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Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

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Advanced Power Dissipation and AC Thermal Analysis

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Intended Audience: Engineers interested in advanced thermal design under AC (variable duty cycle and transient) conditions A basic knowledge of DC thermal analysis is required Topics Covered: Modeling thermal performance with electrical parameters Explanation of thermal RC networks Introduction of the Z th Diagram AC thermal calculations Complex waveform (superposition principle) thermal calculations Expected Time: Approximately 60 minutes

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Advanced Power Dissipation and AC Thermal Analysis Electrical Parameters vs. Thermal Parameters Thermal Resistance and Capacitance Networks Understanding the Z th Diagram Example AC Thermal Calculations Complex Waveforms and Superposition

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Electrical vs. Thermal DC Parameters Electrical ParametersThermal Parameters I R VV + - V = I R R = Electrical Resistance ( ) V = Potential Difference (V) I = Current (A) T = P D R th R th = Thermal Resistance (C/W) T = Temperature Difference (C) P D = Power Dissipated (W) PDPD R th TT + -

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Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance I = Current A = Area d = Thickness = Electrical Conductivity R = Electrical Resistance ( ) I A } d R P D = Power Dissipated A = Area d = Thickness th = Thermal Conductivity R th = Thermal Resistance (C/W) V + - PDPD A } d th R th TT + -

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Electrical Circuits vs. Thermal Circuits Electrical CircuitsThermal Circuits I R VV + - PDPD R th TT + - I = 10A R = 1 V = IR V = (10A)(1 ) = 10V 10V Potential Difference P D = 10W R th = 1C/W T = P D R th T = (10W)(1C/W) = 10C 10C Temperature Difference

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Electrical vs. Thermal Parameters Electrical ParametersThermal Parameters + C th E - TT + C Q - V C = Capacitance (Farads = A-sec / V) C th = Thermal Capacitance (Joules / C = Watts-sec / C)

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Advanced Power Dissipation and AC Thermal Analysis Electrical Parameters vs. Thermal Parameters Thermal Resistance and Capacitance Networks Understanding the Z th Diagram Example AC Thermal Calculations Complex Waveforms and Superposition

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Die Attach C th3 – R th3 Leadframe Silicon C th2 – R th2 Thermal Resistance & Capacitance Example Silicon Wafer Cross Section p + Well C th1 - R th1 Leadframe Metal

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Thermal RC Network - Internal Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Temperature ~ Voltage Power ~ Current

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Thermal RC Network – Total Temperature ~ Voltage Power ~ Current Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Heatsink T case R inteface R heatsink C interface C heatsink

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Junction Temperature Calculations With temperature analogous to voltage, T Is determined by the P D and the RC network Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Heatsink T case

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Junction Temperature Calculations The maximum junction temperature is specified in the absolute maximum section of the data sheet (T j,max ) Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Heatsink T case

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Junction Temperature Calculations The device junction-to-case thermal resistance (R thjc ) is specified in the datasheet and determines T jc. R thjc is usually valid for DC only T jc Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Heatsink T case

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Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Heatsink T case Junction Temperature Calculations The external case-to-ambient thermal resistance, (R thca ) is determined by the heatsink. This determines the temperature change from the case to the ambient. T ca

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DC Junction Temperature Calculations Under DC conditions, power and temperature reach steady state conditions and the thermal capacitors are removed from the circuit model Chip T T junction T ambient R th1 R th2 R th3 PDPD Heatsink T case

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DC Junction Temperature Calculation Power Dissipation P D = I ds 2 R dson = (5A) 2 (24m ) = 0.6W Thermal Resistance R thja = 55 C/W Junction Temperature T junction = T ambient + P D R thja T junction = 85C + (0.6W)(55C/W) T junction = 85C + 33C = 118C DC Calculations are relatively simple

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Z th (j ) = ? Z th (t) = ? AC Junction Temperature Calculation of Transfer Function Chip T T junction C th1 C th2 C th3 T ambient R th1 R th2 R th3 PDPD Heatsink T case

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AC Junction Temperature Calculation of Transfer Function

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Simplified AC Thermal RC Network Chip T T junction T ambient R’ th1 R’ th2 R’ th3 PDPD Heatsink T case C’ th1 C’ th2 C’ th3 Thermal capacitance now in parallel with thermal resistance

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The new RC component values of the simplified network are obtained by mathematical transformations. They do NOT refer to any physical layer. Together, they describe the overall thermal behavior and performance of the device and heatsink. Chip T T junction T ambient R’ th1 R’ th2 R’ th3 PDPD Heatsink T case C’ th1 C’ th2 C’ th3 Simplified AC Thermal RC Network

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Chip T T junction T ambient R’ th1 R’ th2 R’ th3 PDPD Heatsink T case C’ th1 C’ th2 C’ th3 Frequency Domain Time Domain

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AC Temperature Calculation Simplified Transfer Function T( t ) = P D (t) Z th ( t) Tj(t)P(t) Z th

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Advanced Power Dissipation and AC Thermal Analysis Electrical Parameters vs. Thermal Parameters Thermal Resistance and Capacitance Networks Understanding the Z th Diagram Example AC Thermal Calculations Complex Waveforms and Superposition

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Development of the Z th Diagram Create test set-up for integrated circuit package types Power is generated in the integrated circuit for defined lengths of time The resulting temperature rise is measured A thermal impedance (Zth) diagram is generated Tj(t)P(t) Z th

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Z thja (C / W) t pulse (sec) 1E-51E+3 Duty Cycle 50% 20% 10% 5% 2% 1% Single Pulse 1E-31E-11E+1 Z th Diagram for the TO-263 Package

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Z thja (C / W) t pulse (sec) 1E-51E+3 Duty Cycle 50% 20% 10% 5% 2% 1% Single Pulse 1E-31E-11E+1 Single Pulse P(t) PDPD t pulse

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Z thja (C / W) t pulse (sec) 1E-51E-31E-11E+11E+3 Duty Cycle 50% 20% 10% 5% 2% 1% Single Pulse Periodic event: P(t) PDPD t pulse T Duty Cycle: Z th Diagram for the TO-263 Package

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Z th Diagrams for Different Packages TO t p = 1s Z thja 2C/W TO t p = 1s Z thja 4C/W SOT-223 t p = 1s Z thja 30C/W SO-8 t p = 1s Z thja 65C/W

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Advanced Power Dissipation and AC Thermal Analysis Electrical Parameters vs. Thermal Parameters Thermal Resistance and Capacitance Networks Understanding the Z th Diagram Example AC Thermal Calculations Complex Waveforms and Superposition

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PDPD 400W t pulse t pulse = 200µs T iunction (t) T peak T peak = ? Single Pulse in a TO-263 Package 25C

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Duty Cycle 50% 20% 10% 5% 2% 1% Single Pulse Single Pulse in a TO-263 Package Z thja (C / W) t pulse (sec) 1E-51E-31E-11E+11E+3 Duty Cycle 50% 20% 10% 5% 2% 1% Single Pulse t pulse = 2E-4 s Z thja C/W

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Power Dissipation P D = 400W Thermal Resistance Z thja = C/W Junction Temperature T junction,peak = T ambient + P D Z thja T junction,peak = 25C + (400W)(0.083C/W) T junction,peak = 25C + 33C = 58C Single Pulse in a TO-263 Package

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Single Pulse – TO-263 Package Saber Simulation Time ( s) P D,max = 400W 400W 350W 300W 250W 200W 150W 0W 100W 50W 55C 50C 45C 40C 35C 30C 25C T junction,peak = 55C

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50% Duty Cycle in a TO-263 Package PDPD 1.44W t pulse = 200µs t period = 400µs T iunction (t) T peak T peak = ? 25C

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Duty Cycle 50% 20% 10% 5% 2% 1% Single Pulse 50% Duty Cycle in a TO-263 Package Z thja (C / W) t pulse (sec) 1E-51E-31E-11E+11E+3 t pulse = 2E-4 s Z thja 23 C/W

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Power Dissipation P D = 1.44W Thermal Resistance Z thja = 23 C/W Junction Temperature T junction = T ambient + P D Z thja T junction = 25C + (1.44W)(23C/W) T junction = 25C + 33C = 58C 50% Duty Cycle in a TO-263 Package

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Advanced Power Dissipation and AC Thermal Analysis Electrical Parameters vs. Thermal Parameters Thermal Resistance and Capacitance Networks Understanding the Z th Diagram Example AC Thermal Calculations Complex Waveforms and Superposition

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Complex Pulse-Superposition MOSFET Turn On I DS 1.V IN goes HI 2.I DS increases 3.V DS decreases 4.P LOSS spikes 1. V IN 2. I DS 4. P LOSS 3. V DS

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I DS Complex Pulse-Superposition MOSFET Turn On 2. I DS 4. P LOSS 3. V DS Pulse 1: t START 20µS t STOP 50µS Pulse 2: t START 25µS t STOP 50µS Pulse 3(neg): t START 40µS t STOP 50µS Pulse 4(neg): t START 45µS t STOP 50µS 1. V IN

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Complex Pulse-Superposition MOSFET Turn On Power Time t=5µs At t = 5µs: t PULSE,1 = 5µsec t PULSE,2 = t PULSE,3 = t PULSE,4 = 0 T J1 (5µs) = Z TH (5µs)*P PULSE,1

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Complex Pulse-Superposition MOSFET Turn On Power Time t=20µs At t = 20µs: t PULSE,1 = 20µsec t PULSE,2 = 15µsec t PULSE,3 = t PULSE,4 = 0 T J2 (20µs) = Z TH (20µs)*P PULSE,1 + Z TH (15µs)*P PULSE,2

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Complex Pulse-Superpositon MOSFET Turn On Power Time t=25µs At t = 25µs: t PULSE,1 = 25µsec t PULSE,2 = 20µsec t PULSE,3 = 5µsec t PULSE,4 = 0 T J3 (25µs) = Z TH (25µs)*P PULSE,1 + Z TH (20µs)*P PULSE,2 - Z TH (5µs)*P PULSE,3

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Complex Pulse-Superpositon MOSFET Turn On Power Time t=30µs At t = 30µs: t PULSE,1 = 30µsec t PULSE,2 = 25µsec t PULSE,3 = 10µsec t PULSE,4 = 5µsec T J4 (30µs) = Z TH (30µs)*P PULSE,1 + Z TH (25µs)*P PULSE,2 - Z TH (10µs)*P PULSE,3 - Z TH (5µs)*P PULSE,4

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Complex Pulse-Superpositon MOSFET Turn On I DS 4. P LOSS Pulse 1: t START 20µS t STOP 50µS Pulse 2: t START 25µS t STOP 50µS Pulse 3(neg): t START 40µS t STOP 50µS Pulse 4(neg): t START 45µS t STOP 50µS T J1 T J2 T J3 T J4

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Advanced Power Dissipation and AC Thermal Analysis Electrical Parameters vs. Thermal Parameters Thermal Resistance and Capacitance Networks Understanding the Z th Diagram Example AC Thermal Calculations Complex Waveforms and Superposition

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Advanced Power Dissipation and AC Thermal Analysis

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Thank you!

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