Patent Description:
A type of conventional welding-type power supply that is well suited for portability and for receiving different input voltages is a multi-stage system with a pre -regulator to condition the input power and provide a stable bus, and an output circuit that converts or transforms the stable bus to a welding-type output. Such conventional welding-type power supplies using transformers that are subject to magnetic saturation, which may be referred to as a volt-second rating. If the transformer is saturated, the system can become unusable.

<CIT> discloses a welding-type power supply includes a controller, bus, and an output converter. The controller has a preregulator control output and an output converter control output. The controller may receive bus feedback indicative of a plurality of bus voltages. A bus voltage balancing module in the converter includes a scaled correction module responsive to the bus feedback signal, and the converter control output is responsive to the bus voltage balancing module. The controller may receive load feedback indicative of a load output, have a bus voltage balancing module that includes a load proportional gain module responsive to the load.

<CIT> discloses a method and apparatus for detecting an impending saturation in nonlinear magnetic material for particular operating conditions, such as in a core of a power transformer of a switched-mode converter, in response to a varying magnetic field in a principal direction induced by a drive current through its primary winding with its axis aligned in the principal direction. A transverse flux sense winding is used in monitoring the rate of change of flux density of a transverse magnetic field to produce a voltage proportional thereto. That voltage is compared in a comparator with a predetermined threshold voltage characteristic of impending saturation of the core. The output of the comparator is used for controlling drivers to prevent saturation of the switched-mode converter bias switching the drivers off alternately when the sensed voltage reaches the predetermined threshold set at the comparator.

The invention refers to a welding-type power supply according to claim <NUM> and a corresponding method to control a welding-type power supply according to claim <NUM>.

Conventional welding-type power supplies use one or more of the following methods to avoid saturation of the high-frequency transformer: <NUM>) an instantaneous flux limit that restricts the duty cycle in either polarity to an upper limit; <NUM>) flux balancing, which limits how quickly the flux applied is able to change to keep the positive and negative cycles closer to balancing on an instantaneous basis to avoid exceeding the volt-second rating in one direction; and flux centering, in which the magnetic flux applied to the transformer is continually summed during each switching period. Flux centering acts to modify the positive and negative duty cycles over time to maintain the accumulated flux near zero.

One shortcoming of conventional flux centering is that the method assumes that one-half of the bus voltage is applied in both the positive and negative direction. Under some transient conditions, for example when a large load is applied or removed, the voltage across the series capacitor may no longer equal one-half of the bus voltage, and asymmetrical voltage can be applied without knowledge of the flux centering logic. For example, if the bus is at 600V, and the series cap is at 310V, the voltage applied to the transformer during the positive half cycle is 290V, and during the negative cycle it is 310V. This unbalanced voltage results in a volt-second mismatch applied to the transformer. A conventional flux accumulator assumes that 300V is applied in both directions, so the flux accumulator does not identify the voltage imbalance. If the capacitor remains imbalanced for several PWM cycles, a net volt-second imbalance can accumulate and the transformer can be driven into saturation, resulting in an error condition and/or unexpected shutdown of the welding power supply. The present disclosure describes systems and methods for detecting transient conditions that may cause asymmetrical voltage across the series capacitor. The present disclosure also describes systems and methods for responding to such transient conditions by balancing the voltage across the series capacitor.

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, when power is applied thereto, supplying welding, plasma cutting, induction heating, CAC-A and/or hot wire welding/preheating (including laser welding and laser cladding) power, including but not limited to inverters, converters, resonant power supplies, quasi-resonant power supplies, and the like, as well as control circuitry and other ancillary circuitry associated therewith.

As used herein, the term "welding-type voltage" refers to a voltage 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 "positive current" through a transformer refers to a current flowing in a first direction, and the term "negative current" through the transformer refers to a current in a second direction opposite the first direction.

Some examples involve a welding-type power supply comprising a switched mode power supply, comprising: a transformer configured to transform a bus voltage to a welding-type voltage; a capacitor in series with a primary winding of the transformer, the capacitor having a capacitor voltage; and switches configured to control a voltage applied to a series combination of the primary winding of the transformer and the capacitor; and a controller configured to: detect a transient condition in which the bus voltage exceeds a threshold voltage; and control duty cycles of the switches in response to the transient condition to adjust the capacitor voltage based on the bus voltage.

In some examples, the welding-type power supply further includes a flux accumulator configured to determine a net flux in the transformer based on a number of volt-seconds applied to the primary winding of the transformer, and wherein the controller is further configured to control the duty cycles of the switches to balance the net flux when the capacitor voltage is within a threshold voltage range based on the bus voltage.

In some examples, the controller is configured to detect the transient condition based on comparing one of the welding-type voltage, the bus voltage, or the capacitor voltage to a transient threshold voltage. In some examples, the controller is configured to adjust the duty cycles of the switches to unbalance a net flux in the transformer for a threshold time period to adjust the capacitor voltage in response to the transient condition. In some examples, the threshold time period is approximately two milliseconds. In some examples, the controller is further configured to adjust the duty cycles of the switches to balance the net flux in response to at least one of elapsing of the threshold time period or determining that the capacitor voltage is within a threshold voltage range based on the bus voltage.

In some examples, the controller is configured to adjust the capacitor voltage by at least one of: increasing a first duty cycle corresponding to a first period during which a voltage applied to the primary winding is equal to the difference between the bus voltage and the capacitor voltage, or decreasing a second duty cycle corresponding to a second period during which the voltage applied to the primary winding is equal to the capacitor voltage.

In some examples, the controller is configured to adjust the duty cycles of the switches until the capacitor voltage is within a threshold range of one-half of the bus voltage. In some examples, the controller is configured to detect the transient condition by detecting at least one of: a first threshold change in the bus voltage in less than a threshold time period; a second threshold change in the welding-type voltage in less than the threshold time period; or a third threshold change in the capacitor voltage in less than the threshold time period.

In some examples, the threshold time period is less than two switching cycles, each switching cycle comprising a positive current through the primary of the transformer and a negative current through the primary of the transformer. In some examples, the switched mode power supply comprises a stacked full bridge topology.

Some examples involve a welding-type power supply, comprising: a transformer configured to transform a bus voltage to a welding-type voltage; a capacitor in series with a primary winding of the transformer, the capacitor having a capacitor voltage; and switches configured to control a voltage applied to a series combination of the primary winding of the transformer and the capacitor; and a controller configured to: control duty cycles of the switches; detect a transient condition in which the bus voltage exceeds a threshold voltage; and in response to detecting the transient condition, control a capacitor voltage balancing circuit to adjust the capacitor voltage based on the bus voltage.

In some examples, the capacitor voltage balancing circuit is configured to regulate the capacitor voltage within a predetermined range that is based on the bus voltage. In some examples, the capacitor voltage balancing circuit comprises a first switching element and a second switching element coupled in series and configured to selectively couple the capacitor to a reference voltage, the controller configured to control the first switching element and the second switching element to couple the capacitor to the reference voltage in response to detecting the transient condition. In some examples, the capacitor voltage balancing circuit comprises a combination of a diode and a transistor in series, and configured to selectively couple the capacitor to a reference voltage, the controller configured to control the transistor to couple the capacitor to the reference voltage in response to detecting the transient condition.

Some examples involve a method to control a welding-type power supply, comprising: controlling, via control circuitry, switches of a stacked full bridge switched mode power supply to provide current to a primary winding of a transformer to generate a welding-type current output via a secondary winding of the transformer, the primary winding being in series with a capacitor having a capacitor voltage; detecting, via the control circuitry, a transient condition on a bus voltage input to the stacked full bridge switched mode power supply; and in response to detecting the transient condition, controlling duty cycles of the switches via the control circuitry to adjust the capacitor voltage based on the bus voltage.

In some examples, the method further involves determining, via a flux accumulator, a net flux in the transformer based on a number of volt-seconds applied to the primary winding of the transformer, and controlling, via the control circuitry, the duty cycles of the switches to balance the net flux when the capacitor voltage is within a threshold voltage range based on the bus voltage.

In some examples, the circuitry detects the transient condition based on comparing one of the welding-type voltage, the bus voltage, or the capacitor voltage to a transient threshold voltage.

In some examples, the method further comprises adjusting, via control circuitry, the duty cycles of the switches to unbalance a net flux in the transformer for a threshold time period to adjust the capacitor voltage in response to the transient condition. In some examples, the method further comprises adjusting, via control circuitry, the duty cycles of the switches to balance the net flux in response to at least one of elapsing of the threshold time period or determining that the capacitor voltage is within a threshold voltage range based on the bus voltage.

In some examples, the capacitor voltage is adjusted in response to detecting the transient condition by at least one of: increasing a first duty cycle corresponding to a first period during which a voltage applied to the primary winding is equal to the difference between the bus voltage and the capacitor voltage, or decreasing a second duty cycle corresponding to a second period during which the voltage applied to the primary winding is equal to the capacitor voltage. In some examples, the control circuitry detects the transient condition by detecting at least one of: a first threshold change in the bus voltage in less than a threshold time period; a second threshold change in the welding-type voltage in less than the threshold time period; or a third threshold change in the capacitor voltage in less than the threshold time period.

<FIG> is a block diagram of an example welding-type power supply <NUM>, including a switched mode power supply <NUM>, configured to estimate magnetic flux in a transformer of the switched mode power supply <NUM>. The example welding-type power supply <NUM> of <FIG> receives an AC line voltage <NUM> (e.g., AC single-phase or three-phase power) at a rectifier <NUM>.

The rectifier <NUM> rectifies the AC line voltage <NUM>. Example values for the AC line voltage <NUM> can range from <NUM> VAC or lower to <NUM> VAC or higher. The power supply <NUM> may be designed for a single nominal AC line voltage and/or for a range of AC line voltages. The rectifier <NUM> may include a filter capacitor, and provides a rectified line voltage <NUM>.

A pre-regulator <NUM> provides a regulated bus voltage (e.g., Vbus), which may be regulated to a voltage greater than the peak of the rectified line voltage <NUM>. The pre-regulator circuit <NUM> may also contain a power factor correction circuit and/or control to improve the power factor for the current or power drawn from the line voltage <NUM>. The pre-regulator circuit <NUM> may include a boost converter circuit arrangement. In some examples, the pre-regulator <NUM> may be omitted and the rectified line voltage <NUM> provided to the switched mode power supply circuit <NUM> as the bus voltage Vbus (e.g., with or without filtering and/or other conditioning of the rectified line voltage <NUM>).

The switched mode power supply <NUM> receives the bus voltage Vbus and outputs welding-type power <NUM>. As described in more detail below, the switched mode power supply <NUM> includes a high frequency transformer that has a saturation point for magnetic flux.

The example power supply <NUM> includes a controller <NUM> that controls the pre-regulator circuit <NUM> and the switched mode power supply <NUM>. For example, the controller <NUM> may control switching of a power semiconductor in the pre-regulator circuit <NUM> to control the regulated bus voltage Vbus. The controller <NUM> may control the switching of the power semiconductor in the pre-regulator circuit <NUM> so as to provide a regulated bus voltage Vbus as well as to perform power factor correction.

The controller <NUM> is a circuit, including digital and/or analog circuitry, discrete and/or integrated circuitry, microprocessors, DSPs, etc., software, hardware and/or firmware, located on one or more circuit boards, that form part or all of a controller, and are used to control a welding process, or a device such as a power source.

<FIG> is a schematic diagram of an example stacked full bridge circuit <NUM> that may be used to implement the switched mode power supply <NUM> of <FIG>. The switched mode power supply <NUM> of <FIG> receives the regulated bus voltage Vbus <NUM>, controls a voltage provided to a primary side of a high-frequency transformer <NUM>, and outputs the welding-type power <NUM> from a secondary side of the transformer <NUM>.

The switched mode power supply <NUM> of <FIG> includes a capacitor <NUM> in series with the high-frequency transformer <NUM>. The series capacitor <NUM> has a capacitor voltage Vcap approximately half of the bus voltage Vbus <NUM>. The capacitor <NUM> allows for bidirectional current flow in the transformer <NUM>. The switched mode power supply <NUM> further includes switching elements <NUM>, <NUM>, <NUM>, <NUM>. The control terminals of the switching elements <NUM>-<NUM> (e.g., the gates when using transistors for the switching elements) are labeled "A" and "B" in <FIG> to indicate the switching elements <NUM>-<NUM> that are controlled in combination. In some examples, the example switching elements <NUM>-<NUM> may be insulated-gate bipolar transistors (IGBTs).

The transformer <NUM> is driven with a positive voltage for a positive half-cycle by turning on the "A" switching elements <NUM>, <NUM>, which applies a voltage equal to Vbus - Vcap to the primary winding <NUM> of the transformer <NUM>. A negative half-cycle is accomplished by turning on the "B" switching elements <NUM>, <NUM>, which applies a voltage equal to - Vcap <NUM> to the primary winding <NUM> of the transformer <NUM>. The nominal value of Vcap is Vbus /<NUM>, so the positive and negative half-cycles both nominally apply voltages of Vbus /<NUM>, with opposite polarities for the different half-cycles. In each of the positive half-cycle and the negative half-cycle, the magnetic flux in the core of the transformer <NUM> changes in accordance with the applied voltage and current. When the positive half-cycle and the negative half-cycle are on for the same lengths of time, the net magnetic flux (volt-seconds) applied to the transformer <NUM> is zero over the course of one period (i.e., one positive half-cycle and one negative half-cycle) when Vcap <NUM> is Vbus /<NUM>. The transformer <NUM> has a volt-second rating that the transformer <NUM> can withstand before it saturates. While the flux is balanced, the switched mode power supply <NUM> avoids saturating the transformer <NUM>.

The example switched mode power supply <NUM> pre-biases the capacitor <NUM> to have a capacitor voltage Vcap <NUM> of half the bus voltage Vbus <NUM> (e.g., using balancing resistors before the switched mode power supply <NUM> is enabled to provide an output). The capacitance value of the example capacitor <NUM> is such that the capacitor voltage Vcap <NUM> may only change by a few volts above and below one half the bus voltage Vbus <NUM> at twice the switching frequency of the switching elements <NUM>-<NUM> (e.g., a PWM frequency) under normal circumstances. However, under dynamic load conditions, or current commands, the capacitor <NUM> may deviate farther from its nominal voltage.

Returning to <FIG>, to reduce the likelihood of saturating the transformer <NUM> in the switched mode power supply <NUM>, the example welding-type power supply <NUM> includes a flux accumulator <NUM>. The example welding type power supply <NUM> also includes a voltage estimator <NUM> is coupled to the switched mode power supply <NUM> and/or the controller <NUM>. In some example power supplies, the voltage estimator determines the AC-coupled voltage at the capacitor <NUM> by measuring the current flowing through a current transformer in series with the capacitor <NUM>. In some examples, the voltage estimator <NUM> measures the voltage of the capacitor <NUM>, measures the voltage at the bus, measures the voltage at the primary winding <NUM> of the transformer <NUM>, measures a voltage at a secondary winding <NUM> of the transformer <NUM>, and/or measures a voltage at a tertiary winding of the transformer <NUM>. In some examples, the controller <NUM> may compare measured voltages at the capacitor <NUM>, the bus <NUM>, the primary winding <NUM>, the secondary winding <NUM>, and/or a tertiary winding of the transformer <NUM>, to a respective threshold voltage to determine if a transient condition exists.

In some example power supplies, a current detector includes at least one of a current transformer, a Hall effect sensor, a sense resistor, or a magnetoresistive current sensor, to measure current flowing in the bus <NUM>, the capacitor <NUM>, the primary winding <NUM>, the secondary winding <NUM>, and/or a tertiary winding, to identify a transient condition. In some examples, the controller <NUM> controls duty cycles of the switches <NUM>-<NUM> to reduce the value of the net flux from a saturation value while continuing to generate an output from the welding-type power supply <NUM>.

The controller <NUM> may employ one or more techniques to avoid transformer saturation in the switched mode power supply <NUM>. The flux accumulator <NUM> determines a net flux in the transformer <NUM> applied to the primary winding of the transformer <NUM> and/or to a series combination of the capacitor <NUM> and the transformer <NUM> when such a capacitor <NUM> is present. As used herein, the term "net flux" refers to an accumulation (e.g., integration, summation, etc.) of volt-seconds in a core of the transformer <NUM> over one or more processing cycles of the flux accumulator <NUM> (e.g., a half switching cycle, a whole switching cycle, multiple switching cycles, etc.). For example, the flux accumulator <NUM> may integrate the flux in the transformer <NUM> to maintain history of the net flux (e.g., volt*sec) that has been applied to the transformer <NUM>. In some examples, the flux accumulator <NUM> tracks the PWM values output by the controller <NUM> to the switching elements <NUM>-<NUM>. The flux accumulator <NUM> calculates the net flux by adding the positive PWM value and subtracting the negative PWM value, to a running accumulator.

A first technique is an instantaneous flux limit that restricts the duty cycle in either polarity to an upper limit, thereby limiting an amount of flux that can be added or removed from the transformer <NUM> in any given cycle. A second technique involves flux balancing, which limits how quickly applied flux can change, to keep the positive and negative current cycles closer to balancing on an instantaneous basis, to thereby avoid exceeding the volt-second rating of the transformer <NUM> in a single direction. For example, if the switched mode power supply <NUM> is running operating at a <NUM>% duty cycle, and is commanded to change to <NUM>% duty cycle to satisfy a changing load condition, the controller <NUM> executing the control loop will not change the duty cycle to <NUM>% on the next PWM. Instead, the controller <NUM> increases it in uniform or non-uniform increments (e.g., <NUM>%, <NUM>%, <NUM>%, <NUM>%) until the desired command duty cycle is reached over several PWM cycles.

A third technique is a flux centering algorithm. The controller <NUM> continually sums up the flux applied to the transformer <NUM> during each switching period. Dynamically, the controller <NUM> permits the flux to accumulate up to the volt-second limit of the transformer. However, controller <NUM> performs flux centering to slowly modify the duty cycles of either the "A" or "B" pairs of switching elements <NUM>-<NUM> to bring the accumulated flux closer to zero. The flux centering reduces or avoids incremental increase of the flux in the transformer <NUM> to either positive or negative saturation by maintaining the flux to be generally centered at or near zero.

A fourth technique may be used to balance the flux in response to transient conditions that may be caused by changing load conditions. Transient conditions may be created in the switched mode power supply <NUM> by large load changes. Referring to <FIG>, during transient conditions, Vbus <NUM> can experience an overshoot in voltage due to energy stored in the input circuits. Overshoot on Vbus <NUM> causes a volt-second imbalance in the transformer <NUM>, because the capacitor voltage Vcap cannot change instantaneously. Similarly, other load changing conditions may cause an undervoltage at Vbus <NUM>, which can also cause a volt-second imbalance for the same reason. If the capacitor voltage Vcap is not balanced to Vbus /<NUM>, then the voltage applied during the duty cycles of the "A" and "B" pairs of switching elements <NUM>-<NUM> will not be equal, which causes the transformer flux to increase or decrease. Sufficiently large and/or lengthy increases or decreases in transformer flux can eventually saturate the transformer <NUM>, as more volt-seconds are applied during either the "A" or the "B" duty cycle.

To balance the flux after a transient condition, a transient condition is first be detected. In some examples, , the controller <NUM> detects a transient condition by comparing the output voltage <NUM> to an adjustable threshold voltage to determine if a load change has caused a transient condition. In other examples, the controller <NUM> may measure the capacitor voltage Vcap and compare the capacitor voltage Vcap to an adjustable threshold voltage to determine if a transient condition exists. In other examples, the controller <NUM> may measure the bus voltage Vbus and compare the bus voltage Vbus to an adjustable threshold voltage to determine if a transient condition exists.

In some examples, the controller <NUM> detects a transient condition by detecting a threshold change in the output voltage <NUM>, the bus voltage Vbus, and/or the capacitor voltage Vcap in less than a threshold time period. For example, the controller may detect a transient condition by detecting a threshold change in one of the output voltage <NUM>, Vbus <NUM>, or Vcap <NUM> in less than two switching cycles.

In other examples, a dedicated comparator <NUM> may be included in the voltage estimator <NUM>. The comparator <NUM> may be coupled to the controller <NUM> and/or the switched mode power supply <NUM>. The comparator <NUM> may measure the capacitor voltage Vcap and compare the capacitor voltage Vcap to an adjustable threshold voltage to determine if a transient condition exists. In other examples, the comparator <NUM> may measure the bus voltage Vbus and compare the bus voltage Vbus to an adjustable threshold voltage to determine if a transient condition exists. In other examples, the comparator <NUM> is coupled to the controller <NUM> and the welding type power output <NUM>. The comparator <NUM> compares the welding type output <NUM> to an adjustable threshold voltage to determine if a transient condition exists.

When a transient condition is detected, in some examples the controller <NUM> ignores or turns off the balancing techniques disclosed above, and instead uses the fourth technique to balance the flux. The fourth technique involves intentionally unbalancing the flux for a selected period of time, and/or until the capacitor voltage Vcap measures Vbus /<NUM>. For example, in reaction to a step overshoot to the bus voltage Vbus, a small amount of pulse width is added to the "A" duty cycle, or similarly a small amount of pulse width is subtracted from the "B" cycle. Or in reaction to an undervoltage at bus voltage Vbus, a small amount of pulse width is added to the "B" duty cycle, or similarly a small amount of pulse width is subtracted from the "A" cycle. The controller may measure the voltage at the bus voltage Vbus when a transient condition is detected to determine if the transient condition caused a step overshoot or an undervoltage at the bus voltage Vbus. The controller <NUM>, by adjusting the duty cycles of the switches <NUM>-<NUM>, thus unbalances the flux for either a predetermined period of time, or until the capacitor voltage Vcap measures Vbus /<NUM>. Unbalancing the flux in such a manner balances the capacitor voltage Vcap to Vbus /<NUM>. In some examples, a predetermined period of time may be two milliseconds. Adjusting the pulse widths in such a manner does create a small amount of flux walk in one direction. However, as the capacitor voltage Vcap is at or near the Vbus/<NUM> equilibrium point, the transformer <NUM> will not saturate. Once the capacitor voltage Vcap is back at the equilibrium point, the time based flux balancing techniques disclosed above may be used to prevent saturation.

In some examples when a transient condition is detected, the controller <NUM> communicates to the flux accumulator <NUM> that more or less flux has accumulated than has actually accumulated. For example, in reaction to a step overshoot to the bus voltage Vbus, the controller <NUM> may add a small amount to the net flux accumulated calculation tracked by the flux accumulator <NUM>. The flux accumulator <NUM> then calculates that more flux has accumulated during the "B" cycle than during the "A" cycle, even though more flux has actually accumulated during the "A" cycle than during the "B" cycle. In response, a flux centering algorithm as disclosed above adds a small amount of pulse width to the "A" cycle, or similarly subtracts a small amount of pulse width is subtracted from the "B" cycle. This process unbalances the flux, which balances the capacitor voltage Vcap to the Vbus/<NUM> equilibrium point. Adjusting the pulse widths in such a manner does create a small amount of flux walk in one direction. However, as the capacitor voltage Vcap is at or near the Vbus/<NUM> equilibrium point, the transformer <NUM> will not saturate. Once a predetermined period of time has passed, or once the capacitor voltage Vcap is back at the equilibrium point, the controller <NUM> may subtract the same amount of net flux from the flux accumulator <NUM> that the controller <NUM> added previously. Since the capacitor voltage Vcap is then at the Vbus/<NUM> equilibrium point, the time-based flux balancing techniques described above may be used to prevent saturation.

<FIG> is a flowchart representative of example method <NUM> that may be performed by the example welding-type power supply <NUM> of <FIG> to reduce magnetic flux in a transformer <NUM> of the switched mode power supply <NUM> in the welding-type power supply <NUM>.

At block <NUM>, the flux accumulator <NUM> of <FIG> determines a net flux in the transformer <NUM> based on a number of volt-seconds applied (e.g., by the switching elements <NUM>-<NUM>) to the primary winding of the transformer <NUM>.

At block <NUM>, the controller adjusts the duty cycles of the switches <NUM>-<NUM> to balance the flux in accordance with the net flux in the transformer <NUM> determined in block <NUM>. At block <NUM>, the controller <NUM> monitors for a transient condition. The controller may monitor for a transient condition by comparing on of the output voltage <NUM>, the bus voltage Vbus, or the capacitor voltage Vcap to an adjustable threshold voltage. In some examples, the controller <NUM> may detect whether the output voltage <NUM>, the bus voltage Vbus, or the capacitor voltage Vcap exceed a threshold voltage. In other examples, the controller <NUM> may detect whether the output voltage <NUM>, the bus voltage Vbus, or the capacitor voltage Vcap is below a threshold voltage. In some examples, the controller may detect whether the output voltage <NUM>, the bus voltage Vbus, or the capacitor voltage Vcap exceed a first threshold voltage, or is below a second threshold voltage.

At block <NUM>, if the controller <NUM> determines that a transient condition does not exist, then the method loops back to block <NUM> to determine the net flux in the transformer <NUM>. At block <NUM>, if a transient condition does exist, then the method moves on to block <NUM>, where the controller adjusts the duty cycles of switches <NUM>-<NUM> to adjust the capacitor voltage based on the bus voltage.

In some examples, at block <NUM>, the controller controls the switches <NUM>-<NUM> to intentionally imbalance the flux for a predetermined period. For example, the predetermined period may be two milliseconds. In some examples, the flux may be intentionally unbalanced until the capacitor voltage Vcap equals Vbus/<NUM>. When the flux is intentionally unbalanced in this way, the capacitor voltage Vcap adjusts to Vbus /<NUM> and the transformer <NUM> does not saturate. After the predetermined period, the method loops back to block <NUM> to determine the net flux in the transformer <NUM>. When the capacitor voltage Vcap is at the equilibrium position of Vbus /<NUM>, time based flux balancing algorithms can effectively balance the flux in the transformer.

<FIG> is a schematic diagram of an example implementation <NUM> of the switched mode power supply <NUM> of <FIG> wherein hardware has been added to the example implementation <NUM> shown in <FIG> which forces the capacitor voltage Vcap to remain at, or within a predetermined range of, the equilibrium point Vbus/<NUM>. In the disclosed example of <FIG>, a first switching element <NUM> and a second switching element <NUM> are added to the switch mode power supply <NUM>. Other circuitry in the system actively balances a first capacitor <NUM> and a second capacitor <NUM> at equal or substantially equal voltages. When the balance gate <NUM> is turned on at the same time as the A switch <NUM>, the series capacitor <NUM> is in parallel with the second capacitor <NUM>, causing the voltages across the series capacitor <NUM> (the capacitor voltage Vcap) and the second capacitor <NUM> to be equal. Since the capacitors <NUM>, <NUM> are equal, the capacitor voltage Vcap will be at the equilibrium point Vbus /<NUM>.

<FIG> is a schematic diagram that illustrates another example <NUM> of an implementation of the switched mode power supply <NUM> of <FIG> wherein hardware has been added to the example implementation <NUM> shown in <FIG> which forces the capacitor voltage Vcap to stay at, or within a predetermined range of, the equilibrium point Vbus /<NUM>. Other circuitry in the system actively balances the capacitors <NUM>, <NUM> at equal voltages. In the example implementation shown in <FIG>, a diode <NUM> has replaced the switching element <NUM> of <FIG>. Similarly to the example described above, when the balance gate <NUM> is turned on at the same time as the A switch <NUM>, the series capacitor <NUM> is in parallel with the second capacitor <NUM>, causing the voltages across the series capacitor <NUM> (the capacitor voltage Vcap) and the second capacitor <NUM> to be equal.

Referring to <FIG>, in some examples the additional hardware, transistors <NUM> and <NUM> may regulate Vcap within a certain range, the range based on the bus voltage Vbus. The controller <NUM> may monitor the bus voltage Vbus and the capacitor voltage Vcap, and then adjust the switching elements <NUM> and <NUM> to maintain the capacitor voltage Vcap within a certain range of Vbus/<NUM>. Similarly referring to <FIG>, the additional hardware, the transistors <NUM> and the diode <NUM>, may regulate the capacitor voltage Vcap within a certain range, the range being based on the bus voltage Vbus. The controller <NUM> may monitor the bus voltage Vbus and the capacitor voltage Vcap, and then adjust the switching elements <NUM> to maintain the capacitor voltage Vcap within a certain range of Vbus/<NUM>.

Claim 1:
A welding-type power supply (<NUM>), comprising:
a switched mode power supply (<NUM>), comprising:
a transformer (<NUM>) configured to transform a bus voltage to a welding-type voltage;
a capacitor in (<NUM>) series with a primary winding (<NUM>) of the transformer (<NUM>), the capacitor having a capacitor voltage; and
switches (<NUM>-<NUM>) configured to control a voltage applied to a series combination of the primary winding (<NUM>) of the transformer (<NUM>) and the capacitor (<NUM>);
a flux accumulator (<NUM>) configured to determine (<NUM>) a net flux in the transformer (<NUM>) based on a number of volt- seconds applied to the primary winding (<NUM>) of the transformer (<NUM>); and
a controller (<NUM>) configured to:
control (<NUM>) duty cycles of the switches (<NUM>-<NUM>) to balance the net flux when the capacitor voltage is within a voltage range based on the bus voltage;
detect (<NUM>, <NUM>) a transient condition in which the bus voltage exceeds a threshold voltage; and
control (<NUM>) the duty cycles of the switches (<NUM>-<NUM>) in response to the transient condition to adjust the capacitor voltage based on the bus voltage.