Note: Voltage spikes can occur at diode switch off, which can lead to high blocking voltage on the diode and the parallel connected IGBT. The worst case is mostly at low current (<10%*IC) and low temperature. The voltage has to be measured on the diode which is switching off or on the parallel connected IGBT. Sometimes the snubber capacitor is more necessary for the diode switch off than for the IGBT switch off. Short on times of diodes can also cause voltage spikes if the chip is not fully floated with carriers.
The blocking voltage should be measured as close to the IGBT chip as possible. For SKiiP modules, the closest points are the module power terminals. For discrete power modules such as SEMiX and SEMITRANS auxiliary emitter contacts are available which are electrically closer to the chip. Voltages on internalmodule stray inductances between measurement point and IGBT chip have to be added to the measured value to obtain the actual blocking voltage at IGBT chip level.
A practical approach for most applications is to carry out what is known as a “double pulse test” (see Fig. 6). With different values of the load inductance and the pulse length, each load condition from low load to overload can be adjusted. A single pulse test with limited pulse length should be used for a short circuit. In these tests, the driver board receives its input signal from a pulse generator instead of the control board.
• DC-link is fed by an insulated DC voltage source which is limited in output current. Normally a few 100mA is enough. Set the DC-link voltage to the highest possible value in the application. This is usually the value of over voltage protection.
• The short circuit is realized by thick cable from DC plus connection to AC for measuring BOT switch and from DC minus to AC for measuring on TOP switch. The inductance is given by the length of the wire; 1μH corresponds to about 1m length. The short circuit can also be caused by connecting the wire between two AC terminals of two different legs of an inverter circuit. One IGBT (e.g. TOP Phase L1) has to be permanently switched on while the pulse is applied to the other IGBT (e.g. BOT Phase L2).
• A pulse generator with adjustable pulse length is connected on the driver input. The pulse generator can be set to single pulse and double pulse.
• If the over current protection (OCP) is carried out by the control board and not the driver, then the control board OCP error signal has to be monitored to find the point when the input signal would be set to off. This is not necessary for SKiiP modules because the OCP is implemented in the driver board.
• Start with the highest inductance. Carry out a single pulse, increase the pulse length until the OCP switches off. Measure the maximum value of VCE.
• Lower the inductance and repeat the test down to the lowest short circuit inductance specified for the application. Find the maximum value of VCEpeak.
• Carry out leg shot through if the driver has no interlock function.
• Apply a double pulse for investigation of IGBT switch on and diode switch off behaviour. The diode (e.g. BOT) is switched off when the complementary IGBT (e.g. TOP) is switching on while the diode is conducting current. This is when the second pulse is applied.
• Carry out measurements on each IGBT module. Highest values occur on the module which is farthest from the DC link capacitors.
• Perform the test at low and high temperatures. A high temperature can be reached by heating the heatsink e.g. using a heating plate. The junction temperature is approximately the heat sink temperature because the temperature increase due to the single switching is negligible.
Grounding and voltage probe connections:
• Grounding the oscilloscope is necessary for safety and for taking accurate measurements. Therefore the DC power source has to be isolated to prevent a short circuit.
• It is recommended to connect the negative polarity of the voltage probe to DC plus when measuring the VCE of the TOP IGBT because this potential does not change. This reduces common mode noise on the measured signal. If the gate voltage of the TOP IGBT is also measured, the AC can be grounded (Fig. 5) and the minus polarity of the voltage probes have to be connected to this AC.
• Differential (isolated) voltage probes can be used for measurements when they have sufficient bandwidth. When starting the measurements it is recommended to test the behaviour of the differential voltage probe e.g. by comparing the signal at VCE measurement with a passive voltage probe.
• Common mode noise on the measured signals can also be reduced by putting appropriate ferrites across the probes and across the oscilloscope mains cable.
Fig. 4 VCEpeak measurement on BOT IGBT. TOP switch shortened by cable or inductor, double pulse applied to BOT IGBT, DC-grounded
Fig. 5 VCEpeak measurement on TOP IGBT. BOT switch shortened by cable or inductor, double pulse applied to TOP IGBT, AC-grounded
Fig. 6 Typical double pulse waveforms