admissible dissipation diodes, rectifiers Zener diodes which operate f

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admissible dissipation diodes, rectifiers Zener diodes which operate f

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PULSE POWER RATING SEMICONDUCTORS
admissible dissipation diodes, rectifiers Zener diodes which operate from sinusoidal supplies based arithmetic mean value junction temperature power dissipation. Devices which handle pulses capable passing short-term currents excess maximum admissible static dissipation, this case admissible exceed continuous dissipation curve duration each pulse. magnitude admissible current then inversely proportional pulse duty factor, because power dissipated only intermittently, thermal capacity system heat conduction prevent undue rise junction temperature. Some data sheets contain diagrams which allow rating device operating under pulsed conditions determined. Figure which applies diodes rectifiers, maximum admissible pulse current amplitute plotted function pulse duration ambient case) temperature 25°C. device operate higher ambient temperatures, then necessary derate current values derived from this diagram accordance with "admissible dissipation versus temperature" curve. Zener diodes preferable provide plot which gives terminal pulse resistance rather than admissible current amplitude function (the duration rectangular pulse which causes power dissipated), shown Figure operational junction temperature then calculated formula Tamb rthA, additional power continuously dissipated, formula Tamb RthA rthA, diode fitted heat sink, then equation becomes Tamb Ptot RthS rthC, where Ptot mean value pulse dissipation). additional power continuously dissipated, then above equation must extended Tamb Ptot RthS RthC rthC, where Ptot mean value total dissipated power. operation semiconductor device involves dissipation power with consequent rise junction temperature. Because maximum admissible junction temperature must exceeded, careful circuit design with regard only electrical, also thermal performance semiconductor circuit essential.
Figure
Figure
Heat Removal from Semiconductor Components
dissipated power low, then sufficient heat radiated from surface case; dissipation high, however, additional steps have taken promote this process reducing thermal resistance between junction ambient air. This achieved either pushing star- flag-shaped heat dissipator over case, bolting semiconductor device heat sink. power dissipated, junction temperature, Tamb, ambient temperature related formula
Copper Cooling
Rths
Tamb RthA
Tamb RthA RthS
Thickness
where RthA total thermal resistance between junction ambient air. total thermal resistance turn comprises internal thermal resistance between junction mounting base, outer thermal resistance RthS between case surrounding other cooling medium). should noted that only outer thermal resistance affected design heat sink. determine size heat sink required meet given operating conditions, proceed follows: First calculate outer thermal resistance formula
Length edge
Aluminum Cooling
RthS
Tamb
RthC
then, following diagrams, determine size heat sink which provides calculated RthS-value. determine maximum admissible device dissipation ambient temperature limit given heat sink, proceed reverse order that described above. calculations based following assumptions: squareshaped heat sink without finish, mounted vertical position; semiconductor device located centre sink; heat sink operated still subjected additional heat radiation. calculated area should increased factor sink mounted horizontally, reduced factor approximately black finish used. following curves give thermal ambient resistance square vertical heat sinks function side length. assumed that heat applied centre square.
Rths
Thickness
Length edge
Steel Cooling
Rths
Thickness
Length edge

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