Snubber networks for IGBTs
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 Why low inductive DC-link design?
 Due to stray inductances in the DC link, voltage overshoots occur
during switch off of the IGBT:
vovershoot
di
 Lstray 
dt
 These voltage overshoots may destroy the IGBT module because they
are added to the DC-link voltage and may lead to VCE > VCEmax
vCE  vovershoot  vDC link
 With low inductive DC-Link design (small Lstray) these
voltage overshoots can be reduced significantly.
Motivation
2
 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
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 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
4
 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
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 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
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 Comparison of different designs
 Two capacitors in series
 Two serial capacitors in parallel
Typical solution
Low inductive solution
IGBT Moduls
+ -
+ +
+
--
Capacitor
--
+
+
IGBT Moduls
-
+ -
+ -
-
-
-
+
+
-
+
+
-
Capacitor
Low Inductance DC-link Design
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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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 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
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 Why use a snubber?
 Due to stray inductances in the DC link, voltage overshoots occur
during switch off of the IGBT:
vovershoot
di
 Lstray 
dt
 These voltage overshoots may destroy the IGBT module because they
are added to the DC-link voltage and may lead to VCE > VCEmax
vCE  vovershoot  vDC link
 The snubber works as a low pass filter and “takes over” the
voltage overshoot
Motivation
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 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
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 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
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 These capacitors did not work satisfactory as snubber:
Not Sufficient Snubber Capacitors
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 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
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 After optimisation:
 Significantly reduced voltage overshoots
 No oscillations
IGBT switch off
(raise of VCE )
after optimisation
Voltage overshoot
No oscillation
Optimal Snubber Capacitor
15
Snubber networks for IGBTs
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Calculation of a snubber capacitor
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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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