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).The superior characteristics of MOSFETs are.POWER TRANSISTOR – MOSFET Example of power MOSFET parameters

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