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Using of IGBT inUPS

THE MERLIN GERIN KNOW- HOW

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1.abstract

MGE UPS SYSTEMS MGE0123UKI - 06/98 2

Author: Jean-Nol FIORINA

Contents

1.abstract .............................................................................................................................3

2.appropriate configurations for each range ....................................................4

n introduction......................................................................................................................... 4

nthe main functions used in the UPS............................................................................... 4

nthe main configurations .................................................................................................... 5

3. the semiconductors used in the various functions ...................................6

n the thyristor......................................................................................................................... 6

n the GTO (Gate Turn Off Thyristor) ................................................................................. 6

nthe MOS transistor (Metal Oxyde Semiconductor)...................................................... 6

n the bipolar transistor......................................................................................................... 6

nl' IGBT (Insulated Gate Bipolar Transistor).................................................................... 6

4. bipolars / IGBT .............................................................................................................8

n control.................................................................................................................................. 8

n the switching characteristics ............................................................................................8

n reliability.............................................................................................................................. 9

5. a few details on the main UPS functions ......................................................10

n the rectifier........................................................................................................................ 10

nspecific element of a high performing UPS: the PWM inverter................................ 10

n regulation.......................................................................................................................... 11

n the step up converter...................................................................................................... 11

6. impact of inverter chopping frequency on UPS performance ............12

n the losses.......................................................................................................................... 12

n

importance of efficiency.................................................................................................. 13nbehaviour of UPS on non-linear loads ......................................................................... 13

n acoustic noise.................................................................................................................. 14

nexample of a medium power configuration (Comet).................................................. 15

nexample of a high power configuration (Galaxy) ........................................................ 15

7. development and technological watch at MGE UPS SYSTEMS .........16

8.conclusions ...................................................................................................................... 17

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1.abstract


The UPS market is highly competitive. As a result, UPS performance and

reliability requirements are steadily increasing and can only be satisfied

if the components used keep pace.The present article provides a comparative analysis of the different

semiconductors available for UPS components and their respective

applications. Easy control, excellent switching characteristics and high

reliability today make IGBTs the best choice for medium and high-power

UPS. They significantly improve UPS performance, particularly in terms ofefficiency, acoustic noise, size and weight.

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2. appropriate configuration for each range


MGE UPS SYSTEMS employs a staff of 2000 and boasts 4 plants worldwide.

The current range extends from 150 VA to 800 kVA and covers a fieldof applications mainly geared towards the supply of computer sites, but also

including telecommunications, industrial processes, the medical and military sectors.

The ever-increasing demand for performance and reliability calls forthe use of components in turn ever-more reliable and efficient. From this viewpoint,

the IGBT is today the ideal choice in three-phase medium and high power UPS (220

to 460V). The purpose of this paper is to describe the improvements thatthe IGBT make to the UPS.

The UPS ranges

range Pulsarlow powers

Cometmedium power

Galaxyhigh power

powers 0.4 kVA4 kVA 5 kVA 30 kVA 30 kVA several MVA

application microcomputersnetworks up to 5substations

networks > 5substationsminicomputers

telecommunications...

large systems,industrial processes,hospitals,

telecommunications...type off-line (1) / on line (2) on-line on-lineinstallation offices computer room technical roomparametersto optimise

simplicity, cost, acousticnoise

reliability, acousticnoise, overalldimensions, weight

reliability, efficiency,VTHD on non-linearload

(1) an off-line UPS replaces the faulty mains after a switching time.

(2) an on-line UPS continuously supplies the application.

This section provides a brief description of the various functions used in the UPS.

Some of these functions will be described in greater detail in the paragraphs below.

nthe rectifier:Supplies a dc voltage from the mains which will be used to supply

the battery and the inverter;nthe charger:Keeps the battery charged;

nthe inverter:Supplies an ac voltage regulated in voltage and frequency from

the dc voltage of the rectifier/charger. It chops the dc voltage with a Pulse WidthModulation (PWM) mode ; then the signal obtained is filtered to supply the output

sinusodal voltage.

nthe transformer:The inverter can only supply a peak to peak voltage 20 % less

than its supply voltage. The transformer ensures the necessary voltage is suppliedat the output;

nthe step up converter: Used to generate a dc voltage higher than that supplied

by the rectifier or the battery, thus making it possible to produce an output voltageequal to or greater than the input voltage without using a transformer. This option is

advantageous if weight and overall dimensions are priorities;

n the filter:designed to eliminate higher rank harmonics, which cannot be achievedby the inverter regulation without using high chopping frequencies to

the detriment of UPS efficiency;

nthe static switch: If the application requirements exceed the possibilities of

the UPS, the static switch automatically switches the application, without breaking,to the input mains. This is not possible if the UPS is acting as a frequency converter.

introduction

the main functions used

in the UPS

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2. appropriate configuration for each range


n classical solution up to the highest powers

Fig. 01

n configurations allowing a reduction in weight and overall dimensions

without transformer

Fig. 02

A step up/converter (described below) is used which compensates losses in thesemiconductors and ensures output voltages of 240V with an input voltage of 220V.

with H.F. transformer

Fig. 03

This configuration uses a high frequency (H.F) power transformer to reduce

transformer weight.

the main configurations

battery

battery

battery

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3. the semiconductors used in the various functions (cont.)


Some components used for power switching.

Produced for voltages up to 6000V, currents of several thousand amps and a directvoltage drop of approximately 1.5 V, its ignition requires only a small current impulse

whereas its blocking requires cancellation of the entire anode current by branching

off in an auxiliary circuit. Although the thyristor is bulky and costly, it is neverthelessboth reliable and economic to purchase.

This device can be compared to a thyristor equipped with a blocking control whose

gain is however very low. Although its power range is equivalent to that ofthe thyristor, this component is relatively expensive and is mainly used to control

traction engines.

Its main advantages are its voltage control and its switching times of under 100 ns.However it has the drawback of a relatively high direct voltage drop compared with

its competitors. Its limit is around 50A and 500V.

This device, doubtless the oldest, did not really become powerful until around 1985with the triple Darlington modules (3 cascading transistors) of 300 A, 1000 V

and a gain of around 100. Despite this gain, the current control at high powers

continues to be penalising. At high powers, switching times are around 1.5 sand the direct voltage drop is 1.5 V.

From the users standpoint, the IGBT can be roughly likened to a bipolar transistor

monitored by a MOS transistor with the added advantage of a voltage controland very short switching times (300 ns) for power levels similar to those

of the bipolar. Its main disadvantage is its direct voltage drop of around 3 V for 1200

V components

The graph in figure 4 situates each of the components described in a working

power/frequency context.

1990 2000

Fig. 04

the thyristor

the GTO (Gate Turn OffThyristor)

the MOS transistor (MetalOxyde Semiconductor)

the bipolar transistor

the IGBT (Insulated Gate

Bipolar Transistor)

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4. bipolars / IGBT (cont.)


Below two transistors of 300 A, 1000 V for the bipolar and 1200 V for the IGBT are

compared with respect to the following points:ncontrol;

nswitching characteristics;

nreliability.

The IGBT has a voltage control with the result that the power required for its controlis considerably reduced as can be seen in figure 7.

Fig. 07

The control power of a bipolar is more or less constant whatever the frequency,

whereas the control power of an IGBT increases with frequency, since the inputimpedance is mainly capacitive (approximately 20nF) with a negligible leakagecurrent (500 nA max.).

A rapid examination of the curves below shows that the IGBTs superiority

in switching speed is affected by its more modest performance if we look at

the VCEsat.

The global characteristics of the IGBT remain more or less constant according to

collector current whereas those of the bipolar transistor drop from 75 to 100 % ofits rated collector current. Moreover, its long storage time (up to 15 s) results

in considerable limitation of its working frequency (Fig. 8).

Fig. 08

the switching

characteristics

control

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4. bipolars / IGBT (cont.)


n the safety aera:Another advantage of the IGBT is apparent in the safety area.

The IGBT accepts a current twice its rated value without any significant variationin its voltage capacity.

Fig. 09

n the control circuit:If we look at figure 7 which shows the control power ratio

between the IGBT and the bipolar transistor, the simplification resulting from use

of IGBT transistors is evident.

Concretely speaking, the table below compares the parameters of a control board

for IGBT and of a control board for bipolar transistors, taking the IGBT controlas a reference.

IGBT bipolar

number of electronic components 1 3.6number of mechanical parts 1 3.7connections, clamping points 1 2.6wiring no yesdissipated power on the board 1 30

reliability

ipolar

1000 V

I/Ic: ratio of test current over rated collector

current of transistor

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5. a few details on the main UPS functions (cont.)


The progressive energising of the charger prevents the strong inrush currents

caused by the capacitive loads downstream from the charger. The thyristorsfrom now on act as a diode, in the case of Comet, or they regulate the DC voltage in

the case of Galaxy.

Fig. 10

A

Fig. 11

This device converts a dc voltage into an ac voltage whose frequency and distortionratio are kept within very strict limits, compatible with supply of modern sourceswhose current contains very many harmonic components. The diagram in figure 11

shows an H inverter. In the AB and CD arms , each transistor is energised

in turn in dc for 1/2 period 50 or 60 Hz then in H.F. so as to evenly distributeswitching losses between the two transistors.

With a battery voltage Vb, it is theoretically possible to produce an output voltageof 2 Vb peak to peak (minus the transistors direct voltage drop). This value will be

limited to 1.6 Vb in order to retain an operating margin for regulation.

the rectifier

specific element of a highperforming UPS: the PWM

inverter

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5. a few details on the main UPS functions (cont.)


n free frequency regulation: Figure 12 gives a simplified view of the regulation

principle. An envelope curve reference voltage , calibrated in frequencyand amplitude, is applied to a differential amplifier. The other input receives

the output voltage of the UPS. As is shown in the envelope curves

of the diagram below, when the output voltage equals the minimum referencevoltage, the power trans istor becomes conductive, thus causing the output voltage to

rise towards the maximum reference voltage.

Conversely, if the output voltage equals the maximum reference voltage, the powertransistor is blocked.

It can be observed that the chopping frequency, just like the cyclic ratio, are not

really constant but free. They adapt to the difference (battery voltage - outputvoltage). At high currents, frequency decreases whereas the cyclic ratio increases.

Fig. 12

n fixed frequency regulation: This is used for operation above 16 kHz with nodescent into the audible spectrum. The amplifier simply modulates the transistorconduction time.

Fig. 13

During the transistor conduction phase, the choke (not saturable) stores the 1/2 L Is2

energy then restored to the inverter via the diode.

This device which has the advantage of making the transformer not alwaysnecessary, however results in considerable losses (choke, transistor, diode).

The use of the IGBT in this configuration ensures a substantial reduction in weight

without affecting efficiency.

regulation

the step up converter+

Vs

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6. impact of inverter chopping frequency on UPSperformance (cont.)


Losses in a single-phase inverter (H bridge, with 4 transistors) for a UPS of 100 kVA

on inductive load. The values given in the tables below are approximate and aimsolely at giving a general idea.

n dynamic losses

Ton Toff

frequency

0.10 0.30 0.50 0.75 1.00 1.50 2.00 2.50 3.00

0.5 19 58 97 146 194 291 388 485 582

1 39 116 194 291 388 582 777 971 1165

2 78 233 388 582 777 1165 1553 1941 2330

4 155 466 777 1165 1553 2330 3106 3883 4660

8 311 932 1553 2330 3106 4660 6213 7766 9319

16 621 1864 3106 4660 6213 9319 12426 15532 18638

n static losses (VCEsat)

VCEsat 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75 4

P (W) 1000

1250

1500

1750

2000

2250

2500

2750

3000

3250

3500

3750

4000

Example: the following table shows 2 inverters operating with different devices. At 2

kHz, the bipolar transistor provides better efficiency, but the IGBT is superior as from

4 kHz.

chopping frequency > 2 kHz 4 kHz 16 kHz

Transistors VCEsa

t

Ton/offstatics losses total losses total losses total

(V) ( s) losses dyn. dyn. dyn.

IGBT 3 0,3 3000 233 3233 466 3466 1864 4864

bipolar 1.5 1.5 1500 1165 2665 2330 3830 9319 10819

efficiency gain: IGBT / bipolar -0.6 % +0.4 % +6 %

n in conclusion

Switching losses increase with inverter chopping frequency.

Switching losses of an IGBT are 5 times less than those of a bipolar transistor,

whereas its static losses are 2 times greater.

If only efficiency is considered, bipolars may be chosen below 2-3 kHz and IGBT

above this value. However other considerations, in particular componentsand control power may invalidate this choice.

the losses

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The following table gives the annual cost over 5 years in US $ induced by a loss of 1

% in efficiency according to various energy tariffs. If we compare this table tothe one above, we understand that the choice of switching devices for high power

UPS should not be limited merely to technical considerations.

cost of kWH in cents >UPS power in kVA

2 4 6 8 10

1 10 20 30 40

10 70 140 210 280 350

100 700 1400 2010 2800 3500

500 3500 7000 10500 14000 17500

For high power installations, these costs must be marked up by roughly 30 %

to allow for discharge of heat losses.

Improvement of the power/weight ratio in electronic equipment has led to

an extensive use of chopping power supplies. Their input stage made up of

a rectifier and a RC load, forms a non-linear load generating harmonic currents(Fig. 14). The three-phase rectifiers not shown here, eliminate harmonics 3 but

generate a high harmonic distortion of 5 and 7.

Fig. 14

importance of efficiency

behaviour of UPS on non-

linear loads

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n the efficiency/harmonic distortion compromise

The dilemma confronting all UPS manufacturers is frequently the choiceof components and the chopping frequency of the inverter to ensure compatibil ity

with these non-linear loads and to obtain a minimum VTHD (Voltage Total Harmonic

Distortion) at the UPS output.

Increased chopping frequency out enables an increase in regulation gain and thus a

reduction in output impedance at high frequencies. The size and weight of passive

components can thus be minimised.

However, as seen above, this increase in frequency increases switching losses

and reduces UPS efficiency.

As a rule, efficiency is an important parameter for medium and high power

UPS (see cost table over 5 years with 1 % efficiency in paragraph 6.2.).

Consequently, the inverter will work between 2 and 3 kHz with bipolartransistors and above 3 kHz with IGBT transistors.

This is generated by the electromagnetic forces created in the magnetic circuits

(transformers, chokes) or by the electrodynamic forces between conductors.

An inverter operating at 16 kHz will not produce an audible noise at this frequencyand only the 50 or 60 Hz component will remain in the magnetic circuits. Removal

of the transformer will thus be an important factor in noise limitation. This feature will

make it possible for personnel and the UPS to work side by side and meansthe UPS no longer needs to be installed in a technical room (Comet range).

However an inverter operating in a band below 16 kHz at free or pseudo-

random frequency yields a line-free (and thus resonance free) noise spectrumand is far less noisy than the same device with fixed frequency in this band

(Galaxy).

acoustic noise

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Fig. 16

As the main drawback of the IGBT is their relatively high VCE

sat, it is advisableto limit their number in the power path. In Fig. 16, the inverter operates at 16 kHz,uses 2 IGBT and a double voltage source whose mid-point brought to the neutral

yields the same peak to peak value as in the H bridge.

In single-phase, the step up receives a sinusoidal current setpoint, in phase with

the input voltage, making it possible to obtain an input current distortion of 3 %

and a power factor of 0.99.

This set-up optimises weight, overall dimensions, reliability and acoustic noise.

Fig. 17

In this set-up, the step up function is assigned to a transformer which, for highpowers, has an efficiency approaching 98 %.

This set-up optimises reliability, efficiency, behaviour on non-linear loads

and acoustic noise (free frequency regulation).

example of a medium power

configuration (Comet)

example of a high power

configuration (Galaxy)

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7. development and technological watch at MGE UPSSYSTEMS


UPS prices are constantly dropping. A UPS project must incorporate new

technologies which provide enhanced performances and a reduction in cost price.

One of the characteristic features of the technological watch is a close relationship

with component suppliers, a strategy which has proved decisive in development

of high performing, economic products. The diagram below shows the integrationphases of a new component in the MGE UPS SYSTEMS standards where design

and manufacture of a product are governed by the ISO 9001 standard.

Fig. 18

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8. conclusions

MGE UPS SYSTEMS MGE0247UKI 08/99 17

The switching speed, simple control and overload withstand of the IGBT currently

make it a component of considerable interest.

In high power UPS where the inverter operates between 2 and 4 kHz, the main

advantage of the IGBT is simplification of transistor control (increased reliability).

Its efficiency is equivalent to that of bipolar transistors.

In medium power UPS often installed in computer rooms, the acoustic noise criterion

makes it necessary to remove the 50 or 60 Hz transformer and to add

an inverter operating at a frequency of 16 kHz, thus making the IGBT absolutelyindispensable both due to the reduction in number of components required

to control it and due to the gain in weight and overall dimensions.

Creation of new products calls for a careful, rigorous selection of new components.

As the IGBT is still in the development phase, announcements of new components

are frequent. Existing studies should not be questioned as official approval

of a power semiconductor is long and costly.

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