Istvan Somos

Power semiconductors-a new method for predicting the on-state characteristic and temperature rise during multi-cycle fault currents

A concept, test procedure, and evaluation method are described for determining the on-state voltage of thyristors (or diodes) over a very wide temperature range to the limit of normal behavior. The difficulty that the temperature dependence is not constant has been overcome, and a mathematical relationship representative of this dependence as a continuously changing variable, a function of both current and temperature, is derived from the analysis of data. Measurement involves a series of single-cycle test shots at the fault current level or I/sub TSM/ with varying starting temperatures ranging from 25 degrees to 150 degrees C. Combining mathematical representations of the recorded voltage and current traces with a preestablished thermal model, the junction temperature profile or excursion which occurs simultaneously is calculated. The proposed approach is applied to a 5000 V class 100 mm thyristor. >

Behavior of thyristors under transient conditions

The capability of gate-triggered thyristors to withstand steep wavefront, high-current pulses (i.e., di/dt capability) is a function of both junction temperature and frequency of operation. Localized internal heating occurs during turn-on and may lead to thermal runaway. The conditions required for this to occur have been determined by destructively testing many devices. The initial conducting area of a thyristor largely determines di/dt capability, which is not necessarily related to the size of the device but is a function of the design of the gate region. Gate drive is very important for determining the di/dt capability of a thyristor having a conventional gate design. Two devices which have been designed to increase the initial conducting area are discussed. One of these devices, if improperly designed, can lose its effectiveness with high gate drive. This characteristic can be studied by observing the reverse recovery current immediately following short forward current pulses.

A thermal analogue of higher accuracy and factory test method for predicting and supporting thyristor fault suppression ratings

Because sophisticated computer modeling is not easily accessible, a one-dimensional thermal analogue is often used for routine calculations of virtual junction temperature occurring in thyristors when subjected to fault currents. The limitations of this procedure were found while establishing and improving fault suppression ratings of high-voltage (5 kV) thyristors. The thermal model was upgraded to include a number of heat sources within the silicon and some degree of two-dimensional heat spreading, while retaining the relative simplicity of a one-dimensional model. This upgraded model distinguishes the thermal impedance of the cathode junction from the thermal impedance of the anode junction. The supporting bench test was set up to use the calibrated off-state V/sub B0/ characteristic rather than the calibrated on-state drop of the signal current. It was found to be a more discriminating thermometer, responding to any existing hot spot temperature caused by surge current levels. Using certified thyristors, improved fault suppression ratings were established and subsequently proved under real power system conditions.<<ETX>>

Commutation and destructive oscillation in diode circuits

Since the introduction of single crystal rectifier cells, the switching action from the forward- to the reverse-biased condition, in other words commutation, frequently has been analyzed. In the early stages of germanium and silicon rectifiers, destruction of cells was often caused by this commutation action. After the application of parallel capacitors or filtering circuits, the troubles were eliminated, and correctly designed filtering capacitors provided a simple, cheap, and effective way to assure trouble-free operation.

Switching characteristic of silicon power-controlled rectifiers: II — Turn-off action and dv/dt self-switching

THE TURN-OFF process is the regaining of forward blocking ability-after termination of a forward current wave. By the turn-off process the silicon-controlled rectifier (SCR) is brought into the forward blocking condition where turn-on will not occur because of the unavoidable dv/dt slope of the reapplied forward voltage. The problem of the turn-off process is how to avoid unintentional turn-on. Thus, the study of the turn-off and turn-on processes cannot be separated.