MOSFETs - 1 Copyright © by John Wiley & Sons 2003 Power MOSFETs Lecture Notes Outline Construction of power MOSFETs Physical operations of MOSFETs Power.

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Presentation transcript:

MOSFETs - 1 Copyright © by John Wiley & Sons 2003 Power MOSFETs Lecture Notes Outline Construction of power MOSFETs Physical operations of MOSFETs Power MOSFET switching Characteristics Factors limiting operating specfications of MOSFETs COOLMOS PSPICE and other simulation models for MOSFETs

MOSFETs - 2 Copyright © by John Wiley & Sons 2003 Multi-cell Vertical Diffused Power MOSFET (VDMOS)

MOSFETs - 3 Copyright © by John Wiley & Sons 2003 Important Structural Features of VDMOS 1. Parasitic BJT. Held in cutoff by body-source short 2. Integral anti-parallel diode. Formed from parasitic BJT. 3. Extension of gate metallization over drain drift region. Field plate and accumulation layer functions. 4. Division of source into many small areas connected electrically in parallel. Maximizes gate width-to-channel length ratio in order to increase gain. 5. Lightly doped drain drift region. Determines blocking voltage rating.

MOSFETs - 4 Copyright © by John Wiley & Sons 2003 Alternative Power MOSFET Geometries Trench-gate MOSFET Newest geometry. Lowest on-state resistance. V-groove MOSFET. First practical power MOSFET. Higher on-state resistance.

MOSFETs - 5 Copyright © by John Wiley & Sons 2003 MOSFET I-V Characteristics and Circuit Symbols

MOSFETs - 6 Copyright © by John Wiley & Sons 2003 The Field Effect - Basis of MOSFET Operation

MOSFETs - 7 Copyright © by John Wiley & Sons 2003 Drift Velocity Saturation

MOSFETs - 8 Copyright © by John Wiley & Sons 2003 Channel-to-Source Voltage Drop

MOSFETs - 9 Copyright © by John Wiley & Sons 2003 Channel Pinch-off at Large Drain Current

MOSFETs - 10 Copyright © by John Wiley & Sons 2003 MOSFET Switching Models for Buck Converter Buck converter using power MOSFET. MOSFET equivalent circuit valid for off- state (cutoff) and active region operation. MOSFET equivalent circuit valid for on-state (triode) region operation.

MOSFETs - 11 Copyright © by John Wiley & Sons 2003 MOSFET Capacitances Determining Switching Speed

MOSFETs - 12 Copyright © by John Wiley & Sons 2003 Internal Capacitances Vs Spec Sheet Capacitances MOSFET internal capacitances Input capacitance Output capacitance Reverse transfer or feedback capacitance

MOSFETs - 13 Copyright © by John Wiley & Sons 2003 Turn-on Equivalent Circuits for MOSFET Buck Converter

MOSFETs - 14 Copyright © by John Wiley & Sons 2003 MOSFET-based Buck Converter Turn-on Waveforms Free-wheeling diode assumed to be ideal. (no reverse recovery current).

MOSFETs - 15 Copyright © by John Wiley & Sons 2003 Turn-on Gate Charge Characteristic

MOSFETs - 16 Copyright © by John Wiley & Sons 2003 Turn-on Waveforms with Non-ideal Free-wheeling Diode Equivalent circuit for estimating effect of free- wheeling diode reverse recovery.

MOSFETs - 17 Copyright © by John Wiley & Sons 2003 MOSFET-based Buck Converter Turn-off Waveforms

MOSFETs - 18 Copyright © by John Wiley & Sons 2003 dV/dt Limits to Prevent Parasitic BJT Turn-on

MOSFETs - 19 Copyright © by John Wiley & Sons 2003 Maximum Gate-Source Voltage

MOSFETs - 20 Copyright © by John Wiley & Sons 2003 MOSFET Breakdown Voltage

MOSFETs - 21 Copyright © by John Wiley & Sons 2003 MOSFET On-state Losses

MOSFETs - 22 Copyright © by John Wiley & Sons 2003 Paralleling of MOSFETs

MOSFETs - 23 Copyright © by John Wiley & Sons 2003 MOSFET Safe Operating Area (SOA)

MOSFETs - 24 Copyright © by John Wiley & Sons 2003 Conventional vertically oriented power MOSFET COOLMOS™ structure (composite buffer structure, super-junction MOSFET, super multi-resurf MOSFET) Vertical P and N regions of width b doped at same density (N a = N d ) Structural Comparison: VDMOS Versus COOLMOS™

MOSFETs - 25 Copyright © by John Wiley & Sons 2003 COOLMOS™ Operation in Blocking State COOLMOS™ structure partially depleted. Arrows indicate direction of depletion layer growth as device turns off. Note n-type drift region and adjacent p-type stripes deplete uniformly along entire vertical length. COOLMOS™ structure at edge of full depletion with applied voltage V c. Depletion layer reaches to middle of vertical P and N regions at b/2. Using step junction formalism, V c = (q b 2 N d )/(4  ) = b E c,max /2 Keep E c,max ≤ E BD /2. Thus N d ≤ (  E BD )/(q b)

MOSFETs - 26 Copyright © by John Wiley & Sons 2003 COOLMOS™ Operation in Blocking State (cont.) For applied voltages V > V c, vertically oriented electric field E v begins to grow in depletion region. E v spatially uniform since space charge compensated for by E c. E v ≈ V/W for V >> V c. Doping level N d in n-type drift region can be much greater than in drift region of conventional VDMOS drift region of similar BV BD capability. At breakdown E v = E BD ≈ 300 kV/cm ; V = BV BD = E BD W

MOSFETs - 27 Copyright © by John Wiley & Sons 2003 COOLMOS™ Operation in ON-State On-state specific resistance AR on [Ω-cm 2 ] much less than comparable VDMOS because of higher drift region doping. COOLMOS™ conduction losses much less than comparable VDMOS. R on A = W/(q µ n N d ) ; Recall that N d = (  E BD )/(q b) Breakdown voltage requirements set W = BV BD / E BD. Substituting for W and N d yields R on A = (b BV BD )/(  µ n E BD 2 )

MOSFETs - 28 Copyright © by John Wiley & Sons 2003 R on A Comparison: VDMOS versus COOLMOS™ COOLMOS at BV BD = 1000 V. Assume b ≈ 10 µm. Use E BD = 300 kV/cm. R on A = (10 -3 cm) (1000 V)/[ (9x F/cm)(12)(1500 cm 2 -V-sec)(300 kV/cm) 2 ] R on A = Ω-cm. Corresponds to N d = 4x10 15 cm -3 Typical VDMOS, R on A = 3x10 -7 (BV BD ) 2 R on A = 3x10 -7 (1000) 2 = 0.3 Ω-cm ; Corresponding N d = cm 3 Ratio COOLMOS to VDMOS specific resistance = 0.007/0.3 = or approximately 1/40 At BV BD = 600 V, ratio = 1/26. Experimentally at BV BD = 600 V, ratio is 1/5. For more complete analysis see: Antonio G.M. Strollo and Ettore Napoli, “Optimal ON-Resistance Versus Breakdown Voltage Tradeoff in Superjunction Power Device: A Novel Analytical Model”, IEEE Trans. On Electron Devices,Vol. 48, No. 9, pp , (Sept., 2001)

MOSFETs - 29 Copyright © by John Wiley & Sons 2003 COOLMOS™ Switching Behavior Larger blocking voltages V ds > depletion voltage V c, COOLMOS has smaller C gs, C gd, and C ds than comparable (same R on and BV DSS ) VDMOS. Small blocking voltages V ds < depletion voltage V c, COOLMOS has larger C gs, C gd, and C ds than comparable (same R on and BV DSS ) VDMOS. Effect on COOLMOS switching times relative to VDMOS switching times. Turn-on delay time - shorter Current rise time - shorter Voltage fall time1 - shorter Voltage fall time2 - longer Turn-off delay time - longer Voltage rise time1- longer Voltage rise time2 - shorter Current fall time - shorter MOSFET witching waveforms for clamped inductive load.

MOSFETs - 30 Copyright © by John Wiley & Sons 2003 PSPICE Built-in MOSFET Model

MOSFETs - 31 Copyright © by John Wiley & Sons 2003 Lateral (Signal level) MOSFET

MOSFETs - 32 Copyright © by John Wiley & Sons 2003 MOSFET circuit simulation models must take this variation into account. Vertical Power MOSFET

MOSFETs - 33 Copyright © by John Wiley & Sons 2003 Inadequacies of PSPICE MOSFET Model

MOSFETs - 34 Copyright © by John Wiley & Sons 2003 Example of an Improved MOSFET Model Developed by Motorola for their TMOS line of power MOSFETs M1 uses built-in PSPICE models to describe dc MOSFET characteristics. Space charge capacitances of intrinsic model set to zero. Space charge capacitance of DGD models voltage-dependent gate-drain capacitance. CGDMAX insures that gate-drain capacitance does not get unrealistically large at very low drain voltages. DBODY models built-in anti-parallel diode inherent in the MOSFET structure. CGS models gate-source capacitance of MOSFET. Voltage dependence of this capacitance ignored in this model. Resistances and inductances model parasitic components due to packaging. Many other models described in literature. Too numerous to list here.

MOSFETs - 35 Copyright © by John Wiley & Sons 2003 Another Improved MOSFET Simulation Model