Analog and Telecommunication Electronics

Transcription

1 Politecnico di Torino - ICT School Analog and Telecommunication Electronics F1 - Power devices» Schottky and pin diodes» MOS, BJT, IGBT, TRIAC» Safe Operating Area» Thermal analysis 06/06/ ATLCE - F DDC

2 Lesson F1: Power devices Models and parameters for Diodes, PIN, Schottky Zener diodes MOS, BJT, IGBT, SCR, TRIAC Operating limits Safe Operating Area Power dissipation Thermal analysis Reference: Book 1: ch 3, ch 4, ch 5, ch 14 06/06/ ATLCE - F DDC

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 06/06/ ATLCE - F DDC

4 Power devices Diodes Standard P-I-N Schottky Zener Transistors BJT MOS-FET IGBT SCR/TRIAC Subsystems Voltage reference, voltage regulators,. group G 06/06/ ATLCE - F DDC

5 Power device applications Switches Two-state devices» ON /OFF state Power switches Signal switches Amplifier & 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 06/06/ ATLCE - F DDC

6 Switch operation Command signal ON state Short circuit, zero voltage drop (Low resistance, low voltage drop) Control OFF state Open circuit, zero current (High resistance, low current) V I Dynamic (state transition) Types: Mechanical/ Electronic 06/06/ ATLCE - F DDC

7 Switch parameters ON state Ideal: Short circuit, zero voltage drop Actual:» Low resistance Ron» low voltage drop Vsat OFF state Ideal: Open circuit, zero current Actual:» High resistance» Low current (leakage) Ioff Power dissipation Ideal: 0 Actual: not 0 06/06/ ATLCE - F DDC

8 Switch parameters Command signal type (V, I, mechanical, ) Vmax Max voltage OFF state (Vbrk) Imax Max current ON state Pd dissipated power (Max, actual) Tc switching time Delay from command to actual state change ON OFF and ON OFF Direction of V and I Num of cycles Environmental (temp, vibration, radiation, ) EMI (speed of change dv/dt, di/dt) 06/06/ ATLCE - F DDC

9 Mechanical switches Mobile metal contact Open/closed Command signal: Force (finger, coil, other mechancal) 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 Critical issues Number of cycles Arching, Bouncing Delay from command to actual state change 06/06/ ATLCE - F DDC

10 Electronic switches Diodes Two-terminal devices Control on same pins as Power flow (output) ON/OFF controlled by applied V and I Transistors and other devices (DIAC, TRIAC, ) At least 3 pins Separate control signal 06/06/ ATLCE - F DDC

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 06/06/ ATLCE - F DDC

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 06/06/ ATLCE - F DDC

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) 06/06/ ATLCE - F DDC

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 06/06/ ATLCE - F DDC

15 Diode ON state Near threshold Exponential characteristic High currents Resistance of low-doping material and contact 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 06/06/ ATLCE - F DDC

16 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 normally occurs below 5 V. Avalanche breakdown Field strength is sufficient to accelerate the electrons; Electrons collide with atoms, and liberate other electrons, (avalanche effect) Avalanche breakdown usually occurs at voltages > 5 V 06/06/ ATLCE - F DDC

17 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. 06/06/ ATLCE - F DDC

18 Diode equivalent circuits Straight-line models, with different approximation. 06/06/ ATLCE - F DDC

19 Silicon diode parameters Turn-on voltage about 0.5 V Considered ON when I > I S Conduction voltage about 0.7 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 06/06/ ATLCE - F DDC

20 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 1 A 1 A 300 ma 15 ma Maximum Reverse Voltage 50 V 50 V 75 V 70 V Reverse Leak. 25 C, VR = 20 V 50 na 200 na 5 na 200 na Forward Voltage ~0.7 V 1 IF = 1A 1 IF = 10 ma ma 06/06/ ATLCE - F DDC

21 Device data sheet example 06/06/ ATLCE - F DDC

22 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. 06/06/ ATLCE - F DDC

23 PIN diodes Applications of PIN diodes: radio frequency switches and attenuators. 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) 06/06/ ATLCE - F DDC

24 Pin diode band diagram 06/06/ ATLCE - F DDC

25 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 06/06/ ATLCE - F DDC

26 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. 06/06/ ATLCE - F DDC

27 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 06/06/ ATLCE - F DDC

28 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 06/06/ ATLCE - F DDC

29 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) 06/06/ ATLCE - F DDC

30 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 1 A 1 A 300 ma 15 ma Maximum Reverse Voltage 50 V 50 V 75 V 70 V Reverse Leak. 25 C, VR = 20 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 06/06/ ATLCE - F DDC

31 Data sheet Schottky diode 06/06/ ATLCE - F DDC

32 Schottky junction in logic circuits Schottky junctions are used also for BJT to avoid saturation and increase speed (no reverse recovery). 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 06/06/ ATLCE - F DDC

33 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: ZENER diodes Used for Protection circuits Low power voltage regulators Cheap reference voltage sources 06/06/ ATLCE - F DDC

34 Zener diode i(v) characteristic Iz and Vz have inverse sign from Id, Vd of standard diodes I Z V Z Figure 3.21 The diode i v characteristic with the breakdown region shown in some detail. 06/06/ ATLCE - F DDC

35 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 I Pdmax (or Izmax): limited by temperature rise V 06/06/ ATLCE - F DDC

36 Zener diode applications The reverse breakdown voltage can be used to get a voltage reference Breakdown voltage becomes the Zener voltage, Vz Output voltage of circuit shown is equal to Vz, despite variations in input voltage V A resistor is used to limit the current in the diode Discussed in detail in the voltage regulator section. 06/06/ ATLCE - F DDC

37 Zener diode operating point Select I, V sign as for diodes Draw characterisitc of left-side bipole (Vs + R) + Rs Vsu I V Draw Dz I(V) characteristic Operating point at intersection I Evaluate effect of Vsu or Rs changes Vsu/Rs V If Dz operates in breakdown Vo regulation Vsu 06/06/ ATLCE - F DDC

38 Diode circuit analysis The non-linear behavior of diodes makes analysis more difficult Correct: Solve system with nonlinear equations E I I S V D (e V k V R Approximated: D ) V D IR Use linear piecewise model, with two-step analysis» Evaluate ON/OFF switching point» Use separate models for each state 06/06/ ATLCE - F DDC

39 Diode circuits: power rectifier One application of diodes is in rectification half-wave rectifier 4 diodes full wave rectifier Evaluate with different models Effects of leakage current Is Effect of voltage drop on the junction Effect of diode series resistance 06/06/ ATLCE - F DDC

40 Diode circuits: AC DC conversion Half-wave rectifier capacitor used to produce a steadier output Vi is AC Vo is almost DC Basic AC-DC converter (power supply) 06/06/ ATLCE - F DDC

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. 06/06/ ATLCE - F DDC

42 Diode circuits: signal clamping A simple form of signal conditioning Circuits limit the excursion of the voltage waveform Can use a combination of signal and Zener diodes. 06/06/ ATLCE - F DDC

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 06/06/ ATLCE - F DDC

44 Lesson D1: Power devices Models and parameters for Diodes, PIN, Schottky Zener diodes MOS, BJT, IGBT, SCR, TRIAC Operating limits Safe Operating Area Power dissipation Thermal analysis Reference: Book 1 (Sedra): ch 3, ch 4, ch 5, ch 14 06/06/ ATLCE - F DDC

45 Power BJT devices Use vertical technology More current in the same area (higher density) Most relevant parameters (for power circuits): Vcebr C-E breakdown voltage Icmax max collector current β current gain Vcesat C-E voltage drop in saturation I c Thermal parameters» Max power, Thermal resistance Fundamental relation: Ic= β Ib I b V be V ce 06/06/ ATLCE - F DDC

46 Vertical power BJT structure Vertical BJT doping density ( cm^-1) Low doping in base region Wide depletion layer, high brk voltage Low current gain (5 20) High transit time Ft < 10 MHz B E n P 10^16 N 10^14 N+ 10^19 Primary breakdown Avalanche carrier multiplication in the BC junction Secondary breakdown High current in small area (same problem as diodes)» Multiple small devices with current sharing Critical region: Near saturation, high power deep saturation (low β) C 06/06/ ATLCE - F DDC

47 BJT model Ebers-Moll model for BJT Simplified models BE diode + Ic source Ic = β Ib Linear models» Hybrid»Gm» Figure 5.8 The Ebers-Moll (EM) model of the npn transistor. 06/06/ ATLCE - F DDC

48 Switch or amplifier? Use as amplifier Active region Use as switch ON Saturation Use as switch OFF Cutoff 06/06/ ATLCE - F DDC

49 BJT as a switch Operating points are on the load line 06/06/ ATLCE - F DDC

50 BJT operation The current gain β decreases for high currents Need significant driving power Operation is based on minority carriers Slow dynamic Temperature dependence To increase BVceo, base region long and lightly doped Higher epsilon Reduced E field Higher recombination probability Lower current gain High voltage devices have low current gain 06/06/ ATLCE - F DDC

51 Safe Operating Area Limits to Current Voltage Power 06/06/ ATLCE - F DDC

52 Saturation model for BJT V source Vcesat (0.1 V) Series resistor Rcesat (few ohms) Lower Vcesat with C-E inversion (lower β) 06/06/ ATLCE - F DDC

53 Critical saturation region Low current gain (5 20) Critical region: Near saturation, high Ic, residual Vce high power dissipation Design solution Guarantee deep saturation (high Ib drive) Use Darlington connections» Single integrated structure» Npn-npn» Npn-pnp 06/06/ ATLCE - F DDC

54 Darlington devices 06/06/ ATLCE - F DDC

55 Cutoff model for BJT Ib = 0 Ic = 0 (ideal) BC junction leakage current: Icbo If base open, enters as Ib, causing Iceo = β Icbo Iceo causes power dissipation Temperature rise high leakage current temperature rise Thermal runaway Avoid high current density areas (hot spot) Multiple devices, with current partition Steer Icbo away from Base R to GND Reverse bias BE (without breakdown!) 06/06/ ATLCE - F DDC

56 Power MOS-FET Planar structure Low power devices Current and breakdown voltage ratings function of the channel W & L. Vertical structure Voltage rating function of doping and thickness of N-epitaxial layer (vertical) Current rating is a function of the channel W & L A vertical structure can sustain both high V & I 06/06/ ATLCE - F DDC

57 MOS-FET parasitics The vertical structure creates a pn junction from body (S) to substrate (D) S G D Current can always flow from S to D A 1-quadrant switch 4-quadrant with two MOS 06/06/ ATLCE - F DDC

58 MOS-FET parasitics The vertical structure creates also a parasitic transistor 06/06/ ATLCE - F DDC

59 MOS-FET parameters Basic parameters: Vdsbr D-S Breakdown voltage Idmax Max Drain current Vgsth Threshold voltage Rdson ON equivalent resistance Qg total charge injected into the Gate (for a given Vgs) A power transistor may consist of several cells (thousands) Power MOS DMOS,. (double-diffused metal oxide semiconductor) Power MOSFETs are made using this technology 06/06/ ATLCE - F DDC

60 MOS-FET model Model depends on operating point Low Vgs (subthreshold):» Exponential Medium Vgs:» Square law High Vgs:» linear Figure Typical i D v GS characteristic for a power MOSFET. 06/06/ ATLCE - F DDC

61 MOS-FET output characteristic Warning! Saturation in MOS has a different meaning (called active region in BJT) 06/06/ ATLCE - F DDC

62 MOS-FET switching models ON: Equivalent resistance Ron OFF: Leakage current Ioff Dynamic GS capacitance DS capacitance Parasitic towards substrate 06/06/ ATLCE - F DDC

63 MOS-FET gate charge Before threshold: Cgs Active region Miller effect on Cgd Saturation Cgd 06/06/ ATLCE - F DDC

64 MOS-FET vs BJT MOS-FET use majority carriers High switching speed Reduced temperature dependence MOSFET use simpler driving circuit The Gate represents a plate of a capacitor (towards GND); no current after first charging step, but Need to drive a high-capacitance load for fast switching ON state BJT modeled as Vcesat (+Ron) MOS modeled as Ron OFF state: BJT and MOS modeled as current source (leakage) 06/06/ ATLCE - F DDC

65 Four-layer devices Transistors have limitations in switching high currents at high voltages Other devices are specifically designed for such applications: four-layer devices Specific physical structure Can be used only as switches (not for linear amplifiers) A great deal in common with bipolar transistors SCR/Tyristor TRIAC/DIAC 06/06/ ATLCE - F DDC

66 4-layer device operation Circuit with two interconnected BJTs Turning on T2 provides Ic2 as Ib1 to T1, and Ic1 as Ib2. Both devices conducts until the current goes to zero. The two BJTs can be built as a single 4-layer device Tyristor or Silicon Controlled Rectifier (SCR) 06/06/ ATLCE - F DDC

67 SCR in CMOS logic circuits SCR structure intrinsic in CMOS ICs Responsible for latch up Triggered by Input levels out of GND-Vcc range High energy particles pmosfet nmosfet V G DD S D D G S V SS p+ p+ n+ n+ n+ p+ T1 p-well T2 n-substrate 06/06/ ATLCE - F DDC

68 The thyristor Four-layer device with a pnpn structure Three terminals: anode, cathode and gate Gate is the control input. Power flow between Anode and Cathode 06/06/ ATLCE - F DDC

69 Thyristor in AC power control Triggered ON by a pulse on the Gate Stays ON as long as V > 0 (remainder of the half cycle) Returns OFF when V = 0 Varying firing time changes output power Single-wave allows control from 0 50% of full power 06/06/ ATLCE - F DDC

70 The Triac and the Diac A bidirectional thyristor Allows full-wave control using a single device Often used with a diac: bidirectional trigger diode to produce the gate drive pulses The DIAC breaks down at a particular voltage and fires the triac 06/06/ ATLCE - F DDC

71 A simple lamp-dimmer using a triac Current pulse to fire the Triac Phase shift network. Provides trigger voltage for Diac 06/06/ ATLCE - F DDC

72 IGBT The Insulated Gate Bipolar Transistor or IGBT combines bipolar and MOS devices MOSFET gate-drive + high Ic and low Vcesat of BJT isolated gate FET for the control input, bipolar power transistor as a switch, in a single device combines high efficiency and fast switching. Used in medium- to high-power applications switching power supply, motor control, induction heating, Large IGBT modules (many devices in parallel), can handle» high current k 100 A» High voltages k 1000 V. 06/06/ ATLCE - F DDC

73 IGBT structure 06/06/ ATLCE - F DDC

74 IGBT characteristic 06/06/ ATLCE - F DDC

75 Lesson F1: Power devices Models and parameters for Diodes, PIN, Schottky Zener diodes MOS, BJT, IGBT, SCR, TRIAC Operating limits Safe Operating Area Power dissipation Thermal analysis 06/06/ ATLCE - F DDC

76 Operating limits Breakdown voltage If higher, insulating layers are broken Max current If higher, wires or conducting paths can melt Max power Power dissipation causes temperature rise Max temperature Doping distribution is modified changes in parameters Silicon itself can melt Special application parameters Radiation in space,. 06/06/ ATLCE - F DDC

77 Safe Operating Area Any electronic devices can handle limited power, voltage, current For active devices, the region of acceptable V,I is the Safe Operating Area (SOA), defined by Power limit (V x I > Pdmax)» Excess power cause temperature rise, with melting» Secondary breakdown: local heating and thermal runaway Voltage (V < Vbrk)» Excess voltage causes breakdown and insulator perforation Current (I < Imax)» Excess current cause heating and metal evaporation 06/06/ ATLCE - F DDC

78 Safe Operating Area boundaries Too high current Too high V x I (power) - not uniform current flow - high local power dissipation Active & Safe Operating Area (SOA) Too high voltage 06/06/ ATLCE - F DDC

79 SOA for BJT (TIP31) Includes dynamic behavior Pdmax depends on pulse Duty Cycle Log scale! I x V = K is a straigth line 06/06/ ATLCE - F DDC

80 SOA for MOS (IRF640) Dynamic behavior Log scale No secondary breakdown Id limited by Rds 06/06/ ATLCE - F DDC

81 Power dissipation All electric devices dissipate a power Pd = V I Power dissipation increases temperature Any device has temperature limits, therefore power limits The effects of power dissipation can be modeled using thermal equivalent circuits Power current Temperature node voltage Heat conduction capability thermal resistance θr ( /W) Diodes/MOS/BJT power dissipated on the junctions Heat must be brought outside, through a path including» Junction-case defined by manufacturer» Case-ambient controlled using heat sinks 06/06/ ATLCE - F DDC

82 Power derating Manufacturers specify Max power dissipation Pdmax Max junction operating temperature Tjmax Power dissipation causes temperature rise Allowed power dissipation decreases with Ta Ta = Tjmax Pd = 0 06/06/ ATLCE - F DDC

83 Evaluation of temperature rise Electric network model for thermal behaviour Power Pd current source Temperature T node voltage Heat conduction θ thermal resistance θr ( /W) Electrical equivalent circuit Tj Ta = Pd θja 06/06/ ATLCE - F DDC

84 From junction to ambient The thermal path from junctin to ambient consists of: Junction-Case: θ JC» Thermal resistance defined by the package Case-heatsink: θ CS» Case and fixture Heatsink-ambient: θ SA» Heatsink and operating condition (air flow) Designer can control θ CS and θ SA 06/06/ ATLCE - F DDC

85 Thermal specification Power devices specified for No heatsink, Ta specified, Tc? infinite heatsink, Tc = Ta Example datasheet TIP30 06/06/ ATLCE - F DDC

86 Power BJT datasheet (TIP31) 06/06/ ATLCE - F DDC

87 Power MOS datasheet IRF640 06/06/ ATLCE - F DDC

88 Heatsink datasheet example 06/06/ ATLCE - F DDC

89 Dynamic thermal response 06/06/ ATLCE - F DDC

90 Lesson F1: summary Describe the dynamic behavior of pn junctions. Which are the benefits of PIN diodes vs standard diodes? Which are the benefits of Schottky junctions? Draw at least 3 models for semiconductor diodes. What is secondary breakdown? Draw a model for power BJT. Describe differences of low/high power MOSFETs. Which parameters defines the boundary of SOA? How can we evaluate the actual temperature of a power semiconductor junction? Define the infinite heatsink concept. 06/06/ ATLCE - F DDC