1 snubber networks for igbts. 2 motivation nwhy low inductive dc-link design? udue to stray...
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Snubber networks for IGBTsSnubber networks for IGBTs
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Motivation
Why low inductive DC-link design?
Due to stray inductances in the DC link, voltage overshoots occur during switch off of the IGBT:
These voltage overshoots may destroy the IGBT module because they are added to the DC-link voltage and may lead to V CE > VCEmax
With low inductive DC-Link design (small Lstray) these voltage overshoots can be reduced significantly.
dt
diLv strayovershoot
linkDCovershootCE vvv
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Low Inductance DC-link Design
The mechanical design has a significant influence on the stray inductance of the DC-link The conductors must be paralleled
Lstray = 100 %
Lstray < 20 %
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Low Inductance DC-link Design
The mechanical design has a significant influence on the stray inductance of the DC-link The connections must be in line with the main current flow
Lstray = 100 %
Lstray = 30 %
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Low Inductance DC-link Design
The mechanical design has a significant influence on the stray inductance of the DC-link Also the orientation must be taken into regard
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Lstray = 100 %
Lstray = 80 %
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Low Inductance DC-link Design
The mechanical design has a significant influence on the stray inductance of the DC-link A paralleling of the capacitors reduces the inductance further
Lstray = 100 %
Lstray = 50 %
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IGBT Moduls
Capacitor
Low inductive solution
Low Inductance DC-link Design
Comparison of different designs Two capacitors in series Two serial capacitors in parallel
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IGBT Moduls
Capacitor
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Typical solution
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“Low cost” solution For paralleling standard modules a minimum requirement is a
DC-link design with two paralleled bars
Low Inductance DC-link Design
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Low Inductance DC-link Capacitors
Also the capacitors have to be decided Capacitors with different internal stray inductance are available Choose a capacitor with very low stray inductance!
Lstray = ?
Ask your supplier!
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Motivation
Why use a snubber?
Due to stray inductances in the DC link, voltage overshoots occur during switch off of the IGBT:
These voltage overshoots may destroy the IGBT module because they are added to the DC-link voltage and may lead to V CE > VCEmax
The snubber works as a low pass filter and “takes over” the voltage overshoot
dt
diLv strayovershoot
linkDCovershootCE vvv
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Snubber Networks
SEMIKRON recommends for IGBT applications: Fast and high voltage snubber capacitor parallel to the DC link
Not to increase Lstray, the snubber must be located very close to the IGBT module
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Not Sufficient Snubber Capacitors
But still: the snubber networks need to be optimised The wrong snubber does not reduce the voltage overshoots Together with the stray inductance of the DC-link oscillations can
occur
IGBT switch off (raise of VCE )
before optimisation
Voltage overshoot
Oscillation
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Not Sufficient Snubber Capacitors
These capacitors did not work satisfactory as snubber:
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Available Snubber Capacitors
From different suppliers different snubber capacitors are available.
In a “trial and error” process the optimum can be find, based on measurements.
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Optimal Snubber Capacitor
After optimisation: Significantly reduced voltage overshoots No oscillations
IGBT switch off (raise of VCE )
after optimisation
Voltage overshoot
No oscillation
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Snubber networks for IGBTs
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Calculation of a snubber capacitor
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Conclusion
When using latest generations of IGBT modules it is recommended and advantageous to Do a low inductive (“sandwich”) DC-link design Decide for low inductive DC-link capacitors Optimise the snubber circuit
Dealing with IGBT Modules