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Transcript
EE2301: Basic Electronic Circuit Let’s start with diode EE2301: Block C Unit 1 1 EE2301: Basic Electronic Circuit Examples of Diode EE2301: Block C Unit 1 2 EE2301: Basic Electronic Circuit The Basic Property of a Diode Let’s have a demo EE2301: Block C Unit 1 3 EE2301: Basic Electronic Circuit How does it work? EE2301: Block C Unit 1 4 EE2301: Basic Electronic Circuit Block C Unit 1 Outline Semiconductor materials (eg. silicon) > Intrinsic and extrinsic semiconductors How a p-n junction works (basis of diodes) Large signal models > Ideal diode model > Offset diode model Finding the operating point Application of diodes in rectification EE2301: Block C Unit 1 5 EE2301: Basic Electronic Circuit Electrical Materials Insulators Semiconductor Electronics - Unit 1: Diodes SemiConductors Conductors 6 EE2301: Basic Electronic Circuit Semiconductor Applications Integrated Circuit Semiconductor Electronics - Unit 1: Diodes 7 EE2301: Basic Electronic Circuit Semiconductor Applications TFT (Thin Film Transistor) Semiconductor Electronics - Unit 1: Diodes 8 EE2301: Basic Electronic Circuit Intrinsic Semiconductor Si Si Si Si Covalent Bonds Semiconductor Electronics - Unit 1: Diodes 9 EE2301: Basic Electronic Circuit Silicon Crystal Lattice In 3-D, this looks like: Number atoms per m3: ~ 1028 Semiconductor Electronics - Unit 1: Diodes 10 EE2301: Basic Electronic Circuit Growing Silicon We can grow very pure silicon Semiconductor Electronics - Unit 1: Diodes 11 EE2301: Basic Electronic Circuit Conduction Semiconductor Electronics - Unit 1: Diodes 12 EE2301: Basic Electronic Circuit Currents in Semiconductor Source: http://hyperphysics.phy-astr.gsu.edu/HBASE/solids/intrin.html Semiconductor Electronics - Unit 1: Diodes 13 EE2301: Basic Electronic Circuit Carrier Concentration The number of free electrons available for a given material is called the intrinsic concentration ni. For example, at room temperature, silicon has: ni = 1.5 x 1016 electrons/m3 1 free electron in about every 1012 atoms Semiconductor Electronics - Unit 1: Diodes 14 EE2301: Basic Electronic Circuit Doping: n-type 1 Si atom substituted by 1 P atom Si Si Si P has 5 valence electrons (1 electron more) - Si P Si 1 free electron created Electrically neutral Semiconductor Electronics - Unit 1: Diodes 15 EE2301: Basic Electronic Circuit Doping: p-type 1 Si atom substituted by 1 B atom Si Si Si B has 3 valence electrons (1 electron short) + Si B Si 1 hole created Electrically neutral Semiconductor Electronics - Unit 1: Diodes 16 EE2301: Basic Electronic Circuit p-n Junction Semiconductor Electronics - Unit 1: Diodes 17 EE2301: Basic Electronic Circuit Diode Physics + + + + + + + + + + + + + + + + + + + --- +++ - - - --- +++ - - --- +++ - - - - - + - + - + + + + + Semiconductor Electronics - Unit 1: Diodes - - - - - - - - - 18 EE2301: Basic Electronic Circuit Diode Physics + + + + + + + + + + + + + --- +++ - - - --- +++ - - --- +++ - - - - ----+ + + --------+ + + + - +++++ +++++ +++++ - - - - - - + Website: http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/diode.html Semiconductor Electronics - Unit 1: Diodes 19 EE2301: Basic Electronic Circuit Biasing and Conventions vD: Voltage of P (anode) relative to N (cathode) iD: Current flowing from anode to cathode EE2301: Block C Unit 1 20 EE2301: Basic Electronic Circuit Diode Diode equation: ID = I0 [exp(eVD/kT) - 1] Diode begins to conduct a significant amount of current: Voltage Vγ is typically around 0.7V EE2301: Block C Unit 1 21 EE2301: Basic Electronic Circuit Diode Symbol and Operation Forward-biased Current (Large) Reverse-biased Current (~Zero) iD + - - + Forward Biased: Reverse Biased: Diode conducts Little or no current EE2301: Block C Unit 1 22 EE2301: Basic Electronic Circuit Real diode circuits + VD - ID + - VT RT To find VL where VT and RT are known, + VL - First apply KVL around the loop: VT = VD + RTID Then use the diode equation: ID = I0 [exp(eVD/kT) - 1] At T = 300K, kT/e = 25mV We then need to solve these two simultaneous equations, which is not trivial. One alternative is to use the graphical method to find the value of ID and VD. EE2301: Block C Unit 1 23 EE2301: Basic Electronic Circuit Graphical method Equation from KVL 1 vT iD v D RT RT Operating point is where the load line & I-V curve of the diode intersect EE2301: Block C Unit 1 24 EE2301: Basic Electronic Circuit Diode circuit models Simplify analysis of diode circuits which can be otherwise difficult Large-signal models: describe device behavior in the presence of relatively large voltages & currents > Ideal diode model > Off-set diode mode EE2301: Block C Unit 1 25 EE2301: Basic Electronic Circuit Ideal diode model vD > 0: Short circuit In other words, diode is treated like a switch here vD < 0: Open circuit EE2301: Block C Unit 1 26 EE2301: Basic Electronic Circuit Ideal diode model Circuit containing ideal diode EE2301: Block C Unit 1 Circuit assuming that the ideal diode conducts Circuit assuming that the ideal diode does not conduct 27 EE2301: Basic Electronic Circuit Ideal diode example 1 Problems 9.7 and 9.8 Determine whether the diode is conducting or not. Assume diode is ideal Repeat for Vi = 12V and VB = 15V EE2301: Block C Unit 1 28 EE2301: Basic Electronic Circuit Ideal diode example 1 solution Option 1: Assume diode is conducting and find the diode current direction Outcome 1: If diode current flows from anode to cathode, the assumption is true Diode is forward biased Outcome 2: If diode current flows from cathode to anode, the assumption is false Diode is reverse biased Option 2: Assume diode is not conducting and find the voltage drop across it Outcome 1: If voltage drops from cathode to anode, then the assumption is true Diode is reverse biased Outcome 2: If voltage drops from anode to cathode, then the assumption is false Diode is forward biased This slide is meant to be blank EE2301: Block C Unit 1 29 EE2301: Basic Electronic Circuit Ideal diode example 1 solution Assume diode is conducting Forward-bias diode current (ie anode to cathode) = (10 - 12) / (5 + 10) = -2/15 A Assumption was wrong Diode is in reverse bias Assume diode is not-conducting Reverse-bias voltage (ie cathode referenced to anode) = 12 - 10 = 2V Assumption was correct Diode is in reverse bias This slide is meant to be blank EE2301: Block C Unit 1 30 EE2301: Basic Electronic Circuit Ideal diode example 1 solution Assume diode is conducting Forward-bias diode current (ie anode to cathode) = (15 - 12) / (5 + 10) = 1/5 A Assumption was correct Diode is in forward bias Assume diode is not-conducting Reverse-bias voltage (ie cathode referenced to anode) = 12 - 15 = -3V Assumption was incorrect Diode is in forward bias This slide is meant to be blank EE2301: Block C Unit 1 31 EE2301: Basic Electronic Circuit Ideal diode example 2 Problem 9.14 Find the range of Vin for which D1 is forward-biased. Assume diode is ideal The diode is ON as long as forward bias voltage is positive Now, minimum vin for vD to be positive = 2V EE2301: Block C Unit 1 32 EE2301: Basic Electronic Circuit Offset diode model EE2301: Block C Unit 1 33 EE2301: Basic Electronic Circuit Offset model example Problem 9.19 The diode in this circuit requires a minimum current of 1 mA to be above the knee of its characteristic. Use Vγ = 0.7V What should be the value of R to establish 5 mA in the circuit? With the above value of R, what is the minimum value of E required to maintain a current above the knee EE2301: Block C Unit 1 34 EE2301: Basic Electronic Circuit Offset model example solution ID = (E - VD)/R When the diode is conducting, VD = Vγ ID = (5 - 0.7)/R We can observe that as R increases, ID will decrease To maintain a minimum current of 5mA, Rmax = 4.3/5 = 860 Ω Minimum E required to keep current above the knee (1mA), Emin = (10-3 * 860) + 0.7 = 1.56V This slide is meant to be blank EE2301: Block C Unit 1 35 EE2301: Basic Electronic Circuit Rectification: from AC to DC One common application of diodes is rectification. In rectification, an AC sinusoidal source is converted to a unidirectional output which is further filtered and regulated to give a steady DC output. Supply is AC EE2301: Block C Unit 1 DC required 36 EE2301: Basic Electronic Circuit Rectifier with regulator diagram Rectifier Bi-directional input Unidirectional output Filter Regulator Steady DC output We will look at two types of rectifiers and apply the large signal models in our analysis: 1) Half wave rectifier 2) Full wave rectifier EE2301: Block C Unit 1 37 EE2301: Basic Electronic Circuit Half-Wave Rectifier ~ VS RL ~ VS RL We can see that the circuit conducts for only half a cycle VL VS On the positive cycle On the negative cycle Diode is forward biased Diode is reverse biased Diode conducts Diode does not conduct VL will follow VS VL will remain at zero EE2301: Block C Unit 1 38 EE2301: Basic Electronic Circuit Average voltage in a HW Rectifier vL T /2 T 1 V peak sin( t )dt 0dt T0 T /2 2 1 V peak sin( )d 0dt 2 0 st 1 half of V peak 2Vrms period 2nd half of period NB: This is equal to the DC term of the Fourier series EE2301: Block C Unit 1 39 EE2301: Basic Electronic Circuit Full-Wave Rectifier VS ~ Also known as BRIDGE rectifier Comprises 2 sets of diode pairs Each pair conducts in turn on each half-cycle EE2301: Block C Unit 1 40 EE2301: Basic Electronic Circuit Full-Wave Rectifier EE2301: Block C Unit 1 41 EE2301: Basic Electronic Circuit Full-Wave Rectifier Repeats for every half a period: Integrate through half a period vL 2 T 1 T /2 v peak sin( t )dt 0 v peak sin( )d Half a period 0 2v peak 2 2vrms NB: This is equal to the DC term of the Fourier series EE2301: Block C Unit 1 T/2 – time π – phase 42 EE2301: Basic Electronic Circuit Full-Wave Rectifier (offset) VD-on (only one diode is on) 2VD-on (two diodes are on) With ideal diodes With offset diodes EE2301: Block C Unit 1 43 EE2301: Basic Electronic Circuit Ripple filter Anti-ripple filter is used to smoothen out the rectifier output Charging Discharging EE2301: Block C Unit 1 44 EE2301: Basic Electronic Circuit Ripple filter Approximation: abrupt change in the voltage From transient analysis: VMexp(-t/RC) VL Ripple voltage Vr = VM - VL min EE2301: Block C Unit 1 45 EE2301: Basic Electronic Circuit Ripple filter example Problem 9.40 Find the turns ratio of the transformer and the value of C given that: IL = 60mA, VL = 5V, Vr = 5%, Vline = 170cos(ωt) V, ω = 377rad/s Diodes are fabricated from silicon, Vγ = 0.7V EE2301: Block C Unit 1 46 EE2301: Basic Electronic Circuit Ripple filter example solution a) TURNS RATIO: To find the turns ratio, we need to find VS1 and VS2 1 Vm VL Vr 5 0.125 5.125 V 2 1 VL min VL Vr 5 0.125 4.875 V 2 EE2301: Block C Unit 1 47 EE2301: Basic Electronic Circuit Ripple filter example solution But VM is not equal to VS1 due to voltage drop across diodes So we now apply KVL on the secondary coil side: VS1 - VD - VM = 0 VS1 = 5.825 V (VD = 0.7V) Turns ratio, n = Vline / VS1 ~ 29 EE2301: Block C Unit 1 48 EE2301: Basic Electronic Circuit Ripple filter example solution b) Value of C: Need to find the RC time constant associated with the ripple RL = VL/IL = 83.3 Ω We know it decays by VMexp(-t/RC), we now just need to know how long this lasts (t2) VL-min = - VSOcos(ωt2) - VD-on vso is negative at this point EE2301: Block C Unit 1 49 EE2301: Basic Electronic Circuit Ripple filter example solution t2 = (1/ω) cos-1{-(VL-min + VD-on)/VSO} = 7.533 ms Decaying exponential: VL-min = VM exp(-t2/RLC) t2 VLmin C ln RL VM 1.8mF VL-min = - VSOcos(ωt2) - VD-on 2nd half of the sinusoid EE2301: Block C Unit 1 50 EE2301: Basic Electronic Circuit Examples of Diode EE2301: Block C Unit 1 51 EE2301: Basic Electronic Circuit EE2301: Block C Unit 1 52 EE2301: Basic Electronic Circuit Electrical Materials Insulators Electrons are bound to the nucleus and are therefore not free to move With no free electrons, conduction cannot occur Conductors Sea of free electrons not bound to the atoms Ample availability of free electrons allows for electrical conduction Semiconductors Electrons are bound to the nucleus but vacancies are created due to thermal excitation Electrical conduction occurs through positive (called holes) and negative (electrons) charge carriers EE2301: Block C Unit 1 53 EE2301: Basic Electronic Circuit Conduction in Semiconductors Silicon is the dominant semiconductor material used in the electronics industry. In a cubic meter of silicon, there are roughly 1028 atoms. Among these 1028, there will be about 1.5×1016 vacancies at room temperature. This is known as the intrinsic carrier concentration: n = 1.5×1016 electrons/m3. This corresponds to 1 free electron for every 1012 atoms. There will be same number of electrons as holes in intrinsic silicon since it is overall electrically neutral. EE2301: Block C Unit 1 54 EE2301: Basic Electronic Circuit Extrinsic semiconductors A semiconductor material that has been subjected to the doping process is called an extrinsic material. Both n-type and p-type materials are formed by adding a predetermined number of impurity atoms to a silicon base. An n-type material is created by introducing impurity elements that have five valence electrons. In an n-type material, the electron is called the majority carrier. An p-type material is created by introducing impurity elements that have three valence electrons. In a p-type material, the hole is the majority carrier. EE2301: Block C Unit 1 55 EE2301: Basic Electronic Circuit p-n Junction The pn junction forms the basis of the semiconductor diode Within the depletion region, no free carriers exist since the holes and electrons at the interface between the p-type and n-type recombine. EE2301: Block C Unit 1 56 EE2301: Basic Electronic Circuit Response of the depletion region + + + + - + - + - + + + + + + + + - - --- +++ - --- +++ --- +++ - - - - - ----+ --------+ + - +++++ +++++ +++++ - + Forward biased: Reverse biased: Voltage on the p-type side is higher than the n-type side Voltage on the p-type side is lower than the ntype side Depletion width reduces, lowering barrier for majority carriers to move across the depletion region Depletion width increases, increasing the barrier for majority carriers to move across the depletion region Large conduction current Very small leakage current EE2301: Block C Unit 1 57 EE2301: Basic Electronic Circuit Analogy from tides Depletion region Forward Biased Reverse Biased Depletion region EE2301: Block C Unit 1 58