Scattering parameter approach applied to the stability analysis of power IGBTs in short circuit
Introduction
Power IGBT devices are extensively used in motor drives and switching converters thanks to the easy controllability of the on–off status and the good switching performances. However, during the normal operating conditions and/or during overload conditions the device inserted in a circuit can become instable and can compromise the reliability of the entire system [1]. The instable phenomena have been widely studied in short circuit, SC, where the device at the same time holds high voltages and currents. In these test conditions oscillations are observed at the external leads of the device. Several studies [2], [3], [4], [5] have attributed these oscillations to a negative gate capacitance which appears at the input leads of the Device Under Test, DUT. The negative capacitance is evidenced by the fact that a positive variation of the gate voltage corresponds to a negative variation of the gate current [2]. This behavior is due to a layer of holes coming from the collector which accumulates below the gate region. A positive variation of the gate voltage causes the increase of the holes concentration and the consequent increase of the concentration of electrons recalled in the gate polysilicon giving origin to a negative charge variation and then to a negative gate current. The way how the external gate and drain circuits interact with the device in SC to trigger the oscillation was analyzed in [3]. A first attempt to study the stability of the IGBT in SC and to correlate it with the internal device capacitances was proposed in [4] where a simulation was used to study the variation of the small signal low frequency gate capacitance with the gate resistance and the biasing conditions. The test conditions for the negative capacitance to appear were identified and the presence of poles with a positive real part was revealed and the instability of the device was theoretically confirmed. The approach presented in [4] gives the basis for understanding the phenomena involved in the instability but it is very unpractical because it requires the knowledge of device internal parameters like gate-collector and gate-emitter capacitances that are not easy to be measured with the usual methods [5].
The problem of the instability of power semiconductor devices operating in active region has been widely studied for the design of radio frequency power amplifier [6], [7] where the Stern stability factor (K-factor) [8] has been used to simplify the design of the amplifier.
The objective of this paper is to present a new design-oriented approach, based on the employment of scatter parameters of the device and the Stern stability factor (K-factor), for studying the stability of IGBTs and identifying at one glance the operating regions where these devices become instable. The method can be applied to study the effects on the stability due to the parameters of the input and output circuits, namely the impedances of the gate and collector circuits which play a relevant role in the SC instabilities.
The proposed approach can be used both by the devices manufacturer for improving the performances of their devices and by the application designer for preventing the device to oscillate by using the proper parameters of the driving and load circuit.
Section snippets
Instable behavior during short circuit
The schematic of Fig. 1 refers to a circuit which can be used to study the behavior of power IGBTs operating in the active region as it happens in the short circuit. This circuit, which is a modification of the one used in [5] for measuring the input capacitance in SC, allows us to easily change driving and load conditions and to study the device operation in the time domain while keeping it in a SC condition. The biasing supply voltage, Vcc, is applied to the collector through the series
S-parameter approach to the IGBT stability analysis
In order to overtake the problems induced by the time domain analysis, in the following sections we propose an alternative method to perform the stability analysis of power IGBTs based on scatter parameters. The S-parameter matrix (Sij) is typically used for design and analysis of radio frequency amplifiers. It was applied also to power MOSFETs in avalanche condition [9] but, to the authors’ knowledge, not to IGBTs yet, in particular operating in SC.
Simulation results
In this section we report the results of SILVACO TCAD [10] simulations that confirm the validity of the proposed method to study the IGBT stability.
The use of a simulator allows us to extend the analysis to high collector currents, which is not easy to be performed experimentally because of the aforementioned high power dissipation.
Fig. 8 reports the elementary cell of the simulated IGBT having a die area of about 11 mm2. The device has a trench gate – field stop structure [11] and it is rated
Conclusions
We have proposed, for the first time, a stability analysis of IGBTs, particularly in SC, based on the use of scattering parameters. The method was applied to analyze experimentally the stability of the device for different driving and load conditions at low currents. Simulations confirm the obtained results and permit to extend the analysis at any operating conditions.
Thanks to the simplicity of its application the proposed method can be a powerful tool for aiding circuit designers as well as
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