High-Performance Online UPS

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 52, NO. 3, JUNE 2005 889

High-Performance Online UPS UsingThree-Leg-Type Converter

Jin-Ha Choi, Jung-Min Kwon, Jee-Hoon Jung, and Bong-Hwan Kwon, Member, IEEE

AbstractA high-performance single-phase online uninterrupt-ible power supply (UPS) is proposed. The UPS is composed ofa three-leg-type converter which operates as a battery chargerand an inverter. The first leg is controlled to charge the battery,and the third leg is controlled to make the output voltage. Thecommon leg is controlled in line frequency. The charger andthe inverter are controlled independently. The charger has thecapability of power-factor correction while charging a battery.The inverter regulates output voltage and limits output currentunder an impulsive load. The three-leg-type converter reducesthe number of switching devices. As a result, the system has lesspower loss and a low-cost structure. In the determination of thecharger voltage, the nominal voltage is derived using the feedback

linearization concept and then a perturbed voltage is determinedfor the reactive power control. The disturbance of input voltage isdetected using a fast sensing technique of the input voltage. Ex-perimental results obtained with a 3-VA prototype show a normalefficiency of over 87% and an input power factor of over 99%.

Index TermsBattery charger, power-factor correction (PFC),uninterruptible power supply (UPS).

I. INTRODUCTION

UNINTERRUPTIBLE power supplies (UPSs) are used to

supply clean and uninterrupted power to critical loads,

such as computers, comunication systems, and medical support

systems, etc. As such sensitive equipment is used worldwide,their interruption due to a power failure may lead to critical ac-

cidents. The UPS system is indispensible for this reason.

The recent increase in the use of nonlinear loads caused se-

rious concern for power quality and, consequently, on the dis-

turbances tolerated by sensitive electronic loads. IEEE Standard

4461987 [1] describes the voltage tolerance limits for sensi-

tive loads, such as computer power supplies. In IEEE Standard

4461987, shown in Table I, a voltage drop of more than 15%

cannot be tolerated for more than 30 cycles (or 500 ms). Simi-

larly, a 35% voltage drop can be tolerated for only one cycle (or

16.7 ms). In addition, power factor can be tolerated for over 0.8.

In an effort to meet these requirements, voltage regulation andpower-factor correction (PFC) have become very important.

A typical single-phase converter shown in Fig. 1(a) has four

legs; two of them are used for a rectifier/battery charger, and the

Manuscript receivedJune 11, 2003; revised September 1, 2004. Abstract pub-lished on the Internet March 14, 2005.

J.-H. Choi was with the Department of Electronic and Electrical Engineering,Pohang University of Science and Technology, Pohang 790-784, Korea . Heis now with LG Electronics Company Ltd., Seoul 150-721, Korea (e-mail:[email protected]).

J.-M. Kwon, J.-H. Jung, and B.-H. Kwon are with the Department ofElectronic and Electrical Engineering, Pohang University of Science andTechnology, Pohang 790-784, Korea (e-mail: [email protected]).

Digital Object Identifier 10.1109/TIE.2005.847575

others for an inverter. To improve this, a three-leg-type converter

shown in Fig. 1(b) is proposed. According to the recent literature

[2][9], a three-leg-type converter is developed in many ways.

The most surpassing feature is that power losses can be de-

creased by using a common leg for both the pulsewidth-modula-

tion (PWM) rectifier and PWM inverter. In addition, the system

can have excellent input and output characteristics.

The proposed system has such characteristics as high ef-

ficiency, high power factor, and fast response to input and

output disturbances. The disturbance of the input voltage, such

as overvoltage and undervoltage, can cause a system trip and

hardware equipment failure. To overcome these, fast detec-tion of input voltage disturbances and a fast output voltage

compensation technique are necessary to achieve good output

voltage regulation. Here, we propose a fast output voltage

controller by utilizing a fast input voltage detection method and

a feedforward controller. This paper also proposes a new output

voltage recovery technique under an impulsive load [10]. By

using leakage transformers, universal filtering is implemented

without any additional passive filters [11].

Experimental results of a 3-kVA prototype show the perfor-

mance of the proposed UPS. From the experimental results,

it can be seen that a maximum overall efficiency of 87% and

a power factor of 99% can be obtained. Moreover, the inputand output characteristics have good steady-state and dynamic

performance. The main advantages of the proposed approach

are: 1) the rectifier works as an active filter and thus eliminate

harmonics created by nonlinear loads; 2) the inverter regulates

output voltage under any disturbance; 3) it possesses high ef-

ficiency and high power factor; and 4) there is current-limiting

capability for impulsive loads.

II. SYSTEM DESCRIPTION

Fig. 2(a) shows the configuration of the single-phase UPS

using a three-leg-type converter. The proposed UPS uses

leakage transformers to reduce cost. The leakage inductance ofthe leakage transformer is utilized as an inductor of the LCfilter

without using additional inductors. The leakage inductance of

the leakage transformer should be designed to have reasonable

value; otherwise, an additional inductance must be added in

series with the transformer. The leakage inductance can be

handled by controlling the separation degree of the primary

winding and secondary winding of the transformer. Tight

coupling of the primary winding and the secondary winding

reduces the leakage inductance of the transformer. The input

and output capacitors are simultaneously utilized as a capacitor

of the LCfilters for the charger and inverter. Thus, the UPS has

universal filtering capabilities to act as a line-voltage filter, an

0278-0046/$20.00 2005 IEEE


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890 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 52, NO. 3, JUNE 2005

TABLE ITYPICAL RANGE OF INPUT POWER QUALITY AND LOAD PARAMETERS OF MAJOR COMPUTER MANUFACTURERS

Fig. 1. Single-phase converters. (a) Conventional converter with four legs. (b) Three-leg-type converter.

output-voltage stabilizer, a backup voltage filter, and an active

filter compensating for the reactive current.

Fig. 2(b) shows the three-leg-type converter using leakage

transformers. In the three-leg circuit of Fig. 2(b), the rectifier

leg and the common leg operate as a PWM recti fier (boost con-verter) and, at the same time, the common leg and the inverter

leg operate as a PWM inverter (buck converter). By using the

common leg, the system can reduce one switching leg compared

with the conventional four-leg variety. Due to reduced devices

and a simpler structure, it can provide low cost and small size.

The operation of the proposed UPS can be divided into eightmodes. When the input voltage is normal, the operation is di-


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CHOI et al.: HIGH-PERFORMANCE ONLINE UPS USING THREE-LEG-TYPE CONVERTER 891

Fig. 2. Configurations of the proposed UPS. (a) Proposed UPS. (b) Three-leg-type converter using leakage transformers.

vided into four modes, as shown in Fig. 3. The others are in-

verter/backup modes. Mode 1 and Mode 2 show the current

flows and switching states during the positive half cycle of the

input voltage, and Mode 3 and Mode 4 vice versa. Since the

common leg is switched by the polarity of the input voltage, the

switch is consistently turned on during the positive cycle,

while the switch is turned on during the negative cycle. In

Mode 1, the lower switches and are turned on. Then, the

inductor current increases in a positive direction and the mag-

netic energy is stored in . Mode 1a and Mode 1b are deter-

mined by the switching state of the unipolar PWM strategy in

the inverterleg. If isturned oninMode 1a, the dc-link voltage

is applied to the load and flows toward the load. Since the

switch is turned on in Mode 1b, a zero voltage is applied to

the load, and the output current is freewheeling through and

. InMode 2, the switch isoff and ison. Next, the energy

stored in is transferred to the dc-link stage. Since the posi-

tive voltage is applied to the load in Mode 2a, the dc link is

discharged by the amount of the output current . Therefore, it

can be seen that the current charges the dc link during the

Mode 2a. In Mode 2b, zero voltage is applied to the load and the

output current is freewheeling, as in Mode 1b. Similarly, Mode

3 andMode 4 show the operation modes during the negative half

cycle of the input voltage. Obviously, Fig. 2 shows that only theamount of current flows through the common legin

each mode, where is battery charging current. The on-loss

of the common leg switches is further reduced. The other modes

are inverter modes. If the input voltage is lost, only the inverter

is operating without charging the battery. The modes also use a

unipolar PWM strategy. The operation is straightforward.

III. SYSTEM ANALYSIS

A. Active Power Filter Mode of the Charger

The charger in Fig. 2(b) acts to compensate for the reac-

tive current required by the load connected on the output side,

thereby reflecting a linear load to the input, while charging thebattery. The common leg is switched at low switching frequency

depending on the sign of the input voltage. It is assumed that the

inductor current is continuous. In the case of positive half cycle

of the input voltage, if the switches and are turned on

(Mode 1), the inductor current increases and magnetic energy

is stored in the inductor. Similarly, in the negative cycle, the

switches and are turned on (Mode 2) to increase the in-

ductor current in opposite direction. Thus, the following voltage

equation is satisfied:

(1)

where and are the source voltage and current, respectively,and is the inductance of the leakage transformer.


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Fig. 3. Operation modes of the proposed UPS.

In the positive cycle, if the switches and are turned on

(Mode 3), the energy stored in the inductor is transferred to the

dc link. In the same manner, in the negative cycle, the switches

and are turned on (Mode 4) to release the energy to the

dc-link capacitor. Then,

(2)

where is the dc-link voltage, is the turns ratio

of the transformer , and denotes the sign of . It isnoted that under unity-power-factor control.

Depending on the duty ratio of the switch and of

the switch , the average inductor voltage over one switching

period gives the following source current variation :

(3)

(4)

The duty ratios and can be considered to be in each

half cycle. Let the duty ratio be represented by(5)


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Fig. 4. Proposed power-factor controller of the active power filter mode.

which is composed of a nominal duty ratio and a perturbed

duty ratio . Then, the nominal duty ratio and the per-

turbed duty ratio can be represented as

(6)

(7)

where and are the peak source voltage and angular

frequency, respectively. The nominal duty ratio generates

voltage for a feedback linearization of the original nonlinear

system. By contrast, the perturbed duty ratio is used to

control the current required by the inverter. To force the current

of the charger to track its current command , the current

controller is utilized as follows:

(8)

where is the current control gain.

Let be the average charging current over the line period

. Then, total real power of the system is composed of the power

for charging the battery and the output average power

given by

(9)

Therefore, the average input current becomes

where is the peak value of the input voltage. Then

the reference current for the input current of the charger in

Fig. 2(a) is computed as

(10)

The current component is required for charging the

battery and the component for compensating reactive current

is . Fig. 4 shows the proposed control algorithms

of the active power filter mode. The output of the current

controller only generates the inductor voltage drop required to

maintain the sinusoidal source current. With the addition of the

nominal duty to the rectifier which is originally a nonlinear

dynamic system, the relation between the input and output

of the rectifier becomes a first-order linear dynamic system

(7) with easy controllability. Thus, the addition of the nominal

duty relaxes the burden of the current controller and im-proves the input current waveform.

Fig. 5. Peak voltage detectors. (a) Conventionalpeak voltage detector. (b) Fastpeak voltage detector.

With loss of the input power, the static bypass switch is

opened and the charger is disabled. The inverter operates in

backup mode (or inverter mode) and supplies power to the load

using the battery. The unipolar PWM switching scheme is used

for the inverter. It results in a better output voltage waveform

and in a better frequency response than the bipolar PWM

switching scheme, since the effective switching frequency

of the output voltage waveform is doubled and the ripple is

reduced. The common leg is switched at the line frequency

synchronized to the input voltage, as previously stated.

B. Fast Detection of the Input Voltage

With loss of input power, the UPS must transfer instantly to

the backup mode to minimize a transient effect on the output

voltage. To accomplish this, a fast detection technique of the

input voltage is required. In general, the conventional peak

voltage detector with diodes, capacitor, and resistor is used as

a voltage sensing circuit, as shown in Fig. 5(a). When the input

signal is decreased, the capacitor is discharged through the

resistor and when increased, the capacitor is charged directly.

Therefore, the charging speed is faster than the discharging

one. Reducing the resistance increases the discharge speed.

However, the ripple current of the detected signal is increased.

A fast detection technique of the input voltage is shown in

Fig. 5(b) which is composed of a phase shifter, two multipliers,and an adder. The detection technique utilizes the simple

trigonometric theorem as follows:

(11)

The phase shifter delays its input waveform by 90 , i.e., the co-

sine waveform of the input voltage is converted to the sine wave-

form, and each waveform is multiplied and added. The sensed

signal voltage of the input voltage then becomes

(12)

where is the detection gain of the input voltage. Thus, the

detector output can represent the magnitude of the inputvoltage.


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Fig. 6. Control block diagram of the inverter.

Consider a digitized implementation of the detection algo-

rithm of the input voltage. The 90 phase shifter of the input

voltage with the angular frequency has the transfer character-

istic of the first-order all-pass filter as follows:

(13)

Using the following bilinear transformation:

(14)

where is the sample period, the transfer function of the phase

shifter in the discrete domain is given by

(15)

Therefore, the output of the phase shifter in the discrete-

time domain can be obtained as follows:

(16)

where

(17)

Thus, the algorithm (12) for detecting the input-voltage mag-

nitude can be calculated using the input voltage and the

output signal of the 90 phase shifter.

C. Current-Limiting Technique Under the Impulsive Load

Fig. 6 shows the inverter control block diagram. The output

voltage is regulated through the proportionalintegral (PI)-type

voltage controller. The proposed current limiter consists of two

operation modes: a normal mode and an overcurrent mode. The

two modes are decided by the value of the output current. When

the absolutevalueof the output current islowerthan the lim-

ited current value , the inverter operates in normal mode.

In this mode, the duty of the inverter is determined as

(18)

where is the output of the voltage controller and is

obtained from the sine lookup table. Due to the sinusoidal refer-

ence , the inverter generates sinusoidal output voltage in

normal mode. When the impulsive load such as a capacitive rec-tifier load is attached, the inverter supplies excessive current to

Fig. 7. Control algorithm of output current limiter.

charge the load capacitor. This excessive current may damage

the switching devices in the inverter. If a large current is de-

tected, the current limiter reduces the output voltage reference.

A fast p-type controller reduces the reference value and holds

the reference value during a half cycle, then the capacitor ischarged to some degree without increasing the current. As the


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Fig. 8. Overall block diagram of proposed UPS.

capacitor is fully charged in some cycles, no further impulsive

current flows through the inverter. Then, the inverter operates in

normal mode. The output voltage is recovered to its rated value.

The detailed algorithm of the current limiter is shown in Fig. 7.

As the p-type controller is used, various impulsive loads can be

attached to the system. The system will recover its rated output

value immediately thereafter.

IV. EXPERIMENTAL RESULTS

In a microprocessor-based control system, software flexi-

bility facilitates the development and updating of the control

technique and uses control theory to obtain high performance.

Moreover, a single-chip microcontroller can implement the

controller with a lower cost and smaller size than the gen-

eral-purpose microprocessor with accompanying externalcircuits such as an A/D converter, D/A converter, and PWM

generator. The overall control block diagram of the single-phase

online UPS is implemented using a single-chip microcontroller

Intel 80196 MH, as shown in Fig. 8. The switching times of

each device are implemented in software and PWM pulses are

generated through the pulse generator of the microcontroller.

Voltage or current signals are measured by using the 10-bit

analog-to-digital converter in the microcontroller. The imple-

mentation of the voltage or current controllers and PWM pulse

generation is performed every sample period s.

The overall system is divided into two parts: the controller

and power circuit. The controller part includes the micro-

controller running the control algorithms and driver circuits.The power circuit is implemented as in Fig. 8. The rating of

TABLE IISYSTEM PARAMETERS OF THE PROPOSED UPS USED FOR EXPERIMENT

the proposed UPS is designed for up to 3 kVA with 60-Hz

220-V nominal input/output voltages. To handle this power

rating, 50-A/600-V insulated gate bipolar transistors (IGBTs)

are selected as power semiconductor switches. These power

semiconductor switches are operated with a carrier frequencyof 15 kHz and a dead time of 2 s. The system parameters

used for the experiment are given in Table II. The experimental

waveforms of input and output signals of the input voltage

detector for 20% line voltage disturbance are shown in Fig. 9.

This voltage detector gives a fast response to the voltage sag

due to line voltage disturbance.

As the inverter operates independently of the line status, the

UPS transfers seamless power to the load from the battery in line

fault state. Upon the return of the input power, the UPS trans-

fers back to the active filter mode while charging the battery. In

this transition, a fast transfer time is not required. The output

voltage is slowly adjusted to be in phase with the line voltage

using a phase-locked loop. Once synchronism is achieved andthe magnitude of the input voltage is in the working range, the


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Fig. 9. Input and output signals of the input voltage detector for 20% linevoltage disturbance.

Fig. 10. Input current and voltage waveforms of proposed UPS.

UPS starts to operate in the active filter mode while the mag-

netic contactor (MC) connects the line to the charger. Fig. 10

shows the input voltage and the input current of the UPS under

a linear load. In this case, the input current of the UPS is the

sum of the charging current for the battery and the load current.

The line current is exactly in phase with the input line voltage

and nearly sinusoidal. Thus, the input power factor approaches

unity. The result measured by the power meter shows that thetotal efficiency of the UPS is 87% and the power factor is 99%.

Fig. 11 shows the output voltage and the output current, respec-

tively, when the UPS is impacted by a capacitive rectifier load

( F). From the experimental waveforms, it is ob-

served that if a current-limiting technique is employed, the UPS

fulfills the impulsive loading quickly. In addition, it can be seen

that its rated output voltage is recovered within 2 cycles of the

fundamental line period.

V. CONCLUDING REMARKS

We have proposed an improved single-phase online UPS

which uses a three-leg-type converter. The proposed UPScombines low cost with excellent performance. The inverter

Fig. 11. Compensated output voltage and current under impulsive load.

generates output independently on line voltage from the battery,

and the UPS transfers uninterrupted power to the load. Theexperimental results validate that the proposed current-limiting

algorithm has quick voltage recovery characteristics under an

impulsive load. The fast input voltage detector also improves

the dynamic response of the output voltage. As the detector

gives a fast output response to the voltage sag, it is adequate

for the UPS demanding fast input disturbance detection. The

experimental results also show that the proposed UPS gives

good dynamic and steady-state performance.

REFERENCES

[1] IEEE Recommended Practice for Emergency and Standby Power Sys-

tems for Industrial and Commercial Applications, IEEE Standard 446-1987, 1987.[2] N. Hirao, T. Satonaga, T. Uematsu, T. Kohama, T. Ninomiya, and M.

Shoyama, Analytical considerations on power loss in a three-arm-typeuninterruptible power supply, in Proc. IEEE PESC98, 1998, pp.18861891.

[3] S. J. Chiang, T. S. Lee, and J. M. Chang, Design and implementationof a single phase three-arms rectifier inverter, Proc. IEEElect. Power

Appl., vol. 147, no. 5, pp. 379384, Sep. 2000.[4] T. Uematsu, T. Ikeda, N. Hirao, S. Totsuka, T. Ninomiya, and H.

Kawamoto, A study of the high performance single phase UPS, inProc. IEEE PESC98, 1998, pp. 18721878.

[5] I. Ando, I. Takahashi, Y. Tanaka, and M. Ikchara, Development of ahigh efficiency UPS having active filter ability composed of a three armsbridge, in Proc. IEEE IECON97, 1997, pp. 804809.

[6] I. Youichi, I. Satoru, T. Isao, and H. Hitoshi, New power conver-sion technique to obtain high performance and high efficiency for

single-phase UPS, in Conf. Rec. IEEE-IAS Annu. Meeting, 2001, pp.23832388.

[7] H. Pinheiro, R. Blume, and P. Jain, Space vector modulation for singlephase on-line three-leg UPS, in Proc. IEEE Int. Conf. Industrial Elec-tronics, Control and Instrumentation, 2000, pp. 679689.

[8] B. Francois, P. Delarue, A. Bouscayrol, and J. Niiranen, Five-legac-ac power converter: Structure, modeling, and control, in Conf. Rec.

IEEE-IAS Annu. Meeting, Rome, Italy, 2000.[9] B.-R. Lin and D.-J. Chen, Implementation of a single-phase three-leg

AC/AC converter with neutral-point diode-clamped scheme, Proc.IEEElect. Power Appl., vol. 149, no. 6, pp. 423432, Nov. 2002.

[10] Y. Yang, J. Liu, and D. Zhou, Pulse by pulse current limiting techniquefor SPWM inverters, in Proc. IEEE PEDS99, 1999, pp. 10211026.

[11] F. Kamran and T. G. Habetler, A novel on-line UPS with universalfiltering capabilties, IEEE Trans. Power Electron., vol. 13, no. 3, pp.410418, May 1998.

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Jin-Ha Choi was born in Seoul, Korea, in 1972. Hereceived the B.S., M.S., and Ph.D. degrees in elec-tronic and electrical engineering from Pohang Uni-versity of Science and Technology, Pohang, Korea,in 1996, 1998, and 2003, respectively.

Since 2004,he has beenwiththe DALaboratory ofLG Electronics Company Ltd., Seoul, Korea, wherehe is currently a Researcher. His research interests

are UPSs, motor drives, power converter/inverter sys-tems, and microprocessor applications.

Jung-Min Kwon was born in Ulsan, Korea, in 1981.He received the B.S. degree in electrical and elec-tronic engineering from Yonsei University, Seoul,Korea, in 2004. He is currently working toward the

M.S. degree in electronic and electrical engineeringat Pohang University of Science and Technology,

Pohang, Korea.His research interests are photovoltaic systems and

power supplies.

Jee-Hoon Jung was born in Suwon, Korea, in 1977.He received the B.S. and M.S. degrees in 2000 and2002, respectively, from the Department of Elec-tronic and Electrical Engineering, Pohang Universityof Science and Technology, Pohang, Korea, wherehe is currently working toward the Ph.D. degree inelectronic and electrical engineering.

His research interests include motor drives and di-

agnosis, controland signalprocessing with micropro-cessors, and switch-mode power supplies.

Bong-Hwan Kwon (M91) was born in Pohang,Korea, in 1958. He received the B.S. degree fromKyungbuk National University, Taegu, Korea, in1982, and the M.S. and Ph.D. degrees in electricalengineering from Korea Advanced Institute ofScience and Technology, Seoul, Korea, in 1984 and1987, respectively.

Since 1987, he has been with the Department ofElectronic and Electrical Engineering, Pohang Uni-versity of Science and Technology, Pohang, Korea,where he is currently a Professor. His research in-

terests are motor drives, high-frequency converters, and switch-mode power

supplies.

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