IGBT模块在三电平中点钳位式变流器的保护考虑

Fig. 1. 3-level NPC topology
In the beginning stage, some MV-inverters were using GTO and IGCT as the power
switching devices. Recently IGBT is becoming more and more popular in the market. In this
paper, 4500V IGBT application for 3300V MV-inverter using 3-level topology will be
discussed in details, which is popular in 3300Vwind power converters shown in Fig.1.
2. Development of 4500V IGBT modules
2.1 IGBT chip technology

Fig. 2. The cross section of H series and R series
Now both Mitsubishi H series HVIGBT and R series HVIGBT are widely used in the market.
Fig. 2 shows the cross section of H series and R series HVIGBT chips, which are developed
by Mitsubishi Electric. Compared with H series IGBT, the new developed R series HVIGBT
has been improved on 3 pints: a) Enhanced planar gate cell by additional carrier enhanced N
layer; b) Optimized N- drift layer; c) LPT structure [3].
2.2 Safety-Operating Area (SOA) of R series 4500V HVIGBT
(a) RBSOA (b)SCSOA

Fig. 3. SOA of CM1200HC-90R
Fig. 3 gives RBSOA (Reverse Biased Safety Operating Area), SCSOA (Short Circuit Safety
Operating Area) of CM1200HC-90R respectively. RBSOA stands for the ability of turning off
the current under the rated voltage. SCSOA stands for the ability of turning off the current
under short-circuit, which is also related with the positive drive power supply closely. In the
actual application, IGBT should be guaranteed to run in SOA.

 

Fig. 4. Actual test waveform of CM1200HG-90R
Fig.4 gives the actual test results of turn-off waveform of CM1200HG-90R, which shows that
there is much margin for turn-off ability [3].
3. Influence of stray inductance of DC busbar during SC
3.1 Functions of short-circuit
There are two different types of short-circuit for an IGBT: a) short-circuit I, the IGBT is turned
on when a load or bridge short circuit already exists; b) short-circuit II, the IGBT is already
turned on before the short circuit occurs.
3.2 Short circuit test and the influence of the stray inductance of busbar

 

Fig. 5. Test circuit of short circuit
In this paper, short-circuit I will be taken as the example. And the test circuit was shown as
Fg.5.
3.2.1 Short circuit test under large stray inductance busbar
(a) Example layout of busbar (b) Test result (voltage:1kV/div; current: 2kA/div)

Fig. 6. Example layout of busbar and test result of short-circuit
Fig.6(a) gives the example layout of bus bar for short circuit test, in which positive busbar
and negative busbas are connected independently. It is easy to understand that there is
large stray inductance of DC busbar in this layout. And Fig.6(b) gives the test results of short
circuit based on DC busbar layout in Fig.6(a).The test module is CM900HC-90H and DC bus
voltage is 1000V. From it, it could found that the current could be over 11000A, while
Vce(peak) is 3000V. By comparing the test results with the SCSOA, it could be found that the
test result is over SCSOA as shown in Fig.7, which cannot satisfy the actual application.

Fig. 7. SCSOA of CM900HC-90H
Vce voltage
3.2.2 Short circuit test based on laminated busbar
(a) Laminated busbar (b) test results (voltage: 1kV/div; current: 2kA/div)

Fig. 8. Laminated busbar and test results
Fig.8 gives the laminated busbar for short circuit test and the related test results. The test
results showed that the current is 6100A and the Vce(peak) is 3700V. The DC bus voltage is
set up as 2500V, which is similar with the actual application conditions in 3-level NPC
converter. Comparing with SCSOA in Fig.8, it could found the test results are within SCSOA.
From the test results in Fig.6 and Fig.8, it could be concluded that the stray inductance of DC
busbar has a very large influence on keeping IGBT running in SCSOA. This is caused by: a)
larger stray inductance will cause higher Vce(peak); b) larger stray inductance (load
inductance) will cause lower current increasing, which will introduce longer time on protecting
turn-off of IGBT modules.
4. Consideration on reducing the stray inductance of DC bus bar

 

(a) (b)
Fig. 9. Current shifting loop during IGBT turn-off
Vce voltage
current
⑧
⑦
Fig.9 gives the current shifting loop [1]. In Fig.9(a), the peak voltages for T1 and T3 during
turn-off are respectively as the following.
When T1 turn off and T3 turn on, the peak voltage on T1 is
VT1(peak)=Vcc/2+L1*di/dt+LModule*di/dt+L2*di/dt+LDiode*di/dt+L3*di/dt (1)
When T3 turn off and T1 turn on, the peak voltage on T3 is
VT3(peak)=Vcc/2+L1*di/dt+LModule*di/dt+LDiode*di/dt+L3*di/dt (2)
Where, Vcc is the DC bus voltage, Li(i=1,2,3) is the stray inductance of busbar, LModule is the
stray inductance of IGBT, LDiode is the stray inductance of Diode.
From (1) and (2), it could be found that T1 would bear the higher voltage than T3, which is
similar as the actual test results. Based on this, it is recommended to try to reduce the stray
inductance in LOOP A.
On the other hand, it is important to reduce the stray inductance in short-circuit loop (busbar
1, 4, 6, 7, 8), which is to keep IGBT running in SCSOA when short-circuit occurs.
5. Conclusion
With the development of wind power market, 3300V output voltage wind power converters
will be more and more popular. And the 3-level topology and 4500V IGBT modules, including
both H series and R series, could satisfy this development. In this paper, the actual test
results of CM900HC-90H (900A/4500V) were given, which show that the stray inductance of
busbar will have a big influence on running of IGBT module. In order to make IGBT run in
SOA, the design of busbar with low stray inductance should be concerned. And the stray
inductance could be reduced with the analysis of current shifting loop.

（本文转自http://www.igbt8.com/）