Parallel Interleaved Multiphase LLC Current Sharing Control

A multiphase power converter may include a number of LLC converter stages coupled in a parallel interleaved current sharing configuration. The total current provided by the multiphase power converter may be balanced between the different LLC converter stages by sensing a respective output current in each LLC converter stage, with the sensed output current of one of the LLC converter stages used as a reference current, and performing one or more adjustments for each LLC converter stage (other than the reference LLC converter stage), based on the sensed output currents. The adjustments may include adjusting the input voltage provided to the LLC converter stage, the resonant frequency of the LLC converter stage, and/or the effective resonance impedance of the LLC converter stage. The ability to sense the phase current or power makes it possible to achieve balance between different LLC converter stages in a multiphase LLC-stage current sharing configuration.

FIELD OF THE INVENTION

The present invention relates to the field of instrumentation, and more particularly to the design of parallel interleaved LLC power supplies.

DESCRIPTION OF THE RELATED ART

A switched-mode power supply (also referred to as a switching-mode power supply, switch-mode power supply, switched power supply, or switching power supply) is an electronic power supply that incorporates a switching regulator to efficiently convert electrical power. Unlike a linear power supply, the pass transistor of a switching power supply continually switches between low-dissipation, full-on and full-off states, remaining in high dissipation transition states for only brief periods of time, thereby reducing wasted energy. Voltage regulation is typically achieved by varying the ratio of the on-state and off-state of the pass transistor(s). In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. The higher power conversion efficiency is therefore an important advantage of a switching power supply, which may also be substantially smaller and lighter than a linear power supply due to the smaller transformer size and weight.

During the development of square-wave switching power conversion (e.g. pulse width modulated—PWM—switching), concerns regarding suddenly turning OFF current passing through an inductor lead to the design of “resonant topologies”. Switching power supplies featuring one such topology are referred to as LLC power supplies. The LLC topology features a unique combination of two inductors and one capacitor (hence the name, “LLC”), and offers a relatively narrow range of switching frequencies, which make possible the design of standard EMI filters combined with the capability of producing zero-voltage switching (soft-switching) through careful design, significantly improving electromagnetic interference (EMI) and efficiency over a wide load range.

Because LLC converters are part of the resonant converter family, voltage regulation is not performed in the exact same manner as it is in conventional PWM switching power converters. Running at 50% duty-cycle and fixed 180 degree phase shift, regulation is typically obtained through frequency modulation. All primary side switches turn on resonantly—zero voltage switching—resulting in full recycling of the energy contained in the MOSFETs' parasitic output capacitance. Furthermore, all secondary side switches turn off resonantly—zero current switching—to minimize switching losses normally associated with hard switching. Resonant operation of all switching devices in the LLC converter results in minimized dynamic loss, and thus increased overall efficiency, particularly at higher operating frequencies in the hundreds of kHz to MHz range.

LLC Converters are becoming more widely accepted in offline (AC-DC) power supplies due to their high efficiency and lower component count. Adoption has been limited, however, to moderate power levels (e.g. lower than 400W). At higher power levels, traditional approaches make use of multi-phase techniques to divide component stress and reduce the size of the components. In addition, multi-phase arrangements can reduce ripple currents on output filters/capacitors, greatly extending the life of these components. This is especially important in high output-current supplies. LLC converters operate on a variable frequency control scheme to regulate line and load variation, using the slope of the gain/frequency curve to adjust the gain.

Combining the outputs of several (more than one) LLC stages naturally allows sharing the load current if the supply is operating under ZVS (zero voltage switching), the respective inductors of each stage are equal in value (e.g. in inductance) to the corresponding respective inductors of all other stages, and the respective capacitors of each stage are equal in value (e.g. in capacitance) to the corresponding respective capacitors of all other stages. Unfortunately, when the inductances and capacitances of one stage differ from the corresponding inductances and capacitances of other stages by even several percentage points, the sharing of load current can become disparate by a large amount. This difference can also be exaggerated by changes in operating frequency. Normal component manufacturing tolerances (+/−10%) are sufficient to cause unworkable differences in stage current.

Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.

SUMMARY OF THE INVENTION

Various embodiments of systems disclosed herein include a novel topology/circuitry for a parallel interleaved multiphase LLC current sharing power supply system. In various embodiments, a multiphase power converter may include a number of different LLC converter stages coupled in a parallel interleaved current sharing configuration. The total sum current provided by the multiphase power converter may be balanced between the different LLC converter stages by sensing a respective output current in each LLC converter stage, with the sensed output current of one of the LLC converter stages used as a reference current, and performing one or more adjustments for each LLC converter stage other than the reference LLC converter stage, based on the sensed output currents. The adjustments may include adjusting the input voltage provided to the LLC converter stage, adjusting the resonant frequency of the LLC converter stage, and/or adjusting the effective resonance impedance of the LLC converter stage. The ability to sense the phase current, or power, therefore makes it possible to achieve balance between the different LLC converter stages in a multiphase LLC-stage current sharing configuration.

Pursuant to the above, in some embodiments, a method may be devised for controlling a number of parallel interleaved switching power supplies coupled in a current sharing configuration, where each switching power supply sources a respective portion of a total current provided by the current sharing configuration. The method may include balancing the total current between the plurality of parallel interleaved switching power supplies by sensing a respective output current in each switching power supply, where a first sensed output current corresponding to a first switching power supply is used as a reference current, and for each respective switching power supply other than the first switching power supply, adjusting, based on the first sensed output current and the sensed respective output current corresponding to the respective switching power supply, the input voltage provided to the respective switching power supply, the resonant frequency of the respective switching power supply, and/or the effective resonance impedance of the respective switching power supply.

In some embodiments, a multiphase current sharing configuration may include a number of different switching power supply stages, each switching power supply stage sourcing a respective portion of a total current provided by the current sharing configuration. The current sharing configuration may further include a number of different power factor correction stages, with one of the power factor correction stages designated as a reference stage. For each respective power factor correction stage, an output voltage of the respective power factor correction stage may be provided as an input voltage to a corresponding respective switching power supply stage, and a control circuit included in the current sharing configuration may be used to balance the total current between the plurality of switching power supply stages by adjusting the output voltage of each respective power factor correction stage except the reference stage. The output voltage of each respective power factor correction stage (except the reference stage) may be adjusted based on an output current of the respective switching power supply stage corresponding to the reference stage, and an output current of the respective switching power supply stage corresponding to respective power factor correction stage.

In some embodiments, a multiphase power converter may include a first LLC converter stage that provides a first output current having a first value, and may also include a second LLC converter stage coupled in parallel to the first LLC converter stage and providing a second output current having a second value. A total sum current including the sum of the first output current and the second output current may be provided at an output of the multiphase power converter. A control circuit included in the multiphase power converter may be operated to cause the first value to match the second value within a specified tolerance (e.g., the first value may be equal to the second value, or it may not differ from the second value by more than an specified tolerance value, etc.) In order to obtain matching values, the control circuit may obtain the first value and the second value, use the first value as a reference value, and adjust, based on the reference value and the second value, at least one of an input voltage provided to the second LLC converter stage, a resonant frequency of the second LLC converter stage, or an effective resonance impedance of the second LLC converter stage.

Other aspects of the present invention will become apparent with reference to the drawings and detailed description of the drawings that follow.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of improved power supply systems include a parallel interleaved multiphase LLC current sharing configuration providing accurate balancing of the total output current among the different power converter stages connected in the current sharing configuration.

Previous Power Sharing Configurations

There are various power sharing configurations presently in use. The following brief descriptions are intended to provide an overview of two different impedance matching methods used to match the respective inductances and capacitances of each individual LLC power converter to the respective inductances and capacitances of all other individual LLC power converters in a power sharing configuration. As generally used herein, “matching” and “achieving balance” are used to refer to balancing the total sum current output by a multiphase power controller among the individual converters coupled together in a current sharing configuration. Such current/power balancing may include matching the respective LLC components of an LLC power converter to the respective LLC components of other LLC power converters in a power sharing configuration to obtain stage currents (a respective portions of the total current conducted by the LLC power converters in the power sharing configuration) equal to each other, or not to differ from each other by more than a specified (e.g. allowed) percentage.

Matching by Trimming

According to one method, balance can be achieved by careful trimming of the resistive, magnetic and capacitive components of each phase of the LLC (e.g. of each LLC power converter that is a part of the power sharing configuration) to precisely match. However, this represents an expensive process for production, and is feasible only for small quantities. Factors like components aging over time will also affect the balance of the components and current balance during operation.

Matching by ‘Floating’ Star Configuration

According to another previously proposed method, balance may be achieved among the different phases (e.g. among the different individual LLC power converters that are part of a power sharing configuration) by ‘floating’ the return (ground) connection of the phases connected in a ‘star’ configuration. A two-phase (2-phase) example is illustrated inFIG. 1, and a three-phase (3-phase) example is illustrated inFIG. 2.

As shown inFIGS. 1 and 2, Z1, Z2and Z3represent each phase of the LLC power stage (or each phase of the LLC power converter stage) modelled as a load impedance. In case of the 2-Phase configuration102shown inFIG. 1, if the return connection between Z1and Z2is removed, as shown in configuration104, the common node between Z1and Z2automatically achieves a balanced voltage VBAL, and currents through Z1and Z2are equally shared as Z1and Z2now share what is effectively a series connection, as shown in configuration106. This scheme can effectively be extended to multiphase configurations.

A three-phase (3-phase) configuration is illustrated inFIG. 2. In case of the 3-Phase configuration202, if the return connection between Z1, Z2, and Z3is removed, as shown in configuration204, the common node between Z1, Z2, and Z3becomes a floating node, which can present a balanced voltage, with equally shared currents through Z1, Z2, and Z3. While this method may work well, one drawback is that each phase needs to operate at a 360 degrees/N phase shift from each other phase (where N is the number of total phases in the sharing configuration).

One advantage of multiphase LLC in general is the significant reduction of the output noise and ripple currents as the number of phases is increased. However, significant output noise and ripple current reduction may be most effectively obtained when each phase is operated at a more flexible relative phase shift with respect to each other phase than the 360 degrees/N which is required in the prior art configurations illustrated inFIGS. 1 and 2.

Novel Power Sharing Topology

FIG. 3shows the basic structure of an exemplary resonant LLC pulse supply topology (or LLC [switching] power converter topology)300according to some embodiments. The circuit of LLC converter300includes resonance inductor308and resonance capacitor310. As illustrated in circuit300, together with resonance inductor308and resonance capacitor310, voltage source302, primary side upper switch304, primary side lower switch306, secondary side upper switch316, and secondary side lower switch318are used for controlling the voltage VOdeveloped across the load represented by load resistance322, with the output capacitance across the load represented by capacitor320. COrepresents the parasitic capacitance312across the primary winding (or winding on the primary side) of the transformer314.

An LLC converter is essentially a frequency driven power supply, with a resulting gain curve illustrated in the exemplary voltage versus frequency diagram400shown inFIG. 4, according to some embodiments. Diagram400illustrates the relationship between the input voltage and upper voltage versus the operating frequency. The resonant frequency is indicated as Fr1, the minimum frequency is indicated as fmin, and the maximum frequency is indicated as fmax. Diagram400illustrates three operating regions. Region1is the region above resonant zero voltage switching (ZVS), region2is the region below resonant ZVS, and region3is the region below resonant zero current switching (ZCS).

Operation of a single LLC stage is considered simple with the use of a control loop to adjust the gain by varying the frequency of the LLC oscillator, to achieve the desired gain that maintains the desired output voltage (i.e., it provides a regulated output voltage). However, as shown inFIG. 5, when at least a couple of LLC power converter stages are coupled in parallel, the operation of the resulting system is no longer as simple as in the case of a single LLC stage (or single LLC power converter stage).FIG. 5illustrates a multi-stage power sharing configuration500that includes three LLC power converter stages, according to some embodiments. A first LLC stage includes primary side upper switch504, primary side lower switch506, resonant inductor520, resonant capacitor530, a transformer540with primary winding LM1, with the output capacitance of the secondary (output stage) illustrated by capacitor570. A second LLC stage includes primary side upper switch508, primary side lower switch510, resonant inductor522, resonant capacitor532, a transformer550with primary winding LM2, with the output capacitance of the secondary (output stage) illustrated by capacitor572. Finally, a third LLC stage includes primary side upper switch512, primary side lower switch514, resonant inductor524, resonant capacitor524, a transformer560with primary winding LM3, with the output capacitance of the secondary (output stage) illustrated by capacitor574. The supply voltage is provided by input voltage supply502.

The final output voltage of each LLC stage may have slight variations resulting from the tolerances (e.g. slight differences) between respective corresponding resonant components in the different stages, e.g. tolerances between the resonant capacitors (530,532,534), and between the resonant inductors (520,522,524). In a parallel LLC structure as shown inFIG. 5, this may cause a power output imbalance between the parallel stages and reduced efficiency of the parallel stages. In one set of embodiments, balance of (or between) the output stages (the LLC stages) may be achieved according to at least two different methods, both of which are directed to balancing the final output voltage VOUTamongst the output (LLC) stages by adjusting the gain curve(s) previously illustrated inFIG. 4.

The difference between the two methods, which will be further described in detail below, is in the means used to achieve the final output voltage balance between the stages. One of the goals is to maintain the desired phase difference between the stages, to maximize output ripple cancellation, without changing the fundamental operating frequency between the stages. In other words, adjustments to the operation of the stages are made in a manner that ensures that the operating frequency remains the same across the different LLC stages. It should also be noted that the various embodiments of multiphase LLC current sharing control disclosed herein are not necessarily limited to just three-stage LLC multiphase configurations and may be extended from a two-stage LLC multiphase configuration to any number (N) of LLC stages. In both methods, the adjustments may be implemented dynamically via a secondary feedback loop that is slower than the main voltage (primary) feedback loop of the multiphase LLC. While the various embodiments disclosed herein require no initial calibration when performing dynamic adjustments, other embodiments may implement an initial calibration performed to eliminate component imbalances according to the methods described herein without making dynamic adjustments.

In some embodiments, respective power factor correction (PFC) stages may be used to provide input voltages to corresponding the LLC stages in a power supply topology derived from the parallel LLC power sharing configuration500illustrated inFIG. 5. Starting with a parallel multiphase topology (e.g. configuration500) that includes a specified number (N) of LLC power converter stages, N PFC stages may be coupled to the N LLC stages downstream.FIG. 6shows a functional block diagram of an exemplary multiphase current sharing configuration600in which multiphase current balancing is performed via adjustable PFC outputs, according to some embodiments. The illustrated multiphase LLC power sharing configuration600includes LLC power converters (phases)1through N (indicated as610,612, and614, respectively), with control provided by an LLC interleave controller616. The configuration600also includes corresponding PFC stages1through N (indicated as602,604, and606, respectively), with control provided by PFC interleave controller608. The input voltage to each LLC phase (stage) is provided by the corresponding PFC phase as shown. For example, the output voltage of PFC phase1(602) is provided as the input voltage of LLC phase1(610), the output voltage of PFC phase2(604) is provided as the input voltage of LLC phase2(612), and so on up to PFC phase N (606) and LLC phase N (614). Thus, one difference between the power sharing configuration500shown inFIG. 5and the power sharing configuration600shown inFIG. 6is that while power sharing configuration500features a single input voltage source502providing the input voltage to each LLC stage, in the power sharing configuration600shown inFIG. 6, the output voltage of a PFC phase (stage) is used as the input voltage for a corresponding LLC phase. Thus, each LLC phase (stage) receives an individual input voltage instead of a single source (e.g. source502shown inFIG. 5) providing the input voltage to all the LLC phases (stages).

The current output (or output current) of each LLC phase (LLC stage) may be measured, and one of the LLC phases may be used as a reference to obtain error information corresponding to the other LLC phases (or to the respective output currents of the other LLC phases). As shown inFIG. 6, LLC phase1is used as the reference phase, with the measured output current of LLC phase1used as the reference current. The measured output current of LLC phase1may be compared with the measured output current of LLC phase2, e.g. by using a comparator618, and the resulting difference in current values may be reported back to PFC phase (stage)2for making an adjustment, if necessary, to the output voltage of PFC phase2, based on the reported difference in current values. The same may be performed for all respective PFC phase/LLC phase pairs, with a final PFC phase N (606) and LLC phase N (614) using comparator620to compare the measured output current of LLC phase1with the measured output current of LLC phase N. In general, the error information may be used to make adjustments to the respective output voltages of the non-reference PFC phases (e.g. PFC phases2through N in the case shown inFIG. 6) such the output voltage of all the respective PFC phases (602,604,606) are varied slightly to cancel out the current imbalance in the respective outputs of the corresponding LLC phases (610,614,616).

According to this method, the gain curves of the respective LLC phases may still be mismatched, and the input voltage to each of the LLC phases may be adjusted such that the output voltage (DC output) is matched. A matched output voltage between the LLC phases is achieved when the respective error output corresponding to the output current of each non-reference LLC phase is zero, effectively achieving equal power (or achieving a power balance) between all the LLC phases. The PFC phases and the LLC phases need not be linked or operate at the same frequency in the power sharing configuration600shown inFIG. 6.

Parallel Interleaved LLC, Method II: Adjustable Effective Resonance Impedance

In some embodiments, according a second method, balance between the different LLC stages (phases) may be achieved by adjusting the effective resonance impedance (e.g. one or more of the component values of the resonant tank) in each LLC stage as may be necessary to achieve balance between the different LLC stages.FIG. 7shows an exemplary circuit diagram of a resonant LLC pulse supply (LLC stage) topology700according to some embodiments. The resonant LLC supply700includes a transformer730having a primary side (winding) receiving an input voltage input voltage from source708, with primary side upper switch704and lower switch706used for switching control of the output voltage VOacross load734(exemplified by a resistor R, also indicating an output capacitance732) at the secondary side (winding) of transformer730. LLC supply also includes a resonant tank702with resonant capacitor Cr720, resonant inductor Lr722and primary (or main) inductance Lm724as shown. Overall, the resonant tank may be considered as representative of the effective resonance impedance of the LLC stage700.FIG. 8shows an exemplary gain versus frequency characteristic diagram for the LLC stage700ofFIG. 7, featuring different gain versus frequency curves for different quality factor values, according to some embodiments.

The respective value(s) of any one or more of the components in resonant tank702may be adjusted (changed) to adjust (change) the quality factor (Q), a ratio between primary inductance and resonant inductance (M), and the gain of the LLC converter (or Vout/Vin) for a fixed operating frequency. Maintaining a common operating frequency across the multiple LLC phases may ensure output ripple cancellation as previously outlined above.

In some embodiments, the effective value of Cr720may be adjusted to balance the LLC phases. However, in various other embodiments, the respective values of the other LLC resonant components (e.g. Lr722and/or LM724) may be adjusted to achieve the same effect, making dynamic adjustments (changes) to the LLC gain curves. The resonant capacitor values (or resonant capacitances) may be adjusted based upon the current-balance error information (which may be obtained as described above with respect to the first method), effectively adjusting the gain curves of each resonant LLC stage to balance out the tolerances, as illustrated inFIG. 9. Because the value of the resonant capacitor (or the resonant capacitance) directly affects (and hence may be used to adjust) the resonant frequency (Fri) of each of the LLC stages, the gain of each of the LLC phases may be directly adjusted such that a balance between the LLC phases is achieved. As shown in the gain diagram ofFIG. 9, effective adjustments may be made to Cr to align the gain at the operating frequency of multiple LLC phases. When LLC phases are balanced in this manner, the respective resonant frequencies of the LLC phases may slightly differ from each other, but during static operation they all have the same operating gain, and therefore achieve the same output voltage for a fixed input voltage.

Current Sensing (Measurement)

In order to sense the phase currents, or the respective output currents of the LLC phases (stages), traditional resistor sense strategies may be employed. However, when using such traditional strategies, efficiency may suffer due to resistive losses. Thus, in some embodiments, an alternative current sensing strategy may be used. As shown inFIG. 10, sensing resonant capacitor voltage (which varies as a function of load current) provides a low-loss alternative to sensing resistance voltage. Accordingly, using the current sharing configuration500as an example, respective resonant capacitor voltages may be sensed in resonant capacitors530,532, and534as illustrated inFIG. 10. In various embodiments, it may also be possible to use inductive current sensing. The sensors may not be required to directly indicate current, only to respond to the current in an indicative manner, e.g. in a manner that tracks and indicates changes in the current. For example, equal sense signals on all sensors may be indicative of a balanced system within the matching of the sensors.

FIG. 11shows an exemplary circuit diagram1100of a resonant LLC pulse supply topology with switch-controlled capacitors, according to some embodiments. A switch-controlled capacitor circuit including capacitors1102and1104, as well as switch M3(1106) may be used to implement an adjustable effective resonant capacitance Cein LLC supply circuit1100. In an exemplary embodiment, capacitor1102may have a value of 10 nF and capacitor1104may have a value of 33 nF. As shown inFIG. 11, switch M3(1106) is driven with a square wave synchronous to the loop clock (switching frequency). The duty cycle of the waveform may be adjusted, for example between 0 and 0.5, to vary the amount of capacitance that appears in parallel with capacitor1104, as illustrated in diagram1150. In this manner, the effective resonant capacitance Cemay be adjusted to move the gain, and balance the load with other LLC phases. The top switch M1(e.g. a Field Effect Transistor gate) may be driven by a signal with a 50% duty-cycle at the LLC switching frequency, Fsw. If M3is also driven with the correct phase relationship with respect to M1and M2, M3may be operating under ZVS conditions, maximizing efficiency.

It should be noted that M3takes advantage of the positive bias on capacitor1104to make use of only one switching device. Various embodiments featuring a series switch arrangement are also possible and are contemplated. It is also possible to allow the LLC phases to adjust “differentially” in order to maximize the gain control effect. For example, the duty-cycle in one LLC phase may be reduced while the ratio in the other LLC phase is increased, doubling the effect or reducing the required size of capacitor1102by a specified factor, for example by a factor of 2. The ability to sense phase current or power makes it possible to achieve balance between the different LLC stages in a multiphase LLC stage current sharing configuration. In other embodiments (not shown), and extra inductor may be duty-cycle switched (similar to the switch-controlled capacitor circuit) in order to adjust the resonant frequency Fr (in a manner similar to how thr Cr is adjusted inFIG. 11), facilitating balance between phases.