Monday, April 8, 2013

How to Replace a Transistor with a MOSFET

Until mosfets arrived in the field of electronics, transistors ruled the power switching circuits and applications. Though even transistors can not be ignored due to there immense flexibility and low cost, mosfets also have certainly become  hugely popular as far as switching heavy loads is concerned and due to the high efficiency associated with these components.

However even though these two counterparts may look similar with their functions and style, these two components are completely different with their characteristics and configurations.

The main difference between a transistor and a mosfet is that, a transistor operation depends on current and needs to be proportionately increased with the load, whereas a mosfet depends on voltage.

But here the mosfet gets an edge over a transistor, because voltage can be easily manipulated and achieved to the required degrees without much  trouble, in contrast increasing current means greater power thats to be delivered, which results in bad efficiency, bulkier configurations etc.

Another big advantage of a mosfet against the transistor is its high input resistance, which makes it possible to be integrated with any logic IC directly, no matter how big the load may be thats being switched by the device. This advantage also allows us to connect many mosfets in parallel even with very low current inputs (in mA).

Mosfets are basically of two types, viz. enhancement mode type and depletion mode type. In common words these are referred as N-type and P-type mosfets.

The N-type mosfets can be triggered by applying a specified positive voltage at their gate while a P-type mosfet will require just the opposite that is a negative voltage to get triggered.

How to Replace a Transistor with a Mosfet

As explained above a transistor depends heavily on current, and its base current needs to be increased proportionately with increase in its collector load current. But the voltage does not matter as they require just 0.6 to1 volts to get triggered.

With mosfets its just the opposite, you can trigger them with any voltage between 3 and the maximum rated voltage with current as low as 1 to 5 mA. Since the voltage is never an issue with any source like from an IC or any other relevant source and the current being so low, mosfets can be easily interfaced with any source irrespective of the load current.

So it means, in general we can easily and blindly replace a transistor with a mosfet, provided we take care of the relevant polarities.

For an NPN transistor, we may replace the transistor with a correctly specified mosfet in the following manner:

  • Remove the base resistor from the circuit because we dont typically need it anymore with a mosfet.

  • Connect the gate of the N-mosfet directly to the trigger source.

  • Keep the positive supply connected to one of the load terminals, and connect the other terminal of the load to the drain of the mosfet.

  • Lastly, connect the source of the mosfet to ground.......DONE, you have replaced the transistor with a mosfet within minutes. 
The procedure will remain as above even for a PNP transistor to be replaced with a  P-channel mosfet, you will need to just reverse the relevant supply polarities.

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