Thyristor (SCR) Cheatsheet

Cheatsheet Content

1. Introduction to Thyristor (SCR) Definition: A Thyristor, or Silicon Controlled Rectifier (SCR), is a four-layer, three-junction, three-terminal semiconductor device that acts as a bistable switch, conducting current only after being triggered by a gate pulse and remaining ON until the anode current falls below a certain level. Terminals: Anode (A), Cathode (K), Gate (G). Layers: p-n-p-n structure. Analogy: Can be thought of as two transistors (one NPN, one PNP) regeneratively coupled. 2. Thyristor Structure & Symbol Structure (Simplified): P N P N Anode (A) Cathode (K) Gate (G) Circuit Symbol: A K G 3. Operating Modes & V-I Characteristics A thyristor has three operating modes: Reverse Blocking Mode: Anode negative w.r.t. cathode. Junctions $J_1$ and $J_3$ are reverse biased, $J_2$ forward biased. Only a small leakage current flows. If reverse voltage exceeds Reverse Breakdown Voltage ($V_{BR}$), current increases rapidly, potentially damaging the SCR. Forward Blocking Mode (OFF State): Anode positive w.r.t. cathode, gate open or reverse biased. Junctions $J_1$ and $J_3$ are forward biased, $J_2$ reverse biased. Only a small leakage current (forward blocking current) flows. SCR is in OFF state. Forward Conduction Mode (ON State): Triggered by a gate pulse while in forward blocking mode, or by exceeding Forward Breakover Voltage ($V_{BO}$). SCR conducts heavily with a small forward voltage drop ($\approx 1-2V$). V-I Characteristics: V I I Reverse Blocking $V_{BR}$ Forward Blocking $V_{BO}$ Forward Conduction (ON) $I_{G1}$ $I_{G2} > I_{G1}$ $I_H$ $I_L$ $V_{BO}$ (Forward Breakover Voltage): The anode-to-cathode voltage at which the SCR turns ON without a gate signal. $V_{BR}$ (Reverse Breakover Voltage): The reverse voltage at which the SCR breaks down and conducts in the reverse direction. $I_L$ (Latching Current): The minimum anode current required to maintain conduction immediately after turn-on and removal of the gate signal. ($I_L > I_H$) $I_H$ (Holding Current): The minimum anode current required to keep the SCR in the ON state. If anode current falls below $I_H$, the SCR turns OFF. 4. Thyristor Turn-On Methods Gate Triggering (Most Common): Applying a positive voltage pulse to the gate terminal while anode is positive w.r.t. cathode. Forward Voltage Triggering: Increasing anode-to-cathode voltage ($V_{AK}$) beyond $V_{BO}$. (Not recommended, can damage SCR). dv/dt Triggering: A rapid rate of change of anode voltage can turn ON the SCR due to capacitive effects of junction $J_2$. Formula: $I_C = C \frac{dV}{dt}$ where $C$ is junction capacitance. Protection: RC snubber circuit. Temperature Triggering: Increased temperature increases leakage current, reducing $V_{BO}$. Light Triggering (LASCR): Light falling on the junction increases electron-hole pairs, turning ON the SCR. 5. Thyristor Turn-Off Methods (Commutation) Once ON, the gate loses control. SCR turns OFF only when anode current falls below holding current $I_H$. Natural Commutation (AC Circuits): In AC circuits, the anode current naturally drops to zero during the negative half-cycle, turning OFF the SCR. Forced Commutation (DC Circuits): External circuitry is used to reduce the anode current below $I_H$ or reverse bias the SCR. Class A (Self Commutation): Series resonant circuit. Class B (Self Commutation): Parallel resonant circuit. Class C (Complementary Commutation): Uses another SCR to turn off the main SCR. Class D (Impulse Commutation): A charged capacitor is switched across the SCR to momentarily reverse bias it. Class E (External Pulse Commutation): An external pulse source forces current reversal. Class F (Line Commutation): Similar to natural commutation, but for DC systems connected to an AC grid. 6. Ratings & Parameters $V_{DRM}$ (Peak Forward Blocking Voltage): Max instantaneous anode-cathode voltage in OFF state. $V_{RRM}$ (Peak Reverse Blocking Voltage): Max instantaneous reverse voltage. $I_{T(AV)}$ (Average ON-state Current): Max average current in ON state. $I_{TSM}$ (Surge ON-state Current): Max non-repetitive peak current for short duration. $I_L$ (Latching Current): Minimum anode current to sustain conduction after gate signal removal. $I_H$ (Holding Current): Minimum anode current to maintain conduction. $I_{GT}$ (Gate Trigger Current): Minimum gate current required to turn ON the SCR. $V_{GT}$ (Gate Trigger Voltage): Minimum gate voltage required to turn ON the SCR. $t_{ON}$ (Turn-on Time): Time from gate pulse application to full conduction. $t_{OFF}$ (Turn-off Time): Time from anode current falling below $I_H$ to the device regaining its forward blocking capability. Crucial for high-frequency operation. $dv/dt$ Rating: Maximum rate of rise of anode voltage that will not cause false turn-on. $di/dt$ Rating: Maximum rate of rise of anode current that the SCR can withstand without damage during turn-on. Protection: Series inductor (choke). 7. Two-Transistor Analogy A thyristor can be represented as a feedback pair of a PNP and an NPN transistor. A $B_1$ $C_1$ Q1 (PNP) K $B_2$ $C_2$ Q2 (NPN) G $I_A = I_{C1} + I_{C2}$ $I_A = \alpha_1 I_A + I_{CEO1} + \alpha_2 I_A + I_{CEO2} + I_G$ $I_A = \frac{\alpha_1 I_{CEO1} + \alpha_2 I_{CEO2} + I_G}{1 - (\alpha_1 + \alpha_2)}$ Turn-on occurs when $(\alpha_1 + \alpha_2) \rightarrow 1$. This positive feedback drives both transistors into saturation. 8. Applications AC/DC power control (e.g., light dimmers, motor speed control). Solid-state relays. Crowbar circuits (overvoltage protection). Battery chargers. Inverters and choppers (though often replaced by MOSFETs/IGBTs for faster switching). Ignition circuits. 9. Key Considerations Snubber Circuits: RC circuits connected in parallel with the SCR to protect against $dv/dt$ triggering during turn-off and voltage spikes. Gate Protection: Resistors in series with the gate limit current, and diodes can protect against reverse gate voltage. Heat Sinks: Essential for power SCRs due to power dissipation ($P_D = V_{ON} \times I_{ON}$). Turn-off Time ($t_{OFF}$): Limits the maximum operating frequency. Fast SCRs are required for high-frequency applications.