Using of IGBT in
UPS
THE MERLIN GERIN KNOW- HOW
1.abstract
Author: Jean-Noël FIORINA
Contents
1. abstract.............................................................................................................................3
2. appropriate configurations for each range ....................................................4
n introduction.........................................................................................................................4
n the main functions used in the UPS...............................................................................4
n the main configurations ....................................................................................................5
3. the semiconductors used in the various functions ...................................6
n the thyristor.........................................................................................................................6
n the GTO (Gate Turn Off Thyristor) .................................................................................6
n the MOS transistor (Metal Oxyde Semiconductor)......................................................6
n the bipolar transistor.........................................................................................................6
n l'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
n specific 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..................................................................................................13
n behaviour of UPS on non-linear loads .........................................................................13
n acoustic noise..................................................................................................................14
n example of a medium power configuration (Comet)..................................................15
n example 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 of
efficiency, acoustic noise, size and weight.
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2. appropriate configuration for each range
introduction
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 field
of 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 for
the 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 that
the IGBT make to the UPS.
The UPS ranges
range
powers
application
type
installation
parameters
to optimise
Pulsar
low powers
0.4 kVA à 4 kVA
microcomputers
networks up to 5
substations
Comet
medium power
5 kVA à 30 kVA
networks > 5
substations
minicomputers
telecommunications...
off-line (1) / on line (2)
on-line
offices
computer room
simplicity, cost, acoustic reliability, acoustic
noise
noise, overall
dimensions, weight
Galaxy
high power
30 kVAà several MVA
large systems,
industrial processes,
hospitals,
telecommunications...
on-line
technical room
reliability, efficiency,
VTHD on non-linear
load
(1) an off-line UPS replaces the faulty mains after a switching time.
(2) an on-line UPS continuously supplies the application.
the main functions used
in the UPS
MGE UPS SYSTEMS
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.
n the rectifier: Supplies a dc voltage from the mains which will be used to supply
the battery and the inverter;
n the charger: Keeps the battery charged;
n the 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 Width
Modulation” (PWM) mode ; then the signal obtained is filtered to supply the output
sinusoïdal voltage.
n the transformer: The inverter can only supply a peak to peak voltage 20 % less
than its supply voltage. The transformer ensures the necessary voltage is supplied
at the output;
n the 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 voltage
equal 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 achieved
by the inverter regulation without using high chopping frequencies to
the detriment of UPS efficiency;
n the 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.
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2. appropriate configuration for each range
the main configurations
n classical solution up to the highest powers
battery
Fig. 01
n configurations allowing a reduction in weight and overall dimensions
without transformer
battery
Fig. 02
A step up/converter (described below) is used which compensates losses in the
semiconductors 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.
battery
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3. the semiconductors used in the various functions
(cont.)
Some components used for power switching.
the thyristor
Produced for voltages up to 6000V, currents of several thousand amps and a direct
voltage 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 nevertheless
both reliable and economic to purchase.
the GTO (Gate Turn Off
Thyristor)
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 of
the thyristor, this component is relatively expensive and is mainly used to control
traction engines.
the MOS transistor (Metal
Oxyde Semiconductor)
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.
the bipolar transistor
This device, doubtless the oldest, did not really become powerful until around 1985
with 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 µs
and the direct voltage drop is 1.5 V.
the IGBT (Insulated Gate
Bipolar Transistor)
From the user’s standpoint, the IGBT can be roughly likened to a bipolar transistor
monitored by a MOS transistor with the added advantage of a voltage control
and 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
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3. the semiconductors used in the various functions
(cont.)
Figure 5 shows the main switching characteristics of the semiconductors used
in the UPS: the saturation voltage at the on-state VCEsat and the switching times Tr
and Tf. The ideal device naturally has zero VCEsat, Tr, Tf.
Fig. 05
Figure 6 shows, for the two main parameters, the relative position of the various
devices currently used for power switching.
Despite their low saturation voltage, thyristors are being increasingly less used
due to the problem of moving them from the ON to the OFF state.
The remainder of this paper thus takes the form of a comparison between bipolar
transistors and IGBT.
Fig. 06
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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:
n control;
n switching characteristics;
n reliability.
control
The IGBT has a voltage control with the result that the power required for its control
is 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 input
impedance is mainly capacitive (approximately 20nF) with a negligible leakage
current (500 nA max.).
the switching
characteristics
A rapid examination of the curves below shows that the IGBT’’s 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 % of
its rated collector current. Moreover, its long storage time (up to 15 µs) results
in considerable limitation of its working frequency (Fig. 8).
Fig. 08
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4. bipolars / IGBT (cont.)
reliability
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 variation
in its voltage capacity.
bipolar
1000 V
I/Ic: ratio of test current over rated collector
current of transistor
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 control
as a reference.
number of electronic components
number of mechanical parts
connections, clamping points
wiring
dissipated power on the board
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IGBT
1
1
1
no
1
bipolar
3.6
3.7
2.6
yes
30
9
5. a few details on the main UPS functions
the rectifier
(cont.)
The progressive energising of the charger prevents the strong inrush currents
caused by the capacitive loads downstream from the charger. The thyristors
from now on act as a diode, in the case of Comet, or they regulate the DC voltage in
the case of Galaxy.
Fig. 10
specific element of a high
performing UPS: the PWM
inverter
A
Fig. 11
This device converts a dc voltage into an ac voltage whose frequency and distortion
ratio are kept within very strict limits, compatible with supply of modern sources
whose 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 distribute
switching losses between the two transistors.
With a battery voltage Vb, it is theoretically possible to produce an output voltage
of 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.
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5. a few details on the main UPS functions
regulation
(cont.)
n free frequency regulation: Figure 12 gives a simplified view of the regulation
principle. An « envelope curve » « reference voltage », calibrated in frequency
and 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 reference
voltage, 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 power
transistor 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 - output
voltage). 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 no
descent into the audible spectrum. The amplifier simply modulates the transistor
conduction time.
the step up converter
+
Vs
Fig. 13
During the transistor conduction phase, the choke (not saturable) stores the 1/2 L Is 2
energy then restored to the inverter via the diode.
This device which has the advantage of making the transformer not always
necessary, 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.
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6. impact of inverter chopping frequency on UPS
performance (cont.)
the losses
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 aim
solely at giving a general idea.
n dynamic losses
Ton Toff
——————
frequency
0.5
0.10
0.30
0.50
0.75
1.00
1.50
2.00
2.50
3.00
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)
100
0
125
0
150
0
175
0
200
0
225
0
250
0
275
0
300
0
325
0
350
0
375
0
400
0
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
Transistors
VCEsa Ton/off statics
t
(V)
( s)
losses
losses
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: IGB T / bipolar
total
dyn.
losses
16 kHz
total
dyn.
-0.6 %
losses
total
dyn.
+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 components
and control power may invalidate this choice.
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6. impact of inverter chopping frequency on UPS
performance (cont.)
importance of efficiency
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 to
the 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
1
2
4
6
8
10
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.
behaviour of UPS on nonlinear loads
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
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6. impact of inverter chopping frequency on UPS
performance (cont.)
n the efficiency/harmonic distortion compromise
The dilemma confronting all UPS manufacturers is frequently the choice
of components and the chopping frequency of the inverter to ensure compatibility
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 bipolar
transistors and above 3 kHz with IGBT transistors.
acoustic noise
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 frequency
and 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 means
the 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 pseudorandom frequency yields a line-free (and thus resonance free) noise spectrum
and is far less noisy than the same device with fixed frequency in this band
(Galaxy).
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6. impact of inverter chopping frequency on UPS
performance (cont.)
example of a medium power
configuration (Comet)
Fig. 16
As the main drawback of the IGBT is their relatively high VCEsat, it is advisable
to 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.
example of a high power
configuration (Galaxy)
Fig. 17
In this set-up, the step up function is assigned to a transformer which, for high
powers, has an efficiency approaching 98 %.
This set-up optimises reliability, efficiency, behaviour on non-linear loads
and acoustic noise (free frequency regulation).
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7. development and technological watch at MGE UPS
SYSTEMS
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 integration
phases 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
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 absolutely
indispensable 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.
MGE UPS SYSTEMS
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MGE UPS SYSTEMS
140, avenue Jean Kuntzmann
Zirst Montbonnot Saint Martin
38334 SAINT ISMIER Cedex
France
Tel : 33 (0) 4 76 18 30 00
www.mgeups.com
MGE0123UKI
As standards, specifications and designs change from time to
time, please ask confirmation of the information given in this
publication.
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Published by: MGE UPS SYSTEMS -06/98
Designed by: AMEG