pulsed power modulator with active pull-down using diode...

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 35, NO. 3, MARCH 2020 2943

Pulsed Power Modulator With Active Pull-DownUsing Diode Reverse Recovery Time

Su-Mi Park and Hong-Je Ryoo , Member, IEEE

Abstract—This paper presents a pulsed power modulator witha new structure that utilizes the reverse recovery characteristics ofdiodes to achieve an effective pull-down circuit configuration. Theproposed configuration for active pull-down applicable to nega-tive pulsed power applications with a high-voltage direct switchingmethod is implemented by adding a diode to the pulse dischargingpath of the modulator, instead of using a pull-down resistor. Thediode is forward-biased while an output pulse voltage is applied tothe load. When the pulse voltage applied to the load is removed,the residual energy in the load is quickly discharged through thereverse recovery path of the diode so that the pull-down func-tion of the pulsed power supply system is efficiently performed.The operating principle of the proposed system is described, andeach operating mode is analyzed in detail. In addition, the feasi-bility of the proposed system configuration is examined throughvarious simulation results. Finally, it is proven that the proposedsystem can be used effectively in pulsed power application fieldsthrough a practical experiment in plasma source ion implantationapplications.

Index Terms—Plasma source ion implantation (PSII), pull-downcircuit, pulsed power modulator, reverse recovery.

I. INTRODUCTION

R ECENTLY, a lot of pulsed power application fields such asgas or water treatment, medical lasers, and plasma sourceion implantation (PSII) require a high repetition rate [1]–[6].In response to this requirement, research on solid-state pulsedpower modulators has been performed. Applications using acapacitive load have difficulties achieving the required fast pulsefall time, because the voltage applied to the load is maintainedin the load even after the pulse voltage is removed; for example,

Manuscript received May 14, 2019; accepted June 13, 2019. Date of publica-tion June 23, 2019; date of current version December 13, 2019. This work wassupported in part by the Chung-Ang University Graduate Research Scholarshipin 2017 and in part by the National Research Foundation of Korea Grant fundedby the Korea Government (MSIP) (NRF-2017R1A2B3004855). Recommendedfor publication by Associate Editor B. Chen. (Corresponding author: Hong-JeRyoo.)

S.-M. Park is with the Department of Energy Engineering, Chung-Ang Uni-versity, Seoul 06974, South Korea (e-mail: [email protected]).

H.-J. Ryoo is with the School of Energy Systems Engineering, Chung-AngUniversity, Seoul 06974, South Korea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2019.2924586

the plasma reactor load in high-voltage plasma applications isa capacitive load consisting of complex resistor–capacitor (RC)elements.

A simple solution to generate the square-wave pulse voltage isto connect a pull-down resistor in parallel with the load. This de-creases the fall time of the pulse voltage while providing a paththrough which the residual energy in the reactor load can bequickly discharged. However, additional power dissipation oc-curs at the pull-down resistor because current also flows throughthe pull-down resistor during the application of the pulse. In ad-dition, a lower resistance should be used to decrease fall time,which leads to more losses. The push-pull circuit, where thepull-down resistor is replaced by simple stacked switching de-vices can be a solution to this large loss problem [7], but it hasa complicated structure [8]. Although Yu et al. [8] proposes asimpler structure than the push-pull circuit, the structure still re-quires an additional switching device, a “bypass insulated gatebipolar transistor (IGBT),” and a gate driving circuit, leading tohigher cost. For active pull-down methods where the pull-downresistor is replaced with a pull-down switch such as a MOSFET orIGBT, control for the synchronized gate pulses is required andthe dead time between the main switch for pulse discharge andthe pull-down switch should be considered.

In this paper, a simple and new structure to achieve the activepull-down function is proposed, which can be effectively ap-plied to negative pulsed power applications using high-voltagedirect switching without any additional active components suchas gate driving circuits and/or a control circuit for synchroniza-tion. The proposed modulator system includes a fast recoverydiode in the charging path of the storage capacitor and a diodewith relatively slow reverse recovery time in the dischargingpath, instead of using a pull-down resistor. Owing to the re-verse recovery characteristics of the diode, the voltage of thecapacitive load can be discharged quickly without the lossesincurred by a pull-down resistor, and the pull-down operationcan be performed automatically after the main IGBT is turnedOFF. Therefore, an effective low-cost pull-down function withsimple and reliable operation can be achieved, without any de-sign considerations for the dead time circuit required in an activepull-down circuit using semiconductor switches. In addition, theload discharging current during the pull-down operation is lim-ited by the charging current limiting circuit in the high-voltagecapacitor charger (HVCC), so that even if the load capacitanceis somewhat large, no short-circuit current flows through thedevices.

0885-8993 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

https://orcid.org/0000-0001-8657-7350mailto:[email protected]:[email protected]

2944 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 35, NO. 3, MARCH 2020

Fig. 1. Simplified circuit diagrams of conventional pulsed power modulators.(a) Using a pull-down resistor. (b) Using a pull-down switch.

II. OPERATING PRINCIPLE OF PULSED POWER SYSTEM WITHPROPOSED PULL-DOWN CIRCUIT

A. Conventional System With Pull-Down Resistor orPull-Down Switch

Fig. 1 shows the circuit diagram of a conventional pulsedpower system using a pull-down resistor or a pull-down switch.The HVCC can be implemented by a high-voltage charger withcurrent source characteristics, or a voltage source with a currentlimiting component such as a series resistor or inductor. In thisstudy, the dc voltage source Vs and charging resistor Rc pro-vide the simplest example. Cs is the storage capacitor and SW1is a main switching device used to generate the output pulses.Rseries is a parasitic series resistance value. A plasma reactorload, which has capacitive characteristics, is assumed to have anequivalent capacitance of Cplasma. Rpull in Fig. 1(a) is the pull-down resistor and SWpull in Fig. 1(b) is the pull-down switchfor discharging the residual energy in the capacitive load. Af-ter Cs is charged by the HVCC, a high-voltage output pulse isapplied to the load by closing the main switch SW1. The ca-pacitive load is charged by the output pulse voltage during thepulse duration and is discharged through the pull-down resis-tor in Fig. 1(a). When using the pull-down switch, as shown inFig. 1(b), the load voltage can be discharged quickly by closingthe pull-down switch after turning SW1 OFF. In this case, SWpullshould be closed with a proper dead time after SW1 is turned OFF.Typical waveforms of the output pulse voltage for the ideal caseand the practical case without any pull-down circuit are shownin Fig. 2(a). If a pull-down resistor is used to approximate theideal case, the voltage waveforms of the output pulse shownin Fig. 2(b) can be obtained. The smaller the pull-down resis-tance value is, the faster the pulse fall time. Fig. 2(c) shows thepractical waveforms of the load voltage and the current flowingthrough the pull-down switch when a pull-down switch is usedfor the active pull-down. After closing the pull-down switch, theload voltage decreases to zero very quickly because the load isshort-circuited by the pull-down switch. However, as mentionedearlier, a proper dead time is required between the main switch

Fig. 2. Typical waveforms of pulse voltage of the pulsed power modulators.(a) Ideal and practical waveforms of pulse voltage without any pull-down circuit.(b) Practical waveforms of pulse voltage in the conventional system using a pull-down resistor. (c) Practical waveforms of pulse voltage and current through thepull-down switch in the conventional system using a pull-down switch.

and the pull-down switch. If, for the fast pull-down, insufficientdead time is applied so that the pull-down switch is turned ONbefore the main switch is completely turned OFF, a current withhigh peak value flows through the pull-down switch, as shownin Fig. 2(c), and causes very high current stress on many com-ponents and devices. Therefore, a sufficient dead time shouldbe ensured so that the high-voltage main switch is completelyturned OFF, resulting in a time delay of the pull-down operation.Even when sufficient dead time is applied, the residual energy inthe small capacitance of the load is discharged for a short time;therefore, a spike current flows through the pull-down switch,as shown in the magnified current waveform in Fig. 2(c). It ispossible to generate an output pulse approximating a squarewaveform in the conventional active pull-down method usinga pull-down switch, but dead time optimization and additionalcontrol and gate driving circuits are required, and relatively highcurrent stress is generated in the pull-down switch.

Fig. 3 shows the voltage waveform of a single pulse appliedto the plasma reactor load in more detail. During t1, the voltageaccumulated by Cs charges the load capacitance Cplasma. Thetime constant during t1 is determined by Rseries and Cplasma;therefore, t1 can be roughly expressed by (1). In this case, SW1is assumed to be an ideal switch

t1 ≈ Rseries · Cplasma. (1)

PARK AND RYOO: PULSED POWER MODULATOR WITH ACTIVE PULL-DOWN USING DIODE REVERSE RECOVERY TIME 2945

Fig. 3. Practical voltage waveform of a single pulse applied to a plasma reactorload.

After the load voltage is approximately charged to Vs, themagnitude of the load voltage slowly decreases during t2 with avoltage drop of ΔV, as described by (2). t2 has the same valueas the pulsewidth when a pull-down resistor is used and can beexpressed as the sum of the pulsewidth and dead time when apull-down switch is used

ΔV ≈ I · t2CS

where I ≈ VSRplasma

+VSRpull

. (2)

Rplasama represents the value of the equivalent resistance ofthe plasma load for the steady state. For this period, when us-ing a pull-down resistor, the discharge current of Cs flows notonly to the plasma load but also to the pull-down resistor, Rpull.Therefore, the pull-down resistor consumes an average powerapproximated by (3), which causes large heat losses. In (3), fpulseis the pulse repetition rate (PRR)

PRpull ≈ VS2

Rpull· fpulse · t2. (3)

After t2, the residual energy in the load capacitor is dischargedthrough the pull-down resistor or the pull-down switch duringperiod t3. The magnitude of the load voltage, when using thepull-down resistor, gradually decreases and t3 can be approxi-mated by

t3 ≈ Rpull · Cplasma. (4)Equation (4) indicates that a faster fall time of the output pulse

can be achieved by using a pull-down resistor with a lower value,but according to (3), the loss increases as a lower pull-downresistor value is used to reduce the pulse fall time. When usinga pull-down switch, the load voltage is discharged very quicklyso t3 cannot be expressed using (4).

B. Proposed System With Active Pull-Down Circuit UsingDiode Reverse Recovery Time

The circuit diagram of the proposed pulsed power systemwith active pull-down circuit is shown in Fig. 4. The proposedsystem is configured by adding diodes D1 and D2 to the pulsecharging and discharging paths. D1 is a fast recovery diode andD2 is a discharging and pull-down diode with slower reverserecovery time than D1; hence, D2 is used for discharging theload voltage and performing the pull-down function after outputpulses are removed. To avoid the short circuit current that mayflow through Cs and D1 when SW1 is closed, a fast recovery

Fig. 4. Simplified circuit diagram of the proposed pulsed power modulatorwith active pull-down circuit.

Fig. 5. Operation modes of the proposed system. (a) Mode 1: Capacitor charg-ing mode. (b) Mode 2: Pulse discharging mode. (c) Mode 3: Pull-down mode.

diode should be used for D1. The three operating modes of theproposed system are shown in Fig. 5. In mode 1, the voltage ofCs is charged to the value of the required output pulse voltage bythe HVCC, as shown in Fig. 5(a). The current flows through Csand D1; therefore, Cs can be fully charged prior to dischargingthe output pulses. At this time, the maximum current flowingthrough D1 is limited by the charging resistor Rc, as determinedby

ID1,lim = VS/RC . (5)

Mode 2 begins as the main switch SW1 is closed so that thevoltage stored in Cs is applied to the load. The waveform ofthe voltage across the load is similar to that shown in Fig. 2,but when the proposed pull-down circuit is used, the relevantequations are (6) and (7). The maximum reverse voltage stress


Fig. 6. Simulation circuit model for each type of pulsed-power system.(a) Conventional system using pull-down resistor. (b) Conventional system usingpull-down IGBTs. (c) Proposed system.

TABLE ICIRCUIT PARAMETERS AND DEVICE SPECIFICATIONS FOR THE

SIMULATION MODELS

of D1 is given by (8)

t1 ≈ Rseries · Cplasma (6)

ΔV ≈ I · t2CS

where I ≈ VS/Rplasma (7)VD1,r,max ≈ VS . (8)

Fig. 7. Simulation results of the conventional system using a pull-down resis-tor with different resistance values under−10 kV output pulse, 20µs pulsewidth,and 1 kHz PRR conditions. (a) Waveforms of output pulse voltage. (b) Wave-forms of the current flowing through the pull-down resistor.

Fig. 8. Simulation result of the output pulse voltage and current for the pro-posed system under −10 kV output pulse, 20 µs pulsewidth, and 1 kHz PRRconditions.

Immediately after the turn-OFF signal is applied to SW1,mode 2 ends and mode 3 begins. The output pulse applied tothe load is removed in mode 3, but the voltage charge in theload is not eliminated immediately. However, a current path forcharging Cs is formed, as shown in Fig. 5(c), owing to the re-verse recovery characteristics of D2. Therefore, the voltage ofthe load can be quickly discharged during the recovery time ofD2. At this time, it would be best if the discharge of the loadvoltage were completed within the reverse recovery time of D2.


Fig. 9. Power dissipation and efficiency of the pull-down circuit in the conventional and proposed system with variable pulsewidths and PRRs under −10 kVpulse voltage. (a) Variable pulsewidths at 1 kHz PRR. (b) Variable PRRs at 20 µs pulsewidth.

However, even when the load voltage is not discharged com-pletely during D2’s reverse recovery time, the pull-down opera-tion continues as the current path of mode 3 is maintained. Thisis because D1 will be reverse-biased if any residual energy ex-ists, and the charge current of Cs will flow through the parasiticcomponents of D2. Therefore, mode 3 continues until all theresidual energy is discharged, and then mode 1 begins again.Because an anode terminal of D2 is in the floating state in mode1, it is not easy to calculate the reverse voltage of D2 in thismode. In this manner, the proposed system performs the pull-down operation efficiently without using any pull-down resistorsor active driving switches such as IGBTs.

III. SIMULATION AND EXPERIMENTAL RESULTS

A. Simulation Results

Simulation circuits for the conventional and proposed systemsare modeled as in Fig. 6. The operation of the proposed pull-down circuit was verified and comparisons between the conven-tional systems, and the proposed system were performed. Thegeneral plasma reactor load can be equivalently modeled by theparallel connection of a resistor, series-connected RC, and ca-pacitor [6], as shown in Fig. 6. Fig. 6(a) shows the simulationcircuit model of the conventional system using a pull-down re-sistor. Fig. 6(b) shows the conventional system using pull-downIGBTs, and Fig. 6(c) shows the proposed system. Twelve IG-BTs (Z1–Z12) were used to generate 10 kV output pulses byapplying the actual properties of the IGBTs used in the experi-ment. Each of the IGBTs (with 1200 V rating) formed an IGBT

stack connected in series. Diodes D1 and D2 in the proposedsystem were also modeled based on the actual devices used inthe experiment, in 12–piece stacks each (D1_1 to 12 and D2_1 to12). The parasitic inductance was represented by Lseries, whichwas added to more closely approximate the practical circuit.Table I summarizes the circuit parameters and specifications ofthe devices used in the simulation models, including the reverserecovery time trr of the diodes. Each load parameter was initiallyselected in accordance with the load modeling method in [6], butbecause those parameters are largely variable depending on pro-cess conditions such as plasma conditions, gas density, and soon, it is not easy to produce an exact model. In this simulation,RC values for modeling the reactor were adjusted based on theexperimental results.

Initially, the pull-down capability and losses of the pull-downresistor according to the variable pull-down resistance valueswere confirmed through simulation. The waveforms of the out-put pulse voltage and the current flowing though the pull-downresistor for different resistance values are shown in Fig. 7. Inthis case, the selected pulse voltage, pulsewidth, and PRR werefixed to −10 kV, 20 µs, and 1 kHz, respectively. The fall timeof the pulse voltage became faster with decreasing resistanceof the pull-down resistor [see Fig. 7(a)]. However, as shown inFig. 7(b), if a lower value of the pull-down resistor is used toachieve a faster fall time, the current flowing through it will in-crease and larger losses will be generated. The simulation resultsfor the proposed system under the same conditions are shownin Fig. 8, which shows the waveforms of the pulse voltage andpulse current. The results indicate that the load voltage can be


Fig. 10. Simulated power loss and fall time comparison between proposedsystem and conventional system with various pull-down resistor values under−10 kV output pulse, 20 µs pulsewidth, and 1 kHz PRR conditions.

completely discharged within 3 µs as the charging current ofstorage capacitor flows through the load during the reverse re-covery time of D2_1 to D2_12. This result indicates that thefall time of the pulse voltage in the proposed system is muchfaster than that of the conventional system, even with a 1 kΩpull-down resistor. Fig. 9 compares the power dissipation in thepull-down resistor or diodes, and the efficiency of both systemswhen the pulsewidth varied between 5 and 30 µs with a fixedPRR of 1 kHz, and when the PRR varied between 100 Hz and3 kHz with fixed pulsewidth of 20 µs under −10 kV pulses. Forthe conventional system, greater losses occurred with decreasingvalues of the pull-down resistor, as higher current flowed throughit. Moreover, the power dissipated in the pull-down resistor isgreater than the power consumption of the load in the conven-tional system. On the other hand, losses incurred in the proposedsystem are only several watts in all conditions, which are muchlower than the losses in the conventional system. Fig. 10 showsthe results comparing the losses of the pull-down resistor andthe time required for the pulse voltage to reach 10% of its orig-inal value after the IGBT is turned OFF versus the conventionalsystem with varying pull-down resistances from 1 to 10 kΩ. Thevalues of the pulse voltage, pulsewidth, and PRR are −10 kV,20 µs, and 1 kHz, respectively. This result shows that the falltime of pulse voltage can be faster than when a pull-down resistoris used with lower resistance, but the additional losses increasesimultaneously, as mentioned previously. It is also confirmedthat losses are large compared with the results of the proposedsystem, as summarized in Fig. 10.

Fig. 11 shows the simulation results of the conventional sys-tem using pull-down IGBTs [see Fig. 6(b)] when the dead timebetween the gate signals of the main IGBTs and the pull-downIGBTs is optimized as 50 ns. There is a time delay due tothe dead time before the pull-down operation starts, but oncethe pull-down IGBT is turned ON, the output pulse voltage canbe pulled down to zero very quickly, within 100 ns including thetime delay of 50 ns. However, in the case using the pull-downIGBT, a spike current flows through the pull-down IGBT creat-ing high current stress, as shown in the pull-down IGBT currentwaveform in the figure. In the proposed system, however, the

Fig. 11. Simulation results of the pulsed power system using pull-down IGBTsfor active pull-down operation under −10 kV output pulse, 20 µs pulsewidth,and 1 kHz PRR conditions.

Fig. 12. Experimental setup with pulsed power modulator using the proposedstructure for active pull-down operation applied in a PSII system (Left: 1 L PSIIchamber, Right: IGBT stack).

pull-down diode D2 can perform the pull-down function imme-diately after the main IGBT is turned OFF without any delaytime. In addition, the pull-down current is limited by the charg-ing current limiting circuit in the HVCC, leading to low currentstress on the diodes.

B. Experimental Results

In order to verify the performance of the proposed pulsedpower system with active pull-down circuit configuration, thesystem was applied to a PSII system for surface treatment, whichis one application for pulsed power systems. Fig. 12 shows theentire setup for the PSII experiment. The detailed design of theIGBT stack is described in [10] and [11].


Fig. 13. Waveforms of the output pulse voltage and current measured duringthe experiment (−10 kV pulse voltage and 18 µs pulsewidth).

The experimental conditions were 10 kV negative pulses with18 µs pulsewidth. The waveforms of the output pulse voltageand current measured during the experiment are presented inFig. 13. As shown in Fig. 13, the proposed system can quicklyreduce the load voltage during the reverse recovery time of thediode efficiently, and it has a fast pulse voltage fall time in theexperiment.

IV. CONCLUSION

In this paper, a new structure for an active pull-down cir-cuit for the negative pulse modulator with high-voltage directswitching method was proposed. The proposed pull-down cir-cuit can decrease the fall time of the output voltage pulse byusing the reverse recovery characteristics of diodes instead ofa pull-down resistor. The proposed pulsed power system cansolve the problem of the large additional losses incurred by con-ventional pulsed power modulators using a pull-down resistor.Furthermore, comparing the proposed system with another ac-tive pull-down scheme using pull-down switches, the proposedmethod does not require any additional gate driving and/or con-trol circuits for the dead time and synchronization of the gatesignals. In addition, the current stress on the diode is much lowerthan that on the pull-down switch. The results of circuit modelingand simulation of the conventional and proposed systems veri-fied the effectiveness and superior performance of the proposedsystem based on the waveforms of the pulse voltage and cur-rent and on comparison and analysis of the additional losses andpulse fall times of each system. Finally, the pulsed power mod-ulator with the proposed structure was fabricated and appliedeffectively to real load conditions in a practical experiment forPSII applications.

REFERENCES

[1] W. Jiang et al., “Compact solid-state switched pulsed power and its ap-plications,” Proc. IEEE, vol. 92, no. 7, pp. 1180–1196, Jul. 2004.

[2] H. J. Ryoo, G. Gussev, and S. R. Jang, “Development of 60 kV, 300 A,3 kHz pulsed power modulator for wide applications,” Inst. Eng. Technol.,vol. 115, no. 6, pp. 967–970, 2009.

[3] E. J. M. van Heesch et al., “A fast pulsed power source applied to treatmentof conducting liquids and air,” IEEE Trans. Plasma Sci., vol. 28, no. 1,pp. 137–143, Feb. 2000.

[4] H. S. Jin, S. H. Song, C. G. Cho, S. M. Park, and H. J. Ryoo, “Studyof exhaust air treatment from a ship building factory painting facility us-ing pulse plasma technology,” IEEE Trans. Plasma Sci., vol. 46, no. 10,pp. 3552–3556, May 2018.

[5] H. S. Kim, C. H. Yu, S. R. Jang, and G. H. Kim, “Solid-state pulsed powermodulator with fast rising/falling time and high repetition rate for pockelscell drivers,” IEEE Trans. Ind. Electron., vol. 66, no. 6, pp. 4334–4343,Jun. 2019.

[6] M. P. J. Garudreau, J. A. Casey, M. A. Kempkes, T. J. Hawkey, and J. M.Mulvaney, “Solid state modulators for plasma immersion ion implantationapplications,” J. Vacuum Sci. Technol., vol. 17, no. 2, pp. 888–894, Apr.1999.

[7] M. J. Barnes and G. D. Wait, “A 25-kV 75-kHz kicker for measurement ofmuon lifetime,” IEEE Trans. Plasma Sci., vol. 32, no. 5, pp. 1932–1944,Oct. 2004.

[8] C. H. Yu, S. R. Jang, H. S. Kim, and H. J. Ryoo, “Gate driving circuit withactive pull-down function for a solid-state pulsed power modulator,” IEEETrans. Power Electron., vol. 33, no. 1, pp. 240–247, Jan. 2018.

[9] H. J. Ryoo, S. H. Ahn, J. W. Gong, and S. R. Jang, “Design and comparisonof capacitor chargers for solid-state pulsed power modulator,” IEEE Trans.Plasma Sci., vol. 41, no. 10, pp. 2675–2683, Oct. 2013.

[10] J. H. Kim, B. D. Min, S. Shenderey, and G. H. Rim, “High voltage pulsedpower supply using IGBT stacks,” IEEE Trans. Dielectr. Elect. Insul.,vol. 14, no. 4, pp. 921–926, Aug. 2007.

[11] J. H. Kim, M. H. Ryu, S. Shenderey, J. S. Kim, G. H. Rim, and G. Y. Jeong,“Pulse power supply using an IGBT switch stack for plasma source ionimplantation,” in Proc. 35th Annu. Power Electron. Spec. Conf., Aachen,Germany, Jun. 20–25, 2004.

Su-Mi Park received the B.S. degree in energy sys-tems engineering in 2017 from Chung-Ang Uni-versity, Seoul, South Korea, where she is currentlyworking toward the M.S. and Ph.D. degrees with theDepartment of Energy Engineering.

Her research interests include high-voltage powerconverters, bi-directional dc/dc converters, and solid-state pulsed power modulators.

Hong-Je Ryoo (M’17) received the B.S., M.S.,and Ph.D. degrees in electrical engineering fromSungkyunkwan University, Seoul, South Korea, in1991, 1995, and 2001, respectively.

From 2004 to 2005, he was a Visiting Scholarwith WEMPEC, University of Wisconsin–Madison,Madison, WI, USA. From 1996 to 2015, he joined theElectric Propulsion Research Division as a PrincipalResearch Engineer, the Korea Electrotechnology Re-search Institute, Changwon, South Korea, where hewas a Leader with the Pulsed Power World Class Lab-

oratory, a Director of Electric Propulsion Research Center. From 2005 to 2015,he was a Professor with the Department of Energy Conversion Technology, Uni-versity of Science and Technology, Deajeon, South Korea. In 2015, he joined theSchool of Energy Systems Engineering, Chung-Ang University, Seoul, SouthKorea, where he is currently a Professor. His current research interests includepulsed-power systems and their applications, as well as high-power and high-voltage conversions.

Prof. Ryoo is an Academic Director of the Korean Institute of Power Elec-tronics, a Senior Member of the Korean Institute of Electrical Engineers, and theVice President of the Korean Institute of Illuminations and Electrical InstallationEngineers.

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