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Chapter 20

Chapter 20. Thyristors and Optoelectronic Devices. Silicon Unilateral Switch (SUS). Silicon unilateral switch – A two-terminal, four-layer device that can be triggered into conduction by applying a specified forward voltage across its terminals.

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Chapter 20

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  1. Chapter 20 Thyristors and Optoelectronic Devices

  2. Silicon Unilateral Switch (SUS) • Silicon unilateral switch – A two-terminal, four-layer device that can be triggered into conduction by applying a specified forward voltage across its terminals. • The SUS has two p-type and two n-type materials. • Also referred to as a pnpn diode or a four-layer diode.

  3. SUS Equivalent Circuit • The SUS is effectively made up of two transistors: one pnp and one npn. • The collector of each is tied to the base of the other.

  4. Forward Breakover Voltage • Forward breakover voltage, VBR(F) – The value of forward voltage that forces an SUS into conduction. • Once conduction begins: • The device is rapidly driven into saturation. • The value of VAK decreases.

  5. Anode Current Interruption • Holding current – The minimum value of IF required to maintain SUS conduction. • Anode current interruption – A method of driving an SUS into cutoff by breaking the diode current path or shorting the circuit current around the diode.

  6. Forced Commutation • Forced commutation – Driving an SUS into cutoff by applying a reverse voltage to the device. • The SUS turns off when IF drops below the value of IH.

  7. SUS Operating Curve • Forward blocking region – The forward off-state (nonconducting) region of operation. • Forward operating region – The forward on-state (conducting) region of operation.

  8. SUS Specifications • Forward breakover current – The value of IF at the point where breakover occurs. • Average on-state current (IT) – The maximum average (dc) forward current. • Average on-state voltage (VT) – The value of VF when IF = IT.

  9. Temperature and SUS Triggering • As temperature increases, the SUS breaks over into conduction at lower values of VBR(F).

  10. Silicon-Controlled Rectifiers (SCRs) • Silicon-controlled rectifier (SCR) – A three-terminal device very similar in construction and operation to the SUS. • The third terminal, called the gate, provides an additional method for triggering the device.

  11. SCR Triggering • A positive gate pulse triggers the SCR into conduction. • Once conduction begins, the gate input signal has no effect on the device. • The SCR is driven into cutoff by anode current interruption or forced commutation.

  12. SCR Operating Curve

  13. Circuit Fusing (I2t) Rating • Circuit fusing (I2t) – A rating that indicates the maximum forward surge current capability of an SCR.

  14. Critical Rise (dv/dt) Rating • False Triggering – When a noise signal triggers an SCR into conduction. • Critical Rise (dv/dt) – The maximum rate of increase in VAK without causing false triggering.

  15. Preventing False Triggering • False triggering can be caused by a noise signal at the gate terminal. • Problems with gate noise can be eliminated using a gate biasing voltage or a gate bypass capacitor.

  16. Snubber Networks • False triggering can be caused by a noise in VAK. • Snubber network – An RC circuit that is connected between the SCR anode and cathode to eliminate false triggering.

  17. SCR Crowbar • Crowbar – A circuit used to protect a voltage-sensitive load from excessive dc power supply output voltages. • Once conducting, the SCR is turned off by a blown primary fuse in the dc power supply (an example of anode current interruption).

  18. SCR Phase Controller • Phase controller – A circuit used to control the conduction angle through a load, and thus, average load voltage. • Conduction angle – The portion of the input waveform that is coupled to the load. • Firing angle – The point on the input waveform at which the SCR triggers.

  19. Diacs • Diac – A two-terminal, three-layer device with forward and reverse characteristics that are identical to the forward characteristics of the SUS.

  20. Diac Operating Curves

  21. Triacs • Triac – A bidirectional thyristor whose forward and reverse characteristics are identical to the forward characteristics of the SCR. • Also referred to as triodes and bidirectional triode thyristors.

  22. Triac Construction • The primary conducting terminals are referred to as main terminal 1 (MT1) and main terminal 2 (MT2). • The triac is essentially complementary SCRs connected in parallel.

  23. Triac Operating Curves

  24. Triac Triggering

  25. Controlling Triac Triggering

  26. Triac Phase Controller

  27. Unijunction Transistors (UJTs) • Unijunction transistor (UJT) – A three-terminal device whose trigger voltage is proportional to its applied biasing voltage. • Peak voltage (VP) – The value of VEB1 that triggers the UJT into conduction.

  28. UJT Construction • Interbase resistance – The total resistance between the base terminals. • Intrinsic standoff ratio (h) – The ratio of emitter-base 1 resistance (RB1) to interbase resistance.

  29. UJT Operating Curve • Negative resistance region – The region of operation between the peak and valley points on the UJT curve. • Negative resistance – A term used to describe any device with current and voltage values that are inversely related.

  30. UJT Relaxation Oscillator • Relaxation oscillator – A circuit that uses the charge/discharge characteristics of a capacitor or inductor to produce a pulse output.

  31. Programmable UJT (PUT)

  32. Light Emitters and Detectors • Light emitter – Optoelectronic devices that produce light. • The LED is an example of a light emitter. • Light detector – Optoelectronic devices that respond to light.

  33. Light • Light – Electromagnetic energy that falls within a specific range of frequencies.

  34. Wavelength • Wavelength (l) – The physical length of one cycle of a transmitted electromagnetic wave.

  35. Light Intensity • Light intensity – The amount of light per unit area received by a given photodetector. Also called irradiance. • Light intensity decreases as the distance between the light emitter and detector increases

  36. Photodiodes • Photodiode – A diode whose reverse conduction is light-intensity controlled. • Light current (IL) – The reverse current with an active light input present. • Dark current (ID) – The reverse current with no active light input present.

  37. Photodiode Ratings • Wavelength of peak spectral response (lS) – The wavelength that causes the strongest response in a photodetector. • Sensitivity – The response of a photodetector to a specified light intensity, measured in mA/mW/cm2. • Spectral response – A measure of a photodetector’s response to a chance in input wavelength.

  38. Photodiode Curves

  39. Optocoupled Amplifier • Phototransistor – A three-terminal photodetector whose collector current is controlled by the intensity of the light at its optical input (base).

  40. Solid-State Relay

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