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