**Subject :**Power Electronics

**Unit :**Power Semiconductor Devices

## GTO Ratings - Steady state voltage and current rating

**Steady state voltage and current rating**:

** V _{DRM}**:

It is the maximum repetitive forward voltage the GTO can block in the forward direction assuming line frequency sinusoidal voltage waveform. It is important to note that GTO can block rated voltage only if the gate is reverse biased or at least connected to the cathode through a low value resistance. Manufactures usually provide the forward voltage withstanding capacity of the GTO as a function

**V _{RRM}: **

It is the maximum repetitive reverse voltage the GTO is able to withstand. For all asymmetric GTOs this value is in the range of 20-30 V, since it is determined by the gate cathode junction break down voltage. Due to the anode shorted structure of the GTO the anode – base junction (J_{1}) does not block any reverse voltage. Unlike V_{DRM}, V_{RRM }rating may be exceeded for a short time without destroying the device. This “**reverse avalanche**” capability of the GTO is useful in certain situations of the gate cathode reverse voltage (and /or resistance) for a given forward dv/dt.

Ø **V _{DC}:**

This is the maximum continuous DC voltage the device can withstand. Exceeding this voltage does not immediately lead to device failure, but the probability of a cosmic radiation failure increases progressively with the applied dc voltage.

Ø **I _{FAVM }and I_{FRMS}:**

These are maximum average and RMS on state current respectively. They are specified at a given case temperature assuming half wave sinusoidal on state current at power frequency.

Ø **I _{FSM}:**

This is the maximum allowed peak value of a power frequency half sinusoidal non-repetitive surge current. The pulse is assumed to be applied at an instant when the GTO is operating at its maximum junction temperature. The voltage across the device just after the surge should be zero.

Ø ∫**i ^{2}dt:**

This is the limiting value of the surge current integral assuming half cycle sine wave surge current. The junction temperature is assumed to be at the maximum value before the surge and the voltage across the device following the surge is assumed to be zero. The i^{2}t rating of a semiconductor fuse must be less than this value in order to protect the GTO. Plots of both I_{FSM }and ∫ i^{2}dt as functions of surge pulse width are usually provided by the manufacturer.

Ø **V _{F }:**This is the plot of the instantaneous forward voltage drop vs instantaneous forward current at different junction temperatures.

Ø **P _{av }:**

For some frequently encountered current waveforms (e.g. sine wave, square wave) the plot of the average on state power dissipation as a function of the average on state current is provided by the manufacturers at a given junction temperature.

Ø **I _{H}: **

This is the holding current of the GTO. This current, in case of a GTO_{1} , is considerably higher compared to a similarly rated thyristor. Serious problem may arise due to anode current variation because the GTO may “un-latch” at an in appropriate moment. This problem can be avoided by feeding a continuous current into the gate (called the “back porch” current) during ON period of the device. This DC gate current should be about 20% higher than the gate trigger current (I_{GT}) at the lowest expected junction temperature.

Ø **di:dt| _{crit} : **

This is the maximum permissible value of the rate of change of forward current during turn on. This value is very much dependent on the peak gate current magnitude and the rate of increase of the gate current. A substantial gate current ensures that all GTO cathode segments are turned ON simultaneously and within a short time so that no local hot spot is created. The di_{g}/dt and I_{gM }values specified in the operating conditions should, therefore, be considered as minimum values.