Dynamic power balancing among multiple induction heater power units

A modular RF power system allows multiple power supplies to combine their RF output power as a single system and deliver it to a common resonant circuit. For flexibility and commonality, each power supply is designed to be separately powered by AC line voltage (aka AC Mains). The AC voltage supplied to each power supply may differ due to differing AC distribution line length, line impedance, wire gauge, or different supply generation locations.

TECHNICAL FIELD

This disclosure relates to power balancing among power supplies, and, more specifically, it relates to power balancing among multiple (two or more) induction heater power units.

BACKGROUND

Induction heating generally refers to the process of heating an object (usually a metal object) by exposing the object to a time-varying magnetic field and, thereby, inducing a current (e.g., an eddy current) in the object. The induced current creates heat. To create the time-varying magnetic field, an induction heating unit is used. An induction heating unit typically includes the following components: (1) a power unit for producing a radio frequency (RF) signal and (2) a load comprising a coil coupled to the power unit for receiving the RF signal and producing the time-varying magnetic field. The power unit may include: a rectifier (a.k.a., AC/DC converter) for converting alternating current (AC) to direct current (DC) and an inverter for converting the DC produced by the rectifier to an RF signal, thereby producing the RF signal provided to the coil.

In some applications it would be advantageous to couple the coil to multiple power units so that the coil receives an RF signal from the multiple power units. However, in such a system where the coil receives power from multiple power units, there is a need to ensure that the power units operate in unison.

SUMMARY

In one aspect there is provided a modular power supply system, which includes multiple power supply modules, for providing power to a common load. In some embodiments, the modular power supply system includes: a drive signal generator that generates a common PWM drive signal; a first power supply module configured to receive power from a first AC power supply source, the first power supply module comprising: (i) a first AC/DC converter for converting AC power to DC power, (ii) a first PWM control signal generator configured to generate a first PWM control signal, and (iii) a first switching system comprising a first set of switches and a second set of switches, wherein the switching system is configured to (a) receive the DC power, the common PWM drive signal and the first PWM control signal and (b) cycle the switches based on the common PWM drive signal and the first PWM control signal, thereby producing a first output signal for driving the common load; a first monitor for monitoring the output power or temperature of the first power supply module; a second power supply module configured to receive power from a second AC power supply source that is different than the first AC power supply source, the second power supply module comprising: (i) a second AC/DC converter for converting AC power to DC power, (ii) a second PWM control signal generator configured to generate a second PWM control signal, and (iii) a second switching system comprising a first set of switches and a second set of switches, wherein the switching system is configured to (a) receive the DC power, the common PWM drive signal and the second PWM control signal and (b) cycle the switches based on the common PWM drive signal and the second PWM control signal, thereby producing a second output signal for driving the common load; a second monitor for monitoring the output power or temperature of the second power supply module; and a master controller coupled to the first and second monitors, wherein the master controller is configured to (a) cause the first PWM control signal generator to modify the duty cycle of the first PWM control signal in response to the output power or temperature of the first power supply module exceeding a threshold and (b) cause the second PWM control signal generator to modify the duty cycle of the second PWM control signal in response to the output power or temperature of the second power supply module exceeding a threshold.

The modular system allow multiple power supply modules to combine their output power (usually RF output power) as a single system and deliver the combined power to the object to be heated using a common resonant circuit. The object to be heated and the common resonant circuit form the common load of the power system.

The above and other aspects and embodiments are described below with reference to the accompanying drawings.

DETAILED DESCRIPTION

FIG. 1illustrates a modular power supply system100according to some embodiments. As shown inFIG. 1, in some embodiments, system100includes a first power supply module104aand a second power supply module104bfor driving a common load108. That is, load108, which may include a coil, is coupled to the output of module104aand the output of module104b. That is, load108receives drive signals171and172output by modules104aand104b, respectively. In the embodiment shown, system100also includes a drive signal generator102that generates a common PWM drive signal190that is received by the modules of system100. Only two modules104are shown inFIG. 1for the sake of brevity and explanation, however, system100may have more than two modules104that drive the common load108.

The first power supply module104ais configured to receive power from a first AC power supply source101and the second power supply module104bis configured to receive power from a second AC power supply source103. Hence, each module104of system100may be separately powered by AC line voltage (a.k.a., AC mains). The AC signal (e.g. AC voltage) supplied to each module104may differ due to one or more of: differing AC distribution line length, line impedance, wire gauge, and different supply generation locations.

For maximum system efficiency, each power supply module104should deliver its power at precisely the same (resonant) frequency, in phase with every power supply connected to the common load. For maximum utilization, each power supply module104should deliver the same percent of rated power.

Accordingly, as shown inFIG. 1, in some embodiments, system100includes a drive signal generator that generates a common drive signal190so that each power supply will output a drive signal that has substantially the same frequency and phase. That is, drive signal171should have at least substantially the same frequency as drive signal172and the two signals171and172should at least be substantially in phase. In some embodiments, common drive signal190is an RF signal having a square-wave waveform and is pulse width modulated. That is, the width of the pulses of the square-wave is modulated to, for example, increase or decrease the power delivered by system100to load108.

As further shown inFIG. 1, each power supply module may include: (i) an AC/DC converter120for converting the AC power output by source101to DC power; (ii) a pulse width modulation (PWM) control signal generator122configured to generate a PWM control signal131, which may have a square-wave waveform; and (iii) a switching system121, which functions as an inverter, wherein the switching system121is configured to (a) receive the DC power, the common drive signal190and the PWM control signal122and (b) produce an output signal171for driving the common load108, wherein the form of the output signal is based on the common drive signal190and control signal122; and (iv) a monitor123for monitoring one or more of: the output power, temperature, etc. of the power supply module104.

As further shown inFIG. 1, system100includes a master controller110controller coupled to the monitors123of the modules104. In some embodiments, the master controller110is configured, such that, in response to receiving from a monitor123of a module104information indicating that the output power of the module104(and/or the temperate of the module104) is exceeding a predetermined threshold, the master controller causes the PWM control signal generator122of the module104to modify a duty cycle of the PWM control signal131produced by the generator122. By modifying the duty cycle of control signal131, controller110can, at least to some degree, control the output signal171that is produced by switching system. For example, as further explained herein, modifying the duty cycle of control signal131has the effect of causing cycles to be dropped from signal171.

Referring now toFIG. 2,FIG. 2illustrates switching system121according to some embodiments. As shown inFIG. 2, switching system121, in some embodiments, includes: power module bridge circuit290comprising switches (A-D) configured in a full-bridge configuration; a differential receiver (DR)203; and a logic circuit202that outputs a first drive signal (out1) for operating switches B and D and a second drive signal (out2) for operating switches A and C. Switches A-D may be MOSFET transistors or other transistors. As illustrated, DR203receives the common drive signal190, which may be a differential signal (i.e., signal190may comprise two complimentary signals), and generates single ended signals in1and in2. As also shown inFIG. 2, logic circuit202receives as input the control signal131, signal in1and signal in2.

Referring now toFIG. 3,FIG. 3illustrates the relationship between the outputs (out1and out2) and the inputs (signal131, in1, and in2) of logic circuit102according to one embodiment. As shown in the embodiment ofFIG. 3, out1is normally in the HIGH state, but out1will transition to the LOW state whenever in1transitions to the LOW state provided that control signal131is in the HIGH state at the time that in1transitions to the LOW state. Additionally, when out1is in the LOW state, out1will transition to the HIGH state whenever ml transitions to the HIGH state, regardless of the state of the control signal131. Similarly, out2is normally in the HIGH state, but out2will transition to the LOW state whenever in2transitions to the LOW state provided that control signal131is in the HIGH state at the time that in1transitions to the LOW state. Additionally, when out2is in the LOW state, out2will transition to the HIGH state whenever in2transitions to the HIGH state, regardless of the state of the control signal131.

Referring now toFIG. 4,FIG. 4is a circuit diagram showing one possible implementation of logic circuit202. As shown, DR203receives differential drive signals190and generates single ended signals: In1and In2at the operating frequency. The combined cycle of In1and In2comprise one full drive cycle to be presented to the power module bridge circuit290. Two full cycles of these signals are shown inFIG. 5. They are active LOW and in this example they are presented at a 10% duty cycle. Their duty cycle is used to regulate the resultant output level. This type of regulation is referred to as Pulse Width Modulation (PWM). Very low duty cycles result in a very low output level, while a nearly 50% duty (of each) result in maximum output level. In the embodiment shown, In1and In2are 180 degrees out of phase.

These two signals are presented to FF1A with In1driving the (−)SET Input and In2driving the CLOCK input. Resultant FFIA (Q1) waveform is actively HIGH from the time In1becomes active until the time In2becomes inactive—thereby framing the active time of each cycle.

This framing waveform is presented to FF2A CLOCK input. FF2A (−)Q1is determined by the level of the control signal131at the rising edge of the framing signal and it determines whether In1and In2will be either be allowed to reach the power module bridge290(signal131is high at the rising edge of Framing input) or whether these two signals are inhibited (signal131is low at the rising edge of the Framing input). Over a long period of time with respect to the operating frequency, the resultant power switching waveform output from circuit290reflects the desired amount of enabled cycles.

This technique inhibits some number of drive signals over time without changing the PWM duration of the drive signals or their operating frequency. It is able to vary the number of cycles that are inhibited over a very wide range: from inhibiting all of them, resulting in no output; to inhibiting none of them, resulting on no impact to the desired output.

An advantage of this design is that cycles are inhibited asynchronously as the controlling signal131is asynchronous with the operating frequency. Inhibit cycles are usually very short as the controlling signal131generally runs at a higher frequency than the operating frequency. Inhibited cycles are also rarely contiguous (i.e. are spread out), resulting in very little resultant output ripple and plenty of signal for sensing even at low duty drive signals.

Designing the system to accommodate independently AC powered power units provides additional benefits as users seek to minimize THID (Transient Harmonic Current Distortion) on the AC distribution network.

As higher power systems naturally draw more current from the AC distribution network, AC current distortion (especially from nonlinear rectifier circuitry) become more troublesome, interfering with other powered units.

A common practice to minimize THID is to separate the supplied AC power among units and introduce a phase shift to the AC power to each power supply. This approach minimizes the ability of each unit's distortion to fully combine. Phase shifting the AC power not only reduces the combining effect of the units producing distortion, but if implemented correctly, can cancel the largest contributors.

Theoretically, the more points of correct phase shift that are employed, the more distortion harmonics that can be canceled. However, the theoretical advantage is never truly enjoyed as it relies on perfect balancing of the AC Mains' phases and the voltage balance among the supplies it is addressing. Alternative measures used such as harmonic phase balancing to improve the end result are costly.

An advantage of internally balancing the output power of each power supply is that it makes the AC mains appear to be balanced. The result is that traditional, less costly phase shift techniques employed can approach theoretical capabilities even with unbalanced AC lines.