Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

Currently I am doing a project on a voltage regulator. Can you tell me some advantages of MOSFETs over BJT? Apart from a higher power loss, I can not find much. In control engineering it is much more difficult to deal with them. Would be great if you could tell me a few.

share|improve this question
2  
Please elaborate on 'much more difficult to deal with them'. To me, controlling a series-pass MOSFET gate voltage is trivially easy with an op-amp. –  Adam Lawrence Aug 8 at 15:26
    
to get stability with pmos is not as easy as it sounds. They start oscillating very easy. –  Simon Aug 8 at 15:43
1  
Anything with a feedback loop can oscillate very easily. Also please update your question if you specifically want to consider PMOS / PNP. –  Adam Lawrence Aug 8 at 16:45

1 Answer 1

In a linear regulator, switching speed isn't a consideration. However, BJT base current can be. There's no steady-state gate current in a MOSFET so a MOSFET can be lower loss.

Also, a MOSFET in the on state looks resistive, so the dropout voltage can be very small at low currents. A BJT will saturate and may lead to higher dropout voltage at low currents.

At higher currents a BJT can suffer from low current gain.

All of these things can be mitigated by proper design, but they are definitely considerations when choosing a pass element.

share|improve this answer
1  
Just to add one downside of MOSFETS, if you want to use an N-Channel you have to have a way to bring the gate higher that the input voltage, and if you use a P-FET the area will be significantly larger than an N-FET or BJT for similar performance. –  John D Aug 8 at 15:34
    
that's true, so far I have not managed a pmos regulator to stabilize. They start pretty quickly oscillate at higher loads, despite snubber etc. –  Simon Aug 8 at 15:46
1  
Sounds like you don't use appropriate compensation - BTW snubbers are used for suppressing high-frequency switching noise, not in regulator control loops. There's nothing inherently wrong with using a MOSFET as a regulator. –  Adam Lawrence Aug 8 at 16:47

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.