ETC AN5505

AN5505 Application Note
AN5505
Parallel Operation of Dynex IGBT Modules
Application Note
Replaces October 2001, version AN5505-1.2
AN5505-1.3 July 2002
INTRODUCTION
IGBT modules can be connected in parallel to create a switch
with a higher current rating. However, successful paralleling of
IGBT modules requires some care. Depending on the application
the system designer has to consider a number of issues to
ensure that the system is reliable. These issues include the
module characteristics, gate drive circuitry and circuit layout.
The first step is to determine the optimum number of modules to
be connected in parallel to obtain the required current rating for
the system. Any solution must ensure that the safe operating
area of the individual modules is not exceeded. Due to variations
in module characteristics and circuit layout it cannot be assumed
that the parallel connection of N modules each with a current
rating of R amps will have a combined current rating of NR amps.
For this reason it is necessary to derate the modules. This
application note explains the theory behind derating with some
examples and charts. The derating process can be applied to the
IC
Q1
When two or more IGBT modules are connected in parallel a
current imbalance occurs due to the difference in the dynamic
and static characteristics of the individual IGBT modules. In a
steady state condition it is principally the difference in output
characteristics which causes the current to divide unequally
between the modules. This is illustrated in figure 1, which shows
that the device having the lower VCE(sat) (Q1) carries the largest
portion of the total current.
Note that in the static situation the total circuit inductance
including the load also influences the dc current sharing but the
following analysis assumes that differences in each arm are
negligible.
WHAT IS THE STATIC (DC CURRENT)
DERATING FACTOR FOR PARALLEL
CONNECTED IGBT MODULES?
IC1
Q1
STATIC SHARING OF IGBT MODULES
CONNECTED IN PARALLEL
Q2
Tj = 25˚C
IC1
entire range of Dynex IGBT modules. This note covers the static
(conduction) and dynamic (switching) behaviour of parallel IGBT
modules taking into consideration device characteristics only.
If two modules of the same type but with different VCE(sat)
characteristics are connected in parallel the combined current
rating is not twice the nominal current rating of the module (this
assumes the current in neither module exceeds the nominal
current rating). The reduction in effective current capability is
known as the current derating factor. This is defined as:
IC2
Q2
δ = 1−
IC2
IT
n p x IM
[1]
Where,
δ = the derating factor,
∆V1
∆V2
VCE(Sat)
V01 V02
Fig. 1 IGBT module output characteristics
IT = total current sustainable by the parallel combination of
modules,
IM = maximum allowable current for a single module operating
alone (dc current rating),
np = number of modules in parallel.
1/5
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AN5505 Application Note
Example 1:
Suppose we have two IGBT modules with a dc current rating of
800A, and that when operated in parallel one of the devices
conducts 800A and the other 640A. We can calculate the
derating factor from [1] thus:
IT = 800 + 640 = 1440A
Imin = minimum current for a single module operating in a parallel
connection.
We can express [4] as follows:
Imin
= (1 − m )
IM
[5]
IM = 800A
From [1], [3] and [5] we have
np = 2
Hence,
δ = 1 – 1440 / (2 x 800)
δ = 1−
= (1 – 0.9) x 100%
(n p − 1)(1 − m ) + 1
np
[6]
= 10%
Rearranging equation [1] we obtain:
I T = (1 − δ )n p I M
Example 2:
What is the derating factor for 4 devices in parallel with a missharing factor of 20%?
[2]
Using equation [6], we have
δ % = 1 – ((4 – 1)(1 – 0.2) +1)/4 = 0.15 x 100% = 15%.
Knowing the derating factor for a number of modules in parallel,
one can determine the total current that can be sustained by the
parallel system. For a system of np parallel connected modules
where none of them is to exceed the rated current, the worst case
situation is when one of the devices is conducting the maximum
rated current IM and the remaining (np - 1) devices are each
conducting some minimum current Imin. In this case the total
current is given by:
The mis-sharing factor m is an important device characteristic for
parallel connected IGBTs and it is related to the spread in IGBT
output characteristics for a given device type. It is a function of
35
Tj = 25˚C
30
[3]
25
WHAT IS THE MIS-SHARING FACTOR?
Following on from the derating factor, it is possible to define in a
similar way the amount of mis-sharing between devices when
connected in parallel. In this analysis the mis-sharing factor is
a measure of the maximum current compared to the minimum
current:
Derating factor (%)
I T = I M + (n p − 1) Imin
Tj = 125˚C
20
15
10
m=
I M − Imin
IM
[4]
Where
m = mis-sharing factor
IM = maximum allowable current for a single module operating
alone.
5
0
1
2
3
4
Number of modules in parallel
5
Fig. 2 Static derating factor vs number of
DIM800DDM17-A000 IGBT modules in parallel
2/5
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AN5505 Application Note
saturation voltage VCE(sat), junction temperature, device design
and technology. Figure 2 (which is a plot of equation [6]) shows
the derating factors for the Dynex DIM800DDM17 IGBT module
for up to 5 modules in parallel.
Note that de-rating is more severe at lower temperatures. For a
worst case design it is advisable to use derating factors applicable
at 25°C junction temperature. The total system current can be
estimated for up to 5 modules in parallel by reading off a derating
factor from figure 2 and using equation [2].
Example 3: Estimate total current for a system of four
DIM800DDM17 connected in parallel.
From figure 2 the derating factor at 25°C for 4 devices in parallel
is 30%. From equation [2],
conditions. The main reason for current imbalance during
switching (turn-on and turn-off) assuming ideal gate drive
conditions and circuit layout is the difference in the transfer
characteristics (collector current vs. gate-emitter voltage (VGE))
of the individual modules. Referring to figure 3, if the VGE applied
to each of the parallel modules is identical during the switching
transistions, the current divides dynamically according to the
transfer characteristics. The IGBT module with the largest value
of transconductance (i.e. the “steeper” transfer characteristic)
carries the greater portion of the current and incurs the highest
switching losses. The dynamic current rating (IC(PK)) is related to
the IGBT rated junction temperature and hence to the total device
losses. Thus the dynamic current rating depends on the specific
application conditions.
If we define IC(PK)(max) as the maximum allowable peak current for
a single module operating alone in a specific appication and
IC(PK)(min) as the minimum peak current for a single module
operating in a parallel connection, then we can define the partial
current ∆IC(PK) as IC(PK)(max) – IC(PK)(min) and the dynamic current missharing factor by
IT = (1 – 0.3) x 4 x 800 = 2240A
DYNAMIC BEHAVIOUR
The static rating a system of parallel connected IGBT modules
assumes that none of the devices in parallel combination carries
current more than its rated value. A similar approach can be
applied to parallel connected-IGBT modules in dynamic
ϕ = ∆IC(PK) / IC(PK)(max)
[7]
Thus we can obtain the dynamic derating factor:
δ (dynamic) = 1 – [(np – 1)(1 – ϕ) + 1]/np
[8]
35
IC
Static
30
IC(max)
25
Dynamic
∆IC(dynamic)
Derating factor - (%)
g(max)
IC(min)
g(min)
20
15
10
∆VGE
5
VGE
VTH(min)
VTH(max)
VGE(min)
VGE(max)
Fig. 3 IGBT module transfer characteristics
0
1
Operating conditions:
IC(PK) = 1300A, fsw = 4kHz
Tj = 25˚C
2
3
4
Number of modules in parallel
5
Fig. 4 Comparison of static and dynamic derating factors
for DIM800DDM17-A000
3/5
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AN5505 Application Note
Where
δ = dynamic derating factor
np = number of modules in parallel
ϕ = Mis-sharing factor
The dynamic mis-sharing factor ϕ is related to the transfer
characteristics (VTH and gfe) of the IGBT.
Figure 4 compares static and dynamic derating for DIM800DDM17
IGBT modules at 25°C junction temperature under given operating
conditions (in this case a PWM motor drive inverter). Note that
the static derating factor is greater than the dynamic derating
factor. This is true in general and so for most applications users
need only consider the static derating factor when determing the
number of modules required in parallel.
Fig. 5 Static and dynamic collector current imbalance for
two DIM800DDM17-A000 modules in parallel
EXTERNAL INFLUENCES ON SHARING
This note considers only the effects of device characteristics on
static and dynamic sharing. However, it should be noted that
dynamic sharing is more sensitive to external circuit factors
(especially the stray inductance in the gate-emitter circuit loop)
than the IGBT module dynamic characteristics
(transconductance). Figure 5 illustrates how unequal stray
inductances in the gate-emitter loops produce dynamic imbalance
due to non equal gate emitter voltages (i.e. V ≠ V ) during
GE1
GE2
switching and thus an unbalanced current in dynamic switching.
The use of separate gate resistors Rg1 and Rg2 (figure 6) helps to
restore the dynamic balance.
IC1
Gate
Drive
IC2
Rg1
T1
Rg2
T2
VGE1
VGE2
LT1
LT2
SUMMARY
When a number of IGBT modules are connected in parallel, the
total current capability must be derated due to mismatching of
device characteristics and non-symmetrical external circuit layout.
The derating factors for static and dynamic operation are
presented for parallel combinations of up to five modules. Using
the derating charts a system designer can estimate the total
sustainable current for a system of parallel-connected IGBT
modules.
Fig. 6 Use of Rg1 and Rg2 to restore dynamic balance
4/5
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