MOSFET VGS

[Kevin Weddle]

MOSFETS tend to have a high VGS requirement. I see them used a lot in power supplies. But the real benefits in using a MOSFET over a biploar junction transistor, although not immediately noticeable from a schematic, don't appear to be utilized. Might it be better not to use MOSFETS at all? Or are they just a good option because of availability?

 

[Hero999]

MOSFETs have a faster switching speed, lower on loss and are easier to drive than BJTs, the gate also requires no current, at DC.

 

[Kevin Weddle]

Saturated biploar transistors do have a lot of base current, but the power dissipation is the same as a MOSFET. A bipolar transistor can be put into cutoff or saturation by 1V. A MOSFET reguires a high VGS difference between saturation and cutoff, maybe 5V. But I know that biploar transistors require biasing resistors all of the time, where as the biasing for a MOSFET might save power depending on the biasing resistors. Is there a major guideline in choosing between a MOSFET and a biploar transistor for low frequency applications? And I've read that MOSFET linearity is better, or is it marginally better?

 

[Hero999]

MOSFETs can be cheaply purchased with on resistances as low as 5mΩ so the voltage drop is only 50mV at 10A.

I wouldn't say that 5V is a high voltage unless you want to power the device from two AA cells.

Using a MOSFET invariably saves power, especially in DC applications. To achieve a good saturation voltage with a BJT ther base current typically needs to be 1/10th of the collector current and if the circuit driving the base is the same voltage as the collector load, 10% of the power is wasted on driving the transistor. Using a MOSFET in an application such as this will result in a 9% power saving.

BJTs are better at higher voltages. A MOSFET's channel resistance increases exponentially with increasing break down voltage while the saturation voltage of a BJT remains roughly constant. At some point (around 400V) the on voltage loss becomes lower with BJTs compared to MOSFETs and if the base drive circuit is powered from a lower voltage than the collector circuit, then the power wasted driving the transistor drops.

IGBTs are now replacing BJTs now in high voltage switched mode power supplyies so the BJT is becomming less common all the time.

 

[Kevin Weddle]

So there isn't really much difference between using a MOSFET and a bipolar transistor. That is what I expected, however a good design needs to utilize one or the other based on component charateristics and circuit requirements.

 

[indulis]

MOSFETS tend to have a high VGS requirement.

Not true... I use small MOSFET's (Vishay) that have a  Vgs of 1.2V... I wouldn't consider that high.

So there isn't really much difference between using a MOSFET and a bipolar transistor.

No... re-read and UNDERSTAND what Hero999 just said!!!!!

That is what I expected...

If you are really serious when you say that,  I'd recommend you get yourself a good book on semiconductor physics read that, then read-up on BJT's, JFET's and MOSFET's... after you have done that, let us know if you still think there isn't really much difference.

 

[Hero999]

So there isn't really much difference between using a MOSFET and a bipolar transistor. That is what I expected, however a good design needs to utilize one or the other based on component charateristics and circuit requirements.

You're not listening - there's a huge difference between a MOSFET and a BJT.

 

[homeguy]

and I can recommend a great book. Integrated Electronics-by Millman-Halkais.......Just wanted my tuppenceworth !!!!!!