Static output i-v characteristics of a thyristor
Static O/P I-V characteristics of a Thyristor:
- With ig = 0, VAK has to increase up to forward break over voltage VBRF before significant anode current starts flowing. However, at VBRF forward break over takes place and the voltage across the thyristor drops to VH (holding voltage). Beyond this point voltage across the thyristor (VAK) remains almost constant at VH (1-1.5v) while the anode current is determined by the external load.
- The magnitude of gate current has a very strong effect on the value of the break over voltage as shown in the figure. The right hand side figure in the inset shows a typical plot of the forward break over voltage (VBRF) as a function of the gate current (Ig).
- After “Turn ON” the thyristor is no more affected by the gate current. Hence, any current pulse (of required magnitude) which is longer than the minimum needed for “Turn ON” is sufficient to effect control.
- The minimum gate pulse width is decided by the external circuit and should be long enough to allow the anode current to rise above the latching current (IL) level.
- The left hand side of Fig 1 shows the reverse i-v characteristics of the thyristor. Once the thyristor is ON the only way to turn it OFF is by bringing the thyristor current below holding current (IH).
- The gate terminal has no control over the turn OFF process. In ac circuits with resistive load this happens automatically during negative zero crossing of the supply voltage. This is called “natural commutation” or “line commutation”.
- However, in dc circuits some arrangement has to be made to ensure this condition. This process is called “forced commutation.”
- During reverse blocking if ig = 0 then only reverse saturation current (Is) flows until the reverse voltage reaches reverse break down voltage (VBRR). At this point current starts rising sharply.
- Large reverse voltage and current generates excessive heat and destroys the device. If ig > 0 during reverse bias condition the reverse saturation current rises. This can be avoided by removing the gate current while the thyristor is reverse biased.