Realization of High Efficiency AC link Converter System
based on AC/AC Direct Conversion Techniques with RB-IGBT
Jun-ichi Itoh*, Takashi Iida*, Akihiro Odaka***Nagaoka University of Technology
1603-1 Kamitomioka-cho:
Niigata, 940-2188
JAPANitoh@vos.nagaokaut.ac.jp
Abstract –This paper proposes how to realize high efficiency of 95%
for the high frequency ac link converter of three-phase input andoutput. The proposed control method is based on virtual
converter system control method without resonant. Two novel
approaches are proposed in this paper. First, reducing method ofa switching loss is proposed. The proposed method realizes zerovoltage switching for generator side converter without auxiliary
circuit. Second, this experimental converter uses a reverse
blocking IGBT (RB-IGBT) instead of a series connection diodeto realize the Ac switches. The effects of the RB-IGBT ismentioned by the loss analysis using PSIM and experimental
results. At last, the benefit of adopting the RB-IGBT to the ac/ac
direct converter is verified through experiments.
The efficient of the proposed approach are confirmed through
1.5kW experimental system with the RB-IGBT. The clean
sinusoidal waveforms for power line side current and generatorside voltage, and the maximum efficiency of 94.7% are obtained.
The RB-IGBT improves the efficiency of 1.9 point. The
efficiency of around 95 % is very high in case of ac/ac converterwith transformer system at 1kW class. In addition, the motor
drive characteristics with 1.5kW induction motor are also shownin this paper.

I. INTRODUCTION
Recently, a distributed generation system is very
important to protect natural resources. Power sources of a
distributed generation system, for example, a wind power or a
micro-gas turbine, need an interface power converter tochange frequency. The interface power converter requires
isolation between a generator and a power line because it iseasy to protect the power line from accidents and noise. Thus,
the system of the interface converter becomes a three-phase
input and three-phase output with ac link converter. Moreoverthe interface converter also are required to be high efficiency
and small size.
Those requirements are filled with an ac/ac directconverter with a high-frequency transformer. A conventional
system, which consists of a PWM rectifier, a high frequency
link dc/dc converter and a PWM inverter, can also convert
from three-phase input to three-phase output with isolation.
However this system needs a large dc link capacitor and aboost-up reactor. It is inefficient because the number of theconversion is four times in the conventional system. On theother hands, the ac/ac direct converter has no large energy
storage components as an electrolytic capacitor. Besides, thereverse blocking IGBT (RB-IGBT)[11] is newly developed
for ac/ac direct converter. Therefore, the low conduction loss
is realized by only two times conversion and the RB-IGBT.

**Fuji Electric Advanced Technology Co.LTD.
: 1, Fuji-machi,
Hino-city Tokyo, 191-0082
JAPANodaka-akihiro@fujielectric.co.jp
However, the conventional control strategies of input andoutput waveforms for ac/ac direct converters are very
complicated. In addition, an energy management of a leakage
inductance of the high-frequency transformer is difficult. Theenergy of the leakage inductance causes to increase snubber
loss. To obtain high efficiency, some kinds of ac/ac directconverter using resonant phenomena of capacitors andreactors are proposed [1]-[9]. However the resonant converter
constrains generator side voltage region, moreover resonantcapacitors and reactors causes large volume.
On the other hands, the ac/ac direct converter techniques
was presented by [5]. The generator side converter consists ofa single-phase PWM rectifier and three-phase inverter. The
proposed converter does not use a large capacitor in dc stage.
However, the solution of the energy management method forthe leakage inductance and the commutation method of thegenerator side converter are not mentioned. In addition, thismethod is somewhat complicated to make PWM pulses foreach switch in the converter.
To obtain simple controller, a novel control strategy basedon virtual converter system was propped in [10]. The
proposed controller can be divided three virtual parts thatconsists of power line side current control part, high frequency
link part and generator side voltage part, respectively. After
that, PWM pulses of the ac/ac direct converter are obtained byeach part of those PWM pulses. However the effect of loss
suppression and the commutation method were not discussed.
Moreover the conventional IGBT and diodes are used in the
power line and generator side ac/ac direct converter.
This paper proposes novel method, which makes highefficiency for the three-phase input and three-phase output
ac/ac direct converter with the high frequency transformer.
The proposed system realizes the high efficiency without theauxiliary resonant circuit. Two novel approaches are proposedin this paper. First, the low loss and simple commutation
method are proposed. The zero voltage switching and the safe
commutation of the generator side converter are realized by
the power line side converter control. As a result, theswitching loss of the generator side can be reduced, andenergy management of the generator side is realised at thesame time. Second, an experimental converter uses a reverse
blocking IGBT (RB-IGBT)[11][12] instead of a conventionalac switch which consist of a IGBT with series connection
diode. The benefit of adopting the RB-IGBT to the ac/ac
direct converter is verified through loss simulation andexperiments.

H.F.T.:High- frequecy Transformer
Srp Ssp Stp epi
Sup Svp Swp
epoPower line

H.F.T
.

R,S,T

Input
filter

U,V,W

eni

⎡ srp
⎢s
⎣ rn

s sp
ssn

stp ⎤ ⎡ sap sra + san srb=⎢
stn ⎥⎦ ⎣ sbp sra + sbn srb

sap s sa + san s sb
sbp s sa + sbn s sb

sap sta + san stb ⎤sbp sta + sbn stb ⎥⎦(3).
The PWM pulses of the generator side are also obtainedby the same process as Equation (3). It is very easy to calculate
Equation (3) by digital logic hardware to multiply and add
switching functions ("1" or "0"). With this concept, all thecontrol algorithms for the conventional PWM rectifier andinverter system can be applied to the ac/ac direct converter.
The proposed control method separates the waveforms control

Sun Svn Swn

Fig. 1. Main circuit of high-frequency ac link
direct converter with RB-IGBT
Power line

Srp Ssp Stp

H.F.T.:High- frequecy Transformer

Sap Sbp

Scp Sdp Sup Svp Swp
U,V,W

H.F.T.
epi epo

R,S,T

eni

Input
filterSrn Ssn Stn
PWM rectifier

San Sbn

Generator

eno
Scn Sdn Sun Svn Swn

High-frequency aclink converter(H.F.C.)

InverterFig. 2. Main circuit of conventional PWM rectifier and inverter
system with a high-frequency ac link converter.
Pulse cal. by
zero-vector

up/down signal

Input current
commands

dc linkripple cal.
[Ref.[4]CarrierPulse
patternconversion[Ref.[6]]Single leg
modulation[Ref.[4]]1/xCarrier
generator

H.F.C
pulse
commands

Ac/acconverter pulse
commands

H.F.C controller
PWM rectifier controller

Output voltage
commands

II. CONTROL STRATEGY BASED ON VIRTUALCONVERTER SYSTEM
A. Virtual indirect control
Figure 1 shows a main circuit of an ac/ac direct converterwith a high-frequency ac-link. The relation between the
power line side voltage t[vr vs vt] and the motor( or generator)side voltage t[vu vv vw] can be expressed as Equation (1).
⎡ vr ⎤⎡e pi ⎤ ⎡ srp ssp stp ⎤ ⎢ ⎥Power line side: ⎢ ⎥ = ⎢⎥ ⎢ vs ⎥⎣ eni ⎦ ⎣ srn ssn stn ⎦ ⎢v ⎥⎣ p⎦
⎡ vu ⎤ ⎡ sup sun ⎤⎢⎥ ⎡e ⎤
Generator side: ⎢⎢ vv ⎥⎥ = ⎢ svp svn ⎥ ⎢ po ⎥(1)e⎢⎣vw ⎥⎦ ⎢⎣ s wp s wn ⎥⎦ ⎣ no ⎦where smn is the switching function of the switch Smn.
smn=1 when Smn is turned-on, and smn=0 when Smn isturned-off.
Figure 2 shows a main circuit of a conventional PWM
rectifier and inverter system with a high-frequency ac-link
converter. Same as Equation (1), the relation between the
power line side voltage and the generator side voltage infigure 2 can be described as Equation (2).
⎡ vr ⎤⎡e pi ⎤ ⎡ s ap s an ⎤ ⎡ s ra s sa sta ⎤ ⎢ ⎥Power line side: ⎢ ⎥ = ⎢⎥⎢⎥ ⎢ vs ⎥⎣ eni ⎦ ⎣ sbp sbn ⎦ ⎣ s rb s sb stb ⎦ ⎢v ⎥⎣ p⎦
⎡ vr ⎤ ⎡ suc sud ⎤⎡ scp s cn ⎤ ⎡e po ⎤⎢ ⎥Generator side: ⎢ v s ⎥ = ⎢⎢ svc svd ⎥⎥ ⎢(2)ss dn ⎥⎦ ⎢⎣ eno ⎥⎦⎢v p ⎥ ⎢⎣ s wc s wd ⎥⎦ ⎣ dp
⎣ ⎦In general, the operation of a converter can be describedwith the switching function. If the same switching function is
obtained from different topology converters, then the
waveforms of the power line side current and the generatorside voltage in those converters become exactly the same.
Thus, from a comparison of Equations (1) and (2), it canbe summarized that the power line side the ac/ac directconverter PWM pulses are obtained as

Generator

eno

Srn Ssn Stn

Pulse pattern Rectifier
pulse
compositioncommands[Eq.(3)]The efficient of the proposed approach are confirmed
through 1.5kW experiment system. The clean sinusoidal
waveforms for power line side current and generator side
voltage, and the maximum efficiency of 94.7% are obtained.
The efficiency of around 95% is the top class value in case of
three-phase ac/ac converter system with transformer at 1kWclass.

Inverterpulse
commands
Inverter controller

Fig. 3. Control block diagrams of proposed strategy.

of the input and output from the high-frequency link
conversion. In the following discussion, it is note that thepower line side converter is called the rectifier, and thegenerator side converter is called the inverter in figure 1.
Figure 3 shows a control block diagram of the proposed
strategy. The control block consists of a virtual rectifiercontrol, an inverter control, a high-frequency ac-link
converter and a pulse pattern composition of Equation (3).
(a)Rectifier part control:
The strategy of the virtual rectifier control has two topics
to realize simplicity and high-efficiency.
The first one is the adoption of the pulse patternconversion to translate the PWM pulses of the voltage source
type rectifier into the PWM pulses of the current source type.
Basically, the virtual rectifier is expressed as the current
source type, since the ac/ac direct converter has no boost up
reactor. However, the control of the current type rectifier iscomplicated in general. Thus, by utilizing the pulse patternconversion proposed in [16], the control method for theB. Loss suppression techniques
It is the key idea that the zero voltage output period in the
inverter is used to reduce the switching loss. The point of the
proposed method is that the dc-link current must be zero whenthe virtual inverter controller is selecting zero voltage vectors.
To reduce the snubber loss by the leakage inductance of thehigh-frequency transformer, the power line side of thetransformer is made short circuit by Sap,Sbp or San,Sbn during
the zero vector period of the inverter as shown Figure 4. Theenergy of the leakage inductance is discharged to the load. In
addition the output of the inverter pulse keeps same switchingstate during the zero vector because zero voltage of the output
is made by the short circuit the power line side of thetransformer. Moreover, the switching of the high frequency
converter of the generator side works while the inverter

Short circuit of the primary side
H.F.C. pulse
commands

voltage source type rectifier is used, which is much simpler
than the one for the current source type.
The second topic is the switching loss reduction by using asingle-phase modulation. With a conventional two-phasemodulation, the switching period is 2π /3(electrical angle) in a
half cycle of the power line side current. On the other hand, it
is reduced to π/3 with the proposed single-phase modulation.
Thus, the switching loss decreases by a half to theconventional method. Under the single-phase modulation, thedc-link voltage in the virtual circuit must contain the 6ω ripple
to avoid the power line side current distortion.
(b)Inverter control:
The voltage command for the virtual inverter control issimilar to the conventional one. It has dc-link voltage
compensation as the conventional inverter control because thevirtual dc-link voltage contains the 6ω ripple component as
explained above. Also, there is the leakage inductance in thehigh-frequency transformer. Thus, to reduce the snubber loss,
the two-phase modulation is used in the virtual inverter
control.
Figure 4 shows the relation of the carrier signals between
the virtual inverter and rectifier. There is a special carrier
signal modification in the virtual inverter control. The virtual
rectifier is controlled under the condition of constant dc-linkcurrent. However, the virtual dc-link current must be zero
when the virtual inverter controller selects the zero voltage
vectors. In the proposed method, this is achieved bycontrolling the slope of the inverter carrier signal as shown infigure 4. By adopting this method, the zero current period of
the dc-link is distributed by the same ratio to each power line
side current, as described in Equation (4).
Tr 0 Ts 0 Tr 0 + Ts 0
(4)
=
=
T1
T2
T1 + T2
As a result, the average value in the switching period of thepower line side currents are balanced among each phase. The
slope control can be implemented simply by using a controlled
up-down counter.
(c) High-frequency ac-link converter control
Basically, the pulse commands for the high-frequency link
converter synchronize with the up/down signal of the PWM
rectifier carrier. To reduce the switching loss, the switchingpattern in this part is improved as shown in the next section.

Sap

On

Off

On

Off

Sbp

Rectifier side ir *carrier andcommand
Rectifierside pulse

Tr0
T2

ir 0
Input
curret

is 0
it

0

T1
Ts0

Im
dc link
curret 0
Inverterside carrier vu *andvv *commands *vw

Zero voltage output

ON (by short circuit of the primary side)
Pu
Gate pulse
Commands Pvof inverter
Pw OFF (by modified two-phase modulation)
Zero voltage period ininverter side
Commutation
timingFig. 4. Relation among rectifier and H.F.C. pulse commands,
inverter carrier.

outputs the zero vectors. Therefore the high frequency ac link
operation does not generate the switching loss. As a result, theswitching loss of the inverter can be reduced because thenumber of the switching times during one period of the carrierin the generator side is decreased to only two times by thetwo-phase modulation and the zero voltage output method
using the short circuit.
C. Commutation strategy
The commutation strategy for the generator side converter
becomes very simple because the commutation is achieved
during the zero voltage period of the secondary side ofhigh-frequency transformer. The zero voltage period is madeby the short circuit of the power line side converter as
described in aforesaid explains. In usually ac/ac directconverter requires four-step commutation to avoid thecommutation fail [15].
However the number of thecommutation step is only two in this case. The over lap time
to keep the current pass of the inductive load is provided
during the commutation time.
The commutation strategy of the power line side converter isthe same as the conventional commutation. However, thenumber of the switching in the power line side converter isreduced by the single-phase modulation as described in theprevious section.
III. LOSS SIMULATION
We developed a simple simulation method for power
semiconductor loss of the proposed ac/ac converter. This
simulation method is composed by linking a circuit simulator(PSIM, Powersim Technologies Inc) and a DLL file (Dynamic

Link Library).
Figure 5 shows a configuration of the proposed loss
calculation method.
At first, power semiconductor
characteristics for the switching loss and conduction loss aremeasured by a chopper test under the various current andvoltage. Next, the instantaneous values of the current and thevoltage at the switch timing are captured to the DLL file. In
the DLL file, the power semiconductor characteristics and aloss calculation program are described. The switching loss isestimated by referring to the values of the voltage and current,
which are obtained by the chopper test results. The conduction
loss is estimated by the V-I characteristics from the current
value. This method can estimate the power semiconductor loss
regardless of the circuit configuration.
Figure 6 shows the power semiconductor characteristics of
the RB-IGBT and the conventional ac switch obtained by the
chopper test and V-I curve test. The forward drop voltage of
the RB-IGBT becomes about half of the conventional IGBT
although the switching loss is almost same.

+

Switching loss

DLL file

VCE-

Conduction loss

Vce

Ic
capturecaptureTurn-on,Recoveryloss cal.

Turn-off
loss cal.

Chopper
test data

Conduction
loss cal.

Total loss cal.

DLL file

Fig. 5. Loss simulation method with DLL file

B. Effect of the Reverse blocking IGBT
Figure 9 shows the efficiency and the input power factor of
the proposed converter with the RB-IGBT. It is noted that theRB-IGBT is only applied to the power line side and theconventional ac switch are used in the generator side. The
input power factor is more than 99% with 30% load and higher.
The maximum efficiency is 94.7% at 1.5kW output power.
The efficiency of the conventional PWM rectifier and inverter
system with the isolation transformer is less than around 90%
at 1kW class. Therefore this suggests a clear advantage of theac/ac direct converter with the isolation transformer in terms

RB-IGBT

IGBT+FWD

0.8

10Turn on loss [mJ]

Collector Current Ic[A]

12
8
6
4
2
0

0

1

2

0.6
0.4
RB-IGBT0.2

3

0

IGBT+FWD0

2 4 6 8 10 12
Collector Curennt Ic[A]Emitter-Collector voltage Vce[V]

0.4
0.3
0.2
RB-IGBT

0.1
0

IGBT+FWD
02

4 6 8 10 12
Collector Current Ic[A](b) turn-on characteristic.
Reverse recovery loss [mJ](a) Vce-Ic characteristic.

Turn off loss [mJ]

IV. EXPERIMENTAL RESULTS
A. Control performance of proposed method
Table 1 shows the condition of the experiments. Note thata commutation method based on the polarity of the input
voltage [15] for the power line side converter is applied in theexperiments. In addition, in order to control the output
voltage, a V/f control without a voltage error compensation
for the commutation is applied.
Figure 7 shows the waveforms of the power line voltage(phase voltage), the power line current and the output current
in case of the R-L load instead of the generator (motor), output
frequency of 40Hz. Good sinusoidal waveforms are obtained
for the input and the output currents. In addition, it is notedthat power factor of the input is unity. The causes of the
distortion in the input and output waveforms are the voltage
error depending on the current commutation.
Figure 8 shows the T.H.D. characteristics of the powerline side and generator side current under 1kHz harmoniccomponents. The T.H.D. of the power line side current of
2.6% and generator side current of 3.6% are obtained at the
rating output power of 1kW, respectively. The T.H.D. of thepower line side and generator side current are less than 4%,
and 6% with 20% or higher load region, respectively. From
those experimental results, the ac/ac direct converter canachieve a high performance by utilizing the proposed control
strategy.

0.25
0.2
0.15
0.1

RB-IGBT

0.05(c) turn-off characteristic.

00

2

IGBT+FWD
4 6 8 10 12
Collector Current Ic[A](d) recovery characteristic.

Fig. 6. Power semiconductor characteristics of theconventional IGBT and RB-IGBT
Table 1. Experimental conditions
Line voltage

200V

Cut off freq. of
input filter

Switching freq.

10kHz

Load

R-L

50Hz

Load P.F.

0.97

Input freq.

1.1kHzof efficiency, and the efficiency around 95% means on the topof the efficiency in the three-phase input three-phase output
converter.
Figure 10 shows the comparison between the loss
simulation results and the experimental result of the total loss.
The simulation results agree well with the experimental
results. It is confirmed that the loss simulation method iseffective to estimate the converter loss. Next step, the loss
analyses and estimation with the other construction of the ac
switch are discussed by this simulation.
Figure 11 shows loss analysis at 40Hz, 1.5kW with the loss
simulation and experimental results. The left side bar means

30

Input voltage (phase)

Loss (W)

Experimental results

Input current

20

Simulation results
10

Output current

00.5

Fig. 7. Experimental results of proposed control
Strategy (output frequency :40Hz).

1.5

Fig. 10. Comparison of loss between simulation results andexperimental results.

10

140

8

Efficiency: 92.8%

120

Transformer

100

6Secondary sw. loss

Output current

Loss[W]

T.H.D(%)

1

Output power (kW)42

Efficiency: 94.7%

80
Secondary cond. loss

60
40

Input current

snubberPrimary sw. loss

20

0

0.5

1

1.5

2

Output power (kW)

Efficiency, Input Power Factor(%)

Fig. 8. T.H.D of input current and output current in the
proposed converter.

P.F.
95
Efficiency
90

0

0.5

1

1.5

IGBT+FWD

RB-IGBT(Simulation)(Experimental)

Figure 11. Comparison of the loss each ac switch at rated load.

100

85

Primary cond. loss

0
The configurationof ac switch.

2

Output power (kW)Fig. 9. Characteristics of efficiency and input power factor.

the loss simulation results with the conventional ac switch.
Though the ac switches are used the conventional IGBTs and
FWD, the high efficiency of 92.8% is obtained by the
proposed control method. Moreover, the total efficiency of
94.7% is obtained by changing the all the conventional ac
switch to the RB-IGBT. The conduction loss decreases toabout 2/3 of the conventional ac switch. The RB-IGBT can
improve the efficiency of 1.9points against the conventionalac switch. The loss of the transformer accounts for about 20%of the total loss. To improve the efficiency, the design to of thehigh-frequency transformer is important.
Figure 12 shows power line voltage (phase voltage) ,power line current and motor current waveforms in case of thegeneral purpose induction motor instead of the R-L load. The
ratings of the motor are power: 1.5kW, voltage: 200V,
frequency: 50Hz, speed: 1420r/min. The motoring andgenerating operation is confirmed in Figure 12(a) and (b),respectively. In the both case, the power line current is almost
kept sinusoidal waveform and unity power factor. In addition
the motor current is also keeping sinusoidal waveform. It isnoted that the distortion of the power line side and generatorside current are occurred by the commutation error. Thus, thecompensation of the commutation error for the proposed
circuit or the current control using feedback control is needed
to improve the current waveforms.
V.CONCLUSIONS. In this paper, two approaches to make the high efficiency
isolated ac/ac conversion system are proposed. The loss is
suppressed by the following techniques without the restrictionof the PWM pattern as resonant type converter.
•
The number of the switching times in the generator

side is reduced by the two-phase modulation and theshort circuit of the power line side in transformer. The
proposed control method is realized by the virtual
converter system control method which can beseparated into the rectifier, dc/dc high-frequency link
and inverter stage despite the ac/ac direct converter.
•
The commutation of the generator side converter
becomes simple. The number of the proposed
commutation timing is only two because thecommutation is achieved during zero voltage timingof the generator side converter.
•
The effect of the RB-IGBT is confirmed by the loss
simulation and experimental results. As a results, thehigh efficiency of 94.7% is obtained at 1.5kW load in
the experimental results, and it is improved about 1.9
point against a conventional device.
In future works, the input and output waveforms will be
improved by the voltage error compensation, and the details of
the motor drive characteristics will be mentioned.
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Input voltage (phase)

200V/div

Input current

10A/div

00
10A/div0

Output current

10ms/div(a)Motoring

Input voltage (phase)

200V/div

Input current

10A/div

00
10A/div0

Output current

10ms/div(b)Generating
Fig. 12 Input and output current waveforms in case of theinduction motor under ratting torque.

[10] Jun-ichi Itoh, Daisuke Matsumura, Seiji Kondo, Akihiro
Odaka, Hideki Ohguchi: “Novel Control Strategy for
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Kazuhiro Kodachi:” A Novel Approach to Practical
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