Junction Temperature
Calculate the junction temperature of a power semiconductor, T_j = T_a + P × R_th, from the ambient temperature T_a, the dissipated power P and the total junction-to-ambient thermal resistance R_th (°C/W). The result, in °C, is the device's internal temperature (silicon junction), which must not exceed the manufacturer's limit (typically 150 °C) on pain of failure. The thermal resistance adds the junction-to-case, case-to-heatsink and heatsink-to-ambient stages. Lowering R_th (larger heatsink, ventilation, thermal paste) lowers the junction temperature. It is the central calculation of power electronics thermal design. Enter the ambient temperature, the dissipated power and the thermal resistance.
Result
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Temperatura de junção
O ponto mais quente e mais crítico de um componente eletrônico de potência (transistor, diodo, regulador) é a sua junção — a região ativa do semicondutor de silício onde o calor é gerado. Se a temperatura de junção ultrapassar o limite do fabricante (tipicamente 150 °C, às vezes 175 °C para dispositivos modernos), o componente falha — por degradação, fuga térmica ou destruição imediata. Por isso o dimensionamento térmico é tão vital quanto o elétrico, e seu cálculo central é T_j = T_a + P × R_th, onde T_a é a temperatura ambiente, P é a potência dissipada (a soma das perdas de condução e chaveamento) e R_th é a resistência térmica total da junção ao ambiente (em °C/W — quantos graus a junção sobe por watt dissipado). A analogia com circuitos elétricos é perfeita e poderosa: a potência dissipada é como uma 'corrente' de calor, a resistência térmica é como uma resistência elétrica, e a diferença de temperatura é como uma 'tensão'. E assim como resistências em série se somam, a resistência térmica total é a soma das etapas do caminho do calor: junção-encapsulamento (R_jc, interna ao componente), encapsulamento-dissipador (R_cs, através da interface, melhorada com pasta térmica) e dissipador-ambiente (R_sa, a maior e mais controlável). Para baixar a temperatura de junção, ataca-se a resistência térmica: usar um dissipador maior e com mais aletas (reduz R_sa), forçar ventilação (ar ou líquido, reduz drasticamente R_sa), aplicar boa pasta ou pad térmico (reduz R_cs), ou simplesmente reduzir a potência dissipada (componentes mais eficientes). O cálculo inverso é igualmente útil: dado o limite de T_j, a temperatura ambiente máxima esperada e a potência, determina-se a resistência térmica máxima do dissipador necessário — e daí se escolhe o dissipador no catálogo. É o cálculo que decide se um componente sobrevive ou queima. Informe a temperatura ambiente, a potência dissipada e a resistência térmica.
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