Development of bipolar high-current correction magnet power supply for TPS facility

Taiwan Photon Source (TPS) is a globally renowned 3 GeV synchrotron accelerator light source. With a successful decade of operation, various subsystems have continuously improved and maintained an optimal research facility environment. Presently, there is a concerted effort towards energy conservation. This technical report focuses on the future development of the TPS-II Permanent Magnet Trim Coil Power Supply for the correction magnet, emphasizing a Bipole high-current correction magnet power source. According to the prototype specifications, the maximum output current is 20 A with an operating voltage of 48 V. This augmentation increases the amplitude of the magnetic field correction in the permanent magnet-associated Trim core, providing greater flexibility in manufacturing the permanent magnet correction coils. To design a power supply with high current and stability, the system adopts the Danisense DP50-IP-B DC Current Transducers (DCCT) as the current feedback component and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) as power switches in a full-bridge (H-bridge) configuration. The driving frequency is set at 40 kHz. Analog modulation control circuitry and protective circuits ensure precise control loop modulation. In the power laboratory, a hardware prototype circuit was constructed, featuring a 48 V input voltage, 20 A output current, a maximum power of 960W, and a current ripple component maintained within 100 μA. This experiment validates the control loop design of the prototype, showcasing the ability to achieve rapid and stable output current performance. Using a 1 V input reference signal for small-signal testing, the bandwidth displayed a -3 dB bandwidth of 8.51 kHz. Long-term current stability is within ± 10 ppm, and interface compatibility with the existing TPS correction magnet power source interface allows direct operation within the current system.


Introduction
Over the past decade, the TPS has consistently maintained its world-class status with top-notch beam quality and facility performance.However, continuous improvements and modernization of various subsystems are crucial for sustaining and enhancing the research facility environment [1,2].In the foreseeable future, TPS is poised to advance towards energy conservation.For instance, replacing the existing iron-core magnet structure with a permanent dipole, quadrupole, and sextupole magnets in the storage ring is expected to significantly reduce the components' dependence on power.From the perspective of the power system, this transition is anticipated to bring substantial energy savings, projected at 3,071 kilowatts per hour.There is a growing recognition of the potential of permanent magnets in power systems, and collaborative efforts are underway to develop bipolar high-current correction magnet power sources.To meet the requirements of the internal correction coils in permanent magnets, a 20-ampere high-current correction magnet power supply has been designed [3][4][5][6][7].This increased current capacity provides greater flexibility in the magnetic field adjustment during the production of permanent magnets, meeting the demands of the Fast Orbit Feedback (FOFB) system in TPS [8,9].FOFB plays a critical role in maintaining the stability of particle beam trajectories by swiftly correcting any deviations caused by external disturbances or system fluctuations.By incorporating FOFB requirements into the design, the enhanced power bandwidth aims to improve the beam stability and reproducibility within TPS.The primary objective -1 -of this research is to address these challenges by adopting the Danisense DCCT as the current feedback component for the TPS correction magnet power supply.This DCCT seamlessly integrates into the new correction magnet power supply board version while retaining compatibility with the existing TPS correction magnet power source interface and protection logic, ensuring maximum compatibility [10][11][12][13].The development of the Bipolar High-Current Correction Magnet Power Supply (BHCCMPS) system comes with numerous advantages.Firstly, it provides high output current capacity with repeatability and stability, allowing for the verification of the performance and reliability of this power source prototype through experiments.Additionally, insights and design principles from developing the BHCCMPS system extend beyond TPS, influencing power technologies in similar synchrotron accelerator light sources globally.This report delves into the background and significance of TPS, explores the motivations behind this research, introduces the adopted methodologies, discusses the design and implementation of BHCCMPS, and evaluates its performance through experimental results.The study aims to propel advancements in power technologies for synchrotron accelerator light sources and provide valuable insights for future developments [14][15][16][17][18].

Design of the BHCCMPS
In addressing the challenges posed by the future application development of permanent magnet trim coils (PMTC) within the TPS, we have developed the BHCCMPS.The TPS correction magnet power source (CMPS) is pivotal in the storage ring.Its version, shunt, and high-precision zero-flux DCCT have been collaboratively developed with the Industrial Technology Research Institute (ITRI).However, the specifications of the primary current feedback components, LEM ITN 12-P DCCT and Isabellenhutte RUG-Z-R001, are limited to 12.5 amperes, and their costs have steadily increased.Considering these factors, there has been a drive to explore alternative solutions.We adopted the Danisense DP50-IP-B DCCT as the current feedback component to tackle this challenge.It is bipolar, offering high bandwidth, cost-effectiveness, and a compact design.This DCCT provides three current modes for enhanced flexibility.This design reduces costs and expands the current sensing range, ensuring stable modulation under high current output conditions.The entire BHCCMPS architecture includes DC-DC converters, control compensation circuits, and interlock protection circuits, forming the foundation for implementing BHCCMPS.However, due to the BHCCMPS's output power capacity being twice that of the CMPS version, a thorough evaluation and update of the Buck power are still required.In the following sections, we will delve into the experimental setup, methods, and performance evaluation of the BHCCMPS, providing valuable information for power technologies in synchrotron accelerator light sources.

Power stage and PWM algorithm
The overall configuration of the BHCCMPS is depicted in figure 1, encompassing the power stage, buck converter, current feedback circuit, and protection circuit.In the power stage, the full-bridge converter is selected as the DC-DC power converter due to its advantages, primarily its ease of polarity switching and modulation mode operation.This choice produces outstanding output current zero-current crossing characteristics, generating bipolar voltage and current outputs.The analog current error signal is processed through a PI compensator and compared with a triangular carrier to form a 40 kHz pulse-width modulation (PWM) frequency.Under the PWM modulation mode, two UC3525N -2 -components generate positive and negative current PWM signals and a slight bias, enhancing control performance at low currents.This configuration aids in common current control and response speed during the zero-current period, as illustrated in figure 2. Introducing switch losses in the MOSFET concurrently increases the duty cycle during low currents, improving resolution at low currents.The HIP4081A full-bridge driver IC operates four MOSFETs to produce a pulse-modulated voltage waveform.After this modulation process, the output signal passes through an L-C low-pass filter comprising two parallel 100 μH output inductors, six 0.82 μF output capacitors, and a damping load.This arrangement eliminates harmonic voltages generated by high-frequency switching, correcting magnet loads and achieving the desired current output settings.Adjustments to the wire diameter of output inductor L may be necessary depending on the current output specifications.Further details on other aspects of the BHCCMPS will be elaborated in subsequent sections.

Plug-and-play PI compensation card and PWM timing synchronization card
In this BHCCMPS paper, we have implemented a plug-and-play architecture for PI compensation and synchronous PWM timing adjustment, as depicted in figure 3, to minimize harmonic content in the output current.Let's delve into each component separately.

Plug-and-Play PI Compensation Card: this PI compensation card features a PI compensator
circuit integrated into the system, providing real-time compensation for the analog current error -3 -  signal.This facilitates effective adjustment and fine-tuning of the current control loop, enabling it to adapt to varying load parameters.Therefore, the PWM timing synchronization card enhances system stability and control accuracy.Both cards play a crucial role in optimizing our system performance.They facilitate the seamless integration and precise control of the PI compensator and PWM signals.The "plug-and-play" design simplifies the setup process, enhancing system efficiency and reliability.

Redundant buck power supply
In the BHCCMPS system, a redundant parallel configuration of buck converters has been employed.This design allows seamless switching to a redundant unit in case of a failure or any issues in one of the units, ensuring continuous power supply.The objective is to enhance reliability and minimize downtime caused by power supply failures.We opted for the MW RCP-1000-48-C 1 kW power converters as the choice for the buck converters.These converters have three fundamental protection features: overload protection, overvoltage protection, and overheat protection.These protective measures ensure the safe operation of the system in redundant mode.Furthermore, these converters offer 12-volt and 24-volt voltage specifications, allowing flexible selection of the operating voltage for the buck power supply based on the actual MOSFET and output current characteristics.Figure 4 shows three RCP-1000-48-C modules combined to form a 3-kW redundant buck power supply unit, fitting into a compact 1U form factor.This ensures that there is no feedback between the current control loop of the trim coil power supply and the external measurement system, thereby preventing mutual interference.

Protection Logic circuit
The protection logic circuit is crucial for the safe and stable operation of power electronics and converters.In the BHCCMPS, various protection mechanisms have been implemented, including: 1. Input Side Overcurrent Protection (FUSE): when the input current to the DC BUS exceeds 20 A, the fuse will blow, interrupting the circuit.

Output Overcurrent Protection (IOVL):
IOVL protection activates when the output current exceeds 15% of the rated current due to a load short circuit or other uncontrollable factors.

Auxiliary Power Ground Protection (PWM POWER):
PWM POWER protection comes into play when the control ICs UC3525A, HIP4081A, or the 12-volt power supply malfunctions.

Power Switch Heat Sink Protection (T-SINK):
T-SINK protection triggers when the temperature of the MOSFET heat sink exceeds 70 • C.

Output Oscillation Protection Circuit (T1-TEMP & T2-TEMP):
when output current oscillations occur, the temperature of the damping load resistor increases.If it exceeds 70 • C, T1-TEMP and T2-TEMP protection will be activated.Upon the occurrence of any of these fault signals, the BHCCMPS system halts the operation, latches the fault signals, and indicates the fault area through LEDs on the panel.Additionally, the BHCCMPS status is compiled into I2C sequences through PCF8574, assisting maintenance personnel in identifying the direction for repairs.As depicted in figure 5, system operators can read the status of the BHCCMPS system through this interface.

Current feedback components and PCB layout 3.1 Current feedback components
This version's current feedback component is the Danisense DP50IP-B, renowned for its high stability and precision through DCCT technology.Key specifications include the ability to momentarily measure a maximum current of up to 72 amperes on a PCB board, linear accuracy within ten ppm, utilization of zero-flux detection technology, PCB mounting configuration, three selectable rated currents (12.5 A, 25 A, 50 A), 300 kHz high bandwidth, and a compact form factor.Its physical appearance is illustrated in figure 6.The DP50IP-B outputs in current mode, and its signal is converted to a voltage signal through a precise low-temperature coefficient resistor (25R, 0.01%, VFR B1214), entering the current feedback control loop modulation.In terms of cost-effectiveness, the DP50IP-B has proven more economical -6 -than the escalating prices of LEM DCCT and shunt versions.A single Isabellenhutte RUG-Z-R100-0.1-TK1(100 mΩ shunt resistor) costs approximately $500.In comparison, the Danisense DP50IP-B is priced at around $250.It boasts a cost advantage with a selectable measurement range of 25 amperes, making it the preferred choice for the current feedback component in the BHCCMPS system.

Output inductor of low-pass filter
Enhancing the output current specifications of BHCCMPS requires meticulous consideration of various components.For instance, the rated specification of the MOSFET is 150 amperes, and the circuit design adopts the IXFK180N15 model.Monitoring the associated heatsink temperature is crucial.Additionally, the output inductor must be capable of withstanding the required current and ensuring that the magnetic core material remains unsaturated.In this power supply version, we have prepared two types of inductors for testing purposes, capable of handling continuous currents of up to 25 amperes.The specifications for these inductors are 50 μH/25 A and 100 μH/25 A, respectively.

Circuit and PCB layout design
We performed updates and revisions using the current TPS CMPS version as the baseline.The objective was to replace the original Isabellenhütte RUG-Z-R100-0.1-TK1component with the Danisense DP50IP-B and reconfigure the output inductor.The challenge involved planning a structure within limited space to accommodate multiple output inductors and allow for DP50IP-B's current mode selection.Several key considerations include: 1. Component Layout: by integrating DP50IP-B into the corrected magnetic flux design, we effectively ensured space for parallel output inductors to handle existing applications of 25 amperes and 50 amperes.
2. Current Selection Setup: DP50IP-B offers various current switch modes, as the datasheet describes.We configured different current modes through jumper settings, redesigning them into the PCB layout of BHCCMPS to simplify the configuration process (see figure 7).The design includes three horizontal traces for 12.5-ampere output, a square frame for 25-ampere -7 -operation, and traces for 50-ampere output.These jumper connections require customized copper traces to ensure sufficient current-carrying capacity.
3. Enhanced Ground Protection: we strengthened the ground areas on both layers of the PCB to enhance signal integrity and reduce noise interference.This step ensures that BHCCMPS exhibits improved output current characteristics.-8 -

JINST 19 T05004 4 Experimental results
Several experiments were conducted to validate the current output performance of BHCCMPS.These experiments encompassed step response analysis, output current ripple, long-term output current stability, and small-signal bandwidth of the output current.To ensure the accuracy and reliability of the test results, we employed high-precision, high-resolution instruments for measuring external output current and stability.The Dynamic Signal Analyzer (DSA) and Ultrastab Saturn Current Transducers (USCT) played crucial roles in analyzing and measuring the output current spectrum.

Step response
Fine-tuning of the output current performance is easily achievable by adjusting the plug-in PI compensator sub-board.For instance, when subjected to a 2-volt step command, the system undergoes multiple iterations of PI compensator parameter adjustments (resistors and capacitors).This process facilitates the selection of appropriate parameter settings, as illustrated in figure 9.
Step response plots under different parameter configurations demonstrate a rapid rise time of 220 μs with no overshoot.For this load condition, the values of Kp and Ki resistors and capacitors were determined as 10.89 kΩ and 0.1 μF, respectively.

Output current spectrum analysis
We utilized the HP35670A DSA as the measurement tool for conducting spectral analysis of the output current.The study was performed within the frequency range of 10 to 6.4 kHz with a resolution of 1600 lines.Figure 10 illustrates the FFT analysis of the output current provided by the DP50IP-B at 20 amperes, with the FLUKE 741B serving as the reference signal source.The spectral components of the prototype's output current mainly stem from the 120 Hz component, contributing 58.46 μA.The harmonic components are derived from the low-frequency 60 Hz AC power source, with the total output current ripple maintained within 0.4 mA.Given its performance, the DP50IP-B represents an excellent, cost-effective solution.

Stability
The long-term output current stability measurement in BHCCMPS, USCT, and an 8 1/2 digit highresolution digital voltmeter (DVM) was used for recording instruments.The stability was tested using a four-pole magnet to measure ±20 amperes of output current.-10 -

Bandwidth
To measure the frequency response of the BHCCMPS output current using small signal current disturbances, the HP 35670A DSA was employed in frequency sweep sine mode.The source function was configured to set DC offset and scaling levels.Figure 12 illustrates the frequency response of the DP50IP-B magnet power supply system to a 1-ampere signal disturbance.The −3 dB gain yields a bandwidth of 8.51 kHz, while the −45 • phase occurs at 3.25 kHz.Additionally, figure 13 displays the gain and phase curves for output current under different voltage levels (1-10 amperes) disturbances.
It is noteworthy that with increasing disturbance current, the bandwidth decreases.However, the performance is impressive, especially at 10 amperes of output current, where the gain still reaches 2.576 kHz, and the phase is at 1.68 kHz.These results indicate that the system is well-suited for TPS applications, serving as a rapid correction magnet power source.-11 -

Conclusions
The BHCCMPS system significantly enhances magnet power sources' output current capabilities and precision control technology.We have demonstrated this system's outstanding performance and reliability through a comprehensive series of experiments and analyses.The plug-and-play PI compensation card and PWM timing synchronization card are crucial for optimizing system efficiency.They simplify the setup process and enhance the overall efficiency and reliability of the system.The redundant buck power supply automatically takes over in case of a failure, ensuring uninterrupted power supply and minimizing downtime, thus improving system reliability.The protection circuits within the system provide essential safety mechanisms, ensuring the stable operation of power electronics and converters.The Danisense DP50IP-B current feedback component has proven to be an economically efficient and reliable choice, characterized by high stability and accuracy, making it highly suitable for BHCCMPS applications.Furthermore, the system's output current stability exhibits outstanding performance through long-term measurements, reaching ±10 ppm within a ±20 ampere output current range.Additionally, small signal frequency response analysis confirms that the system achieves impressive bandwidth and phase performance even under challenging conditions.The BHCCMPS system is well-suited for future TPS permanent magnet coil power source applications.

Figure 1 .
Figure 1.The architecture of the BHCCMPS.

Figure 2 .
Figure 2. The PWM algorithm with bias voltage circuit.

Figure 3 .
Figure 3.The plug-and-play PI controller card and PWM Timing Sync Card.

2 .
PWM Timing Synchronization Card: it can synchronize PWM signals across all BHCCMPS modules.Since a single chassis can accommodate up to eight BHCCMPS modules, all powered by a standard buck DC voltage source, synchronizing PWM signals among these modules is crucial.Synchronization failure could lead to harmonic components in the BHCCMPS output current, directly impacting output current performance.

Figure 5 .
Figure 5. Latch interlock and Encoded fault state circuit of BHCCMPS.

Figure 8
Figure 8 displays hardware photos of the completed BHCCMPS prototype in our laboratory.The design maintains compatibility with the control interface and TPS CMPS version consistency.

Figure 10 .
Figure 10.The output current spectrum analysis of BHCCMPS.
Figure 11 illustrates the estimated output current stability of BHCCMPS.The output current was set to ±20 amperes, sampled, and recorded every 10 seconds over an eight-hour duration.The variation in output current remains within 400 μA, indicating a current stability of approximately ±10 ppm.

Figure 11 .
Figure 11.Output current stability of the BHCCMPS during 8 hours.

Figure 13 .
Figure 13.The different disturbance currents of BHCCMPS bandwidth.