What is an Intelligent MOSFET

 
 
 

The Intelligent MOSFET
    Traditional MOSFETs have had a reputation for unexplicable failure in use. These failures are often attributed to such things as over voltage, over current, over temperature etc... although after the event it is often difficult to pinpoint the true reason. Careful design of the MOSFET circuit can help enormously but if there is scope for the load conditions to change (eg driving a different motor) then there is also scope to take the MOSFET near to or exceeding its design specifications. This has lead to the alternative meaning of the MOSFET acronym .. 'Magically Obliterated, Smoke and Fire Emitting Transistor'. What was needed was a MOSFET that included some of the device protection circuitry normally present on the circuit board to protect it. This need was met by the "Intelligent MOSFET" sometimes referred to as the "Omnifet" (although this is actually a manufacturers product name).
      A block diagram of such a device is shown below. It can be seen that the device still has the outward appearance of a standard 3-terminal MOSFET even down to the traditional T0-220 package (as illustrated).




    Inside the device, however, there are a number of integrated protection facilities that serve to protect the device from damage. These are.....

Over Temperature.
    During normal operation, the Input pin is electrically connected to the gate of the internal power MOSFET. The device then behaves like a standard power MOSFET. Overtemperature is based on sensing the chip temperature and is not dependent on the input voltage. The location of the sensing element on the chip in the power stage area ensures fast, accurate detection of the junction
temperature. Overtemperature cutout occurs, typically, at a minimum of 150oC. When detected the signal from the input pin is disconnected from the internal MOSFET gate and effectively turns it off. The device is then automatically restarted when the chip temperature falls below 135oC.

Overvoltage Clamp Protection
   This can be important when driving inductive loads such as motors and solenoids. As the MOSFET is turned off, voltage spikes can be generated that can cause damage. Under these conditions an internal voltage clamp operates protecting the MOSFET.

Linear Current Limiter.
    If the current through Drain - Source exceeds the device specifications it is automatically reduced and limited. This operation takes the device into its linear operating region and will inevitably cause an increase in power dissipation within the device. However, even if this extra power dissipation causes the device to overheat the over temperature protection will then come into play, as described above, and protect the device.

Short Circuit Protection
    Connecting different loads to any MOSFET circuit can easily lead to short circuits. In this situation the device is protected again by the overtemperature facility disconnecting the signal to the internal gate and turning it off until the short circuit has been rectified.

   Although you can never say that a MOSFET is 100% protected from abnormal conditions, the above protection facilities address and minimise the risks from the most common sources of damage and make such devices extremely reliable in use. The intelligent MOSFETS are usually more expensive than their ordinary equivalents but this cost can easily be offset against the increased reliability (and consequent life expectancy) of any application using them.

   
                   
    Example of a device using Intelligent MOSFETs    
                   
 

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