Fig. 1 Low inductance snubber capacitors for mounting on IGBT modules
1. Capacitor DC-voltage class.
2. Capacitance value and series inductance
3. Pulse handling capability.
4. RMS voltage and RMS current
Fig. 2 Typical waveform of VCE voltage on IGBT at switching off
Fig. 3 Equivalent circuit diagram of IGBT module connected to DC-link and snubber capacitor
DC voltage class
The maximum continuously applied DC voltage can be the rated DC voltage given in the data sheet to achieve the life expectancy. Semiconductors with 1200V blocking voltage are used with up to 900V DC-link voltage. For these applications, capacitors with a rated voltage of 1000V are recommended. For 1700V semiconductors, 1250V or 1600V capacitors are recommended, depending on the DC-link voltage.
The peak voltage also has to be in the admissible values because otherwise the plastic film could be damaged. Permissible peak voltages are given in the data sheets or have to be requested. Consider also that the applied DC voltage has to be derated when the capacitor is operating at higher temperatures than the rated temperature.
Capacitance and series-inductance
The capacitance value has to be high enough to achieve sufficient voltage spike suppression during switching off. Typical values for these capacitors are from 0.1 μF to 1.0μF. But not only is the capacitance value important for this. Also a low inductive design of the capacitor isimportant. The remaining inductance, caused by the loop between the terminals and the internal connections of the capacitors is responsible for the first voltage spike V2 seen in Fig. 2. A high capacitance value is no guarantee for a low voltage spike if the self-inductance remains. A low self-inductance can be achieved by using capacitors with wide flat terminals that can be screwed directly onto the IGBT module terminals. The capacitor should be designed so that the terminals encircle as small an area as possible and that they are directly connected to the capacitor coil without having internal wires between (see Fig. 1). The choice of the correct snubber should be determined by measurements. Furthermore, metallized polypropylene foil capacitors should be used with plastic case according to UL94V-0.
The inner connections of the capacitor are capable of withstanding only a limited amount of energy at each switching event. The data sheets of the supplier specify limits for pulse operation as i²t or v²t values. These values can be calculated from the oscillating current or voltage waveform of the capacitor. This calculation can easily be carried out using modern digital oscilloscopes.
A capacitor failure can occur only because of very high peak currents, even when the involved voltages are lower than the specified ones. In this situation the critical thing is the involved energy and normally there will be a loss of connection between metal spray and film metallization. Because of the very high energy involved the film metallization will be vaporized on the connection area to the metal spray. This will lead the capacitor to a high loss factor or even to a capacitance loss. The maximum dv/dt values are less critical because of the damped sinusoidal waveform.
RMS voltage and RMS current
A damped oscillation occurs at each switching event (on or off = twice switching frequency of the IGBT) between the snubber capacitor and the bus bar capacitance. The maximum magnitude, V3, for an undamped oscillation and the frequency can be calculated using the formulae in Fig. 3. This RMS current leads to self heating of the capacitor. The capacitor will stabilize at a certain temperature which also depends on the ambient temperature and on the mounting conditions (e.g. temperature of power module terminals).
Data sheets give values for the permissible RMS current and RMS voltage depending on the frequency. The oscillating frequency depends on the DC-link stray inductance and the snubber capacitor value. Typical values are in the range of 100 kHz to 1 MHz. The permissible RMS current decreases with the frequency because the losses increase.
Please see chapter “Measuring capacitor RMS current” which gives tips for practical current measurement on the capacitor.
The capacitor lifetime and failure rate is mainly affected by the operating temperature and operating voltage. The failure criteria differ from supplier to supplier. Check the data sheet and application notes for lifetime and failure rate data.
The most important reliability feature of film capacitors is their property to self-heal that means to clear a defect in the dielectric. The capacitor can be used afterwards without any restrictions. This defect is caused when the dielectric breakdown field strength is exceeded locally at a weak point in the foil.