FREDFET

A fast- recovery epitaxial diode field - effect transistor ( almost - reverse epitaxial diode field - effect transistor, abbreviated FREDFET or FREDFET, English) is a special power field effect transistor which especially for switching inductive loads ( transformers, electric motors) in full bridge circuit ( four-quadrant is or quasi-resonant push-pull switching power supplies ) are suitable.

Construction and application

In this transistor, the semiconductor structure induced by the inverse diode (also called body diode, see structure of an n - MOSFET) with common in comparison with the power MOSFET especially short response times. This is achieved by doping with heavy metals, which significantly reduces the stored charge and the reverse recovery time.

This makes it possible to switch inductive loads in very short time periods. Most FredFETs be used in so-called four-quadrant and other applications with a full bridge (H- bridge ) of MOSFET switches. Without fast inverse diode it would apply in a fast off and reconnection process of inductive loads to a current flow in einschaltenden MOSFET due to the stored charge of the body diode in the opposite MOSFET that can damage the einschaltenden MOSFET. Partly, this can be avoided by the MOSFET at the moment of passing the body diode is turned on, which he takes over the current flow of the diode and it can recover already. At high operating frequencies and low drive level, combined with a high power factor, this time for the diode is not sufficient to freizuwerden. Importantly, therefore, is another Zerstörmechanismus that is because it can result in a not yet freed- body diode and forced her to reverse voltage to the breakthrough of a parasitic transistor structure in this blocking MOSFET, which destroys him.

However have fast rectifier diodes, also known as fast recovery rectifier diodes which are connected externally as a freewheeling diode in parallel with the internal inverse diode, even shorter and therefore better switching behavior. This follows from the fact that an external diode and its structure can be optimized independently of the MOSFET structure of short switching times and forward voltage. This is especially true for fast rectifier diodes based on the semiconductor material, silicon carbide (SiC). At low operating voltages silicon Schottky diodes can be used as freewheeling diodes, which are not only much faster, but completely take over due to their low forward voltage to reverse current flow through the body diode.

Alternative name

The abbreviation FREDFET sometimes called the term freerunning extinction diode FET (FET with freewheeling diode - extinguishing ).