AC Inverter with Active Neutral Balancing

Abstract

A welding type power system provides balanced split-phase AC power or three-phase AC power from a DC power bus, the welding type power system comprising a power supply, a DC power bus, an inverter and a balancing circuit. The inverter and the balancing circuit are configured in a half-bridge topology. The balancing circuit is configured to balance the neutral midpoint of the DC power bus. The balanced split-phase AC power or three-phase AC power is achieved by actively controlling the neutral midpoint of the DC power bus via the balancing circuit.

Description

[0001] Related Applications

[0002] This international application claims priority to U.S. Patent Application Serial No. 15 / 990,009, entitled “AC INVERTER WITH ACTIVE NEUTRAL BALANCE,” filed on May 25, 2018. The entire contents of U.S. Patent Application Serial No. 15 / 990,009 are incorporated herein by reference. Background Art

[0003] Conventional welding-type power systems provide welding power outputs. Some welding-type power systems utilize a direct current (DC) power bus to provide power to a welding power supply to provide welding power outputs, and to provide power to an auxiliary power supply to provide auxiliary outputs. Currently, an alternating current (AC) inverter using a shared neutral line provides an AC output from the DC bus. In the event of an unbalanced load on the auxiliary outputs, the neutral line between the AC outputs may become unbalanced if there is no balancing mechanism. Summary of the invention

[0004] An apparatus, system and method of a welding-type power system is disclosed, the welding-type power system including an AC inverter with an active neutral balancing circuit, configured to provide balanced split-phase AC power or balanced three-phase AC power to an AC load, substantially as shown and described in conjunction with at least one of the figures, and more fully set forth in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 is a block diagram of an example welding-type power system in accordance with aspects of the present disclosure.

[0006] Figure 2 is a schematic diagram of an example welding-type power system with an active balancing circuit in accordance with aspects of the present disclosure.

[0007] Figure 3 is a flow chart of an exemplary method of operating an active balancing circuit according to aspects of the present disclosure.

[0008] The drawings are not necessarily drawn to scale. Where appropriate, similar or identical reference numerals are used to indicate similar or identical components. DETAILED DESCRIPTION

[0009] The present disclosure relates generally to welding systems, and in particular to systems and methods for providing balanced AC output power from a DC power bus to (multiple) AC loads. More specifically, the present disclosure relates to generating balanced split-phase AC output power from a DC power bus by means of an inverter bridge topology. In some aspects, the present disclosure relates to generating balanced three-phase AC output power from a DC power bus by means of an inverter bridge topology. Balanced AC power can be achieved by adding a balancing circuit to control the neutral midpoint of the AC output. In some examples, the neutral midpoint of the AC output is connected to the center point of the DC power bus, which includes two capacitors connected in a series arrangement to form a split capacitance. In some examples, the balancing circuit is an active half-bridge circuit. Although described with respect to a welding-type power system, the systems and methods described in the present disclosure can be used to provide balanced AC output power in any power system that provides AC output power from a DC power bus to an AC load.

[0010] Current systems can provide two half-bridge inverters running a pulse width modulated (PWM) sine wave sequence to generate two out-of-phase AC outputs from a center-tapped DC power bus. For example, each output can be 115 volts AC (VAC), so that 230VAC is seen across the two outputs. A welding inverter can run on the same DC bus. In such an example, an unbalanced AC load may cause the relative voltages of the two AC outputs to become unbalanced, which may cause the neutral line to shift to a point that is not midway between the two AC outputs, which is the neutral midpoint of the DC power bus. For example, an unbalanced AC load may occur when the AC load has a net DC content (e.g., a half-wave rectifier). In view of the deficiencies of the current system, an improved system or device is needed to balance the neutral midpoint of the DC power bus when the AC output is unbalanced.

[0011] The present disclosure provides an apparatus, system, and method to balance a neutral midpoint between AC outputs in a welding-type system. Advantages of the disclosed apparatus, system, and method include the ability to balance a DC power bus midpoint without the need for a center-tapped generator or a Y-three-phase configuration. Further, a single boost circuit can be used to provide power to a DC power bus, rather than using a dual boost converter. Another advantage of the present disclosure is that the disclosed system simplifies the bidirectional converter required to convert power between a bus and an energy storage device in a hybrid power supply system because the balancing circuit balances the DC power bus midpoint, so no bidirectional converter is required to balance the DC power bus midpoint.

[0012] This system for providing balanced AC output power from a DC power bus is not necessarily limited to welding-type power systems. For example, the system can be used to provide balanced split-phase power or three-phase power from a DC power bus to any AC output that provides power to a load.

[0013] As used herein, the term "welding-type power" refers to power suitable for welding, plasma cutting, induction heating, CAC-A and / or hot wire welding / preheating (including laser welding and laser cladding). As used herein, the term "welding-type power supply" refers to any device capable of supplying power to welding, plasma cutting, induction heating, CAC-A and / or hot wire welding / preheating (including laser welding and laser cladding) when power is applied thereto, including but not limited to switch-mode power supplies, inverters, converters, resonant power supplies, quasi-resonant power supplies, etc., as well as control circuit systems and other auxiliary circuit systems associated therewith.

[0014] As used herein, "circuitry" includes any analog and / or digital components, power and / or control elements (such as a microprocessor, digital signal processor (DSP), software, etc.), discrete components and / or integrated components, or multiple parts and / or combinations thereof.

[0015] As used herein, the term "pulse welding" or "pulsed MIG welding" refers to a technique in which a pulsed electrical power waveform is generated, such as to control the deposition of molten metal droplets into an advancing weld puddle.

[0016] As used herein, the term "boost converter" is a converter used in a circuit to increase voltage. For example, a boost converter can be a step-up converter (such as a DC to DC power converter) that can step up voltage while stepping down current from its input (e.g., from a starting battery) to its output (e.g., a load and / or an attached power bus). This is a switch-mode power supply.

[0017] As used herein, the term "buck converter" (eg, step-down converter) refers to a power converter that steps down the voltage (eg, while simultaneously stepping up the current) from its input to its output.

[0018] As used herein, the term "memory" includes volatile and non-volatile memory, and may be an array, a database, a list, and the like.

[0019] As used herein, the term "welding torch" or "welding-type tool" may include a handheld or robotic welding torch, welding gun, or other device for generating a welding arc.

[0020] As used herein, the term "modulation" refers to turning a switch on and off multiple times to produce a desired output, such as pulse width modulation, pulse frequency modulation, hysteresis control, etc.

[0021] The disclosed example welding-type power system includes: a DC bus having a neutral midpoint; a first half-bridge inverter configured to convert power from the DC bus to a first AC power output coupled to the neutral midpoint; a second half-bridge inverter configured to convert power from the DC bus to a second AC power output coupled to the neutral midpoint, wherein the second AC power output has a phase relationship with the first AC power output; and a balancing circuit configured to provide a balanced output that balances the neutral midpoint of the DC bus. In some examples, the phase relationship is 0 degrees. In some examples, the phase relationship is 180 degrees. In some examples, the RMS voltage of the first AC power output is approximately equal to the RMS voltage of the second AC power output. In some examples, the RMS voltage of the first AC power output is approximately one of 100 volts, 115 volts, or 230 volts.

[0022] In some example welding-type power systems, the first half-bridge inverter includes a first switch and a second switch, the second half-bridge inverter includes a third switch and a fourth switch, and the balancing circuit is a half-bridge circuit including a fifth switch and a sixth switch.

[0023] Some example welding power systems further include a controller. In some examples, the controller is configured to: monitor one of the voltage of the DC bus, the DC bus midpoint voltage, the voltage of the first AC power output, or the voltage of the second AC power output; and adjust the duty cycle corresponding to at least one of the first switch, the second switch, the third switch, or the fourth switch to maintain at least one of the amplitude of the first AC power output voltage, the amplitude of the second AC power output voltage, or the phase relationship of the first AC power output and the second AC power output. In some example welding power systems, the controller is configured to: monitor the current through the first half-bridge inverter and the current through the second half-bridge inverter; and when the current through the first half-bridge inverter or the current through the second half-bridge inverter exceeds a threshold, adjust the duty cycle corresponding to at least one of the first switch, the second switch, the third switch, or the fourth switch.

[0024] In some example welding-type power systems, both the fifth switch and the sixth switch have an adjustable duty cycle. In some example welding-type power systems, a controller is included, the controller being configured to: monitor the current through the balancing circuit; and when the current through the balancing circuit exceeds a threshold, adjust the duty cycle of at least one of the fifth switch and the sixth switch. In some examples, when the neutral midpoint is half of the DC bus, the duty cycles of the fifth switch and the sixth switch are approximately equal.

[0025] In some examples, a welding-type power system includes a controller configured to: receive a DC bus midpoint voltage feedback signal indicating whether the DC bus midpoint voltage exceeds a first threshold or is below a second threshold; and modulate a fifth switch if the DC bus midpoint voltage exceeds the first threshold, and modulate a sixth switch if the DC bus midpoint voltage is below the second threshold. In some examples, the controller is further configured to monitor a current through the balancing circuit, and adjust modulation of the sixth switch or the fifth switch when the monitored current exceeds a threshold.

[0026] Some example welding-type power systems further include a bidirectional converter for converting power between the battery and the DC bus. Some example welding-type power systems further include a welding inverter configured to convert power from the DC bus to a welding-type output.

[0027] The disclosed example welding-type power system includes: a DC bus having a neutral midpoint; a first half-bridge inverter configured to convert power from the DC bus to a first AC power output coupled to the neutral midpoint; a second half-bridge inverter configured to convert power from the DC bus to a second AC power output coupled to the neutral midpoint; a third half-bridge inverter configured to convert power from the DC bus to a third AC power output coupled to the neutral midpoint; and a balancing circuit configured to provide a balanced output that balances the neutral midpoint of the DC bus, wherein the first AC output has a first phase relationship with the second AC output, and the first AC output has a second phase relationship with the third AC output. In some examples, the first phase relationship is approximately 120 degrees, and the second phase relationship is approximately 240 degrees. In some examples, the balancing circuit is a half-bridge circuit including a first switch and a second switch, and when the neutral midpoint is half of the DC bus, the duty cycles corresponding to the first switch and the second switch are each approximately 50%.

[0028] In some example welding-type power systems, the balancing circuit is a half-bridge circuit including a first switch and a second switch, and the first switch and the second switch are open during a balanced state. The welding-type power system further includes a controller, and the controller is configured to: receive a DC bus midpoint voltage feedback signal indicating whether the DC bus midpoint voltage exceeds a first threshold or is below a second threshold; and if the DC bus midpoint voltage exceeds the first threshold, modulate the first switch, and if the DC bus midpoint voltage is below the second threshold, modulate the second switch.

[0029] The disclosed example welding-type power system includes: a DC bus having a neutral midpoint; a first half-bridge inverter including a first switch and a second switch, the first half-bridge inverter being configured to convert power from the DC bus to a first AC power output coupled to the neutral midpoint; a second half-bridge inverter including a third switch and a fourth switch, the second half-bridge inverter being configured to convert power from the DC bus to a second AC power output coupled to the neutral midpoint, wherein the second AC power output has a phase relationship with the first AC power output; a balancing circuit including a fifth switch and a sixth switch, the balancing circuit being configured to provide a balanced output that balances the DC bus midpoint; and a controller configured to: receive a DC bus midpoint voltage feedback signal; and adjust modulation of the fifth switch or modulation of the sixth switch in response to the DC bus midpoint voltage feedback signal to balance the DC bus neutral midpoint. In some examples, the phase relationship is 180 degrees. In some examples, the phase relationship is 0 degrees.

[0030] In some example welding-type power systems, the controller is further configured to: determine whether the DC bus midpoint voltage feedback signal indicates that the DC bus midpoint voltage exceeds a first threshold or is below a second threshold; when the DC bus midpoint voltage exceeds the first threshold, adjust the modulation of the fifth switch and the sixth switch to reduce the DC bus midpoint voltage; and when the DC bus midpoint voltage is below the second threshold, adjust the modulation of the fifth switch and the sixth switch to increase the DC bus midpoint voltage. In some examples, the controller sets the modulation of one of the first switch, the second switch, the third switch, the fourth switch, the fifth switch, or the sixth switch corresponding to a lookup table of modulation values.

[0031] Figure 1 is a block diagram of a welding-type power system 100. Figure 1 , solid lines correspond to power connections, while dashed lines correspond to data connections. In some examples, the power connections may also carry data. Example power system 100 includes a power supply 102 that feeds a DC power bus (DC bus) 104. In some examples, power supply 102 may include a variable speed motor 106 connected to a generator 108. In some examples, power supply 102 may feed DC power bus 104 directly through rectifier 110. In other examples, power supply 102 may feed DC power bus 104 through boost converter 110.

[0032] In some examples, the bidirectional converter 112 converts power between the DC power bus 104 and a battery 114 or other energy storage device. In some examples, the DC power bus 104 has a split capacitor, where the auxiliary power circuit obtains power from the DC power bus 104 and has a neutral connection connected to the split capacitor and provides auxiliary power, thereby allowing a user to plug in a device (e.g., lighting, small power tools, etc.) to get auxiliary power, while the total DC power bus 104 can feed the welding inverter 116 that supplies the welding-type power output 120. In some examples, the bidirectional converter 112 can be configured to balance the split capacitor DC power bus 104. In some examples, the bidirectional converter 112 can be a three-level bidirectional converter.

[0033] The controller 122 is connected to the power supply 102, the DC power bus 104, the bidirectional converter 112, and the welding inverter 116. The welding inverter 116 provides a welding-type power output 120. The controller 122 is also connected to auxiliary inverters 124, 126, and 128. The auxiliary inverters 124, 126, and 128 generate AC outputs 130, 132, and 134, respectively.

[0034] In some examples, as described in more detail below, the auxiliary inverter is a half-bridge inverter that runs a PWM sine wave sequence to generate an AC output. In some examples, there are two auxiliary inverters 124 and 126, each of which generates an AC output with equal voltage and out of phase. For example, the output from the auxiliary inverters 124 and 126 can be 180 degrees out of phase. For example, each auxiliary inverter 124 and 126 can output 115VAC with 180 degrees out of phase, so that the output across the auxiliary inverters 124 and 126 is measured to be 230VAC. In some examples, each auxiliary inverter 124 and 126 can output 220VAC with 180 degrees out of phase, so that the output across the auxiliary inverters 124 and 126 is measured to be 440VAC. In some examples, each auxiliary inverter 124 and 126 may output 100 VAC with 180 degrees out of phase, such that 200 VAC will be measured across the outputs of the auxiliary inverters 124 and 126. In some examples, each auxiliary inverter 124 and 126 may output 230 VAC with 180 degrees out of phase, such that 460 VAC will be measured across the outputs of the auxiliary inverters 124 and 126. The auxiliary inverters 124 and 126 share a neutral line.

[0035] The welding-type power system 100 also includes a balancing circuit 136. The balancing circuit 136 is also connected to the controller 122. The balancing circuit 136 can be actively controlled to force the neutral line shared by the auxiliary inverters (124 and 126) to be the midpoint between the AC outputs of the auxiliary inverters (124 and 126). The neutral line is connected to the center point of the DC power bus 104, which includes split capacitors. When the neutral midpoint reaches balance, the center point of the DC power bus 104 should be 1 / 2 of the total voltage VBus across the DC power bus 104. The neutral midpoint can be connected to the chassis, the ground, the metal frame of the welding-type power system 100, as a return path in a complete circuit, etc. In some examples described in more detail below, the balancing circuit 136 is a half-bridge inverter.

[0036] In some examples, the welding-type power system 100 includes three auxiliary inverters 124, 126, and 128, each of which generates an AC output (respectively 130, 132, and 134) with equal voltage and 120 degrees out of phase (the first AC output 130 is 120 degrees out of phase with the second AC output 132, and the first AC output 130 is 240 degrees out of phase with the third AC output 134). The auxiliary inverters (124, 126, 128) share a neutral line. The welding-type power system 100 also includes a balancing circuit 136. The balancing circuit 136 is actively controlled by the controller 122 to force the neutral line shared by the auxiliary inverters (124, 126, 128) to become a midpoint between the AC outputs of the auxiliary inverters (124, 126, 128). In some examples, the neutral midpoint is controlled at a voltage level of 1 / 2 of the total voltage VBus across the DC power bus 104.

[0037] Figure 21 is a circuit diagram of an exemplary welding-type power system, which includes a power supply 102, a boost converter 110 for converting power from the power supply 102 to a split capacitor DC power bus 104, a first half-bridge inverter 124, a second half-bridge inverter 126, and a balancing circuit 136. The first half-bridge inverter 124 runs a PWM sine wave sequence to generate a first AC output 130 from the DC power bus 104. The second half-bridge inverter 126 runs a PWM sine wave sequence to generate a second AC output 132 from the DC power bus 104. Each of the first half-bridge inverter 124 and the second half-bridge inverter 126 is connected to a controller 122. The controller 122 controls the duty cycle of the PWM sequence of each of the half-bridge inverters 124 and 126 to control the AC outputs 130 and 132. In some examples, the controller 122 can set the duty cycle via a duty cycle lookup table to obtain the desired sine wave sequence for each AC output 130 and 132. In some examples, the generated AC outputs (130 and 132) operate at 60 Hz. In some examples, the generated AC outputs (130 and 132) operate at 50 Hz.

[0038] In some examples, the AC outputs (130 and 132) have equal amplitude and are out of phase, which means that each AC output is 180 degrees out of phase with the other AC output. When these AC outputs are out of phase, the voltage amplitude measured across the two AC outputs is twice the voltage amplitude of each AC output. For example, the voltage amplitude of the AC outputs 130 and 132 can each be 115VAC. If the phases of the AC outputs (130 and 132) are 180 degrees out of phase, the voltage measured across the two AC outputs 130 and 132 is 230VAC. In some examples, the AC outputs 130 and 132 are in phase, so that the phase difference between the AC outputs is 0 degrees. In other examples, the AC outputs 130 and 132 can have any other selected phase relationship.

[0039] Each of the half-bridge inverters 124 and 126 includes two switches. The first half-bridge inverter 124 includes switches 202 and 204. The second half-bridge inverter includes switches 206 and 208. The controller 122 can control the duty cycle of each of the switches 202, 204, 206, and 208 to control the PWM sine wave sequence of the first half-bridge inverter 124 and the PWM sine wave sequence of the second half-bridge inverter 126. In some examples, the controller 122 can set the duty cycle via a duty cycle lookup table to obtain a desired sine wave sequence.

[0040] The AC outputs 130 and 132 share a neutral midpoint 210, which is also connected to the neutral midpoint 210 of the split capacitor DC power bus 104. If either of the AC outputs (130 or 132) is connected to a load with net DC content (e.g., a half-wave rectifier), the neutral midpoint 210 may become unbalanced. This will cause the voltages across the capacitors 216 and 218 of the split capacitor DC power bus 104 to be unequal.

[0041] To avoid a situation where the neutral midpoint 210 is unbalanced, the output of the balancing circuit 136 is also connected to the neutral midpoint 210 so as to actively balance the neutral midpoint 210 to the midpoint (VBus / 2) of the voltage across the DC power bus 104. The balancing circuit 136 has a half-bridge topology and includes a first switch 212 and a second switch 214. The balancing circuit 136 is connected to the controller 122. The controller 122 can control the output of the balancing circuit 136 so as to maintain the neutral midpoint 210 at the midpoint (VBus / 2) of the split capacitor DC power bus 104. The controller 122 controls the output of the balancing circuit 136 by controlling the modulation applied to each of the first switch 212 and the second switch 214. The modulation includes turning on and off a switch (e.g., switch 212 or 214). In some examples, the modulation includes setting a duty cycle for the switch (e.g., switch 212 or 214). In some examples, the modulation includes applying a PWM signal to the switch (e.g., switch 212 or 214). In some examples, modulation includes repeatedly turning a switch (eg, switch 212 or 214) on and off at a set frequency. In some examples, controller 122 can use hysteresis control of modulation of switches 212 and 214 to balance neutral midpoint 210 at VBus / 2.

[0042] In a first aspect, the balancing circuit 136 can operate with a complementary PWM signal centered at approximately 50%. In this first aspect, the balancing circuit 136 actively balances the neutral midpoint 210 at all times. This aspect provides a natural open-loop response that effectively keeps the neutral midpoint 210 of the DC power bus 104 balanced. In some examples, the controller 122 can monitor the total voltage VBus across the split capacitor DC power bus 104 and compare the voltage at the neutral midpoint 210 to VBus / 2. In some examples, the controller can receive a DC power bus 104 midpoint feedback signal indicating whether the neutral midpoint 210 voltage is within a threshold range of VBus / 2. If the voltage at the neutral midpoint 210 is not within the threshold range of VBus / 2, the controller 122 can adjust the PWM signal applied to the balancing circuit 136 to bring the voltage at the neutral midpoint 210 to VBus / 2. For example, if the voltage at the neutral midpoint 210 is too high, meaning that the voltage at the neutral midpoint is higher than VBus / 2 by a threshold amount, the controller 122 may increase the pulse width of the lower switch 214 and decrease the pulse width of the upper switch 212. Similarly, if the voltage at the neutral midpoint 210 is too low, meaning that the voltage at the neutral midpoint is lower than VBus / 2 by a threshold amount, the controller 122 may increase the pulse width of the upper switch 212 and decrease the pulse width of the lower switch 214. In some examples, the controller 122 may monitor the current through the balancing circuit 136 and decrease the pulse width of either or both switches 212 and 214 when the magnitude of the current through the balancing circuit 136 exceeds a threshold value so as to reduce the magnitude of the current through the balancing circuit 136.

[0043] In a second aspect, the two switches (212, 214) of the balancing circuit 136 are normally open during the equilibrium state. For example, when the voltage at the neutral midpoint 210 is within a threshold percentage of VBus / 2, the equilibrium state exists. The controller can receive feedback indicating whether the voltage at the neutral midpoint 210 is within a threshold percentage of VBus / 2. For example, the controller 122 can monitor the voltage at the neutral midpoint 210 and compare the voltage at the neutral midpoint 210 to VBus / 2 to determine whether the equilibrium state exists. In this second aspect, if the neutral midpoint 210 voltage is too low (lower than VBus / 2 by a threshold amount), the controller 122 can modulate the upper switch 212, and if the neutral midpoint 210 voltage is too high (higher than VBus / 2 by a threshold amount), the controller 122 can modulate the lower switch 214. The controller 122 can modulate the switch (e.g., 212 or 214), for example, by turning the switch on and then off, turning the switch on and off at a set frequency, applying a PWM signal to the switch, etc. Modulating only the upper switch 212 (e.g., by applying a PWM signal to the upper switch 212) in the case where the neutral midpoint 210 is too low will increase the neutral midpoint 210 voltage, thereby returning the neutral midpoint 210 voltage to VBus / 2. Modulating only the lower switch 214 (e.g., by applying a PWM signal to the lower switch 214) in the case where the neutral midpoint 210 is too high will decrease the neutral midpoint 210, thereby returning the neutral midpoint 210 to VBus / 2.

[0044] In the second aspect, in some examples, the controller 122 monitors the neutral midpoint 210 after each operating cycle of the half-bridge inverters 124 and 126, and adjusts the modulation of the switches 212 and 214 as needed to balance the voltage at the neutral midpoint 210 to VBus / 2. For example, the controller 122 can adjust the PWM signals applied to the switches 212 and / or 214 as needed after each cycle of the half-bridge inverters 124 and 126 to balance the neutral midpoint 210 voltage to VBus / 2. In some examples, the controller 122 can monitor the current through the balancing circuit 136, and adjust the modulation applied to both or either of the switches 212 and 214 when the magnitude of the current through the balancing circuit 136 exceeds a threshold value to reduce the magnitude of the current through the balancing circuit 136.

[0045] In some examples, in response to determining that the neutral midpoint 210 voltage is too high or too low, the controller 122 may apply a PWM signal to the corresponding switch 212 or 214 for a predetermined time period to correct the imbalance of the neutral midpoint 210. The predetermined time period may be a number of PWM cycles applied to the balancing circuit 136. In some examples, the predetermined time period is determined by how much the measured neutral midpoint 210 voltage differs from the VBus / 2 equilibrium point. For example, if the measured neutral midpoint 210 voltage is much lower than the VBus / 2 point, more PWM cycles will be applied to the first switch 212 of the balancing circuit 136 than if the measured neutral midpoint 210 voltage is closer to the VBus / 2 equilibrium point. Alternatively, the controller 122 may increase the PWM pulse width of the first switch 212 or the second switch 214 more for a greater deviation of the detected neutral midpoint 210 voltage from VBus / 2 than for a smaller deviation of the neutral midpoint 210 voltage from the VBus / 2 equilibrium point.

[0046] In some examples, in response to determining that the voltage at the neutral midpoint 210 is too high or too low, the controller 122 may modulate the corresponding switch 212 or 214 , for example by applying a PWM signal to the switch 212 or 214 , until the controller 122 determines that the neutral midpoint 210 is within an acceptable threshold of VBus / 2.

[0047] In some examples, the controller 122 may use hysteresis control to balance the neutral midpoint 210 to VBus / 2. When using hysteresis control, the controller 122 adjusts the qualitative on-time of the upper switch 212 and / or the lower switch 214. For example, when using hysteresis control, in order to increase the neutral midpoint 210 voltage in response to detecting that the neutral midpoint 210 voltage is less than VBus / 2, the controller 122 will turn on the upper switch 212 for a longer time than the lower switch 214 in a given time period. Similarly, in order to reduce the neutral midpoint 210 voltage in response to detecting that the voltage at the neutral midpoint 210 exceeds VBus / 2, the controller 122 will turn on the lower switch 214 for a longer total time than the upper switch 212 in a given time period.

[0048] In some examples, the controller 122 may also monitor the current of the AC outputs 130 and 132. If the current magnitude of the AC output 130 exceeds a threshold, the controller 122 may reduce the PWM pulse width of the switches 202 and / or 204 to reduce the current of the AC output 130. Similarly, if the current magnitude of the AC output 132 exceeds a threshold, the controller 122 may reduce the PWM pulse width of the switches 206 and / or 208 to reduce the current of the AC output 132. If the current through the balancing circuit 136 exceeds a threshold, the controller 122 may reduce the pulse width of the switches 212 and 214 to reduce the current through the balancing circuit 136.

[0049] In some examples, the controller 122 can monitor VBus, the DC bus midpoint voltage, one of the first AC output voltage 130, or the second AC power output voltage 132 to determine whether one of the amplitude of the first AC output 130, the second AC output 132, or the phase relationship between the AC outputs (130 and 132) is outside a threshold acceptable range. In response to determining that one of the amplitude of the first AC output 130, the amplitude of the second AC output 132, or the phase relationship between the AC outputs (130 and 132) is outside the acceptable threshold range, the controller 122 can adjust the pulse width of any of the switches 202, 204, 206, or 208 to maintain the amplitude of the first AC output 130, the amplitude of the second AC output 132, or the phase relationship between the AC outputs (130 and 132). The controller 122 can adjust the pulse width of the switches 202 to 208 via a duty cycle lookup table.

[0050] Figure 3 A flow chart of an exemplary method 300 for actively balancing the midpoint 210 of the DC power bus 104 according to the above system is shown. At block 302, the controller 122 measures the voltage at the midpoint 210 of the DC power bus 104. At block 304, the controller 122 compares the measured voltage at the midpoint 210 to VBus / 2. In some examples, the controller 122 can measure the voltage across the DC power bus 104 and compare the measured voltage across the DC power bus 104 to the measured voltage at the midpoint 210. In some examples, the value of VBus can be programmed into a memory of the controller 122, and the controller 122 can compare the measured voltage at the midpoint 210 to the programmed VBus value.

[0051] If the measured voltages are balanced, meaning that the measured voltage at the DC power bus midpoint 210 is within a threshold amount of VBus / 2, the controller 122 returns to block 302 to monitor the voltage at the midpoint 210 again. In some examples, the controller 122 monitors the voltage at the DC power bus midpoint 210 after each PWM cycle of the inverter (124, 126, 128). In some examples, if the measured voltages are balanced, then in a complementary arrangement, the PWM signals applied to the first switch 212 and the second switch 214 will remain at approximately 50%. In some examples, if the measured voltages are balanced, the first switch 212 and the second switch 214 will remain open.

[0052] If the measured voltage is too low, meaning that the voltage at the DC power bus midpoint 210 is below VBus / 2 by a threshold amount, the controller 122 proceeds to block 306. At block 306, the controller 122 determines how much below VBus / 2 the measured voltage at the midpoint 210 is. The controller 122 determines the PWM signal so that the duty cycle of the PWM signal applied to the first switch 212 is increased and the duty cycle of the PWM signal applied to the second switch 214 is decreased, and the controller 122 determines the number of cycles to apply the PWM signal to the first switch 212 and the second switch 214. In some examples, the duty cycle applied to the second switch 214 may be set to zero, meaning that the switch 214 may remain open. At block 308, the controller 122 applies the PWM signal to the first switch 212 and the second switch 214, which increases the voltage at the midpoint 210. Increasing the voltage at the midpoint 210 balances the midpoint because the controller 122 determined at block 304 that the voltage at the midpoint is too low. After applying the PWM signal at block 308 , the controller 122 returns to block 302 and monitors the DC power bus midpoint 210 voltage.

[0053] If the measured voltage is too high, meaning that the voltage at the DC power bus midpoint 210 is greater than VBus / 2 by a threshold amount, the controller 122 proceeds to block 310. At block 310, the controller 122 determines how much greater the measured midpoint 210 voltage is than VBus / 2. The controller 122 determines the PWM signal so that the duty cycle of the PWM signal applied to the second switch 214 is increased and the duty cycle of the PWM signal applied to the first switch 212 is decreased, and the controller 122 determines the number of cycles to apply the PWM signal to the first switch 212 and the second switch 214. In some examples, the duty cycle applied to the first switch 212 may be set to zero, meaning that the switch 212 may remain open. At block 312, the controller 122 applies the PWM signal to the first switch 212 and the second switch 214, which reduces the voltage at the midpoint 210. Reducing the voltage at the midpoint 210 balances the midpoint because the controller 122 determined at block 304 that the voltage at the midpoint is too high. After applying the PWM signal at block 312 , the controller 122 returns to block 302 and monitors the DC power bus midpoint 210 voltage.

[0054] As mentioned above about Figure 1 As described, in some examples, the welding-type power system may include three auxiliary inverters 124, 126, and 128, each of which generates an AC output (130, 132, and 134, respectively). In such an example, each of the auxiliary inverters 124, 126, and 128 may be a half-bridge inverter connected to the controller 122, similar to the description of FIG. Figure 2 The circuit described. In such an example, each of the half-bridge inverters 124, 126, and 128 runs a PWM sine wave to generate an AC output (130, 132, and 134, respectively). Each AC output is 120 degrees out of phase with the other AC outputs. Each AC output (130, 132, and 134) shares a neutral line connected to the neutral midpoint 210. As described above, the balancing circuit 136 can actively balance the neutral midpoint 210 to deal with any DC distortion on the AC outputs (130, 132, and 134) that may cause the neutral midpoint 210 of the DC power bus 104 to become unbalanced.

[0055] The present method and system can be implemented with hardware, software, and / or a combination of hardware and software. The present method and / or system can be implemented in a centralized manner in at least one computing system, or in a distributed manner with different elements spread over several interconnected computing systems. Any type of computing system or other device suitable for executing the method described herein is suitable. A typical combination of hardware and software may include a general computing system with a program or other code, which controls the computing system when loaded and executed so that the computing system executes the method described herein. Another embodiment may include a dedicated integrated circuit or chip. Some embodiments may include a non-temporary machine-readable (e.g., computer-readable) medium (e.g., a flash drive, an optical disk, a magnetic storage disk, etc.), which stores one or more lines of code executable by a machine, so that the machine executes the process described herein. As used herein, the term "non-temporary machine-readable medium" is defined as including all types of machine-readable storage media and excluding propagation signals.

[0056] As used herein, the terms "circuit" and "circuitry" refer to physical electronic components, any analog and / or digital components, power elements and / or control elements, such as microprocessors or digital signal processors (DSPs), including discrete components and / or integrated components, or parts and / or combinations thereof (i.e., hardware), and any software and / or firmware ("code") that can configure, be executed by, and / or otherwise associated with hardware. As used herein, for example, a particular processor and memory may constitute a first "circuit" when executing a first line or more lines of code, and may constitute a second "circuit" when executing a second line or more lines of code. As used herein, "and / or" refers to any one or more of the multiple items connected by "and / or" in a list. For example, "x and / or y" refers to any element in the three-element set {(x), (y), (x, y)}. In other words, "x and / or y" refers to "one or both of x and y". As another example, "x, y, and / or z" refers to any element in the seven-element set {(x), (y), (z), (x,y), (x,z), (y,z), (x,y,z)}. In other words, "x, y, and / or z" refers to "one or more of x, y, and z." As used herein, the term "exemplary" refers to serving as a non-limiting example, instance, or legend. As used herein, the terms "eg" and "for example" list a list of one or more non-limiting examples, instances, or legends. As used herein, a circuit system is "capable of operating" to perform a function when the circuit system includes the hardware and code (if necessary) necessary to perform the function, regardless of whether the performance of the function is disabled or not enabled (e.g., by user-configurable settings, factory adjustments, etc.).

[0057] Although the present method and / or system has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and may be substituted with equivalents without departing from the scope of the present method and / or system. For example, the frames and / or components of the disclosed examples may be combined, split, rearranged and / or otherwise modified. In addition, many modifications may be made to adapt specific circumstances or materials to the teachings of the present disclosure without departing from the scope of the present disclosure. Therefore, the present method and / or system is not limited to the specific embodiments disclosed. Alternatively, the present method and / or system will include all embodiments that fall within the scope of the appended claims, both literally and according to the doctrine of equivalents.

Claims

1. A welding power system, comprising: A DC bus with a neutral midpoint; a first half-bridge inverter configured to convert power from the DC bus to a first AC power output coupled to the neutral midpoint; a second half-bridge inverter configured to convert power from the DC bus to a second AC power output coupled to the neutral midpoint, wherein the second AC power output has a phase relationship with the first AC power output; a balancing circuit configured to provide a balanced output balancing a neutral midpoint of the DC bus, wherein the balancing circuit is a half-bridge circuit including a first switch and a second switch; and A controller, the controller being configured to: receiving a DC bus midpoint voltage feedback signal, the DC bus midpoint voltage feedback signal indicating whether the DC bus midpoint voltage exceeds a first threshold or is below a second threshold; and If the DC bus midpoint voltage exceeds the first threshold, the first switch is modulated, and if the DC bus midpoint voltage is below the second threshold, the second switch is modulated.

2. The welding type electric power system according to claim 1, wherein: The phase relationship is one of 0 degrees or 180 degrees.

3. The welding type electric power system according to claim 2, wherein: An RMS voltage of the first AC power output is equal to an RMS voltage of the second AC power output.

4. The welding type electric power system according to claim 3, wherein: The RMS voltage of the first AC power output is one of 100 volts, 115 volts, or 230 volts.

5. The welding type electric power system according to claim 1, wherein: The first half-bridge inverter includes a third switch and a fourth switch, and the second half-bridge inverter includes a fifth switch and a sixth switch.

6. The welding-type electric power system of claim 5, further comprising: A controller, the controller being configured to: monitoring one of a voltage of the DC bus, a midpoint voltage of the DC bus, a voltage of the first AC power output, or a voltage of the second AC power output; and Adjusting a duty cycle corresponding to at least one of the third switch, the fourth switch, the fifth switch, or the sixth switch to maintain at least one of the amplitude of the first AC power output voltage, the amplitude of the second AC power output voltage, or a phase relationship between the first AC power output and the second AC power output.

7. The welding-type electric power system according to claim 6, wherein: The controller is further configured to: monitoring a current through the first half-bridge inverter and a current through the second half-bridge inverter; and When the current through the first half-bridge inverter or the current through the second half-bridge inverter exceeds a threshold, a duty cycle corresponding to at least one of the third switch, the fourth switch, the fifth switch, or the sixth switch is adjusted.

8. The welding type electric power system according to claim 1, wherein: Both the first switch and the second switch have an adjustable duty cycle, and the welding-type power system further includes a controller configured to: monitoring current through the balancing circuit; and When the current through the balancing circuit exceeds a threshold, a duty cycle of at least one of the first switch and the second switch is adjusted.

9. The welding-type electric power system according to claim 8, wherein: When the neutral midpoint is half of the DC bus, the duty cycles of the first switch and the second switch are equal.

10. The welding-type electric power system according to claim 1, wherein: The first switch and the second switch are open during an equilibrium state.

11. The welding-type electric power system according to claim 10, wherein: The controller is further configured to monitor current through the balancing circuit and adjust modulation of the first switch or the second switch when the monitored current exceeds a threshold.

12. The welding-type power system of claim 1, further comprising at least one of the following: a bidirectional converter for converting power between a battery and the DC bus; or A welding inverter is configured to convert power from the DC bus to a welding-type output.

13. An electric power system comprising: A DC bus with a neutral midpoint; a first half-bridge inverter configured to convert power from the DC bus to a first AC power output coupled to the neutral midpoint; a second half-bridge inverter configured to convert power from the DC bus to a second AC power output coupled to the neutral midpoint; a third half-bridge inverter configured to convert power from the DC bus to a third AC power output coupled to the neutral midpoint; a balancing circuit configured to provide a balanced output balancing a neutral midpoint of the DC bus, wherein the balancing circuit is a half-bridge circuit including a first switch and a second switch; and A controller, the controller being configured to: receiving a DC bus midpoint voltage feedback signal, the DC bus midpoint voltage feedback signal indicating whether the DC bus midpoint voltage exceeds a first threshold or is below a second threshold; and modulating the first switch if the DC bus midpoint voltage exceeds the first threshold, and modulating the second switch if the DC bus midpoint voltage is below the second threshold, The first AC power output has a first phase relationship with the second AC power output, and the first AC power output has a second phase relationship with the third AC power output.

14. The power system according to claim 13, wherein: The first phase relationship is 120 degrees, and the second phase relationship is 240 degrees.

15. The power system according to claim 14, wherein: The balancing circuit is a half-bridge circuit including a first switch and a second switch, and wherein, when the neutral midpoint is half of the DC bus, duty ratios corresponding to the first switch and the second switch are each 50%.

16. The power system according to claim 14, wherein: The first switch and the second switch are open during an equilibrium state.

17. An electric power system comprising: A DC bus with a neutral midpoint; a first half-bridge inverter, the first half-bridge inverter comprising a first switch and a second switch, the first half-bridge inverter being configured to convert power from the DC bus to a first AC power output coupled to the neutral midpoint; a second half-bridge inverter, the second half-bridge inverter comprising a third switch and a fourth switch, the second half-bridge inverter being configured to convert power from the DC bus to a second AC power output coupled to the neutral midpoint, wherein the second AC power output has a phase relationship with the first AC power output; a balancing circuit, the balancing circuit comprising a fifth switch and a sixth switch, the balancing circuit being configured to provide a balanced output balancing a neutral midpoint of the DC bus; and A controller, the controller being configured to: receiving a DC bus midpoint voltage feedback signal; and determining whether the DC bus midpoint voltage feedback signal indicates that the DC bus midpoint voltage exceeds a first threshold or is below a second threshold; When the DC bus midpoint voltage exceeds the first threshold, adjusting the modulation of the fifth switch and the sixth switch to reduce the DC bus midpoint voltage; and When the DC bus midpoint voltage is lower than the second threshold, modulation of the fifth switch and the sixth switch is adjusted to increase the DC bus midpoint voltage.

18. The power system according to claim 17, wherein: The phase relationship is one of 0 degrees or 180 degrees.

19. The power system of claim 17, wherein: The controller sets modulation of one of the first switch, the second switch, the third switch, the fourth switch, the fifth switch, or the sixth switch corresponding to a lookup table of modulation values.

20. The power system of claim 18, wherein: The controller is further configured to: monitoring current through the balancing circuit; and When the current passing through the balancing circuit exceeds a threshold, a duty cycle of at least one of the fifth switch and the sixth switch is adjusted.

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