thermal resistance from the junction to the case, JC The temperature conduction process may be represented as follows:.maximum operating junction (device) temperature, TJmax.Manufacturers’ data sheet for power devices generally give:.POWER TRANSISTOR Heat sinks (Extra) Temperature of transistor junction Ambient temperature Electrical equivalent circuit of thermal-conduction process Temperature difference Voltage difference Thermal resistance between the junction and ambient Electrical resistance Thermal power through the element Electric current. Additionally, heat sinks can be used to remove the heat developed in the transistor:.Heat may be removed through proper packaging:.This heat, if not properly removed, may cause internal temperature above a safe limit and can cause permanent damage to transistor.The power dissipated in a transistor can cause an internal temperature rise above ambient temperature.More immune to thermal runaway and second breakdown. MOSFET Simple input circuitry because of low input current (voltage-controlled device). POWER TRANSISTOR – Comparison BJT Requires complex input circuitry because of high input current (current-controlled device) More sensitive to temperature variation – thermal runaway and problem of second breakdown. POWER TRANSISTOR – MOSFET The “ON” resistive path between drain and source (rds(on)) is an important parameter in power capability of MOSFET A single power MOSFET may contain as many as 25,000 parallel cells. POWER TRANSISTOR – MOSFET Structure These hexagonal cells can be paralleled to form large-area devices without the need of emitter ballast resistance. POWER TRANSISTOR – MOSFET Structure DMOS process can be used to produce a large number of hexagonal cells on a single chip. POWER TRANSISTOR – MOSFET Transfer characteristics curves for various values of temperature. POWER TRANSISTOR – MOSFET Transconductance versus drain current curves for various values of temperature – less than the variation in BJT current gain. Stable gain and response time over a wide temperature range (Figure on the next slide).
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