Analog and Telecommunication Electronics

Transcription

1 Politecnico di Torino Electronic Eng. Master Degree Analog and Telecommunication Electronics F1 - Power devices: diodes» Switches» pn Junction» Diode models» Dynamic behavior» Zener diodes AY /04/ ATLCE - F DDC 2016 DDC 2013 DDC 1

2 Lesson F1: Power devices Device structure, models, parameters Switches pn Junction Diode models Dynamic behavior Special purpose diodes PIN and Schottky Zener References: Any text on electronic devices 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 2

3 Relevant parameters for power devices Voltage Sustain high voltages Current Handle high current Power Deliver high power to the load, Low dissipated power and temperature rise on control device High efficiency Speed High frequency (RF not here) Fast switching 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 3

4 Power devices Diodes Standard P-I-N Schottky Zener Transistors BJT MOS-FET IGBT SCR/TRIAC Subsystems Voltage reference, voltage regulators,. group G 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 4

5 Power device applications Switches Two-state devices» ON /OFF state Power switches Signal switches Amplifiers & linear applications Analog devices Continuous control output V or I from input V or I Provide power gain» Low input control power Extremes of operation range: ON/OFF states 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 5

6 Switch operation Type: Mechanical Electronic Control Command signal Mech/electric ON state Short circuit, zero voltage drop OFF state Open circuit, zero current Power dissipation: 0 V I 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 6

7 Switch parameters ON state Ideal: Short circuit, 0 voltage drop Actual:» Low resistance Ron» Low voltage drop Vsat OFF state Ideal: Open circuit, 0 current Actual:» High resistance» Low current (leakage) Ioff Power dissipation Ideal: V or I = 0; Pd = 0 Actual: V and I 0 Pd 0 ON voltage Vsat Slope Ron V I Leakage current Ioff 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 7

8 Switch parameters Direction of V and I 1/2/4 quadrant Power handling capabilities Vmax Max voltage OFF state (breakdown voltage) Imax Max current ON state Pd dissipated power (Max, actual) Speed (Tc, switching time) EMI Delay from command to complete state change ON OFF and ON OFF related with speed of change dv/dt, di/dt, 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 8

9 Mechanical switches Mobile metal contact Open/closed Command signal: Force (finger, coil, other mechanical) Vmax Max voltage OFF state (Vbrk): Contact spacing Imax Max current ON state: Contact size Tc switching time: Mechanical time constant Strong points Low Ron, Low Ioff, Low power loss Able to handle high V and I (within limits!) Critical issues Number of cycles Arching, Bouncing Delay from command to actual state change 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 9

10 Electronic switches Diodes Two-terminal devices: Control & Power on same pins ON/OFF controlled by applied V and I Transistors and other devices (DIAC, TRIAC, ) At least 3 pins Separate control signal Smart SWS: Include control and check circuits Strong points Speed Long life (No wear) Critical issues Vmax, Imax Ron, Ioff, 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 10

11 Review of pn Junctions Density gradient cause diffusion of majority charge carriers across the junction. These carriers combine with (and remove) carriers of opposite polarity. free p carriers (holes) free n carriers (electrones) No free carrier here: depletion layer (insulator) free carriers available: conductor 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 11

12 Biased pn Junction Forward bias p-type side positive with respect to n-type side thin depletion layer If forward bias voltage higher than a threshold majority charge carriers can move through the junction current flow, which increases with the voltage Reverse bias p-type side negative with respect to the n-type side wide depletion layer less majority charge carriers through the junction (same amount of minority carriers) small leakage current caused by minority carriers 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 12

13 Semiconductor junction model In a diode current can flow only in one direction Ideal, or first approximation model I = Is (too small to be seen) conduction voltage p n Turn-on voltage (threshold) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 13

14 I(V) characteristic of sem. junction pn junction current I: I S : leakage current reverse current, for V<0 I D V ηv I T 1 s e V: applied voltage V T kt e e: electron charge, k: Boltzmann s constant, T: absolute temperature, η: a constant in the range 1 to 2 (we assume =1) kt At 25 C: V T 26 mv e Resistive behavior for high currents 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 14

15 Linear model and parameters Imax: Max direct current ΔV/ΔI = Ron: ON resistance Is: reverse current (leakage) Vbr (Vzk) Breakdown voltage Von: ON voltage drop Figure 3.8 The diode i v relationship with some scales expanded and others compressed in order to reveal details. 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 15

16 Diode ON state Near threshold Exponential characteristic High currents Resistance of low-doping material and contacts Linear V,I Current (Is) related with temperature» Exponential dependence Current concentration in small area» Hot spots Get wide effective active junction cross-section» Multiple parallel device» Current distribution + Positive feedback 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 16

17 Diode OFF: reverse breakdown The junction cannot sustain too high reverse voltage For V > Vbr insulating layer is broken, and current increases Two types of breakdown Zener breakdown A very high field strength across the junction pulls electrons through the junction, causing large reverse current. Zener breakdown occurs below 5 V. Avalanche breakdown Field strength is sufficient to accelerate the electrons; Electrons collide and accelerate other electrons, (avalanche) Avalanche breakdown occurs at voltages > 5 V 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 17

18 Diode equivalent circuits Straight-line models, with different approximation. 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 18

19 Silicon diode parameters Turn-on voltage about 0.5 V Considered ON when I > I S At operating current V D from 0.7 to 1 V Depends on current, higher for high currents Breakdown voltage depends on device construction 40 V (small-signal) 400 V (power) 4 kv (High Voltage) Max current depends on device construction 100 ma (signal) many A (power) a few ka (High Current) Leakage current Is depends on Material, temperature, device type, (na ma) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 19

20 Comparison of diode parameters Description Sample Device Maximum DC/Average Forward Current Maximum Reverse Voltage Reverse Leak. 25 C, VR = 20 V Forward Voltage General Purpose Rectifier 1N A 50 V 50 na ~0.7 V Small Signal Diode 1N ma 75 V 5 na 1 IF = 10 ma 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 20

21 Device data sheet example Medium power silicon diode: 1N Relevant parameters V F V R /V RRM I F(AV) /I F(max) R θ Vr 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 21

22 Lesson F1: Power devices Device structure, models, parameters Switches pn Junction Diode models and parameters Dynamic behavior Special purpose diodes PIN Schottky Zener Application examples Rectifier Clamp 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 22

23 Increasing reverse breakdown voltage Breakdown comes from high E-field E=V/x Reduce the E-field same V, increase x (distance) Decrease doping to get wider depletion layer (increase x )» Worse parameters in ON state Decrease doping only in a narrow layer: PIN diodes p-intrinsic (insulator)-n junction (PIN diode): OFF state:» Can sustain very high inverse voltages (kv). ON state» Higher losses in the conductive region. 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 23

24 PIN diodes Applications of PIN diodes: radio frequency switches and attenuators (low capacitance). radiation detectors and photodetectors. power electronics (central layer can withstand high voltages) PIN structure used also in other power semiconductors: IGBTs, power MOSFETs, thyristors, The intrinsic layer increases breakdown voltage Drawbacks Higher ON resistance Higher threshold voltage ( 1 V) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 24

25 Pin diode band diagram p n p i n 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 25

26 ON OFF ON transients (ideal) Vi positive Direct bias, ON Current can flow Id Vi Vd Vi Id Reverse bias: Id = 0; Vd = Vi Vi negative Reverse bias, OFF Vd t No current Actual transient depends on parasitic capacitances and charges in the junction ON: charges are stored in the junction region ON OFF: charges must be removed OFF ON: faster transient Forward bias: Vd = 0,6V Id = Vi/R 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 26

27 Junction in the transient Forward bias: a large number of electrons/holes injected into the p/n-material, Switching to reverse bias: Stored minority carriers must return to the opposite material. Storage time ts: current reverses and stays at a constant level. Electrons and holes diffuse and recombine Transition time Tt: current decreases to the reverse current Is Ts + Tt = Trr: reverse recovery time. During trr the diode dissipates energy high frequecy applications diodes with short trr. higher forward voltage drop and higher reverse currents. 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 27

28 ON OFF ON transient (actual) ON OFF OFF ON Vi Id t rr Vd t s t t t 90% final Forward bias Id = Is t rf Minority carrier storage removal Id = Vi/R RC transient 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 28

29 Switching parameters ON OFF Charges stored in the junction must be removed Reverse current flow» High losses (for a short time)» Qrr: reverse injected charge» Trr: reverse recovery time» Irr: reverse peak current OFF ON Faster than ON OFF Relevant timing parameter:» Trf (recovery forward)» Much faster that Trr 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 29

30 Schottky diodes Schottky junction Metal - Si (light doping, if heavy doping becomes a contact) Conduction based on majority carriers (from metal) Benefits: No charge storage in depletion region Fast switch Lower threshold voltage and direct drop (0,3 V), less Pd Schottky junctions used to speed-up BJT circuits (logic ICs) Drawbacks High capacitance (OFF state) Low reverse voltage (<100V) High reverse current (temperature-dependent) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 30

31 Comparison of diode parameters Description General Purpose Rectifier Fast Switching Rectifier Small Signal Diode Schottky Diode Sample Device 1N4001 1N4933 1N4148 ZC2800 Maximum DC/Average Forward Current Maximum Reverse Voltage Reverse Leak. 25 C, VR = 20 V 1 A 1 A 300 ma 15 ma 50 V 50 V 75 V 70 V 50 na 200 na 5 na 200 na Forward Voltage ~0.7 V 1 IF = 1A 1 IF = 10 ma ma Rev Rec Time Trr 2 μs 200 ns 4 ns < 1 ns Cost 20 cent 15 cent 25 cent 100 cent 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 31

32 Data sheet Schottky diode 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 32

33 Schottky junction in logic circuits Schottky junctions are used also for BJT logic gates to avoid saturation and increase speed (no reverse recovery delay). The Shottky junction is placed from B to C; as C voltage goes below B, the diode steers current away from B, and avoids transistor saturation Embedded in gate structure for logic circuits Example: 2-input NAND (74S00) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 33

34 Zener diodes All junctions have breakdown Breakdowns usually change junction parameters to worse values (high local heating, modification of doping) Some devices are designed to operate in breakdown without degradation: Used for ZENER diodes Protection circuits Low power voltage regulators Cheap reference voltage sources 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 34

35 Zener diode i(v) characteristic I Z and V Z have inverse sign from I D, V D of standard diodes I Z Zener operation reverse bias V Z 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 35

36 Zener diode i(v) characteristic I Z and V Z have inverse sign from I D, V D Standard diodes operates in forward/reverse bias» Breakdown is a fault condition Zener operates in reverse bias (breakdown) I D I Z V Z V D breakdown breakdown 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 36

37 Zener diode equivalent circuit Vzo: Vz for I = 0 (linear model) I Rz Rz: ΔV/ΔI (actually differential rz) V + Vzo Izmin: minimum current to exit knee region Pdmax (or Izmax): limited by temperature rise I Slope ΔV/ΔI = Rz V Vzo 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 37

38 Diode circuit analysis The non-linear behavior of diodes makes analysis more difficult Correct: Solve system with nonlinear equations E VD VR VD IR I I S (e k V Approximated: D ) Use linear piecewise model, with two-step analysis» Evaluate ON/OFF switching point» Use separate models for each state 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 38

39 Diode circuits: power rectifier One application of diodes is in rectification half-wave rectifier 4 diodes full wave rectifier Other parameters (evaluated with independent models) Effects of leakage current Is Effect of voltage drop on the junction Effect of diode series resistance 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 39

40 Diode circuits: AC DC conversion Half-wave rectifier Capacitor used to produce a steadier output V I is AC V O is almost DC Basic AC-DC converter power supply (AM detector, measurement systems, ) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 40

41 Diode circuits: signal rectifier Used to demodulate AM signals: AC DC + LPF (smooth) + HPF (remove DC) Also known as an envelope detector Found in a wide range of radio receivers, from crystal sets to superheterodynes. Low-pass filter High-pass filter 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 41

42 Diode circuits: signal clamping A simple form of signal conditioning Circuits limit the excursion of the voltage waveform (b) and (c) use Zener diodes. 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 42

43 Diode circuits: over-voltage protection When switch is open, the inductor tries to make current continue to flow. large back e.m.f. arcing in mechanical switches, breakdown in electronic switches Catch diode low impedance path to dissipate the energy stored in the inductor» SW closed: diode reverse-biased, no current flow» SW opens: the current in L continue to flow through the diode 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 43

44 Inductive load driver Diode clamp to supply voltage Vs to limit the max voltage across active device ON: current flows through the MOS ON OFF: current can continue to flow through the diode V S G D S Mandatory to drive inductive loads (coils, motors, ) 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 44

45 Lesson F1: summary Describe the difference between 1/2/4 quadrant switches. Draw at least 3 models for semiconductor diodes. List the most relevant parameters of a diode. Describe the dynamic behavior of pn junctions. Which are the benefits of PIN diodes vs standard diodes? Which are the benefits of Schottky junctions? Describe the I(V) behavior of Zener diodes Draw a model for zener diode List some applications of diodes 26/04/ ATLCE - F DDC 2016 DDC 2013 DDC 45