Thyristors and SCRs

Power Semiconductor Devices & Commutation Circuits

• Key components include thyristors, Silicon Controlled Rectifiers (SCRs), Power MOSFETs, and Power IGBTs.
• Focus areas are static and dynamic characteristics, turn on/off methods, and commutation circuits.

Thyristors and Silicon Controlled Rectifiers (SCRs)

• SCRs are four-layer solid-state current-controlling devices.
• General Electric's trade name for a type of thyristor is "silicon controlled rectifier".
• Used mainly in electronic devices needing high voltage and power control, especially in AC power operations like motor control.
• Conduct when a gate pulse is applied, similar to a diode.
• Composed of four semiconductor layers (NPNP or PNPN), three junctions (J1, J2, J3), and three terminals (anode, cathode, gate).
• Anode connects to the P-type layer, cathode to the N-type layer, and the gate to the P-type layer.
• The intrinsic semiconductor material is silicon, doped according to the application.

SCR Modes of Operation

• OFF State (Forward Blocking Mode): Anode is positive, gate is at zero voltage, and cathode is negative; J1 and J3 are forward biased, J2 is reverse biased, high resistance and off state.
• ON State (Conducting Mode): Achieved by raising anode-cathode voltage above avalanche level or by applying a positive gate signal; gate voltage is not needed to maintain the ON state.
• SCR is switched off by reducing current flow to the holding current level or by using a transistor across the junction.
• Reverse Blocking: Compensates for forward voltage drop; forward and reverse voltage ratings are equal.

Thyristor Characteristics

• Four-layer, three-junction p-n-p-n semiconductor device for high-power electrical switching.
• It has three basic terminals: anode, cathode, and gate.

V-I Characteristics of a Thyristor

• Anode and cathode connect to the supply voltage through the load, with a secondary supply (Es) applied between gate and cathode.
• Three basic modes: reverse blocking, forward blocking (off-state), and forward conduction (on-state).

Reverse Blocking Mode

• Cathode is positive relative to the anode.
• Junctions J1 and J3 are reverse biased, while J2 is forward biased.
• Behaves like two diodes in series with a small leakage current.
• If reverse voltage increases to the critical breakdown voltage (VBR), avalanche occurs at J1 and J3, rapidly increasing reverse current.
• Working reverse voltage should not exceed VBR to prevent overheating and damage. -It offers high impedance and acts as an open circuit when reverse voltage is less than VBR.

Forward Blocking Mode

• Anode is positive relative to the cathode, with the gate open.
• Junctions J1 and J3 are forward biased, with J2 in reverse bias.
• A small forward leakage current initially flows.
• The thyristor conducts with a small voltage drop when it is triggered from forward blocking mode to forward conduction mode by exceeding forward break over voltage or by a gate pulse.
• Operates as an open switch.

Forward Conduction Mode

• With the gate circuit open, increasing anode-cathode forward voltage causes an avalanche breakdown at junction J2 at forward breakover voltage VBO, turning the thyristor on.
• The point shifts from M to N, indicating the forward conduction mode, with maximum current and minimum voltage drop.

Two Transistor Analogy of SCR

• The SCR can be understood as a combination of PNP and NPN transistors.

Two-Transistor Model

• Describes how the SCR operates based on the interaction of two transistors.

SCR Turn-On Methods

• Triggering is turning on the SCR, shifting it from forward-blocking to forward-conduction state.
  • Forward Voltage Triggering, Thermal Triggering, Light Triggering, dv/dt Triggering, Gate Triggering

Forward Voltage Triggering

• An additional forward voltage is applied between anode and cathode.
• Junctions J1 and J3 are forward biased, while junction J2 is reverse biased when the anode terminal is positive relative to the cathode.
• No current flows except for a small leakage current.

Thermal (or) Temperature Triggering

• The depletion layer width in SCR reduces as the junction temperature increases.
• The device triggers when VAR is close to its breakdown voltage due to increased junction temperature.
• The reverse-biased junction collapses and the device begins conducting as the junction temperature rises.

Radiation Triggering (or) Light Triggering

• For light-triggered SCRs, an internal niche is made inside the inner P layer instead of the gate terminal.
• Free charge carriers are generated when light strikes this terminal.
• The thyristor conducts when the light intensity exceeds a normal value.
• These SCRs are known as LASCR.

dv/dt Triggering

• J1 and J3 are forward biased, and J2 is reverse biased when the device is forward-biased.
• Junction J2 acts as a capacitor because of the charges present.
• The rate of voltage change across the device affects its conduction.
• The device turns ON if the rate of voltage change is significant, even with a low voltage.

Gate Triggering

• The most used method of SCR triggering.
• A forward polarized thyristor turns ON when a positive voltage is applied between the gate and cathode.
• Injecting charge carriers into the inner P-layer reduces the depletion layer's thickness when a positive voltage is applied at the gate.
• The increase in carrier injection reduces the voltage at which forward break-over happens.

Gate Triggering Signals

• Three signal types include DC, AC.

DC Gate Triggering

• A DC voltage with correct polarity is applied between gate and cathode.
• It starts conducting when the applied voltage is sufficient to create the required gate current.
• Drawbacks: There is no isolation between the DC power and control circuits, continuous DC signal application leads to high gate power loss.

AC Gate Triggering

• AC source is utilized for gate signals.
• It offers sufficient isolation between the control and power circuits.
• Drawback: a separate transformer must step down the AC supply.

AC Voltage Triggering

• Has R (resistance) triggering
• RC (resistance-capacitance) triggering

(i) R Triggering

• The gate current is managed using a variable resistance.
• The SCR starts to conduct when the gate current reaches the magnitude sufficient value.
• Called a blocking diode, the diode D stops the gate cathode junction from being harmed during the negative half cycle.
• Assuming that the gate circuit is purely resistive, the gate current and applied voltage are in phase.
• With this method, a maximum firing angle of up to 90° is achievable.

(ii) RC Triggering

• The capacitor is charged by using the variable resistance in the positive half cycle until it reaches the peak values of the input waveform.
• The capacitor’s charging time is regulated by the variable resistor R.
• Once there is enough voltage, it will enable enough gate current so conduct.
• The capacitor C is charged to its negative peak value during the negative half cycle of the input through the diode D2.
• Diode D1 is utilized to safeguard the gate-cathode junction from any reverse breakdown in the event of a negative cycle.

Pulse Gate Triggering

• Gate drive includes a sequence of high-frequency single pulses that appear periodically.
• Carrier frequency gating is also known as "carrier frequency gating".
• Used for isolation is a pulse transformer.
• Benefits: Gate failures are decreased since continuous signals are not needed.

Advantages of Pulse Train Triggering

• At higher gate current, low gate dissipation may be seen.
• A tiny transformer is used to separate the gate.
• There is a chance for reduced dissipation when there is reversed-biased condition.
• Should the initial trigger pulse not trigger the SCR, then afterwards Latching succeeds in the following SCR.
• Inductive circuits with back EMF and Triggering circuits.

Turn-Off Methods of SCR

• SCR is turned on with proper forward voltages however, it can not be turned off through gate. It has to be brought back.
• Reduction of forward current below holding level allows the forwarding blocking state to brought back coming from the forward conduction current.
• Consists of commutation.

SCR Commutation

• SCR turn off is also called commutation, involves transferring currents from one path to another, reducing forward current to zero to turn off the SCR.
• Conditions to be met include: Reducing the anode or forward current of the SCR to below the holding level, And applying enough reverse voltage to block conduction again.
• Excess charge carriers in different layers must recombine to restore forward blocking of an SCR. Applying a reverse voltage speeds this process.

SCR Turn Off Methods

• Depending on the commutation voltage are categorised into types: 1)forced 2) natural
• DC circuits typically require forced commutation because there is no natural current zero. Forced commutation forces the forward current to zero via an external circuit.

Forced Commutation

• In circuits with DC current, the SCR is turned off using forced commutation.

Class A Commutation

• Also known as: Self/Resonant/Load Commutation
• This commutation uses the load to produce the voltage.
• Needs underdamped R-L-C circuit and DC power for a natural zero voltage.
• Connected with either parallel or series to load resistance which shows SCR, voltage and capacitor voltage.
• Load resistance value is selected they under damped resonant so produce 0 naturally. Current will flow through when this happens, so capacitor value will charge the value of E.
• With capacitor voltage charged, the switch turns device off. Capacitor is now discharged to ensure circuit ready for the next cycle. Frequency depends the mentioned values.

Class B Commutation

• Like class A however the SCR is achieved via the LC components. The LC resonant will connect to circuit however doesn't happen with series load (class A!) therefore L and C don't bring the load current.
• The supply of DC is connected, which means the circuit gets with lower plate, ensuring the supply voltage will have an upper plate. Current going through 2 directions: One will go through resistor and the other will commute by L and C.

Class C Commutation

• The core SCR is connected with serial load and further auxiliary SCR is connected which is in parallel. (also called this is commutation)
• Reverse voltage of capacitor turning the switch OFF.

Class D Commutation

• Using Auxillary SCR allows you switch to capacitor that is charged. Diode D load resistance, inductance L
• SCR will commutate along with the capacitor in it. Main SCR and the resistance result power circuit, and the other two will formulate.

Class E Commutation

• Pulse communication externally that is designed so the reverse voltage can be produced along with switch. Pulse helps to make switch on and off therefore need turns off since the equals to the pulse form helps switch for communication.

Natural Commutation

• The source used will make commutation voltage and is given through AC power. Voltage through the supply goes through and ensures a immediate reverse voltage across the switch. And they are the conditions the SCR has to conform with.
• So its also called either source, line, it happens and is consistent.
• Used a lot in the power of AC such as converters, cyclo, inverters

Dynamic Turn OFF Switching Characteristics

• Going from conduction to the block equals also can be said as to reduce the SCR. SCR once starts means that the gates does not retain a control to allow blocking/OFF

• Current decreases down than just ensures that has blocking capabilities or ensure turned OFF

• This takes for charge carriers to become a thing in the layers.

• So 0 time to instant/anode moment makes it what is also called capabilies switch forward. The extra should have taken and this allows SCR circuit for modes

• This will require two phases, where the former has excessive and removes from the former and next takes it from inside and recombines. However these is broken down, reversed etc and equals as well, where gates equal

• With some time is which how characteristics occur along with the time to current and more builds during current along with current eliminates charge

• At instantaneous J this lets what can the reversed do, there still is more carriers in the junction still. That can be and it could through maintain reversed.

• At the moment it takes and where time has places and then where it is completed. So then it is with a smaller margin, those moments otherwise more errors.

Characteristics slow VS fast turns off

• SCRs' with turns takes 50-100 this ensures those with controlled, and etc
• However the more faster ones that take less help the inverters