Scattering parameter approach applied to the stability analysis of power IGBTs in short circuit

Highlights

• We propose a new method for stability analisys of IGBTs.

• We use the K stability factor from Stern.

• We demonstrated that an increase in the collector current reduces stability.

• We demonstrated that an increase in the gate resistance reduces stability.

Abstract

Power IGBTs operating at high voltage and current under particular load and driving conditions, particularly in short circuit, can be affected by unstable phenomena that can reduce the device’s reliability and increase the generation of electromagnetic interference. In this paper we propose, for the first time, the analysis of the device’s stability based on scattering parameter approach applied to power IGBT operated under different load and driving conditions. The Stern stability factor (K-factor) was used to individuate the frequency range where the device can be potentially unstable and investigate about the influence of the external circuit on the circuit stability. The proposed method has been experimentally verified on discrete IGBTs and extended to FEM simulation. It can supply very important suggestions to the device’s designers for improving the device stability and to the circuit designer for preventing the instabilities by using a proper choice of driving and load circuits.

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.