Thyristor - Specification and Ratings

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For reliable and satisfactor operation of the SCR, it is ensured that its current, voltage, power, temperature, dv/dt ratings do not exceed above its manufacturer specification. Ratings and specifications server as links between designer and the user of the SCR.

Some subscripts are associated with voltage and current ratings for convenience. Let X₁₂₃, where X may be voltage(V) or current(I) where the first subscript '1' indicates the direction or state, that is −

• D = forward-blocking region
• T = on state
• R = reverse
• F = forward

Second subscript '2' indicates −

• W = working value
• R = repetitive value
• S = surge/non repetitive value
• T = trigger

Third subscript '3' indicates −

• M = maximum or peak value

V_DWM(Peak Working Forward - blocking Voltage)

It is the maximum value of the forward voltage that an SCR can withstand when it is in the OFF state.

V_DRM(Peak repetitive forward - blocking voltage)

It is the peak transient voltage that an SCR can withstand repeatly when it is in ON or OFF state. It is specified at a maximum allowable junction temperature. It appears when the SCR is turned OFF due to an abrupt in the change in the reverse recovery current and circuit inductance voltage i.e L(di/dt).

V_RRM(Peak Repetitive Reverse Voltage )

It is the maximum value of reverse voltage that may be applied repeatedly to the SCR.

V_T(ON - state Voltage drop)

It is the voltage drop across the SCR when conducting(on-state) at the specified value of forward current and junction temperature. It is about 1.0 - 1.5 V.

V_GT(Gate Trigger Voltage)

It is the maximum gate voltage required to produce the gate triggering current.

Forward dv/dt ratings

It is the maximum rate of rise of the anode voltage that will not trigger SCR if the gate signal is not applied. If dv/dt exceeds the rate values, the SCR may tuned on even in the absence of gate signal due to the flow of charging current, $\mathrm{i_c\:=\:C_j(\frac{dv}{dt})}$

I_TAV(Average on-state forward current)

It determines the power loss in the SCR because the voltage drop across the conducting SCR is low. The current I_TAV depends on the temperature of the case, and date sheets of manufacturers indicates this fact.

I_RMS(RMS on state current)

It is also repetitive type and is specified at the maximum junction temperature.

I_TSM(Impulse Surge current rating)

It is the maximum surge current(non-repetitive) that the SCR can withstand. It is determined by equating the energies involved in one cycle surge and sub-cycle, that is −

$$\mathrm{I_{sub}^2\:=\:I^2T} \\ \mathrm{I_{sub}\:=\:I\sqrt{\frac{T}{t}} }$$

where,

• I = current rating of one-cycle surge, A
• I_sub = current rating of subcycle surge, A
• T = time duration of hslf-cycle, s
• t = time duration of sub-cycle surge, s

For 50Hz, T = 10x10^-3s

$$\mathrm{ I_{sub}\:=\:I\sqrt{\frac{10\times 10^-3}{t}}} \\ \mathrm{I_{sub}\:=\:\frac{0.1I}{t} }$$

I²t rating

It is tha maximum allowable non-repetitive value of a square of instantance current integrated over time.

di/dt rating

This rating of a thyristor indicates the maximum rate of rise of current from anode to cathode without any harm to the device. When a thyristor is turned on, conduction starts at a place near the gate. This small area of conduction spreads to the whole area of junction. If the rate of anode current(di/dt) is large as compared to the spreading velocity of carriers across the cathode junction, local hot spots will be formed near the gate connection on account of high current density.

This causes the junction temperature to rise above the sfe limit and as a consequence, SCR may be damaged permanently.

Therefore, a limit on the value of di/dt at turn-on is specified in amperes per microsecond for all SCRs. Typical values of di/dt are 20 to 500A/µsec