- SCR Definition: An SCR (silicon-controlled rectifier) is defined as a type of thyristor used for controlling power in circuits.
- Gate Characteristics of SCR: The gate characteristics of SCR include the safe limits for gate voltage and current, which are crucial for proper operation.
- Maximum and Minimum Limits: Each SCR has specified maximum and minimum limits for gate voltage and current to ensure safe operation.
- Non-Triggering Voltage (Vng): Vng is the voltage below which noise and unwanted signals should remain to prevent accidental activation of the SCR.
- Load Line and Operating Point: The operating point, determined by the load line, should be close to the maximum average gate power dissipation (Pgav) to minimize turn-on time and avoid accidental activation.
The gate characteristic of a thyristor or SCR relates gate-to-cathode voltage to gate current. It helps a designer choose a trigger pulse that turns the thyristor on throughout its temperature range without overstressing the gate. The device data define maximum gate voltage Vg-max, maximum gate current Ig-max and average gate power Pgav. The graph also shows trigger boundaries, labelled here as Vg-min and Ig-min. In an actual design, use the datasheet’s guaranteed gate-trigger voltage and current under the stated anode voltage and temperature.
The gate non-triggering voltage Vng, often specified as VGD in a datasheet, is the highest gate-cathode voltage that should not trigger the SCR under the stated worst-case conditions.
Residual and interference signals must remain below that non-trigger limit, with a suitable margin, to prevent unintended turn-on.
Curve 1 marks the trigger boundary in this diagram, while curve 2 marks the maximum safe gate voltage. The labelled region bcdefghb represents the permitted trigger area for the stated conditions. Real datasheet curves vary with junction temperature and pulse duration, so the driver must stay inside the applicable boundary.
For the simple resistive trigger circuit,
where
Es is the gate-source supply voltage
Vg is gate-to-cathode voltage
Ig is gate current
Rs is the gate-source resistance
The trigger-source load line is AD. OA equals Es, while OD equals Es/Rs, the prospective short-circuit current of the trigger circuit. Curve 3 is the gate input characteristic, and its intersection with AD gives operating point S. The point must lie between S1 and S2 for every relevant temperature and tolerance. A stronger pulse can reduce turn-on delay, but the point does not need to sit near Pgav. Instead, meet the guaranteed trigger requirement and keep peak and average gate power within their pulse-duration limits. The magnitude of the AD slope is related to source resistance Rs. Choose Rs from the driver voltage, required gate current and permitted Pgav, then verify the result across component tolerances.
