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Snubber networks for IGBTs

Snubber networks for IGBTs. 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 > V CEmax

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Snubber networks for IGBTs

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  1. Snubber networks for IGBTs

  2. 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 VCE > VCEmax • With low inductive DC-Link design (small Lstray) these voltage overshoots can be reduced significantly. Motivation

  3. The mechanical design has a significant influence on the stray inductance of the DC-link • The conductors must be paralleled Lstray = 100 % Lstray < 20 % Low Inductance DC-link Design

  4. 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 % Low Inductance DC-link Design

  5. + + - - • The mechanical design has a significant influence on the stray inductance of the DC-link • Also the orientation must be taken into regard Lstray = 100 % Lstray = 80 % Low Inductance DC-link Design

  6. 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 % Low Inductance DC-link Design

  7. IGBT Moduls IGBT Moduls Low inductive solution + - + - + - + - + + - - - - - + + + + + - - + + - + + - + Capacitor Capacitor - + • Comparison of different designs • Two capacitors in series • Two serial capacitors in parallel Typical solution Low Inductance DC-link Design

  8. “Low cost” solution • For paralleling standard modules a minimum requirement is a DC-link design with two paralleled bars Low Inductance DC-link Design

  9. 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! Low Inductance DC-link Capacitors

  10. 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 VCE > VCEmax • The snubber works as a low pass filter and “takes over” the voltage overshoot Motivation

  11. 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 Snubber Networks

  12. 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 Not Sufficient Snubber Capacitors

  13. These capacitors did not work satisfactory as snubber: Not Sufficient Snubber Capacitors

  14. From different suppliers different snubber capacitors are available. • In a “trial and error” process the optimum can be find, based on measurements. Available Snubber Capacitors

  15. After optimisation: • Significantly reduced voltage overshoots • No oscillations IGBT switch off (raise of VCE ) after optimisation Voltage overshoot No oscillation Optimal Snubber Capacitor

  16. Snubber networks for IGBTs

  17. Calculation of a snubber capacitor

  18. Dealing with IGBT Modules • 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 Conclusion

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