Thyristor Turn ON methods

In the previous chapter, we discussed the two-transistor model of a thyristor (SCR) and concluded that the thyristor can be turned on by increasing the forward current gain of the transistors. Here are some other methods to turn on a thyristor −

• Forward Voltage Triggering
• High dv/dt Triggering
• Temperature Triggering
• Light Triggering
• Gate Triggering

Forward Voltage Triggering

When anode is positive with respect to the cathode, with gate circuit open, thyristor(SCR) is said to be forward biased. In this mode junction J1 and J3 are forward biased and junction J2 is reverse biased.

When the anode voltage(applied voltage) V_a is greater than the forward break over voltage V_BO i.e..,$\mathrm{V_a\:\geq\:V_{BO}}$, then the junction J₂ undergo avalace breakdown then huge I_a current flow and thyristor is turned ON.

As huge I_a current flows the device may damage. And we can also conclude, forward break over voltage V_BO and reverse break-over voltage V_BR are temperature dependent and $\mathrm{V_{BR}\:>\:V_{BO}}$. Therefore, V_BO is taken as highest voltage rating.

High dv/dt Triggering

When the SCR is in forward blocking mode, then the junction J₂ is reversed biased. The depletion layer in J₂ acts as a capacitor(junction capacitor C_jo ) it is formard dur to polaization. We know for any capacitor current can be expressed as −

$\mathrm{i_C\:=\:C\frac{dv}{dt}}$

In this case the forward voltage V_a appears across reverse biased junction J₂, then charging current across the juction is given by −

$\mathrm{i_C\:=\:C_{jo}\frac{dv_a}{dt}}$

When there are some disturbances in the supply there will be high dv/dt so the charging or displacement current across the J₂ will be disturbed and huge anode current I_a flows leading to eventual switching action of thyristor.

Temperature or Thermal Triggering

SCR is a semi-conductor device made with silicon material, which has a negative temperature co-efficient of resistance i.e., as the temperature increases the leakage currents increases.

By increasing the temperature, the width of the depletion layer at J₂ decreases, So when the forward bias voltage is near to V_BO, we can turn ON the SCR by increasing the temperature. Practically, the temperature is less than 125^oC.

Light Triggering

Light triggering is a specialized method used exclusively in Light Activated Silicon Controlled Rectifiers LASCR. When light is incident on the surface of the device, it generates photons. These photons create electron-hole pairs, which reduce the width of the depletion layer and facilitate the turning on of the LASCR. These thyristors are used in HDVC and FACT's controllers

Gate Triggering

When the anode is positive with respective cathode then the SCR is said to be forward biased, when a positive gate pulse is provide at junction J₂ , it will inject some electrons and disturb the depletion layer , which result in increasing the reverse leakage current and hence the breakdown of J₂.

We know that forward break-over voltage V_BO is inversely proportional to the gate current i.e., as the gate current increases the forward break-over voltage decreases.

Among all the triggering methods, the best way to turn on the thyristor is by gate triggering method. While light triggering is used only in specific applications