1_Starkishawa Howard 3080 Power Mosfets

1_Starkishawa Howard 3080 Power Mosfets - Power MOSFETs...

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Unformatted text preview: Power MOSFETs Power MOSFETs Starkishawa Howard ECE 3080 Spring 2011 Dr. Alan Doolittle 2 Introduction Introduction Power MOSFETs were first developed in the late 1970's, first being created by International Rectifier in 1978. While Power MOSFETs are voltage controlled, while Bi-polar transistors are current controlled. In these respects, power MOSFETs approach the characteristics of an ideal switch. They can be found in most power converters, power supplies and low-voltage controllers. 3 Advantages Advantages Power MOSFETs are efficient at lower voltages, and are the most common low-voltage switches. Power MOSFETs utilize a vertical structure, which allows the transistor to sustain high current with high-blocking voltage. The drive circuit design is simple and less expensive. It has a wider SOA than the BJT because high voltage and current can be applied simultaneously for a short duration. This eliminates destructive device failure due to second breakdown. They have superior switching speed, have high input impedance, and they require very little gate drive power because of the insulated gate. Being a majority carrier device they do not suffer from minority carrier storage time effects, thermal runaway or second breakdown. 4 Disadvantages Disadvantages The gate oxide is very thin (100 nm or less), so it can only sustain a limited voltage. The conduction loss of a MOSFET is larger than that of a BJT, which has the same voltage and current rating due to the on-state voltage drop. The drain current is limited by heating due to resistive losses in internal components such as bond wires. The packaging often limits the maximum junction temperature, which must stay under a specified maximum value for the device to function reliably. 5 The Structure of the Power MOSFET The Structure of the Power MOSFET 6 Lateral Channel Design The drain, gate, and source terminal are placed on the surface of a silicon wafer. This is suitable for integration but not for obtaining high power ratings since the distance between source and drain must be large to obtain better voltage blocking capability. Also, the drain-to-source current is inversely proportional to the length. Vertical Channel Design The drain and source are placed on the opposite sides of a wafer. This is suitable for a power device, as more space can be used as source. As the length between the source and drain is reduced, it is possible to increase the drain-to-source current rating, and also increase the voltage blocking capability by growing the epitaxial layer (drain drift region). 7 Intrinsic and Parasitic Elements Intrinsic and Parasitic Elements 8 Body Diode Body Diode The body-drain p-n junction connected between the drain and source forms an intrinsic diode called the body diode....
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1_Starkishawa Howard 3080 Power Mosfets - Power MOSFETs...

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