Need help choosing a Power MOSFET

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Discussion Overview

The discussion revolves around selecting a suitable power MOSFET for an application requiring a maximum power handling of 30W and a resistance range of 1-25 ohms, specifically for use as a voltage-controlled resistor. Participants explore various options, specifications, and considerations related to the choice of MOSFETs.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant seeks recommendations for a power MOSFET that meets specific power and resistance criteria, noting high costs and long lead times for suggested parts from Microsemi.
  • Some participants suggest looking into products from International Rectifier as potential alternatives.
  • Another participant expresses uncertainty about the suitability of off-the-shelf power MOSFETs for the specified resistance range, discussing the concept of R(on) resistance and its implications.
  • One participant proposes that the choice of FET depends on the voltage source and emphasizes the importance of matching the load to the source resistance for optimal power transfer.
  • Concerns are raised about the impact of the body diode present in the selected MOSFET on its functionality as a voltage-controlled resistor.
  • A suggestion is made to consider using multiple FETs to distribute the load, potentially reducing the need for extensive heat sinking.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the body diode in MOSFETs and the best approach to selecting a suitable device. There is no consensus on a specific MOSFET or approach, and multiple competing perspectives remain throughout the discussion.

Contextual Notes

Participants mention various specifications and characteristics of MOSFETs, including R(on) resistance and body diode functionality, but do not resolve the implications of these factors for the specific application. The discussion also highlights the variability in pricing based on voltage ratings.

Who May Find This Useful

This discussion may be useful for engineers or hobbyists looking for guidance on selecting power MOSFETs for specific applications, particularly those involving voltage-controlled resistance and power handling considerations.

atlbraves49
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I need help finding a MOSFET to buy that can handle max power of 30W, and that can achieve a resistance of 1-25ohms (using it as a voltage controlled resistor).

Microsemi originally pointed me to these 2 of theirs:
http://www.microsemi.com/catalog/part.asp?ID=21208
http://www.microsemi.com/catalog/part.asp?ID=21223

Which would work perfectly,
but the cost was $75 each with minimum of 20, and 16 week leadtime.

I know most power mosfets arent nearly as expensive, and with much shorter leadtimes.. but I need help picking one that can achieve the above specs.

Help is greatly appreciated
 
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Have you tried International Rectifier?
https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672"
 
Last edited by a moderator:
dlgoff said:
Have you tried International Rectifier?
https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672"

I guess i need more help with picking out a specific one based on the application (has to handle 30W, and be able to create a resistance across the Drain/Source of 1-25ohms).
 
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You can ask microsemi for samples.

Not being an expert on semiconductor, I believe just about any off the self Power MOSFET will fit that criteria you need of 1-25 Ohms. Correct me if I am wrong, the spec that manufacturer gives usually is maximum R(on) resistance, which is in the miliohms. This is the resistance between the Drain and Source when the channel is present for current to flow. When the FET is off, channel is not there, the resistance is infinite (or at least several megaohms). So you can see how the resistance range of 1-25 ohms will fit in the range of milliohms to megaohms.

Your using the FET for matching impedance to get the most power transfer right? I was thinking about when you first posted your problem. I have a rough idea and want to see what your thoughts are. Others are welcome to chip in of course.
 
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yes Corneo that is right (about our application).
 
Here's my idea's

http://www.flickr.com/photos/35159644@N08/3255121034/

The FET is irrelevant as long as you chose the right one based on the voltage source you got. So if your voltage source is positive, then you need an N channel FET. Since you know that to get maximum power transfer, the load (FET) has to be equal to the source resistance (Rs = 5 ohms in my example). When this occurs you have an even voltage drop as per voltage divider rule. The op amp driving the gate of the FET will automatically find the right gate voltage to set the drain source resistance to match Rs.

The reason I chose something like this is because I know based on my experience that pinchoff voltage on FETs vary several volts as per manufacturer's data sheet. One date code of FETs can have several volts difference from another date code. To make it work for all FETs, you need an circuit to automatically find the right voltage for you.

Since I don't know what exactly are your specs to meet your project, you should make sure the FET you chose can handle the power dissipation and maximum voltage on the drain. Make sure the resistors are chose properly as well.
 
atlbraves49 said:
So i originally chose this one..

http://www.irf.com/product-info/datasheets/data/irf640npbf.pdf

but i notice on the symbol there shows a diode of some sort? Will this impact my usage of this as a voltage-controlled resistor in anyway? or .. what is the purpose for this diode?
I believe this diode is due to the MOSFETs construction which prevents spurious latching. It's called the http://en.wikipedia.org/wiki/Power_MOSFET#Body_diode".
...a diode between the drain (cathode) and the source (anode) of the MOSFET, making it able to block current in only one direction.
 
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Why one FET when more can share the load? Distributing the load means you don't need an unrealistic heat sink.

You don't care much about the intergral body diode unless you want to block back-current, but what voltage??

Voltage plays a major role in price. I'm seeing unit prices around 60 cents per thousand, not 60 dollars.
 
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