RF pulse amplifier with DC / DC converter and method for amplifying RF pulses - Patents.com

Abstract

The RF pulse amplification apparatus comprises an amplifier configured to amplify a pulsed RF signal, a DC link configured to provide a DC bus voltage, and an energy storage device connected to the DC link. The amplifier has an input for receiving a power supply voltage. The amplifier input is connected to the DC link through a switched DC / DC converter configured to step down the DC bus voltage provided at the converter input to a power supply voltage applied to the converter output. A control unit is configured to operate the switched DC / DC converter during amplification of the RF pulses to control the power supply voltage to a predetermined value.

Description

[Technical field]

[0001] The present invention particularly relates to a radio frequency (RF) pulse amplification apparatus comprising an RF amplifier and an energy storage device for providing peak power during amplification of the pulse. [Background technology]

[0002] Pulsed RF applications such as magnetic resonance imaging (MRI) require high peak power for short periods of time, on the order of a few milliseconds, to generate a single pulse. The peak-to-average power ratio in these applications is typically in the range of 5 to 40. This requires either a large energy (capacitor) buffer or a power supply capable of delivering peak power. However, the latter solution is not favored due to its large impact on grid capacitance and the additional cost and footprint of a power supply sized based on peak power instead of average power.

[0003] One drawback of using an energy buffer to deliver peak power is that it leads to voltage droop in the energy buffer if the rate of depletion of the buffer exceeds the rate of regeneration from the power supply. This typically occurs during the generation of a pulse. The RF output power is determined by the relationship: RF =V DD 2 The DC voltage supplied by the energy buffer to the amplifier is directly related to the DC voltage supplied by the energy buffer through / β, where β represents a constant that depends on the load impedance and the power capabilities of the transistors used. If the energy buffer is simply attached to the amplifier, the pulse power will be reduced, leading to RF power droop. This phenomenon occurs especially when the amplifier is operated in compression.

[0004] The RF power droop problem was a result of the previously used VDMOS (vertical double-diffused metal oxide semiconductor) transistor technology (power supply voltage V DD =150V) for RF amplifiers compared to LDMOS (Laterally Diffused Metal Oxide Semiconductor) transistor technology (supply voltage V DDThis has become more evident with the introduction of the 50V supply voltage (=50V). The constant β for LDMOS is 9 times lower compared to VDMOS leading to a much higher supply current for the same RF power. A 1V supply voltage droop for a VDMOS RF amplifier typically leads to a 1.3% RF power droop, while a 1V voltage droop for an LDMOS RF amplifier typically leads to a 4% RF power droop. In other words, to have an equal RF power droop for LDMOS compared to VDMOS, the supply voltage should be lower than 330mV. The RF power droop problem is also evident for GaN transistor devices which also feature a 50V supply voltage.

[0005] US Patent No. 6,072,315, June 6, 2000, describes an RF magnetic field pulse generator with an energy storage device and a switching voltage regulator that regulates the supply voltage of an RF amplifier. The energy storage device is connected to the amplifier through a switching voltage regulator with a local storage capacitor. The supply voltage is determined by the voltage across the local storage capacitor. The switching voltage regulator controls when the local storage capacitor is charged from the energy storage device. The recharge logic is such that the local storage capacitor is charged in synchronism with the pulse train and that the local storage capacitor is only charged between successive pulses, not during the pulse. US Patent No. 6,072,315 recognizes that the supply voltage may change during the course of one pulse. However, the voltage regulator ensures that the supply voltage is restored to the correct level for the next pulse.

[0006] While the technique of US Pat. No. 6,072,315 may be acceptable for ultrashort pulses on the order of a few microseconds, it is not acceptable for longer pulses on the order of a few milliseconds, such as are customary for MRI applications.

[0007] US Patent Application Publication No. 2017 / 0102441, April 13, 2017, describes a pulsating load, such as an RF amplifier power supply, connected to a three-phase AC mains supply through a passive rectifier. An energy storage device, such as an ultracapacitor, is connected through a DC / DC converter to the DC link created by the passive rectifier. The energy storage device is connected downstream of the rectifier and upstream from the RF amplifier power supply through the DC / DC converter. Power is shared between the AC mains supply and the energy storage device during pulsing periods, and power drawn during non-pulsing periods is used to recharge the energy storage device. The release of energy from the energy storage device during peak load power periods leads to a reduction in the power and therefore the current drawn from the AC mains supply.

[0008] One drawback of the above system is that in order to prevent significant RF power droop, a large capacitor bank is required as an energy storage device, leading to high cost and volume. Nevertheless, even in such a case, RF power droop cannot be completely avoided. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 6,072,315 [Patent Document 2] US Patent Application Publication No. 2017 / 0102441 [Patent Document 3] International Publication No. 2020 / 058361 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, there is a need in the art to provide an RF pulse amplifier that can overcome the shortcomings of the prior art. In particular, there is a need for an RF pulse amplifier that can achieve better performance at reduced cost and volume. [Means for solving the problem]

[0011] According to a first aspect of the present disclosure, therefore, there is provided an apparatus for amplifying radio frequency (RF) pulses as set out in the appended claims. In an aspect of the present disclosure, the RF pulse amplification apparatus comprises an amplifier configured to amplify a pulsed RF signal, a DC link configured to provide a DC bus voltage, and an energy storage device connected to the DC link. The amplifier comprises an input for receiving a power supply voltage. The input of the amplifier is connected to the DC link (and thus to the energy storage device) through a switched (or switched mode) DC / DC converter configured to step down the DC bus voltage (provided at the converter input) to a power supply voltage (applied to the converter output). A control unit is configured to operate the switched DC / DC converter during operation of the RF amplifier (i.e. when amplifying the RF pulses) to control the power supply voltage to a predetermined value. Advantageously, the control unit is configured to control the switched DC / DC converter to switch continuously, i.e. during pulse generation or amplification as well as between successive pulses of the pulsed RF signal. Therefore, advantageously the switched DC / DC converter is arranged to switch continuously during operation of the device, independently of whether pulses are applied or not.

[0012] The apparatus described in this disclosure therefore allows for efficient control of the supply voltage at the input of the amplifier to a predetermined value, regardless of variations in the DC bus voltage of the DC link. This DC bus voltage may be allowed to vary over a wide range, relaxing the specifications required for the energy storage device and any upstream mains-to-DC converter with respect to voltage droop. At the same time, the supply voltage can be tightly controlled so that any voltage droop occurring at the DC link does not propagate to the output of the amplifier, effectively preventing RF output power droop. The apparatus as described herein therefore provides high performance at minimal cost and volume.

[0013] Advantageously, the switched DC / DC converter is configured (for example through the control unit) to be operated at a switching frequency substantially higher than the repetition frequency of the RF pulses. The ratio of the switching frequency to the repetition frequency of the RF pulses is at least 10, preferably at least 20, preferably between 50 and 10000. This allows to effectively control the supply voltage to a stable value. Advantageously, the switched DC / DC converter is configured to be operated continuously. The switched DC / DC converter can be a non-isolated converter, for example a half-bridge converter, which provides a very economical pulse generator of minimal volume while ensuring high performance. Alternatively, it can be an isolated converter.

[0014] Therefore, in an apparatus as described herein, the amplifier is connected through a switched DC / DC converter to an energy storage device, which may comprise one or more storage capacitors, such as a capacitor bank. The energy storage device may be connected to a mains power supply, e.g. an AC power supply, through a mains-to-DC converter. The mains-to-DC converter has an output which is advantageously connected to a DC link. The mains-to-DC converter is advantageously configured to supply energy to the energy storage device, the mains-to-DC converter being advantageously rated based on the average output power of the amplifier.

[0015] In some examples, multiple series arrangements of switched DC / DC converters and amplifiers are provided. Each amplifier receives a power supply voltage from a respective switched DC / DC converter. The series arrangements of switched DC / DC converters and amplifiers can be connected in parallel at the output of the amplifier, for example through a power combiner, and / or at the input of the switched DC / DC converter, for example in a DC link. In this way, a higher output power RF pulse amplification device can be provided.

[0016] According to a second aspect of the present disclosure, there is provided an apparatus for generating an RF electromagnetic field, such as a magnetic resonance imaging apparatus, a plasma generating apparatus, a particle accelerator or a laser apparatus, comprising an apparatus as described herein.

[0017] According to a third aspect of the present disclosure, there is provided a method for amplifying RF pulses as set out in the appended claims. The method according to the present disclosure comprises the steps of storing energy in an energy storage device at a DC bus voltage and stepping down the DC bus voltage to a power supply voltage using a switched DC / DC converter. The power supply voltage is applied to an amplifier which amplifies the RF pulses. The switched DC / DC converter is operated during amplifying the RF pulses. Advantageously, the amplifier outputs RF pulses at a pulse repetition frequency and the switched DC / DC converter is operated at a switching frequency, the ratio of the switching frequency to the pulse repetition frequency being at least 10, preferably at least 20, preferably between 50 and 10000.

[0018] Advantageously, the ratio of the DC bus voltage to the power supply voltage is at least 1.5, preferably at least 2. A larger ratio of the DC bus voltage to the power supply voltage allows for larger voltage variations in the DC bus voltage during pulse generation (e.g. due to voltage droop). The DC bus voltage can be between 20V and 500V, in particular between 70V and 300V. The power supply voltage is advantageously between 5V and 200V, preferably between 12V and 150V or between 28V and 100V.

[0019] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which like reference numbers illustrate like features, and in which: [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram of an apparatus for amplifying RF pulses according to an embodiment of the present disclosure. [Diagram 2]FIG. 2 is a diagram of a non-isolated DC / DC converter that can be used in an apparatus for amplifying RF pulses according to an embodiment of the present disclosure. [Diagram 3] FIG. 1 is a schematic diagram of an isolated DC / DC converter that can be used in an apparatus for amplifying RF pulses according to an embodiment of the present disclosure. [Figure 4] FIG. 1 is a schematic diagram of an apparatus for amplifying RF pulses according to an embodiment of the present disclosure comprising multiple amplifiers and an RF power combiner. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] With reference to Fig. 1, an apparatus 10 for amplifying RF pulses according to the present disclosure is configured to provide an amplified RF pulsed signal at output 16, which may comprise one or more pulses. The RF pulsed signal is generated by a signal generator 153 and fed to an RF amplifier 15 at a signal input 152. The RF amplifier 15 may be any suitable power amplifier as known in the art capable of amplifying a pulsed RF signal, preferably a class B or class E amplifier, configured to amplify the pulsed RF signal received at the signal input 152 to provide an amplified signal at output 16. The RF pulses are advantageously AC pulses and may have a pulse duration on the order of a few milliseconds, such as between 1 ms and 100 ms, preferably between 1 ms and 50 ms, although even shorter pulses are possible, for example on the order of between 50 μs and 1 ms. The pulse duty cycle may range between 0.5% and 25%, typically between 1% and 10%. The power delivered by the pulses may range between 1 kW and 100 kW, possibly between 2 kW and 60 kW, such as between 5 kW and 50 kW, for example about 20 kW, which is typical for MRI applications.

[0022] The RF amplifier 15 is driven by a power supply voltage V DD For RF amplifiers based on LDMOS or GaN transistors, the power supply voltage V DDcan range between 12V and 100V, typically V DD =50V. The power delivered by RF amplifier 15 at output 16 is typically directly related to the power supply voltage.

[0023] The input 151 of the RF amplifier 15 is connected to the DC link 13 through a DC / DC buck converter 14. The DC link is connected to a supply voltage V DD Bus voltage higher than V BUS The DC / DC converter operates at the bus voltage V BUS supply voltage V DD The 10-V 100-MHz ...

[0024] An energy storage device 12 acting as an energy buffer is connected to the DC link 13. The energy storage device 12 may comprise a capacitor bank, or one or more ultracapacitors, or any other energy buffering system as known in the art. The energy storage device typically receives the bus voltage V of the DC link 13. BUS The energy storage device 12 buffers the electrical energy at 100 V. The energy storage device is charged via a mains power supply 9, which typically provides AC power. An AC / DC converter 11 is connected between the mains power supply 9 and the energy storage device 12. In particular, the AC / DC converter 11 has a DC output that is connected to a DC link 13, and the energy storage device 12 is connected to the DC link 13. The AC / DC converter 11 can be an active or passive rectifier as known in the art.

[0025] Advantageously, AC / DC converter 11 is rated based on the average power required by RF amplifier 15. The output power provided by AC / DC converter 11 can be limited to or rated against the average power required by RF amplifier 15. Energy storage device 12 and DC / DC converter 14 are sized to be capable of delivering the required peak power, whereas the peak power required by RF amplifier 15 during pulse generation is typically 5 to 40 times higher.

[0026] The bus voltage V during pulse amplification due to the discharging energy storage device BUS A voltage droop of V is unavoidable. BUS The voltage droop of the RF amplifier power supply voltage V DD To prevent the bus voltage from spreading to the DD The switch (or switch mode) DC / DC buck converter 14 is chosen to be higher than the bus voltage V BUS supply voltage V DD The DC / DC converter 14 is therefore configured to provide a controlled and substantially stable supply voltage V DD Therefore, when power is drawn from the RF amplifier, particularly during amplification of the pulse, the bus voltage V BUS can vary, whereas the DC / DC converter has a constant supply voltage V DD V BUS As a result, the apparatus according to the present disclosure avoids RF power droop resulting from voltage droop in the energy buffer bus voltage.

[0027] The specifications of the energy storage device 12 with respect to voltage droop can therefore be relaxed, since voltage fluctuations will be absorbed by controlling the operation of the switched DC / DC converter 14. The energy storage device 12 can therefore be designed as a much smaller energy buffer compared to prior art solutions, allowing the volume and cost of the device according to the present disclosure to be reduced compared to existing RF pulse amplification devices. In addition, the specifications of the AC / DC converter 11 can also be relaxed, since the converter 11 needs to be designed for the average output power of the device 10. The AC / DC converter 11 provides a predetermined voltage V BUS 1. The bus voltage V can be operated continuously, regardless of the amplification of the pulses by the RF amplifier 15, in order to charge the energy storage device 12 and compensate for any idle losses (e.g., of the DC / DC converter 14) in order to maintain the bus voltage V. BUS Appropriate control logic can be provided to operate the AC / DC converter 11 based on the sensed voltage level.

[0028] ratio V BUS / V DD (nominal value) is advantageously at least 1.3, advantageously between 1.5 and 20, possibly between 1.7 and 10, such as 2, 2.5 or 3. BUS / V DD The larger the V BUS The allowable voltage fluctuations on V during the pulse are large. BUS In some cases, a voltage fluctuation of at least 20% above V BUS A voltage fluctuation of at least 30% or at least 50% above can be tolerated.

[0029] With reference to FIG. 2, an embodiment of the switched DC / DC converter 14 is provided as a non-isolated half-bridge converter. The half-bridge 143 comprises a controllable high-side switch T1, which can be a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) switch device with unidirectional or bidirectional current blocking capability, and a low-side switch T2, which can be a controllable switch (e.g., MOSFET or IGBT as with T1) or a passive device with current blocking capability, such as a diode. The upper node of the half-bridge 143 is connected to an input terminal 141 of the DC / DC converter 14. The input terminal 141 can be connected to the positive rail of the DC link 13. The lower node of the half-bridge 143 can be connected to the negative rail of the DC link 13 or to a common ground, as illustrated in FIG. 2. The middle node 144 of the half-bridge 143, between the switches T1 and T2, is connected to an output terminal 142 of the DC / DC converter 14 through an inductor L.

[0030] A local input capacitor C1 may have its positive terminal connected to the input terminal 141. The negative terminal of C1 may be connected to a common ground or, possibly, to the negative rail of the DC link 13. The bus voltage V BUS is therefore provided across the local input capacitor C1. The local input capacitor C1 is used to provide local decoupling. It is not configured in principle to buffer energy. A local output capacitor C2 may have its positive terminal connected to the output terminal 142. The negative terminal of C2 may be connected to a common ground or, possibly, to the negative input terminal of the RF amplifier 15. The supply voltage V DD is provided across the local output capacitor C2. Like C1, the local output capacitor C2 is provided merely for local decoupling. It is not, in principle, configured to buffer energy.

[0031] A control unit 17 controls the operation of the switches T1 and possibly T2 (if T2 is controllable) of the half-bridge 143, for example through pulse width modulation (PWM). DD and provides a feedback signal 171 to the control unit 17. In this way, the control unit 17 controls the supply voltage V through appropriate adaptation of the duty cycle of T1 (and possibly T2). DD The bus voltage V BUS The control unit 17 can control the power supply voltage V DD An input 172 may be provided for receiving a setpoint value for such a half-bridge converter. Such a half-bridge converter provides high performance at minimal cost and volume.

[0032] 1, the control unit 17 can additionally control the operation of the RF amplifier 15, and in particular the generation of the RF pulsed signal. Additionally or alternatively, the control unit 17 can control the operation of the AC / DC converter 11.

[0033] Other converter topologies can be used for the DC / DC converter 14. By way of example, and referring to FIG. 3, it may be advantageous to provide an isolated DC / DC converter 24, such as including a transformer 248 providing galvanic isolation between the input terminal 141 and the output terminal 142 of the DC / DC converter 24. The DC / DC converter 24 comprises a first full-bridge converter circuit 246 connected to the input terminal 141 and a first ground node G1, and a second full-bridge converter circuit 247 connected to the output terminal 142 and a second ground node G2. The first and second full-bridge converter circuits 246, 247 are coupled through a transformer 248 providing galvanic isolation, for example having a 1:1 turns ratio, although any other suitable turns ratio may also be used. The switches in the bridge legs of the first full-bridge converter circuit 246 may be active semiconductor switching devices, in which case they are operated through the control unit 17. The switches in the bridge legs of the second full-bridge converter circuit 247 can be active or passive semiconductor switching devices. It is convenient to note that the ground nodes G1 and G2 should be galvanically isolated. Many isolated DC / DC converter topologies can be used, such as dual active bridges, flyback converters, series or parallel resonant converters, and half-bridge or full-bridge converters.

[0034] Referring to FIG. 4, the RF pulse generator 20 includes a number of RF power amplifiers 151 to 155. N Each of the RF amplifiers 151 to 15 may include a power combiner 18 for combining the outputs 16 of the RF amplifiers 151 to 15 into a combined output 26. N DC / DC converters 141 to 144 each provide a power supply voltage to each RF amplifier. N The RF pulse generator (such as block 153 in FIG. 1) is not shown in FIG. 4 for the sake of clarity. All of the DC / DC converters 141 to 14 N The input terminals of all the DC / DC converters 141 to 14 are connected to the DC link 13. Nis the same bus voltage V BUS , and each of the N DC / DC converters is supplied with a given supply voltage V, which can be the same or different. DD,1 ~V DD,N A suitable combiner is disclosed in WO 2020 / 058361.

[0035] Comparative Example In this comparative example, the diagram of FIG. 1 was considered, but without the DC / DC converter 14 and with the bus voltage V BUS is the power supply voltage (V DD ), i.e. V BUS =V DD The allowable voltage droop (dV BUS ) is limited to 0.1V, and V DD = 50V. The peak power output by the RF amplifier, P RF_SUP = 10 kW and pulse duration T RF The case of = 5 ms was considered. The required capacitance C of the capacitor bank for the energy storage device 12 BUS was calculated as follows: C BUS =[(P RF_SUP / V BUS )×T RF ] / dV BUS =[(10kW / 50V)×5ms] / 0.1=10F

[0036] Working Example In this example, the diagram of FIG. 1 was considered, including the DC / DC converter 14. The bus voltage V BUS = 100V is considered, and the allowable voltage droop dV BUS =25V(V BUS The RF amplifier power supply voltage V DD = 50V needs to be provided by the output of the DC / DC converter 14. As in the comparative example, the peak power output by the RF amplifier P RF_SUP = 10 kW and pulse duration T RF= 5 ms. The required capacitance C of the capacitor bank for the energy storage device 12 BUS was calculated as follows: C BUS =[(P RF_SUP / V BUS )×T RF ] / dV BUS =[(10kW / 50V)×5ms] / 25=40mF This means that an energy storage device can be designed with a capacitance 250 times smaller than the comparative example.

[0037] Advantageously, the ratio of the capacitance of the energy storage device 12 to the peak power output of the RF amplifier 15 in a pulse generator according to an embodiment of the present disclosure (C BUS / P RF_SUP ) is less than 0.100 mF / W, advantageously less than 0.050 mF / W, advantageously less than 0.025 mF / W, advantageously less than 0.5.10 -3 between 0.020 mF / W and 1.10 -3 The (rated) bus voltage V in the pulse generator according to the embodiment of the present disclosure is between 0.010 mF / W and 0.010 mF / W. BUS is advantageously at least 50V, at least 70V, at least 75V, at least 100V or at least 120V. BUS is advantageously 1000V or less, advantageously 500V or less, 300V or less or 200V or less.

[0038] RF pulse generators as described in this disclosure can be used to generate RF electromagnetic fields. They may find application as amplifiers in MRI equipment, as plasma generators, CO2 lasers or particle accelerators. To this end, the output 16 or 26 of the pulse generators 10, 20, respectively, can be coupled to a tank circuit 30, which may include an RF coil for generating a magnetic field. [Explanation of symbols]

[0039] 9 Main power 10. Apparatus, RF pulse generator 11 AC / DC converter 12 Energy Storage Devices 13 DC Link 14 DC / DC converter 141,14 N DC / DC Converter 15 RF Amplifier 151,15 N RF Power Amplifiers 16 Output 17 Control Unit 18 Power combiner 20 Apparatus, RF pulse generator 24 Isolated DC / DC Converter 26 Combined Outputs 30 Tank Circuit 141 Input terminal 142 Output terminal 143 Half Bridge 144 intermediate nodes 145 Voltage Sensor 151 Input 152 Signal Input 153 Signal Generator 171 Feedback Signal 172 Input 246 First Full-Bridge Converter Circuit 247 Second Full-Bridge Converter Circuit 248 Transformer C1 local input capacitor C2 local output capacitor G1 First Grand Node G2 Second ground node L Inductor T1 High Side Switch T2 Low Side Switch V BUS Bus Voltage V DD Power supply voltage V DD,1 ,V DD,N Power supply voltage

Claims

1. 1. An apparatus (10, 20) for amplifying radio frequency (RF) pulses, comprising: An amplifier (15) configured to amplify the RF pulsed signal, the amplifier (15) being connected to a power supply voltage (V DD an amplifier (15) having an input (151) for receiving DC bus voltage (V BUS a DC link (13) for supplying an energy storage device (12) connected to the DC link; a switched DC / DC converter (14, 24) having a converter input (141) connected to the DC link and a converter output (142) connected to the input (151) of the amplifier; a control unit (17) configured to operate the switched DC / DC converter; Equipped with The switching DC / DC converter (14, 24) controls the DC bus voltage (V BUS ) at the converter output. DD ) The control unit (17) is configured to continuously operate the switched DC / DC converters (14, 24) to control the power supply voltage at a predetermined value during generation of the RF pulsed signal. An apparatus (10, 20) characterized in that:

2. 2. The apparatus of claim 1, wherein the switched DC / DC converter (14) is configured to operate at a switching frequency, the ratio of the switching frequency to the repetition frequency of the RF pulses being at least 10.

3. The power supply voltage (V DD ) to the DC bus voltage (V BUS 2. The device of claim 1, wherein the ratio of the rated values ​​of

4. 2. The apparatus of claim 1, wherein the control unit (17) is configured to switch the switched DC / DC converter (14) during amplification of pulses of the RF pulsed signal.

5. 2. The apparatus of claim 1, further comprising a mains-to-DC converter (11) having an output connected to the DC link (13), the mains-to-DC converter (11) being configured to supply energy to the energy storage device (12).

6. 6. The device according to claim 5, wherein the mains-to-DC converter (11) is an AC-to-DC converter.

7. The apparatus of claim 1 , wherein the energy storage device (12) comprises one or more storage capacitors.

8. 8. The apparatus of claim 7, wherein the ratio of the capacitance of the energy storage device (12) to the peak output power of the amplifier is between 0.5 mF and 100 mF.

9. A plurality of said switch DC / DC converters (14 1 , 14 N ) and a plurality of said amplifiers (15 1 , 15 N 10. The apparatus (20) of claim 1, comprising: a plurality of amplifiers each connected in series with a respective one of the plurality of switch DC / DC converters to receive the supply voltage from a respective one of the plurality of switch DC / DC converters; and the plurality of amplifiers connected in parallel to an output (26) of the apparatus.

10. 10. The apparatus of claim 9, wherein the output (26) of the apparatus comprises a power combiner (18) connected to the outputs of the plurality of amplifiers.

11. 10. The apparatus of claim 9, wherein the plurality of switched DC / DC converters are connected in parallel to the DC link (13).

12. 12. Apparatus for generating an RF electromagnetic field, comprising a device according to any one of claims 1 to 11.

13. 13. The device of claim 12, comprising a coil for generating the RF electromagnetic field, the coil being coupled to the device (10, 20).

14. The device of claim 12, which is a magnetic resonance imaging device.

15. The device of claim 12, which is a plasma generating device.

16. 1. A method for amplifying radio frequency (RF) pulses, comprising: DC bus voltage (V BUS ) storing energy in an energy storage device (12); A switching DC / DC converter (14) is used to convert the DC bus voltage to a power supply voltage (V DD ) applying the power supply voltage to an amplifier (15) that amplifies the RF pulse; Including, The method wherein the power supply voltage is controlled to be at a predetermined value by continuously switching the switched DC / DC converter (14) while amplifying the RF pulse.

17. 17. The method of claim 16, wherein a plurality of the RF pulses are amplified, the plurality of RF pulses having a pulse repetition frequency, and the switched DC / DC converter (14) is operated at a switching frequency, and a ratio of the switching frequency to the pulse repetition frequency is at least 10.

18. The power supply voltage (V DD ) to the DC bus voltage (V BUS 17. The method of claim 16, wherein the ratio of

19. generating the RF pulse; applying the RF pulse to the amplifier; 17. The method of claim 16, further comprising:

20. The method of claim 16 , wherein the energy in the energy storage device is stored in one or more capacitors.

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