Power Electronics: Power Semiconductor Devices, Commutation, and Controlled Rectifiers

Flashcards covering power semiconductor devices (BJT, SCR, TRIAC, DIAC), triggering methods, commutation classes (Class A to F), dynamic switching characteristics, circuit protections, and controlled rectifier equations based on lecture notes.

Circuit configuration of an RC gate triggering circuit for an SCR

An AC voltage source connected to the load and thyristor, with a parallel gate-control network consisting of a variable resistor $R$, a capacitor $C$, and two diodes: diode $D_1$ in series with the gate terminal and diode $D_2$ connected across the gate-cathode junction.

Conduction path and operating mechanism of an RC gate triggering circuit

During the positive half-cycle, capacitor $C$ charges through the variable resistor $R$ until the capacitor voltage reaches the threshold required to inject gate current through diode $D_1$, triggering the SCR. During the negative half-cycle, diode $D_2$ charges capacitor $C$ negatively to protect the gate-cathode junction from excessive reverse breakdown, while $D_1$ prevents reverse current flow into the gate.

Circuit configuration of Class A (Self / Resonant / Load) commutation

A DC source ($V_{dc}$) connected to an underdamped $RLC$ circuit where the commutating elements—inductor $L$ and capacitor $C$—are connected either in series with load resistance $R$ or with load $R$ connected in parallel across capacitor $C$.

Conduction path and operating mechanism of Class A commutation

When the SCR is triggered, forward current flows through the resonant path $V_{dc}^+ \rightarrow \text{SCR} \rightarrow L \rightarrow C \rightarrow R \rightarrow V_{dc}^-$. Due to the underdamped resonant response, capacitor $C$ charges to a voltage exceeding the supply voltage, naturally driving the oscillatory current to zero and reverse-biasing the SCR to turn it off.

Circuit configuration of Class B (Resonant-Pulse / Current) commutation

A DC supply voltage connected in series with load resistor $R$ and the main SCR, while a separate resonant commutating $LC$ branch is connected directly in parallel across the main SCR.

Conduction path and operating mechanism of Class B commutation

The capacitor initially charges to $V_{dc}$ with upper plate positive. Triggering the SCR establishes the load current path $V_{dc}^+ \rightarrow R \rightarrow \text{SCR} \rightarrow V_{dc}^-$ while discharging the capacitor through the resonant path $C^+ \rightarrow L \rightarrow \text{SCR} \rightarrow C^-$. Once the resonant current reverses polarity, the discharge current opposes and exceeds the forward load current ($I_c > I_L$), forcing the SCR anode current below holding current to turn it off.

Circuit configuration of Class C (Complementary) commutation

A DC voltage source supplying two parallel branches: the main thyristor ($SCR_1$) in series with load resistor $R_1$, and a complementary thyristor ($SCR_2$) in series with resistor $R_2$, with a commutating capacitor $C$ connected across the anodes of both thyristors.

Conduction path and operating mechanism of Class C commutation

Triggering $SCR_1$ completes the load path $E^+ \rightarrow R_1 \rightarrow SCR_1 \rightarrow E^-$ and charges capacitor $C$ via path $E^+ \rightarrow R_2 \rightarrow C^+ \rightarrow C^- \rightarrow SCR_1 \rightarrow E^-$. When $SCR_2$ is triggered, the positive terminal of $C$ is grounded through $SCR_2$, applying a negative transient voltage across $SCR_1$ to commutate it off while $SCR_2$ conducts.

Circuit configuration of Class D (Auxiliary) commutation

A power circuit with a main thyristor ($SCR_1$) and load resistor $R$ across a DC source, accompanied by an auxiliary commutation circuit comprising an auxiliary thyristor ($SCR_2$), a commutating capacitor $C$, an inductor $L$, and an antiparallel discharge diode $D$.

Conduction path and operating mechanism of Class D commutation

Auxiliary $SCR_2$ is fired first to charge capacitor $C$ via path $E^+ \rightarrow C^+ \rightarrow C^- \rightarrow SCR_2 \rightarrow R \rightarrow E^-$. Main $SCR_1$ is then triggered to power the load and reverse the capacitor charge via the resonant loop $C^+ \rightarrow SCR_1 \rightarrow L \rightarrow D \rightarrow C^-$. Re-triggering $SCR_2$ discharges $C$ through $C^+ \rightarrow SCR_2 \rightarrow SCR_1 \rightarrow C^-$, opposing the forward load current to force $SCR_1$ off.

Circuit configuration of Class E (External Pulse) commutation

A main DC circuit with a thyristor and load connected to a DC supply, interfaced with an external pulse generator that supplies a pulse transformer whose secondary winding is connected across the main SCR via a commutating capacitor $C$.

Conduction path and operating mechanism of Class E commutation

While the main SCR carries forward load current, a high-frequency turn-off pulse is applied to the primary of the pulse transformer. The induced voltage across the secondary winding applies a reverse-bias voltage across the SCR via capacitor $C$, driving the thyristor anode current below holding current for a duration exceeding turn-off time ($t_q$).

Circuit configuration of an SCR snubber circuit for dynamic protection

A protective network consisting of a series damping resistor $R_s$ and capacitor $C_s$ connected directly in parallel across the thyristor, alongside a small inductor $L_s$ connected in series with the thyristor.

Conduction path and operating mechanism of an SCR snubber circuit

When an abrupt voltage transient occurs, uncharged capacitor $C_s$ behaves as a temporary short, diverting transient current away from the SCR via path $V_s^+ \rightarrow R_s \rightarrow C_s \rightarrow \text{Load}$ to suppress $\frac{dv}{dt}$. Inductor $L_s$ limits the rate of anode current rise ($\frac{di}{dt}$), while $R_s$ dissipates stored energy and limits discharge current when the SCR turns on.

Circuit configuration of a single-phase half-wave controlled rectifier with resistive load

An alternating sinusoidal voltage source ($v_s = V_m \sin(\omega t)$) connected in series with a single thyristor (SCR) and a purely resistive load $R$.

Conduction path and operating mechanism of a single-phase half-wave controlled rectifier with resistive load

During the positive half-cycle ($0 \le \omega t \le \pi$), the thyristor is forward biased and triggered at delay angle $\alpha$, conducting load current along $v_s^+ \rightarrow \text{SCR} \rightarrow R \rightarrow v_s^-$. At $\omega t = \pi$, the source voltage falls to zero, bringing the anode current to zero and naturally turning off the SCR due to natural line commutation throughout the negative half-cycle.

Circuit configuration of a single-phase half-wave controlled rectifier with an R-L load

A sinusoidal AC voltage source ($v_s = V_m \sin(\omega t)$) connected in series with a single Silicon Controlled Rectifier and a series combination of a load resistor $R$ and inductor $L$.

Conduction path and operating mechanism of a single-phase half-wave controlled rectifier with an R-L load

When triggered at firing angle $\alpha$, current flows along path $v_s^+ \rightarrow \text{SCR} \rightarrow R \rightarrow L \rightarrow v_s^-$, storing energy in inductor $L$. When the AC source reverses at $\omega t = \pi$, the collapsing magnetic field in inductor $L$ induces a positive voltage ($L \frac{di}{dt}$) that maintains forward current conduction until extinction angle $\beta$ ($\beta > \pi$), where current drops to zero and commutates the SCR off.