IEEE PEDS 2017, Honolulu, USA
12 – 15 December 2017

Development of High-Speed and High-Voltage
Pulse Generator forNOx Decomposition Plasma Reactor
Toshihiko Noguchi, Hafidz Elmana, Ryota Kitamoto and Kazuki Shimizu
Shizuoka University
noguchi.toshihiko@shizuoka.ac.jp, hafidz.elmana.16@shizuoka.ac.jp

Abstract-This paper proposes a novel high-speed and highvoltage pulse generator for a NOx decomposition plasma reactor
for automotive application, which is composed of a DC/DC stepup converter and a multiple-toroidal-core technique. The
DC/DC converter steps up 12-V battery voltage to 420 V with anisolated high-frequency transformer. 12 ferrite toroidal cores
are connected to the output of the DC/DC converter, and they
are simultaneously excited by turning on 12 MOSFETs. The
synchronous switching is achieved within only ±2-ns time
deviation. The secondary high-voltage winding is placed throughthe 12 toroidal cores, and generates 10,000 V across theterminals in 50 ns. In order to make this high-speed and highvoltage pulse generation possible, the system has uniqueconfiguration and special appearance. In the paper, an
experimental prototype and test results are demonstrated toshow the performance of the proposed system.

I.

INTRODUCTION

In recent years, various regulations for environmental
conservation are imposed as environmental pollution isgetting a serious matter. Many sorts of research anddevelopment activities are promoted to satisfy therequirements of the regulations. In some part of themetropolitan areas in some developed countries, it ismandatory to mount an exhaust depurator on a vehicle todecompose nitrogen oxide (NOx) contained in the exhaust gas
from a diesel engine. NOx is an air pollution material thatcauses photochemical smog and acid rain not only in urban
areas but also in rural areas. Particularly, high-density
nitrogen dioxide (NO2) is strictly restricted by theenvironmental standard because it affects detrimentally
respiratory organs of human being [1].
Market of diesel engine based vehicles is growing in thedeveloped countries such as Japan and Western European
countries, where the brand-new vehicles are sold with socalled clean diesels. In general, diesel engine vehicles candeliver higher torque and reduce the carbon dioxide (CO2)emission, resulting in higher combustion efficiency withlower fuel consumption, compared with gasoline based
vehicles. However, it is still demanded to improve the diesel
engine performance, especially quality of exhaust gas that
contains NOx and particulate matters, acoustic noise,mechanical vibration and so forth. There have been manymethods reported to decompose NOx and its relevantmaterials as follows: 1) a selective catalytic reduction (SCR)
method which employs carbon hydride as catalyst; 2)
decomposition method of nitrous oxide (N2O) which is a byproduct of SCR process; and 3) a low-temperature plasma
decomposition method using corona discharge [2] [3].
This paper focuses on the low-temperature plasma
decomposition method, and an application of powerelectronics is attempted to generate ultra-high-speed highvoltage pulse for the plasma reactor. It is already known thatthe low-temperature plasma decomposition method requires
high-voltage pulse generator, of which rise time of thevoltage pulse is a key factor for efficient decomposition
quantity of NOx. It is reported that decomposition rate ofNOx can be over 95 % by achieving high-rise-time within
100 ns, high-peak-voltage over 10 kV, and repetitive periodof 1 kHz, for example. In addition, other most important
points are physical dimensions of the equipment and thepower source voltage because both of them are severely
restricted to implement on the actual vehicle.
The
transformer in the newly developed high-speed and highvoltage pulse generator employs a toroidal multi-coreapproach to optimize placement of the transformer cores andto reduce the line inductance to obtain less than 100-ns risetime and higher than 10-kV peak voltage [4]-[7]. The paper
discusses practical implementation techniques to reduce theline inductance, to achieve simultaneous high-speed
switching of multiple parallel-connected MOSFETs, and to
make the compact mechanical configuration possible.
Feasibility of the developed system has been confirmed
through experimental tests using a prototype, which proves
usefulness of the proposed approach [8].
II.

DESIGN OF HIGH-SPEED AND HIGH-VOLTAGE PULSE
GENERATOR USING TOROIDAL MULTI-CORE
TRANSFORMER

Fig. 1 and TABLE I show a main circuit schematic
diagram and its components of the high-speed and highvoltage pulse generator. As can be seen in the figure, thevoltage boost is achieved by using a two-stage configuration.
The first stage employs a combination of a push-pull DC/DC
converter and a high-frequency step-up transformer, and thesecond stage uses a toroidal multi-core transformer withconnecting its primary side in parallel and its secondary side

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978-1-5090-2364-6/17/$31.00 ©2017 IEEE

III.

IMPLEMENTATION OF PROTOTYPE WITH LOW
INDUCTANCE FOR HIGH-SPEED SWITCHING

A. Required Specifications and Reduction of Inductance
It is required for the prototype to satisfy the voltage risetime less than 100 ns to the peak over 10 kV because thehigher the rise time is, the more the NOx can be decomposed.

Tr2

Tr1
C1

Q13

Tr12

Tr12

V1
Q1

VB

Tr3

・
・
・

Q2

Q3

Q12

Tr3

Q14

V2

Rec1
Tr2
VB = 12 V, V1 = 420 V, V2 = 10 kV
Tr1

Fig. 1. Configuration of main circuit.

Symbols
Q1-Q12
Q13 and Q14
Rec1
C1
Tr1-Tr12

TABLE I COMPONENTS IN MAIN CIRCUIT.
Components
Specifications
1000 V, 12 A
DE375-102N12A
ton: 3 ns
IRFB61N15D
150 V, 60 A
FMG-G2CS
1000 V, 30 A
EMC
600 V, 100 F
PC95
T38×14×22

Quantities
12
2
4
2
12

TABLE II DESIGN PARAMETERS OF TOROIDAL MULTI-CORE
TRANSFORMER.
Parameters
Symbols
ValuesPrimary voltage
V1
420 V
Secondary voltage
V2
10×103 V
Primary number of turns
N1
4
Secondary number of turns
N2
8
Number of multiple coresn12
Duty cycle
D
0.005
Magnetic flux density
Bm
0.5 T
Section areaS
112×10-6 m2
Operation frequencyf1×103 Hz12
Output voltage (kV)2
Bm SN 2 f ,(2)D
where V1 is a primary voltage, V2 is a secondary voltage, N1 isa primary number of turns, N2 is a secondary number of turns,
Bm is magnetic flux density, S is a section area of thetransformer, f is an operating frequency, n is a toroidal core
count. As indicated in (2), n-times high-voltage can beobtained on the secondary winding because the secondaryside of each toroidal core can be regarded to be equivalent tothe series connection. Major design parameters of the
prototype are listed in TABLE II.
The peak voltage of the output pulse can be determined by
the turn ratio of the transformer and the number of thetoroidal cores. Assuming that the transformer is ideal, therelationship between the primary and the secondary voltages
are simply expressed asN(3)V2  n 2 V1 .
N1
The transformer employs the ferrite toroidal cores in order toimprove the frequency characteristics and to reduce theleakage inductance at the same time.
V2  n

1：2
1：35
・
・
・

like in series. The primary sides of the toroidal multi-coretransformer have the high-speed switching devices tomagnetize the multiple cores simultaneously.
The voltage is stepped up through the first stage from 12
VDC to 420 VDC. The push-pull inverter has a 12-V
automotive battery as a power supply, and operates at thefrequency of 50 kHz to step up the voltage. The inverter
output is connected to the ferrite core step-up transformer,
whose turn ratio is 35. A simple full-bridge diode rectifier isused to obtain the DC bus voltage V1 followed by a LC filter.
On the second stage, MOSFETs Q1-Q12 are employed toachieve simultaneous switching, and are connected to theprimary windings of the toroidal multi-core transformer.
There are twelve ferrite toroidal cores used as a transformer.
Their primary windings are connected to the MOSFETs one
by one. On the other hand, a single secondary winding iswound around through the twelve ferrite toroidal cores in a
torus shape. The total voltage step-up ratio of the second
stage is 24 times. All of the twelve MOSFETs Q1-Q12 areturned on for 500 ns every 1 ms, and generate high-speed andhigh-voltage pulse across the secondary winding terminals.
The following equations are used for the design of thetoroidal multi-core transformer:
2
V1  Bm SN 1 f , and
(1)
D

10
8
6
4
2
0

0

20

40
60
Time (ns)80

100Fig. 2. Simulation result of output pulse transient behavior.

Therefore, it is indispensable to reduce the line inductancedisturbing the high-speed switching. Fig. 2 shows a computer
simulation result to check the influence of the line inductanceto the voltage rise time. The simulation was conducted with
an ideal transformer with the primary windings connected inparallel and the secondary windings connected in series togenerate over 10 kV pulse. By intentionally inserting the line
inductance to the ideal transformer, the maximum value ofthe line inductance was found to satisfy the required
specifications. As can be found in Fig. 2, 10-kV output
within 50 ns is possible by limiting the line inductance lessthan 100 nH.

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B.

Reduction of Inductance by Means of Optimized
Implementation of Components
As Fig. 1 shows, each of twelve toroidal cores Tr1-Tr12 isconnected to and is switched over by one of the twelve
MOSFETs Q1-Q12. The line inductance can be reduced byplacing the toroidal core close to the switching device. The
twelve MOSFETs must be turned on at the almost same time,so the MOSFETs and their drive circuits are placed in a ring
shape to make the current path perfectly symmetrical. In
addition to that, the front and the rear sides of the printed wireboard (PWB) are used for the current path, and the line
inductance is effectively reduced by making the current flowin opposite directions on the both sides. Furthermore, themain current root is separated from the gate drive circuit toprevent the conduction noise caused by the MOSFETs to thegate drive ICs. Fig. 3 shows turn-on waveforms of the
MOSFETs, where the twelve waveforms are synchronously
measured and are superimposed in the figure. It is confirmed
from the figure that all of the waveforms turn on within the 1ns time deviation.

Fig. 3. Turn-on waveforms of MOSFETs.
Fig. 3. Turn-on waveforms of MOSFETs.

Primary windings

Secondary winding

Primary windings

Secondary winding

Fig. 4. Winding configuration of toroidal multi-core transformer.
Table 3. Specifications of winding.
Voltage
Outer diameter ofComponents
ratings
conductors

Windings
Primary
winding
Secondarywinding

C3/RV-90 LF

2000 V

1.3 mm

UL323930 kV

2.0 mmTroidal4.
ferritecore
Fig.
Winding
configuration of toroidal multi-core transformer.
MOSFET

MOSFET drive IC

Smoothingcapacitor
TABLE III SPECIFICATIONS OF WINDING.
Voltage
Outer diameter ofComponents
ratings
conductors

Printed wire board
DC-DC Converter
Windings

Primary
winding
Secondarywinding

C3/RV-90 LF

2000 V

1.3 mm

UL323930 kV

2.0 mmTroidal ferrite core
MOSFET
MOSFET drive IC

200 mmSmoothing capacitor
Printed wire board
Fig. 5. Overview of high-speed and high-voltage pulse generator prototype.
DC-DC Converter

135 mm

D. Compact Design with 3D CAD
Fig. 5 illustrates a CAD model of the prototype, which has
been designed to make the total volume minimum, assuming
an actual application to the automotives. In order to makeevery wiring as short as possible, the first and the second
stages are stacked up in the vertical way. The push-pull
DC/DC converter and the LC filter across the DC bus areplaced under the PWB, while the toroidal multi-cores areplaced on the PWB. As described previously, the wire length
between the MOSFETs and the toroidal core is as short as 10mm. The dimensions of the whole system are 200 mm indiameter and 135 mm in height, and the weight isapproximately 2 kg, which allows mounting this system close
to the actual automotive exhaust system. Since the wire
length at every part is designed as short as possible owing tothe optimized configuration described above, the turn-off

5 ns

5 ns

135 mm

C. Reduction of Inductance with Multi-Core Transformer
Fig. 4 illustrates a CAD model that has a winding
configuration employed in the prototype. In the second stage
step-up transformer, each primary winding is wound around a
toroidal core, respectively, and the secondary winding iswound through all the twelve toroidal cores in a torus shape.
It is impossible to draw the electric circuit diagram of thesecondary winding connection of the multi-core transformer;
hence, Fig. 1 does not reflect the actual winding configuration.
Taking this configuration contributes to enhance the magnetic
coupling, which leads to efficient reduction of the leakage
inductance. TABLE III shows specifications of the windings.
Because a high-withstanding voltage is required on thesecondary winding, a 30-kV electric wire with silicon
insulator is employed. The silicon sheath is mechanically
flexible enough to wind around the wire through the multiple
toroidal cores with a high-space factor, which also effective
to increase the magnetic coupling coefficient.

100 V

100 V

200 mmFig. 5. Overview of high-speed and high-voltage pulse generator prototype.

serge voltage across the DC bus due to the line inductancecan be reduced down to 5 % of the DC bus voltage.

1,082

PROTOTYPE AND EXPERIMENTAL TEST RESULTS

B. Output Voltage Characteristic with Respect toToroidal Core Counts of Multi-Core Transformer
Experimental tests have been conducted with changing thetoroidal core counts of the multi-core transformer to 4, 6, 8,
and 12 under a condition of a 420-VDC constant DC bus
voltage and a 4:8 constant turn ratio. The peak value and therise time of the output voltage are shown in Figs. 7 and 8,
respectively. As described in the previous sections, theoutput voltage can be adjusted not only by changing the turnratio but also by changing the number of the toroidal coresused. It is found that the output voltage is proportional to thetoroidal core counts from (2) and (3), but the rise time of thevoltage pulse is supposed to be longer as the number of thetoroidal cores increases because of more leakage magnetic
flux or more leakage inductance. Increasing the number ofturns can detrimentally affect the leakage inductance,
resulting in the slow voltage rise due to the longer time
constant determined by the leakage inductance.
It can be found from Fig. 7 that 4.5-kV is available acrossthe output terminals even though the toroidal core counts are
four. The theoretical value of the output peak voltage is 3.4
kV, and the actual value is higher than the theoretical one
because of the serge voltage imposed on the output. As thenumber of the toroidal cores is increased, however, the output
voltage is also proportionally increased as the theoreticalexpressions suggest. Fig. 8 represents that the rise time of theoutput voltage is also almost proportional to the toroidal core
counts. This is because of the larger leakage inductancedepending on the secondary wire length that is proportional to
the number of toroidal cores. As Fig. 8 demonstrates, the risetime is only 50 ns even when the toroidal core counts are
twelve to generate over 10-kV peak output voltage.
C. Output Voltage Characteristic with Respect toPrimary DC Bus Voltage
Other experimental tests have been conducted withchanging the primary DC bus voltage to 100 V, 200 V, 300 V,
and 420 V under a condition of the twelve toroidal cores usedin the multi-core transformer and their 4:8 constant turn ratio.
Figs. 9 and 10 show the characteristics of the output peakvoltage and the rise time, respectively. As (3) indicates, the

MOSFET
MOSFETdrive
driveICICIXISRFIXISRF DEIC420
DEIC420MOSFET
MOSFETIXISRFIXISRF DE375-102N12A
DE375-102N12A

Fig. 6. Photographs of prototype.
14
12
Peak output voltage (kV)

A. Prototype
Fig. 6 shows photographs of the high-speed and highvoltage pulse generator prototype. TDK PC95 T38x14x22
ferrite toroidal cores are selected on the basis of the design
parameters calculated by (1)-(3), and are employed in the
prototype. The turn ratio of the second stage multi-coretransformer is 4:8. Since the number of toroidal cores istwelve, the voltage step-up rate of the second stage is 12*8/2= 24 times.
Several experimental tests have been conducted to examine
the operation characteristics, using the prototype. In thefollowing sections, feasibility of the design is verified
through the test results.

Troidal
Troidal core
core
TDK
TDK PC95
PC95 T38×14×22
T38×14×22

10
8
6
4
2
0
2

4

6
8
10
12
Multiple count of transformer

14

Fig. 7. Characteristic of output peak voltage with respect to toroidal core
counts of multi-core transformer.
60
50
40
Rise time (ns)

IV.

30
20
10
0
2

4

6

8

10

12

14

Multiple count of transformer

Fig. 8. Characteristic of output voltage rise time with respect to toroidal
core counts of multi-core transformer.

output voltage increases as the primary voltage increases if
the turn ratio and the toroidal core counts are fixed at constantvalues. It is confirmed in Fig. 9 that the output peak voltage
is proportionally increases as the primary DC bus voltage isincreased. However, every output peak voltage value isslightly higher than the theoretical value because the serge

1,083

E. Energy Conversion Efficiency and Power
Consumption
Fig. 15 shows efficiency characteristic with respect to theload power. Efficiency of the DC/DC converter in the first
voltage step-up stage reaches 80 % when the load is relatively
heavy, while the efficiency of the second voltage step-up
stage is around 57 % at the same load. Therefore, the total
efficiency is 47 % at most. The reason why the second stage
has such low efficiency is as follows:
1) switching loss of the MOSFETs;
2) conduction loss caused by on-voltage of the
MOSFETs;

Peak output voltage (kV)

12
10

8
6
4
2
0
0

100

200
300
Primary voltage (V)

400

500

Fig. 9. Characteristic of output peak voltage with respect to primary DC bus
voltage.
60

Rise time (ns)

50
40
30
20
10
0

0

100

200
300
Primary voltage (V)

400

500

Fig. 10. Characteristic of output voltage rise time with respect to primary
DC bus voltage.
14
12
Peak output voltage (kV)D. Output Voltage Characteristic with Respect to Load
Resistance
Other experimental tests have been conducted withchanging the load resistance to 2.8 k, 5.6 k, 8.4 k, and11.2 k under the condition of the twelve toroidal cores usedin the multi-core transformer, their 4:8 constant turn ratio,
and 420-V constant primary DC bus voltage. The peak value
and the rise time of the output voltage are shown in Figs. 11
and 12, respectively. Fig. 11 shows the output peak voltage
is kept almost constant over 10 kV regardless of the load
resistance. The rise time of the output voltage pulse can also
be found to be almost constant around 50 ns despite the load
resistance variation.
From these characteristics, it isconfirmed that the output voltage characteristics are hardly
affected by the load current, which implies that the internal
impedance of the designed prototype is quite low. The
MOSFETs used in the experimental tests have the draincurrent rating of 12 A because the 3-k load resistance isassumed for an actual plasma reactor application. If the
MOSFETs have the higher current rating, more instantaneous
power can be obtained by reducing the load resistance value
as long as the MOSFETs have similar switching speed as
those used in the prototype.
An output voltage pulse train waveform is shown in Fig. 13,
and its expanded waveform is shown in Fig. 14, which has
been measured in the experimental tests under the conditionof the twelve toroidal cores used in the multi-core transformer,
their 4:8 constant turn ratio, 420-V constant primary DC bus
voltage, and 5.6 k load resistance. It is confirmed fromthese results that the 1-ms cyclic voltage pulse with as fastrise time as 50 ns and as high voltage as 10 kV is obtainedwith the prototype.

14

10
8
6
4
2
0
2

4

6
8
Load resistance (k)1012Fig. 11. Characteristic of output peak voltage with respect to load
resistance.
60
50

Rise time (ns)voltage pushes up the voltage level as explained previously.
As for the rise time of the output voltage pulse, it is known
that the rise time is slightly affected by the primary DC bus
voltage. This rise time variation seems to be caused by theconstant load resistance. In other words, increased load
current caused by the increased primary DC bus voltage andthe constant load resistance makes the rise time slightly
longer. However, the difference of the rise time between
100-V and 400-V DC bus voltages is only 6 ns; hence it canbe confirmed that the load current value hardly affects the rise
time.

40
30
20
10
0
2

4

6
8
Load resistance (k)1012Fig. 12. Characteristic of output voltage rise time with respect to load
resistance.

1,0843) power loss in the snubber circuits; and4) iron loss in the toroidal multi-cores.
The major part of the total power loss is occupied by thesnubber loss and the conduction loss of the MOSFETs. It isnecessary to investigate the power loss further and to improve
the total efficiency in the future work.
2 kV

F. Discussion on Output Voltage Characteristics
The experimental test results demonstrated output voltage
characteristics with the rise time less than 100 ns to reach 10
kV, which proves feasibility of the design approach.
However, it has been confirmed through the tests that the risetime becomes longer as the number of the toroidal multicores increases, which has an opposite trend, compared withthe experimental results confirmed in the past study of theauthors’ [4]. The difference between the current prototype
and the past model is the number of turns on the secondaryside in the toroidal multi-core transformer.
In theexperimental tests described in this paper, the number of turns
is fixed at eight regardless of the toroidal core counts. In thepast tests, however, as the toroidal core counts increase, thesecondary number of turns is reduced to keep the magnetic
flux density in each toroidal core constant. In other words, as
more toroidal cores are used, the length of the secondary wire
becomes longer in this paper, but becomes shorter in the past
test results. Therefore, the length of the secondary winding in
the toroidal multi-core transformer is a dominant factor to
influence the rise time of the output voltage pulse. It is
important to take balance between the secondary number ofturns and the number of the toroidal cores for boosting up thevoltage and to reduce the secondary number of turns as low
as possible to speed up the voltage rise. In addition to that, avoltage oscillation of 4 MHz is observed in Fig. 14 when thetoroidal core counts are increased to twelve. This oscillation
is supposed to be generated by the resonance between the line
inductance and the stray capacitance. The more the toroidal
cores are connected, the more the stray capacitance comes
into prominence.

200 μsFig. 13. Output voltage pulse train waveform.

2 kV
100 nsFig. 14. Expanded waveform of output voltage.
100
90

Efficiency (%)

80

[1]

White Paper and Annual Report on the Environment and Sound
Material Cycle Society in Japan, Ministry of the Environment.

30

0
0

DC/DC converter
Multiple core transformer
Total
5

10
15
Load (W)20

25

Fig. 15. Efficiency characteristic with respect to load power.

[3]
[4]
[5]
[6]
[7]

[8]

REFERENCES

40

10

CONCLUSION

This paper discussed a compact high-speed and highvoltage pulse generator for a NOx decomposition plasma
reactor. The target application of the pulse generator is an
exhaust depurator for automotives. The pulse generator takes
advantage of the two-stage voltage step-up configuration andthe toroidal multi-core transformer in the second stage toachieve ultra fast rise time less than 100 ns to reach over 10kV output voltage. It has been confirmed through several
experimental tests that the pulse generator prototype achieved
as fast-rise-time as 50 ns and as high-voltage as 12 kV almost
regardless of the load current.

60
50

20

[2]

V.

70

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