temperature determined ambient temperature heat dissipated total therm

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temperature determined ambient temperature heat dissipated total therm

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COMPUTING TEMPERATURE RISE
temperature determined ambient temperature heat dissipated total thermal resistance This total thermal resistance comprised three individual component resistances: chip lead frame heat sink
TEMPERATURES RISE
Heat enemy integrated circuits-particularly power devices. Here's thermal ratings determine safe operation. Excessive heat shortens life reduces operating capability. Until recently, were capable operating only lowpower applications requiring perhaps milliwatts power. now, handle several amperes drive devices such relays, solenoids, stepping motors, incandescent lamps. These high power levels increase temperatures substantially capable destroying devices unless appropriate precautions taken.
THERMAL CHARACTERISTICS
thermal characteristics determined four parameters. Maximum allowable chip junction temperature thermal resistance specified manufacturer. Ambient temperature power dissipation determined
Reprinted permission from June 1977 issue MACHINE DESIGN, Copyright 1977 Penton/IPC Inc., Cleveland, Ohio.
COMPUTING TEMPERATURE RISE
user. Equation expresses relation these parameters.
Junction temperature usually limited +150°C silicon ICs. Devices operate momentarily slightly higher temperatures, device life expectancy decreases exponentially extended hightemperature operation. Usually, lower junction operating temperature, greater anticipated life Ambient temperature traditionally limited either 70°C 85°C plastic dual in-line packages (DIPs) 125°C hermetic devices. Again, objective operate junction temperature practical. Thermal resistance basic thermal characteristic ICs. usually expressed terms °C/W represents rise junction temperature with unit power applied still air. reciprocal thermal resistance thermal conductance, derating factor, expressed W/°C. Thermal resistance consists several distinct components, which specified thermal resistance. typical these components thermal resistance 0.5°C/W unit thickness silicon chip, 3°C/W unit length lead frame, 2,000°C/W unit thickness still surrounding DIPs used more than other type packaging copper-alloy lead frames provide superior thermal rating over standard iron-nickel-cobalt alloy (Kovar) lead frames. However, power also available other packages such PLCCs, SOICs, power-tabs. power that safely dissipate usually depends size chip type packaging. Most common copper-frame DIPs dissipate about although some special-purpose types have ratings high
Total power dissipated depends input current, output current, voltage drop, duty cycle. Thus, many industrial digitalcontrol ICs, logic-gate power (typically less than output power must determined find total power dissipated. Total power dissipation these logic devices n(VCCICC) (SAT)IC)
where logic-gate supply voltage, logic-gate supply current, VCE(SAT) output saturation voltage, output load current, number logic gates. Manufacturers usually list typical maximum values these voltages currents. thermal considerations best maximum values that worst-case power dissipation determined. duty cycle device longer than peak power dissipation logic-gate power output power logic state alone. time less than however, average power dissipation must calculated from instantaneous power POFF from DPON (1-D) POFF
CORRECTIVE ACTIONS
junction temperature required power dissipation calculated greater than maximum values specified manufacturer, device reliability operating characteristics possibly will reduced. Possible solutions are: Modify partition circuit design required dissipate much power. Reduce thermal resistance using heat sink forced-air cooling. Reduce ambient temperature moving heat-producing components such transformers resistors away from Specify different with improved thermal electrical characteristics available).
Northeast Cutoff, 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000
COMPUTING TEMPERATURE RISE
SETTING CIRCUIT
MEASURING TEMPERATURE
Sometimes junction temperature cannot calculated readily instead must measured. Measurement should made when there insufficient data with which calculate, when effects external variables such forced-air cooling enclosure size must determined, check manufacturer's specifications regarding package thermal resistance. most popular technique measuring temperature uses characteristic diode reduce forward voltage with temperature. Many chips have some sort accessible diode-parasitic, input protection, base-emitter junction, output clamp. With this technique, "sense'' diode calibrated that forward voltage direct indicator diode junction temperature. Then, current applied some other component chip simulate operating conditions produce temperature rise. Because thermal resistance silicon chip low, temperature sense diode assumed same rest monolithic chip. sense diode should calibrated over least expected junction operating temperature. Apply accurately measured, current about through sense diode measure forward voltage 25°C increments after stabilization each temperature. This calibration provides enough data least points construct diode-forwardvoltage versus junction-temperature graph specified forward current. typical 25°C forward voltage between decreases mV/°C. power levels above necessary more than single transistor only device saturation voltage sink current used. higher power desired, keep output saturation. Measuring sense-diode forward voltage require considerable waiting period minutes) thermal equilibrium. event, instant measurement, heating power have disconnected because erroneous readings result from drop circuit common leads. Various circuit connections (such four-point Kelvin) arranged reduce eliminate this source error. junction temperature determined comparing voltage measurement with internal power source against voltage measurement with temperature chamber.
CALIBRATING SENSE DIODE
COMPUTING TEMPERATURE RISE
FINDING SAFE OPERATING LIMITS
Here's calculate safe operating limits first examples simple calculations involving maximum allowable power straightforward. third fourth examples more complex involve logic power, output power, duty cycle. Problem: Determine maximum allowable power dissipation that handled safely 16-lead Kovar with 125°C/W ambient temperature 70°C. Solution: From Equation maximum allowable power dissipation this 150°C 70°C Thus, total worst case power dissipation plus 1.225 From Equation maximum junction temperature 70°C (1.225 (16.67 mW/°C) 143.5°C Problem: Determine acceptable duty cycle power driver with thermal resistance 100°C/W ambient 85°C which controlling load currents each four outputs. Solution: From Equation allowable average power dissipation this 150°C 85°C
125°C/W 0.64 Problem: Determine maximum allowable power dissipation that handled 14-lead copper with derating factor 16.67 mW/°C ambient 70°C. Solution: Because derating factor reciprocal thermal resistance maximum allowable power dissipation from Equation (150°C 70°C) (16.67 mW/°C) 1.33 Problem: Calculate maximum junction temperature quad power driver wlth thermal resistance 60°C/W ambient 70°C which controlling load each four outputs. Solution: determine maximum (worst case) junction temperature this maximum total power dissipation must determined from data listed data sheet. specifications usually listed typical minimum maximum values. important maximum voltage current limits ensure adequate design. Common maximum values industrial power driver 5.25 VCE(SAT) From Equations worst case logic output power dissipation (5.25 (0.7
100°C/W 0.65
This means that there 0.65 limit average power, but, instantaneous power. duty cycle enough, time more than about average power dissipation considerably lower than peak power. peak power, determined from data sheet maximum values VCC, ICC, VCE(SAT) specified load current From Equations logic-gate power output power state (5.5 26.5 (0.7 Instantaneous power state, 1.283 power primarily power dissipated logic state, found using maximum rated current listed specification sheet. power dissipated output stage calculated from leakage current supply voltage VCE. From Equations Iogicgate power output power state (5.5 (100
Northeast Cutoff, 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000
COMPUTING TEMPERATURE RISE
Instantaneous power POFF state, From equation acceptable duty cycle POFF 0.65 0.205 1.283 0.205
WHAT CURVES SHOW
junction temperature depends several factors, including thermal resistance operating duty cycle. Graphs showing relationship these factors often useful specifying Typical thermal-resistance ratings still range from 60°C/W 140°C/W. slope each curve this graph equal derating factor which reciprocal thermal resistance ambient temperature 50°C, typical 14-lead flatpack with 140°C/W dissipate about typical DIP, however, with copper-alloy leads dissipate almost 50°C. highest allowable package power dissipation shown here Other special-purpose packages available with power dissipation ratings high 45°C/W). package limitations, chip dissipation might greater than ambient temperature 70°C. Although curve plastic DlPs goes +150°C, they ordinarily used ambients above +85°C because traditional package limitations.
Duty cycle important calculating junction temperature because average power-not instantaneous power-is responsible heating convert from peak power average power, multiply peak power dissipation duty cycle. averagepower rating then used with thermal-resistance rating calculate junction temperature. Thus, duty cycles allow peak power high without exceeding 150°C junctiontemperature limit. However, this consideration applies only times less than second.

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