IFB270 Advanced Electronic Circuits Chapter 11: Thyristors Prof. Manar Mohaisen Department of EEC Engineering
Review of the Precedent Lecture To introduce several concepts on capacitance in amplifiers To introduce the Miller s Theorem To discover the decibel as a unit to measure power or gain To investigate the low-frequency amplifier response To investigate the high-frequency amplifier response To analyze the frequency response of the multistage amplifier Keywords 2
Introduce the four layer diode Lecture Objectives Introduce the silicon-controlled controlled rectifier (SCR) Introduce the light-activated SCR (LASCR) Introduce several applications of the SCR/LASCR Introduce the diac and triac Introduce the Unijunction Transistor (UJT) and Programmable UT 3
The Four Layer Diode Structure of the 4-layer diode (a.k.a. Shockly diode and SUS) The pnpn structure can be seen as a pnp transistor and an npn transistor The pnp transistor (Q 1 ) consists of layers 1 ~ 3 The npn transistor (Q 2 ) consists of layers 2 ~ 4 When the Anode (A) is positively biased with respect to the Cathode (K) The emitter-base (Q 1 ) and the base-emitter emitter (Q 2 ) junctions become forward-biased The base-collector (common for both Q 1 and Q 2 ) becomes reverse-biased 4
Characteristic curve Two regions The Four Layer Diode contd. Forward-blocking region: The diode is in the off state (high input resistance, an open circuit) The anode current is less than the switching current (I S ) Forward-conduction region: The diode is in the on state (a short circuit) The current is larger than the holding current (I H ) 5
The Four Layer Diode contd. Characteristic curve contd. Two critical currents Switching current: The current at which the diode switches from the forward-blocking region (off state) to the forward-conduction region (on state) Holding current: The anode current below which the diode switches from the forwardconduction region to the forward-blocking region 6
The Four Layer Diode contd. Characteristic curve contd. The forward-breakover voltage (V BR(F) ) For V AK varies from 0toV BR(F), the anode current gradually increases After V AK = V BR(F), V AK drops suddenly, The anode current increases (higher than I H ), and the 4-layer enters the forward-conduction region 7
Example 11-1 The Four Layer Diode contd. V AK = 20 V, in the forward-blocking region I A = 1 ua, Find the diode resistance R AK V 20 V 20M = AK I = A 1μA = Ω Example 11-2 The 4-layer diode is on (forward-conduction region) V AK dropped to 0.9 V Find I A I A VR = I = S = 20V 0.9V = R 19.1mA S R 1kΩ S 8
An application The Four Layer Diode contd. When switch is closed, the capacitor charges through R up to V BR(F) The diode switches to the on state (forward-conduction region; small resistance) The capacitor rapidly discharges through the diode The diode enters the forward-blocking region when I A < I H (holding current) This operation is repeated leading the shown voltage across the capacitor 9
Structure The Silicon-controlled Rectifier (SCR) 10
The SCR contd. SCR equivalent circuit Similar to the 4-layer diode with the exception that it has the gate terminal 11
Turning the SCR on V G = 0 The SCR contd. I G = 0 and Q 2 is off, therefore Q 1 is off and I A = 0 Triggering the gate: V G 0 (I G 0) I B2 turns Q 2 on, making a path for I B1, thus turning Q 1 on Even if the trigger is removed, Q 2 will remain on, due I B2 Note that V AK must be positive! 12
Turning the SCR on contd. The SCR contd. Without I G, the SCR can be switched on/off by controlling V AK Same operation of the 4-layer diode However, as I G increases, the V AK required to turn the SCR on decreases Also, high values of V AK will not damage the device if the current is controlled Nonetheless, this situation must be avoided to keep control of the SCR 13
The SCR contd. Turning the SCR off Even though the trigger is removed, the SCR remains in the on state The anode current must drop below I H to in order to turn off the SCR There are two methods to turn the SCR off Anode current interruption: This can be done by either a momentary series interruption (open circuits) or parallel switching arrangement (short circuit) Forced commutation: An external circuit is used to generate a current opposite to the anode current in direction where the summation is below the holding current anode current interruption forced commutation 14
Operation The Light-activated SCR (LASCR) Similar to the SCR with the exception that it can be triggered by light The LASCR is most sensitive to light when the gate terminal is open Therefore, if necessary a resistor is placed from the gate to the cathode to reduce the sensitivity Circuit example 15
SCR Applications Application I: On-off control of current Closing SW1 triggers the SCR and it switches to the on state SW1 can be opened but the SCR will remain in the on state The load will still have a high current Closing SW2 switches the SCR to the off state The SCR is short circuited The anode current is reduced below the holding current and the SCR enters the off state 16
SCR Applications contd. Application II: Half-wave power control This application is used in ac power control for Lamp dimmers, electric heaters, electric motors, R 2 is used for triggering the SCR via the diode The triggering can be at any instant of the positive half cycle 17
SCR Applications contd. Application II: Half-wave power control contd. 18
SCR Applications contd. Show the voltage waveform across the SCR In relation to the load current for 180, 45, and 90 degrees conditions. 19
Sawtooth generator When the switch is closed SCR Applications contd. The capacitor charges until triggering the SCR Then, the capacitor rapidly discharges via the SCR This is repeated to generate the sawtooth signal shown below 20
The diac The Diac and Triac The top and bottom layers contain both n and p materials The right side of the stack is a pnpn structure with same characteristics of the 4-layer diode The left side is an inverted 4-layer diode with an npnp structure The diac, therefore, conducts the current in both directions 21
The Diac and Triac contd. The diac (equivalent circuit) Modeled as four transistors (a) The bias in (b) makes Q 1 and Q 2 forward-biased, Q 3 and Q 4 reverse biased The pnpn structure is in operation The bias in (c) makes Q 1 and Q 2 reverse-biased, Q 3 and Q 4 forward-biased The npnp structure is in operation 22
The Triac The Diac and Triac contd. The triac can be simply considered as two SCRs connected in parallel Therefore, it does not require a breakover voltage to turn it on The gate can trigger the triac to switch to the on state Then, the voltage across the triac defines the direction of the current flow 23
The Diac and Triac contd. Triac characteristic curves Note that the breakover voltage decreases as the gate current increases Same as the SCR 24
Triac s bilateral operation The Diac and Triac contd. 25
Applications The Diac and Triac contd. Triac as a control device (average power control to a load) 26
The Diac and Triac contd. Applications contd. D 1 conducts in the positive half-cycle, R 1 sets the trigger point D 2 conducts in the negative half-cycle, R 1 sets the trigger point 27
Structure of the SCS The Silicon-controlled Switch Same as that of the SCR with the exception that it has two gates; the cathode gate and the anode gate Therefore, SCS is a four-terminal thyristor The two gates are used to trigger the SCS on and off Unlike the SCR, where the gain is only used for triggering g it on 28
The Silicon-controlled Switch contd. Operation of the SCS (the on state) The SCS can be turned on using either gate terminal A positive pulse on the cathode gate turns the SCS on Turns Q 2 on, which provides a path for the base current of Q 1 and turn it on A negative pulse on the anode gate turns the SCS on Turns Q 1 on, which provides base current of Q 2 and turn it on 29
The Silicon-controlled Switch contd. Operation of the SCS (the off state) The SCS can be turned off using either gate terminal A negative pulse on the cathode gate turns the SCS off Turns Q 2 off, which blocks the base current of Q 1 which turns off as a result A positive pulse on the anode gate turns the SCS off Turns Q 1 off, which blocks the base current of Q 2 which turns off as a result 30
The Silicon-controlled Switch contd. Operation of the SCS (the off state) In this method, the BJT is used to reduce the anode current below the holding current. (a) series interruption, when the BJT is off, the anode current = 0 and the SCS is turned off (b) parallel interruption, when the BJT is on, the anode current = 0 and the SCS is turned off 31
The Unijunction Transistor Structure of the UJT It has three terminals; Emitter, Base 1, and Base 2 Note that that UJT has different characteristics due to its special structure Only one pn junction unlike the BJT and the FET 32
The Unijunction Transistor contd. Equivalent circuit of the UJT A diode that represents the junction, two resistances; r B1 and r B2 r B2 varies inversely with I E The interbase resistance r = r + r Voltage divider ' ' ' BB B1 B2 V r' B1 = Standoff ratio r η = r ' B1 ' BB r r ' B1 ' BB V BB The point at which the junction is forward-biased V EB1 must satisfy V = V +η V EB1 pn BB The equality point is referred to as the peak-point voltage 33
The Unijunction Transistor contd. The characteristic curve of the UJT Fixed V BB 34
The Programmable Unijunction Transistor Structure of the PUT It is more similar to an SCR than to a UJT As a difference with the SCR, the anode-to-gate voltage can be used to turn on and turn off the device 35
Introduced the four layer diode Lecture Summary Introduced the silicon-controlled controlled rectifier (SCR) Introduced the light-activated SCR (LASCR) Introduced several applications of the SCR/LASCR Introduced the diac and triac Introduced the Unijunction Transistor (UJT) and Programmable UT 36