Investigation for High Output of 2.5MHz
Power Supply Constructed from Multi-CoreTransformers and a Multi-Phase Inverter andApplication for Wireless Power Transfer
Koji Orikawa

Yusuke Fujita

Jun-ichi Itoh

Department of Electrical Engineering
Nagaoka University of Technology
Nagaoka, Japanorikawa@vos.nagaokaut.ac.jp

Department of Electrical Engineering
Nagaoka University of Technology
Nagaoka, Japanfujita_y@stn.nagaokaut.ac.jp

Department of Electrical Engineering
Nagaoka University of Technology
Nagaoka, Japan
itoh@vos.nagaokaut.ac.jp

Abstract— This paper discusses a development for high output
of 2.5 MHz inverter without fast-switching semiconductor
switches. This inverter consists of a multi-phase inverter using
conventional silicon semiconductor switches and multiple coretransformers. In this paper, a principle of drop in inverter
output voltage which reduces the inverter output voltage due toa relationship between dead-time and a leakage inductance of atransformer is clarified. In addition, two methods which arebifilar wound transformers and a LC series resonance areapplied to the proposed circuit in order to achieve higher output
power. The validity of two methods for higher output power isconfirmed by experiments. Finally, the proposed circuit isapplied to a wireless power transfer when a lamb bulb is used asa load.

I.

INTRODUCTION

Recently, wireless power transmission systems are
increasingly received attentions for the application of EV
chargers [1-7]. In wireless power transmission, there are twokinds of method, which are an electromagnetic induction
method and a magnetic resonance method. In theelectromagnetic induction method, low frequency such as
several tens kHz is used for power transmission. As a result,the power transmission distance which can achieve highefficiency is only several centimeters. Also, the efficiency isdrastically decreased when the position of a receiving coil ischanged. Therefore, the correspondence of the receiving coilbecomes a problem when the electromagnetic induction
method is applied for a battery charger of EV. On the other
hand, in the magnetic resonance method which has highquality factor for resonance, it is possible to achieve the highefficiency power transmission even though the coupling
coefficient is low. Therefore, the magnetic resonance method

is proper the battery charger of EV which requires the larger
power and middle-large power transmission distance. In
addition, Industry-Science-Medial (ISM) frequency band suchas around 13.56 MHz is one of candidate for this applicationin respecting to the standard of the magnetic coupling from theview point of a radio law and the downsizing of coils.
Therefore, power supplies which output MHz band are
required.
Conventional high-frequency power supply systems use
vacuum tube and high frequency FET, which is categorized inclass C linear amplifier. A drawback of this method is low
efficiency. Furthermore, the size of the systems is large
because a cooling system which includes a heat sink and a fan
becomes large.
By the recent technology in semiconductor devices, wide
gap semiconductors such as silicon carbide (SiC) and gallium
nitride (GaN) are used widely due to fast-switching and low
conduction loss under a high-temperature operation [8-9].
However, power consumption of a gate drive circuit becomeslarge in order to drive wide gap semiconductor [10]. In
addition, it is difficult to control a gate signal of each switch
due to low gate threshold voltage of SiC and GaN. Due to theswitching noise, the self turn-on phenomenon sometimes
occurs. Therefore, power converters using wide gap
semiconductors have some problems in order to output highfrequency and large power. Moreover, wide gap
semiconductors are still expensive compared to theconventional semiconductors.
The authors have been proposed frequency multiplying
circuit, which utilizes the neutral point voltage fluctuation of
the multi-phase voltage-type inverter and can use conventional
low-switching semiconductor switches. The proposedconverter can output high frequency voltage depending on thenumber of the phase in the inverter. There is no problems havementioned previous paragraph in the proposed circuit. So far,the fundamental operation of the proposed circuit haveconfirmed by simulations and experiments [11].
In this paper, two methods are investigated in order toachieve higher output power of the proposed converter. First,
the principle of the proposed frequency multiplying method is
described. Next, problems which result in the decreasing of
the inverter output voltage and output power in the proposed
circuit is introduced. Thirdly, the design of the leakage
inductance of the multi-core transformer and the seriesresonance are applied to the proposed circuit in order toovercome the problems of the proposed circuit. Finally,
fundamental experiments and experiments for wireless powertransfer using spiral antennas are shown.
II.

Vin
2

u

Vin

v

o
Vin
2

Snu

Snv

Spx

Spw

Spv

Spu

Iu

Vuo

Vout

w
Snw

Spy

Snx

x

Sny

y

Fig. 1. Proposed circuit.

PROPOSED CIRCUIT

A. Principle of the multiplying method
Fig. 1 shows the proposed circuit in a five-phase inverter
model. The primary side of the transformer is connected inparallel, and the secondary side is connected in series. As aresult, the multi-phase inverter can output high frequency overthe switching frequency.
Fig. 2 shows the principle of the proposed frequency
multiplying method. In the proposed method, the variation ofneutral point voltage in a voltage-type inverter is focused. In
the five phase voltage-type inverter, each of the voltage phases
is shifted by 72 degree and operated with square wave
modulation. As a result, the output frequency fout is obtainedby Eq. (1) using the switching frequency fsw. The output
voltage with N times frequency as larger as the switchingfrequency of the inverter is achieved.

f out = N・ f sw

(1)

Here, it is noted that any parasitic resistances and leakage
inductances are not considered. On the other hand, theamplitude of output voltage is determined by turn ratio of themulti-core transformer.
B. Multicore Transformer
Fig. 3 shows the diagram of multi-core transformers [12,13]. In Fig. 3(a), secondary winding is wired through eachcore in order to reduce a wire length and copper losses.
However, a leakage inductance becomes large due to low
degree of coupling in this method. In this paper, a bifilar
wound transformer is applied to the proposed circuit in orderto reduce a leakage inductance. In a bifilar wound transformer,
a primary winding and a secondary winding ware twisted.
Fig. 4 shows the photograph of the multicore transformers.
Toroidal cores are used for the multicore transformers. In ageneral bifilar wound, the turn ratio of the winding is 1:1. In
this paper, in order to achieve the output voltage which hassame amplitude to the input voltage, the number of thesecondary winding turn is two. As a result, the turn ratio of thewinding becomes 1:2.

Fig. 2. Principle of the proposed circuit (Turn ratio=1:1).
Secondary PrimaryPrimary
Secondarywinding
winding
winding
windingTo
To loads
u
u
Core
loads
Core
o
o
v
o

v

w
o

w

x
o

x

y
o

y

o

o

o

To
loads

o
To
loads
(a) Secondary wiring through cores. (b) Bifilar wound transformer.
Fig. 3. Multi-core transformer per a core.

III.

DROP IN INVERTER OUTPUT VOLTAGE

Fig. 5 shows the experimental circuit and the single-phaseequivalent circuit. L is inductance components transformed tothe primary side of the transformer, which includes a leakage

inductance. R is resistance components transformed to theprimary side of the transformer, which includes a wire
resistance. Cp is parasitic capacitances of MOSFETs.
Fig. 6 shows the operation patterns of the equivalentcircuit shown in Fig. 5. In Fig. 6, the switch Sp is changed
from off-state to on-state. Fig. 6 (b) and (c) show the period ofdead-time during both of switches of Sp and Sn are off. Whenthe drop in the inverter output voltage occurs, the operation
pattern is changed according to Fig. 6 (a), (b), (c) and (d). On
the other hand, when the drop in the inverter output voltage
does not occur, the operation pattern is changed according to
Fig. 6 (a), (b) and (d).
First, after the operation pattern is changed from Fig. 6(a)to (b), the current of L flows to the input voltage side throughthe free wheel diode of the upper switch Sp. During dead-time,
when the current of L keeps to flow, the drop in the inverter
output voltage does not occur and keep Vin/2. As a result, after
the period of dead-time, the operation pattern is changed to (c).
On the other hand, during dead-time, when the current of L
becomes zero, the operation pattern is changed to Fig. 6(c). As
a result, the resonance occurs between the L and parasitic
capacitances of MOSFETs Cp. Therefore, the drop in the
inverter output voltage occurs and cannot keep Vin/2.
The condition that the drop in the inverter output voltage
does not occur is obtained by (2) using the single-phaseequivalent circuit shown in Fig. 5(b).

(a) Secondary wiring through cores.
(b) Bifilar wound transformer.
Fig. 4. Multi-core transformer.

(2)

Cp
L

Fig. 1 Vload

Rload

iinvout
R

Vin/2

Sn

Cp

(a) Experimental circuit. (b) Single-phase equivalent circuit.
Fig. 5. Equivalent circuit.
Vin/2

Sp
L

Vin/2

Sp

Vin/2

Sn

L

R

iinvout
Vin/2

⎛
⎞
2
⎜
⎟
Td < τ ln⎜
T− sw ⎟⎜⎟⎝ 1 + e 2 Nτ ⎠

Sp

Vin/2

Iout

R

iinvout

Sn(a) Sp:OFF, Sn:ON.

(b) Sp:OFF, Sn:OFF(After (a)).

where τ=L/R is time constant, Tsw =1/fsw is switching
period and N is the number of the phase.
When the leakage inductance is small, time constantbecomes small. As a result, the drop in the inverter output
voltage occurs. On the other hand, when the leakage
inductance is large, the drop in the inverter output voltage
does not occur. However, if the leakage inductance is too large,
the operation of the proposed circuit becomes a constantcurrent source. As a result, the output voltage cannot be
controlled constant according to the load. Therefore, it isimportant to design a suitable leakage inductance which does
not cause the drop in the inverter output voltage and cancontrol the output voltage according to the load in order to
output desired output voltage. Moreover, the high output ofthe proposed circuit with LC resonance using the leakage
inductance actively, is also investigated.
IV.

PRINCIPLE OF HIGH OUTPUT USING LC RESONANCE

Fig. 7 shows the experimental circuit and the equivalentcircuit when a resonance capacitor is connected to thesecondary side of the transformer shown in Fig. 7(b). Fig. 7(b)is a LC resonant circuit. Therefore, from Fig. 7(b), theresonance capacitance which can realize LC resonance isobtained by (3).

(c) Sp:OFF, Sn:OFF(After (b)).
(d) Sp:ON, Sn:OFF.
Fig. 6. Operation patterns.

(a) Experimental circuit.
(b) Equivalent circuit.
Fig. 7. Equivalent circuit when a resonance capacitor is connected to thesecondary side of the transformer.

Cr =

1

(2πf r ) (Nn 2 L )2(3)

Table 1. Experimental conditions for the prototype circuit.

where fr is a resonance frequency. By the LC resonance,
the load voltage can be a sinusoidal waveform and increased
even though there is a leakage inductance.

(a) Td:85 ns, L:1.8 μH.

The quality factor for resonance Q is obtained by Eq. (4).
From Eq. (4), it is confirmed that Q is decided by theresonance frequency and the time constant shown in Eq. (2).
The load voltage becomes the sinusoidal waveform when Q islarge.

Q=V.

2πf r L= 2πf rτR(4)(b) Td:160 ns, L:1.8 μH.

EXPERIMENTAL RESULTS

Fig. 8 shows experimental waveforms; (a): Td=85 ns,
L=1.8 μH, (b): Td=160 ns, L=1.8 μH, (c): Td=85 ns, L=2.6 μH
and (d): Td=85 ns, L=3.1 μH. Table I shows the experimental
conditions. Note that the bifilar wound transformer is used.
First, comparing (a) and (b), it is confirmed that the period of
the drop in the inverter output voltage increases due to theincreasing of the dead-time period based on the principle
described in pervious chapter. On the other hand, comparing(a), (c) and (d), it is confirmed that the drop in the inverter
output voltage does not occur because the inductor current
keeps to flow through the body diode of the MOSFET due tothe increasing of the leakage inductance. Therefore, it isconfirmed that the drop in the inverter output voltage isreduced by designing the leakage inductance properly.
Fig. 9(a) shows experimental waveforms when LC
resonance is applied and a resistance of 50 Ω. A resonancecapacitance connected to the load in series. From Fig. 9(a), it
is confirmed that the load voltage is almost sinusoidal
waveform due to LC resonance. Fig. 9(b) shows the variationof the load voltage subjected to the value of the resonancecapacitance. The resonance point where maximum voltage
occurs in the load exists under the capacitance Cr of 100 pF.
However, over the capacitance Cr of 100 pF is used because
the withstand voltage of used capacitor is low. From Fig. 9(b),it is confirmed that the load voltage decreases according to theincreasing of the resonance capacitance based on the principleof LC capacitance.
Fig. 10 shows the comparison of the load voltage related
the input voltage when the secondary winding is wired
through each core, the bifilar wound transformer and LC
resonance are used. From Fig. 11, when the secondary
winding is wired through each core, the load voltage is very
low compared with the input voltage even though the turnration of the transformer is designed in order to achieve the(c) Td:85 ns, L:2.6 μH.
U phase voltage
Vuo
0(50V/div)
U phase current
Iu 0(2A/div)
Output voltage
Vout 0(100V/div)
Output current
Iout 0(1A/div)
400 (ns)(d) Td:85 ns, L:3.1 μH.
Fig. 8. Experimental results when inductance component is changed.

same amplitude to the input voltage. This is because theleakage inductance is large. As a result, the proposed circuit isoperated as a constant current source. On the other hand, inother two cases, the load voltage is larger because the leakage
inductance component is reduced.
Fig. 11 shows the comparison of the output current and theload voltage in three cases. From Fig. 11, the output current isalmost same when the secondary winding is wired througheach core because of the large leakage inductance even though
the load resistance is changed. On the other hand, in other twocases, according to the changing the load resistance, the output
current and load voltage is changed. According to theincreasing of the load resistance, the output current isdecreased. As a result, the voltage drop in the leakage

(a) Rload: 50 Ω.
60Fig. 10(a)Load voltage Vload [V]50
Fig. 10. Comparison of the load voltage related the input voltage.

40
Rload: 100

60

30
LC resonance ( )

50
20
10

40

Rload: 33
Rload: 50

30
400
500
300
600
200
Resonance capacitance Cr [pF](b) Load voltage related resonance capacitance.
Fig. 9. Experimental results with resonance capacitance.
0100

inductance is decreased and the load voltage is increased.
Fig. 12 shows the load power when the secondary winding
is wired through each core, the bifilar wound transformer and
LC resonance using the resonance capacitor are used. The
condition of the input voltage is 100 V. From Fig. 12, when
the secondary winding is wired through each core, it isconfirmed that the proposed circuit works as a constantcurrent source because the leakage inductance is very large.
On the other hand, in case of the bifilar wound transformer
and LC resonance, the load voltage and current becomes
higher compared with the condition when the secondary
winding is wired through each core. As a result, the validity of
two methods for higher output power is confirmed byexperiments.

20

Bifilar wound transformer ( )When the secondary winding iswired through each core ( )

10

0

20

40

60
80
Load Rload ( )

100

120

(a) Load voltage.
800

Bifilar wound transformer ( )

700
600
500

LC resonance ( )

400
300

When the secondary winding iswired through each core ( )200Fig. 13 shows the efficiency characteristics in three100
conditions. The efficiency of the prototype circuit is calculated
from the input and output power measured from the
0
20
40
60
80
100
120
waveforms. Noted that the output power is changed by
Load Rload ( )changing the input voltage 50, 80 and 100V in this experiment.
(b) Output current.
From Fig. 14, using the bifilar wound transformer and theFig. 11. Comparison of the output voltage and current related resistance ofseries resonance, the output power and the efficiency areload.
increased. The two methods overcome the problems of theinseriesinordertoadjusttheresonance frequency as 2.5
standard multi-core transformers. Especially, in the seriesMHzbecausetheresonance
frequencyof the spiral antenna isresonance method, the maximum efficiency of 84.7 % is
2 MHz. Note that, the LC resonance is not used and the bifilar
obtained by the experiment.
wound transformer is used in this experiment. Experimental
Fig. 14 shows the experimental setup during wireless parameters are shown below; input voltage is 100 V, thepower transfer using spiral antennas. In order to confirm the capacitance of the variable capacitor is 605 pF.
wireless power transfer easily, bulb of 60 W is used as a load.
Fig. 15 shows experimental waveforms of the output
In addition, a variable capacitor is added to the spiral antenna

25
Bifilar wound transformer ( )Load power Pout [W]20
15

LC resonance ( )

10
5

20

(a) Diagram

40

60
80
100
120
Load Rload ( )Fig. 12. Comparison of output power related resistance of load.

Efficiency [%]0

When the secondary winding iswired through each core ( )(b) PhotographFig. 14. Experimental setup of wireless power transfer.

Fig. 13. Efficiency characteristics.

voltage and current of the proposed circuit and the secondary
voltage and current of the receiver coil. From Fig. 15, thesecondary voltage and current of the receiver coil is almost
sinusoidal waveform because the quality factor for resonance
Q is high. Therefore, it is confirmed that the proposed circuit
can be operated for wireless power transmission.
VI.

CONCLUSION

This paper discussed the development for high output of
2.5MHz inverter without fast-switching semiconductor
switches. In this paper, the principle of drop in inverter output
voltage which reduces the inverter output voltage due to a
relationship between dead-time and a leakage inductance ofthe multi-core transformer was clarified. In addition, theexperimental results confirmed that two methods which arebifilar wound transformers and the LC series resonance canachieve higher output power of the proposed circuit. Finally,
the proposed circuit is applied to a wireless power transfer
when a lamb bulb is used as a load.
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