit, a preregulator controller, and a control power circuit. The input circuit receives input power at an input frequency and an input magnitude, and provides a signal having a magnitude responsive to the input magnitude to the preregulator. The preregulator provides a dc signal having a preregulator magnitude independent of the input magnitude to the output circuit. The output circuit provides a welding type output power signal having an output frequency independent of the input frequency and having an output voltage independent of the input voltage. The preregulator controller is connected to the preregulator, and receives power from the control power circuit. The control power circuit derives power from the dc signal and provides control power to the controller that has a control power magnitude independent of the input magnitude and a control frequency independent of the input frequency. 
     The input circuit includes a rectifier in one embodiment. 
     The preregulator magnitude is greater than the first magnitude, and the preregulator includes a boost converter in various alternatives. The boost converter may include a slow voltage switched switch and a slow current switched switch. 
     The output circuit includes an inverter, which may include a switched snubber in other alternatives. 
     The preregulator magnitude is greater than the control power magnitude, and/or the control power circuit includes a buck converter in additional embodiments. 
     According to a second aspect of the invention a method of providing welding type power from a range of input voltages and frequencies, includes receiving an input power signal having an input frequency and an input magnitude. A first signal having a magnitude responsive to the input magnitude is provided. The first signal is converted into a dc second signal having a second magnitude independent of the input magnitude. A welding type power signal derived from the dc second signal has an output frequency independent of the input frequency and further has an output voltage independent of the input voltage. The dc second signal is converted into control power having a control power magnitude independent of the input magnitude. 
     The input signal is rectified in one embodiment. 
     The second magnitude is greater than the first magnitude, and converting the first signal into a dc second signal includes boost converting the first signal in other embodiments. Boost converting may include slow voltage switching and slow current switching a switch. 
     The output power signal is provided by inverting the dc second signal, and/or using a switched snubber in various alternatives. 
     The second magnitude is greater than the control power magnitude, and/or converting the dc second signal into control power includes buck converting the dc second signal in additional alternative. 
     According to a third aspect of the invention a welding type power source capable of receiving a range of input voltages and frequencies includes a dc bus. An output circuit receives the dc bus, and provides a welding type output power signal. The output power is voltage and frequency independent of the input power. A controller is connected to the output circuit. Power for the controller comes from the dc bus, through a control power circuit. 
     According to a fourth aspect of the invention a method of providing welding type power from a range of input voltages and frequencies includes receiving a dc bus and providing welding type power at a magnitude independent of the bus magnitude, but derived from the dc bus. The dc bus is also converted into control power whose magnitude is independent of the dc bus magnitude. 
     According to a fifth aspect of the invention a method of starting to provide providing welding type power from a range of input voltages and frequencies, includes receiving an input power signal and providing a first dc signal at magnitude responsive to the input&#39;s magnitude. A second dc voltage whose magnitude is less than the first dc magnitude is derived from the first dc magnitude. A control converter is controlled with the second dc voltage such that the control converter produces a control dc voltage. An output converter is controlled with the control dc voltage to produce an output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a welding power supply constructed in accordance with the present invention; 
     FIG. 2 is a circuit diagram of one embodiment of the preregulator of FIG. 1; 
     FIG. 3 is a circuit diagram of one embodiment of an inverter with a switched snubber used in the output circuit of FIG. 1; 
     FIG. 4 is a circuit diagram of one embodiment of the controller power circuit and portions of the controller of FIG. 1; and 
     FIG. 5 is a circuit diagram of one embodiment of the aux power circuit and portions of the controller of FIG.  1 . 
    
    
     Before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. Like reference numerals are used to indicate like components. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention will be illustrated with reference to a particular power supply, having particular components, and used in a particular environment, it should be understood at the outset that the invention may-also be implemented with other power supplies, components, and used in other environments. 
     Referring now to FIG. 1, a welding power source is  100  includes an input circuit  101 , a preregulator  102 , an output circuit  103 , a controller  104 , a controller power supply  105 , and an aux power supply  106 . 
     Input circuit  101  receives input utility or generator power, and provides a signal to preregulator  102 . The input is ac, and the input circuit includes a rectifier and capacitor bank in the preferred embodiment. Thus, the output of the input circuit is a dc (uni-polar) signal, having a frequency twice that of the input frequency. Input circuit  101  is comprised of other components in alternative embodiments. 
     Preregulator  102  receives the signal from input circuit  101  and provides preregulated signal. Preregulator  102  includes a boost converter and boosts the rectified signal to be a dc bus (about 800 Vdc) in the preferred embodiment. Preregulator  102  is controlled so that, regardless of the input, the dc bus voltage is about 800 V. Thus, the magnitude of the dc bus voltage is independent of input magnitude. (As used herein a second voltage is independent of first voltage when magnitude of the second voltage is controlled to be a value which is not proportional to or a function of the first voltage). Also, the dc bus frequency (substantially zero, but with ripple) is independent of the input frequency. (As used herein a second voltage is independent of first voltage when magnitude of the second voltage is controlled to be a value which is not proportional to or a function of the first voltage). 
     Preregulator  102  includes other types of converters, such as an inverter, a series resonant converter, etc., in other embodiments. Converter, as used herein, includes a power circuit that receives or provides an ac or dc signal, and converts it to the other of an ac or dc signal, or to a different frequency. Inverter, as used herein, includes a power circuit that receives or provides a dc bus signal that is inverted to be an ac signal. 
     If a dc input signal is received, input circuit  101  simply passes the dc signal to preregulator  102 , or is omitted altogether. If the dc input signal is of a sufficient magnitude, preregulator  102  may pass simply provide the dc input as the dc bus, or be omitted altogether. 
     Output circuit  103  receives the dc bus and provides an output suitable for welding/heating/cutting. Output circuit  103  includes, in the preferred embodiment, an inverter, followed by a transformer, followed by a rectifier and an output inductor. The output power is also frequency and voltage independent of the dc bus and the input signal. Output circuit  103  is comprised of other components in other embodiments, and may provide an ac or dc output. 
     Controller  104  includes control circuitry similar to that known in the prior art, and causes the boost and inverter switches to switch in response to feedback and a setpoint (such as 800V for the boost converter and a user setpoint for the output inverter). 
     Control power is provided to the controller by controller power supply  105 . Controller power supply  105  derives power from the output of preregulator  102 , in the preferred embodiment. Controller power supply  105  includes a buck converter that steps down the 800V to 15V dc. The magnitude and frequency of the output of controller power supply is thus independent of the dc bus, and the input power. It is easily seen that, once the 800V dc bus is present control power is easily derived from the bus rather than from a control power transformer, any input voltage and frequency is acceptable. Thus, linking, such as adjusting transformer taps, need not be performed. 
     The circuitry that controls preregulator  102 ., output circuit  103  and controller power supply  105  is collectively called controller  104  because of the common function (controlling). However, in practice they may form distinct and remotely located circuits, they may share circuitry, they may reside in a common microprocessor or DSP, and they may share control signals and feedback. 
     One potential difficulty at start up is a result of the switching of theebuck converter in controller power supply  105  being controlled by controller  104 : The 800V dc bus is not created until the controller causes the boost converter switch to switch on and off, but the controller cannot control the switch until it has power, and the power for the controller is derived from the dc bus. 
     This difficulty is overcome in the preferred embodiment because even before the boost converter begins to switch to create the 800V dc bus, the dc bus will have the same magnitude as the rectified input voltage,.which is typically at least 110V rms. Since the buck converter steps down the bus voltage to 15V dc, even a relatively low magnitude input voltage (110V ac. e.g.) is sufficient to create the controller power. Also, control power for the buck converter is derived from a floating 15 volt supply, by bleeding current from the bus. 
     The start up sequence will be described in detail below, but generally is as follows. At start up the dc bus quickly rises to the rectified input voltage, through a precharge resistor. The precharge resistor is bypassed after the bus is charged. Current bled from the bus charges capacitors which provide power for the buck converter controller. The buck converter controller controls the buck converter, causing it to produce 15V dc power for all of controller  104 . Controller  104  controls the boost converter in preregulator  102  to step up the rectified input and produce an 800V dc bus. Thus, the 800V dc bus is created, and can provide power to output circuit  103  when the user begins to weld. 
     Additionally, aux power supply  106  includes an inverter, and produces a synthetic aux power, i.e., a desired output voltage at a desired output frequency (110V ac at 60 Hz, e.g.). Controller  104  also controls aux power supply  106 . 
     Input circuit  101 , preregulator  102 , output circuit  103 , and the portions of controller  104  that control them, are implemented using the circuitry shown in U.S. Pat. No. 5,601,741 (and correspond to like numbered features of the drawings therein) in one embodiment. However, a wide variety of circuits may be used to implement this part of the present invention, and the details of will not be described in detail herein. 
     A particular switching circuit is used for the preregulator in another embodiment because it provides for efficient slow voltage switching and slow current switching. This circuit is described in patent application Ser. No. 09/111,950, filed, Jul. 9, 1998, entitled Power, Converter With Low Loss Switching, and owned by the owner of this invention. Slow voltage/current transitions or switching (SVT and SCT) as used herein, describe transitions where the voltage or current rise is slowed (rather than held to zero), while the switch turns off or on. 
     The circuit used in the preferred embodiment to implement preregulator  102  is shown in FIG. 2 (along with input circuit  101  and voltage source  109 ). The embodiment of FIG. 2 uses a 90-250 volt ac power line as input voltage  109 . Input circuit  101  is comprised of diodes D 60 , D 70 , D 80 , and D 9 , which rectify the input voltage to provide a single polarity sinusoidal input voltage. 
     A power factor correction portion (described below) of preregulator  102  functions best when the input voltage is sinusoidal, although it could be another alternating input. Thus, a small (10 μF) capacitor (not shown) is provided across the input rectifier in one embodiment to smooth the input line voltage. 
     The rectified input voltage is applied to a boost inductor L 10  (750 μH) which is connected with a boost switch Z 1  (preferably an IGBT) to form a boost convertor, An anti-parallel diode D 50  is connected across switch Z 1  to protect switch Z 1  during transitions. The portion of the circuit which provides the lossless switching includes a snubber inductor L 2  (3.9 μH) a pair of capacitors C 100  (1 μF) and C 200  (0.068 μF), and diodes D 10 , D 20 , D 30 , and D 40 . Switch Z 1  is switched in a known manner such that the output of preregulator  102  is a desired voltage, no matter what the input voltage is. The output is provided across a capacitor C 50  (2000 μF) that provides a stable voltage source (200 volts in the preferred embodiment) for the downstream convertor. Also, capacitor C 50  prevents the voltage from being dangerously high and damaging switch Z 1 . 
     The portion of preregulator  102  that provides power factor correction is a power factor correction circuit  204  (FIG.  2 ), and generally senses the input voltage waveform, and conforms the shape of the current waveform to be that of the line voltage waveform. This provides a power factor of very close to 1, 0.99 in the preferred embodiment. Power factor correction circuit  204  may be implemented using an integrated circuit, such as a UC3854 or an ML4831, or with discrete components, such as those shown in the above-referenced Power Converter With Low Loss Switching, incorporated herein by reference. 
     Power factor correction circuit  204  receives as inputs the output voltage from input circuit  101 , the output voltage from preregulator  102 , and the output current of preregulator  102  (using a CT). Because the frequency of preregulator  102  (25 Khz) is much higher than that of the line (60 Hz) the pre-regulator current can be made to track the input line voltage shape by sensing the shape of the input voltage, and controlling the input current in response thereto. 
     Output circuit  103  can include a conventional inverter, output transformer, output rectifiers, and an output inductor such as in U.S. Pat. No. 5,601,741. However, in one embodiment, the inverter is a switched snubber, such as that described in Power Converter With Low Loss Switching, and shown in FIG.  3 . 
     The invertor implemented with a switched snubber includes a dc voltage source  1501 , a pair of switches  1502  and  1504 , with a pair of anti-parallel diodes  1503  and  1505 , a pair of capacitors  1507  and  150 . 8  (1410 μF), a transformer  1509 , a capacitor  1512  (0.099 μF), an output rectifier including diodes  1510  and  1511 , and an output inductor  1513 . 
     Capacitor  1512  is switched across transformer  1509  by switches  1502  and  1504 . Switches  1402  and  1403  are used to soft switch switches  1502  and  1504 . Switches  1402  and  1403  do not need any special timing, and run with the main clock at effectively 50% duty cycle. For example, switches  1502  and  1402  turn on together, and switch  1502  delivers current to transformer  1509 , while switch  1402  does nothing. When switch  1502  turns off, switch  1402  remains on, and current is directed through switch  1402  and diode  1405  into capacitor  1512 , thus giving an SVT (Slow Voltage Transition) turn off. Switch  1402  is turned off after the transition and diode  1405  prevents the back flow of current from capacitor  1512 . This occurs in complimentary fashion with switches  1502  and  1402  and diode  1405 . Thus, this circuit provides full-wave transformer usage, PWM control, complete capacitor balance control with no extra circuitry, and efficient use of switches with SVT. An alternative embodiment includes using a full bridge version of the snubber. 
     The specific circuitry used to control the switched snubber may be conventional control circuitry, such as that described in Power Converter With Low Loss Switching. 
     A circuit used to implement controller power supply  105 , and a portion of controller  104  that controls controller power supply  105  is shown. Controller power supply  105  includes a buck converter in the preferred embodiment, and includes a switch  401 , a freewheeling diode  403  and a buck inductor L 1 , configured in a conventional buck arrangement, and a resistor  404  (0.5 ohms). 
     The circuitry that controls the buck converter (or regulator), in the preferred embodiment, is also shown on FIG. 4, and is part of controller  104 . One skilled in the art will readily recognize that the control-circuitry may be located on the same control board as the portion of controller  104  that controls preregulator  102  and output circuit  103 , or it may be located remotely therefrom, for example on the PC board for controller power supply  105 . 
     Generally, the buck converter is controlled such that at startup current is bled from the DC bus to charge a capacitor, thus providing sufficient power to turn on and off the buck switch. The voltage across the capacitor is a floating voltage, and is sufficient to operate the buck converter control circuitry. The control circuitry causes the buck switch to turn on and off repeatedly to create a control power of about 15 volts DC. The 15 volts DC is then used to power the remaining control circuitry. 
     More specifically, when the power supply is turned on the DC bus will have a voltage equal to the peak voltage of the input rectified signal (about 200 volts DC for an input having 140 volts RMS, e.g.). Current bleeds from the bus through a pair of resistors R 5  (150 Kohms) and R 4  (150 Kohms) to charge a pair of capacitors CS (0.1 μF) and C 2  (100 μF). The voltage across capacitors C 2  and C 5  is called the BUCK-COM and BUCK+15V, and is the floating voltage supply for the circuitry that controls the buck converter is Proper selection of the resistance of resistors R 5  and R 4  (and other components described below) determines the minimum voltage needed on the dc bus to operate the buck converter control circuitry. In the preferred embodiment the minimum voltage is no higher than that obtained by rectifying 110V ac power. 
     When the voltage across capacitors C 5  and C 2  reaches approximately 11.7 volts a switch Q 1  turns on. 
     Switch Q 1  is used to enable (or disable) the logic or control circuitry for the buck converter. When the voltage across capacitors C 2  and C 5  is less than about 11.7 volts, then switch Q 1  off, and the logic circuitry is disabled. A resistor R 2  (10 Kohms), a resistor R 3  (100 Kohms) and a zener diode D 1  (11 volts) are associated with switch Q 1 , and create the turn on voltage. Thus, the resistance of resistor R 3  also sets the minimum voltage needed to operate the circuitry that controls the buck controller. 
     The logic circuitry includes a plurality of NAND gates U 4 A, U 4 B, U 4 C, AND U 4 D, and associated circuitry capacitors C 1  (0.1 μF) and C 3  (0.001 μF), resistors R 6  (20 Kohms), R 7  (332 Kohms) and R 8  (20 Kohms). This circuitry controls a plurality of paralleled NOT gates U 3 , whose output is the on/off signal to the base of buck switch  401 , through a resistor R 12  (10 ohms). The supply voltage for the logic circuitry is the floating BUCK-COM/BUCK+15V voltage supply. 
     When NOT gates U 3  output is 1, buck switch  401  is on, and when NOT gates U 3  output is 0, buck switch  401  is off. NOT gates U 3  output is 1 when their input is 0, which requires both inputs of NAND gate U 4 D to be 1. 
     At start up, before the bus charges the BUCK-COM/BUCK+15V voltage supply to 11.7 volts switch Q 1  is off. Thus, an input to NAND gate U 4 A is 0, and the output of NAND gate U 4 A is 1. This output is fed through diode D 3  and resistors R 6  and R 7  to the inputs pin  2  of NAND gate U 4 B, and the output of NAND gate U 4 B is thus 0. The output of NAND gate U 4 B is fed to input pin  2  of NAND gate U 4 A, thus holding the output of NAND gate U 4 D high (and holding switch  401  off). Also, at start up the output of NAND gate U 4 C is 1 because both its inputs from NAND gate U 4 B is 0. 
     When the bus charges the BUCK-COM/BUCK+15V voltage supply to 11.7 volts switch Q 1  is turned on, and input pin  9  of NAND gate U 4 A goes high, thus enabling the output of NAND gate U 4 A to go low, and enabling the output of NAND gate U 4 B to go high and the output of NAND gate U 4 D to turn on switch  401  (through gates U 3 ). Also; when the output of NAND gate U 4 A goes to 0, capacitor C 3  discharges through resistor R 7  (with a relatively long RC time constant). When capacitor, C 3  has discharged, the output of NAND gate U 4 B goes to 1 causing the output of NAND gate U 4 D to go 0, turning on buck switch  401 . 
     The associated circuitry causes the logic to latch until input pin  8  of NAND gate U 4 A goes to zero. This happens because when switch  401  is on current through inductor L 1  increases, turning on a switch Q 3  through a pair of resistors R 11  (2 Kohms) and R 10  (100 Kohms) and a capacitor C 4  (0.001 μF). When switch Q 3  is turned on, input pin  8  of NAND gate U 4 A is 0 and the output of NAND gate U 4 D is 1, causing (eventually) the output of NAND gate U 4 B to be 0, the output of NAND gate U 4 C to be 1, and the output of NAND gate U 4 D to be 1, turning off switch  401 . 
     The process of turning switch  401  on and off is repeated, and limited after the necessary 15V dc bus is created on a pair of outputs PRECOM and PRE+15V. The current through inductor L 1  charges a pair of capacitors C 13  and C 14  (2200 μF). Outputs PRECOM and PRE+15V are connected across capacitors C 14  and C 14 , thus, when they have charged to +15V, the needed control power is provided. A zener diode D 8  prevents the magnitude of the voltage across outputs PRECOM and PRE+15V from getting to high. 
     A resistor R 13  (1 Kohm), a pair of zener diodes D 5  and D 2  (6.8V), an opto-isolator OC 3 , and a resistor R 1  (1 Mohm) limit the turning on of buck switch  401  when the voltage across outputs PRECOM and PRE+15V reaches 15 volts. When the voltage drop across diodes D 5  and D 6  and opto OC 3  reaches 15 volts, opto OC 3  turns on, pulling up the inputs to NAND gate U 4 B, turning off switch  401 . 
     A circuit used to implement aux power supply  106 , and a portion of controller  104  that controls aux power supply  106  is shown in FIG.  5 . One skilled in the art will readily recognize that the control circuitry may be located on the same control board as the portion of controller  104  that controls preregulator  102  and output circuit  103 , or it may be located remotely therefrom, for example on the PC board for aux power supply  106 . 
     Aux power supply  106  includes an inverter in the preferred embodiment, and operates much like a motor drive, or typical AC inverter circuit (the output follows a pattern of high, zero and negative, zero, high, zero, . . . ). The output is, in the preferred embodiment, a synthetic AC, 60 Hz, 575V supply (dependent on magnitude of the 800V bus). The 575 Volt supply may be transformed to any desired magnitude. Alternatively, the magnitude may be controlled to a lower value by having a different bus value, or by bucking the bus value down to a desired level. The regulated level could be preset or user selected. Also, the frequency can be user selected (50 or 60 Hz e.g.) or preset. 
     The inverter includes 4 switches  501 - 505 , which are turned on and off to produce ac power on a pair of outputs AC/ 2  and AC/ 1 . Specifically, normally switches Q 3  and Q 4  are on and freewheeling (when there is no applied voltage difference across outputs AC/ 1  and AC/2). AC/ 1  is made high (and AC/ 2  low) by turning switch  502  on and switch  504  off. Thus, the conduction path is from the bus, through switch  502 , to AC/ 1  and the load, and then from AC/ 2  through switch  503  to ground (PRECOM). Conversely, AC/ 1  is made low (and AC/ 2  high) by turning switch  501  on and switch  503  off. Thus, the conduction path is from the bus, through switch  501 , to AC/ 2  and the load, and then from AC/3 through switch  504  to ground (PRECOM). 
     The remaining circuitry on FIG. 5 is control circuitry, and is powered by PRECOM and PRE+15V. The circuitry operates in a conventional manner and includes drive circuitry, level shifters, current limiters and an enable circuit. 
     The gate drives for the switches operate in a conventional fashion, and include resistors R 16  (10 ohms), R 17  (5.11 Kohms), R 18  (1 Mohms), R 19  (22.1 ohms), R 20  (10 ohms), R 21  (1 Mohms), R 22  (5.11 Kohms), R 23  (22.1 ohms), R 24  (2 Kohms), R 25  (2 Kohms), R 26  (2 Kohms), R 27  (2 Kohms), R 28  (20 Kohms), R 29  (20 Kohms), R 30  (20 Kohms), and R 31  (20 Kohms), capacitors C 6  (100 μF), C 7  (0.1 μF), C 10  (100 μF), C 11  (0.1 μF), switches Q 6 , Q 7 , Q 8 , Q 9 , Q 10  and Q 11 , diodes D 61  and D 7 , and opto-isolators OC 1  and OC 2 . 
     Latch circuitry current limits each cycle, and operates in a conventional fashion. The latch circuitry includes level shifter U 1 A ( 40109 ), gates U 2 A, U 2 B, U 2 C, U 2 D, resistors R 32  (22.1 Kohms), R 33  (22.1 Kohms), R 34  (20 Kohms), R 35  (20 Kohms), R 36  (3.01 Kohms), R 37  (5.11 Kohms), R 38  (16 Kohms), R 39  (1 Kohms) and R 40  (20 Kohms), a capacitor C 15  (0.000 μF) and a pair of switches Q 12  and Q 13 . A pair of level shifters U 1 B and U 1 C ( 40109 ) are also provided. 
     An enable circuit includes opto-isolator OC 4 , resistors R 41  (1 Mohms) and R 42  (20 Kohms), and capacitor C 16  (0.1 μF). A voltage regulator circuit includes voltage regulator VR 1 , capacitor C 18  (0.1 μF), capacitor C 17  (0.1 μF) and regulator VR 1 , and produces a regulated +5V supply form the +15V supply generated by the buck regulator. 
     A timing circuit sets the clock for the control circuitry and aux frequency. The timing circuitry includes a microprocessor MPU 1 , capacitor C 19  (22 pF) and capacitor C 20  (22 Pf), a crystal oscillator Y 1  (4.096 MHz), and a level shifter U 1 D. 
     These components cooperate in a known manner to produce the desired 575V ac, 60 Hz output. As stated above, the circuit could be modified to allow the user to select the voltage magnitude and/or frequency. 
     Numerous modifications may be made to the present invention which still fall within the intended scope hereof. Thus, it should be apparent that there has been provided in accordance with the present invention a method and apparatus for providing welding type power from any typical input voltage or frequency that fully satisfies the objectives and advantages set forth above. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives,.modifications and variations that fall within the spirit and broad scope of the appended claims.