Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THPH028

DEVELOPMENT PROGRESS OF THE 4TH GENERATION OF SWITCHING POWER SUPPLY FOR CIRCULAR INDUCTION ACCELERATORS
Katsuya Okamura, High Energy Accelerator Research Organization, Tsukuba, Japan
Yi Liu*, Institute of Fluid Physics, CAEP, Mianyang, China
Ken Takayama, High Energy Accelerator Research Organization, Tsukuba, Japan
Akira Tokuchi, Pulsed Power Japan Laboratory Ltd., Kusatsu, JapanAbstract
A novel synchrotron called an induction synchrotron (IS)was developed at KEK in 2006. In the IS, charged particles
are accelerated and confined by bipolar high-voltage
pulses generated by switching power supply (SPS), which
is one of the key technologies for IS. In this work, the 4th
generation of SPS, based on full-bridge circuit with 3.3 kV
high-speed MOSFET is developed. Compared with previous prototype, better waveforms have been achieved since
the structure has been redesigned with a better balance ofparasitic inductance and capacitance in each arm. The heatdissipation performance at high frequency has been improved through applying MOSFETs in parallel, and confirmed by single shot test and 50 kHz test. Temperature at
500 kHz has been also estimated by simulation.

BACKGROUNDKEK digital accelerator (KEK-DA) [1-3] is the first circular induction accelerator [4] in the world, which utilizes
induction cells instead of RF cells to accelerate particle
bunches. Essentially, the induction cell is a one-to-one
pulse transformer, of which the particle bunch is the secondary coil. The induction cell accelerates particle bunches
that are passing by, just like the RF cell does in a synchrotron. The induction cell is driven by switching power supply (SPS) directly, without any waveguide. So it has no frequency bandwidth limitation compared with RF cell.
Therefore, the circular induction accelerator, either Synchrotron or Microtron, can works without an injector.
As the heart of circular induction accelerator, SPS generates positive pulses to drive the induction cells, while
negative pulses to reset the magnetic core. Up to now, 3generations of switching power supplies have been developed. The voltage rating of the Si-MOSFET of the 1st generation [5] is only 1.0 kV, so 7 MOSFETs in series wereneeded for each arm. Years later, 1.2 and 2.4 kV SiC-JFET
were utilized for the 2nd [6-8] and 3rd [9, 10] generation
respectively. In recent years, the 4th generation, which utilized 3.3 kV SiC-MOSFET, was under development [11].
After heat dissipation analysis and structure redesign [12],the updated 4th generation of SPS has been manufactured
and tested in this work.

UPDATED DESIGN AND MANUFACTURE
The 4th generation of SPS is based on full-bridge circuitwith 3.3 kV high-speed MOSFETs. Bipolar high-voltagepulses are generated within an operation period. On theload, positive pulse is generated when ARM 1 and 4 are onbut ARM 2 and 3 are off, while negative pulse is generated
when ARM 2 and 3 are on but ARM 1 and 4 are off.
Some problems emerged in the previous design of the4th generation of SPS, as seen in Fig. 1. Firstly, each ARM
contained only one MOSFET. The switching loss of theMOSFETs was high and heat dissipation was serious. Secondly, buses are made of wide copper plate and stacked together, with isolating paper sandwiched between them. The
parasitic capacitance of buses inevitably increased the risetime and fall-time of the output pulse. Thirdly, parasitic capacitance of buses was unbalanced because of its asymmetric structure. The 𝑉 of each MOSFET was not half of HV
before switching on and could not return to half of HV afterswitching off, unless an extra equalizing CR circuit wasapplied.
The problems mentioned above have been improved in
the updated design, as seen in Fig. 2. Two parallel
MOSFETs have been applied in each ARM, to reduce theFigure 1: Circuit schematics, structure of buses and picture of the previous design of 4th generation of SPS.
___________________________________________* yiliu.ifp@outlook.com- 669 -

Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

23 cm

PASJ2019 THPH028

50 cm

Figure 2: Circuit schematics, structure of buses and picture of the updated design of 4th generation of SPS.
switching loss. The buses have been redesigned. The parasitic capacitance of UV reaches almost 0. The parasitic capacitance of UN, VN, UP and VP has been reduced as low
as possible and is almost the same. Thus, the rise-time andfall-time of the output pulse have been kept low, and theequalizations of output voltage and 𝑉 of ARMs havebeen achieved. The details of the experiment results are described in the next Section.

switch-on period. Switch-off loss was defined as the device
loss generated from the time point of drain voltage risingfrom 10% to 100% of the recovery voltage in the switchoff period. The switch-off loss was higher than the switchon loss, because the rise-time of 𝑉 was longer than theSINGLE SHOT PERFORMANCE
Waveforms and switching loss
A single shot test of the updated manufactured 4th generation of SPS was conducted with an 80 Ω resistive load.
The pulse width was set as 300 ns and the delay time between positive and negative pulses was set as 500 ns. The
𝑉 of each ARM was measured one by one, by using a pairof HV differential probes. The current of load, which wasapproximated as the current of each ARM (𝐼 ), was measured by a current transformer at the same time. Figure 3
shows the 𝑉 waveform, 𝐼 waveform, switching power(𝑃 ) and switching loss (𝐸 ) of ARM 1 as an example,
at a HV varying from 0.5 kV to 2.5 kV. 𝑃 was calculated𝑉 𝐼 ) and the 𝐸
by 𝑉 and 𝐼 waveforms ( 𝑃was the integration of 𝑃 ( 𝑉 𝐼 𝑑𝑡). The waveforms of𝑉 were improved significantly, compared with the previous design. Because of the fairly low and balanced parasitic capacitance of buses, 𝑉 of each ARM was half of
HV before switching on and returned to half of HV afterswitching off as expected. The 𝐼 was around 31 A and thefall-time of 𝑉 of each ARM was around 25 ns at 2.5 kV
HV and 80 Ω load.
Figure 4 (a) shows the switch-on loss, switch-off loss
and total switching loss of each ARM. The total switchingloss increased more at higher HV. Switch-on loss was defined as the device loss generated from off state to the time
point of drain voltage falling to 10% of the DC value in theFigure 3: 𝑉
ARM 1.

- 670 -waveform, 𝐼waveform, 𝑃and 𝐸ofProceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, JapanPASJ2019 THPH028

(a)

(b)

Figure 4: (a) Switch-on loss, switch-off loss and total switching loss of each ARM at different HV. (b) Switching loss
comparation of the previous and the updated SPS.
fall-time of 𝑉 . Note that the parasitic capacitance of UV
was almost zero, the time constant, which was determinedby the product of the capacitance and resistance of the load,
was ensured as low as possible. So the fall-time and risetime of 𝑉 in this test could furthest reflect the MOSFET
characteristics.
As seen in Fig. 4 (b), in the same condition (2 kV HV, 80
Ω load), compared with the previous design, the total
switching loss has been reduced from 2607 μJ to 1581μJ.
Almost 40% improvement on the previous design has beenachieved. Furthermore, at 2.5 kV HV the total switchingloss is 2518 μJ.

calibration factors applied

low-pass FFT filter applied

Current monitor calibration
In the updated design, two 0.1 Ω sampling resistors werein series with ARM2 and 4 respectively. Their voltage wasextracted by coaxial cables with matching resistors to theoscilloscope. Then the sampling resistor could be used as acurrent monitor (CM) to measure the conducting current ofARMs and monitor the operation state. A calibration test
was executed, in which the measurement results from current transformer (CT) were treated as standard value. The
calibration factor should be 20 theoretically. In our test, thecalibration factors were 22 and 21 for these current monitors respectively as seen in Fig. 5. On the other hand, highfrequency oscillations were observed as noise signal in thewaveform. A low-pass FFT filtering process was applied at35 MHz, which was determined by an empirical formula𝑓 0.35/𝑡 , where 𝑡 was 10 ns to 25 ns in our experiment. The results were shown in Fig. 6. The solid line
waveforms were from current transformer (CT), while theFigure 6: Comparation of the standard waveforms fromcurrent transformer and the processed waveforms fromcurrent monitor.
dot line waveforms were from current monitor (CM). In thetop graph, only calibration factor was applied. In the second graph, the low-pass FFT filtering process was added.
The waveforms from CT and CM became almost the same.
The third and bottom graph shows the calculated 𝑃 and𝐸 . The value from CM was a little higher than that fromCT, because the rise-time and fall-time were slightly longer,
which were caused by the resistance and inductance of thesampling resistor inevitably.

CONTINUOUS OPERATION

Figure 5: The indications from current transformer andcurrent monitors.

A low frequency continuous operation test was conducted successfully with an 80 Ω resistance load, 2.5 kV
HV and 50 kHz. As seen in the Fig. 7, the output voltageof the load was almost the same as that in a single shot test.

- 671 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THPH028

ACKNOWLEDGEMENTS
The authors wish to thank Rohm Co., Ltd for provisionof Prototype MOSFETs.

REFERENCES

Figure 7: Output voltage of the load at 50 kHz and single
shot.
Thanks to the updated design, the heat power of theMOSFET was around 17.5 W (50 kHz) and the temperature increment of the cases of two randomly selected
MOSFETs were 5.9 C and 6.8 C respectively. A heat dissipation simulation based on multi-physics and finite element method, in which the model had been confirmed previously, was carried out. As seen in Table 1, the results ofexperiment and simulation matched well. Furthermore, ifthe SPS operate at 500 kHz, the estimated heat power ofeach MOSFET would be 175 W. According to the simulation, the junction temperature would be as high as around120 C, but still lower than the rated value 175 C. So, theMOSFETs` survivability of the updated SPS was optimistic at the condition of 500 kHz, 2.5 kV, 31 A and double
300 ns pulse.
Table 1: Heat Dissipation in Experiment and Simulation
Settings
Air temperature
Water temperature
TIM
Water flow / L/min
Frequency / kHz
Heat power / W
Results

Experiment≈22 C26.3 C≈0.1 mmIndium
≈ 0.7550≈ 17.5∆T of cases / C

5.9 & 6.8

∆T of water / CJunction
temperature /C0.5-Simulation
22 C26 C
0.1 mmIndium
0.75
50500
17.5
1755.2 &5.4
0.635.3 &35.5

17.7 &
20.7
6.6
119 &
121

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CONCLUSION
The 4th generation of Switching Power Supply for Circular Induction Accelerators is redesigned and manufactured. The heat dissipation and output waveform are improved by parallel MOSFETs and new structure respectively. A current monitor component is applied and calibrated. Stable 50 kHz operation is achieved, and the junction temperature of MOSFET at up to 500 kHz is estimatedand the result is optimistic.

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