NL2022622B1 - Electric power converter with inductively coupled parallel power stacks - Google Patents

Electric power converter with inductively coupled parallel power stacks Download PDF

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Publication number
NL2022622B1
NL2022622B1 NL2022622A NL2022622A NL2022622B1 NL 2022622 B1 NL2022622 B1 NL 2022622B1 NL 2022622 A NL2022622 A NL 2022622A NL 2022622 A NL2022622 A NL 2022622A NL 2022622 B1 NL2022622 B1 NL 2022622B1
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Netherlands
Prior art keywords
stack
bridges
electric current
current converter
full
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NL2022622A
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Dutch (nl)
Inventor
Johan Van Straalen Joost
Terlouw Jozua
Johan Hubert Slaats Noud
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Prodrive Tech Bv
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Application filed by Prodrive Tech Bv filed Critical Prodrive Tech Bv
Priority to NL2022622A priority Critical patent/NL2022622B1/en
Priority to EP20705093.1A priority patent/EP3928110A1/en
Priority to PCT/EP2020/054616 priority patent/WO2020169797A1/en
Priority to CN202080015655.XA priority patent/CN113678013A/en
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Publication of NL2022622B1 publication Critical patent/NL2022622B1/en

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers
    • H03F3/21—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers with semiconductor devices only
    • H03F3/217—Class D power amplifiers; Switching amplifiers
    • H03F3/2178—Class D power amplifiers; Switching amplifiers using more than one switch or switching amplifier in parallel or in series
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
    • G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
    • G01R33/3852—Gradient amplifiers; means for controlling the application of a gradient magnetic field to the sample, e.g. a gradient signal synthesizer
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/49—Combination of the output voltage waveforms of a plurality of converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/0077—Plural converter units whose outputs are connected in series

Abstract

Electric power converter with inductively coupled parallel power stacks Electric power converter (10), comprising a first stack (131) and a second stack (132). The first stack and the second stack each comprise a plurality of controllable power switches (81-84) arranged in at least one full H-bridge and/or a plurality of half H-bridges (15). The H-bridges are connected in series within each stack. The first stack (131) and the second stack (132) are parallel connected through an inductive component (14), in particular a coupling inductor.

Description

Electric power converter with inductively coupled parallel power stacks Technical field
[0001] The present invention is related to an electric power converter. In particular, the present invention is related to a multi-level electric power converter overcoming problems related to dead-time of the transistor switches at low current levels. Electric power converters according to the present invention are particularly suitable for use as amplifiers for driving gradient coils in Magnetic Resonance Imaging (MRI) systems. Background art
[0002] Gradient coils in an MRI system provide a magnetic field that is used for performing imaging measurements. These gradient coils require a high voltage and high current that must be controlled. The voltage is typically on the order of 2000 V and above. The required peak current is on the order of 600 A and above. The quality and resolution of the image depend on how precisely the magnetic fields are controlled. Therefore, a high accuracy on the currents is required to prevent image artifacts
[0003] Gradient amplifiers in MRI-scanners drive the gradient coils that typically have an inductance of several hundred pH to 1 mH. The gradient amplifier is constructed to drive this gradient coil to specific magnetic fields and typically comprises switching elements arranged in so called switching legs, together forming an H-bridge. The switching elements can be formed of IGBT switches with anti-parallel diodes, or any other electronically controlled switch with a parallel diode can be used instead. The switches are controlled with a pulse width modulation (PWM) scheme suitable for an H- bridge (for example unipolar or bipolar PWM).
[0004] In addition to ramping the current up and down fast, the gradient amplifier needs to generate a bidirectional high voltage and current. Because of this high voltage and current, high power components are used limiting the switching frequency. In order to switch the required voltage and current with a high frequency, it is known from US 2017/0045596 to use cascaded H-bridges. This allows to distribute the power demand over the H-bridges. Furthermore, the effective switching frequency is increased as well as the number of switching levels.
[0005] However, the H-bridge topology experiences dead-time {or blanking time) effects. The dead-time is a time where both top switches and bottom switches (in other words: all switches of one leg) are commanded to be off. This dead-time is added to prevent short circuiting the voltage source (shoot through) during a transition and create margin for cross-conduction due to the turn-on and turn-off delays of the switches. Due to the required margins on the dead-time, there is a moment where both the top and bottom switch of a single switching leg are off. During this time, the output voltage of the H-bridge is determined by the sign of the current in each switching leg, as this determines which parallel diodes will conduct. This current dependent output voltage has a negative effect on the converter output signal quality. The effect also generates a dead-band in the response of the output voltage/current on a changing control signal, specifically at low load currents. When using multiple H-bridge cells, the effect of the dead-time becomes larger as each H-bridge cell requires a certain dead-time. Because of the dead- band at low output currents, the output current is difficult to control, providing a poor output quality with a large error between the load current and the reference signal.
[0006] US 2017/00445596 describes to overcome dead-time effects by injecting a bias current in the centre of the switching legs of the H-bridge. The injected bias current forces the direction of the current through the switches at low output current levels, resulting in a well-defined switching transition at these low currents. Consequently, the dead-time effect is shifted to higher positive and negative load current, the effect is reduced and compensated by the controller. A drawback of this method is that it requires additional auxiliary circuits for each half H-bridge. This increases the complexity of the hardware, the number of components, the probability of component failure and the cost of the amplifier. Summary of the invention
[0007] It is an aim of the invention to provide an electric power converter which has at least equal and advantageously improved performance over electric power converters of the prior art, in particular at low output current levels. It is an aim of the invention to achieve this equal or improved performance with less hardware and/or at lower cost.
[0008] According to a first aspect of the invention, there is therefore provided an electric power converter as set out in the appended claims.
[0009] Electric power converters according to the invention comprise a first stack. The first stack comprises a plurality of controllable power switches arranged in one or more first H-bridges which are cascaded in series. The one or more first H-bridges can be at least one full H-bridge or a plurality of half H-bridges, or a combination of both.
[0010] According to the invention, the electric power converter comprises a second stack. The second stack comprises a plurality of controllable power switches arranged in one or more second H-bridges which are cascaded in series. The one or more second H-bridges can be at least one full H-bridge or a plurality of half H-bridges,
or a combination of both. According to the invention, the first stack and the second stack are parallel connected through an inductive component. The inductive component can be implemented in various ways. In one particularly advantageous way, the inductive component comprises at least one coupling inductor.
[0011] Electric power converters according to the invention allow for further reducing the dead-time effects compared to the converter of US 2017/00445596 in that it can shift the dead-time effects of the power switches to even higher current levels, where operation is less problematic. Additionally, the improved operational performance is obtained with less hardware and hence a reduced hardware complexity, which also lowers manufacturing cost.
[0012] Advantageously, the first and second stacks are operated in an interleaved manner, e.g. through operation of the controllable power switches of both stacks. This further allows to increase the number of voltage levels and the effective switching frequency, reducing ripple. One particular way of interleaving is configured to adapt a modulation parameter, such as a duty cycle, applied to the first stack and the second stack as described herein. The modulation parameter is applied to the pulse width modulation control signal to adjust an output voltage of each H-bridge.
[0013] According to a second aspect of the invention, there is provided a gradient amplifier for a magnetic resonance imaging apparatus comprising the electric power converter according to the first aspect.
[0014] According to a third aspect of the invention, there is provided a magnetic resonance imaging apparatus comprising a gradient coil and an electric power converter according to the first aspect. The electric power converter is operably coupled to the gradient coil.
[0015] According to a fourth aspect of the invention, there is provided a method of operating an electric power converter, as described herein.
[0016] Further advantageous aspects are set out herein below and in the dependent claims. Brief description of the figures
[0017] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:
[0018] Figure 1 represents a topology of an electric power converter according to the present invention;
[0019] Figure 2 represents the topology of a power stack that is used in the electric power converter of Fig. 1;
[0020] Figure 3 represents a first embodiment of inductive coupling between two parallel power stacks according to the present invention;
[0021] Figure 4 represents an equivalent electric circuit of the topology of Fig. 3 for the common mode current;
[0022] Figure 5 represents an equivalent electric circuit of the topology of Fig. 3 for the differential mode current;
[0023] Figure 6 represents a second embodiment of inductive coupling between two parallel power stacks according to the present invention;
[0024] Figure 7 represents an equivalent electric circuit of the topology of Fig. 6 for the common mode current;
[0025] Figure 8 represents an equivalent electric circuit of the topology of Fig. 6 for the differential mode current;
[0026] Figure 9 represents a third embodiment of inductive coupling between two parallel power stacks according to the present invention using a coupling inductor;
[0027] Figure 10 represents a layout of the coupling inductor used in Fig. 9 realized with the aid of a transformer;
[0028] Figure 11 represents the equivalent circuit of the coupling inductor of Fig. 10;
[0029] Figure 12 represents an equivalent electric circuit of the topology of Fig. 9 for the common mode current;
[0030] Figure 13 represents an equivalent electric circuit of the topology of Fig. 9 for the differential mode current;
[0031] Figure 14 represents another topology of an electric power converter according to the present invention;
[0032] Figure 15 represents a block diagram of the electric power converter of Fig. 14;
[0033] Figure 16 represents a diagram of the control structure for controlling the electric power converters of the present invention;
[0034] Figure 17 represents a block diagram of the power stacks of the electric power converter of Fig. 14;
[0035] Figure 18 represents a PWM scheme implementation of the electric power converter of Fig. 14, showing the voltage levels of the H-bridges of the two power stacks, the output voltage and the differential mode current circulating between the power stacks.
Description of embodiments
[0036] Referring to Fig. 1, one possible topology of the electric power converter according to the present invention comprises a power stage 11 and an output filter 12 coupled between the power stage 11 and a load 9. The power stage 11 5 comprises a pair of power stacks 131, 132.
[0037] Referring in more detail to Fig. 2, each power stack 131, 132 of the power stage 11 of Fig. 1 comprises a plurality of H-bridges 15. Each H-bridge 15 comprises power switches S1 to S4, such as IGBT switches, arranged in two switching legs 151, 152. Each switching leg comprises two power switches in series. Diodes D1 to D4 are coupled in antiparallel with the respective switches S1-S4. Alternatively, half H- bridges can be used instead of the full H-bridges 15. Each H-bridge 15 is connected to a voltage source 16, in particular a DC voltage source, advantageously an isolating DC- DC converter which provides a stable bus voltage Vous. A capacitor buffer 161 is advantageously coupled parallel to the voltage source 16 to counter sudden power demands. The switches S1-S4 are advantageously controlled with a pulse width modulation scheme suitable for an H-bridge (for example unipolar or bipolar PWM), as will be explained further below.
[0038] Each H-bridge 15 can switch its output voltage Vsns between -Vous, OV and +%us. The H-bridges 15 within a power stack are cascaded such that the outputs of the H-bridges are connected in series, between the output terminals 133 and 134. Cascading the H-bridges 15 within a single power stack 131, 132 combines the outputs Ventir..n OF all the H-bridges to one output voltage Vsn:, Vsn2 of the power stack 131, 132 respectively. The power stack output voltage Vsn:, Vsnz has an output range from -nvpys to +nVpus With increments of Vous, where n is the number of cascaded H-bridges.
[0039] Referring back to Fig. 1, the two power stacks 131, 132 are coupled in parallel by means of an inductive component 14. Each stack is advantageously configured to supply half of the output power, so the power components required to obtain the output power can be equally divided over the two stacks. During operation, the power stacks can sometimes have a same output voltage and sometimes a voltage difference. A difference in output voltage between the two power stacks 131, 132 causes a circulating current ía;;; (differential mode current) to flow between the two power stacks. In order to generate a controlled current and maintain this during switching, the inductive component 14 added between the power stacks 131, 132 will limit the current rise and fall in case of a voltage difference between the power stacks. In addition, the common mode current in is maintained when there is no voltage difference applied.
[0040] The inductive component 14 can be implemented in various ways. A first possible implementation is shown in Fig. 3. The inductive component comprises two inductors 141, 142, coupled symmetrically between the respective power stack 131, 132 and the output terminal 111. The equivalent electric circuits for the common mode current and the differential mode current are represented in Fig. 4 and Fig. 5 respectively.
[0041] A second possible implementation is represented in Fig. 6. The inductive component of Fig. 6 differs from the one of Fig. 3 in that a third inductor 140 is coupled in parallel to the inductors 141 and 142. By so doing, the inductance is reduced from a differential mode perspective. The equivalent electric circuits for the common mode current and the differential mode current are represented in Fig. 7 and Fig. 8 respectively.
[0042] A third possible implementation is represented in Fig. 9, wherein the inductive component 14 is a coupling inductor. A coupling inductor generally refers to a pair of magnetically coupled inductors 141, 142 which have one terminal that is short circuited between one another. Fig. 10 shows a possible implementation of the coupling inductor by means of a transformer. The inductor 141 forms the primary winding and the inductor 142 forms the secondary winding which are magnetically coupled through the transformer core 148. One terminal 144 of the primary winding 141 and one terminal 146 of the secondary winding 142 are electrically connected to each other and to an output terminal 147. Fig. 11 shows the equivalent electric circuit of the coupling inductor of Fig. 10, wherein L refers to the inductance of the primary winding, advantageously identical to the inductance of the secondary winding, and M = kVL.L = k. L refers to the mutual inductance. The equivalent electric circuits for the common mode current and the differential mode current are represented in Fig. 12 and Fig. 13 respectively. In common mode only the leakage inductance L,.,; occurs, whereas in differential mode only the magnetizing inductance L‚4 is seen by the differential current.
[0043] By coupling the two power stacks in parallel through the inductive component, a topology is obtained able to circulate current through the power stacks without influencing the output current or voltage. Therefore, when the output current ins must become low, the current i; is “circulated” between the power stacks 131 and 132 guaranteeing a well-defined switching transition. In addition, the number of switching levels can almost be doubled to 4n +1 by taking the average of v,,; and Vsn2.
[0044] The operation of the power stacks 131, 132 is furthermore advantageously interleaved, resulting in doubling the effective switching frequency of the output voltage vn.
[0045] Referring to Fig. 14, an alternative power stage 21 is depicted which differs from power stage 11 of Fig. 1 in that a second inductive component 24 is added in the loop of power stack 131 and power stack 132. The inductive component 14 is coupled between the upper output terminals of the power stacks 131, 132, whereas the second inductive component 24 is coupled between the lower output terminals of the power stacks. In contrast, in Fig. 1, the lower output terminals of the power stacks 131, 132 are connected to ground. Inductive components 14 and 24 can be identical.
[0046] The output voltage v applied at the output terminals 110 - 111 of the power stages 11 and 21 is advantageously filtered by an output filter 12 before applying to the load 9. The output filter 12 can be a passive second order LC-low pass filter. This results in a smoothed output voltage v,,,,, that is applied to the load 9.
[0047] Referring to Fig. 15, the electric power converter 10 comprises a controller 17. Controller 17 is operably connected to the H-bridges 15 of each power stack 131, 132. The power switches S1-S4 of each H-bridge are advantageously controllable power switches, and their operation is controlled by controller 17, advantageously implementing a PWM scheme, in particular through a unipolar PWM scheme. The controller can further be coupled to the output filter 12, e.g. for measuring an output current and/or output voltage of the power stage 11, 21. Alternatively, or in addition, a voltage and/or current sensor 171 can be provided at the output of the electric power converter, and coupled to the controller 17.
[0048] The electric power converter 10 can comprise an AC to DC converter 18 which is coupled to an external power supply for receiving power. The AC to DC converter supplies power to the isolating DC-DC converters 16.
[0049] Controller 17 advantageously operates the power switches of the H- bridges 15 so as to interleave the outputs of the power stacks 131 and 132. This increases the number of switching levels and the effective switching frequency.
[0050] Controller 17 advantageously implements a PWM scheme for operating the power switches S1-S4 of the H-bridges 15. The PWM scheme is advantageously based on a unipolar pulse width modulation. Each H-bridge 15 of the power stacks 131, 132 advantageously has its own PWM carrier. These PWM carriers may have the same waveform, but be phase-shifted. One advantageous PWM scheme implements modulation parameters d; and dg for the power stacks 131, 132 respectively. The modulation parameters can refer to a duty cycle of the PWM scheme. This means that the output voltage of the power stacks 131, 132 can respectively be defined as Vy, = danVpus and Vy, = dn, with vs the output voltage of the DC-DC converters 16 and n the number of cascaded H-bridges 15 within a power stack, with n being an even number or advantageously an odd number.
[0051] In one possible control implementation of controller 17, the power stacks 131, 132 are regarded as voltage sources, e.g. as described above in the preceding paragraph, which are integrated in a state space model of the power stage and output filter. Such a state space model may have the modulation parameters d, and dp as inputs and the currents i{;,,4 and igs; as defined in Fig. 1 as outputs. In an alternative control implementation, the modulation parameters are redefined as d, = deom + dairy and dg = doom — daf It can be shown that by so doing the modulation parameter dcom can be used to control the output current i;,,4, whereas dai can be used to control the circulating current (differential mode current) i.
[0052] Referring to Fig. 16, controller 17 can implement control scheme 170, comprising a feedback loop controller 173 for controlling a reference current ief that is to be applied to the load 9, e.g. based on sensing of i;,,4 through e.g. sensor 171. The feedback loop controller 173 advantageously provides a reference value for d
[0053] Advantageously, control scheme 170 comprises a second controller 174 for controlling the circulating current i, through adjustment of the modulation parameter dgi;;. The second controller 174 is configured to ensure that i4;;; stays within bounds, preventing damage to the power stacks and to the inductive components. The second controller can use a classic control strategy based on a measured and/or estimated value of the circulating current íg;;;. By way of example, the second controller 174 can determine an ideal value for the circulating current i; and adapt the modulation parameter dg; rr accordingly.
[0054] The reference values for modulation parameters dcom and daiff output from controllers 173, 174 are fed to PWM module 175 which generates the PWM control signals that are applied to the power switches S1-S4 of the H-bridges 15 of power stacks 131, 132.
[0055] Through modulation parameter d‚9m, the voltage level of the output voltage vg, is adjusted. By way of example, the modulation parameter dom can for example be set to vary between -1 and +1. This means, when dcom is set to 0, vg, will be 0 as well, when d,,,,, is set to 0.5, vn will be set to half the maximum output voltage, etc.
[0056] Advantageously, the PWM module 175 implements an offset do fet to dom for each of the H-bridges 15 allowing for controlling the circulating current
(differential mode current) i4;;; flowing between the two power stacks 131 and 132 in addition to, or in the alternative of the modulation parameter daiff.
[0057] Advantageously, the offset parameter doffset has a fixed value. Advantageously, doffser = 1/(27n), where n is the number of cascaded h-bridges. This allows for a desired amount of interleaving taking place.
[0058] Alternatively, the value of d‚ffser can be variable, e.g. it can depend on the value of the modulation parameter dom. or it can depend on the amplitude of the circulating current. Advantageously, the magnitude of the offset d,,.; is dependent on the modulation parameter dcom. By way of example, the offset doffset is selected such that the sum of dom and doser Will never exceed 1 in absolute value. This is particularly relevant at the boundaries of d‚9m, €.9. in proximity of the range limits +1 and -1. At the range limits of dom, €.9. at dcom = +1 and -1, the offset doffset is advantageously zero. By way of example, doffset can have a constant value x while dn, is between -1+x and +1-x, and gradually decrease towards 0 in the d«9m- range between -1 and -1+x, and between +1-x and +1, so that the sum of the two never exceeds 1 in absolute value. Advantageously, x= 1/(2*n), as defined above, allowing for doubling the effective switching frequency.
[0059] The value (magnitude) of doffser can also be made to change between the H-bridges of an H-bridge pair, i.e. a different offset (in magnitude) is applied to the two H-bridges within an H-bridge pair. Alternatively, or in addition, the value (magnitude) of doffset can also be made to change between the H-bridges of a same power stack 131, 132.
[0060] Advantageously, the offset doffser is applied to dcom in a time- varying manner. Advantageously, the offset do;fset is applied to corresponding H- bridges of the two power stacks 131, 132 in an inversed way, in particular by adding doffset 10 dcom for one H-bridge and subtracting doffset from dcom for the corresponding H-bridge of the other power stack. To this end, advantageously, each of the two power stacks comprises an equal and advantageously odd number of cascaded H-bridges. Alternatively, the offset do;fset can be applied to each H-bridge 15 depending on the PWM carrier wave. During ramp up of the PWM carrier wave, the offset do;fset can be positive, whereas the offset d,s. can be negative, though possibly equal in magnitude, during ramp down of the PWM carrier wave of the respective H-bridge.
[0061] In one example implementation, corresponding H-bridges 15 between the two power stacks 131 and 132 are considered to form H-bridge pairs as seen in Fig. 17. The H-bridges are schematically drawn as sources Vi: — Vian for the first power stack 131 and Vy,, — Vg2, for the second power stack 132. The first H-bridges Vi and Vy,, are considered as a first pair, the second H-bridges V4, and Vy,, are considered as a second pair, etc. The offset doffset is advantageously applied to each pair (Vi . Via), Wai, Vaan), ooo Vian» Vien), in a complementary manner. By way of example, for V‚11 the offset d,s... is added to dom, whereas in the other H-bridge Vj, of the pair the offset dofset is subtracted from decom. Hence the resulting modulation parameter applied to Vg: dyn = dcom + doffser. and the resulting modulation parameter applied to V‚;21: dygz21 = dcom — doffset. Additionally, the offset dojfset is advantageously alternated between the different H-bridge pairs. Yet additionally, the offset is advantageously alternated between the H-bridges within a same pair after each cycle. The can be obtained when the PWM carrier waves are properly interleaved between H-bridges of a pair, such as when identical PWM carrier waves are used which are phase shifted by 180° between H-bridges of a pair, and the offset dr... is positive or negative depending on whether the PWM carrier wave ramps up or down, respectively. Fig. 18 shows the outputs of the different H-bridges and resulting circulating current for the case n = 5, dcom = 0.05 and dopfset = 0.1.
[0062] Applying the offset doffset may render use of the modulation parameter dr; superfluous, and this modulation parameter may not be implemented. Alternatively, the modulation parameter da; is advantageously set to O under ‘normal’ operation conditions and adjusted only when unexpected problems occur, in particular when the circulating current i; exceeds a predetermined threshold.
[0063] It can be seen from Fig. 18 that the circulating current (differential mode current) í4;; is large at low output voltage level. This allows to have a well-defined soft switching behaviour at these low output voltage levels. The amplitude of the circulating current is further influenced by the inductance L,,, of the coupling inductor. By appropriate choice of the inductance L,,,, it can be obtained that the circulating current is large enough when the modulation parameter d_,,,, is zero.
[0064] By implementing a PWM scheme as above, it was observed that well defined soft switching transitions occur when the modulation parameter dcom is O. Furthermore, the electric power converters of the present invention allow for reducing ripple on the output current and voltage due to the increase in effective switching frequency and the creation of additional switching levels due to the cascaded dual power stack topology.
[0065] It will be convenient to note that the present invention contemplates electric power converters comprising more than two stacks which are coupled in parallel through inductive components, such as coupling inductors.
In such case, the inductive component can comprise an inductive element for each of the stacks, and these inductive elements are inductively coupled with one another.

Claims (20)

-12- Conclusies L. Elektrische-stroomomvormer (10), omvattende: een eerste stapel (131) die meerdere regelbare vermogenschakelaars (S1-S4) omvat die in ten minste één eerste volledige H-brug en/of meerdere eerste halve H- bruggen (15) opgesteld zijn, waarbij de ten minste ene eerste volledige en/of de meerdere halve H-bruggen in serie verbonden zijn, met het kenmerk dat de elektrische-stroomomvormer een tweede stapel (132) omvat die meerdere regelbare vermogenschakelaars (S1-S4) omvat die in ten minste één tweede volledige H-brug en/of meerdere tweede halve H-bruggen (15) aangebracht zijn, waarbij de ten minste ene tweede volledige en/of de meerdere halve H-bruggen in serie verbonden zijn, en dat de eerste stapel (131) en de tweede stapel (132) parallel verbonden zijn via een inductief onderdeel (14).-12- Claims L. Electric current converter (10), comprising: a first stack (131) comprising a plurality of controllable circuit breakers (S1-S4) arranged in at least one first full H-bridge and / or a plurality of first half H-bridges (15), the at least one first full and / or the plurality of half H-bridges being connected in series, characterized in that the electric current converter comprises a second stack (132) comprising a plurality of controllable circuit breakers (S1-S4 ) arranged in at least one second full H-bridge and / or a plurality of second half H-bridges (15), the at least one second full and / or the plurality of half H-bridges being connected in series, and that the first stack (131) and the second stack (132) are connected in parallel via an inductive member (14). 2. Elektrische-stroomomvormer volgens conclusie 1, waarbij het inductief onderdeel tussen de eerste stapel (131) en de tweede stapel (132) gekoppeld is, hetgeen toelaat om een gemeenschappelijke-modusstroom (ips) vanaf de eerste stapel (131) en de tweede stapel (132) uit te voeren, en een verschilmodusstroom (igirr) tussen de eerste stapel en de tweede stapel te circuleren.An electrical current converter according to claim 1, wherein the inductive member is coupled between the first stack (131) and the second stack (132), allowing common mode current (ips) to be transmitted from the first stack (131) and the second stack (132), and circulate a differential mode current (igirr) between the first stack and the second stack. 3. Elektrische-stroomomvormer volgens conclusie 1 of 2, waarbij het inductief onderdeel een paar inductoren (141, 142) omvat, waarbij elke inductor van het paar inductoren in serie verbonden 1s met de respectievelijke eerste en tweede stapel, en waarbij een gemeenschappelijke-modusstroom (ip) uitgevoerd vanaf de eerste stapel (131) en de tweede stapel (132) uit een tussenpunt tussen het paar inductoren getrokken wordt.An electric current converter according to claim 1 or 2, wherein the inductive member comprises a pair of inductors (141, 142), each inductor of the pair of inductors connected in series with the respective first and second stack, and wherein a common mode current (ip) output from the first stack (131) and the second stack (132) is pulled from an intermediate point between the pair of inductors. 4. Elektrische-stroomomvormer volgens conclusie 3, die een derde inductor (140) omvat die parallel verbonden is met het paar inductoren (141, 142).An electric current converter according to claim 3, comprising a third inductor (140) connected in parallel with the pair of inductors (141, 142). 5. Elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies, waarbij het inductief onderdeel (14) een eerste koppelingsinductor omvat dieAn electric current converter according to any of the preceding claims, wherein the inductive member (14) comprises a first coupling inductor which -13 - met de eerste stapel (131) en de tweede stapel (132) verbonden 1s.-13 - 1s connected to the first stack (131) and the second stack (132). 6. Elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies, die een regelaar (17) omvat die werkzaam verbonden is met de meerdere regelbare vermogenschakelaars (S1-S4) van de eerste stapel en van de tweede stapel, waarbij de regelaar ingesteld is om de eerste volledige en/of halve H-bruggen (15) en de tweede volledige en/of halve H-bruggen op een verscherfde wijze te bedienen.An electric current converter according to any one of the preceding claims, comprising a regulator (17) operably connected to the plurality of controllable circuit breakers (S1-S4) of the first stack and of the second stack, the regulator being set to operate the first full and / or half H-bridges (15) and the second full and / or half H-bridges in an interleaved manner. 7. Elektrische-stroomomvormer volgens conclusie 6, waarbij de regelaar (17) ingesteld is om de meerdere regelbare vermogenschakelaars (S1-S4) van de eerste stapel en van de tweede stapel te bedienen via een pulsbreedtemodulatieschema dat een modulatieparameter (decom) definieert die een gemeenschappelijke- modusspanningsniveau (ve) van de eerste stapel en van de tweede stapel instelt.An electric current converter according to claim 6, wherein the controller (17) is set to operate the plurality of controllable circuit breakers (S1-S4) of the first stack and of the second stack via a pulse width modulation scheme that defines a modulation parameter (decom) that defines a common mode voltage level (ve) of the first stack and of the second stack. 8. Elektrische-stroomomvormer volgens conclusie 7, waarbij het pulsbreedtemodulatieschema een offset (domser) implementeert die de modulatieparameter (dcom) in pulsbreedtemodulatieregelsignalen van de eerste en tweede volledige en/of halve H-bruggen (15) selectief aanpast, wat het regelen van een verschilmodusstroom die tussen de eerste stapel (131) en de tweede stapel (132) stroomt, mogelijk maakt.An electric current converter according to claim 7, wherein the pulse width modulation scheme implements an offset (domser) that selectively adjusts the modulation parameter (dcom) in pulse width modulation control signals of the first and second full and / or half H-bridges (15), thereby controlling a differential mode current flowing between the first stack (131) and the second stack (132). 9. Elektrische-stroomomvormer volgens conclusie 8, waarbij het pulsbreedtemodulatieschema ingesteld is om de modulatieparameter (dcom) selectief aan te passen middels het afwisselen van optelling en aftrekking van de offset (dost) tussen de eerste en tweede volledige en/of halve H-bruggen (Van, Vm) die op overeenkomstige locaties in de eerste stapel (131) en in de tweede stapel (132) ingericht zijn.An electric current converter according to claim 8, wherein the pulse width modulation scheme is set to selectively adjust the modulation parameter (dcom) by alternating addition and subtraction of the offset (dost) between the first and second full and / or half H-bridges. (Van, Vm) arranged at corresponding locations in the first stack (131) and in the second stack (132). 10. Elektrische-stroomomvormer volgens conclusie 8 of 9, waarbij het pulsbreedtemodulatieschema ingesteld is om de modulatieparameter (dcom) selectief aan te passen middels het afwisselen van optelling en aftrekking van de afwijking (doser) tijdens respectievelijk een oploop en een afloop van het pulsbreedtemodulatieregelsignaal van de respectievelijke eerste of tweede H-brug.An electric current converter according to claim 8 or 9, wherein the pulse width modulation scheme is set to selectively adjust the modulation parameter (dcom) by alternating addition and subtraction of the deviation (doser) during a ramp-up and a ramp-down, respectively, of the pulse width modulation control signal of the respective first or second H-bridge. -14 --14 - 11. Elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies, waarbij het inductief onderdeel een tweede koppelingsinductor (24) omvat die tussen de eerste stapel (131) en de tweede stapel (132) bij tegenovergestelde aansluitklemmen (133) verbonden is in vergelijking met de eerste koppelingsinductor (14).An electric current converter according to any one of the preceding claims, wherein the inductive member comprises a second coupling inductor (24) connected between the first stack (131) and the second stack (132) at opposite terminals (133) as compared to the first coupling inductor (14). 12. Elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies, die een bovenste tussenaansluitklem (111) en een onderste tussenaansluitklem (110) omvat, waarbij het inductief onderdeel verbonden is met de bovenste tussenaansluitklem (111), en optioneel met de onderste tussenaansluitklem (110), en waarbij de elektrische-stroomomvormer een uitvoerfilter (12) omvat dat met de bovenste en onderste tussenaansluitklemmen gekoppeld is.An electrical current converter according to any one of the preceding claims, comprising an upper intermediate terminal (111) and a lower intermediate terminal (110), the inductive part being connected to the upper intermediate terminal (111), and optionally to the lower intermediate terminal (111), 110), and wherein the electric current converter includes an output filter (12) coupled to the upper and lower intermediate terminals. 13. Elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies, waarbij elke van de eerste en tweede volledige en/of halve H-bruggen (15) verbonden is met een spanningsbron (16).An electric current converter according to any one of the preceding claims, wherein each of the first and second full and / or half H-bridges (15) is connected to a voltage source (16). 14. Elektrische-stroomomvormer volgens conclusie 13, waarbij de spanningsbron (16) een isolerende DC/DC-omvormer is.An electric current converter according to claim 13, wherein the voltage source (16) is an isolating DC / DC converter. 15. Gradiéntversterker voor een magnetische-resonantie-afbeeldingsapparaat die de elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies omvat.A gradient amplifier for a magnetic resonance imaging device comprising the electric current transducer according to any one of the preceding claims. 16. Magnetische-resonantie-afbeeldingsapparaat dat een gradiëntspoel en een elektrische-stroomomvormer volgens eender welke van de voorgaande conclusies omvat, waarbij de elektrische-stroomomvormer werkzaam verbonden is met de gradiëntspoel.A magnetic resonance imaging device comprising a gradient coil and an electric current converter according to any one of the preceding claims, wherein the electric current converter is operatively connected to the gradient coil. 17. Werkwijze voor het bedienen van een elektrische-stroomomvormer (10), omvattende: het inrichten van meerdere regelbare vermogenschakelaars (S1-S4) in meerdere volledige en/of halve H-bruggen (15) die in twee parallelle stapels (131, 132) in serieA method of operating an electric current converter (10), comprising: arranging a plurality of controllable circuit breakers (S1-S4) in a plurality of full and / or half H-bridges (15) arranged in two parallel stacks (131, 132 ) in series -15 - cascadeopstelling geschakeld zijn, waarbij elke stapel ten minste één volledige H-brug en/of meerdere halve H-bruggen omvat, waarbij de parallelle stapels verbonden zijn via een inductief onderdeel (14, 24), en het bedienen van de meerdere regelbare vermogenschakelaars (S1-S4) om een gemeenschappelijke-modusstroom (inu) uitgevoerd vanaf de twee parallelle stapels (131, 132) en een verschilmodusstroom (iaif) die tussen de twee parallelle stapels circuleert te verschaffen.-15 - cascade arrangement, each stack comprising at least one full H-bridge and / or multiple half-H-bridges, the parallel stacks being connected via an inductive part (14, 24), and operating the multiple controllable circuit breakers (S1-S4) to provide a common mode current (inu) outputted from the two parallel stacks (131, 132) and a differential mode current (iaif) circulating between the two parallel stacks. 18. Werkwijze volgens conclusie 17, waarbij de regelbare vermogenschakelaars (S1-S4) van de meerdere volledige en/of halve H-bruggen bediend worden via pulsbreedtemodulatie wat het verkrijgen van verscherfde werking van de meerdere volledige en/of halve H-bruggen (15) tussen de twee parallelle stapels (131, 132) mogelijk maakt.A method according to claim 17, wherein the controllable circuit breakers (S1-S4) of the plurality of full and / or half H-bridges are operated via pulse width modulation to provide interleaved operation of the plurality of full and / or half H-bridges (15 ) between the two parallel stacks (131, 132). 19. Werkwijze volgens conclusie 17 of 18, waarbij de regelbare vermogenschakelaars (S1-S4) bediend worden via een regelschema dat een modulatieparameter (decom) definieert die een gemeenschappelijke- modusspanningsniveau (vs) van de twee parallelle stapels (131, 132) instelt, waarbij het regelschema een offset (dure) implementeert die de modulatieparameter (decom), die op de meerdere volledige en/of halve H-bruggen toegepast wordt, selectief aanpast, hetgeen het regelen van een verschilmodusstroom (iaifr) die tussen de twee parallelle stapels (131, 132) stroomt, mogelijk maakt.A method according to claim 17 or 18, wherein the controllable circuit breakers (S1-S4) are operated via a control scheme that defines a modulation parameter (decom) that sets a common mode voltage level (vs) of the two parallel stacks (131, 132), wherein the control scheme implements an offset (decom) that selectively adjusts the modulation parameter (decom) applied to the multiple full and / or half H-bridges, thus controlling a differential mode current (iaifr) flowing between the two parallel stacks ( 131, 132) flows. 20. Werkwijze volgens conclusie 19, waarbij de modulatieparameter (decom) selectief aangepast wordt middels het afwisselen van optelling en aftrekking van de offset (dofrser) tussen H-bruggen (Vai, Vrai) van de meerdere volledige en/of halve H-bruggen die op overeenkomstige locaties in de twee parallelle stapels (131, 132) opgesteld zijn.A method according to claim 19, wherein the modulation parameter (decom) is selectively adjusted by alternating addition and subtraction of the offset (dofrser) between H-bridges (Vai, Vrai) of the plurality of full and / or half H-bridges are arranged at corresponding locations in the two parallel stacks (131, 132).
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