Thyristor Gate Characteristics
Thyristor Gate Characteristics:
- The gate circuit of a thyristor behaves like a poor quality diode with high on state voltage drop and low reverse break down voltage. This characteristic usually is not unique even within the same family of devices and shows considerable variation from device to device.
- Each thyristor has maximum gate voltage limit (Vgmax), gate current limit (Igmax) and maximum average gate power dissipation limit(Pgav|Max).These limits should not be exceeded in order to avoid permanent damage to the gate cathode junction. There are also minimum limits of Vg (Vgmin) and Ig (Igmin) for reliable turn on of the thyristor. A gate non triggering voltage (Vng) is also specified by the manufacturers of thyristors. All spurious noise signals should be less than this voltage Vng in order to prevent unwanted turn on of the thyristor. The useful gate drive area of a thyristor is then b c d e f g h.
- Referring to the gate drive circuit in the inset the equation of the load line is given by
Vg = E - Rgig
- The actual operating point will be somewhere between S1 & S2 depending on the particular device.
- For optimum utilization of the gate ratings the load line should be shifted forwards the Pgav|Max curve without violating Vg|Max or IgMax ratings. Therefore, for a dc source E c f represents the optimum load line from which optimum values of E & Rg can be determined.
- Maximum power dissipation curves for pulsed operation (Pgm) allows higher gate current to flow which in turn reduces the turn on time of the thyristor. The value of Pgm depends on the pulse width (TON) of the gate current pulse. TON should be larger than the turn on time of the thyristor. For TON larger than 100 μs, average power dissipation curve should be used. For TON less than 100 μs the following relationship should be maintained.
- The magnitude of the gate voltage and current required for triggering a thyristor is inversely proportional to the junction temperature.
- The gate cathode junction also has a maximum reverse (i.e, gate negative with respect to the cathode) voltage specification. If there is a possibility of the reverse gate cathode voltage exceeding this limit a reverse voltage protection using diode.