Switching Loss Reduction of AC-AC Converter
using Three-level Rectifier and Inverter for UPS.
Kazuki Yoneda, Hiroki Takahashi and Jun-ichi Itoh
Dept. of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japan
yoneda@stn.nagaokaut.ac.jp, thiroki@stn.nagaokaut.ac.jp and itoh@vos.nagaokaut.ac.jp
Abstract— This paper proposes an AC-AC converter, which
consists of T-type three-level rectifier and inverter, for an on-line
UPS. The switching loss of the proposed AC-AC converter isdrastically reduced because the proposed converter is driven at avery low switching frequency which is six times of input side
frequency. The T-type rectifier separates the maximum phasevoltage, medium phase-voltage and minimum phase-voltage fromthe input voltage. Next the output waveform is built by the T-typeinverter from each maximum phase-voltage, middle phasevoltage and minimum phase-voltage. The proposed circuit canachieve not only high efficiency, but also short instantaneousinterruption time. Furthermore, the proposed AC-AC converter
compensates a voltage dip with changing an operation mode of arectifier. In this paper, the fundamental operation of the
proposed converter is confirmed by simulations and experiments.
In addition, the power loss of the proposed converter is compared
to a conventional on-line UPS and the efficiency of the proposed
converter is 97.1% at rated load in an experiment.

high switching frequency. Therefore the reduction of theswitching loss at a stable grid condition is limited.

Keywords— Uninterruptible power systems; Power electronics;
Circuit topology; Modulation; Power conversion; voltage dip; ACAC converter

This paper is organized as follows. Firstly a control method
and operations of the proposed converter are described;
secondly, simulation results for the stable operation, thevoltage dip and the interruption of the grid are presented. As aresult of a loss analysis, the proposed converter obtains anefficiency of 97.7% while an efficiency of a conventional online UPS is 94.6%. Finally, experimental results for the stable
operation are presented. An efficiency of 97.1% is confirmedby an experimental result at 3 kW load.

I. INTRODUCTION
Recently, the demand of uninterruptible power supplies(UPS) for many server rooms and line factories has been
increased [1]. Due to the use of UPSs, the stability of powersources has increased and the damage on instantaneous power
failure such as a data loss in data centers or a halt of the
production in factories is avoided. The configurations of UPSs
are divided into a standby type and an on-line type. The
standby type has an advantage that when a grid is in a normal
state, a provided current flows only through AC switches.
Therefore the power loss is very small. However, when the grid
fails, the detection of the voltage drop takes several ms and theload voltage is interrupted during the detection period [2]. On
the other hand, the on-line type provides no interrupted power
even if the grid fails because the on-line type UPS operates
constantly. However, this operation requires a PWM control
which generates switching loss even when the grid is in thenormal state. In order to solve the problem, a method that
reduces the switching loss by increasing levels of the converter
and reducing rating voltages of switching devices has been
proposed [3-5]. This method is also effective for UPS
application [6]. However, the use of a PWM control implies aThis paper proposes an AC-AC converter using T-type
three-level converters in order to reduce the switching loss. The
proposed converter is driven at six times of the grid frequency.
In addition, when the grid voltage decreases, the rectifier in the
proposed converter is controlled as a boost mode with PWM
operation. As a result, a constant output voltage is maintained.
Moreover, the load power is supplied from a battery throughthe proposed converter without a voltage drop during
interruption of the power grid. Therefore, the proposed
converter is appropriate for an on-line type UPS because theproposed converter reduces the switching loss in the stable
operation of the grid and compensates the grid failure.
Additionally, the proposed converter has high reliability andlong lifetime because the proposed converter uses film
capacitors at a DC-link instead of any electrolytic capacitors.

II. SYSTEM CONFIGURATION
A. Conventional UPS circuits
Fig. 1 shows conventional circuit configurations for UPSs.
Fig. 1(a) shows the standby type while Fig. 1(b) shows the online type. In the standby type, when the grid is stable, thepower is supplied to the load from the grid through AC
switches. Therefore, the power loss is very low. When the grid
fails, the AC switches are opened and the inverter in the UPS
starts to operate. As a result, the load power is supplied fromthe battery by the inverter. However, there is an interruption
time of several ms at changing the operation mode.
On the other hand, on the on-line method, CVCF (constantvoltage constant frequency) power is supplied to the load byusing a rectifier and an inverter. Therefore, the interruption
time of this method is zero, and the on-line type UPS is usedfor applications that require high reliability. However, there is aproblem in this method, which the PWM control of the rectifier
and the inverter are needed regardless of the grid condition.
Therefore, the switching loss is generated at all the time.
B. Proposed UPS circuit
Fig. 2 shows the proposed circuit diagram. The proposed
circuit consists of a three-phase three-level T-type AC-DC-AC
converter [7], [8], batteries and a buck chopper. The proposed
converter has no large electrolytic capacitors because theproposed method utilizes a voltage ripple at six times of thegrid frequency (300 Hz) in a DC-link voltage. This also results
in reducing the switching loss significantly. Specifically, themaximum voltage vmax, the medium voltage vmid and theminimum voltage vmin in the input three-phase voltage areconnected to p, o and n points in the DC-link without asmoothing by the rectifier. It should be noted that bidirectionalswitches of the rectifier turns on to conduct vmid whereas thediode bridge selects vmax and vmin automatically. Finally, athree-phase voltage is provided to the load by the inverter
which turns each 60 degrees of an output voltage phase.
Therefore, the proposed converter reduces the switching loss
drastically when the grid condition is stable. In addition, when
the grid voltage is decreased, the rectifier boosts up the input
voltage and the DC-link voltage. As a result, the output voltage
is maintained at constant. Moreover, for an interruption of thegrid, the load power is supplied from the battery through thebuck chopper and the inverter without any voltage drop.

(state number). Table 1 shows a switching table of the rectifier
and the inverter. In Fig. 3, the rectifier and inverter of the
proposed circuit are driven by open loop control using state
number to decide switching patterns. In Fig. 4, state number isdetermined along a magnitude relation of the input phasevoltage vr, vs and vt. The state number changes each 60 degreeof the input voltage phase and switching patterns of therectifier and the inverter are decided by referring state number
and Table 1. It should be noted that P1 and P2 in Table 1 areswitching signals for the voltage dip only, and these are set to 0
during the stable grid condition.
B. Power supply method using batteries for interruption
In the proposed circuit, the output power is supplied fromthe batteries by the buck chopper when the interruption occurs.
The buck chopper is connected at the DC-link in parallel with
Rectifiervr, vs, vt

CDC1

S 1, S 2, S 3
CDC2

Grid

Grid

u
Load
v
w

S5, S8, S11
n vminS6 S9 S12

vr vsEmergency Power Supply
Battery
Vb1

S13

vmax vmid vmin
0

Lb2

S15

Vb2

vt

0

Lb1

S14
BatteryLoadvr, vs, vt
o vmidIII. CONTROL METHOD
A. CVCF control with switching each 60 degrees for a stable
grid condition
Fig. 3 shows a control block diagram when the grid isstable. Fig. 4 shows the correspondence of input voltage phaseand a variable that is used for the judgment of the control area

Inverter
S4 S7 S10

p vmax

vu

vv

vw

0

S16
Fig. 2. Proposed AC-AC converter for UPS.

vr
vs
vt

state number
Judgement(max,mid,min)AC switch

SUP, SVP, SWP

RectifierSwitching table

S1~3

Inverter
Switching table

S4~12

Fig. 3. Control block diagram of CVCF.

L

I
State
number vr

Battery

II

III
vs

IV

V

VI

vt

SUN, SVN, SWN(a) Standby type.

200V
50Hz

Grid

0°

Rectifier Vdc=350 V Inverter
SRP SSP STP
SUP SVP SWPLL

SRN SSN STN

200V
50Hz

Load

Battery

SUN SVN SWN(b) On-line type.
Fig. 1 Conventional circuits for UPS.

60° 120° 180° 240° 300° 360°

Fig. 4. Correspondence of input voltage to state number.
TABLE I.

SWITCHING TABLE OF AC-AC CONVERTER CONTROL.

Rectifierstate no. S1 S2 S3I
P1 1 P2II
1 P1 P2III
P2 P1 1IV
P2 1 P1V
1 P2 P1
VI
P1 P2 1Inverterstate no. S4 S5 S6 S7 S8 S9 S10 S11S12
I
1 0 0 0 1 0 0 0 1
II
0 1 0 1 0 0 0 0 1
III
0 0 1 1 0 0 0 1 0IV
0 0 1 0 1 0 1 0 0
V
0 1 0 0 0 1 1 0 0
VI
1 0 0 0 0 1 0 1 0the rectifier. Hence, in order to compensate the interruption
without changing the 60 degree switching operation of theinverter, the buck chopper needs to output the DC voltage witha ripple of 300 Hz during the interruption as well as the normal
condition. Specifically, the DC-link voltage vmax-vmid and vmidvmin are controlled by the buck chopper. Then, the output
voltage of the chopper is controlled by the following equations.

vmax

mid(vr,vs,vt)

vmid

min(vr,vs,vt)

vmin

Fig. 5. Equivalent circuit of rectifier at the grid voltage dip compensation.
r, s, tv max  v mid  d13Vb1

(vr, vs, vt)

v mid  v min  d16Vb 2

(vu_ref, vv_ref, vw_ref) u, v, w

d14  1  d13d15  1  d16

S16.

max(vr,vs,vt)

a, b
1
0



Where, d13, d14, d15, d16 are the duty ratio of the S13, S14, S15,

C. Voltage dip compensation
Fig. 5 shows an equivalent circuit of the rectifier used forthe compensation of the voltage dip of the grid. From Fig. 5,
the rectifier is able to be assumed as a boost up rectifier. Hence,for the compensation, two bidirectional switches at vmax andvmin carry out PWM operation in order to boost up thedecreased input voltage.
Fig. 6 shows a control block diagram for the grid voltage
dip compensation. In Fig. 6, a boost up ratio is calculated byusing the amplitudes of the input voltage vector and the output
voltage reference vector. The boost up ratio is modulated by acarrier comparison method with a triangle wave. Finally, theswitching signal P1 and P2 is used for the voltage dip
compensation.
IV. SIMULATION RESULTS
A. Input and Output waveforms
In order to confirm the fundamental operation of the
proposed circuit, this chapter shows simulation results using
the simulator PLECS (Plexim). Table 2 shows simulation
conditions of the proposed circuit. The input voltage and the
input frequency are set to 200 V, 50 Hz, sinusoidal waveform.
The load is an RL-load of 1-5 kW.
Fig. 7 shows operation waveforms of the CVCF control. In
these results, it is confirmed that the output voltage of 200 V,
50 Hz is achieved by regulating the DC link voltage at 300 Hz
even when the switches are driven at a very low frequency of
300 Hz.
Fig. 8 shows operation waveforms when an interruption isemulated by decreasing the input voltage to zero suddenly. Itshould be noted that a detection method of the interruption in[9] is used. From Fig. 8, the proposed converter supplies power
to the load by maintaining the DC-link voltage even if the input
voltage becomes zero suddenly due to the interruption. In
addition, the proposed converter changes the current pathway
from the grid to the battery without output voltage drop in thesame manner with the conventional on-line type UPS. It should
be noted that the output voltage oscillates by 22% at thea, ba2  b 2
a b
2

*

2

Carrier 1
Carrier

Carrier 2

-1

+
-

P1

+
-

P2

Fig. 6. Control block diagram of the grid voltage dip compensation.
TABLE II.

SIMULATION CONDITIONS OF THE PROPOSED CIRCUIT MODEL.

Input line voltage 200V(rms) Rated output voltage 200V(rms)
Rated power
3 kW
Load resistance
13 Ω
grid frequency
50 Hz
Load inductance
1 mH
Input inductunce
0.1 mH
Vb1, Vb2
300V
L b 1, L b 2
0.1 mHC DC 1, C DC 2
1 uF
switching frequency
CVCF control
300 Hzswitching frequency
Buck chopper control
100 kHzswitching frequency
100 kHz
Boost mode control
input line voltage step
-0.5 p.u.

400 Input line Voltage [V] vrs
200
0
-200
-400
300 DC link Voltage Vmax-Vmid [V]

200
100
0
DC link Voltage Vmid-Vmin [V]
300
200
100
0
Load line Voltage [V]
vuv
400
200
0
-200
-400
1.0
1.5
2.0
2.5

vst

vtr

vvw

3.0

vwu

3.5

4.0 × 1e-2

Fig. 7. Operation waveforms of the proposed circuit by simulation when thegrid is not fail.

interruption because of a resonance between inductors Lb1, Lb2of the buck chopper and the DC-link capacitor CDC1, CDC2.
Fig. 9 shows operation waveforms when a voltage dip isemulated by decreasing the input voltage to 0.5 p.u. suddenly.
From Fig. 9, the proposed converter maintains the output
voltage of 200 V, 50 Hz because the rectifier boosts up thedecreased input voltage with the voltage dip compensation
method.

A. Configulation of an experimental circuit
This chapter evaluates the operation of the proposed CVCF
control using a prototype circuit in steady state. Fig. 12 shows
the circuit diagram of the prototype and Table 3 shows
specifications of the prototype circuit. A prototype of 3kW isconstructed to confirm the fundamental operation of the
proposed converter. The T-type three-level rectifier and theinverter consist of a 6in1 IGBT module (fwd), three 2in1 IGBT
modules and twelve MOSFETs as bidirectional switches. The
proposed circuit does not need any large electrolytic capacitors
and uses only two film capacitors (2.2 μF) at DC-link.
Therefore, downsizing and long lifetime are achieved by using
the proposed circuit, compared to conventional on-line UPSs
that use a PWM control. The prototype circuit does not include
the buck chopper and the battery because this chapter confirmsthat the CVCF control is achieved by low speed switchingfrequency of six times of the grid frequency when the grid isstable. Experimental verifications of the emergency power
supply and the voltage dip compensation will be reported in the
future. An RL-load is used as a load in Fig. 12. The load
power is changed by adjusting a load resistance (76 Ω to 13 Ω)and the rated power is 3 kW.
Then, the rated voltage of IGBTs and the bidirectionalswitches are calculated by following equations.

VIGBT  2Vac  200 2  283[V ]Vbidirectional _ switch 33
Vac  200
 245[V ]
2
2




where, Vac is an effective value of the input line voltage. In the
proposed circuit, the DC-link voltages are not smoothed andare oscillated at six times frequency by one of the grid.
Accordingly, the medium phase switches of the proposed
circuit require higher rated voltage, compared with a general Ttype three-level converter that uses a constant DC-link voltage.

vvw

vwu

3.0

3.5

4.0 × 1e-2

Fig. 8. Operation waveforms of the proposed circuit by simulation when thegrid is fail.
400 Input line Voltage [V]
vst
vrs
200
0
-200
-400
300 DC link Voltage Vmax-Vmid [V]
200
100
0
DC link Voltage Vmid-Vmin [V]
300
200
100
0
Load line Voltage [V] vuv
vvw
400
200
0
-200
-400
1.0
1.5
2.0
2.5
3.0

vtr

vwu

3.5

4.0 × 1e-2

Fig. 9. Operation waveforms of the proposed circuit by simulation during thegrid voltage dip.
100.0

Proposed circuit

99.0

Efficiency [%]V. EXPERIMENTAL RESULTS

line Voltage [V]
400 Inputv
vtr
200 st
0
-200
vrs
-400
300 DC link Voltage Vmax-Vmid [V]
200
100
0
300 DC link Voltage Vmid-Vmin [V]
200
100
0
400 Load line Voltage [V] vuv
200
0
-200
-400
1.0
1.5
2.0
2.5

98.0
97.0
96.0

Conventional circuit

95.0
94.0
93.0

0

1

2
3
4
Output Power [kW]5

6

Fig. 10. Efficiency characteristic with respect to power.

Power loss
[W]

B. Loss analysis
In order to confirm the validity of the proposed converter
from the viewpoint of the efficiency, this section compares theefficiency of the conventional on-line type UPS as shown in
Fig. 1 (b) and the proposed converter. Fig. 10 shows anefficiency characteristic with respect to the load power and Fig.
11 shows a loss analysis result. The load power is changed byadjusting a load resistor and the rated power is 3 kW. The
switching frequency of the conventional on-line type converter
is set to 20 kHz. Because the switching frequency of the
proposed circuit is 300 Hz, the switching loss of the proposed
converter is reduced by 99% compared to the conventional
circuit at the stable grid condition. In contrast, conduction
losses of the diode in the rectifier and the IGBT in the inverter
are increased because the proposed converter employs more
semiconductor devices. However the effect of reducing theswitching losses is much higher than the increase of the
conduction loss. As a result, the proposed circuit achieves theefficiency of 97.7% while the efficiency of the conventional
converter is 94.6% at the rated power.

300
250
200
150
100
50
0

inv IGBT sw.
inv diode sw.
rec IGBT sw.
rec diode sw.

0

1
2
3
4
Output Power [kW]Proposed circuit

Fig. 11. Loss analysis of simulation result.

inv IGBT cond.
inv diode cond.
rec IGBT cond.
rec diode cond.

5

0

1
2
3
4
5
Output Power [kW]Conventional circuit

As a result, in the prototype, 600V-IGBTs and 500VMOSFETs are selected.
A cut-off frequency fc between the input inductances L andthe DC-link capacitors C are designed not to affect the triangle
waves of the DC-link voltages. A fundamental frequency of thetriangle waves of the DC-link voltages ftri is six times that of
the grid.

f tri  6 f grid [ Hz ]RectifieroAdditionally, a cut-off frequency fc is calculated byfollowing equations. In the proposed circuit, the switchingfrequency fsw is the same as ftri and fgrid is 50 Hz. Therefore, fsw
is equivalent to 300 Hz. Thus, parameters of L and C are set to0.11 mH and 2.2 μF to make fc higher than 300Hz.

fc 

1
2 LC

[ Hz ]  10.2[kHz ]  300[ Hz ]Sum, Svm,
Swm
DC2 n
vmin

Fig. 14 shows the inverter waveforms and u-phaseswitching patterns Sup, Sum, Sun of the T-type three-level inverter
when the grid is stable. From the experimental results, due tothe switching operation of the inverter at 300Hz, it is confirmedthat the output voltage becomes the same as the input voltage
of 200V 50Hz. In other words, the proposed converter achieves
the sinusoidal load voltage without using PWM modulation. As
a result, the proposed circuit can achieve downsizing and loss
reduction of an EMI filter. However, the output voltage
waveform has a surge voltage and the switching pulses havepulse noise periodically. The cause of the surge voltage will befound out and suppressed in the future.
C. Efficiency evaluation
In order to evaluate the proposed converter from theviewpoint of the efficiency, this section compares an efficiencyof simulation results and experimental result. Fig. 14 shows theefficiency characteristics with respect to output power. The
efficiency in range from 1 kW to 3 kW of the output power is

Load
Sun

Svn Swn

Fig. 13. Configuration of Prototype circuit.
TABLE III.

SPECIFICATIONS OF PROTOTYPE CIRCUIT

Input and Output line voltage V ac
Rated power
3 kW Load resistancegrid frequency
50 Hz Load inductance
Input inductance 0.11 mH C DC 1, C DC 2
6in1 IGBT module (fwd)
Rectifier2in1 IGBT module
S up ,vp ,wp , S un ,vn ,wn
MOSFETS rm ,sm ,tm , S um ,vm ,wm

B. CVCF control
Fig. 13 shows operation waveforms and switching patternsof the CVCF control which uses T-type three levels rectifier
when the grid is stable. In the proposed circuit, the maximum
phase and the minimum phase of the input voltage are rectified
due to a diode bridge rectifier. Furthermore, the medium phasevoltage is rectified by using the medium phase switches Srm,
Ssm, Stm, as shown in bottom of Fig. 13. As a result, the DC-link
voltage vmax - vmid and vmid - vmin oscillates at 300Hz as six timesof the frequency of the grid. A phase difference of 30 deg.
between vmax - vmid and vmid - vmin is confirmed because a crosspoint of vmax - vmid and one of vmid - vmin are generated at every
30deg.. In addition, the medium phase switches have no deadtime because the diode rectifier prevents a short circuit.

vu, vv, vw

vmid

Srm, Ssm,
Stm C

Grid





InverterSup Svp Swp

CDC1

vr, vs, vt

Next, a cut-off frequency fc is designed higher than ftri forkeep the waveform of triangle waves.

f c  f tri  6 f gridp vmax200V(rms)
12.6 Ω
2 mH
2.2 μF
6MBP50NA060-01
2MBI50N-060
2SK3522-01

Input phase Voltage vr [V]

100[V/div]

DC link line voltage Vmax – Vmid, Vmid - Vmin [V]

100[V/div]00
Switching pattern of Srm, Ssm, Stm

Fig. 14. Operation waveforms of the proposed circuit by experiment when thegrid is not fail.

100[V/div]
DC link line voltage Vmax – Vmid, Vmid - Vmin [V]0Output phase Voltage vu [V]

100[V/div]0

Switching pattern of Srm, Ssm, Stm
Fig. 15. Operation waveforms of the proposed circuit by experiment when thegrid is not fail.

The conduction losses of the semiconductor devises and theinductors affect the efficiency mainly because the switchinglosses are almost reduced in the proposed circuit. Additionally,
the conduction losses increase with square of current
proportionally. Therefore, the efficiency characteristic
decreases exponentially with increasing of load power.

Proposed circuit (sim.)

100.0
99.0

Efficiency [%]compared. From Fig.14, it is understood that the efficiency
decreases with the increase of load power, and reaches 97.1%
at rated power 3kW. Besides, the error between the simulation
results and the experimental results is -0.6%. This is because
the cupper loss and iron loss of input inductors are notconsidered in simulation models. Therefore, the efficiency
characteristic of the prototype circuit is decreased bysimulation result.

Proposed circuit (exp.)

98.0

97.8%

97.0
96.0

97.6%

97.1%

Conventional circuit (sim.)

95.0
94.0

0

1
2
Output Power [kW]3

4

Fig. 16. Efficiency characteristic with respect to power.

VI. CONCLUSION
This paper proposes an AC-AC converter for an on-line
UPS and confirms a fundamental operation with a simulation.
As a result, the proposed converter achieves CVCF operation
with the switching frequency of 300 Hz for the grid of 50 Hz.
In addition, the compensations for the interruption and the gridvoltage dip without the output voltage drop are confirmed.
Moreover, the power loss of the proposed converter iscompared to a conventional on-line UPS and the efficiency ofthe proposed converter is improved to 97.7%. From the resultsof the loss analysis, the switching loss in the proposed
converter is decreased 99% comparing to that of theconventional one. From the simulation results, the increase of
the efficiency to 3.3% is confirmed when the grid is stable . In
addition, with the 3kW prototype, the efficiency of 97.1% was
confirmed. In the future, the experimental operation in thevoltage dip compensation mode and the input voltage interrupt
mode will be shown in order to confirm the usefulness of the
proposed converter.

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