Download Low Inductance DC

Snubber networks for IGBTs
1
 Why low inductive DC-link design?
 Due to stray inductances in the DC link, voltage overshoots occur
during switch off of the IGBT:
di
vovershoot  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
3
 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
5
 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
6
 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
7
 “Low cost” solution
 For paralleling standard modules a minimum requirement is a
DC-link design with two paralleled bars
Low Inductance DC-link Design
8
 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
9
 Why use a snubber?
 Due to stray inductances in the DC link, voltage overshoots occur
during switch off of the IGBT:
di
vovershoot  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
10
 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
11
 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
12
 These capacitors did not work satisfactory as snubber:
Not Sufficient Snubber Capacitors
13
 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
14
 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
16
Calculation of a snubber capacitor
17
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
18