Rating of SCR or Thyristor

Thyristor ratings, or SCR (silicon controlled rectifier) ratings, set the electrical and thermal limits for a device. Reliable operation requires the selected thyristor to remain within its voltage, current, junction-temperature, gate-drive and switching limits under normal and fault conditions. These include static limits, , , turn-on time and turn-off time. Use the manufacturer’s current data sheet and its stated test conditions.

Anode Voltage Rating

Anode-voltage ratings specify the forward and reverse off-state voltages that the SCR can withstand under defined gate, temperature and waveform conditions.

Peak Working Forward Blocking or Forward OFF State Voltage (V_DWM)

This older term describes the maximum forward off-state voltage permitted during normal operation. Modern data sheets usually centre the selection on repetitive peak off-state voltage and the associated leakage-current conditions.

Peak Repetitive Forward Blocking Voltage (V_DRM)

It specifies the maximum repetitive peak forward off-state voltage allowed across the SCR. The manufacturer also specifies leakage current and junction-temperature conditions for this limit. Switching transients such as must remain below V_DRM unless a separate non-repetitive rating and its pulse conditions apply.

Peak Non-Repetitive or Surge Forward Blocking Voltage (V_DSM)

It is the maximum non-repetitive forward off-state voltage permitted for the pulse duration and conditions stated by the manufacturer. A switching event may create such a transient. Do not assume that this limit is 130% of V_DRM; the data sheet defines the relationship and the allowed pulse. It must also remain below any destructive or uncontrolled breakover region V_BD.

Peak Working Reverse Voltage (V_RWM)

This older term describes the maximum reverse voltage permitted during normal operation with the anode at a lower potential than the cathode. The value is a device limit, not automatically the peak of every applied sinusoidal waveform.

Peak Repetitive Reverse Voltage (V_RRM)

This rating gives the maximum repetitive peak reverse voltage allowed across the SCR under the data-sheet conditions. Manufacturers specify it with reverse leakage current at one or more junction temperatures. V_DRM instead applies in the forward off state.

Peak Non-Repetitive Reverse Voltage (V_RSM)

It is the maximum non-repetitive reverse off-state voltage permitted for a specified short pulse. Do not assume a fixed percentage above V_RRM; use the manufacturer’s value and pulse conditions. The applied transient must remain within the device’s reverse blocking capability V_BR.
Forward on-state voltage drop (V_T)

Manufacturers measure forward on-state voltage drop (VT) between anode and cathode at a stated on-state current, pulse duration and junction temperature. The value is device-specific. Use the maximum VTM value or the manufacturer’s on-state model when calculating conduction loss.

Forward dv/dt Rating

With forward voltage applied, junctions J₁ and J₃ are forward biased while junction J₂ is reverse biased. Junction produces displacement current when anode voltage changes. That current rises with and may trigger the SCR without a gate pulse if the critical rate is exceeded. The junction capacitance, temperature, gate circuit and applied voltage all affect immunity. At a sufficiently high this mechanism can trigger conduction, but it differs from normal avalanche breakdown at J₂. The critical limit is measured under stated conditions. Keep the circuit’s forward below that limit and control transients with suitable layout, gate bias or a snubber. Exceeding can cause unwanted turn-on and damage.

Voltage Safety Factor of SCR (V_SF)

This design margin can be expressed as the ratio of peak repetitive reverse voltage V_RRM to the maximum reverse voltage expected in the circuit.

Finger Voltage of SCR (V_FV)

“Finger voltage” is not a standard rating in current general-purpose SCR data sheets. A conventional SCR cannot normally be turned off through its gate. Use the specified holding current, turn-off time and commutation conditions for turn-off design. Do not use this term as a selection limit unless the manufacturer defines it for a particular device.

Current Rating of SCR

An SCR is a semiconductor device with limited junction temperature and current density. Continuous current, repetitive pulses and fault surges heat it in different ways. The current rating of SCR must therefore be checked with the waveform, conduction angle, cooling arrangement, case or ambient temperature and transient duration.

Maximum RMS Current Rating (I_RMS)

The rms current rating limits heating associated with the device and its connections. A simple resistive component follows I_RMS²R, but total SCR conduction loss also depends on its nonlinear on-state characteristic. The applied I_RMS must remain within the rating for the specified thermal conditions.

Maximum Average Current Rating (I_AV)

Maximum average current rating (IAV) is the maximum average on-state current under the specified waveform and thermal conditions. A manufacturer may show I_AV against case temperature I_C, conduction angle and waveform. The rms limit, on-state loss and maximum junction temperature must also be satisfied.

Maximum Surge Current (I_SM)

Faults and inrush events can produce current above the repetitive rating. The manufacturer therefore specifies a non-repetitive surge peak on-state current for defined initial temperature, waveform and duration. Repetition or a longer event reduces the permissible value, and recovery between surges also matters. Use the selected device’s table or curve rather than generic examples.
I_SM must be checked for the stated half-cycle duration.
I_SM must also be checked for any multi-cycle event.
I_SM does not replace the repetitive current and thermal limits.
A plot of I_SM against duration or cycle count may be provided for surge coordination.

I²t Rating of SCR

The I²t rating describes the short-duration current-squared time withstand associated with a specified surge waveform. Its unit is A²s. For basic coordination, the pre-arcing I²t of an electrical fuse should be below the SCR’s I²t capability under compatible conditions. Complete protection design must also check prospective fault current, clearing time, arc energy, voltage and temperature.

di/dt Rating of SCR

When an SCR starts to turn on, conduction begins near the gate and spreads across the junction area. If anode current rises faster than the conducting area formed by the charge carriers, the high local current density represented by can create a hot spot and damage the device. The manufacturer therefore specifies a maximum turn-on rate of rise of current, , with gate-drive and test conditions.

Latching Current of Thyristor

Latching current is the minimum anode current required to keep the SCR on immediately after the gate signal is removed, under stated conditions. The gate pulse should remain present until anode current has risen above the maximum specified latching current with suitable margin.

Holding Current of Thyristor

Holding current is the minimum anode current required to maintain the on state after the SCR has fully latched. To turn off a conventional SCR, the external circuit must reduce anode current below this value and provide the specified recovery conditions.

Gate Specification of SCR

Gate Current to Trigger (I_GT)

IGT is the maximum gate current required to trigger the device under the stated anode voltage, load, pulse and junction-temperature conditions. The driver must supply adequate current without exceeding the gate’s peak current, voltage or power limits.

Gate Triggering Voltage (V_GT)

VGT is the maximum gate-cathode voltage associated with the specified trigger current under stated test conditions. The gate driver must account for both I_GT and VGT across temperature while staying within absolute gate ratings.

Non Triggering Gate Voltage (V_NG)

Modern data sheets may specify a gate non-trigger voltage, often labelled VGD, below which the SCR must remain off under defined anode-voltage, temperature and gate-resistance conditions. Noise immunity also depends on current, impedance, dv/dt and layout, not voltage alone.

Peak Reverse Gate Voltage (V_GRM)

This is the maximum reverse voltage permitted across the gate-cathode junction. Exceeding it can damage the gate, even if average gate power remains low.

Average Gate Power Dissipation (P_GAR)

This is the maximum average power that the gate junction may dissipate under the data-sheet conditions. Peak gate current, peak gate voltage, pulse width, duty cycle and temperature must also remain within their limits. There is no universal 100 microsecond boundary.

Peak Forwarded Gate Current (I_GRM)

This is the maximum permitted forward gate-current pulse under its stated duration and temperature conditions. The gate driver must not exceed it.