# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 138

126
intersects the reference voltage u
ref
(t), a control signal for the IGBT is generated.
Depending on the reference voltage being higher or lower than the triangular voltage at
each cross point a pulse is generated turning on or off the IGBTs T
1
or T
2
respectively.
This concept is detailed i
The sub-harmonicmethod considered heremay be
realisedwith synchronous or non-synchronous pulsing. Synchronous in this casemeans
that the switching frequency f
SW
is an integer (whole number) multiple of the output
frequency f
out
and is synchronised in at least one zero crossing of f
out
with the frequency
of u
tri
(t).
The ratio of the switching frequency f
SW
to the fundamental of the output frequency f
out
is
called themodulation factorm
f
.
1, out
SW
f
f
f
m
=
Eq. 3.4
The ratio of the amplitudes of the triangular voltage
tri
to the reference voltage
ref
is
called themodulation factorm.
tri
ref
m
=
Eq. 3.5
The amplitude of theoutput voltage then results from:
2
Um Uˆ
DC
out
⋅ =
Eq. 3.6
Generally the value rangeofm can bedivided into three sections:
1m
correlates to the linear region of the PWM, i.e. the amplitude of the
output voltage
out
rises linearly with the modulation factor m and is
independent of m
f
.
π
< <
4m1
correlates to over-modulation of the PW
. The amplitude of the
output voltage
out
does not follow the modulation factor m linearly in this
sector, as detailed in
In this region a dependency results tom
f
; even
if a large value is chose
for this.
2
Synchronous PWM is used as a guideline with a value of m
f
21. It has to bementioned at this stage that m
f
should preferably be of an odd number value in this case in order to reduce the influence of output harmonics.
With values abovem
f
> 21 non-synchronous PWM can be used as the influence of output harmonics is reduced
from this value onwards.
3
Tobe precise, this only bears influenceon the fundamental frequencyof the output voltage. All harmonics of the
output voltageU
out
aredisregardedhere.
4
This mode can often be found in inverters for motor drives, while the linear mode is used for example in
uninterruptable power supplies (UPS). The reason for this is the appearance of the harmonic voltages which are
superimposed on the output voltage and are undesirable particularly in power supply systems. Therefore when
over-modulating, independently of what is chosen form
f
, often synchronous PWM is used.
5
Thederivation of the dependencywill not be detailedat this point, but is described in
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